Foundation Design for Braced Frames
Foundation Design for Braced Frames
(OP)
I am working on the foundation design of a 3-story special concentric braced frame building in California. The bedrock is very shallow at the site, so the grade beams will bear on bedrock. The geotechnical engineer recommends that I use micropiles or rock anchors to resist uplift loads. My question is what force level do I design the micropiles?
1) Force resulting from elastic analysis from equivalent lateral force procedure, or
2) Same as (1), but increased by overstrength factor, or
3) Vertical component of the brace strength in tension
I did not see anything in ASCE 7 or the Californa Building Code that requires anything above (1), but I presume I just missed something.
Thanks for any suggestions.
1) Force resulting from elastic analysis from equivalent lateral force procedure, or
2) Same as (1), but increased by overstrength factor, or
3) Vertical component of the brace strength in tension
I did not see anything in ASCE 7 or the Californa Building Code that requires anything above (1), but I presume I just missed something.
Thanks for any suggestions.






RE: Foundation Design for Braced Frames
Long answer:
Technically, you only need to design your foundation for the loads determined from analysis. They do not need to be amplified to expected/overstrength values.
AISC 341 F2.3 indicates that you can limit the force on columns using the resistance of foundation overturning uplift. Therefore you do not need to design the foundation for overstrength or expected strength. You still need to design the pile/pile cap anchorage for increased loads per ASCE 12.13.6.5.
But, NEHRP recommends that you design for the amplified load. So you can do so if you feel it should be done.
https://www.curee.org/projects/nehrp-jv/publicatio...
(search for the word "uplift")
RE: Foundation Design for Braced Frames
RE: Foundation Design for Braced Frames
RE: Foundation Design for Braced Frames
That's my recollection as well. That the connection between foundation and superstructure needs to be designed for the amplified forces But, that the foundation does not. In practice, this means anchor rods for steel columns need to be designed for the higher loads, but not the foundation itself.
RE: Foundation Design for Braced Frames
RE: Foundation Design for Braced Frames
RE: Foundation Design for Braced Frames
RE: Foundation Design for Braced Frames
Typical factors of safety for soil (i.e. bearing resisting downward structural reactions) don't help the structural system when your spread footing is hopping like a bunny rabbit. Uplift and vertical movement is very damaging to structural systems.
The body of codes we have to work with are pretty good, but this exception seems inconsistent.
RE: Foundation Design for Braced Frames
an overstrength factor irregardless of what the codes indicates....