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

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The a-sub-p factor in the 1997 UBC represents the dynamic amplification factor,
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 value
based on whether the component is either "rigid" or "Flexible", a-sub-p of 1.0
and 2,5 respectively.  This is analogous to the 1994 UBC method of multiplying
C-sub-p times 2 for flexible items.

If your equipment is rigid, and not on vibration isolators (see footnote 14),
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 on
      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 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