Repair of Corroded Tube Column Bottom and Base Plate

[KirkGH]

This is an existing building with a column in need of repair. The remaining rust can be scraped and corrosion inhibitor applied. A nearby drain outlet is the cause and will be dealt with.

The column is HSS 6 x 6 and supports about 53k (D+.25L) at the moment. W 16 x 26 beams frame into each side and one side has a diagonal HSS 5 x 5 framing in (see pictures).

I'm looking for input on two repair options and shoring guidelines:

Option 1: shore column, remove rusted bottom of the column, install base plate/anchors, weld plates to side of HSS column and base plate on each side.

Option 2: remove plate on one side of column, install L shaped plate (welded 1/2" plates or something... to be calculated) welded to column and anchored to footing, cut plate on 2nd side, install L shaped plate... etc. until all 4 sides are supported. The goal of this option is to eliminate shoring requirements.

Shoring: My client is the contractor on the job and I proposed Option 1 which, after inspecting the area for himself, said was the same thing he was thinking. He said he could do it. Myself, I'm worried about the shoring, especially with the diagonal tube preventing jacks from getting a good bearin surface.

I'm not familiar with shoring methods. Is this a clean and clear cut shoring job? Would I just check the beam shear connections for strength as well as beam shear and web crippling/buckling? How does the diagonal effect things? Is Option 2 valid?

I appreciate any guidance on the matter. I didn't see any old thread on the topic but direction towards one would be appreciated as well.

Kirk G Hall, PE

 

[KirkGH]

Some additional repair may be required after looking at the pictures again. Comments?

Kirk G Hall, PE

 

[BAretired]

Hard to say from photos alone, but one of them (DSC00253) seems to indicate the HSS wall has corroded completely through the steel. I don't think you can afford to take the chance of performing a repair without shoring. Shores will be quite short and not unduly expensive.

BA

 

[bridgebuster]

53 k isn't a lot to support. If you can't shore it from one side, you can cut a tapered shim out of wood to put below the diagonal to provide a level surface. Another alternative is to put a bracket coming off the W16 and use it as a bearing point.

You may not have to jack if you can bust out the concrete below the base plate. Then all you need to do is install some cribbing and shim it tight against the W16.

 

[KirkGH]

I thought about the trim wood piece. It seems like it would just slide off; analysis-wise the shores would need lateral support normal to the slope of the diagonal. I like the gist of it. I thought of an idea to weld on a capped HSS 5 x 5 to the underside of the diagonal to provide for horizontal bearing. Only problem is with the corrosion, what's the capacity of the beam and diagonal for force transfer? Yuck. An old supervisor of mine suggested welding on some elements directly to the column to provide bearing for the shores.

Kirk G Hall, PE

 

[BAretired]

I like the idea of welding something to the column to provide bearing for shores. Such welding must be done in an area which has not been subjected to loss of section due to corrosion.

BA

 

[StrucPatholgst]

I've done repairs like this in chemical plants, on short tank legs and long building columns destroyed by chemical corrosion. The easy part in your case is the shoring. Weld on some gussets and plate to the bottom of the diagonal in a couple spots for a flat surface, use two 20-ton bottle jacks on each beam, crib both sides for safety, etc. Your gusset side plates should sandwich the tube, and be welded to the flange of the beam as well as the sides of the tube. The hard part is going to be getting new anchors in. Those are likely J-bolts tied to a rebar cage, and they look shot. Even if you knocked the grout away and cut them flush with the footing, you'll likely hit rebar if you try drilling in new chem anchors, if it doesn't destroy the footing in the process. And if you spin your new baseplate 45 degrees to avoid rebar, trying to hammer drill holes for 4 new chem anchors will be tough, since there's not much head room to maneuver a hilti in there under the beams. Also, in retro's like this, don't skimp: go the whole 9 yards and use jamb nuts and a leveling plate. When you weld the joints on the tube, there will undoubtedly be shrinkage as the welds cool, and taking a shortcut by avoiding jamb nuts and a leveling plate could lead to problems. Also, if this were me, the repair would replicate what's there (a new stub of 6x6 HSS tube with weld joint to the original column and new bp, a new baseplate, etc), plus splice plates on the four sides of the tube at the joint.

 

[TXStructural]

This is a common problem in wet regions, where they insist on putting drains inside gas station canopy supports, especially when they fail to fill the tube with grout up to the hole for the drain (still a bad detail, but it is how things are done. You can also splice the column by cutting a flat surface at some height and fillet welding a plate to the column above and the stub below (always better to weld the bottom stub before actually putting it in place so it can be downhand.) It is not as attractive as a CJP weld, but far more economical in most cases. Consider having the new column bottom stub and baseplate galvanized, adn use a slightly heavier section to reduce the effect of corrosion should it occur again. I fully agree with StructPath on jam nuts, which gets the baseplate away from the water, but it looks like that is the original design anyway.

It also look like the tube has a source of water inside, so sort that out to prevent a repeat.