Ductile Behavior of Reinforced Concrete Arch Ribs – Plastic Hinging under High Compressive Axial Loads

Arch bridges offer unique functionality in bridging over canyons, rivers, and large industrial facilities in addition to their esthetic value. Arch bridges, when subjected to earthquakes, most likely will form plastic hinges in the spandrel columns and arch ribs. These ribs should be detailed to provide the ductility needed by earthquakes, similar to columns in ordinary standard bridges. In general, arch bridges are considered Ordinary Non-Standard (ONS) bridges per the definition of California Department of Transportations' (Caltrans’) Seismic Design Criteria (SDC). Typical Ordinary Standard (OS) bridge columns carry an axial load ratio of 5 to 15 percent of the ultimate axial capacity (f’c x Ag). Where f’c is the concrete strength and Ag is the gross cross section area of the column. Arch ribs typically carry 25 to 40 percent of f’c Ag. Most Caltrans’ column tests were conducted with low axial loads. Ductility of the arch rib under high axial load needs a special study, including proof tests to demonstrate adequate ductility capacity. Rectangular confinement of arch ribs with cross ties, typical of old design, may not be adequate under high axial loads. Circular cores may be needed in the arch rib to provide a factor of safety comparable to the OS columns.Use of simplified analysis and design tools assuming first mode (longitudinal and transverse) response of arch bridges has been the typical practice of the past. Large factors of safety had to be used to cover the uncertainty in the design. One solution to this problem has been the use of non-linear time history analysis to remove the excessive conservatism associated with the simplified approach. Complicated response of the arch bridge in higher modes is expected to produce member demands beyond those identified in the first mode analysis. Plastic hinging in the spandrel columns and in the arch rib is expected, however, the extent of the damage depends on member proportioning. Proof testing of arch rib capacity will assist the designer in proper proportioning of the bridge elements.