Civil Engineering Reference
In-Depth Information
flexural capacity is assessed on the basis of the plane sections theory which
not only is generally considered to describe realistically the deformational
response of the beams, but is also formulated so that it provides a design
tool noted for both its effectiveness and simplicity.
However, an RC beam may exhibit a number of different types of failure
that may occur before flexural capacity is attained. The most common of
such failures are those which may collectively be referred to as shear types
of failure and may be prevented by complementing the initial (flexural)
design so that the shear capacity of the beam is not exhausted before the
flexural capacity is attained, while other types of failure such as, for
example, an anchorage failure or a bearing failure (occurring in regions
acted upon by concentrated loads), are usually prevented by proper
detailing.
Although a generally accepted theory describing the causes of shear
failure is currently lacking, there are a number of concepts which not only
are widely considered as an essential part of such a theory, but also form the
basis of current design methods for shear design. These concepts are the
following:
i)
shear failure occurs when the shear capacity of a critical cross-
section is exceeded
ii)
the main contributor to shear resistance is the portion of the cross-
section below the neutral axis, with strength, in the absence of shear
reinforcement, being provided by “aggregate interlock” and “dowel
action”, whereas for a beam with shear reinforcement the shear
forces are sustained as described in iii) below
iii)
once inclined cracking occurs, an RC beam with shear
reinforcement behaves as a truss with concrete between two
consecutive inclined cracks and shear reinforcement acting as the
struts and ties of the truss, respectively, and the compressive zone
and tension reinforcement representing the horizontal members.
A common feature of both the above concepts and the plane section theory
that form the basis of flexural design is that they rely entirely on uniaxial
stress-strain characteristics for the description of the behaviour of concrete.
This view may be justified by the fact that beams are designed to carry
stresses mainly in the longitudinal direction, with the stresses developing in
at least one of the transverse directions being small enough to be assumed
negligible for any practical purpose. As will be seen, however, such a
reasoning underestimates the considerable effect that small stresses have on
the load-carrying capacity and deformational response of concrete. Ignoring
the small stresses in design does not necessarily mean that their effect on
structural behaviour is also ignored. It usually means that their effect is
attributed to other causes that are expressed in the form of various design
assumptions.
