Civil Engineering Reference
In-Depth Information
If wire or bar tendons are used, these rates of prestressing steel should be factored
inversely with respect to their working stress. For twin rib type decks, statically
determinate decks and incrementally launched bridges built without intermediate
supports, add 15-20 per cent.
Clearly, for very slender decks or decks carrying heavy rail or other exceptional
loads, these rates should be increased, while for very deep or very lightly loaded decks
the fi gures will be over-conservative. A quick preliminary design, using the techniques
described in this topic, will allow the prestress quantity to be refi ned.
8.5 Choice of most economical span
When the span length is not dictated by the obstacles to be crossed, the choice of
the most economical span is not straightforward. Any analytical approach is likely to
discover that the graph of cost against span length does not show a distinct minimum.
Clearly one must consider the costs of both the deck and the substructure.
As described above and shown in Figure 8.8, the quantities of materials in a well-
designed deck are more or less constant up to a span of about 50 m. This refers to a
mean of all types of deck; if one were to study each deck type, more distinct trends
would appear. For instance, consider twin rib decks built span-by-span. On the one
hand, the average deck thickness, and hence the quantity and cost of materials in the
deck rises with increasing span. As the deck becomes heavier, the falsework necessary
for its construction will also become more costly as will the cost of the foundations
necessary to carry the self weight of the deck. On the other hand, as the span increases
and the number of spans reduces, the labour required to strip the shutter and advance
the falsework will diminish. For the British live loading code, which is dominated by
a single heavy vehicle, the fewer the foundations the less often this vehicle is carried,
and consequently the foundations required to carry the live loading become cheaper as
the spans increase. Putting all these trends together would probably show a minimum
cost lying somewhere at a span of about 25 m, although the curve of cost against span
is likely to be quite fl at. As the span increases beyond 40 m, the weight increase of the
deck will tend to dominate and costs will rise more steeply.
If one were to apply a similar analysis to decks made of precast Tee beams, the
determining factor is likely to be the increasing cost of moving and launching the
beams as they become longer and heavier, with costs rising steeply once they exceed
a span of about 45 m and a weight in excess of 130 tons. Thus, each deck type and
method of construction has its own internal logic.
As long as the cost of the foundations is proportional to the weight of the deck, they
will not have a disproportionate infl uence on the choice of the most economical span.
This would be the case for foundations consisting of pads or of short driven piles.
On the other hand, the cost of bored piles per ton of load carried typically reduces
as the pile size is increased, favouring longer spans. This will be most marked where
foundation conditions are diffi cult, requiring for instance very deep piles or piles in the
sea. Where a bridge is founded on large-diameter bored piles, it may well be that the
optimum span is infl uenced by the maximum utilisation of a single pile or of a two-pile
group of a particular diameter which is competitively priced by local contractors.
The author's experience is that the height of the pier shafts does not infl uence the
most economical span until it exceeds about 30 m, when it starts to become economical
