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)

202 Shear, bond and torsion
face of the beam. As the force V is increased, the diagonal crack
would propagate simultaneously towards the loading and support
points. When the crack has penetrated sufficiently deeply into the
concrete zone at the loading point, or, more frequently, at the
support point, crushing failure of the concrete occurs. For a deep
beam failure mode, the ultimate load is of ten several times that at
diagonal cracking.
The mechanisms of shear transfer in a cracked concrete beam are
illustrated in the free-body diagram in Fig. 6.2-5. The shear force V is
resisted by the combined act ion of the shear V cz in the uncracked concrete
compression zone, the shear V d from the dowel action of the longitudinal
reinforcement, and the shear Va which is the vertical component of the
force due to aggregate interlock (sometimes called the interface shear
transfer). Thus
(6.2-2)
Quantitative evidence [10] is now available that for a typical reinforced
concrete beam the shear force Vis carried in the approximate proportions:
compression zone shear V cz = 20-40%
dowel action V d = 15-25%
aggregate interlock Va = 35-50%
According to Taylor [10], as the applied shear force is increased, the dowel
action is the first to reach its capacity, after which a proportionally large
shear is transferred to aggregate interlock. The aggregate interlock
mechanism is probably the next to fail, necessitating a rapid transfer of a
large shear force to the concrete compression zone, which, as a result of
this sudden shear transfer, of ten fails abruptly and explosively. The above
description suggests that the shear failure of a reinforced concrete beam
is affected by a number of shear parameters besides the avf d ratio discussed
earlier. The effects of the main parameters may be summarized as folIows:
(a)
concrete strength. The dowel-action capacity, the aggregate-interlock
capacity and the compression-zone capacity generalIy alI increase
with the concrete strength. Figure 6.2-6 shows the effect of concrete
strength (and the steel ratio) on the nominal ultimate shear stress
Diagonal
crack
I _Concrete
, compression
Vcz
--+
Steel tension
v
Fig. 6.2-5