Span Selection for Steel Bridges

Introduction:
Steel has traditionally been recognized as the material of choice for longer spans. Because of this reputation, the steel spans selected are often far in excess of the optimum. For greatest economy, substructure and superstructure costs need to be evaluated with respect to span lengths. The purpose of this technical bulletin is to show a proven method of determining the optimum span length for a steel bridge, where pier locations are not predetermined by site conditions. Both the method and the actual project example were provided by the Norfolk, VA office of PBQD.

The method involves calculating the individual super- and substructure costs for a varying set of conditions. Substructure costs vary primarily by pier height, with their unit costs ($/SF) decreasing as the span increases. Superstructure costs, on the other hand, increase as the spans get larger. At some point, the combined cost for a given pier height is lowest. This then is the optimum steel span, and may not be significantly different from the optimum concrete span. This can best be illustrated by example.

Figure 1 shows the bridge cross-section selected.

Superstructure Cost:
Using this cross-section, a superstructure cost estimate is prepared for various span lengths. In order to account for variable overall bridge lengths due to different spans, all comparisons are on a per square foot basis. By plotting the calculated costs versus span length, the following curve for superstructure cost is generated (Figure 2).

Substructure Cost:
Using the same procedure, substructure costs can be determined for various pier heights. The pier costs can then be plotted by both span length and pier height (Figure3).

Bridge Cost:
Once the curves for both substructure and superstructure have been developed, it is then possible to construct curves representing total bridge cost by span and by pier height (Figure 4). The lowest point on each of these curves is then the optimum span length for the given pier height. It is interesting to note that for this example, the optimum span length for a 20-foot high pier is only 130 feet, while the optimum span for piers 90 feet high is about 180 feet.

Conclusions:

• The optimum steel span length is very dependent upon substructure cost, which in turn is mainly dependent upon pier height.
• Increasing span length means larger superstructure costs.
• On projects where pier locations are not predetermined by site conditions, an analysis of both sub- and superstructure cost is necessary to select the most economical steel spans.
• The optimum steel span length could be very close to that of the concrete scheme. Do not automatically assign a larger span length to steel!
• Bethlehem Steel's Preliminary Girder Optimization Program can provide optimized girder designs for the various spans used in computing superstructure costs.