Steel chevron brace into reinforced concrete frame

[Olivaw]

Im working on a 6 story plus 2 basements shear wall building on a high risk seimic area. Due to architectural concerns we can't add more walls on the first floor to resist the seismic load. So we came up with the idea to add a steel chevron brace on some concrete frames on the perimeter of the building to deal with those loads.

The thing is we have some problems on the connection between the braces and the concrete beams (see attached). I was planning on having some anchor bolts welded to a preinstaled steel plate but the connection fails through shear breakout (parallel to the

Ultimate shear through the connection is 692 kip (Im not from the US, I think those are used in the US right?). If I use 12 bolts then I have to account 58 kip per bolt. ACI 318 chapter 17 gives me 17.5 kip of shear breakout resistance per bolt, nowhere near enough resistance.

My only hope is to use supplementary reinforcement but I don't know where to place them since this situation is not your typical connection. Will concrete shear stirrups count as supplementary reinforcement per ACI 318?

 

[PROYECTOR]

Hi Olivaw,

Your load path is not correct (incomplete), the unbalanced vertical load is missing (which can be huge for SCBF, specially for the post-buckling case). The proposed connection detail won't work, no matter how much supplementary reinforcement you provide.

If this was my project and the use of chevron braces was unavoidable, I would try to do the following:

1) Use buckling restrained braces (BRB) to minimize the unbalanced vertical load.

2) Use steel members instead of reinforced concrete. You can still maintain the reinforced concrete shear walls as the primary lateral force resisting system (or use steel-concrete composite shear walls).

3) If option 2 is not possible, consider the use of encased composite beams and columns at the braced bays for a more direct and efficient load path.

Hope this help!

 

[r13]

Will adding metal bracket at beam-column joint possible? Could help this situation.

 

[jayrod12]

Will adding metal bracket at beam-column joint possible? Could help this situation.

In what way do you envision this helping? If I'm understanding the issue correctly, he's having a problem where the Chevron brace frames into the beam at mid-span. Nothing at the beam/column joint will help this.

 

[r13]

jayrod12,

Why he needs the brace at the first place? There is no way to resist the forces from the brace, if the forces are correct as stated. Therefore, some other measures are required to share the loads, stiffen the frame element been one of the options.

 

[jayrod12]

I don't understand what you are suggesting though, a knee brace at the beam/column joint? Doesn't that just result in changing the location of the problem?

I want to understand what you're suggesting so I can use it in another scenario if it ever comes up.

 

[PROYECTOR]

Why he needs the brace at the first place? There is no way to resist the forces from the brace, if the forces are correct as stated. Therefore, some other measures are required to share the loads, stiffen the frame element been one of the options.

I agree, adding steel braces is not a good idea in the first place. It is difficult to get good results if the design of a new structure is approached as a retrofit.

Maybe if Olivaw shares more information about the project (a floor plan may be helpful), he can get more specific guidelines.

 

[r13]

jayrod12,

The bracket increases moment of inertia (I) at beam-column joint, make it more rigid and draw more forces. In turn, the increased forces are distributed to the joint that has larger dimensions due to presence of the bracket.

 

[jayrod12]

But mixing steel and concrete at that point in the way I'm envision you're describing will not help. By the time the steel is engaged, the concrete will have failed.

 

[r13]

PROYECTOR,

The laborious bracket (bull leg) in new design is not a good idea these days, because of its high costs (labor and material), not because of the concept. It is employed frequently in retrofit works.

 

[r13]

Embed bearing plates on beam and column faces, then field fit/weld stub to the plates. Welding would be difficult, but can be done by skilled welders.

 

[PROYECTOR]

This may be an option, but as I said, adding steel braces to an RC frame is usually not the best solution.

 

[r13]

The bracket will be externally placed at all joints at the level, or levels above/below, to minimize the sway mechanism. By diverting the forces away, the braces will then experience lesser force, and the attaching detail might not be destructive. The effectiveness shouldn't be discounted before conducting a detailed evaluation.

 

[KootK]

I was thinking along the same lines as PROYECTOR's latest post: partial, embedded shapes to facilitate the connections. I know this stuff is done but it must be murder on form work, right?

 

[PROYECTOR]

I was thinking along the same lines as PROYECTOR's latest post: partial, embedded shapes to facilitate the connections. I know this stuff is done but it must be murder on form work, right?

Yes, constructability is not the best feature of this solution.

Here is another option, a little bit easier for construction.

Here is the reference:

https://journals.sagepub.com/doi/abs/10.1260/1369-4332.16.4.759

 

[KootK]

The one on the left is another option that I'd been mulling. Essentially, it is to extend the original anchors to the top of the beam such that it become a reinforce concrete problem rather than a shallow anchorage problem. In this sense, it is the best of "supplemental reinforcing". I could get behind that if the numbers panned out and it were acceptable in terms of beam top side aesthetics, thermal performance, and attachment of facade stuff.

 

[KootK]

I've actually been dreaming of it with shear lugs though to really get the shear taken care of robustly. As I understand it, ACI even has shinny new shear lug design provisions that could be applied.