(2) The concrete between the flexural reinforcement is capable of
resisting direct shear stress. The concrete remains effective because these
elements are subjected to comparatively low blast loads and are designed to
attain small support rotations. The magnitude of the ultimate direct shear
force, Vd which can be resisted by a beam is limited to:
Vd = 0.18 f'dc bd
The total support shear produced by the applied loading may not exceed Vd.
Should the support shear exceed Vd, the depth or width of the beam or both
must be increased since the use of diagonal bars is not recommended. Unlike
slabs which require minimum diagonal bars, beams do not require these bars
since the quantity of flexural reinforcement in beams is much greater than
(1) In addition to the flexural effects considered above, concrete
beams may be subjected to torsional moments. Torsion rarely occurs alone in
reinforced concrete beams. It is present more often in combination with
transverse shear and bending. Torsion may be a primary influence but more
frequently it is a secondary effect. If neglected, torsional stresses can
cause distress or failure.
(2) Torsion is encountered in beams that are unsymmetrically loaded.
Beams are subject to twist if the slabs on each side are not the same span
or if they have different loads. Severe torsion will result on beams that
are essentially loaded from one side. This condition exists for beams
around an opening in a roof slab and for pilasters around a door opening.
(3) The design for torsion presented in this section is limited to
rectangular sections. For a beam-slab system subjected to conventional
loading conditions, a portion of the slab will assist the beam in resisting
torsional moments. However, in blast resistant design, a plastic hinge is
usually formed in the slab at the beam and, consequently, the slab is not
effective in resisting torsional moments.
(4) The nominal torsional stress in a rectangular beam in the
vertical direction (along h) is given by:
and the nominal torsional stress in the horizontal direction (along b) is