[National Emergency Specifications for the Design of Reinforced Concrete Buildings]
[From the U.S. Government Publishing Office, www.gpo.gov]
WAR PRODUCTION BOARD NATIONAL EMERGENCY SPECIFICATIONS FOR THE DESIGN OF REINFORCED CONCRETE f BUILDINGS .
DATED NOVEMBER 10, 1942
WAR PRODUCTION BOARD
Part 905—Specifications
[Directive 9]
NATIONAL EMERGENCY SPECIFICATIONS FOR THE DESIGN OF REINFORCED CONCRETE BUILDINGS
Pursuant to the authority vested in me by Executive Orders No. 9024 of January 16, 1942, No. 9040 of January 24, 1942, and No. 9125 of April 7, 1942, and pursuant to the policy stated in the Joint Directive of the War Production Board and the War and Navy Departments dated May 20, 1942, and the Army and Navy Munitions Board “List of Prohibited Items for Construction Work,” dated April 1, 1942, as revised June 29, 1942, the following policy is prescribed (1) for the War Production Board and for the Army, Navy, Maritime Commission, Reconstruction Finance Corporation, National Housing Agency, and (2) for all other Departments and Agencies in respect to war construction and the financing of war construction.
§ 905.2 National Emergency Specifications for the design of reinforced concrete buildings—Adoption of specifications, (a) National Emergency Specifications for the Design of Reinforced Concrete Buildings issued by the War Production Board on October 5, 1942, shall apply to and shall govern the designing of reinforced concrete buildings which are constructed by, or the construction of which is financed by, or the construction of which must be approved by any of such departments or agencies, the contracts for which are placed after a date sixty days after the issuance of this directive. Such departments and agencies are, however, empowered to put such emergency specifications into immediate effect wherever possible.
(b) With respect to any such contracts already placed by any of said departments or agencies or entered into prior to a date sixty days after the issuance of this directive, the department or agency concerned shall review the contract promptly and shall change to said emergency specifications unless such change will result in any substantial delay in the war effort.
(c) The department or agency undertaking or approving the construction shall obtain from the person in responsible charge of the design of each such building a certificate to the effect that such emergency specifications have been complied with. In cases where Forms PD-200 and PD-200A must be filed with the War P&duction Board in order to obtain authorization to begin construction of such building, such certificate shall be filed with said forms.
(d) Authority to depart from the provisions of this directive may, upon specific request, be granted by the Director General for Operations of the War Production Board, or by such person or persons as he may designate for this purpose.
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(P.D. Reg. 1, as amended, 6 F.R. 6680; W.P.B. Reg. 1, 7 F.R. 561; E.O. 9024; 7 F.R. 329; E.O. 9040, 7 F.R. 527; E.O. 9125, 7 F.R. 2719; sec. 2 (a), Pub. Law 671, 76th Cong., as amended by Pub. Laws 89 and 507, 77th Cong.)
Issued this 5th day of October 1942.
Donald M. Nelson, Chairman, War Production Board.
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NATIONAL EMERGENCY SPECIFICATIONS FOR THE DESIGN OF REINFORCED CONCRETE BUILDINGS
Chapter 1—General Requirements and Working Stresses
100—Notation.
yc=Compressive unit stress in extreme fiber of concrete in flexure.
y'c=Ultimate compressive strength of concrete at age of 28 days unless otherwise specified.
/s=Tensile unit stress in longitudinal reinforcement; nominal working stress in vertical column reinforcement.
-u=±=Bond stress per unit of surface area of bar.
v— Shearing unit stress.
ve=Shearing unit stress permitted on the concrete.
101—General Requirements.
(a) Reinforced concrete designs and details shall be so selected as to use a minimum amount of steel reinforcement. To accomplish this, the design shall embody a maximum of symmetry and simplicitv of lay-out and a minimum of ornamentation and shall comply with the requirements of the Directive for War-Time Construction, dated May 20, 1942, and the “List of Prohibited Items for Construction Work” of the Army-Navy Munitions Board, dated June 29, 1942, as amended. Nonreinforced concrete or masonry shall be used in footings, walls and piers of substructures, gravity or semigravity type retaining walls and buttresses in lieu of reinforced concrete construction wherever practicable. Fill under concrete slabs shall be thoroughly consolidated so that the reinforcement may be reduced to a minimum or eliminated entirely.
Wherever practicable, the width and depth of members shall be increased to avoid the use of compressive reinforcement and to minimize the use of web reinforcement and special anchorage. Wherever prestressed reinforced concrete construction can be used to advantage with resulting savings in steel, it shall be adopted.
The amount of reinforcement in concentrically loaded columns shall be kept to a minimum by—
1. Using rodded columns in preference to spiral columns.
2. Using not less than 0.5 percent and not more than 2.0 percent of longitudinal reinforcement.
3. Using high-strength concrete.
102—Allowable Unit Stresses in Concrete.
(a) The unit stresses in pounds per square inch on concrete to be used in the design shall not exceed the following values where f 'c equals the minimum specified ultimate compressive strength at 28 days, or at the earlier age at which the concrete may be expected to receive its full load.
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Table 102 (a).—Allowable Unit Stresses in Concrete
Flexure fe:
Extreme fiber stress in compression_______________________________ f c
but not to exceed 900 p. s. i.
(Upon certification by the designer that an over-all saving of steel will be effected, this allowable stress may be increased for special members or special details to a maximum of 0.45 f'e but not to exceed 1,200 p. s. i.) Extreme fiber stress in tension
Wherever plain concrete is used in flexure the tension in the extreme fiber shall be 0.04 f'e
Shear v:
Beams with no web reinforcement and without special anchorage of longitudinal steel_________________________________________ vc=0.02 /' c
Beams with no web reinforcement but with special anchorage of longitudinal steel________________________________________ t>c= 0.03 /'c
Beams with properly designed web reinforcement but without special anchorage of longitudinal steel______________________ v =0.06 f'e
Beams with properly designed web reinforcement and with special anchorage of longitudinal steel______________________ v =0.12 f'e
*Flat slabs at distance d from edge of column capital or drop panel. _ ve—0.03 f'e
**Footings________________________________________________________ vc=0.03 f'e
J Bond u:
In beams and slabs and one-way footings:
Plain bars_________________________«=0.044 f'e but not to exceed 160 p. s. i.
Deformed bars______________________«=0.055 f'e but not to exceed 200 p. s. i.
In multiple-way footings:
Plain bars (hooked)________________«=0.050 f'e but not to exceed 160 p. s. i.
Deformed bars______________________«=0.062 fe but not to exceed 200 p. s. i.
Bearing
On full area_______________________/c=0.25 f'c
On one-third area or lessf_________/c = 0.375/'c
103—Allowable Unit Stresses in Reinforcement.
The unit stresses in lb. per sq. in. on reinforcement shall not exceed the following values. Lower unit stresses than those herein specified shall not be used in the calculations for the design of the structure.
(a) Tension in Longitudinal and Web Reinforcement
Structural grade steel bars_________________________f,=20,000 p. s. i.
Intermediate grade steel bars, and hard-grade bars (billet steel, rail steel, axle steel, and cold-drawn steel wire)__________________________________________f„ = 24,000 p. s. i.
(ó) Tension in One-Way Slabs of Not More than 12 Feet Span
For the main reinforcement, % inch or Ifss in diameter, in oneway slabs, 60 percent of the minimum yield point specified for the particular kind and grade of reinforcement used, but in no case to exceed 30,000 p. s. i.
(c) Compression in Vertical Column Reinforcement
Forty percent of the minimum yield point specified for the particular kind and grade of reinforcement used, but in no case to exceed 30,000 p. s. i.
•See Section 407.
••See Sections 505 and 408.
tThe allowable bearing stress on an area greater than one-third but less than the full area shall be interpolated between the values given.
{Where special anchorage is provided (see Section 503), one and one-half times these values in bond may be used in beams, slabs, and one-way footings, but in no case to exceed 200 p. s. i. for plain bars and 250 p. s. 1. for deformed bars. The values given for two-way footings include an allowance for special anchorage.
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Emergency specifications for reinforced concrete buildings
(d) Compression in the Metal Core of Composite and Combination Columns
Structural steel sections____________ 16,000 p. s. i.
Cast iron sections___________________ 10,000 p. s. i.
Steel pipe___________________________ See limitations of Section 706 (b).
(e) Compression in Flexural Members
For compression reinforcement in flexural members see Section 306 (b).
Chapter 2—Design'—General Considerations
200—Notation.
/%=Ultimate compressive strength of concrete at age of 28 days, unless otherwise specified.
n =Ratio of modulus of elasticity of steel to that of con-
, Eg , .. 30,000
Crete = 7^; assumed as equal to —77—.
J c
201—Assumptions.
(a) The design of reinforced concrete members shall be made with reference to working stresses and safe loads. The accepted theory of flexure as applied to reinforced concrete shall be applied to all members resisting bending. The following assumptions shall be made:
1. The steel takes all the tensile stress.
2. In determining the ratio n for design purposes, the modulus of elasticity for the concrete shall be assumed as 1000/'c, and that for steel as 30,000,000 p. s. i.
202—Design Loads.
(a) The provisions for design herein specified are based on the assumption that all structures shall be designed for all dead- and live-loads coming upon them, the live-loads to be in accordance with the general requirements of the building code of which this forms a part, with such reductions for girders and lower story columns as are permitted therein.
203—Resistance to Wind Forces.
(a) The resisting elements in structures required to resist wind forces shall be limited to the integral structural parts.
(b) The moments, shears, and direct stresses resulting from wind forces determined in accordance with recognized methods shall be added to the maximum stresses which obtain at any section for dead- and live-loads.
(c) In proportioning the component parts of the structure for the maximum combined stresses, including wind stresses, the unit stresses shall not exceed the allowable stresses for combined live-and dead-loads provided in Sections 102, 103, and 710 by more than one-third. The structural members and heir connections shall be so proportioned as to provide suitable rigidity of structure.
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Chapter 3.—Flexural Computations
300—Notation.
