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What are the factors that
affect the cost of precast
pre-tensioned bridge beams, and how
easy is it to design according to an
optimised cost criteria? Two examples
are presented here which demonstrate
that the answer is not always what it
may seem.
Example 1 - Which of the
following two beams costs
less?
The natural instinct of most bridge
engineers would be to design for the
minimum beam size. Both beams were
designed for a typical 20m span bridge.
The smaller beam has 29 tendons, and
the larger beam has 25 tendons with an
additional 0.715m3 of concrete. With no
other considerations such as overall
depth constraints, the larger beam is
likely to be more cost effective.
Example 2 - Which of the
following 2 beams costs less, the
larger beam in example 1, or a similar
beam shown next?
 Again both beams are
designed for the same 20m span. They
are the same size, but the first has
25 tendons, and the second has 27
tendons. The second is clearly more
expensive. The difference between
the two beams however is that the
first requires a concrete strength
at transfer that is 10N/mm2 higher
than the second. The two additional
tendons could therefore enable a
much lower occupancy time of the
precasting bed, and consequent
improved efficiency in manufacture.
The tendon optimisation algorithm in
Sam was used to generate and
investigate both these examples in less
than an hour. Such is the efficiency
that can be achieved with the new and
highly sophisticated optimised tendon
tool.
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