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RE: ASCE 7-98 Wind Pressure Questions

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One thing that hasn't been mentioned yet, the IBC places a 1.3 load factor
on the ASCE wind forces for ASD.  Maybe it the intent was to fix both load
conditions (LRFD/ASD), but somehow it got missed in ASCE 7.

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
Sent: Tuesday, November 05, 2002 8:27 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: ASCE 7-98 Wind Pressure Questions


>Note that ASCE 7-98 Commentary, Section
>C2.3.2 indicates (in the middle of
>the 3rd paragraph page 219), "This
>directionality effect was not taken into
>account in ASD".  This is stated in
>the context of how the directionality
>factor was applied in ASCE 7-95.  This
>would explain why the ASD load
>combinations (of 7-98) did not change
>in a similar fashion as the strength
>design load combinations of 7-98 (i.e.,
>increased by 1/0.85).  Therefore,
>the design wind pressures of ASCE 7-95
>used for ASD would result in a more
>conservative design unless a Kd factor
>of 1.0 is used in the ASCE 7-98
>velocity pressure equation.

You've established the crux of my issue. Either ASCE 7-95 was conservative
for ASD wind loads or an error has been made that has reduced the wind loads
on buildings in the service-load combinations. I fear it is the latter
because...

We know that ASD and LRFD generally result in design strengths that satisfy
the relationship

    LRFD design strength = 1.5 * ASD allowable strength

And for all other loads, we know (again in general) that

    LRFD loads = 1.5 * ASD loads

Alternatively, if it is the formar, let's put the LRFD wind load factor back
to 1.3 and call it a day.

Charlie







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