A = Span length between opposite supports in one direction. B — Span length at right angles to A.
b = Width of rectangular beam or width of flange of T-beam. b' = Width of web in beams of I or T sections.
d = Depth from compression face of beam or slab to center of longitudinal tensile reinforcement; the diameter of a round bar or side of a square bar.
eA = Factor modifying rA, used in obtaining an equivalent uniform load for bending moments on span A.
Cb— Factor modifying rB, used in obtaining an equivalent uniform load for bending moments on span B.
E — The modulus of elasticity of concrete in compression.
FaA — The distance between lines of inflection in span A, considering span A only to be loaded.
FbB — The distance between lines of inflection in span B, considering span B only to be loaded.
Fa — Ratio of the distance between assumed inflection points of the span A to span A in an isolated strip extending the entire width of the structure when a uniformly distributed load is applied to span A only.
Fb = Ratio as defined above, but applying to span B.
h = Unsupported length of a column.
I — Moment of inertia of a section about the neutral axis for bending.
K = The stiffness factor, that is, the moment of inertia divided by the span.
Ka — Stiffness factor for span A of panel AB.
Kb = Stiffness factor for span B of panel AB.
Kar = Stiffness factors for any span adjacent to and continuous with span A.
Kbr = Stiffness factors for any span adjacent to and continuous with span B.
I — Span length of slab or beam.
l'=Clear span for positive moment and the average of the two adjacent clear spans for negative moment (See Section 301).
2V—The sum of the lengths of those edges of panel AB which are also edges of adjacent panels continuous with AB.
QA = 6rA (1— eA).
^b—^b (1 ^b) •
rA=Proportion of the total load carried by span A of slab.
rB=Proportion of the total load carried by span B of slab. h=Minimum total thickness of slab.
w=Uniformly distributed load per unit of length of beam or per unit area of slab.
x—Distance from face of support to point in span.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
301—General Requirements.
(a) All members of frames or continuous construction shall be designed to resist at all sections the maximum moments and shears produced by dead load, live load, and wind load, as determined by the theory of elastic frames in which the simplified assumptions of Section 302 may be used.
(6) Approximate methods of frame analysis are satisfactory for buildings of usual types of construction, spans, and story heights.
(c) In the case of two or more approximately equal spans (the larger of two adjacent spans not exceeding the shorter by more than 20 percent) with loads uniformly distributed, where the unit live load does not exceed three times the unit dead load, design for the
following moments and shears is satisfactory: Positive moment at center of span:
End spans_____________________________________________________
Interior spans______________________________________________________
Negative moment at exterior face of first interior support:
Two spans___________________________________________________
More than two spans_________________________________________
Negative moment at other faces of interior supports
Negative moment at face of all supports for, (a) slabs with spans not exceeding ten feet, and (6) beams and girders where ratio of sum of
column stiffnesses to beam stiffness exceeds eight___________________________
Shear in end members at first interior support_____________________________
Shear at other supports____________________________________________
302—Conditions of Design.*
(a) Arrangement of Live Load
1. The live load may be considered to be applied only to the floor under consideration, and the far ends of the columns may be assumed as fixed.
2. Consideration may be limited to combinations of dead load on all spans with full live load on two adjacent spans and with full live load on alternate spans.
(6) Span length
1. The span length, I, of members that are not built integrally
•Chapter 3 deals with floor members only. For moments In columns see Section 708.
487471e—42----2
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with their supports shall be the clear span plus the depth of the slab or beam but shall not exceed the distance between centers of supports.
2. In analysis of continuous frames, center to center distances, I and h, may be used in the determination of moments. Moments at faces of supports may be used for design of beams and girders.
3. Solid or ribbed slabs with clear spans of not more than ten feet that are built integrally with their supports may be designed as continuous slabs on knife edge supports with spans equal to the clear spans of the slab and the width of beams otherwise neglected.
(c) Stiffness
1. The stiffness, K, of a member is defined as EI divided by I or h.
2. In computing the value of I of slabs, beams, girders, and columns, the reinforcement may be neglected. In T-shaped sections allowance shall be made for the effect of flange.
3. Any reasonable assumption may be adopted as to relative stiffness of columns and of floor system. The assumption made shall be consistent throughout the analysis.
(d) Haunched Floor Members
1. When members are widened near the supports, the additional width may be neglected in computing moments, but may be considered as resisting the resulting moments and shears.
2. When members are deepened near the supports, they may be analyzed as members of constant depth provided the minimum depth only is considered as resisting the resulting moments; otherwise an analysis taking into account the variation in depth is required. In any case, the actual depth may be considered as resisting shear.
(e) Limitations
1. Wherever at any section positive reinforcement is indicated by analysis, the amount provided shall be not less than 0.005 b'd except in slabs of uniform thickness.
2. Not less than 0.005 b'd of negative reinforcement shall be provided at the outer end of all members built integrally with their supports.
3. Where analysis indicates negative reinforcement along the full length of a span, the reinforcement need not be extended beyond the point where the required amount is 0.0025 b'd or less.
4. In slabs of uniform thickness the minimum amount of reinforcement in the direction of the span shall be—
For structural, intermediate and hard grades and rail steel___ 0.0025 bd For steel having a minimum yield point of 56,000 p. s. i______ 0.002 bd
303—Depth of Beam or Slab.
(a) The depth of the beam or slab shall be taken as the distance from the centroid of the tensile reinforcement to the compression face of the structural members. Any floor finish not placed monolithically with the floor slab shall not be included as a part of the structural member. When the finish is placed monolithically with the structural
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
slab in buildings of the warehouse or industrial class, there shall be placed an additional depth of one-half inch over that required by the design of the member.
304—Distance between Lateral Supports.
(a) The clear distance between lateral supports of a beam shall not exceed thirty-two times the least width of compression flange.
305—Requirements for T-Beams.
(a) In T-beam construction the slab and beam.shall be built integrally or otherwise effectively bonded together. The effective flange width to be used in the design of symmetrical T-beams shall not exceed one-fourth of the span length of the beam, and its overhanging width on either side of the web shall not exceed eight times the thickness of the slab nor one-half the clear distance to the next beam.
(6) For beams having a flange on one side only, the effective overhanging flange width shall not exceed one-twelfth of the span length of the beam, nor six times the thickness of the slab, nor one-half the clear distance to the next beam.
(c) Where the principal reinforcement in a slab which is considered as the flange of a T-beam (not a joist in concrete joist floors) is parallel to the beam, transverse reinforcement shall be provided in the top of the slab. This reinforcement shall be designed to carry the load on the portion of the slab assumed as the flange of the T-beam. The spacing of the bars shall not exceed five times the thickness of the flange, nor in any case eighteen inches.
(d) Provision shall be made for the compressive stress at the support in continuous T-beam construction, care being taken that the provisions relating to the spacing of bars, and to the placing of concrete shall be fully met.
(e) The overhanging portion of the flange of the beam shall not be considered as effective in computing the shear and diagonal tension resistance of T-beams.
(/) Isolated beams in which the T-form is used only for the purpose of providing additional compression area, shall have a flange thickness not less than one-half the width of the web and a total flange width not more than four times the web thickness.
306—Compression Steel in Flexural Members.
(a) Compression steel in beams, girders, or slabs shall be anchored by ties or stirrups not less than % inch in diameter spaced not farther apart than 16 bar diameters, or 48 tie diameters. Such stirrups or ties shall be used throughout the distance where the compression steel is required.
(&) The effectiveness of compression reinforcement in resisting bending may be taken at twice the value indicated from the calculations assuming a straight-line relation between stress and strain and the modular ratio given in Section 201, but not of greater value than the allowable stress in tension.
307—Shrinkage and Temperature Reinforcement.
(a) Reinforcement for shrinkage and temperature stresses normal to the principal reinforcement shall be provided in floor and roof slabs where the principal reinforcement extends in one direction only.
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Such reinforcement shall provide for the following minimum ratios of reinforcement area to concrete area bd, but in no case shall such reinforcing bars be placed farther apart than five times the slab
thickness nor more than eighteen inches:
Floor slabs where plain bars are used__________________________________ 0.0025
Floor slabs where deformed bars are used_______________________________ 0.002
Floor slabs where wire fabric is used, having welded intersections not farther apart in the direction of stress than twelve inches_________;__0.0018
Roof slabs where plain bars are used___________________________________ 0.003
Roof slabs where deforjned bars are used_______________________________ 0.0025
Roof slabs where wire fabric is used, having welded intersections not farther apart in the direction of stress than twelve inches____________ 0.0022
308—Concrete Joist Floor Construction. *
(a) Concrete joist floor construction consists of concrete joists and slabs placed monolithically with or without burned clay or concrete tile fillers. The joists shall not be farther apart than thirty inches face to face. The ribs shall be straight, not less than four inches wide, nor of a depth more than three times the width.
(6) When burned clay or concrete tile fillers, of material having a unit compressive strength at least equal to that of the designed strength of the concrete in the joists are used, and the fillers are so placed that the joints in alternate rows are staggered, the vertical shells of the fillers in contact with the joists may be included in the calculations involving shear or negative bending moment. No other portion of the fillers may be included in the design calculations.
(c) The concrete slab over the fillers shall be not less than one and one-half inches in thickness, nor less in thickness than one-twelfth of the clear distance between joists. Shrinkage reinforcement in the slab shall be provided as required in Section 307.
(d) Where removable forms or fillers not complying with (b) are used, the thickness of the concrete slab shall not be less than one-twelfth of the clear distance between joists and in no case less than two inches. Such slab shall be reinforced at right angles to the joists with a minimum of 0.049 sq. in. of reinforcing steel per foot of width, and in slabs on which the prescribed live load does not exceed fifty lb. per sq. ft., no additional reinforcement shall be required.
(e) When the finish used as a wearing surface is placed monolithically with the structural slab in buildings of the warehouse or industrial class, the thickness of the concrete over the fillers shall be one-half inch greater than the thickness used for design purposes.
