Impact Studies on Small Composite Girder Bridge

Two impact studies on a small span, composite girder highway bridge have given widely scattered impact fractions, with a maximum of 1.32, compared with the AASHTO code value of 0.30 for this bridge. Field strains were used to measure maximum midspan bending moments for 170 trucks in normal traffic, and these were compared with equivalent static values computed from axle weights measured by a weighbridge. Impact fractions varied from −0.08 to +1.32 for gross vehicle weights from 27–44 ton. Large impact values occurred for both light and heavy vehicles and were repeated in two independent series of tests. The scatter of the results suggests that impact is vehicle dependent, and that it may vary with suspension geometry. Possible causes of high impact are examined.