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RE: re/ Fp Values

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The connection force at the parapet is based on the Fp detmined from the
height of attachment to the diaphragm for wall anchorage purposes only
(i.e. hx=hr). Therefore, the connection does not need to be designed to
resist two different force criteria, one above the roof and one below.

The wall design however, must meet each force criteria. (i.e. rebar in
the tilt-up panel or cmu wall)

In regards to the hx definition, it is extremely vague. It is geared for
wall anchorage when it uses the verbage of "component attachment
elevation with respect to grade". For wall anchorage there is not much
debate, hx=hr. For wall design there is the trapezoidal method everyone
is talking about. I use the wall midheight between the F.F. and Top of
wall as my hx term. I have not compared this to the average of the
values at the top & bottom, but this is the method used in the book
Seismic Design of Building and Bridges to the 1997, authored by the Head
Structural Engineer at the State of California, and is the recommended
text in the back of the 1997 Seismic Design Manual Vol I. The author
(Alan Williams, PhD & S.E.) defines hx as the "average height of the
element above grade".

This is my opinion & design method, do as you please. I have not
received any plan check comments challenging this yet, but if I do, I'll
say "the State Head Honcho does it this way."

Gerard Madden, P.E.
Civil Engineer, Associate
CRJ Associates, Inc.
email: gerardm(--nospam--at)
tel: 650.324.0691
fax: 650.324.0927

> -----Original Message-----
> From:	Charles Greenlaw [SMTP:cgreenlaw(--nospam--at)]
> Sent:	Thursday, September 16, 1999 11:46 AM
> To:	seaint(--nospam--at)
> Subject:	Re: re/ Fp Values
> Whoever is going to answer this question, what about the extra
> connection
> force at roof level due to the extra lateral force given the
> cantilever
> parapet of this same tilt-up wall: 
> Is the additional lateral load on the parapet, just because it is a
> parapet,
> used in lieu of the force it would have been assigned the wall at that
> roof
> diaphragm level, and if so, is the wall's anchorage design load
> increased
> accordingly, as rational use of principles of mechanics would
> indicate? 
> And is the parapet's cantilever moment effect at the next level down,
> which
> acts opposite to the wall's local tributary connector design load,
> properly
> subtractive from that connector design load down there?
> May the negative cantilever parapet moment be used to reduce the
> positive
> moment in the wall between diaphragm levels below? If so, is this
> parapet
> negative moment taken as the special large one, or a lesser one based
> on the
> Fp value for the wall that this thread seeks to clarify, or yet
> something
> else?  
> Lastly, from what established authority's chapter and verse do the
> answers
> find support?
> Charles O. Greenlaw SE   Sacramento CA
> >Rick,
> >
> >Many people have been stumbling over the right seismic force to use
> for a
> >building component that spans between elevations.  The 1997 UBC is
> unclear,
> >in this case, about the proper definition of h sub x.  Outside of the
> Blue
> >Book commentary, there have been recommendations that a trapezoidal
> >distribution or an average of the top and bottom values be used.  In
> >researching this issue, I found that in the 1997 NEHRP provisions,
> the
> >variable z (same as h sub x) is defined as the highest point of
> attachment. 
> >
> >My question is; should this be the correct interpretation?  And what
> about
> >the 2 vs. 3 difference?
> >
> >The answer could significantly affect design of tilt up or CMU wall
> panels.
> >
> >TIA,
> >
> >Curt La Count
> >Jacobs Engineering
> >Portland, OR
> >