Wind Gust Factor for Flexible Buildings

[woody1235]

Thank you for looking at this question!

We are trying to repair an existing wood framed picnic shelter canopy that was damaged by wind. It uses wood trusses supported by GL beams supported by GL columns that have corner braces. Unfortunately, it is located close to an escarpment, so we have a 1.4 Kzt factor.

I also fear we have a very large Gust factor. I consider this to be a flexible structure, since the wood columns have no lateral stiffness, other than the bracing at the upper corners. I estimated the natural frequency to be about 0.20 Hz by considering the stiffness of the columns (combined) based on a cantilever with a mass at the ends (the column bases are pinned, not fixed, but the bracing acts to add some moment fixity at the column tops, I think)

Using a wind pressure program, rather than grinding through all the flexible Gust factor equations by hand, we are getting a Gust factor of about 2.0 (using MECAWind program)

The resulting wind pressures are huge, using ASCE7-05 for open structures (I'm considering this to be "obstructed" since we have no control over what might be placed under this canopy) 100 mph (C) x 1.4 Kzt x 2.0 G, etc

Does a Gust factor of about 2 seem realistic? or am I likely to be way off base here. ( I have not come up with more than about 1.2 or 1.3 in past, but different structures, so I have no reference experience) With the resulting uplift pressures, nothing about the existing framing is working; truss holdowns, support beams, (although in good shape currently - it was the columns that failed in wind)

Sorry for the long winded message - just after a reality check.

 

[JAE]

I don't have all your exact geometry but I got something on the order of Gf = 0.93 based on Exposure C and using ASCE 7, Section 6.5.8.2.

 

[woodman88]

Do you know which building code it was originally design under? How far off are your numbers (by 3, 2, 1.4) vs the original design for the truss holdowns, beams, connections? If its 1.4 it may be that the original design did not include the escarpment factor.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[woody1235]

Thanks for the input. I think the Gf is some number other than the 0.93. It's that value times a lot of other factors that relate to the natural frequency, which I calculate to be quite low.

As for the original design code, I'm not convinced it was properly designed to any code. Clearly, some design thought went into it - the roof is intact, in good condition, and is well constructed, but our client has no record of any permit or design info. We can probably repair it if we were just over by the 1.4 Kzt, but that and the 2.0 G factor I have right now is killing us. Thus the reality check for the 2.0 Gust