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

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I would suggest that anyone interested in this topic should review chapter 6
of the 1997 NEHRP Provisions and the Commentary.  

Rick Drake is being a bit modest by not mentioning that he and Bob Bachman
did a ton of work on this topic.  It is well presented in the NEHRP

Harold Sprague
The Neenan Company

-----Original Message-----
From: Rick.Drake(--nospam--at) [mailto:Rick.Drake(--nospam--at)]
Sent: Monday, August 30, 1999 8:01 AM
To: From the desk of Joe Otto
Cc: Seaint@Seaint. Org
Subject: Re: Table 16-O - Horizontal Force Factors, ap and Rp

The a-sub-p factor in the 1997 UBC represents the dynamic amplification
relating the dynamic properties of the component to that of the structure.
Knowing the fundamental period of both the component and the structure, this
value can be exactly determined using basic dynamic principles.

However, since this value is not practical, a-sub-p has been assigned a
based on whether the component is either "rigid" or "Flexible", a-sub-p of
and 2,5 respectively.  This is analogous to the 1994 UBC method of
C-sub-p times 2 for flexible items.

If your equipment is rigid, and not on vibration isolators (see footnote
a-sub-p of 1.0 is appropriate.  If not, a-sub-p of 2.5 is appropriate.

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA


From: "From the desk of Joe Otto" <jmo_engineering(--nospam--at)> AT fdinet
      08/26/99 02:01 PM EDT

To:   "Seaint@Seaint. Org" <seaint(--nospam--at)> AT fdinet@ccMTA-fdlncta10
cc:    (bcc: Rick Drake/AV/FD/FluorCorp)

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

On one of our jobs, we're installing some air conditioning package units on
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
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
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
truly intended to be used.

Thanks in advance.

Joseph M. Otto, PE
Ireland Engineering
Fremont, CA