(/) Where the slab contains conduits or pipes, the thickness shall not be less than one inch plus the total over-all depth of such conduits or pipes at any point. Such conduits or pipes shall be so located as not to impair the strength of the construction.
309—Floors with Supports on Four Sides. W(2)
(a) This construction, consisting of floors reinforced in two directions and supported on four sides, includes solid reinforced concrete slabs; concrete joists with burned clay or concrete tile fillers, with or without concrete top slabs; and concrete joists with top slabs placed monolithically with the joists. The supports for the floor slabs may be walls, reinforced concrete beams, or steel beams fully encased in concrete.
See footnotes 0) and (>) at bottom of pages 11 and 12.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
(6) Minimum Slab Thickness
The slab thickness shall satisfy prescribed working stresses and shall be not less than 4 inches nor less than
(c) Bending Moments and Shears
The bending moment at any section shall be determined with coefficients derived as prescribed for one-way construction (Sections 301 and 302), using the following equivalent uniform load per unit length of span considered:
Slab: Strip of unit width, span A___________________(exrx)w-------------------- (2)
Beam: Span A, carrying one-half of load from panel width B,
Footnotes:
0) For comparative use the moment of inertia of a slab shall be taken as that of the total plain concrete section.
(2) Formulas for Fa, Fb, ¿a. «b-ta, tb. (See “Slabs Supported on Four Sides” by J. DiStasio and M. P. van Buren, Journal of the A. C. I., January-February, 1936).
End Span, continuous at one end only:
Interior continuous span with Kar the same for both adjacent spans continuous with A:
For interior spans where the spans adjacent to and in continuation of the span A under consideration differ in stiffness, for Fa use the average of the two values, one obtained using Kar for the span in continuation on one end of the span A, and the other obtained by using the value of Kar for the span at the other end.
To obtain Fb replace Ka with Kb and Kar with Kbr.
The total load carried by a strip of slab of unit width, span A, equals tawA and is considered to vary in intensity from rAw (3ex—2) at the center of the span, to taw (4—3«a) at the supports.
The total load carried by a beam of span A, one-half panel tributary width, equals
.................................... -......................................................... (10)
and varies uniformly in intensity from (l+2rx—ScaTa)
at the center of the span to
at the supports.
When considering the B spans use the above expressions, replacing A with B, B with A, va with re,
, and tA with cb.
Table I.—Fa and Fb.
The values given in the table are for Fa directly. They are also the values for Fb when the designation Ka/Kar is replaced by Kb/Kbr
Span A Ka Kar 0.00 0.25 0.50 0.67 0.80 1.00 1.25 1.50 2.00 4.00 <9
Interior* Fa = 0.58 0.65 0.69 0.72 0.74 0.76 0.78 0.80 0.83 0.89 1.00
End Fa = 0.75 0.80 0.83 0.84 0.85 0.87 0.88 0.89 0.91 0.95 1.00
Simple Fa = 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
*For interior spans where the spans adjacent to and in continuation of the span A under consideration differ in stiffness, for Fa use the average of the two values, one obtained using Kar for the span in continuation on one end of the span A, and the other obtained by using the value of KarIot the span at the other end.
For values of KaIKar between 2/3 and 3/2 the values of Fa may be taken as 0.76 for interior spans and 0.87 for end spans.
(Footnote (*) continued next page).
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The shear at any section at a distance x from the face of the support shall be taken as:
Slab: Strip of unit width, span A:
Beam: Span A, carrying one-half of load from panel, width B:
For span B, use the above expressions substituting A for B, B for A, eB for eA, rA for rB, and qB for qA.
The factors eA,rA, etc., may be taken from Table 2, footnote (2) below after the ratio FAAfFBB or FbBIFaA on which they depend has been determined by the aid of Table 1, footnote (2); or the several factors may be computed from the formulas which appear in the footnote (2).
(d) Arrangement oj Reinforcement
1. In any panel, the reinforcement per unit width in the long direction shall be at least one-third of that provided in the short direction.
2. The positive moment reinforcement adjacent to a continuous edge only and for a width not exceeding one-fourth of the shorter dimension of the panel may be reduced twenty-five percent.
3. At a noncontinuous edge negative moment reinforcement per unit width in amount at least as great as one-half of that required for maximum positive moment for the center one-half
(Footnote (2) continued from previous page.)
Table 2.
The value of e4 or eB shall be taken as unity for the computation of shear and bending moment in slabs and beams where the span in direction under consideration is not rigidly attached to the supports at one or both ends of the span
FaA FbB Ta or 1—tb ex sat a 1—exrÀ QA = erAd—eA)
0.00 1.00 1.00 1.00 0.00 0.00
0.50 0.89 1.00 0.89 0.11 0.00
0.55 0.86 0.92 0.79 0.21 0.41
0.60 0.82 0.86 0.71 0.29 0.69
0.65 0.78 0.81 0.63 0.37 0.89
0.70 0.74 0.78 0.58 0.42 0.98
0.80 0.66 0.73 0.48 0.52 1.07
0.90 0.58 0.69 0.40 0.60 1.08
1.00 0.50 0.67 0.33 0.67 1.00
1.10 0.43 0.65 ,0.28 0.72 0.90
1.20 0. 37 0.63 0.23 0.77 0.82
1.30 0.31 0.62 0.19 0.81 0.71
1.40 0.27 0. 61 0.16 0.84 0.63
1.50 0.23 0.60 0.14 0.86 0.55
1.60 0.20 0.59 0.12 0.88 0.49
1.80 0.15 0.58 0.09 0.91 0.38
2.00 0.11 0.57 0.06 0.94 0.28
3.00 0.04 0.55 0.02 0.98 0.11
FbB FaA tb or cb ears 1—ears 6^(1—sb) —Qb
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
of the panel shall be provided across the entire width of the exterior support.
4. The spacing of the reinforcement shall be not more than three times the slab thickness and the ratio of reinforcement shall be at least 0.0025.
310—Maximum Spacing of Principal Slab Reinforcement.
(a) In slabs other than concrete joist floor construction or flat slabs, the principal reinforcement shall not be spaced farther apart than three times the slab thickness, nor shall the ratio of reinforcement be less than specified in Section 307(a).
Chapter 4—Shear and Diagonal Tension
400—Notation.
^4e=Total area of web reinforcement in tension within a distance of s (measured in a direction parallel to that of the main reinforcement), or the total area of all bars bent up in any one plane.
a=Angle between inclined web bars and axis of beam.
6=Width of rectangular beam or width of flange of T-beam. b'—Width of web in beams of I or T sections.
d= Depth from compression face of beam or slab to center of longitudinal tensile reinforcement.
Ultimate compressive strength of concrete at age of 28 days unless otherwise specified.
A=Tensile unit stress in web reinforcement.
j=Ratio of distance between centroid of compression and centroid of tension to the depth d.
8=Spacing of stirrups or of bent bars in a direction parallel to that of the main reinforcement.
i2=Thickness of flat slab without drop panels, or the thickness of flat slab through the drop panels where such are used.
/3=Thickness of flat slab (with drop panels) at points outside the drop panel.
. v=Shearing unit stress.
V— Total shear.
V'=Excess of the total shear over that permitted on the concrete.
401—Shearing Unit Stress.
(a) The shearing unit stress v, as a measure of diagonal tension, in reinforced concrete flexural members shall be computed by formula (11):
--------------------------------------------------------------(11)
(6) For beams of I or T section, b' shall be substituted for b in formula (11).
(c) In concrete joist floor construction, where burned clay or concrete tile are used, b' may be taken as a width equal to the thickness of the concrete web plus the thicknesses of the vertical shells of the concrete or burned clay tile in contact with the joist as in Section 308 (b).
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(d) When the value of the shearing unit stress computed by formula (11) exceeds the shearing unit stress ve permitted on the concrete of an unreinforced web (see Section 102), web reinforcement shall be provided to carry the excess.
402—Types of Web Reinforcement.
(a) Web reinforcement may consist of—
1. Stirrups or web reinforcement bars perpendicular to the longitudinal steel.
2. Stirrups or web reinforcement bars welded or otherwise rigidly attached to the longitudinal steel and making an angle of 30 degrees or more thereto.
3. Longitudinal bars bent so that the axis of the inclined portion of the bar makes an angle of 15 degrees or more with the axis of the longitudinal portion of the bar.
4. Special arrangements of bars with adequate provisions to prevent slip of bars or splitting of the concrete by the reinforcement (See Section 404 (f)).
(6) Stirrups or other bars to be considered effective as web reinforcement shall be anchored at both ends, according to the provisions of Section 504.
403—Stirrups.
(a) The area of steel required in stirrups placed perpendicular to the longitudinal reinforcement shall be computed by formula (12):
(6) Inclined stirrups shall be proportioned by formula (14) (Section 404 (d)).
(c) Stirrups placed perpendicular to the longitudinal reinforcement shall not be used alone as web reinforcement when the shearing unit stress (v) exceeds 0.08jf'c.
404—Bent Bars.
(a) When the web reinforcement consists of a single b.ent bar or of a single group of bent bars the required area of such bars shall be computed by formula (13):
(&) In formula (13) V shall not exceed 0.040 j'cbjd.
(c) Only the center three-fourths of the inclined portion of such bar, or group of bars, shall be considered effective as web reinforcement.
(d) Where there is a series of parallel bent bars, the required area shall be determined by formula (14):
(e) When bent bars, having a radius of bend of not more than two times the diameter of the bar are used alone as web reinforcement, the allowable shearing unit stress shall not exceed 0.060 j'c. This
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
shearing unit stress may be increased at the rate of 0.01 f'e for each increase of four bar diameters in the radius of bend until the maximum allowable shearing unit stress is reached. (See Section 102 (a).)
(f) The shearing unit stress permitted when special arrangements of oars are employed shall be that determined by making comparative tests, to destruction, of specimens of the proposed system and of similar specimens reinforced in conformity with the provisions of this code, the same factor of safety being applied in both cases.
405—Combined Web Reinforcement.
