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)

224 Shear, bond and torsion
stresses. In the first and second editions (1975 and-1980 respectively) ofthis
book, the authors wrote: 'Provided the anchorage length is sufficient, the
local bond stress does not seem to have much significance.... It is
desirable that (the requirement to check) local bond stresses will be
dropped in future revisions of CP 110.'
6.7 Equilibrium torsion and compatibility torsion
BS 8110 implicitly differentiates between two types of torsion: equilibrium
torsion (or primary torsion), which is requi.red to maintain equilibrium in
the structure, and compatibility torsion (or secondary torsion), which is
required to maintain compatibility between members of the structure. To
distinguish between the two types, it is helpful to note that (a) in a statically
determinate structure, only equilibrium torsion can exist; (b) in an
indeterminate structure both types may exist, but if the torsion can be
eliminated by releasing redundant restraints then it is a compatibility
torsion.
In general, where the torsional resistance or stiffness of members has
not been explicitly taken into account in the analysis of the structure,
no specific ca1culations for torsion will be necesary. Thus, BS 8110:
Part 2: Clause 2.4.1 states that: 'In normal slab-and-beam or framed
construction specific ca1culations are not usually necessary, torsional
cracking being adequately controlled by shear reinforcement.' In other
words, compatibility torsion may, at the discretion of the designer, be
ignored in the design ca1culations; however, equilibrium torsion must be
designed for.
6.8 Torsion in plain concrete beams
It is only recently that a substantial amount of experimental data has
enabled engineers to obtain a reasonable working knowledge of torsion in
structural concrete [33-37].
Figure 6.8-1 shows a plain concrete beam subjected to pure torsion. The
torsional moment T induces shear stresses which produce principal tensile
stresses at 45° to the longitudinal axis. When the maximum tensile stress
reaches the tensile strength of the concrete, diagonal cracks form which
tend to spiral round the beam [38). For a plain concrete beam, failure
immediately follows such diagonal cracking.
Fig. 6.8-1 Diagonal cracking due to torsion [38]