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RE: Seismic loads on mechanical and electrical components
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- Subject: RE: Seismic loads on mechanical and electrical components
- From: "Gunnar Hafsteinn Isleifsson" <gunnarhi(--nospam--at)post4.tele.dk>
- Date: Thu, 6 Dec 2001 01:04:38 +0100
I don't have the UBC97 at hand at the moment, but I seem to recall that the worst case ratio of ap to Rp equals 2.5/1.5 which results, at base level, in Fp = 1.67*Ca*Ip*Wp or far higher than the minimum base shear. Whether to use height of CG or height of attachment point would also have a big impact on the seismic force, especially for a tall component. The UBC97 says that this force shall be used in LRFD or ASD load combinations with "rho" equal to one. I understand this so that the seismic effects, E = rho*Eh + Ev, shall be calculated as Horizontal effects: Eh = Fp Vertical effects: Ev = 0.5*Ca*Ip*Wp (for LRFD, can be taken as zero in ASD) Regards, Gunnar Hafsteinn Isleifsson Denmark -----Original Message----- From: Gerard Madden [mailto:gmadden(--nospam--at)duquette-eng.com] Sent: Wednesday, December 05, 2001 21:35 To: 'seaint(--nospam--at)seaint.org' Subject: RE: Seismic loads on mechanical and electrical components Gunnar, I can speak intelligently only in regard to the 1997 UBC values... In your example, the h in the equation is roof height with respect to grade. The ratio of x/h (height of CG or Attachment point)/(Roof Height) is the way to calculate it. If you are on the ground or outside of a structure, the minimum base shear will govern which is Fp = 0.7CaIpWp AS far as vertical components of seismic forces, the UBC says only horizontal. However, OSHPD (Agency in California that Hospitals standards are designed to) utilize 1/3 of the lateral force as a concurrent vertical load. (ie. Fv=1/3Fp) Hope that helps, -gerard SJ, CA -----Original Message----- From: Gunnar Hafsteinn Isleifsson [mailto:gunnarhi(--nospam--at)post4.tele.dk] Sent: Wednesday, December 05, 2001 12:15 PM To: seaint(--nospam--at)seaint.org Subject: Seismic loads on mechanical and electrical components We have been asked by one of our clients to make design specification for seismic loads on mechanical and electrical components/equipment, based on either ANSI/ASCE 7-95 or UBC97, and choosing the one that results in higher loads. I have been comparing the two, and there is one thing that strikes me as very odd. The method for determining the seismic force Fp (ASCE 7-95, section 9.3.1.3 and UBC97, section 1632.2) is basically the same in both standards, but ASCE 7-95 states that; "The force (Fp) shall be applied independently vertically, longitudinally, and laterally..." whereas UBC97 says that; "Forces shall be applied in the horizontal directions..." Earthquake loads in ANSI/ASCE 7-95 are based on the 1994 edition of the NEHRP provisions (FEMA 222), which I do not have access to, and have been unable to find on the Internet, but both the 1997 edition (FEMA 302) and the 2000 edition (FEMA 368) say that; "The force (Fp) shall be applied independently longitudinally and laterally..." The UBC, 1994 edition is identical to the 97 edition wrt applicable directions of seismic forces. So, finally, my question. Is this an error in ASCE 7-95, that the seismic force Fp shall be applied vertically. I know that UBC97, and the 1997 and 2000 editions of NEHRP, include vertical seismic effects, but the magnitude of those effects is nowhere near the magnitude of the seismic force, Fp. One other thing. For all "normal" structure periods, the seismic force in both standards can be found with the equation Fp = ap/Rp*Ca*Ip*[1 + 3*x/h]*Wp ASCE defines "x" as "Elevation in structure of center of gravity of component relative to grade elevation" UBC97 defines "x" as "Element or component attachment elevation with respect to grade". As far as I can see this means, to take an example using ASCE 7-95, that for a component of uniform density, standing at base level, with an height hp equal to the structure height, i.e. Elevation of center of gravity "x" = 1/2*hp = 1/2*h then the part of the equation above, enclosed in brackets, equals 2.5 In UBC97 "x" would be zero (attachment at base level) and the bracket part of the equation equals unity. Using ASCE 7-95 thus results in seismic loads that are 2.5 times higher than those in UBC97. Is this correct or am I misinterpretating something here, and how then shall this equation be applied to, for example, a transformer standing on its own baseplate outside the structure. Regards, Gunnar Hafsteinn Isleifsson Denmark ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. 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