Resolution No. 4814 - Laserfiche WebLink

Home Browse Search

Eugene City Hall R32.19.04 Rev. B. September 9, 2003 3.0 Prior Seismic Evaluations The City of Eugene has had three seismic vulnerability studies performed on the City Hall building over the past eight years. The first analysis was by Dames and Moore (ref. Dames and Moore 1995); the second by Berry Architects (ref. Berry Architects 1995); and the latest by URS (ref. URS 2001). The following paragraphs highlight the key findings from those reports. o The 1995 Dames and Moore report (ref. 1995a) predicted earthquake-induced losses for the City Hall complex using ground motions measured by the Modified Mercalli Intensity scale (MMI) and loss models from ATC-13. The buildings were characterized as reinforced concrete structures. Three deterministic scenario earthquakes were considered for seismic loss estimates: Cascadia Subduction Zone (CSZ) Interplate M 8.5 (PGA = 0.27g), CSZ Intraplate M 7.5 (PGA = 0.16g) and a local random crustal M 6.0 event (PGA = 0.20g). The projected losses for the City Hall Complex were 100%, 13% and 22%, for these three earthquakes, respectively. Note that Dames and Moore assumed that a loss requiring repairs costing 25% or more of the building replacement value would in fact be totally replaced, thus leading to the 100% loss estimate for the CSZ M 8.5 events. o The 1995 Berry Architects report (ref. 1995b) evaluated the City Hall complex using he FEMA 178 checklist methods coupled with a linear static-force analysis of the structure by Kanda and Tso Associates. The K&T analysis used the following general method: Site PGA = 0.05g; V = 0.045W; consideration of torsion and relative stiffness of all walls and piers. The K&T analysis made the very conservative assumption that essentially all the existing masonry walls in the building were unreinforced and ungrouted. Given these assumptions, the K&T analysis showed that some wall elements were as much as 20 times overloaded as compared to nominal code allowables. o For the current effort, we make the observation that the findings in the K&T analysis are not realistic. Even for a building designed in 1962, there would have been some provision for lateral loads, whether earthquake (about V=0.05W) or wind (at least V = 0.02W or so). K&T's assumptions about the masonry walls is based on lack of drawings, not a confirmed knowledge. If in fact the walls are unreinforced / unconnected and ungrouted, then this building should "probably" have failed the original plan check reviews / structural engineer's approval. c> The K&T analysis has a second major assumption, in that the as-is analysis is for a level of ground shaking of just PGA = 0.05g. Even in 1995, when K&T performed their analysis, it was widely recognized that Eugene could be exposed to PGA = 0.2 to 0.3g given a Cascadia Subduction Zone event. G&E Engineering Systems Inc. Page 23