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)

220 Shear, bond and tors ion
diagonal crack, which is represented by the dotted line in
Fig. 6.5-1;
a = the angle (Fig. 6.5-1) between the bar being considered and
the diagonal crack (n12 > a > O);
n = the total number of web bars, including the main longitudinal
bars, that intercept the critical diagonal crack;
b = the beam width; and
av and h are as explained in Fig. 6.5-1.
As regards flexure, the design bending moment M should not exceed
fy
fy
M = 0.6As-h or 0.6As-I (6.5-2)
Ym
Ym
whichever is less, where As is the tension steel area. Equation (6.5-2)
assumes that the lever arm is 0.6h for l/h > 1 or 0.61 for l/h < 1. This is
conservative because in deep beams the lever arm is unlikely to fali below
0.7h or 0.71 [18]. It should be noted that ali the main longitudinal bars
provided in accordance with eqn (6.5-2) also act as web bars; that is, the
laws of equilibrium are unaware of the designer's disctimination between
bars labelled as 'flexural reinforcement' and bars labelled as 'shear
reinforcement' .
The design method can be extended to deep beams with openings if the
lever arm, 0.6h in eqn (6.5-2), is replaced by 'lkzh, and the quantities a v
and h in the first term of eqn (6.5-1) are replaced by kJa v and kzh
respectively, where k1av and kzh are as defined in Fig. 6.5-1(b).
The above design method is based on a structural idealization explained
in detail in Reference 16, which also gives a list of design hints. A worked
example, in which the calculated collapse load for a large beam is
compared with the actualload observed in a test to destruction, is given in
Reference 17. Design examples illustrating the use of this method, both for
deep beams with and without openings, are given in References 14 and 18,
as well as in Reynolds and Steedman's Reinforced Concrete Designer's
Handbook [13].
Readers interested in the practic al design of deep beams should consult
CIRIA's Deep Beam Design Guide [12], which also gives comprehensive
recommendations regarding the buckling and instability [15] of slender
deep beams.
6.6 Bond and anchorage (BS 8110)
Bond stress [29-32] is the shear stress acting parallel to the reinforcement
bar on the interface between the bar and the concrete. Bond stress is
directly related to the change of stress in the reinforcement bar; there can
be no bond stress unless the bar stress changes and there can be no change
in bar stress without bond stress. Where an effective bond exists, the strain
in the reinforcement may for design purposes be assumed to be equal to
that in the adjacent concrete. Effective bond exists if the relevant
requirements in the code of practice are met; in BS 8110, these requirements
are expressed in terms of certain nominal stresses, as we shall see