design for gusset plate not at a column

[jeg1976]

Can anyone point me to any references regarding how to design a gusset plate when not at a beam-column joint (i.e. for a knee brace away from a column or for chevron/x-bracing at the midspan of a beam)? I've read that the UFM (Uniform Force Method) or KISS method are not appropriate for this type of connection. Thanks.

 

[racookpe1978]

Draw a quick sketch of your problem - Your description may become clear to others, or may be clear to others already, but I cannot picture it from your description.

 

[WillisV]

AISC Design Examples that come with the manual have a center of chevron beam gusset example (using UFM), though I believe it is being updated.

 

[jeg1976]

racookpe1978 - Attached is a sketch of what I'm talking about. A question would be is there a moment at the beam/gusset and/or column/gusset interface (due to the eccentricity to the centerline of the beam/column from the interface)?

WillisV - Thanks for the tip, EXAMPLE II.C-4 from AISC Design Examples V14.0 is probably the closest thing to what I'm looking for.

 

[BAretired]

No sketch there.


BA

 

[jeg1976]

Sorry about that...here's the sketch.

 

[BAretired]

In the sketch shown, there appears to be a moment at the beam/gusset interface. Not so much if any at the column/gusset interface.

BA

 

[hokie66]

As far as the beam is concerned, there is just a force, which results in a beam moment. I think most would not bother about the eccentricity on the gusset. If the gusset is made concentric about the bolt line by removing the redundant part, there is no moment on the gusset, so why worry?

 

[spats]

I have to disagree with you hokie (I thought I'd never say that)... even if you still have no moment on the gusset, you still have brace shear and tension to deal with. Removing the "redundant" part of the gusset greatly reduces the length available to resist the shear and tension. I, for one, would not ignore the moment in the gusset. It's truly required for equilibrium. It is not a fabricated equilibrium condition like the uniform force method.

 

[hokie66]

spats,
I like to have people disagree with me. My post assumed that the concentric part of the gusset was adequate to develop the brace, so the eccentric part was there for some other reason...a situation which commonly occurs. The brace force can be resolved into components, but it is still an axial force. The gusset will have a moment only if the brace is in bending, and I don't think it would be modeled that way.

 

[spats]

Dang, I like this... I have to disagree again. If the line of action of the brace does not pass through the center of the gusset length where the gusset connects to the beam, then there is bending in the gusset.

 

[hokie66]

Yes, but it is incidental bending, which in my opinion need not be considered. If the part of the gusset making the centroids not coincide is removed, problem solved. So the redundant part is just that, redundant. Maybe we will have to agree to disagree, which is not a bad thing.

 

[slickdeals]

thread507-322006

I also got a email from Dr. Thornton with some clarifications.

 

[slickdeals]

In your case, the weld that attaches the gusset to the beam will have to be designed for a moment = horizontal component of brace force * half the depth of the beam.

 

[hokie66]

slick,
So did Thornton say where the moment comes from? Is he assuming that the brace is in bending?

 

[BAretired]

I believe he is resolving the forces in the two diagonals and finding the moment about the middle of the gusset plate at the junction of the beam, then using the relationship f = P/A ± M/S to determine stress at the gusset/beam junction.

BA

 

[hokie66]

In the OP's example, there is only one diagonal, so that is the situation I am talking about. Makes no sense to me that there is a moment in the gusset when there is none in the brace. And the moment magnitude which slick quoted as horizontal component x 1/2 beam depth makes no sense either...double the beam size and double the moment on the gusset? Maybe everything I learned about steel trusses is wrong.

 

[BAretired]

Hokie, in the OP's example there is only one diagonal but the gusset plate is loaded eccentrically which means it has an axial force and a moment where gusset meets beam. The stress at the edges of the gusset plate is P/A ± M/S. Since M = P*e, this boils down to f = P(1/A ± e/S). But A = b*t and S = b²t/6 so f = (P/bt)*(1 ± 6e/b)

BA

 

[hokie66]

I don't see it that way. It is an axial force which goes straight through. The eccentric part of the plate can be removed. Are you saying that the redundant part of the plate makes the connection worse?

 

[gwynn]

Why not just provide weld on the concentric portion of the gusset, leave a 1/4" gap over the rest and avoid any disagreement? Or remove the the eccentric portion of the gussset if possible.