Under-Reported Risks from Earthquakes in High-Rise Buildings
•••••••••• March 2023 ••••••••••
Even a cursory look at San Francisco’s building quality and standards reveals that the potential for deaths and injuries from a major earthquake is much higher than most people realize. Much of the possible devastation is preventable, but the City is not taking appropriate actions—apparently for the benefit of developers and property owners.
Current building standards are intended to keep 90% of tall buildings standing after a severe earthquake, but that indicates the collapse of 10%—approximately 16 high rises, possibly fully occupied—is acceptable to the City. That could be worse than 9/11—even if the 90% goal is achieved. Who made that life-and-death decision? One estimate predicts half of the downtown buildings could be uninhabitable. More robust building codes and retrofitting could reduce this devastation. Other countries are actively retrofitting their tall buildings, but San Francisco is hardly getting started. Many companies, especially in the high-tech community, will not remain in San Francisco waiting for buildings to be rebuilt or rehabilitated.
That 90% goal seems only to consider how new buildings are designed—and as if they will all be on rock-solid ground—it does not consider the effects of liquefaction. Nor do defects in older buildings, such as faulty welding techniques, seem to be a consideration. All of these issues can cause buildings to collapse. Some buildings are probably vulnerable to multiple problems. San Francisco leaders should consider four distinct problem areas individually and collectively:
- 1. Effects of liquefaction
- 2. Inferior frame welding
- 3. General engineering quality
- 4. Two inconsistent sets of building standards
Facts, Figures Concealed
Unfortunately, studies, articles, and discussions often address only one of these categories or jump around between them. They can allow special interests to conceal problems. For example, a New York Times article quoted an engineer as saying San Francisco buildings are well-designed and will probably not collapse during a significant earthquake. That may sound reassuring, but it ignores categories one, two and four (listed above) and is inadequate for category three. As noted, some buildings may be vulnerable to all four problems. Whether one wishes to design, regulate, acquire, work in, or live in a high-rise, information must be available that covers all three categories.
Liquefaction has been a known condition for a long time. The problem of inferior welds has been recognized for almost 30 years, and it has been four years since the New York Times exposé of the problem. For decades, the City has allowed weaker standards for buildings shorter than 240 feet. City leaders show no signs of seriously considering these structural deficiencies.”
- Effects of liquefaction. It is the most serious prospective problem and the most overlooked. According to an online, interactive database maintained by the SF Office of Resilience and Capital Planning (ORCP), approximately 114 downtown high rises are on soil with a “very high” potential for liquefaction. And according to a letter from the ORCP, “most” buildings' foundations do not reach down to bedrock. (The number, and even an estimate, is concealed.) It is hard to imagine how even the strongest, best-engineered building could be immune from this problem when the entire structure, including its foundation, rests on top of soil that could liquefy. Obviously, these buildings will not float, and bedrock is not flat. The ORCP database also names 43 buildings, including the Transamerica Pyramid, that have what is called “mat” or “floating raft” foundations that do not reach bedrock.
(Twenty-five of these are rated “very high” for liquefaction.) While they may “float” on dry soil, the case has, of course, not been made that they will stand up in liquified soil. From the Ground Up, a well-researched source written by Douglas Frantz, names several other substantial buildings that have pilings that do not reach down to bedrock: One Market Plaza, pictured above (including the Spear Tower at 43 stories and Steuart Tower at 27 stories), and Rincon Center (twin 22-story towers at 88 Howard that include 320 apartments and five floors of Google offices). The Millennium Tower is an example of a newer building designed not to reach bedrock, and it has had problems even without an earthquake. We have never seen a description of what is likely to happen to tall buildings like these during liquefaction.
It appears the City is concealing this problem. The ORCP’s database of high-rises includes significant other information about their foundations, such as the type of foundation, depth to bedrock, and the likelihood of liquefaction. What is omitted is alarming: the simple (but critical) yes/no indication whether the foundation reaches bedrock. Again, according to the ORCP, most buildings do not touch bedrock. Finding an answer to this question requires scheduling a visit to a city office to research the building. The ORCP should have included that information in the database, which is available online. The Millennium Tower is fixing its problem, reportedly, by inserting pilings that will reach bedrock. New York City restricted high-rises to its downtown and midtown because those areas have higher bedrock. The City should hold hearings to determine why buildings in the “very high” liquefaction downtown zone of San Francisco were permitted without foundations reaching bedrock.
- Inferior frame welding. Both the US Geological Survey and the New York Times have documented this serious condition at 39 San Francisco high rises (and briefly copied in local news media). It does not need to be described again here. The potential for severe structural failure is alarming. The problem could compound categories one and three as well. This problem has been known for nearly 30 years, and some California cities have already demanded corrective actions. Unfortunately, the problem seems to be forgotten in San Francisco, and the ORCP indicates the City may “begin” to address it in a couple of years.
