Seismic Retrofit and Instrumentation of Los Angeles City Hall

by Nabih Youssef and Owen Hata

Youssef, Nabih, and Owen Hata (2005). Seismic Retrofit and Instrumentation of Los Angeles City Hall. SMIP05 Seminar on Utilization of Strong-Motion Data, p. 115 - 130.

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The construction for the seismic rehabilitation of the Los Angeles City Hall was completed in 2001. An integral part of the project is the installation of seismic instrumentation throughout the building. The instrumentation program was a collaborative effort involving the Los Angeles City Department of Public Works Bureau of Engineering, California Geologic Services Strong Motion Instrumentation Program (CGS-SMIP) and the Engineering Design team. The future data recorded by these sensors will provide valuable insight on the actual response of the building during an earthquake. This information will aid the structural engineering community to better understand the behavior of base isolated structures with supplemental damping.

The building was originally constructed in 1926 and was the first building to exceed the 150 feet height limitation for all privately constructed buildings in Los Angeles. It is 32 stories (460 feet) tall. The original building was designed prior to the enactment of explicit seismic design requirements and therefore, was not specifically designed to resist earthquake generated forces.

Over the past 75 years, regional earthquakes have caused damage to the building. Terra cotta cladding has been cracked, broken or destroyed in portions of the building's exterior. With every significant earthquake, unanchored masonry debris has been scattered about the building's interior. Large cracks in the masonry walls appeared at the 24th floor after the 1971 Sylmar Earthquake, the 1987 Whittier Earthquake and the 1994 Northridge Earthquake.

The building has been seismically rehabilitated in order to preserve life safety, mitigate damage, maintain the integrity of the building's exterior facade, and protect the historic interior fabric from damage. Base isolation with supplemental damping was used to enhance its seismic performance. This approach was determined to be the most effective strengthening scheme based on performance and cost.

This paper presents an overview of the rehabilitation project including the development of seismic performance goals, identification of inherent seismic deficiencies of the original building, description of the final seismic strengthening and instrumentation program.