Building Design and Construction

Shear in Joists

The factored shear force Vu at a section without shear reinforcement should not exceed

Based on satisfactory performance of joist construction, the ACI 318 Building Code allows the nominal shear strength Vc for concrete in joists to be taken 10% greater than for beams or slabs. The width bw can be taken as the average of the width of joist at the compression face and the width at the tension reinforcement. The slope of the vertical taper of ribs formed with removable steel pans can safely be assumed as 1 in 12. For permanent concrete block fillers, the shell of the block can be included as part of bw, if the compressive strength of the masonry is equal to or greater than that of the concrete.

 

If shear controls the design of one-way concrete-joist construction, tapered ends can be used to increase the shear capacity. The Concrete Reinforcing Steel Institute’s ‘‘CRSI Design Handbook’’ has comprehensive load tables for one-way concrete- joist construction that indicate where shear controls and when tapered ends are required for simple, end, and interior spans.
For joists supporting uniform loads, the critical section for shear strength at tapered ends is the narrow end of the tapered section. Shear need not be checked within the taper.
Reinforcement for shear must be provided when the factored shear force Vu exceeds the shear strength of the concrete Vc. The use of single-prong No. 3 stirrups spaced at half depth, such as that shown in Fig. 9.25, is practical in narrow joists; they can be placed between two bottom bars.

 

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