Civil Engineering Reference
In-Depth Information
Guide and the Equivalent-Column method of Section 10.4 this chapter. In
Table 10.2,
the predictions of the CIRIA GuideÓs Supplementary Rules
supersede those published earlier (Kong
et al.,
1986a; Kong and Wong,
1986) which were incorrect, as explained elsewhere (Kong
et al.,
1987;
Wong 1987a). The concrete stress-strain relationship used to calculate the
equivalent-column loads is that proposed by Desayi and Krishnan (1964).
With reference to Table 10.2, several observations can be made:
i)
All the three CIRIA methods were safe and conservative. When
used in conjunction with BS 8110, the mean factors of safety are:
R
SR
=35.63,
R
SP
=13.06 and
R
TP
=7.06. Therefore, the relative
conservatism of the CIRIA Guide method was in the descending
order: the supplementary rules, the single-panel method and the
two-panel method. As shown in Table 10.2, the supplementary rules
and the single-panel method were often too conservative,
particularly for the very slender beams of
h/b
ratio of 33 or more,
and could lead to factors of safety exceeding 60. The two-panel
method gave the most realistic results; the
R
TP
values ranged from
about 2 to 15, with many values in the region of 8.
ii)
A closer scrutiny of Table 10.2 shows that the conservatism of the
CIRIA methods increased sharply as the height/thickness ratio
h/b
increased, and decreased gradually as the load-eccentricity/
thickness ratio
e/b
increased.
iii)
Of the three methods given by CIRIA Guide, the supplementary
rules are the easiest to use, the single-panel method is more difficult
to use, and the two-panel method even more so. Table 10.2 shows
that the two-panel method gave the most realistic results, while the
supplementary rules gave the least realistic results. In practical
design, therefore, it is worthwhile to move straight to the two-panel
method, by-passing the supplementary rules and the single-panel
method. Even when the deep beam is such that the easier to use
supplementary rules are applicable, the supplementary rules should
be used merely as a preliminary check of the adequacy of the deep
beam against buckling failure.
iv)
The Òequivalent-columnÓ method generally gave comparatively
better predictions than those obtained by the CIRIA Guide.
v)
The last three columns of Table 10.2 show that Cases 2, 3 and 4
(mean
R
EC2
=2.65;
R
EC3
=3.03;
R
EC4
=2.02) lead to quite realistic results,
indicating that the equivalent-column approach is potentially a useful
tool for the buckling analysis and design of slender concrete deep
beams. It should be noted that Cases 3 and 4 of
Figure 10.18
suggests
that the buckling strengths of slender deep beams would increase
with the width
c
of the bearings, which is yet to be confirmed by tests
(Wong, 1987a). The Case 1 results (mean
R
EC1
=0.72) show that the
effective width
b
eff
in Figure 10.18a is too large, as expected.
