Damping ratio for Gust Effect factor (ASCE 7 wind)

[smvk3]

I am trying to calculate the gust effect factor for a four-legged exterior steel portal frame that supports a large piece of mechanical equipment. The frame consists of steel moment frames in each orthogonal direction (directly welded flanges at HSS tubes) and has bolted shear connections at members that support bar grating for access around the equipment

I have calculated that the frame is flexible, which forces me to do the calculation for the gust effect factor. In the commentary of ASCE 7, it states that damping ratios of 1 to 2 percent are typical for steel framed buildings and 0.15 to .5 percent for "steel support structures for signs, chimneys, and towers."

Since this frame can't really be characterized as a "building" and also isn't as slender as "steel support structures for signs, chimneys, and towers," because it is 37' (width) x 45' (length) x 33' (height), what would be a reasonable damping ratio for this structure?

 

[JAE]

I would say it would be closer to the 0.15% to 0.5% since it sounds like it is a pure steel frame with no other components, walls, or misc. items attached to dampen it.

Alternatively, you could do a parametric study using both ends of the range to see how sensitive the results are to the damping ration - design for the worst, etc.


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[smvk3]

JAE,

To calculate the natural period of this frame I used the formula T = 2*Pi*SQRT.(m/k) where m is the weight (assumed to be lumped at the top of the frame) and k is the stiffness of the frame (which I calculated by applying a "dummy" load and seeing how much the frame would deflect).

Do you think this is a valid way of calculating the natural period for the gust effect calculation?

My software doesn't have the capability to do this.

 

[BUGGAR]

I agree with JAE in bracketing the range. Sometimes it matters, sometimes not so much.