Innovative Hi-Tech Solution for a Concrete Residential High-Rise Tower

Abstract

This paper describes the innovative seismic retrofit of the Webb Tower located on the campus of the University of Southern California in Los Angeles. The existing 14-story residential structure was constructed circa 1972 using light-weight concrete. The lateral system for the building consists of perimeter non-ductile post-tensioned concrete moment frames supported on isolated spread footings. A preliminary evaluation of the building revealed several seismic deficiencies, including the following: (1) non-ductile detailing, (2) excessive building deflection and (3) joint shear overstress. After considering several alternate retrofit schemes, a unique retrofit scheme was developed utilizing a single bay of buckling-restrained steel braces in combination with a reinforced concrete beamcolumn frame on each side of the building. The selection of this system provides several advantages for this project over other retrofit schemes, including: (1) provides partial views from the existing windows, (2) requires significantly less demolition than an interior scheme, (3) more cost-effective than an interior solution, (4) reduces building drift and demands on the non-ductile moment frames, and (5) buckling-restrained braces do not exhibit strength degrading and aesthetically unacceptable post-buckling deformations. A three-dimensional nonlinear computer model was developed capable of capturing the nonlinear behavior of the buckling-restrained braces, the existing concrete beams and columns with consideration for the post-tensioning effect, and the new concrete beams and columns. The seismic performance of the retrofitted building was investigated by time history analyses using seven sets of ground motion records.