Seismic Upgrading - FAQ

The occasional small earthquake in the Bay Area is a frequent reminder that many homes are currently vulnerable to damage in the event of a larger quake. The US Geological Survey provides many interactive maps that can help assess your home’s risk for earthquake damage based on your location:

Soil type and shaking hazard: http://earthquake.usgs.gov/regional/nca/soiltype/

Liquefaction hazard maps: http://earthquake.usgs.gov/regional/nca/qmap/

Below are some typical questions that our customers ask about their homes in relation to the potential for earthquake damage, along with our recommendations:

Q. How do I know if my home is at high risk of being damaged during a moderate or strong earthquake?

A. If your home is:

  • An older home,
  • Built on a hillside,
  • Built on a raised foundation,
  • Built on landfill, or
  • Has living space over a garage,

it may be at risk for serious damage during a strong earthquake.

Photograph of crawl space under Bay Area structure built in the early 1930s. Poorly- attached horizontal and vertical beams would easily allow building to slide off of its foundation during a moderate or strong quake

Photograph of crawl space under Bay Area structure built in the early 1930s. Poorly- attached horizontal and vertical beams would easily allow building to slide off of its foundation during a moderate or strong quake

Q. What is seismic upgrading?

A. “Seismic upgrading,” also called “earthquake retrofitting” is a process designed to strengthen the structural frame of your house to help prevent it from racking (moving or tilting from side to side), or sliding, lifting, or rotating off of its foundation during a strong earthquake.

Seismic upgrading typically involves using a system of wood, metal connectors, fasteners and shearwalls to connect the structural frame – i.e. the roof, walls, floors, and foundation – of the house together. This effectively redistributes external forces from an earthquake by transferring these forces from the frame of the house to the foundation.

Photos of crawl space  under 1930’s structure shows horizontal beams fastened with nails only

Crawl space  under 1930’s structure shows horizontal beams fastened with nails only

Photos of crawl space  under 1930’s structure shows horizontal beams fastened with nails only

Photo of crawl space under 1930's structure shows reinforcement of all beams and cripple walls.

Q. What if my house is built on a hill or landfill?

A. In order to properly reinforce a house to prevent or minimize damage during an earthquake, it is sometimes necessary to do more than connect the structural frame of the house together. In particular, houses built on hillsides may not have foundations that are deep enough to be secured into stable ground, and therefore will be subject to movement with the hillside soil during an earthquake. It is often necessary to install “piers”, or reinforced concrete footings, underneath the foundation to secure it into more stable earth.

The soil around all homes undergoes a process called ‘natural settling’ in the years after construction. For a home built for today’s standards, this settlement does not normally impact the structure. However, many of the Bay Area’s older homes as well as homes built on landfill have significant problems caused by natural settling that leaves them vulnerable to earthquake damage. In some cases, this is compounded by poor drainage that leads to saturated soil and therefore compromised stability during even a moderate quake. The most appropriate repair or mitigation strategies for a particular site condition is best determined by a soil analysis, performed by a qualified, licensed Soil Engineer working in conjunction with your contractor.  Some commonly-employed strategies are retaining walls, foundation repairs, foundation buttresses, lime treatment, grout injection, or a Shotcrete and soil-nailing combination. Glynn Construction partners with a number of talented specialty engineering professionals and will assign the appropriate specialist to address your home’s particular needs.

A series of retaining walls hold back hillside soil while providing options for attractive decking and plantings in what was a very steep and unusable backyard

A series of retaining walls hold back hillside soil while providing options for attractive decking and plantings in what was a very steep and unusable backyard

Concrete piers are used to secure a new home’s foundation into stable earth

Concrete piers are used to secure a new home’s foundation into stable earth

Q. Where do I start if I think my home needs seismic upgrading?

A. No home is 100% ‘earthquake- proof’, but you can take steps to prevent or minimize damage in the event of a moderate or strong quake. In order to properly retrofit your home, it’s critical to hire a licensed building contractor. In the case of a high-risk or high-value project, you may also require the services of a Structural Engineer, a Civil Engineer, and/or a Soils Engineer. After a complete analysis of conditions and risks, the most appropriate solution for your specific case can be developed, a building permit is obtained, and retrofit work can begin.

In cases such as hillside homes requiring reinforced footings, or those needing drainage modifications or retaining wall repair/replacement/construction, the homeowner will also require the services of a General Engineering Contractor.  It’s certainly possible to hire two separate contractors and attempt to coordinate their work, but this places greater risk and responsibility on the homeowner. The  most cost-effective and efficient solution involves working with a single contractor who will reinforce the structural frame of your home as well as reinforce any footings, modify drainage patterns, or build retaining walls (i.e. a contractor holding both California “A” and “B” licenses). This allows a single party–the contractor–to manage the risk associated with a multi-faceted retrofit, and ensure that the entire project is orchestrated and managed in the most efficient manner possible.

Michael Glynn, Principal and founder of Glynn Construction, holds both A and B contractor licenses and has extensive experience in retrofitting homes across the Bay Area. His knowledge and skill in remodeling and historic preservation have resulted in many beautifully restored/renovated and seismically-sound homes.  His customers all agree that hiring a single, qualified and experienced contractor to manage the entire multi-faceted project made for greater efficiency, fewer headaches, and resulted in an attractively finished dwelling.

Q. How can I get the best value out of my retrofit project?

A. Many home-owners know that a major remodel is a good opportunity to also address structural deficiencies in their home. The flip-side is also true–planning a major seismic upgrade also offers tremendous potential to enlarge or change a home’s living space. The many options at our disposal to strengthen homes to withstand earthquake forces can open up numerous possibilities for enhancement:

  • A steel ‘moment frame’ (see Photo below) is often used in seismic upgrading to provide support against shear (lateral) stress as well as load stress from upper floors of a building. This structure opens up the possibility to integrate larger windows or create a more open floor plan that incorporates additional natural light.
  • Reinforcing foundation footings requires significant excavation underneath a home. This can provide the opportunity to add living space or even a garage to what was formerly a crawl-space.

Moment frames come in a variety of sizes and open up the possibility to integrate larger windows or create a more open floor plan that incorporates additional natural light

Moment frames come in a variety of sizes and open up the possibility to integrate larger windows or create a more open floor plan that incorporates additional natural light
Image courtesy Simpson Strong-Tie Company Inc.

Use of a moment frame to support against shear stress along a wall allows for large picture windows at the corner of this home

Use of a moment frame to support against shear stress along a wall allows for large picture windows at the corner of this home

Please contact us to discuss your earthquake retrofitting needs.