Civil Engineering Reference
In-Depth Information
1.4.2.3
Bearing strength
CIRIA's 'Supplementary Rules' allow the bearing
stress limit (=0.4
f
cu
in Section 1.4.1.4) to be increased to 0.6
f
cu
at the end
supports and to 0.8
f
cu
under concentrated loads, provided the concrete in the
stress zones is adequately confined, as specified in clause 3.4.3 of the CIRIA
Guide.
1.4.2.4
Comments on Eqn (1.11) of Section 1.4.2.2
i)
Eqn (1.11), taken from clause 3.4.2 of the CIRIA Guide (1977), is
essentially the Kong
et al.
(1972b and 1975) equation. CIRIA,
however, has modified the numerical values of the coefficients
l
1
and
l
2
to introduce the necessary factor of safety for the design
purpose.
ii)
According to the CIRIA Guide (1977), Eqns (1.11) and (1.12) apply
only over the range 0.23 to 0.70 for
x
e
/
h
a
. This is because the test
data then available (Kong
et al.,
1972b; 1975) were limited to this
range of
x
e
/
h
a
. However, as a result of more recent tests (Kong
et al.,
1986), the authors believe that Eqns (1.11) and (1.12) can be
applied to an extended range of
x
e
/
h
a
from 0 to 0.70.
iii)
On the right-hand side of Eqn (1.11), the quantity is a
measure of the load-carrying capacity of the concrete strut, along
the line Y-Y in
Figure 1.3.
From the figure, it is seen that the
capacity increases with the angle
in Eqn (1.11), the factor (1-
0.35
x
e
/
h
a
) allows for the experimental observation of the way in
which this capacity reduces with
f
, (i.e. with an increase in the
x
e
/
h
a
ratio). When the load carried by the concrete strut is high enough, a
splitting failure occurs, resulting in the formation of the diagonal
crack along Y-Y in Figure 1.3. In Eqn (1.11), the quantity is a
measure of the splitting strength of the concrete. After the formation
of the diagonal crack, the concrete strut becomes in effect two
eccentrically loaded struts. These eccentrically loaded struts are
restrained against in-plane bending by the web reinforcement.
f
iv)
On the right-hand side of Eqn (1.11), the second term represents the
contribution of the reinforcement to the shear strength of the beam. The
reinforcement helps the split concrete strut (iii)) to continue to carry
loads, by restraining the propagation and widening of the diagonal
crack. The beam has a tendency to fail in a mechanism in which the
end portion of the beam moves outwards in a rotational motion about
the loading point (Kong and Sharp, 1973). Thus, the lower down the
reinforcement bar intersects the the diagonal crack, the more effective it
would be in restraining this rotation. Hence in Eqn (1.11), the steel
