This evening, fifteen BTH residents attended a presentation and lively discussion about the process of screening our own building for seismic risks, the results, and the implications. The issue of seismic vulnerability at BTH arises from the fact that it is about 50 years old, predating major additions to seismic knowledge, and it is within a mile of the Hayward Fault, whose next major earthquake is expected to be disastrous.
The slides from my presentation summarize the history of arguments about this building’s seismic strength, then describe two methods of do-it-yourself seismic screening of reinforced concrete buildings, which I recently applied to BTH. FEMA 154 (“Rapid Visual Screening”, or RVS) is a simple method promoted by FEMA. P25 is a more complex rapid-screening method developed by academic seismic engineers in Turkey and Italy.
These two methods generally agree for BTH. The scores that they assign to BTH are high enough that buildings with these scores almost never collapse in major earthquakes. The standard recommendations from both methods are that buildings with such scores are not at enough risk of collapse to justify further expert seismic evaluation. Buildings with BTH’s score on the P25 method have often suffered moderate or slight damage, and often have remained undamaged, while rarely suffering heavy damage. The P25 method suggests we be prepared for damages costing up to $1.5 million to repair.
There are sources of uncertainty about these conclusions, including (1) the inherent crudeness of such empirical, statistically based methods, (2) special sources of strength and weakness in the design of BTH that may make it atypical, and (3) errors in judgment or calculation that I may have made in my application of these methods.
I have communicated with four highly reputable seismic engineers over the last few months about work that they might be able to do to help BTH assess its seismic risks. I don’t recall any of them suggesting these or similar do-it-yourself methods to help us decide whether to retain expert consulting services. In one case I raised this subject and the engineer’s reply dismissed all such methods as nearly uninformative and largely a waste of time. However, it is also reputable seismic engineers who have developed and are advocating the use of such methods. Thus, there appears to be dissensus in the profession, and we are left to decide for ourselves.