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What is soft-story seismic retrofitting?


Protecting Soft-story Buildings From Earthquakes
Buildings with multiple unsupported openings on the ground floor can be in trouble when an earthquake rolls into town. This California house faced some serious reconstruction after the 1994 Northridge earthquake.
Buildings with multiple unsupported openings on the ground floor can be in trouble when an earthquake rolls into town. This California house faced some serious reconstruction after the 1994 Northridge earthquake.
Photo courtesy FEMA News

The first steps in any soft-story seismic retrofitting are analyzing the structure and determining the best way to strengthen the property. Structural engineers and contractors have to weigh several factors when deciding the best way to approach such a project. Not only do they have to ensure the building will meet the structural standards required of retrofitted buildings, but they also have to minimize the impact on the building's functionality. As easy (and potentially unsightly) as it might be to reinforce buildings by filling their ground floor parking spaces with braces or walling over their picture windows, both city zoning laws and wary building owners ensure that's not an option. Instead, engineers and contractors typically use a few different approaches to complete a soft-story seismic retrofit.

"One can strengthen existing walls, add new 'shear walls' or add a steel frame in the soft areas of the building," says structural engineer Adan. To strengthen existing walls, finishes like drywall or stucco are replaced with sturdier plywood. Anchoring walls to the foundation is also part of the process. Adding shear walls -- walls built for the sole purpose of adding lateral stability -- can also be very effective, provided they don't interfere with the structure's function. If they do, a steel frame may pose the best option, though such frames may be more expensive and dangerous to install (thanks to the fire hazard and noxious fumes generated from welding them into place) than other techniques.

Rather than making buildings earthquake-proof, retrofitting aims to make them earthquake-safe, meaning that they'll still be standing when the shaking stops. Adan is quick to point out that, even with these improvements, retrofitted buildings still won't meet the structural standards of modern construction.

"In most cases, the [International Code Council] Guidelines for the Seismic Retrofit of Existing Buildings require that soft-story retrofits provide at least 80 percent strength of the level above (the vulnerable story) and that the retrofitted stiffness be approximately 60 percent of that required for new construction," he says.

We'll get into how much retrofits cost next.


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