huangyhg

flexible structures damping ratios
there seems to be a gross lack of good data that provides guidance for what damping ratio to use for tall, flexible structures. i'm talking about 100-150 ft monopoles with natural frequencies in the range of 0.3 to 0.9.
some sources mention using values of 0.01 for steel poles and 0.02 for concrete without any real reference as to why. what i do know is that the gust response factors required in asce 7 can vary all over the map, depending on which value is used.
for example, a tall slender steel stand-alone monopole (meaning no long span wires attached to it to help the damping effect) of a given mass and a fn =0.4 can produce a gust response of 1.43 using a damping ratio of 0.01. merely changing that ratio to 0.02 will reduce the g factor to 1.20. since this directly affects the resultant wind pressure to be used, it seems like asce 7 has provided a lot of sophisticated tools with which to fine tune a wind load analysis, but then leaves you totally in the dark as to how to apply some of them.
anybody out there got a better feel for the logic behind these ratios, or is the solution to just possibly be stuck with overdesign structures by 20%?
no, the solution is to go and belt a similar structure with a hammer and measure the decay rate.
it does vary a great deal, depending on the terminations and joint types.
cheers
greg locock

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