19.1: The Hazard of Buildings

Earthquakes Don't Kill People; Buildings Do

Imagine you are walking your dog in an open field, enjoying the ubiquitous California sunshine. At that moment, you feel violent earthquake waves. Are you in danger? Outside of losing your balance, you are not likely to encounter any health hazard in the middle of a field. Seismic waves don’t directly hurt the human body. In the open, you can experience the biggest shaking the Earth can offer and nothing will happen to your body other than a few cavity fillings loosening.

If you are inside a building, however, the story can be quite different. Much depends on the type of building.

If an exact repeat of the 1872 Lone Pine quake occurred today, we would not lose 88% of buildings, as we did in Lone Pine; in fact, it is safe to posit that no place in California would lose such a high percentage. Over 60% of Californians live in free-standing, wood-framed single family homes; because of the flexibility of such structures, it is expected that they will perform much better in shaking than older, multi-story masonry structures.

Masonry (bricks or larger blocks) is spectacularly unsuited to earthquake country (see Figure \(\PageIndex{1}\)). While masonry has a lot of strength in the up-down direction of compression from gravity, individual blocks can easily shift side-to-side and shear in seismic events. Masonry bears load effectively only in the up-down orientation; when bricks shift position even slightly, they lose the ability to hold up buildings. Some municipalities require retrofitting of masonry buildings–attaching anchors, or cross-bars–but these efforts should be understood as a band-aid rather than a fix. The only comprehensive solution would be to tear down and rebuild vulnerable structures to a higher standard, but given the economic impracticality of such a cure, most municipalities seem to hope that the preventable loss of life will not be too great when the quake comes, and then post-quake reconstruction will occur to those higher engineering standards.

Figure \(\PageIndex{1}\): Unreinforced brick wall in downtown Oakland, 19th and Webster streets. "19th & Webster, Oakland," by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Newer buildings, constructed according to scientifically-informed engineering standards, are much more resilient than older buildings. For example, the new City Hall of Hayward, CA, was finished in 1998 and replaced an earlier structure that was, quite literally, being slowly ripped apart by movement of the Hayward fault. The new City Hall sits less than 200 meters from the fault. In order to isolate this building from the inevitable shaking, the base of the structure sits on dozens of giant ball bearings. These isolation bearings mean that as the ground moves, the ball bearings dampen the movement and allow the building to stay in place, with limited transfers of movement from the quake to the building.

The problems, then, comes down to the dangerous antiquity of many older buildings, and especially the construction style known as soft story buildings.

Soft Story Buildings

A soft story building has an open, poorly-braced ground level. This design performs very poorly in earthquakes, where seismic motion transfers to the building through the ground level. This weakened first story then becomes a risk for the collapse of the entire building.

In the 1989 Loma Prieta and the 1994 Northridge quakes, many of the damaged and collapsed buildings were soft story structures. Figure \(\PageIndex{2}\) shows the heavily damaged Marina District in San Francisco and how motion from the 1989 quake tilted the building laterally due to the collapsed street-level soft story parking garages.

Figure \(\PageIndex{2}\): A Marina District collapsed soft story building destroyed during the 1989 Loma Prieta quake. Corner of Beach and Divisadero Streets, San Francisco. "Soft story" by J.K. Nakata, USGS is in the public domain. Access a detailed description.

In Figure \(\PageIndex{3}\):, we see an existing soft story building and can examine the problematic masonry supports holding up the structure. Even a slight side-to-side motion has the potential of shifting masonry, weakening these load-bearing columns.

Figure \(\PageIndex{3}\):, A soft story apartment building with masonry supports in Oakland, CA. (Image modified for privacy and lighting exposure.) "Soft story" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

What Figure \(\PageIndex{3}\): shows is an apartment building with the entire lower level opened to accommodate parking. The effect of this is to weaken the most critical level for seismic building survival. If cross-bracing were installed across the parking spots, then the building would have greater flexibility; as it stands now, a few narrow posts are all that keeps the living areas standing above the parking spots.

Why build this way? Why build a weak soft story in earthquake country? Some soft story buildings have retail shops on the foot-traffic level, necessitating open spaces for displays. But there is a far more common reason for this problematic architecture: parking.

Many cities require new apartments to provide on-site parking. This is an ill-considered response to local overpopulation and consequent street parking shortages. The thinking is, rather than pushing more cars onto already crowded streets, when multi-family apartment buildings are constructed, parking should be accommodated cars on-site. This could be accomplished with a secondary garage or an adjacent lot, but rather than these expensive options, developers often choose to put resident cars underneath the apartments.

Science has developed a wealth of information about the prevention of death and injury from earthquake; however, rather than adopting these scientific findings, many municipalities fail to take the science into account, and in the case of allowing and even requiring soft story parking architecture, actively ignore well-established dangers.

According to the Association of Bay Area Governments, San Francisco has approximately 6,700 soft story buildings. In 2013, San Francisco implemented regulations requiring soft story building owners to retrofit. Many building owners have complied, despite the extraordinary cost, which can run into hundreds of thousands of dollars. However, in the case of noncompliant property owners, the only actions taken to date have been posting "shame notices" of code violation outside the affected buildings.

Query \(\PageIndex{1}\)

References

1. Dal Pino, J. (2019, March 1). The San Francisco Soft-Story Ordinance. Structure Magazine. https://www.structuremag.org/?p=14310
2. Gutierrez, H. (2023, July 6). Hundreds of San Francisco buildings are behind on earthquake retrofits, putting lives at risk. Retrieved June 27, 2024, from https://www.nbcbayarea.com/investigations/soft-story-retrofits-in-san-francisco/3267556/