Civil Engineering Reference
In-Depth Information
5.3.2
Mode of failure 2: flexural-shear-compression failure
The second mode, flexural-shear-compression failure is similar to mode 1
failure except that crushing of the compression flange will occur before the
full tensile capacity is realised. This will happen when the compression
capacity is less than the tension capacity (i.e. the beam is over-reinforced
flexually). This type of failure occurs when: i) the horizontal component of
the diagonal splitting force is (a) greater than the capacity of the tensile
reinforcement (b) greater than the compression capacity; ii) the capacity of
tensile reinforcement is greater than the compression capacity.
5.3.3
Mode of failure 3: diagonal splitting failure
Diagonal splitting failure, the third mode of failure, occurs by diagonal
splitting and excessive cracks in web in beams with thin webs and a
moderate amount of reinforcement in the top and bottom flanges.
When these beams are loaded cracking may develop on the bottom flange
at or near the mid span of the beam. These flexure cracks do not substantially
grow during the subsequent increment of loading. Additional increments will
increase the shear in the web until the limiting tensile strength is reached.
When this happens, the diagonal splitting will occur.
The splitting is generally located at mid-depth of the beam as shown in
Figure 5.5b.
Additional increments of load result in more cracks of this type.
Failure occurs when the first crack at the centre of the web grows sufficiently
large. The beam may be termed 'unserviceable' in this state. At the last stages
of web splitting, concrete in the web near the load or the support may spall
and crush if further load is applied. The crack will penetrate into either the
tension flange near the support or the compression flange near the edge of
load and lead to the crushing of the compression zone.
This type of failure mode will occur when: the horizontal component of
the diagonal splitting force is; i) less than the capacity of the tensile
reinforcement; ii) less than the compression capacity. If the compression
capacity is greater than the capacity of tensile reinforcement, the diagonal
crack may penetrate into the tensile flange.
5.3.4
Mode of failure 4: splitting with compression failure
The fourth mode of failure splitting with compression is similar to mode 3
except that the diagonal crack may eventually penetrate into the compression
flange. In other words, crushing of the compression flange will occur.
This type of failure will occur when: i) the horizontal component of
diagonal splitting force is; a) less than the capacity of tensile reinforcement, b)
less than the compression capacity; ii) the capacity of tensile reinforcement is
greater than the compression capacity.
The modes of failure are also summarised in
Table 5.2.
In the Table it is
assumed that the amount of horizontal web reinforcement is small and that:
