Gusset Connection Eccentricity Problem 9

[dik]

I have a new project to design bracing for an existing building. The force in the bracing is Cf = Tf = 103K... big forces!

The problem is that the gusset will be secured at the centroid of the resisting beams and columns because there is an existing masonry wall located to the centerline of the connection. The EOR want me to design the connection for an HSS 10x4x0.5 (located at the face of the wall), welding the HSS to the face of the gusset, creating an eccentricity of 2" (from the HSS) + half the thickness of the gusset.

I've designed it using elastic combined stresses, based on a Whitmore width for compression. If it weren't for the compressive load, I'd have used the plastic section. I'm looking for some comments how others would design this for the eccentricity. I've suggested going to plate that is roughly 1"x12", in lieu of the HSS, to minimise eccentricity or possibly using the existing masonry wall for lateral forces.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[GC_Hopi]

Probably need a sketch to get better input on this.

 

[DrZoidberWoop]

*Post updated per Dik's input in later post.
Is this the concept you are describing? If so, probably gonna need a lot of weak-axis stiffening @ the beam and gusset.

For the weld design and interaction of axial, biaxial bending, and shear in the gusset, I would look at "Combined Stresses in Gusset Plates" by W.A. Thornton and "Establishing and Developing the Weak-Axis Strength of Plates Subjected to Applied Loads" by Carter, Muir, & Dowswell. And as a minimum, I'd add stiffeners @ the beam too. Maybe even a "T-shaped" stiffener on the gusset, like this:

 

[dik]

Something like... The gusset is centered on the centrelines of the beam and column.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[phamENG]

Ask the EOR for his preliminary calculations that prove this arrangement is feasible...

EDIT: thought Dr.Z's sketch was dik's.

Haven't had to deal with that before. Can you weld stiffeners to the plate to help with bending and resolve the eccentricity there?

 

[dik]

Doc... the gusset is on the other side of the HSS, adjacent to the wall.

pham... that's another part of the problem. The EOR wants me to reduce the plate thickness (from what's calculated) so it is not so stiff and doesn't transfer as much moment into the connection.


It's close... the gusset just moves over to the centreline of the beam and columns.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[dik]

Thanks for the update, dr... I'd thought of adding a stiffener to reduce the thickness of the gusset. The EOR wants to 'reduce' the moment going into the connection, by using a thinner gusset.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[dik]

I don't know where the masonry wall is relative to beam and column centers, and since it's brick it is likely multi-wythe.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[DrZoidberWoop]

If he wanted to reduce the weak-axis moment generated by the eccentric axial load, something like a WT brace might be better than an HSS.

 

[dik]

I hadn't considered a WT... I suggested a 1x12 or 3/4x16 plate secured to the wall to prevent buckling to minimise the eccentricity.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Deker]

Since the weak-axis stiffness of the brace is orders of magnitude higher than the gusset, does the gusset need to be designed to carry any moment? You could proportion some of the total moment to the gusset, but I think an equally valid approach is to design the brace for the entire moment, design the gusset for in-plane forces only, and detail the gusset-beam/column connections to develop the weak-axis flexural capacity of the plate. This reminds me of this thread where Bill Thornton provided his comments on a similar detail at the end of the thread: thread507-268731.

 

[dik]

Decker: Are you suggesting to design the connection of the HSS to the gusset for both the moment and the axial load, but design the gusset for only the axial load?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Deker]

Dik: The HSS for axial + moment, the HSS to gusset connection for shear only, the gusset plate for in-plane forces only, and detail the gusset to beam/column connections to develop the weak-axis capacity of the plate. See the attached comments from Bill Thornton on a similar WT brace connection from the thread I linked above.

 

[dik]

You are suggesting that this is the model?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Deker]

Yes. If you swap out the WT for an HSS, doesn't this match your condition?

 

[dik]

Yes it does...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[dik]

And the added moment caused by the shear to resist the moment?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[GC_Hopi]

In the OP you mentioned that this connection will take a compression force from the brace and your concern is that this compression will take the plate into a plastic mode.

 

[dik]

To clarify... I treated the combined stresses elastically. If the load were tension only, I'd have considered the plastic section.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Deker]

In your first sketch, are you asking about the unbraced length of the gusset? I'd minimize that distance as much as possible for this detail. If for some reason you can't, you can add a stiffener on the gusset to prevent the plate from hinging and folding over...although for a gusset supported on two edges I don't think that's a major concern.

Regarding your second sketch, there is technically zero shear in the direction you're showing since the eccentricity induces pure bending in the brace. However, there will be some incidental moment in the gusset. This is the main reason I propose detailing the gusset to beam/column connections to develop the capacity of the plate. This allows you to take advantage of the lower bound theorem to proportion your forces any way you see fit that satisfies statics.