T

permitted in a singly reinforced section is 0.714pb, compared to 0.75pb of the 1999 Code.

• A significant revision has been made in the permissible design tension stress, f, for prestressed flexural members. Recall that ACI 318-99, like its predecessors, typically limits the extreme fiber stress in tension, ft, in precompressed tensile zone to . This can be increased to provided deflection analysis is based on transformed cracked sections and on bilinear moment-deflection relationships. Now, under provisions of the 2002 edition, prestressed members are classified into three classes with corresponding allowable tension stress as shown in Table 7.1. It should be noted that serviceability requirements get progressively stringent from class U to class C. (The designer is referred to Table R18.3.3 of the 2002 code for serviceability check requirements for each class.) Note that prestressed two-way slab systems must still be designed as class U. This restriction is to prevent the possibility of punching shear failure in two-way systems.

It should be noted that although the examples given here are based on ACI 318-99, they comply with the 2002 edition, because Appendix C of this edition continues to permit the use of load and strength reduction factors of ACI 318-99.

7.2.1. One-Way Slabs

One-way slabs are discussed here to illustrate the simplifications commonly made in a design office to analyze these systems.

Figure 7.10 shows a uniformly loaded floor slab with intermediate beams that divide the slab into a series of one-way slabs. If a typical 1-ft width of slab is cut out as a free body in the longitudinal direction, it is evident that the slab will bend with a positive curvature between the supporting beams, and a negative curvature at the supporting beams. The deflected shape is similar to that of a continuous beam spanning across transverse girders, which act as simple supports. The assumption of simple support neglects the torsional stiffness of the beams supporting the slab. If the distance between the beams is the same, and if the slabs carry approximately the same load, the torsional stiffness of the beams has little influence on the moments in the slab.

However, the slab twists the exterior beams, which are loaded from one side only. The resistance to the end rotation of the slab offered by the exterior beam is dependent on the torsional stiffness of the beam. If the beam is small and its torsional stiffness low, a pin support may be assumed at the exterior edge of slab. On the other hand, if the exterior beam is large with a high torsional rigidity, it will apply a significant restraining moment to the slab. The beam, in turn, will be subjected to a torsional moment that must be considered in design.

Figure 7.10. One-way slab example: (a) typical 1-ft strip; (b) slab modelled as a continuous beam; (c) design moments.

7.2.1.1. Analysis by ACI Coefficients

Analysis by this method is limited to structures in which: 1) the span lengths are approximately the same (with the maximum span difference between adjacent spans no more than 20%); 2) the loads are uniformly distributed; and 3) the live load does not exceed three times the dead load.

ACI values for positive and negative design moments are illustrated in Figs. 7.11 and 7.12. Observe that ln equals the clear span for positive moment and shear, and the average of adjacent clear spans for negative moment.

Example. One-way mild steel reinforced slab.

Given. A one-way continuous slab as shown in Fig. 7.13.

Ultimate load = 0.32 kip/ft

Required. Flexural reinforcement design for interior span between grids C and E.

Solution. Use Table 7.2 to determine the minimum slab thickness required to satisfy deflection limitations. Using l = center-to-center span = 16 ft, l 12 x 16

min 28 28 2

Analyze a 1-ft width of slab as a continuous beam using ACI coefficients to establish design moments for positive and negative steel (Fig. 7.13). Using a clear span ln = 12.5 ft

Figure 7.11. ACI positive moment coefficients: (a) interior span; (b) exterior span, discontinuous end integral with supports: (c) exterior span, discontinuous end unrestrained.

for the first bay,

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