Civil Engineering Reference
In-Depth Information
may be as high as 60, so flanged beams are considerably more
slender than conventional deep beams.
Table 5.1
lists the
h/t
ratio of
the beams used by the different investigators.
ii)
Accommodation of main tensile reinforcement
In conventional deep beams, the main reinforcement is accommodated
within the plane of the web. Flanged deep beams have wider flanges to
accommodate them.
The usual mode of failure of slender reinforced concrete flanged beams
involves the diagonal splitting of the web between the edge of the loading
plate and the support. A segment of the web between the load and the
support is subjected to a stress field equivalent to pure shear, as shown in
Figure 5.4. This produces diagonal tension and compression in the web.
When the principal tensile stress due to shear reaches the limiting tensile
strength of the concrete, rupture occurs in the web. The limiting tensile
strength of concrete could be defined as the maximum tensile in a biaxial
compression-tension field. The total splitting force in the web consists of a
contribution from the web concrete and the web reinforcement at compatible
strain. Four parameters affect the modes of failure of flanged deep beams:
the strength of concrete
f'
c
the amount of tensile and compressive
reinforcement, the amount of web reinforcements, and the geometry of
beam. Based on the relative values and amounts of various parameters, the
modes of failure could be classified into four types.
Figure 5.4
Biaxial state of stress due to a pure shear
5.3.1
Mode of failure 1: flexural-shear failure
The first mode, flexural-shear failure, occurs in a beam with a very small
amount of main tensile reinforcement. At first, flexural cracks develop on
the bottom flange at or near the midspan of the beam. (
Figure 5.5a)
As load
is increased, more flexural cracks follow accompanied by diagonal web
cracking. Finally failure occurs due to the diagonal cracks in the web and the
flexural cracks in the bottom flange near the support. The main
