ELF, Cd, PDelta, ASCE 7-16, RISA

[engjg]

With respect to ASCE 7-16 is one supposed to include Cd in calculating PDelta effects? How would one do using RISA?

If one is not supposed to include Cd if operating in chapter 12, what about if you are chapter 15 and in reading §15.4.5?

 

[JoshPlumSE]

Take a look at the help file section titled "Drift" that was one of my projects when I worked for RISA. There is a section titled "Seismic Drift Checks" that I probably wrote. It says the following:

"When the ASCE and IBC codes are used, the Story Drift for the strength level combinations will account for the inelastice deflection (i.e. the Cd factor) by amplifying the joint deflections at each level by Cd/ I per section 12.8.6 of ASCE-7."

One thing to keep in mind is that the amplification is purely post processing. So, any additional P-Delta effect caused by the inelastic drifts is NOT taken into account.

 

[engjg]

Thanks Josh glad you responded!

I read that help section and that is what I understood...that it is was only included as a post processing for drift results...was trying to confirm that point thanks.

I guess my real question is what is the intent of the PDelta requirements in ASCE 7, do they want you to evaluate PDelta with the amplified deflections? My previous understanding was that it is generally accepted not to include as the PDelta affects would get really large...and folks smarter than me would start talking about the stabilizing effect of the dynamic loading or such...But then I read ASCE 7-16 15.4.5 "P-delta effects, when considered, shall be based on displacements determined by an elastic analysis multiplied by Cd/Ie" which made believe otherwise...? Any thoughts?

Also regarding the MRSA, DAM, & PDelta what are your thoughts on doing PDelta on the MRSA results combination as opposed to a PDelta on each mode shape (don't know how you would actually do) as far as meeting the intent of the code (AISC/ASCE 7). I note a 2011 3rd Qtr AISC journal article seems to suggest you theoretically need to run PDelta on each mode shape and then combine the effects in the member....not take combined loads and run one PDelta...