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 with 30 units of HB load allowed. The Y3 beam has 29 tendons, and the Y4 beam has 25 tendons with an additional 0.715m3 of concrete. With no other considerations such as overall depth constraints, the Y4 beam is likely to be more cost effective.

Example 2 - Which of the following 2 beams costs less, the Y4 beam in example 1, or the Y4 beam shown next?

Again both beams are designed for the same 20m span. Both are Y4 beams, 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 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 the forthcoming version of SAM-LEAP5 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 in SAM LEAP5.