The Importance of Resilient Communities During Earthquakes

Imagine that it’s 4:30 a.m. and suddenly you’re awakened by strong shaking in your home. Half asleep, you hang on to your bed hoping that the shaking will stop soon. All of a sudden, the floor gives away and you fall. You think, “What just happened? How could this have possibly occurred? Am I alive?”

These could have been the thoughts of Southern California residents living in one of the many apartment buildings, which collapsed on January 17, 1994, during a 6.7 magnitude earthquake. The Northridge Earthquake brought awareness to buildings in our communities with a structural weakness known as a soft story, a condition that exists where a lower level of a multi-story structure has 20% or less strength than the floor above it. This condition is prevalent in buildings with tuck-under parking and is found in multistory structures throughout San Francisco, Los Angeles and other cities (see Figure 1). These structures are highly susceptible to major damage or collapse during a large seismic event (see Figure 2).

Soft Story building
Figure 1: Multi-unit wood-frame building with first weak story.
Aftermath of an earthquake
Figure 2: Collapsed soft story tuck under parking building. Image courtesy of LA Times

Soft story retrofits help to strengthen our communities and make them more resilient to major disasters. There are several resources available to structural engineers that need to retrofit weak-story buildings. Some of these resources are mentioned in our September 18 blog post.

During the 2014 SEAOC Convention held in Indian Wells on September 10-13, speakers discussed different methods, analysis and research that address the behavior of various materials and construction types during seismic events along with approaches to retrofit historically poor performing structures. This information can be viewed from the convention’s proceedings available at www.seaoc.org.

On October 20, 2014, the Structural Engineers Association of Southern California (SEAOSC) will be hosting their 4th annual Strengthening Our Cities BAR Summit in downtown Los Angeles. This event brings together many different stakeholders in our built environment, including public officials, building owners and managers, business owners, insurance industry representatives, emergency managers and first responders, and design professionals.

Many prestigious thought leaders, including USGS Seismologist Dr. Lucy Jones will be speaking at the summit, discussing such topics as tools and analysis methods for retrofitting vulnerable buildings and the Building Occupancy Resumption Program (BORP).

Expect a great day full of useful information about ways to strengthen our communities and prepare for major earthquakes as well as opportunities to network with like-minded peers. For additional information and to register, visit www.barsummit.org. We also hope you’ll visit our booth. We look forward to speaking with you there.

Remembering Loma Prieta

We all know that earthquakes physically shape the landscape here in California, but they shape careers as well.  Earthquakes I felt while growing up in California’s southern San Joaquin Valley got me thinking about engineering as a career while in high school. When the Loma Prieta earthquake struck on October 17, 1989, like many of you I was watching the World Series live on television and thus got to see the earthquake live as well. I was in my senior year of college at the time, studying Civil Engineering with a structural emphasis. This earthquake cemented the direction I would take in my career. I wanted to be a structural engineer, and I wanted to design buildings that would not fall down in earthquakes.

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Applying new FEMA P-807 Weak Story Tool to Soft-Story Retrofit

We have written about San Francisco’s Soft-Story Retrofit Ordinance and Soft-Story Retrofits before on the blog. I wanted to discuss in more detail the issues with soft story buildings and FEMA’s new tool for addressing them. Under the San Francisco Ordinance, wood-framed residential structures that have two or more stories over a “soft” or “weak” story require seismic retrofit. So far, more than 6,000 property owners have been notified about complying with the mandate.Continue Reading

Home Seismic Retrofit

The 6.0 magnitude earthquake that struck Napa, CA, in August caused more than 200 injuries and structural damage to many homes and businesses throughout the area. The quake was the largest to hit the San Francisco Bay Area since the Loma Prieta earthquake (6.9 magnitude) in 1989, prompting the governor to declare a state of emergency.

I have done several posts about San Francisco’s Soft-Story Retrofit Ordinance and some of NEES-Soft testing related to soft-story retrofits. The soft-story ordinance only addresses multi-unit residential units and does not require retrofit of single-family homes. Cities are reluctant to mandate seismic evaluation and retrofit of single-family homes for a number of reasons that I won’t discuss here. The draft Earthquake Safety Implementation Program (ESIP) for San Francisco will not recommend mandatory retrofit of single-family homes until 2030.

CAPSS Implementation Priority Worksheet

The good news is homeowners can retrofit their homes without waiting for the government. A couple years ago in this post, I discussed some of the tools available to retrofit existing buildings.

One of these tools is the 2012 International Existing Building Code (IEBC). The IEBC has provisions for repair, alteration, addition or change of occupancy in existing buildings and for strengthening existing buildings. For alterations, these provisions may not comply with current IBC requirements, but they are intended to maintain basic levels of fire and structural life safety. The IEBC also provides prescriptive provisions for strengthening existing buildings against earthquake damage, which include strengthening residential houses on raised or cripple wall foundations.

Cripple Wall Reinforcing Schematic

Cripple Wall Retrofit Schematic and Installation
Cripple Wall Retrofit Schematic and Installation

Cripple wall failures are a common type of damage observed in older homes, caused by inadequate shear strength in the cripple wall. An additional failure point is the attachment of the wood sill plate to the foundation. Having a strong connection between the wood structure and the concrete foundation is critical in an earthquake. Since the work required to strengthen these connections is typically performed in a crawlspace or unfinished basement, it is a relatively low-cost upgrade that is extremely beneficial to structural performance.

Retrofit with UFP Foundation Plate in Napa
Retrofit with UFP Foundation Plate in Napa

Our website has information for retrofitting your home. The Seismic Retrofit Guide has information about how earthquakes affect a home and the steps to take to reinforce the structural frame of a house. The Seismic Retrofit Detail Sheet is intended to help building departments, contractors and homeowners with seismic retrofitting. It includes common retrofit solutions for reinforcing cripple walls and foundation connections.

One business owner in Napa chose to retrofit her building when she purchased it. You can see her video narrative here.

Ignore Seismic Requirements When Wind Controls?

Prior to joining Simpson Strong-Tie, my career involved the design of projects in California’s San Francisco Bay Area. When designing the primary lateral force resisting system, I would have several pages of seismic base shear calculations and, oh yeah, a one- or two-line calculation of the wind forces – just to show that seismic governed. There was no need for complete wind analysis, since the seismic design and detailing requirements were more restrictive. Of course, building components such as parapets, cladding or roof screens needed a wind design. Unfortunately, when wind appears to control, meeting the seismic requirements is not so simple.

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