SKEW EFFECTS ON BACKFILL PRESSURES AT FRAME BRIDGE ABUTMENTS

The abutments of frame bridges are integrally connected to the deck without expansion joints. Active soil pressures are normally considered in design despite the movement of the abutments into the soil from thermal expansion of the deck. Many abutments are located on a skew, but possible effects of this skew on the backfill soil pressures are not considered in design. To improve the knowledge of soil pressures behind a skewed integral abutment for use in designing this type of bridge, soil pressures were measured on an installed project for 33 months. The soil pressure measurements were taken using total pressure cells in the backfill on each side of the centerline for both abutments of a 20-degree skewed bridge in Maine. A total of 16 pressure cells plus temperature indicators have been monitored four times a day using a data acquisition system since October 1989. Expansion of the deck causes the pressure to increase well above the active conditions on the upper part of the abutment wall. Skew effects on the pressures that develop near the deck level behind the abutment wall of an integral abutment are substantial. When the greatest deck expansion occurs, the pressures at 3 m (10 ft) from centerline on the obtuse side reach almost three times the value at the corresponding distance on the acute side. The horizontal variation of pressure is greater than the vertical variation. A design envelope is proposed.