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Seismic loads on mechanical and electrical components

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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 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.

Gunnar Hafsteinn Isleifsson

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