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design for gusset plate not at a column

design for gusset plate not at a column

design for gusset plate not at a column

(OP)
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.

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

(OP)
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.

RE: design for gusset plate not at a column

No sketch there.

BA

RE: design for gusset plate not at a column

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

RE: design for gusset plate not at a column

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?

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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

RE: design for gusset plate not at a column

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

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

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

RE: design for gusset plate not at a column

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?

RE: design for gusset plate not at a column

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.

RE: design for gusset plate not at a column

gwynn,
No reason, except that is "not the way it is done". Ignoring the eccentric part has the same effect as not welding it or removing it. You could make the gusset any size, but the answer is the same when one line of bolts delivers an axial force.

RE: design for gusset plate not at a column

I agree with Hokie. If the part of the gusset concentric with the single bolt line is adequate to develop forces then it the weld associated with the concentric portion can be designed solely for the axial force and shear components of the line of action.

RE: design for gusset plate not at a column

Quote (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.

This is not correct. The moment here, Ph*eb (where Ph is the horizontal component of the brace force, and eb is half the beam depth), will be countered by another moment, Pv*ev (where Pv is the vertical component of the brace force, and ev is the horizontal distance from the work point, where the brace work line intersects the beam centerline, to the center of the weld group). As Hokie mentions, if the weld is concentric about the brace work line, then these two components are equal, and of opposite signs, so the moment is zero.

I also agree with Hokie that if the concentric portion of the weld and gusset provide adequate capacity, that you haven't made the connection worse by adding some extra redundant gusset on the right-hand side of the connection. If you check this longer weld length, then you do have a moment on the weld, but you also have more weld. The result shouldn't be a weaker weld. But you have made the analysis more complicated, perhaps with no gain in connection efficiency.

RE: design for gusset plate not at a column

Well, hokie, WillisV, nutte and slickdeals, you are not entirely correct. From an elastic point of view, you have made the situation worse by adding material to the gusset. That is to say, the maximum elastic stress has been increased in an eccentric gusset from that of a concentric gusset.

I agree that, under static loading, from an ultimate strength point of view that the situation has not been made worse. But under cyclic loading, I am not so sure. I would suggest that the higher elastic stress in the elastic condition may produce undesired effects under cyclic loading.

BA

RE: design for gusset plate not at a column

(OP)
slickdeals, I agree with your sketch. The only time there won't be a moment is if the brace angle is 45 degrees and e_v = e_h.

Note: The below statement is only in regards to single angle bracing, where the bracing workline is not at the member centerline

Per standard detailing (per my experience - where the gusset is usually offset a fixed distance from the edges of the single angle brace), the case where the midpoint of the gusset at the beam interface coincides with the intersection of the bracing workline and the beam interface, rarely if ever occurs for single angles, since the bracing workline is not centered on the the brace. Hence, in general, for single angle bracing, a moment exists at the interface point (whether this moment can be ignored is left to engineering judgement). Attached are sketches for reference.

RE: design for gusset plate not at a column

JEG, your statement about 45 degrees is not right. Sketch it out for a different angle and see. If the weld is centered on the brace work line, the moment is still zero. Your eccentricities are no longer equal to each other (as for the 45 degree case), but your load components Pv and Ph aren't, either. They're all related to each other, based on the sine and cosine of the brace angle.

To look at it a different way, the brace work line goes right through the weld CG. The eccentricity is zero. How can there be moment?

RE: design for gusset plate not at a column

(OP)
nutte: I was assuming that the weld length is equal to the gusset length at the interface. I'm not a detailer, so I can't say for sure if they would typically make the weld symmetric about the brace workpoint at the interface, or just weld the full length of the gusset edge. If it's the latter, then I stand by my original statement.

RE: design for gusset plate not at a column

BA,
Make the gusset infinitely long in one direction. Do you still think the redundant material makes the connection worse?

By the way, my assumption in all of this is that the brace is a double angle, i.e. symmetric about the gusset in the other plane. If a single angle, then out of plane bending of the gusset becomes an important consideration, as that type connection has been found wanting.

RE: design for gusset plate not at a column

hokie, I don't believe that standard beam theory holds when a member becomes infinitely deep, so the answer to your question is it depends. For a very thin gusset, the mode of failure is not clear. But we are not talking about plates with infinite width.

Consider a plate 1 by 4 units (doesn't matter whether it's inches, cm or mm). If the plate is loaded concentrically with load P, the maximum stress is uniform throughout and is equal to P/4 or 0.25P.

Now, let's consider a plate 1 by 6 units with load applied, as before, 2 units from one edge such that the eccentricity is 1 unit. Now, A = 1*6 = 6 and S = 1*62/6 = 6. The maximum stress is P/A + M/S = P/6 + P*1/6 = P/3 or 0.3333P which is greater than 0.25P. It is not my opinion...it is a fact that the maximum stress is greater than it was for the concentric case. Of course, the minimum stress is P/6 - P/6 = 0. The average stress is P/6 which is only two thirds of the average stress in the concentric plate but if the loading is cyclic, I am suggesting this case may be more critical than the uniformly loaded plate.

If the load is static tension, then the concentric plate is considered to fail at A*Fy or 4Fy for the 1x4 plate. The 1x6 plate fails when the entire section is stressed to yield, mostly in tension but partially in compression. This occurs when P = 4.325Fy and M = 4.325Fy, slightly in excess of 4Fy for the smaller plate. So the eccentrically loaded 1x6 plate carries about 8% more load than the concentrically loaded 1x4 plate.

BA

RE: design for gusset plate not at a column

BA,
Eloquently argued. It would be interesting to see that tested in a lab. Does anyone know of such a test? We use a lot of simplifications in our modeling of structures, and oftentimes the structure is smarter than we are.

RE: design for gusset plate not at a column

Quote (jeg)

slickdeals, I agree with your sketch. The only time there won't be a moment is if the brace angle is 45 degrees and e_v = e_h.

Not true. There will be no moment in the gusset if the force is applied through its centroid irrespective of the brace angle.

BA

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