Two girder designs are under consideration for a bridge for a 1,200-foot crossin

Question

Two girder designs are under consideration for a bridge for a 1,200-foot crossing. The first design is expected to result in a superstructure weight per foot of 22(S) + 800, where S is the span between piers. The second design should result in superstructure weight per foot of 20(S) + 1,000. Piers and two required abutments are estimated to cost $220,000 each. Let X = the number of piers plus abutments. The superstructure will be erected at a cost of $0.55 per pound. Choose the girder design that will result in a minimum cost and specify the optimum value for X. Hint: X must be an integer. An answer of X = 12.678 cannot be correct.

Explanation / Answer

Ans:

Bridge Crossing 1200 feet $220,000 $0.55 Design Span Constant No. of Piers + Abutments weight Pier + Abutments Cost Cost due to weight Total Cost Total Bridge Length 1st Design 22 800 55 2010 $6,050,000 $1,105.50 $6,051,106 1210 2nd Design 20 1000 60 2200 $6,600,000 $1,210.00 $6,601,210 1200 So choose 1st Design as cost is less

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