Column Research Council Symposium on Metal Compression Members: Effect of Floor Systems on Pony Truss Bridges

An analytical method for determining the buckling load of top chords of single-span pony truss bridges has been developed by Edward C. Holt, Jr., M.ASCE. Ten model bridges were tested for failure to supplement this analysis. In Holt's studies, the effect of floor-system participation on the top-chord stresses was neglected in both the analytical and experimental work. The participation of the floor system relieves the stresses in the bottom chord and reduces the effective depth of the truss, thus increasing the top-chord stresses. This paper presents the results of an analytical and experimental study of the effect of floor-system participation on top-chord stresses in pony truss bridges. For the experimental portion of the study, a floor system was added to the model bridge used in the buckling studies. This floor system consisted of four continuous stringers clamped to the top flange of the floor beams. The model bridge was tested at working loads with and without the floor system, and the results compared. This experimental work indicated that the top-chord stresses are increased by 2% or 3%, under normal working loads, due to the effect of floor-system participation. An analytical method is developed to calculate the increase in top-chord stresses due to floor-system participation. The analytical and experimental results compare reasonably well. Although the results of the tests apply only to a single-span pony truss bridge model with continuous stringers, these results can probably be extended to include full-scale bridges. It is concluded from the test results and from measurements made on actual structures that the effect of floor-system participation cannot be neglected.