(3) For the design of concrete beams subjected to exterior blast
loads, it is recommended that the ultimate resisting moment be computed
using Equation (34) even though a considerable amount of compression
reinforcement is required to resist rebound loads. It should be noted that
a large amount of compression steel that does not yield due to the linear
strain variation across the depth of the section, has a negligible effect on
the total capacity.
Minimum Flexural Reinforcement.
(1) To insure proper structural behavior under both conventional and
blast loadings, a minimum amount of flexural reinforcement is required. The
minimum reinforcement required for beams is somewhat greater than that
required for slabs since an overload load in a slab would be distributed
laterally and a sudden failure will be less likely. The minimum required
quantity of reinforcement is given by:
p = 200/fy
which, for 60,000 psi yield strength steel, is equal to a reinforcement
ratio of 0.0033. This minimum reinforcement ratio applies to the tension
steel at mid-span of simply supported beams and to the tension steel at the
supports and mid-span of fixed-end beams.
(2) Concrete beams with tension reinforcement only are not
permitted. Compression reinforcement, at least equal to one-third the
required tension reinforcement, must be provided. This reinforcement is
required to resist the ever present rebound forces. Depending upon the
magnitude of these rebound forces, the required compression reinforcement
may equal the tension reinforcement.
(1) The nominal shear stress, vu, as a measure of diagonal tension
is computed from:
vu = nominal shear stress, psi
Vu = total shear at critical section, lb
The critical section is taken at a distance, d, from the face of the support
for those members that cause compression in their supports. The shear at
sections between the face of the support and the section d therefrom need
not be considered critical. For those members that cause tension in their
supports, the critical section is at the face of the supports.
(2) The shear stress permitted on an unreinforced web of a beam
subjected to flexure only is limited to: