Civil Engineering Reference
In-Depth Information
to lengthen the spans, above all if this can lead to shorter columns, such as when
crossing deep V-shaped valleys.
Finally, the cost of a bridge will depend on how each contractor prices the various
components, which will be coloured by his past experience. Four contractors pricing
the same bridge are likely to show a range of at least 15 per cent in their conclusions.
Consequently, it is vain to attempt to be either too scientifi c or too dogmatic about
the most economical span for any particular situation. It is very much a matter of
experience and intuition, and once the choice is made, of designing as tightly as
possible. A well-designed bridge of the 'wrong' span is likely to be considerably more
economical than a poorly designed bridge of the 'correct' span.
In general, for a viaduct that is crossing reasonably level ground with unexceptional
foundation conditions, the most economical span length for a twin rib deck is likely
to fall between 25 m and 35 m. For a box section deck of constant depth it is likely
to lie between 30 m and 45 m, while if the deck is provided with a haunch, the most
economical span will lie between 40 m and 60 m.
