Bridge Expansion Joint for Bridges with Expansion Piers

Bridges with expansion piers use sliding bearings at the pier tops to allow the deck to move freely under thermal loading without generating large horizontal forces in the piers. The expansion joint design must account for the movement distribution between the expansion piers and the fixed piers. Expansion piers have sliding bearings at the top that allow the deck to move longitudinally. The sliding bearing consists of a PTFE sliding surface on a stainless steel plate, allowing free movement with minimal friction. The friction force from the sliding bearing is typically 2-5% of the vertical load, which is small compared to the thermal force in a fixed pier. Fixed piers have fixed bearings at the top that prevent longitudinal movement of the deck. The thermal movement of the deck is resisted by the fixed pier, generating a horizontal force that must be resisted by the pier and its foundation. The fixed pier must be designed for this horizontal force in addition to the vertical load. Movement distribution between expansion piers and fixed piers depends on the relative stiffness of the piers and the bearing friction. For a bridge with one fixed pier and multiple expansion piers, the fixed pier resists all the horizontal thermal force, and the expansion piers allow free movement. The expansion joint at each abutment must accommodate the full thermal movement of the deck. Bearing maintenance for expansion piers requires periodic inspection of the sliding surfaces and replacement of worn PTFE pads. The PTFE pads wear under repeated movement cycling, increasing the friction coefficient. When the friction coefficient exceeds the design value, the bearing must be serviced or replaced. Seismic design for bridges with expansion piers must consider the behavior of the sliding bearings under seismic loading. The sliding bearings provide limited lateral resistance, so the seismic forces must be resisted by the fixed piers and the abutments. The expansion joint must accommodate the seismic displacement that occurs when the sliding bearings move under seismic loading.