This chapter shall apply to member design for minimum serviceability, including (a) through (d):
(a) Deflections due to service-level gravity loads (24.2)
(c) Shrinkage and temperature reinforcement (24.4)
(d) Permissible stresses in prestressed flexural members (24.5)
Members subjected to flexure shall be designed with adequate stiffness to limit deflections or deformations that adversely affect strength or serviceability of a structure.
Deflections calculated in accordance with 24.2.3 through 24.2.5 shall not exceed the limits in Table 24.2.2.
Table 24.2.2—Maximum permissible calculated deflections
|Member||Condition||Deflection to be considered||Deflection limitation|
|Flat roofs||Not supporting or attached to nonstructural elements likely to be damaged by large deflections||Immediate deflection due to maximum of Lr, S, and R||ℓ/180|
|Floors||Immediate deflection due to L||ℓ/360|
|Roof or floors||Supporting or attached to nonstructural elements||Likely to be damaged by large deflections||That part of the total deflection occurring after attachment of nonstructural elements, which is the sum of the time-dependent deflection due to all sustained loads and the immediate deflection due to any additional live load||ℓ/480|
|Not likely to be damaged by large deflections||ℓ/240|
Limit not intended to safeguard against ponding. Ponding shall be checked by calculations of deflection, including added deflections due to ponded water, and considering time-dependent effects of sustained loads, camber, construction tolerances, and reliability of provisions for drainage.
Time-dependent deflection shall be calculated in accordance with 24.2.4, but shall be permitted to be reduced by amount of deflection calculated to occur before attachment of nonstructural elements. This amount shall be calculated on basis of accepted engineering data relating to time-deflection characteristics of members similar to those being considered.
Limit shall be permitted to be exceeded if measures are taken to prevent damage to supported or attached elements.
Limit shall not exceed tolerance provided for nonstructural elements.
Immediate deflections shall be calculated using methods or formulas for elastic deflections, considering effects of cracking and reinforcement on member stiffness.
Effect of variation of cross-sectional properties, such as haunches, shall be considered when calculating deflections.
Deflections in two-way slab systems shall be calculated taking into account size and shape of the panel, conditions of support, and nature of restraints at the panel edges.
For nonprestressed members, effective moment of inertia, Ie, shall be calculated by Eq. (22.214.171.124a) unless obtained by a more comprehensive analysis, but Ie shall not be greater than Ig.
where Mcr is calculated by
For prestressed Class T and Class C slabs and beams as defined in 24.5.2, deflection calculations shall be based on a cracked transformed section analysis. It shall be permitted to base deflection calculations on a bilinear moment-deflection relationship or Ie in accordance with Eq. (126.96.36.199a), where Mcr is calculated as:
Unless obtained from a more comprehensive analysis, additional time-dependent deflection resulting from creep and shrinkage of flexural members shall be calculated as the product of the immediate deflection caused by sustained load and the factor λΔ
In Eq. (188.8.131.52.1), ρ' shall be calculated at midspan for simple and continuous spans, and at the support for cantilevers.
If composite concrete flexural members are shored during construction so that, after removal of temporary supports, the dead load is resisted by the full composite section, it shall be permitted to consider the composite member equivalent to a monolithically cast member for calculation of deflections.
Deflections resulting from differential shrinkage of precast and cast-in-place components, and of axial creep effects in prestressed members, shall be considered.
Spacing of bonded reinforcement closest to the tension face shall not exceed the limits in Table 24.3.2, where cc is the least distance from surface of deformed or prestressed reinforcement to the tension face. Calculated stress in deformed reinforcement, fs, and calculated change in stress in bonded prestressed reinforcement, Δfps, shall be in accordance with 184.108.40.206 and 220.127.116.11, respectively.
