Seismic Provisions - Additional Analysis Requirements + Anchoring SCBF 1

[lacyma1]

I'm designing a small building in a SDC D. The layout will include two Special Concentrically Braced Frames in one of my orthogonal directions but the loads applied to these frames end up being rather small (one story building, no rooftop equipment, steel roof deck no concrete). Design of my columns, beams, and connections in these frames seems to be largely dictated by the Seismic Provision - F2.3 a) and b) analysis requirements.

My braces are going to be Pipe 5 Std arranged in an X-Brace configuration. My question is this: my expected brace strength in tension (224.56k) applied with the expected post-buckling strength of my brace (11.67k) results in large uplifts. Does my column anchorage need to be designed for these uplifts? Do my foundations need to be sized to deal with this uplift?

 

[jittles]

Paraphrasing here, but 341 tells you to carry expected strength through to the connection/anchorage to the foundation. I include local checks like punching shear in the footing if that's critical to my anchorage design. Often we have an anchor plate buried at the bottom of footing, so the anchorage design includes that anchor plate punching upward under those expected strength uplift demands.

Yes to anchorage, including concrete modes of failure if they govern.
No to foundation size.

 

[OldDawgNewTricks]

See Exception b1 to F2.3. "The required strength of columns need not exceed... the forces corresponding to the resistance of the foundation to overturning uplift."

 

[RPMG]

In SCBF's, the braces are supposed to buckle first. It's the fuse the pops and breaks the static behavior of the building, allowing the seismic energy to dissipate. By the principle of capacity based design, yes, the anchorage should be designed for that uplift. There is usually an exception to use an overstrength factor though. In which case, that would apply to the load combination analysis case. I don't recall if there are special considerations for anchorages and the use of the overstrength factor.

 

[lacyma1]

Thank you everyone for your responses. It was my strong suspicion that I needed to account for this in my anchorage. Not great news, but good to have the proper path forward. Per ASCE I will be using my overstrength Seismic in LRFD Load Combos 6 and 7 to design everything from my diaphragm down to my anchors except for my brace.

 

[OldDawgNewTricks]

Use of the overstrength factor does not satisfy the requirements for Special Concentric Braced Frames. For SCBFs, you use a capacity-based approach resulting from the expected strengths of the brace members. But as mentioned previously, there are exceptions that allow you to limit the connection forces to the maximum load that can be transferred by the system (including foundation uplift resistance, which usually controls for shallow foundations) that you can take advantage of if you choose to.

 

[jittles]

I agree with OldDawg's interpretation, although in my personal experience my mentors never had me invoke that exception, and now in my own designs I also do not.

OldDawg, I suppose you can take that as a competitive advantage if you will. I don't disagree with utilizing it, just based on my training I have not utilized it myself.

 

[OldDawgNewTricks]

Jittles, it may not be a big deal for your particular designs. But sometimes it makes a lot of sense to use those exceptions. Say your brace member is much larger than usual for some reason (long unbraced length, aesthetic reasons, etc.). Then your connection design load could become unreasonably large. It would not make sense to spend an inordinate amount of money for the uplift anchorage when a simple free body diagram would prove that there is no possible way that the weight of the footing could resist that load anyway. A little conservatism is sometimes warranted, but overconservative designs that don't provide any tangible performance benefits can make the engineer look bad if the client has another engineer value engineer the design.

 

[jittles]

Your username is hilariously relevant to this.

Some Old Dawgs taught me one way, and I've never been inclined to try to New Tricks...

 

[lacyma1]

Apologies if I was unclear before, I'm still using capacity design requirements to design my braced frame. I was just stating that I am using the overstrength as well and taking the greatest strength required for each component from all my load combinations and capacity design requirements of the Seismic Provisions.

But this raises the question: Do I need to account for the capacity design requirements in the design of my chords and collectors somehow? I have looked at as many examples as I can find and this is never mentioned.

 

[lacyma1]

All this talk about the exception I think I understand, but want to make sure. So my foundation should be sized for my regular load combinations (no overstrength, no capacity design per Seismic Provisions), then I can design my columns + anchorage using the maximum overturning resistance the weight of my foundations provide?

 

[RPMG]

Just to follow up, I agree that the SCBF design should be capacity based. I was making a general comment about exceptions based on my memory.