Case Studies : Cooper River Bridge : Preliminary Design Conclusions and Recommendations

The cost of each of the three steel alternatives is given in Table 1.

Based on the investigations, alternate preliminary designs, and other considerations previously presented in this report, the following conclusions were reached for each of the main span crossings and the approaches.

The cost estimates shown in Table 1 rank the alternatives (taking into account equivalent approach span costs as previously described) as follows:

  1. Tied Arch = $18,953,000
  2. Parallel Chord Continuous Truss = $20,620,000
  3. Variable Depth Continuous Truss = $23,854,000

The cost of the parallel chord continuous truss and the variable depth continuous truss is 9% more and 26% more, respectively, than the tied arch alternative.

A simple span tied arch is a suitable alternative for the main span crossing the Cooper River. This type of structure presents an elegant appearance and gives the motorist a feeling of openness which is achieved by the use of light bridge strand hangers to support the roadway deck from the arch rib. However, a number of serious problems have occurred in recent years with this type of structure. The FHWA has issued a memorandum indicating that there are 82 tied arches currently in service in the U.S. (as obtained from bridge inventory data), of which six have had cracks developed in the tie girder or related welded connections. Since the ties are non-redundant members, a crack would probably require closure of the span to effect repairs, and could even lead to a catastrophic failure. While a crack in a member of a continuous truss would also be serious, the effect should be less critical than in the simple span tied arch because of the inherent longitudinal and transverse redundancy of a continuous truss configuration.

In view of the problems with the simple span tied arch and the current requirement that approval to design a simple span tied arch bridge must be obtained from the Washington, D.C. office of the FHWA, there is reluctance to recommend the tied arch alternative based on economy alone. If the tied arch is the selected structure type, means should be developed to provide some redundancy in the tie. Furthermore, additional requirements should be specified for testing the steel. Whatever is developed, it is clear that the cost will be increased. Consequently, the 9% cost differential between the tied arch and the parallel chord truss will be decreased. Moreover, the cost differential is a relatively insignificant part of the entire project cost.

Aesthetics is also important because of the prominence of the bridge. While arches are usually pleasing in appearance, it is felt that a parallel chord truss with its clean lines will be an interesting and appealing structure.

Taking all of the foregoing into consideration, the parallel chord continuous truss is recommended for the main spans of the Cooper River crossing. (ref. 4)

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