Movers and Shakers: About Seismic Retrofitting

Earthquakes may seem like a relatively rare hazard, but they happen frequently enough to affect how we construct buildings. In California, the most earthquake-prone region in the United States, laws require a certain degree of earthquake safety in all structures that are to be used commercially or residentially. These laws are built on history, and in a state where many wonder when “the big one” will hit, these extra steps for earthquake safety are priceless. As a residential demolition contractor from a demolition service provider like Nielsen Environmental can explain, although newer buildings are built to code, the same can’t be assumed for old buildings that were constructed years in the past. Fortunately, seismic retrofitting can make these old buildings safer.

Why Retrofitting Matters

Making buildings that can resist earthquakes has been a goal for architects and engineers for years – especially in California. As a result of the 1906 San Francisco earthquake (and resulting fires), 80% of the city was destroyed, and needed to be rebuilt better. 3,000 people were reported dead, and with a city in shambles, the new buildings would need to be able to withstand any future quakes. With the progression of time came the progression of construction techniques, but several structures were still damaged when a 6.9 magnitude earthquake struck nearby Santa Cruz county in 1989. Further south, and five years later, Los Angeles and the San Fernando Valley experienced a 6.7 magnitude earthquake that collapsed highways and buildings, just like the Loma Prieta quake a few years earlier.

Large earthquakes can knock down the sturdiest of buildings, so architects and engineers need to build smarter, not just stronger. Older buildings are more at risk than newer ones that were built according to established codes, but what makes them so dangerous in earthquakes, and what can be done to improve their safety in the event of the next big quake?

Earthquake Vulnerability Explained

Older buildings were constructed before the effects of earthquakes could be properly studied. As such, they are made with certain vulnerabilities that need to be addressed in order to be made safe.

  • Brick buildings are rigid. While this might seem like it would make for a sturdy building, in the event of an earthquake, the opposite is true. A brick structure is unable to flex as easily as a wooden structure, which oftentimes results in the building shaking itself apart during a particularly bad earthquake.
  • Some buildings have soft stories. A soft story is a floor of the building that is relatively empty, but below one or more floors that are heavier and denser. This is most commonly seen in buildings in which apartments or offices sit above a parking garage. The garage is a soft story, and can be crushed by the weight of the building above it during earthquakes.

There are many other factors to consider when planning seismic retrofitting. The materials used for internal supports, the type of foundation, and the ground beneath a structure can all affect how a retrofit proceeds. In terms of the above examples, old brick buildings can receive braced frames – a sort of building within a building that can keep the structure flexible while retaining the brick exterior. Soft stories can be reinforced with steel frames to support the structures that sit above them. Seismic retrofitting takes planning, and no two cases are the same.

A qualified contractor can assist with a seismic retrofit. These jobs are never simple, and often require a degree of careful demolition to allow space for the proper retrofitting. Fortunately, demolition contractors are knowledgeable and trained with preparing spaces for seismic retrofits, and can carefully examine a space to ensure old buildings are properly safe for earthquakes of the future.