Strength and behaviour of deep beams - Reinforced Concrete Deep Beams - page 45

than the size of the internal horizontal actions ( Figure 2.20) . Such behaviour
has been found to result from the fact that the force sustained by the tension
reinforcement of a deep beam at its ultimate limit state is constant
throughout the beam span (Rawdon de Paiva and Siess, 1965).
It may be deduced therefore that if an RC deep beam at its ultimate limit
state cannot rely on beam action to sustain the shear forces, it would have to
behave as a tied arch. However, the word 'arch' is used in a broad context; it
is considered to describe any type of frame-like structure that would have a
shape similar to that of the compressive force path shown in Figure 2.16. It
appears, therefore, that the concepts of tied arch action and compressive
force path are compatible in the sense that while the former identifies the
internal actions providing ultimate resistance to the structure, the latter
provides a qualitative description of the causes of structural failure.
2.5.3 Effect of transverse reinforcement
2.5.3.1 Type III behaviour As discussed in Section 2.5.1.1, for type III
behaviour failure is associated with a large reduction of the size of the
compressive zone of the cross-section coinciding with the tip of the main
inclined crack. Such a reduction in size will lead to the development of
tensile stresses within the compressive zone for the reasons described in
item ii) of Section 2.4. Failure, therefore, will occur when the strength of
concrete under the combined action of compressive and tensile stresses is
exceeded. This type of failure may be prevented either by providing
transverse reinforcement that would sustain the tensile stresses that cannot
be sustained by concrete alone, or by reducing the compressive stresses.
The effectiveness of transverse reinforcement in sustaining the tensile
stresses that develop within the compressive zone is indicated by the fact
that such reinforcement prevented the extension of the inclined crack into
the compressive zone of beam D in Figure 2.8a and allowed the beam to
attain its flexural capacity ( Figure 2.9a) . However, the amount of
reinforcement required to sustain the tensile stresses is difficult to assess,
because the tensile stresses are difficult to calculate. Figure 2.9a also
indicates that provision of transverse reinforcement only within the shear
span can be equally effective (beam C in Figure 2.8a). Such reinforcement
reduces the compressive stresses that develop in the cross-section which
coincides with the tip of the inclined crack, as it sustains a portion of the
bending moment developing in that section ( Figure 2.21) . A method for
designing such reinforcement is discussed in Section 2.6.2.
However, the presence of transverse reinforcement beyond the critical
section is essential, as it has been shown experimentally that reinforcing
with stirrups only to the critical section does not safeguard against brittle
failure (Kotsovos, 1987a). This is because the inclined crack is likely to
extend deeply into the compressive zone and, although the presence of such
reinforcement within this region inhibits crack opening (and therefore may