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- To: seaint(--nospam--at)seaint.org
- Subject: Code min. Design requirements
- From: Lynn <lhoward(--nospam--at)silcom.com>
- Date: Sun, 01 Aug 1999 18:58:30 -0700
As a result of recent dialogues that I have been involved in on this list, I think I would like to open a new topic that has been mentioned, but not specifically dealt with. What standard of performance do we expect from structures that are designed and constructed in conformance with the 1997 UBC? I believe that most of the engineering design community believes that a building designed and constructed in accordance with current Code only provides a minimum Life-Safety level of performance. The Code itself states the following: UBC 1626.1 Purpose. The purpose of the earthquake provisions herein is primarily to safeguard against major structural failures and loss of life, not to limit or maintain function. This section of the 1997 UBC would indicate that the authors of the Code also believe that a Code complying building will only provide a minimal life-safety level of performance. The underlying message is; if your design and or construction fall short of the standards in the 1997 UBC, then the life or safety of the persons occupying the building is in danger. There is a problem with this view, in that there are standards put out in recent years for engineers to use in the evaluation of existing structures. The process of evaluation of existing structures is something that the UBC was not intended for. These documents are FEMA 273 or the NEHRP guidelines and FEMA 310. FEMA 310 defines a life-safety level of performance as follows: Building performance that includes significant damage to both structural and non-structural components during a design earthquake, though at least some margin against either partial or total collapse remains. Injuries may occur, but the level of risk for life threatening injury and entrapment is low. This sounds somewhat familiar to the wording used in the UBC. FEMA 273 gets more detailed, and in fact, in my opinion, starts confusing the issue. Among other levels of performance, they have the following levels of performance that pertain to this discussion: Life Safety: Structure remains stable and has significant reserve capacity; hazardous nonstructural damage is controlled. Limited Safety: Extends between life Safety and Collapse prevention. Collapse prevention: The building remains standing, but only barely; any other damage or loss is acceptable. FEMA 273 collapse prevention sounds like the UBC and FEMA 310 Life Safety definition. My main problem is this, FEMA 310 states that wood light framed structures (residential, commercial, and industrial) designed and constructed in accordance with the 1976 UBC will provide a Life Safety level of performance. This includes multi-story wood framed buildings, as well as building utilizing shear walls constructed with stucco and drywall. As we all know, there is a VAST difference in design requirements between these two Codes. Yet the 1997 UBC also seems to state that design to the current Code standards will only provide a life safety level of performance. So which is it? Whom are we to believe. Both of these documents are fairly new, and had major updates to them as a result of the 1994 Northridge earthquake. I guess there are really 3 questions: 1. What level of performance should the Code require as a bare minimum for seismic design? 2. What is the definition of Life Safety we want to use, UBC and FEMA 310, or FEMA 273, (which I believe will be more in line with the new year 2000 IBC)? 3. With the definition of Life Safety being very close for the UBC and FEMA 310, why are they MILES apart on the level of design that is required for these buildings? All input is appreciate, as I am struggling to understand all this myself. Lynn
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