The cost of Drift

[McQSE]

It amazes me every time I size my structure for gravity loads and then go back to do the lateral analysis at how much larger my members need to be due to drift limitations. For instance, in my current project (one-story retail type steel building) it has doubled the size (by weight) of my members. Wind usually controls in my part of the country and I have read the many posts that have discussed the topic of the allowable amount of drift for serviceability. I am using the 10-year wind pressure as recommended by AISC Design Guide 3. I am interested in some feedback on what process other engineers use for optimizing a moment frame to reduce drift. Do you have any rules-of-thumb that may apply to proportioning the column size to beam size for maximized stiffness?

 

[JAE]

Are you using the 0.7 factor on wind loads per IBC?

 

[msquared48]

Regardless of any drift limitations, I limit the lateral movement to 1/2" in any moment frame in any single story window wall application. No exceptions.

Mike McCann
MMC Engineering

 

[McQSE]

I am using the .7 factor but it has been my interpretation that is (.7)(50-year wind event). According to AISC Design Guide 3, that is approximately the same thing as the 10-year event.