Column braced frame base plate to foundation connection

[tbone73]

For the base plate to foundation connection at braced frame columns, what order of magnitude for tension do you consider significant enough to warrant a more robust connection than just anchor bolts and shear plates? I'm working on a project that has around 450 kips net uplift (ULT). I'm considering slightly recessing the column into the footing to transfer shear through bearing. For the large uplift I was thinking weld plates from the base plate to an embed plate. The embed plate would have welded rebar with hooks to fully develop the tension. Others in the office think this may require something even more robust. I appreciate your input in advance. Thanks.

 

[mike20793]

I've used something similar before and, if I recall correctly, NEHRP recommends something similar in their braced frame design guide. I typically will use rebar with Lenton Terminators to lap deeply embedded anchors. Then I detail the steel in the grade beam/pile cap/ spread footing to restrain my breakout failure modes.

 

[KootK]

I'll keep using anchor bolts and shear lugs right up until they stop working by the numbers. 450 kip is a BIG number of course. Some questions for you:

1) Interior footing condition?
2) Any grade beams connecting your footings to other stuff?
3) Column connection is directly to footing? No pier / pedestal?
4) You're assuming that shear is resisted by friction at the underside of the footing and or passive soil resistance on the side?
5) Wide flange column?
6) What seismic class of braced frame? Special? Intermediate? Ordinary?

A big concern in these situations is uplift punching shear in the footing. If you go with hooked welded rebar, you'll need to give the punching shear force transfer mechanism some serious thought. More on that once we've got a better picture of what's going on.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tbone73]

Thanks mike 20793. I reviewed the NEHRP publication and one recommendation is to embed a plate near the bottom of the footing connected to the anchor bolts and the uplift is resisted through punching shear of the embedded plate.

KootK

1) yes
2) I will use some tie grade beams but this is to engage more dead load to resist sliding through friction.
3) yes
4) I'm assuming sliding is resisted by friction between the footing base and the soil.
5) yes
6) using "steel systems not specifically detailed for seismic resistance" SDC is "B"

 

[mike20793]

Here's what I used on a project that was ordinary braced frames and had about 490 kips of uplift. Here, they wanted as few number of pours as possible, so we could not do the embedded plate option since it required an extra pour.

 

[KootK]

Here's a detail that I've found to have a high capacity, be relatively constructable, and be fairly simple to assess numerically. I like sticking with anchor bolts at the column to ease erection and plumbing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tbone73]

KootK, thanks for the time and effort. I was going to do the embedded plate as you have shown for tension resistance. The shears are not so large, so I was going to recess the column into the footing a few inches and rely on concrete bearing for the shear resistance. But I do like your double angle/embed plate connection for shear resistance, I'll have to keep that in my back pocket. Thanks again.

 

[KootK]

@tbone: two things regarding my sketch:

1) I should have shown the base plate extended under the gusset plate and welded to it.

2) You either have to separate the anchor bolts and embedded plate such that their failure cones do not over lap OR you have to give some consideration to the fact that the do over lap.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tbone73]

mike20793, thanks for the detail. It looks like you are resisting both shear and tension by the bolts. I would be a little concerned about the bolts trying to transfer the shear through the grout bed. In other words as the shear acts upon the bolts I would think they would undergo some localized bending trying to transfer the shear to the foundation.

Kootk, thanks I picked up on that.

One point I would like to further discuss is both KootK your detail and mike20793 your NEHRP recommendation both show a plate recessed to the bottom of the footing and basically resisting tension by a punching shear approach. Logically this makes sense to me, but we are basically saying don't worry about ACI Appendix D. If we were to consider Appendix D our capacities would be significantly less. In fact mike20793, for your latest detail that you sent I don't know how you were be able to resist 490 kips tension by Appendix D.

Thanks again gentlemen.

 

[KootK]

Logically this makes sense to me, but we are basically saying don't worry about ACI Appendix D.

Definitely throwing out appendix D. Although, with my detail, I suppose it would technically be a hybrid. Appendix D for the embed shear plate and another approach for the tension. I don't feel that appendix D is really a suitable tool for the kind of loads that you're dealing with.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tbone73]

KootK the NEHRP publication on Steel Special Concentrically Braced Frame Systems would tend to agree with what you're saying.