Civil Engineering Reference
In-Depth Information
Figure 8.4
Truss analogy.
in which shear forces are transmitted. For example, the web reinforcing does increase the
shearing strength of a beam, but it has little to do with shear transfer in a beam before in-
clined cracks form.
The Code requires web reinforcement for all major beams. In Section 11.5.5.1, a
minimum area of web reinforcing is required for all concrete flexural members except
slabs and footings, certain concrete floor joists, and in wide shallow beams whose total
depths are not more than 10 in. or more than times their flange thicknesses or one-half
their web widths, whichever is greatest. Various tests have shown that shear failures do
not occur before bending failures in shallow members. Shear forces are distributed across
these wide sections. For joists the redistribution is via the slabs to adjacent joists.
Inclined or diagonal stirrups lined up approximately with the principal stress directions
are more efficient in carrying the shears and preventing or delaying the formation of diago-
nal cracks. Such stirrups, however, are not usually considered to be very practical in the
United States because of the high labor costs required for positioning them. Actually, they
can be rather practical for precast concrete beams where the bars and stirrups are preassem-
bled into cages before being used and where the same beams are duplicated many times.
Bent-up bars (usually at 45
2
2
angles) are another satisfactory type of web reinforcing
(see Figure 8.5). Although bent-up bars are commonly used in flexural members in the
United States, the average designer seldom considers the fact that they can resist diagonal
tension. Two reasons for not counting their contribution to diagonal tension resistance are
Figure 8.5
Bent-up bar web reinforcing.