- General engineering quality. Assuming issues #1 and #2 are not applicable, the City's building codes are supposedly designed to keep 90% of tall buildings standing after an earthquake. But this does not mean they will remain habitable. The burden of repairing or replacing red-tagged buildings will be considerable. Furthermore, adjacent buildings must be vacated until a red-tagged structure has been removed or repaired. They could remain unoccupied for many months. The City must adopt standards to keep all buildings upright and habitable after a big earthquake.
- Two inconsistent sets of building standards. Having a tougher set of earthquake standards for buildings over 240 feet is arbitrary and inappropriate. Some shorter buildings are very large in the horizontal direction and stretch for an entire block. The ORCP database does not include the two substantial buildings shown here, presumably built with generally weaker earthquake standards. (One is also on the list of 39 buildings with inferior steel frame welding. It served as the corporate headquarters of the Charles Schwab Corporation until recently. The other currently houses the main offices of the State Bar of California.)
Again, it is crucial that discussions, articles and hearings thoroughly consider all potential problems. Experts should not be allowed to cherry-pick some issues and ignore others. It is also important to understand why inferior building codes were allowed in the past (and currently) so mistakes are not repeated. A thorough analysis of all tall buildings must be conducted and published so people know the risks. Nothing should be concealed.
Bottom line: Installing a sophisticated sliding foundation (“base isolation”) system won’t help much if a building’s steel welds are coming apart. Strengthening the welds won’t do much good if the entire building is sinking. Adding pilings down to bedrock is no big deal if the building is shaking apart. All potential problems need to be considered and addressed.
A committee of the 50-year-old Coalition for San Francisco Neighborhoods, which represents neighborhood groups across the City, agrees the City must take prompt and meaningful action (April 2022 newsletter).
Time for action
Liquefaction has been a known condition for a long time. The problem of inferior welds has been recognized for almost 30 years, and it has been four years since the New York Times exposé of the problem. For decades, the City has allowed weaker standards for buildings shorter than 240 feet. City leaders show no signs of seriously considering these structural deficiencies. The first step is to acknowledge the problems, then develop solutions. Citizens should know what city officials know. The following steps are critical:
- A list of high-rise structures that do not reach bedrock must be publicized.
- A better understanding of high-rise buildings that do not rest on bedrock and descriptions of what is likely to happen during a severe earthquake in high-liquefaction zones is required.
- Corrective actions are required now. The City must formally notify tenants in the 39 buildings with welding deficiencies
- The Board of Supervisors must call hearings of Department of Building Inspection and Planning Department officials to explain how these problems developed.
- The owners of buildings with these safety risks must be required to develop and implement corrective measures.
- Strong earthquake standards also need to apply to buildings shorter than 240 feet. A 22-story building is not insignificant.
- Buildings must be closed every 25 years for refitting with the latest earthquake standards and precautions.
- The database maintained by the ORCP needs to indicate which buildings do not reach down to bedrock.
- Local news media need to give these issues more attention and should report the progress of retrofits every month.
- Buildings should be rated so that their inhabitants know the risks. For example, buildings should be penalized one point for each potential problem: in a high liquefaction zone, and the foundation does not reach bedrock, one of the 39 with inferior welds, older without retrofitting, built to weaker standards (shorter than 240 feet). Owners must post these ratings (the smaller, the better) in a highly visible location.
Questions that should be asked and answered include:
• How many of the 39 buildings known for years to have weak structural welds have been fixed?
• How many large buildings on fill with foundations that do not touch bedrock have had improvements added in recent years to address their shortcoming?
• Which large buildings are on fill and do not have foundations resting on bedrock? What is likely to happen to these during a strong earthquake?
Why are tall buildings still being built in close proximity to other tall buildings?
• Why are buildings shorter than 240 feet still being built under weaker standards? How did these deficiencies come about?
Many buildings have been largely vacant for the past two years, and many structural improvements could have occurred with minimal disruption. Ongoing work on the Millennium Tower is evidence that structural improvements can be made. According to recent reports, the Transamerica Pyramid will soon be undergoing major structural work – but the reasons for this work are unclear. Will its foundation still “float?” At least one other building owner claims to have strengthened its building. So it can be done. No building is meant to last forever, and building owners should have factored in the need for major structural upgrades every 25 years or so. Some of the vulnerable buildings are 60 years old now.
It's no exaggeration to say that inaction on these problems could create the biggest news story out of San Francisco this century. City officials cannot claim they didn't know about these problems. It will be their legacy.
David Osgood is a native of Santa Cruz and has lived on the Embarcadero for over 30 years. He worked as a lobbyist for a tech firm and became politically active working for Rep. Leon Panetta. His great-great-grandfather was a 49er. Contact Dave.