Civil Engineering Reference
In-Depth Information
10.3.3.1 Comments on the Single-Panel Method
The Single-Panel Method is rather convenient to use, but may be too
conservative where the actual stresses vary abruptly, e.g. when concentrated
loads or reactions are applied. This is because the Single-Panel Method may
require an unnecessarily large amount of reinforcement in areas of low stresses.
In these cases, the CIRIA Guide recommends the use of the two-panel method.
10.3.4
CIRIA Guide Appendix C: Two-Panel Method
The Two-Panel Method is the second method in Appendix C of the CIRIA
Guide (1977). The Two-Panel Method is rather similar to the Single-Panel
Method, except that the former analyses and designs a braced panel as two
individual panels. The design procedure for the Two-Panel Method may be
outlined as follows.
Steps 1
and 2:
As Steps 1 and 2 of the Single-Panel Method.
Step 3:
Determination of equivalent applied stresses.
In the Two-Panel Method, the equivalent loads adopted to select the
effective height and the effective length differ in the two panels. For
Panel 1, the equivalent load consists of an upper-bound horizontal
stress, a lower-bound vertical stress and a constant shear stress. For
Panel 2, the equivalent load consists of a lower-bound horizontal
stress, an upper-bound vertical stress and a constant shear stress.
Some recommendations are given in the CIRIA Guide (1977) on the
choice of the lower-bound and upper-bound equivalent stresses.
Step 4:
The critical stresses are determined for the two panels as in Step 4
of the Single-Panel Method.
Step 5:
Following Step 5 of the Single-Panel Method, the effective height
h
e
is calculated for Panel 1 and the effective length
L
e
is determined
for Panel 2.
Step 6:
The vertical and horizontal strips of unit width are designed as
slender columns (Wong and Kong, 1986; Kong and Wong, 1988)
using the
actual
axial stress distributions.
10.4
The equivalent-column method
The CIRIA GuideÓs methods for the buckling design of slender concrete
deep beam consist essentially of replacing the deep beam by equivalent
panels. CIRIA then uses (previously CP110) the slender column approach of
BS 8110 for assessing the strength of the equivalent panels as slender
columns. A close examination (Cranston, 1972; Kong
et al.,
1986b; Kong
and Wong, 1987; Wong 1987a) of the relevant BS 8110 and CP 110 Clauses
shows that these are really intended for the material failure of slender
columns and not for their instability failure. It will be shown (Section 10.6)
