Todd Jones, CC BY 2.0

Virtual Shaker: assessing seismic risk to high-rise buildings in LA

The 1994 Northridge Earthquake revealed significant structural performance deficiencies in steel moment-resisting frame buildings around Los Angeles, California, but a detailed study of the collapse risk of the current structural steel building code was not then undertaken.

In collaboration with the California Institute of Technology (Caltech), Arup carried out a region-wide seismic assessment of steel buildings in Los Angeles.  The study filled an important data gap by addressing the likely seismic performance of existing high-rise structures in the area. The study assessed the expected seismic performance of the existing mid- and high-rise structural steel building stock and confirmed concerns about structural deficiencies in steel buildings constructed before 1994. This knowledge will be used to help develop Los Angeles’ disaster mitigation plan.

The analyses of the response of the building stock to earthquake shaking were undertaken numerically, hence the project title ‘virtual shaker’The project team was careful to quantify the assessed risk of seismic damage or loss in language that can be easily understood and interpreted by the buildings’ owners and operators, so that they can use this information to make decisions about insurance and even building purchases.

 

 

Using LS-DYNA™ software in conjunction with Arup’s proprietary element library, we calculated the expected seismic performance of the steel building stock in the city of Los Angeles.

Based on the analyses, we established a methodology for categorising the seismic vulnerability of buildings in the city based on, amongst other factors, the date of construction (and hence the seismic design code that was applicable at construction), building height, structural form and construction materials.

By responding to the knowledge gap that surrounded the seismic performance of steel moment-resisting frame buildings in Los Angeles, this research generated an immensely informative assessment of existing steel buildings in the city.  The project team gained detailed knowledge of the existing building stock, which can enable rapid numerical risk assessment of mid- and high-rise buildings in Los Angeles.

Although this project focused on Los Angeles and its particular design codes and buildings, the methodology is applicable elsewhere. Categorising the seismic response of buildings based on construction era and construction materials will readily facilitate the quantification of seismic risk in clients’ real estate portfolios in cities elsewhere in North America and around the world.

This project demonstrated that LS-DYNA™ together with Arup’s proprietary element library are powerful, effective useful tools for seismic assessment and retrofit design of tall buildings, as well as the design for new tall buildings in high seismicity areas.

This research supports and reinforces the City of Los Angeles’ vision to improve seismic resiliency.  It can also motivate other municipalities in regions susceptible to earthquake activity to conduct similar analysis of the seismic vulnerability of their existing building stock.