What Is Seismic Retrofitting?
Earthquake retrofitting, otherwise known as seismic retrofitting, is the modification of an existing building to make it more resistant to earthquake activity. Building codes have changed as engineers have learned more about how earthquakes affect buildings. There are now many guidelines on retrofitting structures to resist not only earthquakes but also high winds and other impacts of severe weather and hurricanes.
The benefits of seismic retrofitting include:
• Damage to buildings, including displacement from foundations and structural collapse, can be prevented.
• Injuries and deaths due to building damage can be prevented, keeping occupants of a structure safe.
• Homes can remain habitable after an earthquake or another major disaster.
• Repair costs after a seismic event can be reduced or damage may be completely prevented.
• A reduction of earthquake insurance premiums.
What Are the Steps to a Seismic Retrofit?
Before starting a seismic retrofit, you must understand the structural risks an earthquake poses to your home. The process also requires knowing and evaluating the geological hazards present, including ground slope, nearby fault lines, soil conditions, etc. These are some techniques used for building earthquake retrofitting:
• Cripple Wall Bracing: Cripple walls rest on the foundation to support a structure’s floor and exterior walls, providing bracing that can prevent shifting due to ground motion. Short cripple walls increase stability, thereby strengthening the structure and helping to minimize damage.
• Foundation Bolting/House Bolting: During an earthquake, up-and-down and side-to-side ground motion can shake a house off its foundation. House bolting secures it to its foundation; this is done by drilling holes through the sill plate and then using steel plates, anchor bolts, and threaded rods to attach the foundation to the rest of the house.
• Anchoring to Mud Sill: The mud sill is the wood on top of the foundation. It was not typically bolted to the foundation before the 1950s. As a result, an older building can slide during an earthquake. The Uniform Building Code now requires bolts to be placed every six feet. Anchoring your home to the mud sill usually costs just a few hundred dollars.
• Soft Story Reinforcements: If a lower story of a building is less strong than upper levels, soft story failure can occur. This often happens with residential structures built over garages or office buildings with ground-level retail space. Steel reinforcements, plywood sheathing, shear walls, or trusses can strengthen a building. Garages may be secured by moment frames, while home garage doors can be retrofitted with a steel frame.
• Exterior/Masonry Reinforcement: Poor wall anchorage can be improved to strengthen a building. A home’s exterior walls and masonry can be supported by continuous ties on the roof, stronger connections between the roof and floor joists, and other methods of infilling to connect the masonry to the structural frame.
What Is the ROI for a Seismic Retrofit?
Earthquake retrofitting can cost anywhere from $3,000 to $7,000, but it can cost more to retrofit larger homes, those on hillsides, or homes with basements or rooms over garages. In general, the average price is 1% to 3% of a home’s cost. By comparison, it can cost $25,000 or more to repair a foundation.
Contact Derk Harmsen Construction
We provide professional seismic retrofitting and other construction services in the greater Houston area. To learn more, get a free estimate, or schedule a consultation, call 281-479-3400.