Design of baseplates with gussets
Design of baseplates with gussets
(OP)
Greetings, I have a question as to how to adequately design baseplates with gusset plates.. Specifically for braces. I attached a PDF showing my situation.
In general (not for baseplates) I have used the Uniform Force Method but I know this method is not applicable to situations like this. I am confused as to how the shear, and tension/compression at the gusset to baseplate interface should be obtained; and also if there will be any moment due to the geometry (Even though the axis of the brace and the column are concentric). Basically my question boils down to this.. How do I calculate the shear, axial and moment at the interfaces between gusset and baseplate and gusset and column face.
Thanks.
In general (not for baseplates) I have used the Uniform Force Method but I know this method is not applicable to situations like this. I am confused as to how the shear, and tension/compression at the gusset to baseplate interface should be obtained; and also if there will be any moment due to the geometry (Even though the axis of the brace and the column are concentric). Basically my question boils down to this.. How do I calculate the shear, axial and moment at the interfaces between gusset and baseplate and gusset and column face.
Thanks.






RE: Design of baseplates with gussets
Assuming an HSS column, I typically assume that the work point is at the junction of the HSS wall and the top of the base plate. Then, I assume that the vertical component of the brace force is dealt with entirely in shear at the gusset / column interface and the horizontal brace for is dealt with entirely in shear at the gusset / base plate interface. Both of the shears mentioned intersect at a common point with the applied load without moments at the interfaces and, thus, UFM would be satisfied.
Using this method, there would be moment on the column. I feel that it's prudent, however, because neither the HSS wall nor the base plate is particularly stiff for transverse load. Much depends on the anchor bolt layout however.
To assume that the column takes no moment, I've used the following, more expensive, details in the past:
1) Slot the gusset plate through the HSS so that engages both sides of the column:
1) Use a WT for the gusset to reduce the flexibility of the HSS wall.
It's important to recognize that we usually don't "know" the moment and shear at the interfaces of our steel connections. We assume a path, design for those loads, and try to ensure that alternate load paths are ductile enough to get out of the way. Some assumptions are obviously better than others and there are cases where the ductility argument is a bit dubious (buckling modes, anchor bolts, etc). That's how it works though.
Your gusset plate's a little funny looking. I assume it's just rough at this point. Let us know if you need help with that.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Design of baseplates with gussets
Thanks, I have tried to use the UFM for this situation but I´ve never had complete confidence in the result. Usually what I do is set the height of the "beam" to be the same as the base plate thickness. With the resulting geometry most of the vertical reaction ends up in the column and horizontal force ends up being twice in the baseplate that in the column. Since this is the UFM I assume that there's no moment in the interfaces. And yes you are right the drawing was just some quick sketch I put together so I could upload here. With my current configuration I am getting a gusset plate that is twice as Tall as it is wide, but that probably has as much to do with the angle of the brace as well as the base-plate.
For the HSS wall problem what I do is weld a reinforcing plate flat to the wall... I believe it should be cheaper than using slotted plates.. but the plates get rather thick 1/2" up to 1".
I watched one of the AISC seminars on base-plate design and the presenter was very clear in saying the UFM was not conceived to be used for baseplates... The argument was that the UFM method assumes the beam and column to be very rigid and that base-plates don't fulfill this requirement.
I still would very much like to hear inputs on this.
RE: Design of baseplates with gussets
http://www.aisc.org/content.aspx?id=19858
RE: Design of baseplates with gussets
Thanks, That is precisely the webminar i referred in my post. Its very good but it just points out all the issues to take account when designing.. not to actually design a base-plate.
RE: Design of baseplates with gussets
I wonder if the lecturer modeled any of those L-shaped base plates that you find at corner condition sometimes. I'd love to see the over-stress on those...
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Design of baseplates with gussets
Yes the webminar is very good at explaining the issues with baseplates... but it also doesn't solve the fundamental problem that is how to actually design baseplates. I found it to be disappointing in that regard.
RE: Design of baseplates with gussets
Design of Welded Structures by Blodgett is a must in any engrs library..it is clear, direct, practical and dirt cheap...available fron Lincoln Electric.
RE: Design of baseplates with gussets
that is my line of thinking with hss as well... do anything possible to take the load from the column face and transfer it to the sidewalls.. I am curious however, does matching brace and column and adding gusset plates still gives you enough ductility in the connection? Im talking about seismic design mostly.. perhaps you dont have that issue in your area. It is my understanding that part of the benefit of the slotted plate connection is that its very ductile and ensures that the failure occurs at the brace rather than the connection, basically the brace buckles and the plate bends sideways to accomodate the bending... a double gusset plate would restraint this movement considerably I believe.
Also, I would like to ask you... how do you usually approach the design of the plates themselves? I'm talking about sizing the plate (width and height, not thickness...)and the load transfer between brace and column and brace and base-plate. Do you use UFM or some other method?
RE: Design of baseplates with gussets
@Sail3: I have mister Blodgett book and it is wonderful.
RE: Design of baseplates with gussets
Otherwise, when using the reinforcing pl on the col face I would approach it as follows:
1. Make a section cut of the gusset pl @ the col face.
2. Convert brace load into shear and normal forces on this section.Normal force will also result in a
moment on the section. Design gusset for these loads.
3. To obtain load on reinforcing pl, use max normal stress on above section and multply it by gusset
thickeness to get line load on reinf pl.
4. Take a 1" strip of reinforcing pl and assume it is pinned at each end and subjected to this max
line load @ center. After it is designed check for combined max stress when combined with shear.
Two options for the width of this reinf pl.
a. Stop pl just short of the beginning of the radius on col so one can get a fillet weld in there.
b. Extend pl past beginning of radius on col so that one can get a partial flare-bevel weld and back-up
fillet weld.
The governing height would be either that reguired by the gusset designed above or as often happens by the particular geometry involved.
It is much more beneficial for an engingineer to understand the basics/background of conn design than to rely entirely on a canned method found in the codes.Blodgett is a good place to start.