F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

234 Shear, bond and torsion
6.11 Torsional resistance in design calculations
(BS 8110)
The design procedure in BS 8110 is based on the principles explained in
the preceding sections; it may be summarized as follows (see also
comments at the end).
Step 1 The Torsional moment T
In a determinate structure, the torsional moment T due to ultimate
loads will be given directly by the equations of statics. In analysing an
indeterminate structure to determine T, the f1exural rigidity and the
torsional rigidity may be obtained as follows:
(a) Flexural rigidity EI: E may be taken as the appropriate value of Ee
in Table 2.5-6 and las the second moment of area of the nominal
cross-section.
(b) Torsional rigidity GC: G may be taken as 0.42E c and C as O.5K,
where K is the St Venant value in Table 6.8-1.
Step 2 Torsional shear stress VI
For a rectangular section, the torsional shear stress VI may be calculated
from the equation
2T
hmin[hmax -
VI = ~2~--~~----~
hmin/3]
where T = the torsional moment due to ultimate loads;
hmin = the smaller dimension of the section; and
hmax = the larger dimension of the section.
(6.11-1)
Then proceed to Step 5, unless the member consists of a f1anged section
(or a box section), in which case proceed to Step 3 (or Step 4).
Step 3 Flanged sections
T-, L- or I-sections may be treated by divid ing them into their
component rectangles. This should normally be done so as to maximize
the function 2: (h~inhmax) which will generally be achieved if the
widest rectangle is made as long as possible. The torsional shear stress
VI carried by each component rectangle may be calculated by treating
it as a rectangular section subjected to a torsional moment of
Ti = T h~i;hmax (6.11-2)
2: (hminhmax)
where Ti is the torsional moment for a typical component rectangle (see
also Step 8(f».
Step 4 Box sections
Box and hollow sections in which wall thicknesses exceed one-quarter of
the overall thickness of the member in the direction of measurement
may be treated as solid rectangular sections.
Step 5 The torsional shear stress VIu
In no case should the sum of the shear stresses resulting from shear
force and torsion exceed the value VIu from Table 6.11-1, nor in the case