Two-Way Slabs on Beams

The ACI 318 Building Code provides for use of beams on the sides of panels, on column centerlines. (A system of slabs and beams supported by girders, however, usually forms rectangular panels. In that case, the slabs are designed as one-way slabs.)
Use of beams on all sides of a panel permits use of thinner two-way slabs, down to a minimum thickness h = 31⁄2 in. A beam may be assumed to resist as much as 85% of the column-strip moment, depending on its stiffness relative to the slab (see the ACI 318 Building Code). A secondary benefit, in addition to the direct advantages of longer spans, thinner slabs, and beam stirrups for shear, is that many local codes allow reduced service live loads for design of the beams. These reductions are based on the area supported and the ratio of dead to live load. For service live loads up to 100 psf, such reductions are usually permitted to a maximum of 60%.
Where such reductions are allowed, the reduced total panel moment Mo (Art. 9.59.1) and the increased effective depth to reinforcing steel in the beams offer savings in reinforcement to offset partly the added cost of formwork for the beams.

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