(a) Where more than one type of reinforcement is used to reinforce the same portion of the web, the total shearing resistance of this portion of the web shall be assumed as the sum of the shearing resistances computed for the various types separately. In such computations the shearing resistance of the concrete shall be included only once, and no one type of reinforcement shall be assumed to resist more 2 V'
than ——• o
406—Spacing of Web Reinforcement.
(a) Where web reinforcement is required it shall be so spaced that every 45 degree line (representing a potential crack) extending from the mid-depth of the beam to the longitudinal tension bars shall be crossed by at least one line of web reinforcement. If a shearing unit stress in excess of 0M.fc is used, every such line shall be crossed by at least two such lines of web reinforcement.
407—Shearing Stress in Flat Slabs.
(a) In flat slabs, the shearing unit stress on a vertical section which lies at a distance t2—l% in. beyond the edge of the column capital and parallel or concentric with it, shall not exceed the following values when computed by formula (11) (in which d shall be taken as ¿2—1^ in.):
1. 0.03 i'c, when at least 50 percent of the total negative reinforcement in the column strip passes directly over the column capital.
2. 0.025 j'c, when 25 percent or less of the total negative reinforcement in the column strip passes directly over the column capital.
3. For intermediate percentages, intermediate values of the shearing unit stress shall be used.
(6) In flat slabs, the shearing unit stress on a vertical section which lies at a distance of i3—1M in. beyond the edge of the drop panel and parallel with it shall not exceed 0.03/'c when computed by formula (11) (in which d shall be taken as ¿3—1% in.). At least 50 percent of the cross-sectional area of the negative reinforcement in the column strip must be within the width of strip directly above the drop panel.
408—Shear and Diagonal Tension in Footings.
(a) In isolated footings the shearing unit stress computed by formula (11) on the critical section (see 805 (a)),shall not exceedO.OS/'«, nor in any case shall it exceed 75 p. s. i.
487471°—42---3
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Chapter 5—Bond and Anchorage
500—Notation.
d=Depth from compression face of beam or slab to center of longitudinal tensile reinforcement.
i'e—Ultimate compressive strength of concrete at age of 28 days unless otherwise specified.
j=Ratio of distance between centroid of compression and centroid of tension to the depth d.
So=Sum of perimeters of bars in one set.
u—Bond stress per unit of surface area of bar.
V= Total shear.
501—Computation of Bond Stress in Beams.
(a) In flexual members in which the tensile reinforcement is parallel to the compression face, the bond stress at any cross section shall be computed by formula (15):
______________________________________________________________(15)
So jd
in which V is the shear at that section.
(6) Adequate end anchorage shall be provided for the tensile reinforcement in all flexural members to which formula (15) does not apply, such as footings, brackets, and other tapered or stepped beams in which the tensile'reinforcement is not parallel to the compression face.
502—Ordinary Anchorage Requirements.
(a) Tensile negative reinforcement in any span of a continuous, restrained, or cantilever beam, or in any member of a rigid frame shall be adequately anchored by bond, hooks or mechanical anchors in or through the supporting member. Within any such span every reinforcing bar shall be extended at least twelve diameters beyond the point at which it is no longer needed to resist stress. In cases where the length from the point of maximum tensile stress in the bar to the end of the bar is not sufficient to develop this maximum stress by bond, the bar shall extend into a region of compression and be anchored by means of a standard hook or it shall be bent across the web at an angle of not less than 15 degrees with the longitudinal portion of the bar and either made continuous with the positive reinforcement or anchored in a region of compression.
(5) Of the positive reinforcement in continuous beams not less than one-fourth the area shall extend along the same face of the beam into the support a distance of ten or more bar diameters, or shall be extended as far as possible into the support and terminated in standard hooks, or other adequate anchorage.
(c) In simple beams, or at the outer or freely supported ends of end spans of continuous beams, at least one-half the positive reinforcement shall extend along the same face of the beam into the support a distance of twelve or more bar diameters, or shall be extended as far as possible into the support and terminated in standard hooks.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
503—Special Anchorage Requirements.
(a) Where increased shearing or bond stresses are permitted because of the use of special anchorage (See Section 102,) every bar shall be terminated in a standard hook in a region of compression, or it shall be bent across the web at an angle of not less than 15 degrees with the longitudinal portion of the bar and made continuous with the negative or positive reinforcement.
504—Anchorage of Web Reinforcement.
(a) Single separate bars used as web reinforcement shall be anchored at each end by one of the following methods:
1. Welding to longitudinal reinforcement.
2. Hooking tightly around the longitudinal reinforcement through 180 degrees.
3. Embedment above or below the mid-depth of the beam on the compression side, a distance sufficient to develop the stress to which the bar will be subjected at a bond stress of not to exceed 0.044f'e on plain bars nor 0.055 on deformed bars.
4. Standard hook (see Section 506 (a)), considered as developing 10,000 p.s.i., plus embedment sufficient to develop by bond the remainder of the stress to which the bar is subjected. The unit bond stress shall not exceed that specified in Table 102 (a). The effective embedded length shall not be assumed to exceed the distance between the mid-depth of the beam and the tangent of the hook.
(6) The extreme ends of bars forming simple U or multiple stirrups shall be anchored by one of the methods of Section 504(a) or shall be bent through an angle of at least 90 degress tightly around a longitudinal reinforcing bar not less in diameter than the stirrup bar, and shall project beyond the bend at least twelve diameters of the stirrup bar.
(c) The loops or closed ends of such stirrups shall be anchored by bending around the longitudinal reinforcement through an angle of at least 90 degrees, or by being welded or otherwise rigidly attached thereto.
(d) Hooking or bending stirrups or separate web reinforcement bars around the longitudinal reinforcement shall be considered effective only when these bars are perpendicular to the longitudinal reinforcement.
(e) Longitudinal bars bent to act as web reinforcement shall, in a region of tension, be continuous with the longitudinal reinforcement. The tensile stress in each bar shall be fully developed in both the upper and the lower half of the beam by one of the following methods:
1. As specified in Section 504(a), (3).
2. As specified in Section 504(a), (4).
3. By bond, at a unit bond stress not exceeding 0.044 f e on plain bars nor 0.055 f'c on deformed bars, plus a bend of radius not less than two times the diameter of the bar, parallel to the upper or lower surface of the beam, plus an extension of the bar of not less than twelve diameters of the bar terminating in a standard hook. This short radius bend extension and hook shall together
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not be counted upon to develop a tensile unit stress in the bar of more than 10,000 p. s, i.
4. By bond, at a unit bond stress not exceeding 0.044 f'c on plain bars nor 0.055 f'c on deformed bars, plus a bend of radius not less than two times the diameter of the bar, parallel to the upper or lower surface of the beam and continuous with the longitudinal reinforcement. The short radius bend and continuity shall together not be counted upon to develop a tensile unit stress in the bar of more than 10,000 p. s. i.
. 5. The tensile unit stress at the beginning of a bend may be increased from 10,000 p. s. i. when the radius of bend is two bar diameters, at the rate of 1,000 p. s. i. tension for each increase of 1% bar diameters in the radius of bend, provided that the length of the bar in the bend and extension is sufficient to develop this increased tensile stress by bond at the unit stresses given in Section 5Q4(e), (3).
(/) In all cases web reinforcement shall be carried as close to the compression surface of the beam as fireproofing regulations and the proximity of other steel will permit.
505—Anchorage of Bars in Footing Slabs.
(a) All bars in footing slabs shall be anchored by means of standard hooks. The outer faces of these hooks shall be not less than three inches nor more than six inches from the face of the footing.
506—Hooks.
, (a) The terms “hook” or “standard hook” as used herein shall mean either
1. A complete semicircular turn with a radius of bend on the axis of the bar of not less than three and not more than six bar ! diameters, plus an extension of at least four bar diameters at the free end of the bar; or
»■: 2. A 90° bend having a radius of not less than four bar diam-
? eters plus an extension of twelve bar diameters.
Hooks having a radius of bend of more than six bar diameters shall be considered merely as extensions to the bars, and shall be treated as in Section 504(e), (5).
(6) In general, hooks shall not be permitted in the tension portion of any beam except at the ends of simple or cantilever beams or at the freely supported ends of continuous or restrained beams.
(c) No hook shall be assumed to carry a load which would produce a tensile stress in the bar greater than 10,000 p. s. i.
(d) Hooks shall not be considered effective in adding to the comprèssi ve resistance óf bars.
(e) Any mechanical device capable of developing the strength of the bar without damage to the concrete may be used in Heu of a hook. Tests must be presented to show the adequacy of such devices.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
Chapter 6—Flat Slabs—With Square or Rectangular Panels
600—Notation.
A=The distance from the center line of the column, in the direction of any span, to the intersection of a 45-degree diagonal line from the center of the column to the bottom of the flat slab or drop panel, where such line lies wholly within the column, capital, or bracket, provided sucn capital or bracket is structurally capable of resisting shears and moments without excessive unit stress. In no case shall A be greater than one-eighth the span in the direction considered.
Aav—Average of the two values of A for the two columns at the ends of a column strip, in the direction of the spans considered.
c=Diameter or width of column capital at the under side of the slab or drop panel. No portion of the column capital shall be considered for structural purposes which lies outside the largest right circular cone, with 90 degrees vertex angle, that can be included within the outlines of the column capital.
Z=Span length of slab center to center of columns in the direction of which bending is considered.
Af0=Sum of the positive and the average negative bending moments at the critical design sections of a flat slab panel. See Section 603(b).
W= Total dead and live load uniformly distributed over a single panel area.
War=The average of the total load on two adjacent panels.
x—Coefficient of span L which gives the distance from the center of column to the critical section for negative bending in design according to Section 602(a).
601—Scope.
(a) The term flat slab shall mean a reinforced concrete slab supported by columns with or without flaring heads or column capitals, with or without depressed or drop panels and generally without beams or girders.
(&) Recesses or pockets in flat slab ceilings, located between reinforcing bars and forming cellular or two-way ribbed ceilings, whether left open or filled with permanent fillers, shall not prevent a slab from being considered a flat slab; but allowable unit stresses shall not be exceeded.
