SEISMIC ANALYSIS OF A REINFORCED CONCRETE EXCAVATION SHORING SYSTEM ADJACENT TO A PARTIALLY EMBEDDED BUILDING

摘要

We performed a seismic response-history analysis of a shoring system designed to provide lateral support for a planned construction excavation that will expose one side of an existing building. The excavation is about 16 m deep and extends up to 25 m from the building. The site conditions consist of soft soil for the first 8 m and weathered rock underneath. The adjacent building is a five-story 19 m by 28 m rectangular reinforced concrete structure. Three stories are embedded 12.25 m underground, and two stories extend 7.25 m above ground. The excavation shoring system consists of a vertical piled retaining wall to hold back the soil at the far end of the excavation and a reinforced concrete space frame that laterally supports the retaining wall and channels the forces back to the exposed walls of the existing structure on the other end. The response-history analysis used the finite element method to model the adjacent structures and soil excavation. The force distributions in the retaining wall and shoring system were computed for ground motions corresponding to twice the Safe Shutdown Earthquake. We used an efficient hybrid approach that combined two-dimensional (2D) response history analysis in computer program SASSI2000 and 2D and three-dimensional (3D) quasi-static analyses in computer program SAP2000. This hybrid approach was used in order to provide estimates of the seismic demands for the shoring system design whose schedule would have been significantly delayed by a full 3D analysis. The analysis identified two distinct modes dominating the shoring system dynamic response: (1) inertia loading of the soft soil layers bearing onto the vertical retaining wall, and (2) bearing of the shoring system onto the excavation boundary due to the seismic response of the adjacent building. Critical member force demands were obtained by modal combination of equivalent static analyses which exhibited a good match to dynamic analysis results.