Period of Wood Structure Calculated to Justify Lower Earthquake Loads

[Mike Mike]

Has anyone ever calculated the period of a 4 or 5 story wood structure by hand? I calculated a higher period by the Rayleigh method than by the approximate method (ASCE 7-10 12.8.2.1), and was able to reduce my loads by a factor of 1.7. Still double checking my numbers but is this typical in your experience?

 

[JoshPlumSE]

Isn't there a restriction on how much of a period difference you can use when using the rayleigh method?

Yeah.... Table 12.8-1 of ASCE-7. You can use a period of between 1.4 and 1.7 times the "approximate period" to get your base shear, depending on the Sd1 of your site. I am skeptical how this could be properly applied and get a 1.7 force reduction in the type of structure you describe. That just seems like higher of a force reduction than I'd expect.

Also, I question how much of these provisions have really been "tested" with the taller wood structures that we're seeing today. Heck, I question the deflections that we calculate for wood shear wall buildings too.

I'm not saying that you're wrong. Just that something sounds a little off and that this is something that might require some caution.

 

[Mike Mike]

Thanks for your thoughts Josh. I talked to a few more of the top engineers in our office and they said they haven't ever taken advantage of the code's permission to use the calculated period for a 4 story wood structure, but they wouldn't be against the idea of dividing the loads by 1.7. I checked the numbers, read thru the literature again, and I think this route makes sense. In my opinion the approximate period calculation (12.8.2.1) is extremely conservative for wood structures. It assumes a concrete shear wall structure is just as stiff as a wood framed shear wall structure. Both fall into the "all other structural systems" category of table 12.8-2