Table 24.3.2—Maximum spacing of bonded reinforcement in nonprestressed and Class C prestressed one-way slabs and beams
|Reinforcement type||Maximum spacing s|
|Deformed bars or wires||Lesser of:|
|Bonded prestressed reinforcement||Lesser of:|
|Combined deformed bars or wires and bonded prestressed reinforcement||Lesser of:|
Change in stress, Δfps, in bonded prestressed reinforcement at service loads shall be equal to the calculated stress based on a cracked section analysis minus the decompression stress fdc. It shall be permitted to take fdc equal to the effective stress in the prestressed reinforcement fse. The value of Δfps shall not exceed 36,000 psi. If Δfps does not exceed 20,000 psi, the spacing limits in Table 24.3.2 need not be satisfied.
If there is only one bonded bar, pretensioned strand, or bonded tendon nearest to the extreme tension face, the width of the extreme tension face shall not exceed s determined in accordance with Table 24.3.2.
If flanges of T-beams are in tension, part of the bonded flexural tension reinforcement shall be distributed over an effective flange width as defined in accordance with 6.3.2, but not wider than ℓn/10. If the effective flange width exceeds ℓn/10, additional bonded longitudinal reinforcement shall be provided in the outer portions of the flange.
The spacing of bonded flexural reinforcement in nonprestressed and Class C prestressed one-way slabs and beams subject to fatigue, designed to be watertight, or exposed to corrosive environments, shall be selected based on investigations and precautions specific to those conditions and shall not exceed the limits of 24.3.2.
If shrinkage and temperature movements are restrained, the effects of T shall be considered in accordance with 5.3.6.
The ratio of deformed shrinkage and temperature reinforcement area to gross concrete area shall satisfy the limits in Table 18.104.22.168.
Table 22.214.171.124—Minimum ratios of deformed shrinkage and temperature reinforcement area to gross concrete area
|Reinforcement type||fy, psi||Minimum reinforcement ratio|
|Deformed bars||< 60,000||0.0020|
|Deformed bars or welded wire reinforcement||≥ 60,000||Greater of:|
At all sections where required, deformed reinforcement used to resist shrinkage and temperature stresses shall develop fy in tension.
For one-way precast slabs and one-way precast, prestressed wall panels, shrinkage and temperature reinforcement is not required in the direction perpendicular to the flexural reinforcement if (a) through (c) are satisfied.
(a) Precast members are not wider than 12 ft
(b) Precast members are not mechanically connected to cause restraint in the transverse direction
(c) Reinforcement is not required to resist transverse flexural stresses
Prestressed flexural members shall be classified as Class U, T, or C in accordance with Table 126.96.36.199, based on the extreme fiber stress in tension ft in the precompressed tension zone calculated at service loads assuming an uncracked section.
Table 188.8.131.52—Classification of prestressed flexural members based on ft
|Assumed behavior||Class||Limits of ft|
|Transition between uncracked and cracked||T|
Prestressed two-way slabs shall be designed as Class U with .
For Class U and T members, stresses at service loads shall be permitted to be calculated using the uncracked section.
Calculated extreme concrete fiber stress in compression immediately after transfer of prestress, but before time-dependent prestress losses, shall not exceed the limits in Table 184.108.40.206.
|Location||Concrete compressive stress limits|
|End of simply-supported members||0.70fci'|
|All other locations||0.60fci'|
Calculated extreme concrete fiber stress in tension immediately after transfer of prestress, but before time-dependent prestress losses, shall not exceed the limits in Table 220.127.116.11, unless permitted by 18.104.22.168.1.
Table 22.214.171.124—Concrete tensile stress limits immediately after transfer of prestress, without additional bonded reinforcement in tension zone
|Location||Concrete tensile stress limits|
|Ends of simply-supported members|
|All other locations|
For Class U and T members, the calculated extreme concrete fiber stress in compression at service loads, after allowance for all prestress losses, shall not exceed the limits in Table 126.96.36.199.
|Load condition||Concrete compressive stress limits|
|Prestress plus sustained load||0.45fc'|
|Prestress plus total load||0.60fc'|