Building Design and Construction Handbook - Merritt - Ventech!

9.100 SECTION NINE
9.66 REINFORCEMENT FOR TORSION
AND SHEAR
Any beam that supports unbalanced loads that are transverse to the direction in
which it is subjected to bending moments transmits an unbalanced moment to the
supports and must be investigated for torsion. Generally, this requirement affects
all spandrel and other edge beams, and interior beams supporting uneven spans or
unbalanced live loads on opposite sides. The total unbalanced moment from a floor
system with one-way slabs in one direction and beams in the perpendicular direction
can often be considered to be transferred to the columns by beam flexure in one
direction, neglecting torsion in the slab. The total unbalanced moment in the other
direction, from the one-way slabs, can be considered to be transferred by torsional
shear from the beams to the columns.
Under the ACI 318 Building Code, factored torsional moment, T u, is resisted by
reinforcement (Art. 9.48). No torsion is assumed to be resisted by concrete. When
T u exceeds the value computed by Eq. (9.43) for non-prestressed members, the
effects of torsion must be considered.
The required area A t of each leg of a closed stirrup for torsion should be computed
from Eq. (9.48). Stirrup spacing should not exceed p h/8 or 12 in, where p h
is the perimeter of the centerline of the outermost closed stirrup.
Torsion reinforcement also includes the longitudinal bars shown in each corner
of the closed stirrups in Fig. 9.16 and the longitudinal bars spaced elsewhere inside
the perimeter of the closed stirrups at not more than 12 in. At least one longitudinal
bar in each corner is required. [For required areas of these bars, see Eqs. (9.50)
and (9.52).] If a beam is fully loaded for maximum flexure and torsion simultaneously,
as in a spandrel beam, the area of torsion-resisting longitudinal bars A t
should be provided in addition to flexural bars.
For interior beams, maximum torsion usually occurs with live load only on a
slab on one side of the beam. Maximum torsion and maximum flexure cannot occur
simultaneously. Hence, the same bars can serve for both.
The closed stirrups required for torsion should be provided in addition to the
stirrups required for shear, which may be the open type. Because the size of stirrups
must be at least No. 3 and maximum spacings are established in the ACI 318
Building Code for both shear and torsion stirrups, a closed-stirrup size-spacing
combination can usually be selected for combined shear and torsion. Where maximum
shear and torsion cannot occur under the same loading, the closed stirrups
can be proportioned for the maximum combination of forces or the maximum single
force; whichever is larger.
9.67 CRACK CONTROL IN BEAMS
The ACI 318 Building Code contains requirements limiting flexural reinforcement
spacing to regulate crack widths when the yield strength ƒ y of the reinforcement
exceeds 40,000 psi (Art. 9.50). Crack width is proportional to steel stress. The
tensile area of concrete tributary to and concentric with each bar, and thickness of
the concrete cover are important to crack control. The minimum concrete cover
requirements of the ACI 318 Building Code for reinforcement in beams and girders
are given in Table 9.22.