Brief
Explore construction retrofitting strategies to enhance seismic safety in concrete tilt-up buildings and reduce earthquake damage.
Insight
Construction retrofitting plays a crucial role in safeguarding buildings against earthquake damage, especially for structures built under outdated design codes. Among the most vulnerable are tilt-up concrete buildings, which combine rigid concrete walls with flexible timber roof diaphragms. These structures were not adequately represented in early building codes and demonstrated serious deficiencies during earthquakes such as the 1971 San Fernando and 1994 Northridge events.
Originally, tilt-up buildings suffered from weak connections between the roof and walls, leading to out-of-plane wall failures and even roof collapses. Subsequent code changes attempted to address these weaknesses by gradually increasing anchorage requirements, but they often responded to previous failures instead of addressing root causes.
A more effective approach to construction retrofitting of tilt-ups involves performance-based design. This includes strengthening the roof diaphragm and improving the connections between the roof and supporting walls. Typically, retrofitting is executed in two phases. The first targets wall anchorage and diaphragm cross-ties using standard hardware. The second is usually done during re-roofing and focuses on improving diaphragm nailing patterns.
Many California cities have introduced retrofit ordinances, with the City of Fremont and San Francisco leading the way. These ordinances aim to improve safety by retrofitting older buildings, particularly those built under pre-1978 codes. Retrofits involve adding structural reinforcements such as hold-down anchors, shear walls, or steel moment frames, depending on the building’s configuration and use.
The key to successful construction retrofitting lies in understanding the building’s dynamic response to seismic forces. Engineers now recognise that the roof diaphragm is the primary yielding component, and wall elements are generally rigid. As such, retrofitting efforts must focus on diaphragm strength and connections rather than wall components alone.
Various resources, including FEMA P-1026 and guidelines by SEAOC, support engineers in designing and executing retrofits. These documents provide valuable insights into the seismic behaviour of rigid wall-flexible diaphragm buildings and highlight cost-effective, practical solutions.
Ultimately, retrofitting improves not only life safety but also community resilience by protecting vital infrastructure and housing stock from future seismic events.
Highlight
- A more effective approach to construction retrofitting of tilt-ups involves performance-based design. This includes strengthening the roof diaphragm and improving the connections between the roof and supporting walls.
- Typically, retrofitting is executed in two phases. The first targets wall anchorage and diaphragm cross-ties using standard hardware. The second is usually done during re-roofing and focuses on improving diaphragm nailing patterns.
- Engineers now recognise that the roof diaphragm is the primary yielding component, and wall elements are generally rigid. As such, retrofitting efforts must focus on diaphragm strength and connections rather than wall components alone.
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