MCEER HIGHWAY PROJECT
Task E1-1: Behavior and Displacement Capacity of Pile and Pile-to-Cap Connections
Subject Area: Geotechnical
and Foundation Engineering
|Principal Investigator(s) and Institution(s)
Yan Xiao and Geoffrey Martin, University of Southern California
The strength and deformability limit states of existing pile foundations, particularly due to soil-foundation interaction in the inelastic range, are presently not well understood. The objective of this task is to investigate the soil-pile interaction effects on the ductility capability of common pile types and the strength and ductility of pile-to-cap connections for short-to-medium span bridges as well as long span bridges.
Inertia loads in bridges caused by earthquake ground shaking must be transferred to the ground. This is normally achieved through pile foundations. In cases where the pier consists of a series of strong columns or is a wall-type structure, the piled foundation is often the weakest link in the chain of lateral load resistance. This implies that under large lateral loads, in the limit, a plastic mechanism must form within the piled foundation. Two sets of plastic hinges must form: an upper set at the pile-to-cap connection, and a lower set some four to eight pile diameters below the bottom of the cap in the soil. This type of inelastic behavior is not well understood in terms of either seismic demands or capacities. This task will assess the inelastic capacity (particularly the rotational limit states) of plastic hinges in piled bridge foundations.
Some work was initiated on this topic as part of the prior FHWA/MCEER Contract DTFH61-92-C-00106 where the rotational capacity of steel pile-to-cap connections was experimentally investigated. Results show that considerable inelastic rotation can be achieved, even with small pile embedments. For good performance, however, it is desirable to have at least 2.5 pile diameter embedment into a reinforced concrete cap. (Note - if thinner caps are desired or required, special anchorage and confinement detailing should be provided.)
Year 1 of this research task extended the above-mentioned earlier work by investigating the seismic resistance of two other types of pile caps as follows:
During Year 2 of this task the following research activities are planned:
Seismic evaluation guidelines for assessing the rotational/ductility capacity of plastic hinges for piles within soil and for assessing the strength and inelastic rotational capacity of existing pile-to-cap connections; detailing recommendations for improving the ductility capability of new pile designs; and retrofit guidelines for improving the inelastic rotational capacity of existing pile-to-cap connections.
A large variety of piled foundation types exist. This makes generalization into design and retrofit guidelines difficult. The main challenge is to keep the recommendations fundamental, elegantly simple, and yet easy to implement for the widest variety of design and retrofit situations.
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