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[SEAOC] Re: [SEAOC] Seismic retrofit criteria[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: kent_estes(--nospam--at)wdi.disney.com
- Subject: [SEAOC] Re: [SEAOC] Seismic retrofit criteria
- From: IteUrsi(--nospam--at)aol.com
- Date: Thu, 19 Dec 1996 19:32:27 -0500
- Cc: seaoc(--nospam--at)seaoc.org
>My question is, what are the latest criteria when dealing with modifications to an >existing building that triggers a seismic upgrade? To get even more specific, does >shifting an existing brace to an adjacent bay necessitate a seismic upgrade of the >whole building? If the same brace is simply relocated on the same column line for >an interior remodel, I would think not. If the use of a building is altered such that >new and more mechanical units are required, does this increase in mass (and >possibly new mechanical floor area inside a building) require a upgrade >to the latest code? Does a change in occupancy trigger a seismic >upgrade? What is the latest thinking? Specific criteria from building officials would >be interesting also. Ken Estes: The basic requirement I and many building officials start with is modeled on the Hippocratic Oath: "Above all else, do no harm." If only a few component members or walls are to be modified, removed or relocated, the engineer needs to evaluate how that will affect the loads and stresses on the reconfigured de facto lateral force-resisting components or system, and on changes in the dynamic behavior of the structure. Then provide a package of mitigations to offset any harm. When the magnitude of the work exceeds some level, compliance with a recognized or local retrofit standard will be triggered. But it is sometimes difficult to draw a bright line in the sand for that level, and the engineer and the building official must then engage in give and take on project-specific situations. I've in the past agreed to something like your "75% solution" when it appeared to be a best-faith attempt for the circumstances, and getting the last 25% would raise costs by another 100%. In 1971, I wrote San Francisco's notable (or infamous, depending on one's perspective) Section 104.F, with input from the SEAONC seismology committee. One trigger was adding a story or significantly altering the mass of the structure such that the base shears and loads to the existing de facto lateral force resisting components were increased. A horizontal addition also would be a trigger unless the two parts were seismically isolated from each other. Another was any change of occupancy or use that increased the calculated occupant load by more than 10 people (based on then Table 33-A). A third trigger was performing structural work or alterations that affected more than one-third of the total floor area, cumulative from the original construction and occupancy. And a fourth trigger was extensive remodeling/refurbishment of a non-structural nature to two-thirds of the floors of a building, even if no change of use or occupancy occurred. This included such work as hvac and electrical installations/upgrades, suspended ceilings, non-bearing partitions, elevators, storefronts, lighting fixtures, floor/wall covering, etc. The rationale behind the triggers generally are obvious and remain valid today, and the section has changed little over the years except for some clarifications. The rationale for the fourth trigger may not be as obvious, and was based on the following thinking: At any point in the lifecycle of a building that is seismically deficient, the building exposes the public to a "quantity of risk" that is a function of the magnitude of the deficiencies, and the remaining economic useful life of the building in its current state. While it is difficult to reliably establish values for these two parameters, it is reasonable to assume that the economic useful life of the building will be extended if the owner invests significantly in alteration or refurbishment work. If the work weren't performed due to high seismic upgrade costs, the building would reach the end of its lifecycle sooner, be demolished, and replaced by a seismically complying structure. If the seismic deficiencies weren't reduced to offset the extended lifecycle, then the total risk to the public would be increased. To maintain a financially level playing field for owners of seismically-complying buildings, and to not increase the quantity of risk for the public, I felt it was good public policy that such capital investments in a structure should be a trigger. It wasn't popular, and many folks had reservations about it, including a number of SEAONC members. However, this was at a time just before Nixon and Watergate became 80-point headlines on the front pages, and the "question authority" bumpersticker mindset was not yet in full force among citizens. The trigger reflects a bit of a Father-Knows-Best attitude, but did get codified and remains in effect in SF. In hindsight, and coincident with the implications from the CDMG/USGS maps released today to the media, that was good policy. Franklin Lew, SE Building Official ...
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