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RE: Seismic loads on mechanical and electrical components

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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,

-----Original Message-----
From: Gunnar Hafsteinn Isleifsson [mailto:gunnarhi(--nospam--at)]
Sent: Wednesday, December 05, 2001 12:15 PM
To: seaint(--nospam--at)
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

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 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

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.

Gunnar Hafsteinn Isleifsson

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