Truss Bridge Report Page 4

Prepared by Construction Marketing * Bethlehem Steel Corporation * Bethlehem, PA 18016

Modern Methods of Analysis and Design Aid in Economical Solution to River Crossing

Local Factor Design was used for both superstructure and substructure design. By utilizing the current criteria for trusses, the need for modified live loads for longspan bridges (those exceeding 500 feet) is recognized by AASHTO. It acknowledges the lesser probability of live load occupying the entire critical portions of a bridge and the inherent redundancy of steel structures by allowing smaller factors for those members whose dead load exceeds 77% of the total load. It is estimated that the depth of the chord members is three inches less than that which would be conventionally required due to Load Factor Design and the increased maximum allowable bolt stress of 25 ksi. This not only saves on the weight of main members, but also contributes to smaller gusset plates at the premium priced truss joint connections

With two main load-carrying trusses, the bridge is by definition a nonredundant structure. In order to identify the fracture-critical members, the truss system, including not only the trusses but also the floor system and bracing, was modeled in three dimensions. Each truss tension member was removed one-at-a-time, resulting in a unique structure. The resulting truss configuration was then loaded and the resulting force in each member compared to the capacity of that member as defined by its yield strength and/or buckling capacity. Should any member of this unique structural system fail, the member removed was considered as fracture-critical. All fracture-critical members were identified on the drawings.

Computerized design and drafting have had a demonstrable effect upon the economy of the truss system. By modeling the entire structure, including the two fixed main river piers, it was possible to determine the response of the structure to all non-gravitational loads, and to take advantage of the elastic properties as well as the relative stiffness of both superstructure and substructure bridge elements in resisting the loads. Layout of the rather complex truss joints was facilitated by automatic drafting techniques that resulted in geometrically correct orientation of the various elements.

The means and methods of constructing the bridge were considered from the preliminary stage to final detailing phase of the design process. As a result, all floor system members are essentially identical for the interior panels of the truss. The vertical curve was located at the center of the middle span, thus assuring symmetry. Early on it was determined that, by judicious layout of the truss joints, the allowable 15% loss of section could be taken advantage of to eliminate the costly process of upsweeping the main truss members at the joint locations. As a result of considering the various constraints involved, the most probable method of erection of the truss was established, and consideration of such was made from conceptual layout to the final design and detailing phase. This effort was rewarded when the Contractor’s method was submitted for review, with no increase in member size required due to erection stresses.

Steel Fabricators:
Stupp Brothers Bridge and Iron Co., St. Louis, MO
Vincennes Steel Corporation, Vincennes, IN

Summary

Longspan Steel trusses continue to offer Bridge Owners a cost-competitive option, with proven durability. Attention to concrete deck details and careful selection of the paint system can reduce maintenance costs. As is evident with both the Central Bridge and the Sixth Street Bridge, attractive designs are attainable.