A President's Message From SEAOC President Rafael Sabelli
Tuesday, January 28, 2020
By Rafael Sabelli, SE, SEAOC President 2020
In this month’s column I would like to begin a discussion concerning seismic design. Specifically, I would like to address our relationship as structural engineers to the loads we are required to design. As some may already know, the seismic-design base shears obtained using ASCE 7 2016 for many sites are significantly larger than those computed using ASCE 7 2010. The issue for this month’s column is what it means to structural engineers to have these changes.
There are several factors involved in computing the base shear, but one of the most significant relates to the spectral shape for certain soft-soil sites. (See ASCE 7 2016 Section 11.4.7—yes, I am getting that technical—which requires a site response analysis for many cases, unless the period-dependent portion of the design spectrum is increased substantially. Such a site response analysis would address the effect in this period range and would presumably result in a similar, although perhaps smaller, increase.) This change represents our (that is, the profession’s) collective best understanding of expected ground motions and what it takes to for a structure to have sufficient resistance to perform adequately. Such understandings evolve with more information, including seismological research, earthquake reconnaissance, and seismic-response analysis.
For buildings yet to be designed, the change may mean larger members than we have been used to. This may take us aback, but each of our designs is different, and (by definition) each of our designs is engineered. Of course, many buildings are similar, and some of us will be explaining to architects and owners why the new building can’t have quite as many windows as a previous, similar building design.
Changes in design standards do not invalidate our previous work, although increases in requirements do make designs that were (only just) adequate for previous code editions “substandard” with respect to new ones. This may make for interesting conversations with building owners, especially when changes to facilities require revisiting the seismic design. It is one thing to explain that the design was based on a ground motion with a return period of 1000 years, and it is now anticipated that that ground motion actually has a return period of 800 years and thus the (somewhat obscure) risk is larger than anticipated. It is quite another to explain that there are real costs required to upgrade a building to meet a new standard when that building is only a few years old.
Structural engineers always have to consider future conditions in design. As climate change becomes more apparent, we have started to discuss such future conditions as sea-level rise, changing water tables (including due to depletion), loss of permafrost (not in California, of course), fires, and even human-induced ground motions. Perhaps we need to add to this list unknown changes in our understanding of seismicity, at least for certain owners who need to maintain current compliance with some standard. Do we recommend “future-proofing” designs by building in margins for larger loads? Do we recommend that institutional owners use something more steady than “current code” as the basis of their performance targets? (And is there a Prop 13 for seismic design, us to keep using the same building code until retirement?) The alternative to anticipating change and dealing with the associated risk and consequences is to decide not to learn anything new, or not put that knowledge into effect.