Foundation Design for Braced Frames

[davidfi]

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.

 

[DETstru]

Short answer: #1

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/publications/nistgcr13-917-24.pdf

(search for the word "uplift")

 

[wannabeSE]

I don’t have references in front of me. I think you are ok provided it isn’t a DSA or OSHPD using chapter 16A. Also, look in ASCE 7 chapter 12 foundation section. You may be able to reduce the overturning forces for the foundation design

 

[davidfi]

Unfortunately, it is DSA as this is a school building. It looks likes CBC 1616A.1.16 adds a section that requires the superstructure to foundation connection be designed for the strength of the system or overstrength forces. However, I do not see anything additional regarding the foundation design.

 

[JoshPlumSE]

Davidfi -

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.

 

[sandman21]

You are reading 1616A.1.16 wrong, the section applies to both foundations and connections to foundation are required to be designed for the three options and if a referenced standard has a higher load you must use the higher load. As an example AISC 341 D2.6b does not allow the use of overstrength the shear for the anchorage would need to be based on this load. So the question comes down to what part you are designing. Sizing and proportioning based on CBC load combinations, as the geotech is required to have a FOS of at least overstrength Strength checks for the foudnation system will requrie at least overstregnth. DSA also has revisions to typical micropiles and be aware that of section 1905A.1.3.

 

[davidfi]

I reread 1616A.1.16 and realized I missed that the foundations need to be designed for the lesser of the three options as well. But this applies only to the concrete/reinf of the foundations, correct? The bearing pressures and mircropile friction can be sized using ordinary level forces, correct?

 

[sandman21]

Sizing and proportioning based on CBC load combinations; 1605A.2, 1605.3. Geotech. is required to have factor of safety greater than overstrength, so it is already covered

 

[ATSE]

Using overstrength (omega) on the anchor bolts and then not designing for net uplift on a spread footing is indeed what the code allows, but it sure doesn't make sense when net uplift governs and net uplift is high. OSHPD also allows this discontinuity in load path explicitly.
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.

 

[SAIL3]

from what the OP described there is no soil involved or conc slab.....uplift is resisted by anchoring directly into bedrock....if the Geotech supplies the resistance factors for the bedrock and it has a additional FOS equal to or greater than omega, then fine..if not, IMO. I would use
an overstrength factor irregardless of what the codes indicates....