Civil Engineering Reference
In-Depth Information
without vertical reinforcement, sustained 130 kN before it cracked
horizontally, forming a secondary beam at the lower level of the wall, whose
flexural rigidity continued to carry load. A small increase in the cracking
load can be observed in specimen W2 with 0.8% of vertical reinforcement.
For the range of vertical reinforcement from 0.8 to 2.0%, the load at first
horizontal crack was virtually constant. The cracking load noted for
specimen W3 is inconsistent with other values and is thought to be due to a
delay in detecting the initial crack.
Table 6.4
Effect of vertical reinforcement on cracking load and comparison with CIRIA ultimate
load prediction for wall-beams loaded at the soffit only (loading L2)
6.10
Combined top and bottom loading
When combined top and bottom loading is present the CIRIA Guide states that
Eqn (6.4) should be applied. The equation controls the permissible amounts of
top and bottom load for a given deep beam and clearly is of interest here. In
Figure 6.13
the ultimate test loads are compared with the CIRIA values; with
the exception of the bottom loaded wall-beams, all tests show an ultimate load
of at least twice the design ultimate value. Bearing in mind that in tests the
ultimate loads in shear of the stronger wall-beams were limited by local
crushing failures (
Table 6.2)
,
the CIRIA values are obviously quite
conservative for wall-beams with a high proportion of top loading.
Adopting the CEB criteria of a 0.1 mm crack width as a serviceability
limit state,
Figure 6.14
compares the corresponding loads in tests of wall-
beams W1, W2 and W3 and the CIRIA ultimate load values calculated from
Eqns (6.1) and (6.2). All test loads corresponding to a maximum crack width
of 0.1 mm are in excess of the CIRIA predictions of ultimate load. The load
factor is enhanced as the proportion of top-loading increases above 50% and
also with the percentage of vertical reinforcement.
If Schütt's equation (Eqn (6.4)) is applied to these beams all of the results
for ultimate load fall within the range 85-112 kN and although individual
values are influenced by the ratio of top/bottom loading, the effects are
small. Moreover the volume of shear reinforcement is not taken into account
and, in comparison with test results, values are ultra-conservative. Use of
this equation is not recommended.
