Seismic retrofit for damaged 7-story laboratory building
Barker Hall is a 6,860 m2, 7-story biological research laboratory at the north-west corner of the U.C. Berkeley campus. The building was moderately damaged during the Loma Prieta Earthquake in 1989, and received a seismic rating of “Very Poor” in the course of the University’s campus-wide seismic evaluation process in 1997-98.
The building is a 1960 vintage reinforced concrete structure with both precast and cast-in-place elements. The interior columns bear on individual spread footings. The perimeter basement wall bears on a continuous spread footing. The existing foundations rest on dense sandy clay material with relatively high load bearing capacity. Lateral resistance was originally provided only by several weak core elements within the building and by an unusual and brittle precast concrete cladding panel system, in which the panel tops and sides were doweled to the perimeter beams and columns, but were not connected at the base.
Forell-Elsesser Engineers was asked in 1998 by the University to perform a detailed investigation to develop various means of retrofitting the building, considering the following:
The building would remain at least partially occupied;
Neither the spaces nor the functions could be affected by the retrofitting process;
The MEP systems and lab service systems, which were extremely condensed, could not be significantly disrupted by the retrofitting process.
Forell-Elsesser developed several possible systems, which could be constructed largely on the exterior perimeter of the building. Finally, a system of eight shear walls (two per each face of the building) with drilled pier foundations was selected.
Ultimately, the foundation design was changed to a unique continuous posttensioned concrete grade beam or “belt” system, in lieu of drilled piers and pier caps. The post-tensioned belt beam, which is 1.83 m wide and 3.35 m deep, bears directly on the soil approximately 3.05 m beneath the existing foundation. This close-in construction of a deep foundation beam required the entire perimeter to be underpinned, but was still less costly than the competing drilled pier foundation.
The seismic rocking of the belt foundation system was evaluated using 3-d non-linear dynamic analysis methods.
Due to perimeter gravity support concerns relating to the small (25.4 cm) sectional dimension of the existing perimeter columns, pairs of supplementary steel HSS 8 x 4 columns were provided at each column, for the entire height of the building above the basement level.
The structural construction work was completed in summer of 2002.