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RE: Seismic loads on mechanical and electrical components

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The design criteria is meant to be all-encompassing. I have though excluded chimneys and tanks. Worst case scenario for mechanical or electrical components, acc. to ASCE 7-95, table,  is thus when ap/Rp equals 2.5/3.0 (stacks, panelboards, etc). In my case Ca = 0.3 and Ip = 1.0, so the design equation reduces to

	Fp = ap/Rp*Ca*Ip*[1 + 3*x/h]*Wp = 0.25*[1 + 3*x/h]*Wp
	Fp = 0.25*Wp	at grade level (using x = level of attachment)
	Fp = 1.0*Wp		at roof level (x = h)

This reminds me of the reason I wrote in the first place  8-)

Do any of you guys have the 1994 NEHRP Provisions (FEMA 222) at hand to check if it is an error in ASCE 7-95 when it says that Fp shall be applied both vertically and laterally. As mentioned before; UBC94, UBC97, 1997 NEHRP (FEMA 302) and 2000 NEHRP (FEMA 368) all say that Fp shall only be applied horizontally.

Gunnar Hafsteinn Isleifsson

-----Original Message-----
From: Gerard Madden [mailto:gmadden(--nospam--at)]
Sent: Thursday, December 06, 2001 01:36
To: 'seaint(--nospam--at)'
Subject: RE: Seismic loads on mechanical and electrical components

The 2.5 ap is for flexible equipment. Do you really have that? If you are on
springs, vibration isolators, or inertia pads then yes. Is your equipment
tall and slender and able to wobble? I would venture to guess that most
equipment is rigid if it comes pre-assembled from a manufacturer and
contained within sheet metal walls on a base frame.

Otherwise, ap =1.0 and Fp=1/1.5*Ca*Ip*Wp=0.67CaIpWp

Otherwise, your statement is accurate.


-----Original Message-----
From: Gunnar Hafsteinn Isleifsson [mailto:gunnarhi(--nospam--at)]
Sent: Wednesday, December 05, 2001 4:05 PM
To: seaint(--nospam--at)
Subject: RE: Seismic loads on mechanical and electrical components

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)

Gunnar Hafsteinn Isleifsson


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