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RE: Roof height for A-Frame

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It's an interesting question.  For a dynamic analysis, period = 2*pi / natural circular frequency, which is the square root of (stiffness / mass).  So for "regular" buildings you would model in 2 directions w/ mass lumped at the diaphragm levels.  This is because the natural circular frequency is calculated where the stiffness is calculated: stiffness = force*displacement, and displacement is storey drift.  For gable walls I would still calc storey drift at plate level, to make it easier.

So what about the "approximate fundamental period" method of ASCE7 12.8.2.1?  The way it is written is confusing: "...h.n is the height in ft above the base to the highest level of the structure..."  At first reading one would think it's to the top of the building, but it really makes no sense unless you read it with an emphasis on "level", i.e. highest diaphragm level.  For the Equivalent Lateral Force Procedure, period is in the denominator of the response coefficient, so taking T larger by using larger h.n results in lower design forces (unconservative).


regards,
Gordon Goodell



-----Original Message-----
From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com] 
Sent: Tuesday, November 11, 2008 7:09 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Roof height for A-Frame

So what roof level should be used when calculating the period of the
building for wind or seismic loads? I am trying to think about it
behaving dynamically with the modes of freedom it would have along
either axis. Could the gable roof building have different periods in
each direction?



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