Wind Turbine Structural Design

[VYordan]

I am checking a wind turbine design which should comply w/ ASCE 7-05, Chapter 15. This type of structure does not correspond fully to the table that has the R factors for different types of structures. It could be 1 or 2 depending in how one interprets
the type description. If more than 50% of the mass is at the top it should be 1, but in this case, it is not.

My question is regarding the OS factor omega. The designer intends to use it for the whole structure in the lateral force calculation for the entire structure. I always thought this OS factor was to applied only to designated components described in the code.

I can see to use an OS factor of 2 in the design around the shell opening at the bottom, etc, but not to calculate the total seismic shear.

In a Wind Turbine Seismic design paper by Messrs. Ntambakwa and Rogers from Garrard Hassan America this is covered, but no mention of the OS factor is given.

Thanks in advance.

 

[JAE]

The use of the overstrength OS factor is intended to be a tool the desginer uses to ensure that the weak link in the structural chain is relegated to elements with ductile failure/behavior.

Using the OS factor for everything, in my view, is a mis-use of it in that you are up-sizing elements that you WANT to go ductile.

This would be similar to a designer using the OS factor on a bolted gusset plate connection on an X-brace, then usiing the OS factor on the brace itself as well. With a major seismic event, the designer has then failed to draw the failure into the ductile brace member, but instead has created a condition where the more brittle connection elements would fail first. This would create an overall non-ductile failure.

I don't see the point in using the OS for everything. Doesn't make sense.