r

The results of the first three phases of a research project to study the behavior of a prestressed steel beam-concrete slab bridge unit specifically designed for county road use are presented. The units are constructed by steel fabricators in an upsidedown position, trucked to the site, and turned over and set in one operation. During fabrication, heavy steel forms are hung from the beams and their weight plus the weight of the concrete provide the prestressing. The advantages of this type of construction are that it requires less structural steel, less reinforcing steel, and less concrete for the deck. In addition, because the units are prefabricated, the labor force and the construction time at the job site are reduced. Data are presented on concrete, reinforcing bar and steel beam strains, and vertical displacements for a 1-year period of observation with the unit under sustained loading. Behavior of the unit when subjected to 500,000 cycles of repeated loading is described. Data from a static 50 percent overload test are also presented. The test unit performed satisfactorily, and county road bridges are currently being constructed in Oklahoma using three or four of the units. An additional 2-year period of sustained load observation followed by static tests to failure is planned.

A research program is being conducted at the Fears Structural Engineering Laboratory, University of Oklahoma, under the sponsorship of the Oklahoma Department of Transportation, to study the strength and stiffness characteristics of a full-scale precast, prestressed steel beam composite bridge unit. Two, three, or four of the units can be used to construct county road bridges for small stream crossings. The test unit is 55 ft long and 6 ft 9.5 in. wide and consists of two W2lx50 steel beams and a