Design of Rectangular Beams and One-Way Slabs - Design of Reinforced Concrete - page 101

Placing concrete slab. (Courtesy of Bethlehem Steel Corporation.)

4.8 CANTILEVER BEAMS AND CONTINUOUS BEAMS

Cantilever beams supporting gravity loads are subject to negative moments throughout
their lengths. As a result, their reinforcement is placed in their top or tensile sides, as
shown in Figure 4.9. The reader will note that for such members the maximum moments
occur at the faces of the fixed supports. As a result, the largest amounts of reinforcing are
required at those points. You should also note that the bars cannot be stopped at the sup-
port faces. They must be extended or anchored in the concrete beyond the support face.
We will later call this development length. The development length does not have to be
straight as shown in the figure, because the bars may be hooked at 90 .

Up to this point, only statically determinate members have been considered here. The
very common situation, however, is for beams and slabs to be continuous over several
supports as shown in Figure 4.10. Because reinforcing is needed on the tensile sides of the
beams, we will place it in the bottoms when we have positive moments and in the top
when we have negative moments. There are several ways in which the reinforcing bars
can be arranged to resist the positive and negative moments in continuous members. One
possible arrangement is shown in Figure 4.10(a). These members, including bar arrange-
ments, are discussed in detail in Chapter 14.

or 180

Figure 4.9 Cantilever beam.