Presented at 4th International
Symposium on Robotics and Artificial Intelligence in Building Construction, Haifa, Israel,
June 1987.
Chris J. Burgoyne
Dept of Civil Engineering, Imperial College of Science
and Technology, London, UK
The design of prestressed concrete
beams is arguably one of the most complex tasks facing the structural engineer.
With most materials, the designer has to choose the shape of the cross section;
in many cases this means the choice of web and flange thicknesses for one of a
restricted range of standard cross-section shapes.
In prestressed concrete, there are
more unknowns. Not only is the shape variable, but the amount and position of
the prestress can be varied to suit the particular application. Even the
selection of the section shape is more complex, since the use of concrete
allows the use of non-prismatic sections, with tapered flanges and webs, more
easily than with steel sections.
If the structure is statically
indeterminate, the designer must consider the effects of parasitic (or secondary)
moments set up by the action of the prestressing force. The shape of the cable
profile along the whole length of the beam (which controls the parasitic
moments) must be taken into account when designing each cross-section.
In the hands of an expert designer,
these complexities can appear no more than minor irritations, and indeed, can
be turned to advantage. For example, the double-T section is widely used by one
UK
consultant for continuous road bridges. Similar sections are frequently used
for simply supported floor beams, since they have a large top flange, which
gives adequate compressive strength in sagging bending, when the prestressing
tendons can be placed at the bottom of the webs.
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