Civil Engineering Reference
In-Depth Information
Truss models should be used to determine the principal reinforcement
requirements. The reinforcement should be distributed and detailed
(anchored) in accordance with the model analysis.
Crack control under service loads is expected to be satisfied by the use of
greater than normal minimum reinforcement. For large long numbers
subjected to significant shrinkage, temperature variations and restraint, one
may wish to increase the horizontal and vertical web reinforcement ratios to
0.006. This represents three to four times normal minimum reinforcement.
As a practical matter, this additional reinforcement has little impact on total
project cost as it permits much longer concrete placements (in excess of 30
m) with fewer construction and control joints.
Service load deflection predictions may be based on an elastic analysis of the
truss model duly adjusted for creep and shrinkage. The deflections due to
deformation of the web members of the truss correspond to shear deformations,
while the chord deformations account for the flexural deformations. Appendix E
of CIRIA Guide 2 (1977) should be consulted for further details.
4.6
Design example
Consider the design of the transfer girder shown in Figure 4.7. It is
continuous at one end and supports a uniformly distributed load as well as a
major point load. The specified concrete strength is 35 MPa and the
specified steel yield strength is 400 MPa.
Figure 4.7
Loads and spans for design example
Use dead and live load factors of 1.25 and 1.5 respectively. Use an
effective concrete strength equal to 75% of the specified strength times a
concrete performance factor of 0.6. This produces a final effective
permissible concrete stress equal to 45% of the specified compression
strength. Limit strut angles to between 25 and 65° from the horizontal.
Select the overall size of the beam to give an ultimate shear between 0.5 and
0.67 times the square root of the specified concrete strength.
Normally, one would consider a series of loading cases and a series of
support settlement cases. For simplicity, only one load case will be considered.