(c) This chapter provides for two methods of design of flat slab structures.
1. Any type of flat slab construction may be designed by application of the principles of continuity, using the method outlined in Section 602, or using other recognized methods of elastic analysis. In either case, the design must be subject to the provisions of Sections 605(a) and (c), 606, 608, and 609.
2. The common cases of flat slab construction described in
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Section 603 may be designed by the use of moment coefficients, given in Sections 603 and 604, and subject to the provisions of Sections 605, 606, 607, 608, and 609.
602—Design of Flat Slabs as Continuous Frames.
(a) Except in the cases of flat slab construction where specified coefficients for bending may be used, as provided in Section 603, bending and shear in flat slabs and their supports shall be determined by an analysis of the structure as a continuous frame, and all sections shall be proportioned to resist the moments and shears thus obtained. In the analysis, the following assumptions may be made:
1. The structure may be considered divided into a number of bents, each consisting of a row of columns and strips of supported slabs, each strip bounded laterally by the center line of the panel on either side of the row of columns. The bents shall be taken longitudinally and transversely of the building.
2. Each such bent may be analyzed in its entirety; or each floor thereof and the roof may be analyzed separately with its-adjacent columns above and below, the columns being assumed fixed at their remote ends. Where slabs are thus analyzed separately, in bents more than four panels long, it may be assumed in determining the bending at a given support that the slab is fixed at any support two panels distant therefrom beyond which the slab continues.
3. The joints between columns and slabs may be considered rigid and this rigidity may be assumed to extend in the slabs a distance A from the center of the columns, and in the column to the intersection of the sides of the column and the 45 degree line defining A. The change in length of columns and slabs due to direct stress, and deflections due to shear, may be neglected. Where metal column capitals are used, account may be taken of their contributions to stiffness and resistance to bending and shear.
4. The supporting columns may be assumed free from settlement or lateral movement unless the amount thereof can be reasonably determined.
5. The moment of inertia of slab or column at any crosssection may be assumed to be that of the gross section of the concrete. Variation in the moments of inertia of the slabs and columns along their axes shall be taken into account.
6. Where the load to be supported is definitely known, the structure shall be analyzed for that load. Where the live load is variable but does not exceed three-quarters of the dead load, or the nature of the live load is such that all panels will be loaded simultaneously, the maximum bending may be assumed to obtain at all sections under full live load. Elsewhere, maximum positive bending near mid-span of a panel may be assumed to obtain under full live load in the panel and in alternate panels; and maximum negative bending at a support may be assumed to obtain under full live load in the adjacent panels only.
7. Where neither beams nor girders help to transfer the slab load to the supporting column, the critical section for negative
20
emergency specifications for reinforced concrete buildings
bending may be assumed as not more than the distance xL from the column center, where
In slabs supported by beams, girders, or walls, the critical section for negative bending shall be assumed at the face of such support.
8. The numerical sum of the maximum positive and the average maximum negative bending moments for which provision is made in the design in the direction of either side of a rectangular panel shall be assumed as not less than
9. The bending at critical sections across the slabs of each bent may be apportioned between the column strip and middle strip, as defined in Section 605, in the ratio of the specified coefficients which affect such apportionment in the special cases of flat slabs provided for in Section 603.
10. The maximum bending in columns may be assumed to obtain under full live load in alternate panels. Columns shall be proportioned to resist the maximum bending combined with the maximum direct load consistent therewith; and for maximum direct load combined with the bending under full load, the direct load subject to allowable reductions, in the manner provided in Chapter 7. In computing moments in columns at any floor, the far ends of the columns may be considered fixed.
(5) The foregoing provisions outline the method to be followed in analyzing and designing flat slabs in the general case. In all instances the design must conform to the requirements for panel strips and critical design sections, slab thickness and drop panels, capitals and brackets, arrangement of reinforcement and openings in flat slabs, as provided in Sections 605(a) and (c), 606, 608, and 609.
603—Design of Flat Slabs by Moment Coefficients.
(a) In those cases of flat slab construction which fall within the following limitations as to continuity and dimensions, the bending moments at critical sections may be determined by the use of specified coefficients as provided in Section 604.
1. The ratio of length to width of panel does not exceed 1.33.
2. The slab is continuous for at least three panels in each direction.
3. The successive span lengths in each direction differ by not more than twenty percent of the shorter span.
(6) In such slabs, the numerical sum of the positive and negative bending moments in the direction of either side of an interior rectangular panel shall be assumed as not less than
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(c) Three-fourths of the width of the strip shall be taken as the width of the section in computing compression due to bending, except that, on a section through a drop panel, three-fourths of the width of the drop panel shall be taken. Account shall be taken of any recesses which reduce the compressive area. Tension reinforcement distributed over the entire strip shall be included in the computations.
(d) The design of slabs under the procedure given in this section is subject to the provisions of all subsequent sections of this chapter (Sections 604 to 609).
604—Bending Moment Coefficients.
(a) The bending moments at the critical sections of the middle and column strips of an interior panel shall be assumed as given in Table 604(a).
Table 604(a).—Bending moments in interior flat slab panel
With drop panel: Column strip Negative moment 0.50M.
Postive moment 0.20M.
Middle strip Negative moment 0.15M.
Positive moment 0.16M.
Without drop panel: Column strip Negative moment 0.46M,
Positive moment 0.22M.
Middle strip .......... ------- Negative moment 0.16M,
Positive moment 0.16M.
Table 604(6).—Bending moments in exterior flat slab panel
With drop panel: Exterior negative 0.45M»
Middle strip Without drop panel: Column strip Middle strip Positive moment Interior negative..— Exterior negative Positive moment.................. Interior negative Exterior negative Positive moment ... Interior negative Exterior negative Positive moment Interior negative 0.25M. 0.55M. 0.10M. 0.19M, 0.165M. 0.41M. 0.28M. 0.50M. 0.10M« 0.20M. 0.176M.
Table 604(c).—Bending moments in panels with marginal beams or walls
Marginal Beams with Depth greater than Bi times the Slab Thickness; or Bearing Wall. Marginal Beams with depth IJi times the Slab Thickness or less.
(a) Load to be carried by Marginal Beam or Wall Loads directly superimposed upon it plus a uniform load equal to one-quarter of the total live and dead panel load. Loads directly superimposed upon it exclusive of any panel load.
(6) Moment to be used in the design of Half Column Strip adjacent and parallel to Marginal Beam or Wall. (c) Negative Moment to be used in Design of Middle Strip continuous across a Beam or Wall. Neg. Pos. Neg. With Drop 0.125Mo 0.05M« 0.195M» Without Drop 0.115M. 0.055M. 0.208M. With Drop 0.25Mo 0.10M. 0.15M. Without Drop 0.23M. 0.11M. 0.16M.
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(6) The bending moments at critical sections of strips, in an exterior panel, at right angles to the discontinuous edge, where the exterior supports consist of reinforced concrete columns or reinforced concrete bearing walls integral with the slab, the ratio of stiffness of the support to that of the slab being at least as great as the ratio of the live load to the dead load and not less than one, shall be assumed as given in Table 604(6). Where a flat slab is so supported by a wall providing restraint at the discontinuous edge, the coefficient for negative bending at the edge shall be assumed more nearly equal in the column and middle strips, the sum remaining as given in Table 604(6), but that for the column strip shall not be less than 0.30 Mo. Bending in middle strips parallel to a discontinuous edge, except in a corner panel, shall be assumed the same as in an interior panel. Mo shall be determined as provided in Section 603(6) for an interior panel.
(c) The bending moments at critical sections of strips, in an exterior panel, at right angles to the discontinuous edge, where the exterior supports are masonry bearing walls or other construction which provide only negligible restraint to the slab, shall be assumed as given in Table 604(6) with the following modifications.
1. On critical sections at the face of the exterior support, negative bending in each strip shall be assumed as 0.05 Mo.
2. The coefficients for positive bending shall be increased by forty percent.
3. The coefficients for negative bending at the first interior columns shall be increased thirty percent.
(d) The bending moments in panels with marginal beams or walls, in the strips parallel and close thereto, and in the beams, shall be determined upon the basis of assumptions presented in Table 604(c).
(e) For design purposes any of the moment coefficients of Tables 604(a), 604(6), and 604(c) may be varied by not more than six percent, but the numerical sum of the positive and negative moments in a panel shall not be taken as less than the amount specified.
(/) Panels supported by marginal beams on opposite edges shall be designed as solid one or two-way slabs to carry the entire panel load.
(g) The ratio of reinforcement in any strip shall not be less than 0.0025.
GENERAL REQUIREMENTS
605—Panel Strips and Critical Design Sections.
(a) A flat slab panel shall be considered as consisting of strips in each direction as follows:
A middle strip one half panel in width, symmetrical about panel centerline and extending through the panel in the direction of the span for bending.
A column strip consisting of the two adjacent quarter-panels either side of the column centerlines.
(6) The critical sections for bending are located as follows:
Sections for negative bending shall be taken along the edges of the panel, on column centerlines between capitals, and around the perimeters of column capitals.
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Sections for positive bending shall be taken at midspan of the strips.
(c) Only the reinforcement which crosses a critical section within a strip may be considered effective to resist bending in the strip at that section. Reinforcement which crosses such section at an angle with the center line of the strip shall be assumed to contribute to the resistance of bending only its effective area in the direction of the strip. 606—Slab Thickness and Drop Panels.
(a) The thickness of a flat slab and the size and thickness of the drop panel, where used, shall be such that the compressive stress due to bending at the critical sections of any strip and the shear about the column capital and the drop panel shall not exceed the unit stresses allowed in concrete of the quality used.
(6) The shearing stresses in the slab outside the capital or drop panel shall be computed as provided in Section 407.
(c) Slab thickness shall not, however, be less than
with drop panels
or
— without drop panels OU
(d) The thickness of the drop panel below the slab shall not be more than one-fourth the distance from the edge of the column capital to the edge of the drop panel.
607—Capitals and Brackets.
(a) Where a column is without a flaring concrete capital the distance c shall be taken as the diameter of the column. Structural metal embedded in the slab or drop panel may be regarded as contributing to resistance in bending and shear.
(b) Where a reinforced concrete beam frames into a column without capital or bracket on the same side with the beam, the value of c may be taken as the width of the column plus twice the projection of the beam above or below the slab or drop panel for computing bending in strips parallel to the beam.
(c) Brackets capable of transmitting the negative bending and the shear in the column strips to the columns without excessive unit stress may be substituted for column capitals at exterior columns. The value of c where brackets are used shall be taken as twice the distance from the center of the column to a point where the bracket is 1/2 inches thick, but not more than the thickness of the column plus twice the depth of the bracket.
(d) The average of the diameters c of the column capitals at the four corners of a panel shall be used in determining the bending in the middle strips of the panel. The average of the diameters c of the two column capitals at the ends of a column strip shall be used in determining bending in the strip.
608—Arrangement of Reinforcement.
(a) Slab reinforcement shall be provided to resist the bending and bond stresses not only at critical sections, but also at intermediate sections.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
(5) Bars shall be spaced evenly across strips or bands and the spacing shall not exceed three times the slab thickness.
(c) In exterior panels the reinforcement perpendicular to the discontinuous edge for positive bending, shall extend to the edge and have embedment of at least six inches in spandrel beams, walls, or columns. All such reinforcement for negative bending shall be bent, hooked, or otherwise anchored in spandrel beams, walls, or columns.
609—Openings in Flat Slabs.
Openings of any size may be cut through a flat slab if provision is made for the total positive and negative resisting moments, as required in Sections 602 or 603, without exceeding the allowable, stresses as given in Sections 102 and 103.
Chapter 700—Reinforced Concrete Columns and Walls
700—Notation.
y4c=Area of core of a spirally reinforced column measured to the outside diameter of the spiral; net area of concrete section of a composite column.
=The over-all or gross area of spirally reinforced or tied columns; the total area of the concrete encasement of combination columns.
21r=Area of the steel or cast-iron core of a composite column; the area of the steel core in a combination column.
As— Effective cross-sectional area of reinforcement in compression in columns.
C= Ratio of allowable concrete stress,/a, in axially loaded column to allowable fiber stress for concrete in flexure.
factor, usually varying from 3 to 9. (The term R as used here is the radius of gyration of the entire column section.)
d=The least lateral dimension of a concrete column.
Eccentricity of the resultant load on a column, measured from the gravity axis.
L Yield point of pipe /CI o
F=--------------ZgriAn (See Section 706(b)).
45,000 v x "
ja—ANOxago, allowable stress in the concrete of an axially loaded reinforced concrete column.
/c=Computed concrete fiber stress in an eccentrically loaded column.
f'c= Ultimate compressive strength of concrete at age of 28 days, unless otherwise specified.
/„=Maximum allowable concrete fiber stress in an eccentrically loaded column.
fT=Allowable unit stress in the metal core of a composite column.
f'r= Allowable unit stress on unencased steel columns and pipe columns.
/s=Nominal working stress in vertical column reinforcement.
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Pi=Useful limit stress of spiral reinforcement.
h=Unsupported length of column.
K— Least radius of gyration of a metal pipe section (in pipe columns).
N=Axial load applied to reinforced concrete column.
p'=Ratio of volume of spiral reinforcement to the volume of the concrete core (out to out of spirals) of a spirally reinforced concrete column.
Ratio of the effective cross-sectional area of vertical reinforcement to the gross area Ag.
P=Total allowable axial load on a column whose length does not exceed ten times its least cross-sectional dimension.
P'~ Total allowable axial load on a long column.
R—Least radius of gyration of a section.
t—Over-all depth of column section.
701—Limiting Dimensions.
(a) The following sections on reinforced concrete and composite columns, except Section 707(a), apply to a short column for which the unsupported length is not greater than ten times the least dimension. When the unsupported length exceeds this value, the design shall be modified as shown in Section 707(a). Principal columns in buildings shall have a minimum diameter of twelve inches, or in the case of rectangular columns, a minimum thickness of ten inches, and a minimum gross area of 120 sq. in. Posts that are not continuous from story to story shall have a minimum diameter or thickness of six inches.
702—Unsupported Length of Columns.
(a) For purposes of determining the limiting dimensions of columns, the unsupported length of reinforced concrete columns shall be taken as the clear distance between floor slabs, except that
1. In flat slab construction, it shall be the clear distance between the floor and the lower extremity of the capital.
2. In beam and slab construction, it shall be the clear distance between the floor and the under side of the deeper beam framing into the column in each direction at the next higher floor level.
3. In columns restrained laterally by struts, it shall be the clear distance between consecutive struts in each vertical plane; provided that to be an adequate support, two such struts shall meet the column at approximately the same level, and the angle between vertical planes through the struts shall not vary more than 15 degrees from a right angle. Such struts shall be of adequate dimensions and anchorage to restrain the column against lateral deflection.
4. In columns restrained laterally by struts or beams, with brackets used at the junction, it shall be the clear distance between the floor and the lower edge of the bracket, provided that the bracket width equals that of the beam or strut and is at least half that of the column.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
(5) For rectangular columns, that length shall be considered which produces the greatest ratio of length to depth of section.
703—Spirally Reinforced Columns.
(a) Allowable Load—
The maximum allowable axial load, P, on columns with closely spaced spirals enclosing a circular concrete core reinforced with longitudinal bars shall be that given by Formula 19:
P = Ag(0.225/'c+APi)-------------------------------------------- (19)
Wherein Ag =the gross area of the column.
f'c = compressive strength of the concrete.
= nominal working stress in vertical column reinforcement, to be taken at forty per cent of the minimum specification value of the yield point; viz., 16,000 p.s.i. for intermediate grade steel and 20,000 p.s.i. for rail or hard grade steel.*
pg = ratio of the effective cross-sectional area of vertical reinforcement to the gross area, Ag.
(b) Vertical Reinforcement—
The ratio pt shall not be less than 0.005 nor more than 0.02. The minimum number of bars shall be six, and the minimum diameter shall be % in. The center to center spacing of bars within the periphery of the column core shall not be less than 2% times the diameter for round bars or three times the side dimension for square bars. The clear spacing between bars shall not be less than 1% inches or 1M times the maximum size of the coarse aggregate used. These spacing rules also apply to adjacent pairs of bars at a lapped splice; each pair of lapped bars forming a splice may be in contact, but the minimum clear spacing between one splice and the adjacent splice should be that specified for adjacent single bars.
(c) Splices in Vertical Reinforcement—
Where lapped splices in the column verticals are used, the minimum amount of lap shall be as follows:
1. For deformed bars—with concrete having a strength of 3000 p.s.i. or above, twenty-four diameters of bar of intermediate grade steel and thirty diameters of bar of hard grade steel. For bars of higher yield point, the amount of lap shall be increased in proportion to the nominal working stress. When the concrete strengths are less than 3000 p.s.i., the amount of lap shall be one-third greater than the values given above.
2. For plain bars—the minimum amount of lap shall be twenty-five percent greater than that specified for deformed bars.
3. Welded splices or other positive connections may be used instead of lapped splices. Welded splices shall preferably be used in cases where the bar diameter exceeds 1% inches. An approved welded splice shall be defined as one in ‘Nominal working stresses for reinforcement of higher yield point may be established at forty percent of the yield point stress, but not more than 30,000 p.s.i., when the properties of such reinforcing steels have been definitely specified by standards of A.S.T.M. designation. If this is done, the lengths of splice required by Section 703 (c) shall be increased accordingly.
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which the bars are butted and welded and that will develop in tension at least the yield point stress of the reinforcing steel used.
4. Where changes in the cross section of a column occur, the longitudinal bars shall be offset in a region where lateral support is afforded by a concrete capital, floor slab or by metal ties or reinforcing spirals. Where bars are offset, the slope of the inclined portion from the axis of the column shall not exceed 1 in 6 and the bars above and below the offset shall be parallel to the axis of the column.
(d) Spiral Reinforcement—
The ratio of spiral reinforcement, p', shall not be less than the value given by Formula 20:
Wherein p'=ratio of volume of spiral reinforcement to the volume of the concrete core (out to out of spirals).
J's—useful limit stress of spiral reinforcement, to be taken as 40,000 p.s.i., for hot rolled rods of intermediate grade, 50,000 p.s.i. for rods of hard grade, and 60,000 p.s.i. for cold drawn wire.
The spiral reinforcement shall consist of evenly spaced continuous spirals held firmly in place and true to line by at least three vertical spacer bars. The spirals shall be of such size and so assembled as to permit handling and placing without being distorted from the designed dimensions. The material used in spirals shall have a minimum diameter of % in. for rolled bars or No. 4 W. & M. gage for drawn wire. Anchorage of spiral reinforcement shall be provided by 1% extra turns of spiral rod or wire at each end of the spiral unit. Splices, when necessary shall be made in spiral rod or wire by welding or by a lap of 1% turns. The center to center spacing of the spirals shall not exceed onesixth of the core diameter. The clear spacing between spirals shall not exceed 3 in. nor be less than 1% in. or 1% times the maximum size of coarse aggregate used. The reinforcing spiral shall extend from the floor level in any story or from the top of the footing in the basement, to the level of the lowest horizontal reinforcement in the slab, drop panel or beam above. In a column with a capital, it shall extend to a plane at which the diameter or width of the capital is twice that of the column.
(e) Protection of Reinforcement—
The column reinforcement shall be protected everywhere by a covering of concrete cast monolithically with the core, for which the thickness shall not be less than 1 % in. nor less than 1 % times the maximum size of the coarse aggregate, nor shall it be less than required by fire protection and weathering protection.
(f) Isolated Column with Multiple Spirals—
In case two or more interlocking spirals are used in a column, the outer boundary of the column shall be taken as a rectangle of which the sides are outside the extreme limits of the spiral at a distance equal to the requirements of Section 703(e).
