Understanding Soft-Story Seismic Retrofitting

On January 17, 1994, a 6.7-magnitude earthquake hit Reseda, a neighborhood in Los Angeles. Known as the Northridge Earthquake, this disaster highlighted a critical structural weakness in numerous buildings across earthquake-prone regions in the U.S. During the quake, the ground floor of the Northridge Meadows apartment complex gave way, crushing the two upper floors. Tragically, 16 first-floor residents lost their lives [source: Reich]. The collapse was attributed to the building's soft-story design, which made it vulnerable compared to other structures.

Soft-story buildings are characterized by their less rigid ground floors compared to the upper levels, making them highly vulnerable to seismic activity. These structures often feature large, unsupported openings on the first floor, such as parking areas, wide windows, or spacious lobbies, typically built with wood frames. Without adequate reinforcement, they struggle to resist the lateral forces—side-to-side movements—generated during earthquakes. When the ground floor fails, the upper levels collapse onto it, causing catastrophic damage.

This poses a significant challenge in densely populated, earthquake-prone regions such as Los Angeles and San Francisco, as well as other high-density cities with similar construction styles. Despite the risks, thousands of soft-story buildings in these areas still require upgrades. A study by the California Institute of Technology revealed that out of an estimated 20,000 soft-story structures in Los Angeles, only 800 had been retrofitted [source: Jessup]. San Francisco is in a comparable predicament, with approximately 10,800 soft-story buildings [source: Association of Bay Area Governments]. Alarmingly, experts anticipate a major earthquake in the Bay Area within the next few decades. Such an event could result in 80 percent of San Francisco's soft-story buildings collapsing or becoming irreparably damaged [source: Selna].

How can such a catastrophe be averted? This is where structural engineers like Dr. Scott Adan play a crucial role. As the founder and principal of Adan Engineering, he focuses on a technique called soft-story seismic retrofitting.

"Lateral stiffness and strength are essential for ensuring buildings can withstand earthquakes. In soft-story structures, these elements were either overlooked or insufficient to handle the forces generated during seismic events," Adan explains.

Soft-story seismic retrofitting corrects these deficiencies by incorporating the necessary structural reinforcements to keep buildings intact during earthquakes. Continue reading to learn about the retrofitting process and how cities are incentivizing property owners to implement these upgrades.

Protecting Soft-story Buildings From Earthquakes

The initial phase of soft-story seismic retrofitting involves a thorough structural analysis to identify the most effective reinforcement methods. Engineers and contractors must consider multiple factors, such as meeting retrofitting standards while maintaining the building's functionality. Simply filling parking spaces with braces or covering large windows is neither practical nor compliant with zoning laws. Instead, professionals employ various techniques to achieve a successful retrofit.

"Existing walls can be reinforced, new shear walls can be added, or a steel frame can be installed in vulnerable areas," explains structural engineer Adan. Strengthening walls often involves replacing materials like drywall with robust plywood and securing walls to the foundation. While shear walls enhance lateral stability, they must not disrupt the building's use. In some cases, a steel frame is the best solution, despite higher costs and risks like fire hazards from welding.

Retrofitting doesn't make buildings earthquake-proof but aims to make them earthquake-safe, ensuring they remain standing post-quake. Adan emphasizes that even retrofitted buildings won't match the structural integrity of modern constructions.

"According to the [International Code Council] Guidelines for the Seismic Retrofit of Existing Buildings, most soft-story retrofits must achieve at least 80 percent of the strength of the upper level (the vulnerable story) and approximately 60 percent of the stiffness required for new constructions," he explains.

Next, we'll explore the costs associated with retrofitting.

How Much Will That Retrofit Run Me?

The expense of a soft-story retrofit can vary significantly based on the project's scale, with larger buildings sometimes exceeding $100,000 for retrofitting [source: Selna].

While soft-story seismic retrofitting is undeniably costly, it often proves to be a worthwhile investment. A Caltech study revealed a 7:1 benefit-to-cost ratio for shear walls and a 4:1 ratio for steel frame retrofits [source: Association of Bay Area Governments]. Despite these benefits, many building owners are reluctant due to the expense and complexity, prompting cities to implement measures to incentivize or mandate retrofitting.

In Berkeley, for example, owners of soft-story structures are required to have their buildings evaluated for structural vulnerabilities. While addressing these issues is optional, those who choose not to proceed must display a notice alerting residents and visitors that the building may not withstand an earthquake. Other cities promote retrofitting by relaxing zoning rules for parking, offering tax benefits, or restricting other renovations until seismic upgrades are completed. In Fremont, owners of pre-1978 soft-story buildings must retrofit or face significant penalties [source: Community Action Plan for Seismic Safety]. These policies aim to highlight the importance of retrofitting without waiting for another disaster.

Continue reading for more earthquake-related information that will shake up your understanding.