Compression X Bracing Middle Connection

[HDStructural]

Hello all,

First time posting here.

I am working on a project that has W-shape x-bracing that was designed for tension and compression. They cannot be considered tension only. Normally, we would make one of the braces continuous and the other brace would be cut in half, framing into the continuous brace. The continuous brace would be checked to make sure it is stiff enough to "brace" the two partial braces framing into it. However, the steel has already been produced and instead of having one continuous brace and one broken up brace, both braces were broken up and must connect in the middle.

I believe the best way to design this connection would be to have gusset plates on the outside of the flanges. The built up section properties (I, r, etc.) of the two gusset plates should exceed the properties of the W shape braces to ensure that there will be no global buckling of the brace. Additionally, the individual gusset plates must be checked so that they wouldn't buckle between the connection points. I think this could be done using a k of 0.65 since the connections to the W shape flanges would be rigid (probably a C shaped weld with all dimensions being greater than 10"). The gap between the braces for the connection is up to 84" at some locations, so the plates could get pretty hefty based on kl/r.

Do you think that this should also be analyzed as a built up member? That would impose much more stringent kl/r requirements on the individual gusset plates that would be difficult to meet (0.86 instead of 0.65, individual kl/r must be less than 3/4 of the gross sections kl/r, etc.).

How would you design this connection? Am I missing anything?

 

[human909]

Am I missing anything?

A clear picture.

 

[HDStructural]

See attached for clarification.

 

[TLHS]

You get a whole boatload of restraining force from tension effects in the tension brace. If you want to validate that, do a P-Delta analysis of the bracing system with them pinned at the intersection point.

 

[DrZoidberWoop]

Do a cost-savings analysis w/ your sales/estimating department. The amount of welding (labor) required to fabricate the central connection, via built-up plates w/ equivalent sectional characteristics, may (will) end up costing more than the raw cost to buy a full-length steel member.

 

[human909]

I agree with DrZoidberWoop.

It seems absurd to design and fabricate a bizarre 4 way moment connection splice when you could just fabricate another beam. Steel is cheap, labour both design and fabrication is expensive.

 

[HDStructural]

I agree that it is more economical to just replace the one brace so that it can be continuous. I still need to provide them with a design, which will hopefully show them that they should fabricate new steel.

What are your thoughts on the design of the connection? Do you think that the built up section in the steel manual would apply here?

 

[human909]

I still need to provide them with a design, which will hopefully show them that they should fabricate new steel.

What is your role in this job if you need to waste time and money for no outcome. My thoughts on the connection, I've already said, supply a new member. If you want to play the above game then I'd say site full penetration to create two continuous braces.

This is the center. Likely the most critical area and even most rigid bolted connections are less stiff than a continuous member. Hence my secondary suggestion of site welding to make it continuous.

 

[DrZoidberWoop]

Here's a doodle to get you started. You have many options. I'm not good at uploading pics here. Hope it comes through.

Yep. I screwed up the first upload. [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1714050205/tips/Concept_X_ig0r5l.pdf[/url]

 

[HDStructural]

I am the structural engineer and I'm tasked with coming up with a fix for these connections. They want to know what the connection would be if we used cover plates so that they can price it. My main concern is whether or not this middle connection detail would qualify as a built up member, which would impose more stringent requirements on the gusset plates.

I was planning on welding the gusset plates to the W braces w/ 5/16 fillet welds, along the sides of the flange (probably 15" each side) and across the flange at the end of the gusset plate (about 15" long). I would design the welds to make sure it develops the axial load in the braces. Braces are W14x90 and heavier.