Building Design and Construction

Reinforcement of Joists for Flexure

Reinforcement required for strength can be determined as indicated in Art. 9.46, by treating as a beam a section symmetrical about a rib and as wide as the spacing of ribs on centers.

Minimum Reinforcement. For not greater than 4400 psi, reinforcement (both ƒc positive and negative) with a yield strength ƒy should have an area equal to or greater than 200/ƒy times the concrete area of the rib bwd, where bw is the rib width and d  rib depth. For exceeding 4400 psi, the area of reinforcement should be ƒc at least equal to 3 (squerƒ’cbw) d/ƒy. Less reinforcement can be used, however, if the areas c w of both the positive and negative reinforcement at every section are one-third greater than the amount required by analysis. (See also Art. 9.55.)
Maximum Reinforcement. Positive- and negative-moment reinforcement ratios must not be greater than three-quarters of the ratio that produces balanced conditions (Art. 9.46). The positive-moment reinforcement ratio is based on the width of the top flange, and the negative-moment reinforcement ratio is based on the width of the rib bw.
Reinforcement for one-way concrete-joist construction consists of straight top and bottom bars, cut off as required for moment.
For top-slab reinforcement, straight top- and bottom-bar arrangements provide more flexibility in attaining uniform distribution of top bars to control cracking in the slab than straight and bent bars.
Requirements for structural integrity included in the ACI 318 Building Code affect detailing of the bottom bars in the ribs. Over supports, at least one bottom bar should be continuous or lap spliced to a bottom bar in the adjacent span with a Class A tension lap splice (Art. 9.49.7). At exterior supports, one bottom bar should be terminated with a standard hook.
For development (bond) of reinforcement, see Art. 9.49.
Figure 9.24b shows rebar quantities, lb / ft2 of floor or roof area, for continuous interior spans of one-way concrete-joist construction made with normal-weight concrete for superimposed factored live load of 170 psf, for preliminary estimates.

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