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)

238 Shear, bond and tors ion
longitudinal bars. The validity of eqn (6.9-3) depends on eqn (6.9-1)
being satisfied, in which case the longitudinal steel As and the links Asv
would yield simultaneously. BS 8110's equation (6.11-6) permits As
to exceed that required by eqn (6.9-1) and hence permits the links to
yield first.
The reader should refer to Fig. 6.1O-1(b) and note the implication of the
BS 8110 procedure according to which the longitudinal bars for torsion and
for bending are calculated separately without considering interaction
effects, and then added together. Similarly, according to BS 8110, Iinks
for torsion and for shear are also calculated separately and added together.
Comments on Step 8
The important subject of detailing for torsion is discussed in an interesting
paper by Mitchell and Collins (48).
Example 6.11-1
Explain, with reference to the concept of the shear centre, how to
determine the design torsional moment for the section in Fig. 6.11-3.
SOLUTION
For the general case of a beam of arbitrary section, the torsional moment is
obtained by taking moments, not about the centroid of the section, but
about its shear centre. Only T-, L- and I-sections are explicitly referred to
in BS 8110; for such sections it would seem adequate to take the shear
centre as Iying on the centre line of the web. For other asymmetrical
sections, reference may be made to handbooks such as R. J. Roark and W.
C. Young's Formulas for Stress and Strain (McGraw-HiII). However, it
should not be forgotten that BS 8110's recommendations are essentially
empirical, and it may not be wise to apply them to beams of unusual
sections which are not adequately covered by the tests upon which such
recommendations are based. Instead of meticulously locating the shear
centre by elastic analysis (which may in any case be incompatible with the
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web
I
Fig. 6.11-3