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Re: canopy wind

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I have noticed in few codes only wind up case is mentioned for open canopies in pressure coefficients.
and also some additional coefficients for facia if provided in canopies.
Do you think canopies designed for wind down case also.
i.e DL+LL
Is Third combinations is important ?? As i notoced few consultants design only for first and 2nd case.As coeffiecients for case 3 are not mentioned.
Thanks in advance for ur guidance
----- Original Message -----
Sent: Thursday, January 27, 2005 10:25 PM
Subject: Re: canopy wind

Is it really about pressure tables, or are the failures you are seeing the result of dynamic amplification?  I would suspect that these types of structures have very little inherent damping...far less than the 5% usually "assumed".  The increase in deflection between a 5% damped structure and a 0.5% damped structure (those are the normally accepted bounds for structures without intentional damping). The amplification due to a driving source at the resonant frequency is (2xDamping Ratio)^-1, so in this case you're looking at a 10-fold increase in deflection at 5%, and a 100-fold increase at 0.5%.  Those are mighty big numbers when compared to a complete lack of dynamic amplification accounted for in the applied wind loads in ASCE7. 

The key is that these are unstable structures.  If you think of a flat roof on a cantilevered pole, as the deflection of the pole increases, the wind area increases, and becomes a flat airfoil with a positive angle of attack.  This increases the deflection until the pole/foundation fails or the wind subsides a bit. Then the stored energy in the pole accelerates the canopy back to vertical, reducing the angle of attack, and hence the windforce.  In the worst case, the canopy passes vertical due to inertia and the angle of attack goes to negative. While this increases the restoring force, it is not quite as great, as the vertical component of the windforce on the canopy is negative. Think of a sinusoidal driving function, take the absolute value, then add 10% of the same sinusoid (+ an -) to the function.  I'm not going to tru and do it in ascii, but you get a high-bump, low-bump, high bump pattern.

I did a small refueling station a couple of years ago and settled on a stress increase factor of 5, figuring that the driving frequency was actually not sinusoidal, and wouldn't sustain long enough to really attain maximum deflection. It was small, so I could get away with the extra steel, and it also was not in a coastal area (though we do get the remnants of hurricanes every couple of years that top 70MPH.)


At 09:35 AM 1/27/2005 -0500, you wrote:
I guess by research I meant I would like to see more wind tunnel testing to develop better pressure tables for those types of structures. But I can tell you from my own casual observation that many gast station type canopies did fail in bending or buckling failure at the base of the column.
Andrew Kester, PE
Structural Engineering Consultant
Lake Mary, FL

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