retug September 09, 2021

ASCE 41 - What Gives?!?

Lately I have found myself doing a lot of seismic retrofits of existing structures.

These buildings are often analyzed utilizing ASCE 41 and I have a few complaints with the standard that I have not found answers too... hopefully someone reading this site can help me out with these.

  • Soil bearing pressures

    • In traditional ASCE 7 seismic design, you get to divide your equivalent lateral forces (ELF) or MRSA forces by an "R" factor... In ASCE 41, there is no "R" factor. There is an "m" factor that is ~"R" from my understanding, but these two terms should not be mixed and matched.
    • Given that your seismic loads from ASCE 41, e.g. BSE-1E or BSE-2E, are not divided by "R", the loads are MASSIVE! If I were to report these loads to my geotech engineer, they would laugh at me
    • I thought that maybe ASCE 41 would allow me to divide by the "m" factor, but instead, you get a measly factor of 3 when design your foundations
    • Coming from a high seismic area where I typically have an "R" of 8 and get to reduce my soil pressures by ~%10 with ASCE 7, only getting to divide by 3 is a real pain on my foundations. Contractors do not like these massive foundations.

  • Drift Limits

    • From the reading I have done, there are no drift limits in ASCE 41! What's up with that? It makes my life easier as a designer, but I have to feel like it should be considered.
    • Lots of moment frame designs according to ASCE 7 are drift controlled...

  • M Factors

    • M factors and R factors between ASCE 41 and ASCE 7 often do not match at collapse prevention. For example, BRB "m" factor is 7.5, while a BRB "R" factor is "8", what's the difference?

  • Force displacement curves

    • As innocent looking as this graph is, it is often extremely confusing to decipher
    • I will dig into this graph in the future

All images are taken from ASCE 41, I do not own these images and all rights belong to ASCE 41.

ASCE 41

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  • Latest Comments
  • retug on Diaphragm Slicer New and Improved -

    Thanks Trenno.

    I will work to get metric units incorporated, that's a great idea.

    On your second point, I think something like that could be done, it's just a bit awkward with how CSI currently has it setup. In order to make section cuts, I need to unlock the model, ...

  • Trenno on Diaphragm Slicer New and Improved -

    Awesome work S&T!

    Firstly, I'd recommend if you're going to feature the tool on CSI's website then maybe incorporate metric units (aka real units!) 

    Secondly, I'm wondering if there would be a way to set up the workflow using something akin to RAM Concept design strips. Perhaps the strip width ...

  • e95kwon on Transfer Diaphragm Design -

    My understanding is that for a 5-story building you would have:

    • (1) load case A (ELF)
    • (1) load case B for each level except the roof so total (4)
    • therefore (4) load combos for diaphragm design

    I believe the intent is to envelope higher mode actions as you mentioned ...