Column base plate retrofit 1

[jeffhed]

I have a PEMB steel building that the contractor did not order a special inspection. The building is 100% complete and the lack of special inspection only came up after the contractor went to the city to get their CO. We have since found out that the installed anchor bolts were only embedded half as deep as the plans required. I need to increase the size of the existing base plate to accomodate the new bolts we will be epoxying into the footing. The added portions of base plate will be the same depth as the original base plate and will run parallel with the column web. I originally thought to support the new sections of base plate with gusset plates perpendicular to the column web. A second option would be to overlay the existing base plate with plate that is the thickness I need and weld it to the column flanges and web (I could even remove the existing nuts and bolt this new plate to the existing plate in addition to the welding). Have others done similar retrofits? If so, is there some input you could provide as to why or why not one of these solutions would be acceptable? Obviously better ideas are definitley welcome.

 

[TxPESE]

Jeff,
I have a pretty good idea of what you are thinking, but would like to see a sketch before commenting
Ram

 

[ToadJones]

I would consider welding channels or angles parallel to the web of the column across the tips of the existing column flanges and also weld them to the existing baseplate. Run them out past the existing baseplate to catch the new anchors. Basically it would look like an old mill style base assembly. ....just an idea

Blodgett's book should give you some ideas.

 

[jeffhed]

TxPESE,
A sketch is attached. ToadJones, I think I follow what you are saying, can you elaborate a little more? Maybe a sketch too?

 

[ToadJones]

I was under the impression you needed moment capacity. It looks as if it is really just a pinned base. I am assuming that you only need uplift anchors and not moment.
My idea is likely overkill for your application and it might add fixity to the base that was not part of the original design of the building or foundation.

I don't think I like the idea in your sketch. The weld will be in the worst possible spot and doing a full pen weld against the foundation will be next to impossible.

One idea might be to weld angles in against the web of the column and extend them out to catch the new anchors. Blodgett's book calls them "hold down angles".

 

[SAIL3]

TJ's idea would work, just leave a 1/2" gap between the channels and base plate..that would eliminate the extra fixity concern...

 

[jeffhed]

ToadJones,
This is a PEMB, only uplift forces, no moment. I was only trying to show where my new bolts needed to be. If I added to the base plate, I would provide stiffeners perpendicular to the web so the weld wouldn't be trying to resist moment. However, increased fixity at the base is also a concern. I could overlay a plate over the existing baseplate and extend the new plate out to the new bolts. If I am understanding you correctly, then I think this would be similar to the angles that you mentioned. I don't have Blodgett's book, so I was hoping you could provide a sketch to make sure I am picturing it correctly.

 

[ToadJones]

some thing like this....you could stiffen the angles too, but this would make the field welding easy.

 

[jeffhed]

Thanks ToadJones. That is what I thought you were describing. I think I will have to stiffen the angles to keep the thickness down. My cantilever length from the column web to the bolts is getting somewhat large, or at least larger than I am used to dealing with. Thanks for your help TJ!

 

[ToadJones]

you are welcome

 

[ToadJones]

you can probably check your cantilever bending of the angle using only the distance that the new angle over hangs the existing baseplate so long as you weld the ends of the new angle to the baseplate....might help your calc...

 

[jeffhed]

I thought of that but wouldn't that increase the bending force in my existing base plate? We could make the horizontal leg the same width as the existing base plate and weld the leg to the column flanges.

 

[Duwe6]

Essentially zero bending force on the downwind side of the existing baseplate - the grout handles the compression. The upwind side gets a significant load, but you can use the existing [short] anchors plus the new ones. Will probably need one triangular vertical gusset added between the bolts. I really like ToadJones' fix.

 

[jeffhed]

Duwe6,
You're right. The plate should be fully bearing on the slab or grout. I really like ToadJones' fix too. In fact I have already moved forward with sizing the angles.

 

[ToadJones]

Make sure you check how close your new anchors are to the existing...break out strength etc.

 

[jeffhed]

I have. That is what is pushing my anchors out farther than what I am used to. That and the epoxy spacing requirements/penalizations.

 

[bones206]

Maybe you could cut off the original anchors flush with the base plate after installing the new epoxy anchors. Then you could waive the spacing requirements since they won't be acting in tension.

Not sure what the net benefit/penalty of that would be for your design if you are considering the combined tensile capacity of the new and old anchors, especially if the adhesive anchor spacing will still be pushing the cantilever distance out.