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
(g) Limits of Section of Column Built Monolithically with Wall—
For a spiral column built monolithically with a concrete wall or pier, the outer boundary of the column section shall be taken either as a circle at least 1% in. outside the column spiral or as a square or rectangle of which the sides are at least 1% in. outside the spiral or spirals.
(h) Equivalent Circular Columns—
As an exception to the general procedure of utilizing the full gross area of the column section, it shall be permissible to design a circular column and to build it with a square, octagonal, or other shaped section of the same least lateral dimension. In such case, the allowable load, the gross area considered, and the required percentages of reinforcement shall be taken as those of the circular column.
704—Tied Columns.
(a) Allowable Load—
The maximum allowable axial load on columns reinforced with longitudinal bars and separate lateral ties shall be 80 percent of that given by Formula (19). The ratio, pa, to be considered in tied columns shall not be less than 0.005 nor more than 0.02. The longitudinal reinforcement shall consist of at least four bars, of minimum diameter of % inch. Splices in reinforcing bars shall be made as described in Section 703(c).
(6) Lateral Ties—
Lateral ties shall be at least % in. in diameter and shall be spaced apart not over 16 bar diameters, 48 tie diameters or the least dimension of the column. When there are more than four vertical bars, additional ties shall be provided so that every longitudinal bar is held firmly in its designed position and has lateral support equivalent to that provided by a 90-degree corner of a tie. (c) Limits of Column Section—
In a tied column which for architectural reasons has a larger cross section than required by considerations of loading, a reduced effective area, Aa, not less than one-half of the total area may be used in applying the provisions of Section 704(a).
705—Composite Columns.
(a) Allowable Load—
The allowable load on a composite column, consisting of a structural steel or cast-iron column thoroughly encased in concrete reinforced with both longitudinal and spiral reinforcement, shall not exceed that given by Formula (21):
P = 0.225 Aef'e+faA.+frAry.------------------------------------21
Wherein Ac=net area of concrete section
=Aa—A,—Ar
As=cross-sectional area of longitudinal bar reinforcement.
Ar= cross-sectional area of the steel or cast-iron core.
fT= allowable unit stress in metal core, not to exceed 16,000 p. s. i. for a steel core; or 10,000 p. s. i. for a cast-iron core.
The remaining notation is that of Section 703.
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(b) Details of Metal Core and Reinforcement—
The cross-sectional area of the metal core shall not exceed 20 percent of the gross area of the column. If a hollow metal core is used it shall be filled with concrete. The amounts of longitudmal and spiral reinforcement and the requirements as to spacing of bars, details of splices and thickness of protective shell outside the spiral shall conform to the limiting values specified in Section 703(b), (c) and (d). A clearance of at least three inches shall be maintained between the spiral and the metal core at all points except that when the core consists of a structural steel H-column, the minimum clearance may be reduced to two inches.
(c) Splices and Connections of Metal Cores—
Metal cores in composite columns shall be accurately milled at splices and positive provision shall be made for alignment of one core above another. At the column base, provision shall be made to transfer the load to the footing at safe unit stresses in accordance with Section 102(a). The base of the metal section shall be designed to transfer the load from the entire composite column to the footing, or it may be designed to transfer the load from the metal section only, provided it is so placed in the pier or pedestal as to leave ample section of concrete above the base for the transfer of load from the reinforced concrete section of the column by means of bond on the vertical reinforcement and by direct compression on the concrete. Transfer of loads to the metal core shall be provided for by the use of bearing members such as billets, brackets or other positive connections; these shall be provided at the top of the metal core and at intermediate floor levels where required. The column as a whole shall satisfy the requirements of Formula (21) at any point; in addition to this, the reinforced concrete portion shall he designed to carry, in accordance with Formula (19), all floor loads brought onto the column at levels between the metal brackets or connections. In applying Formula (19), the value of Ag, shall be interpreted as the area of the concrete section outside the metal core, and the allowable load on the reinforced concrete section shall be further limited to 0.35 f'cAg. Ample section of concrete and continuity of reinforcement shall be provided at the junction with beams or girders.
(d) Allowable Load on Metal Core Only—
The metal cores of composite columns shall be designed to carry safely any construction or other loads to be placed upon them prior to their encasement in concrete.
706—Combination Columns.
(a) Steel Columns Encased in Concrete—
The allowable load on a structural steel column which is encased in concrete at least 2}£ inches thick over all metal (except rivet heads) reinforced as hereinafter specified, shall be computed by Formula 22:
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
Wherein Ar=cross-sectional area of steel column.
J'r—allowable stress for unencased steel column. =total area of concrete section.
The concrete used shall develop a compressive strength, J'e, of at least 2000 p. s. i. at 28 days. The concrete shall be reinforced by the equivalent of welded wire mesh having wires of No. 10 W. and M. gage, the wires encircling the column being spaced not more than four inches apart and those parallel to the column axis not more than eight inches apart. This mesh shall extend entirely around the column at a distance of one inch inside the outer concrete surface and shall be lap-spliced at least forty wire diameters and wired at the splice. Special brackets shall be used to receive the entire floor load at each floor level. The steel column shall be designed to carry safely any construction or other loads to be placed upon it prior to its encasement in concrete.
(6) Pipe Columns—
The allowable load on columns consisting of steel pipe filled with concrete shall be determined by Formula 23.
P = 0.225 f eAc +fr A,_________________________________— (23)
The value of/'r shall be given by Formula 24.
Wherein /',=allowable unit stress in metal pipe. h=unsupported length of column.
K = least radius of gyration of metal pipe section.
If the yield point of the pipe is not known, the factor F shall be taken as 0.5.
707—Long Columns.
(a) The maximum allowable load, P', on axially loaded reinforced concrete or composite columns having a length, h, greater than ten times the least lateral dimension, d, shall be given by Formula 25:
where P is the allowable axial load on a short column as given by Formulas 19 and 21.
The maximum allowable load, P', on eccentrically loaded columns in which exceeds ten shall also be given by Formula 25, in which P is the allowable eccentrically applied load on a short column as determined by the provisions of Sections 709 and 710. In long columns subjected to definite bending stresses, as determined in Section 708, the ratio shall not exceed twenty.
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708—Bending Moments in Columns.
(a) The bending moments in the columns of all reinforced concrete structures shall be determined on the basis of loading conditions and restraint and shall be provided for in the design. When the stiffness and strength of the columns are utilized to reduce moments in beams, girders, or slabs, as in the case of rigid frames, or in other forms of continuous construction wherein column moments are unavoidable, they shall be provided for in the design. In building frames, particular attention shall be given to the effect of unbalanced floor loads on both exterior and interior columns and of eccentric loading due to other causes. Wall columns shall be designed to resist moments produced by
1. Loads on all floors of the building;
2. Loads on a single exterior bay at two adjacent floor levels; or 3. Loads on a single exterior bay at one floor level.
Resistance to bending moments at any floor level shall be provided by distributing the moment between the columns immediately above and below the given floor in proportion to their relative stiffnesses and conditions of restraint.
709—Determination of Combined Axial and Bending Stresses.
(a) In a reinforced concrete column, designed by the methods of this Chapter, which is (1) symmetrical about two perpendicular planes through its axis and (2) subject to an axial load, TV, combined with bending in one or both of the planes of symmetry (but with the ratio of eccentricity to depth, eft, no greater than 1.0 in either plane), the combined. fiber stress in compression may be computed on the basis of recognized theory applying to uncracked sections, using Formula 26:
Equating this calculated stress, ft,to the allowable stress,/,,, in Formula 28, it follows that the column can be designed for an equivalent axial load, P, as given by Formula 27*:
When bending exists on both axes of symmetry, the quantity
is to be computed as the numerical sum of the
quantities in the
two directions.
(5) For columns in which the load, N, has an eccentricity, e, greater than the column depth, t, or for beams subject to small axial loads, the determination of the fiber stress fc shall be made by use of recognized theory for cracked sections, based on the assumption that no tension exists in the concrete. For such cases the tensile steel stress shall also be investigated.
*For approximate or trial computations, D may be taken as eight for a circular spiral column and five for a rectangular tied or spiral column.
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
710—Allowable Combined Axial and Bending Stress.
(a) For spiral and tied columns, eccentrically loaded or otherwise subjected to combined axial compression and flexural stress, the maximum allowable compressive stress, fp, is given by Formula 28:
Wherein the notation is that of Sections 703 and 709, and, in addition fa is the average allowable stress in the concrete of an axially loaded reinforced concrete column, and C is the ratio of fa to the allowable
fiber stress for members in flexure. Thus
for spiral
columns and 0.8 of this value for tied columns, and
711—Wind Stresses.
(a) When the allowable stress in columns is modified to provide for combined axial load and bending, and the stress due to wind loads is also added, the total shall still come within the allowable values specified for wind loads in Section 203(c).
712—Reinforced Concrete Walls.
(a) The allowable working stresses in reinforced concrete bearing walls with minimum reinforcement as required by Section 712(i), shall be 0.25/'«. for walls having a ratio of height to thickness of ten or less, and shall be reduced proportionally to 0.15/'c for walls having a ratio of height to thickness of twenty-five. When the reinforcement in bearing walls is designed, placed and anchored in position as for tied columns, the allowable working stresses shall be on the basis of Section 704, as for columns. In the case of concentrated loads, the length of the wall to be considered as effective for each shall not exceed the center to center distance between loads, nor shall it exceed the width of the bearing plus four times the wall thickness. The ratio pg shall not exceed 0.02.
(6) Walls shall be designed for any lateral or other pressure to which they are subjected. Proper provision shall be made for eccentric loads and wind stresses. In such designs the allowable stresses shall be as given in Section 102(a) and 203(c).
(c) Panel and enclosure walls of reinforced concrete shall have a thickness of not less than five inches and not less than one thirtieth the distance between the supporting or enclosing members.
(d) Bearing walls of reinforced concrete in buildings of fire-resistive construction shall be not less than six inches in thickness for the uppermost fifteen feet of their height; and for each successive twenty-five feet downward, or fraction thereof, the minimum thickness shall be increased one inch. In two story dwellings the walls may be six inches in thickness throughout.
