Civil Engineering Reference
In-Depth Information
2
Strength and behaviour of deep
beams
M.D.KOTSOVOS, Imperial College, London
Notation
f
c
cylinder compressive strength of concrete
a
shear span
L
effective length of beam
d
distance of centroid of tension reinforcement from extreme compressive fibre
M
f
section flexural capacity
M
c
maximum moment sustained by cross-section through tip of inclined crack
2.1
Introduction
While current design concepts are based on uniaxial stress-strain
characteristics, recent work has shown quite conclusively that the
ultimate limit-state behaviour of reinforced concrete (RC) elements
such as, for example, beams in flexure (or combined flexure and
shear), can only be explained in terms of multiaxial effects which are
always present in a structure. It is the consideration of the multiaxial
effects that has led to the introduction of the concept of the
compressive-force path which has been shown not only to provide a
realistic description of the causes of failure of structural concrete, but
also to form a suitable basis for the development of design models
capable of providing safe and efficient design solutions. In the
following, the work is summarised and the concept of the compressive-
force path is used as the basis for the description of the behaviour of
RC deep beams of their ultimate limit state. The implications of the
application of the concept in RC deep beam design are also discussed
and a simple design method is proposed.
2.2
Current concepts for beam design
It is a common design practice first to design an RC beam for flexural
capacity and then to ensure that any type of failure, other than flexural (that
would occur when the flexural capacity is attained), is prevented. The
