Civil Engineering Reference
In-Depth Information
v)
Calculate the top and bottom chord forces assuming that the bars
are at yield at points of maximum moment and reduce the chord
force by the horizontal component of each strut at each stirrup,
vi)
Use any chord tension force remaining to equilibrate a direct
concrete compression strut.
vii)
Check that the truss as drawn in step 2 is still appropriate and revise
if necessary. Occasionally two or three iterations may be required,
viii)
The shear capacity is the sum of the vertical components of each
strut which comes down at the support in question.
This detailed procedure is illustrated in
Figure 4.8.
In steps v) and vi), the
addition of stirrups reduces the load which can be supported by the direct
compression strut. With sufficient stirrups, the direct compression strut will
not form. In deep beams, most of the load is supported by the direct
compression strut hence additional stirrups are not entirely effective in
supporting additional load until the direct compression strut is eliminated.
4.5
Design of continuous deep beams
Loading and support conditions are perhaps the most important
considerations in the design of continuous deep beams. Continuous deep
beams are very sensitive to support movements and without heavy stirrup
reinforcement, they may not be ductile enough to permit a design for one set
of moments and support reactions. The designer should select reinforcement
which can accommodate all reasonable distributions of moments and
support reactions. The distributions will depend on the specific application,
but consideration should be given to foundation settlement, column
shortening, and so on. For ideal support conditions at least the following two
distributions should be considered:
i)
A distribution based on an elastic analysis that includes support
settlements but ignores shear deformation effects (e.g. moment
distribution)
ii)
A distribution in which the negative moments from the first
distribution are reduced by 40% and the remaining positive moments
adjusted accordingly (this comes from experimental observations).
To ensure some ductility and reduce variability in behaviour, a well
distributed minimum reinforcement should be provided. This minimum
reinforcement should be at least twice as great as current minimum
horizontal and vertical web reinforcement. Reinforcement ratios of at least
0.003 in the horizontal and vertical directions would be appropriate. In the
case of stirrups, they should be increased if required to support at least 30%
of the direct compression strut capacity.