(e) In buildings of non-fire-resistive construction bearing walls of reinforced concrete shall not be less than one and pne-third times the
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thickness required for buildings of fire-resistive construction, except that for dwellings of two stories or less in height the thickness of walls may be the same as specified for buildings of fire-resistive construction.
(j) Exterior basement walls, foundation walls, fire walls and party walls shall not be less than eight inches thick whether reinforced or not.
(g) Reinforced concrete bearing walls shall have a thickness of at least one twenty-fifth of the unsupported height or width, whichever is the shorter; provided however, that approved buttresses, built-in columns, or piers designed to carry all the vertical loads, may be used in lieu of increased thickness.
(h) Reinforced concrete walls shall be anchored to the floors, columns, pilasters, buttresses and intersecting walls with reinforcement at least equivalent to three-eighths inch round bars twelve inches on centers, for each layer of wall reinforcement.
(i) Reinforced concrete walls shall be reinforced with an area of steel in each direction, both vertical and horizontal, at least equal to 0.0025 times the cross-sectional area of the wall, if of bars, and 0.0018 times the area if of electrically welded wire fabric. The wire of the welded fabric shall be of not less than No. 10 W. & M. gage. Walls more than ten inches in thickness shall have the reinforcement for each direction placed in two layers parallel with the faces of the wall. One layer consisting of not less than one-half and not more than two-thirds the total required shall be placed not less than two inches nor more than one-third the thickness of the wall from the exterior surface. The other layer, comprising the balance of the required reinforcement, shall be placed not less than three-fourths inches and not more than one-third the thickness of the wall from the interior surface. Bars, if used, shall not be less than the equivalent of three-eighths inch round bars, nor shall they be spaced more than eighteen inches on centers. Welded wire reinforcement for walls shall be in flat sheet form.
(/) In addition to the minimum as prescribed in 712(i) there shall be not less than two five-eighths inch diameter bars around all window or door openings. Such bars shall extend at least twenty-four inches beyond the corner of the openings.
(k) Where reinforced concrete bearing walls consist of studs or ribs tied together by reinforced concrete members at each floor level, the studs may be considered as columns, but the restrictions as to minimum diameter or thickness of columns shall not apply.
Chapter 8—Footings
801—Scope.
(a) The requirements prescribed in Sections 802 to 809 apply only to isolated footings.
802—Loads and Reactions.
(a) Footings shall be proportioned to sustain the applied loads and induced reactions without exceeding the allowable stresses as prescribed in Sections 102 and 103, and as further provided in Sections 805, 806, and 807.
(6) In cases where the footing is concentrically loaded and the mem-
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS ber being supported does not transmit any moment to the footing, computations for moments and shears shall be based on an upward reaction assumed to be uniformly distributed per unit area or per pile and a downward applied load assumed to be uniformly distributed over the area of the footing covered by the column, pedestal, wall, or metallic column base.
(c) In cases where the footing is eccentrically loaded and/or the member being supported transmits a moment to the footing, proper allowance shall be made for any variation that may exist in the intensities of reaction and applied load consistent with the magnitude of the applied load and the amount of its actual or virtual eccentricity.
(d) In the case of footings on piles, computations for moments and shears may be based on the assumption that the reaction from any pile is concentrated at the center of the pile.
803—Sloped or Stepped Footings.
(a) In sloped or stepped footings, the angle of slope or depth and location of steps shall be such that the allowable stresses are not exceeded at any section.
(6) In sloped or stepped footings, the effective cross-section in compression shall be limited by the area above the neutral plane.
(c) Sloped or stepped footings shall be cast as a unit.
804—Bending Moment.
(a) The external moment on any section shall be determined by passing through the section a vertical plane which extends completely across the footing, and computing the moment of the forces acting over the entire area of the footing on one side of said plane.
(6) The greatest bending moment to be used in the design of an isolated footing shall be the moment computed in the manner prescribed in Section 804(a) at sections located as follows:
1. At the face of the column, pedestal or wall, for footings supporting a concrete column, pedestal or wall.
2. Halfway between the middle and the edge of the wall, for footings under masonry walls.
3. Halfway between the face of the column or pedestal and the edge of the metallic base, for footings under metallic bases.
(c) The width resisting compression at any section shall be assumed as the entire width of the top of the footing at the section under consideration.
(d) In one-way reinforced footings, the total tensile reinforcement at any section shall provide a moment of resistance at least equal to the moment computed in the manner prescribed in Section 804(a); and the reinforcement thus determined shall be distributed uniformly across the full width of the section.
(e) In two-way reinforced footings, the total tensile reinforcement at any section shall provide a moment of resistance at least equal to eighty-five percent of the moment computed in the manner prescribed in Section 804(a); and the total reinforcement thus determined shall be distributed across the corresponding resisting section in the manner prescribed for square footings in Section 804(f), and for rectangular footings in Section 804(g).
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(/) In two-way square footings, the reinforcement extending in each direction shall be distributed uniformly across the full width of the footing.
(tj) In two-way rectangular footings, the reinforcement in the long direction shall be distributed uniformly across the full width of the footing. In the case of the reinforcement in tiie short direction, that portion determined by formula (29) shall be uniformly distributed across a band-width (B) centered with respect to the center line of the column or pedestal and having a width equal to the length of the short side of the footing. The remainder of the reinforcement shall be uniformly distributed in the outer portions of the footing.
In formula (29), S' is the ratio of the long side to the short side of the footing.
805—Shear and Bond.
(a) The critical section for shear to be used as a measure of diagonal tension shall be assumed as a vertical section obtained by passing a series of vertical planes through the footing, each of which is parallel to a corresponding face of the column, pedestal, or wall and located a distance therefrom equal to the depth d for footings on soil, and one-half the depth d for footings on piles.
(6) Each face of the critical section as defined in Section 805(a) shall be considered as resisting an external shear equal to the load on an area bounded by said face of the critical section for. shear, two diagonal lines drawn from the column or pedestal corners and making 45° angles with the principal axes of the footing, and that portion of the corresponding edge or edges of the footing intercepted between the two diagonals.
(c) Critical sections for bond shall be assumed at the same planes as those prescribed for bending moment in Section 804(b); also at all other vertical planes where changes of section or of reinforcement occur.
(d) Computations for shear .to be used as a measure of bond shall be based on the same section and loading as prescribed for bending moment in Section 804(a).
{e) The total tensile reinforcement at any section shall provide a bond resistance at least equal to the bond requirement as computed from the following percentages of the external shear at the section:
1. In one-way reinforced footings, 100 percent.
2. In two-way reinforced footings, 85 percent.
(f) In computing the external shear on any section through a footing supported on piles, the entire reaction from any pile whose center is located six inches or more outside the section shall be assumed as producing shear on the section; the reaction from any pile whose center is located six inches or more inside the section shall be assumed as producing no shear on the section. For intermediate positions of the pile center, the portion of the pile reaction to be assumed as producing shear on the section shall be based on straight-line interpolation
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EMERGENCY SPECIFICATIONS FOR REINFORCED CONCRETE BUILDINGS
between full value at six inches outside the section and zero value at six inches inside the section.
0) For allowable shearing stresses, see Sections 102 and 408.
(h) For allowable bond stresses, see Sections 102 and 501 to 505.
806—Transfer of Stress at Base of Column.
(a) The stress in the longitudinal reinforcement of a column or pedestal shall be transferred to its supporting pedestal or footing either by extending the longitudinal bars into the supporting member, or by dowels.
(6) In case the transfer of stress in the reinforcement is accomplished by extension of the longitudinal bars, they shall extend into the supporting member the distance required to transfer to the concrete, by allowable bond stress, their full working value.
(c) In cases where dowels are used, their total sectional area shall be not less than the sectional area of the longitudinal reinforcement in the member from which the stress is being transferred. In no case shall the number of dowels per member be less than four and the diameter of the dowels shall not exceed the diameter of the column bars by more than one-eighth inch.
(d) Dowels shall extend up into the column or pedestal a distance at least equal to that required for lap of longitudinal column bars (see Section 703) and down into the supporting pedestal or footing the distance required to transfer to the concrete, by allowable bond stress, the full working value of the dowel.
(e) The compressive stress in the concrete at the base of a column or pedestal shall be considered as being transferred by bearing to the top of the supporting pedestal or footing. The unit compressive stress on the loaded area shall not exceed the bearing stress allowable for the quality of concrete in the supporting member as limited by the ratio of the loaded area to the supporting area.
(/) For allowable bearing stresses see Table 102(a), Section 102.
(g) In sloped or stepped footings, the supporting area for bearing may be taken as the top horizontal surface of the footing, or assumed as the area of the lower base of the largest frustum of a pyramid or cone contained wholly within the footing and having for its upper base the area actually loaded, and having side slopes of one vertical to two horizontal.
807—Pedestals and Footings (Plain Concrete).
(a) The allowable compressive unit stress on the gross area of a concentrically loaded pedestal shall not exceed 0.25/'c. Where this stress is exceeded, reinforcement shall be provided and the member designed as a reinforced concrete column.
(6) The depth and width of a pedestal or footing of plain concrete shall be such that the tension in the concrete shall not exceed 0.04/,c, and the average shearing stress shall not exceed 0.02/'c taken on sections as prescribed in Sections 804 and 805 for reinforced concrete footings..
808—Footings Supporting Round Columns.
(a) In computing the stresses in footings which support a round or octagonal concrete column or pedestal, the “face” of the column or
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pedestal shall be taken as the side of a square having an area equal to the area enclosed within the perimeter of the column or pedestal.
809—Minimum Edge-Thickness.
(a) In reinforced concrete footings, the thickness above the reinforcement at the edge shall be not less than six inches for footings on soil, nor less than twelve inches for footings on piles.
(6) In plain concrete footings, the thickness at the edge shall be not less than eight inches for footings on soil, nor less than fourteen inches above the tops of the piles for footings on piles.
U. S. GOVERNMENT PRINTING OFFICE: 194?
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