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RE: Torsion Analysis - Wind vs. Seismic

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I guess the only comment I that jumps out at me is the point of application
of the wind forces can be/should be different than the application of
seismic forces. While the application of the seismic force occurs at the
centroid of mass, the wind force is applied at the centroid of "sail area".
Based on this, eccentricities will be/should be different.


Bill Allen, S.E.
Laguna Niguel, CA

> -----Original Message-----
> From: Seaintonln(--nospam--at) [mailto:Seaintonln(--nospam--at)]
> Sent: Thursday, August 19, 1999 1:15 PM
> To: seaint(--nospam--at)
> Subject: Torsion Analysis - Wind vs. Seismic
> In flexible diaprhagm analysis (FDA) I typically analyze for
> the worst case
> condition due to wind or seismic in each line of shear. I am
> not sure how
> this is addressed in torsional analysis. It was my intent to take the
> conservative total shear to the structure at each level due to the
> combination of wind and seismic that governs and apply this
> load to the
> diaphragm in the rigid diaphragm analysis (RDA).
> In a previous discussion with another engineer I was advised
> that this was
> overly conservative and that it would also not yield correct
> distribution of
> shear by stiffness. I was advised to determine the total shear to the
> structure from either wind or seismic and apply the worst
> condition to the
> entire diaprhagm area. He also suggested that I may want to
> run the torsional
> analysis twice - once for wind and once for seismic to
> determine where the
> worst conditon will occur.
> This seems counterproductive since the purpose of the RDA is
> to determine the
> correct balance from the single force applied (wind or
> seismic). To "beef up"
> the panels were wind might govern would essentially unbalance
> the system.
> It seems that my first assumption is more valid since the
> total combined load
> would be applied at one time at a given point (point of
> applied load) which
> would yield a truer distribution of shear by torsion. The
> load, although
> combined, would be uniformly distributed by torsional
> analysis rather than
> arbitrarily increased in isolated lines of shear resistance.
> I hope I'm made this clear. Can anyone advise me what
> standards of practice
> dictate in RDA?
> Dennis S. Wish PE