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Re: Table 16-O - Horizontal Force Factors, ap and Rp

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In a message dated 8/26/99 4:02:33 PM EST, jmo_engineering(--nospam--at)email.msn.com 
writes:

<< On one of our jobs, we're installing some air conditioning package units on
 top of a typical panelized roof.
 
 We used a value of 1.0 for ap as required by 3.B in Table 16-O (page 2-33).
 
 We received a plan check comment indicating that the value of ap should be
 2.5 because the attachment of the unit is below its center of mass as is
 required by 3C.  If I use the 2.5 value, I get a value which is greater than
 that required for emergency power systems (3.D) and containers with
 flammable hazardous materials (3.E).
 
 Both parts of the code appear to apply?  Can anyone clarify when 1.0 or 2.5
 are truly intended to be used.
 
 Thanks in advance.
 
 Joseph M. Otto, PE
 Ireland Engineering
 Fremont, CA
  >>

I also have been designing equipment attachments to building roofs lately.

I believe the ap value = 1.0 applies when the anchorage does not occur below 
the center of mass of the object (example: a tank or piece of equipment being 
strapped to a wall where anchorage points occur both above and below the 
center of mass).  In a the case where the attachments only occur at the base 
of the equipment (below the center of gravity), regardless of what it is 
(Table 16-0, sections 3A,3B,3D and 3E)  then you have to use the ap value = 
2.5.  

Typically most A/C units sit on a curb, therefore there attachment is below 
the center of mass.  One thing to look out for is Rp footnote #14 which 
requires Rp = 1.5 when you sit the equipment on spring isolators.  If the A/C 
unit, or other equipment sits on a frame supported by isolators (example: 
cooling towers), then you are supposed to use Rp = 1.5. 

If Rp = 1.5, then equation 32-1 (4.0 Ca Ip Wp) will likely be the equation 
which you can use to limit the design force.  Note that when equipment is on 
the roof, equation 32-2  becomes (ap Ca Ip/Ra) (1 + 3(1/1))Wp  which reduces 
to (4.0 ap Ca Ip Wp/Ra) and the ratio of ap/Ra can be greater than 1.0 
(example: 2.5/1.5) when you are on spring isolators.  Using the 1997 UBC, I 
have been having to design the attachments and steel supporting platforms for 
around 1.6 G for base attached equipment sitting on a spring isolated pads.

As a side note regarding cooling towers, you may want to provide the cooling 
tower manufacturer with the building site specific earthquake data (Ca, Z, 
ap, Ra) so they can calculate the overturning forces and shears for you.  If 
they give you the center of mass location you can do the calculation, but the 
manufacturer can probably give you a more reliable number based upon how the 
internal loads are distributed inside the unit and frame strength to transfer 
loads to the equipment support points.

The typical manufacturer equipment cut sheets showing preapproved seismic 
design values I believe are no longer valid using the 1997 UBC.

Michael Cochran S.E.