Base Plate work point

[BAGW]

Hi,

For braced frames, where should the workpoint (WP) at base plate be? CL of column and CL of brace intersection at top of baseplate or CL of column and CL of brace intersecting at bottom of baseplate? I have seen it done both ways. What are the differences between these two? If UFM is used, forces for the gusset plate to column and base plate connection will change with respect to WP.

Contractor likes to have WP at top of base plate. Is there any additional consideration for the design other than the change in connection forces between gusset plate and column/base plate if workpoint is kept at top of base plate?

Thanks

 

[jayrod12]

Is your baseplate that thick that it makes a large difference one way or the other? I'm a top of baseplate guy myself.

 

[BAGW]

Its a 4in base plate. Even with this it does not make much difference either way. Any reason for top of baseplate?

 

[Deker]

It may not make much difference in the final design, but the main consideration when choosing the work point location is to minimize the eccentricity for a particular component in the connection. Section 3.3a of this document provides some discussion (Link).

 

[Once20036]

We typically do top of slab on grade, roughly 9" above the top of base plate.

 

[KootK]

- Agree with Deker that it depends on what you're trying to accomplish.

- Agree with Once that, in many instances, you want the work point to coincide with the center of action of that which is assumed to provide the shear resistance.

- Agree that the thickness of the base plate makes little difference.

- For the common case where shear is transferred through the anchor bolts, particularly via a weld washer, I believe that there is some technical merit to using the top of base plate as Jayrod and your contractor suggest. Ideally, you'd like your column and gusset plate welds to see only tension and shear forces, no moments. By definition, you've got a little moment in the welds with the work point at the bottom of the base plate. This is a slight thing, of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAGW]

@ KootK

Shear lug is being used to resist the shear. Irrespective of the WP position (whether at the top or bottom of baseplate), the shear lug design will be the same right?

 

[KootK]

The shear lug introduces an eccentricity on the shear force and a moment that goes into the assembly... someplace (column, brace, anchor bolts). It usually doesn't add up to terribly much. If I'm worried about it, sometimes I'll design both the column and the anchor bolts for that moment.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAGW]

Can you explain this a little bit with a diagram if you dont mind. Thanks

 

[KootK]

We'll see how the day goes. Sketch + scan + post = 0.5 hr billable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAGW]

LOL.

Anyways, shear lug size will not change with change in WP. This is what I am trying to get to.

 

[KootK]

Agreed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.