Civil Engineering Reference
In-Depth Information
a
total axial force ratio,
N/(f
cu
bh)
(Eqns 10.3 and 10.19)
e
Ó
c
concrete strain at the extreme
fibre of the least highly com-
pressed face for an uncracked
section
a
c
concrete axial force ratio,
N/(f
cu
bh)
a
max
total axial force ratio at ulti-
mate condition
e
cu
ultimate concrete strain
e
si
steel strain of ith layer of re-
inforcement
a
unity
total axial force ratio at
e
c
/
e
cu
=1 and
x/h
=1
e
yi
steel yield strain of ith layer
of reinforcement
¦
resistance moment ratio,
M/(f
cu
bh
2
)
[
e
c
/
e
cu
]
min
minimum concrete strain ratio
below which Eqn 10.23 is not
solvable
¦
t
external moment ratio,
M
t
/(f
cu
bh
2
)
e
concrete strain
[
e
c
/
e
cu
]
x/h
=1
concrete strain ratio at
x/h
=1
e
c
concrete strain at the extreme
fibre of the more highly com-
pressed face
r
i
steel ratio of
i
th layer of re-
inforcement,
A
si
/bh
t
shear stress
10.1
Introduction
In the past three decades, much of the research on the ultimate load
behaviour of reinforced concrete beams has been concentrated on their
bearing, flexural and shear strengths (Albritton, 1965; C & CA, 1969;
CIRIA, 1977, Kong, 1986a). At a recent lecture given at Ove Arup and
Partners, (Kong, 1986b; Whittle, 1986), it became clear that deep beam
buckling is a failure criterion that needs to be considered in design.
Indeed, with the expected advances in materials technology (ACI
Committee 363, 1984; Clarke and Pomeroy, 1985; Kong
et al,
1983)
deep beam designers will find it possible to use much smaller cross
sections in the future. This would clearly allow more slender deep
beams. As with other thin- walled and slender members such as thin
plates and slender columns, stability rather than strength requirements
will probably dictate the design of slender deep beams. Of the four main
deep beam design documents, namely, the Canadian Building Code
CAN3-A23.3-M84 (CSA, 1984), the American Building Code ACI 318Î
83 (1983), the CEB-FIP Model Code (1978) and the CIRIA Guide No. 2
(1977), the only one that gives direct recommendations on the buckling
strength of concrete deep beams is the CIRIA Guide. However, because
of the lack of experimental data, the CIRIAÓs buckling recommendations
had to be based on theoretical studies and engineering judgement; at the
end of the CIRIA GuideÓs Appendix C:
Buckling strength of deep beams,
it is pointed out that Òthere is no experimental evidence to substantiate
these proceduresÓ (CIRIA, 1977).
This chapter explains the behaviour of slender concrete deep beams and
presents recent test results which show that the CIRIA Guide (1977)
