Replace/extend building column footing

[ztengguy]

I need to replace a footing or extend a footing down for a building column so the owner of the building can add docks to the building. I had thought about shoring up the column, and just excevate the old one out, and pour a new one. This would be a two part pour, the spread footing, then a pilaster up to the column to support existing column. The pilaster could be part of the outside wall that would be needed for the dock wall.

What I am trying to decide, is how to support the column. Some sort of scaffolding under the beams (but then how do you excavate?) Was also thinking a couple channels welded to the sides and bridge across the excavation, but that densest seem to be very stable without alot of monkeying around on the ends of the channels.

Last I thought about pouring a whole new wall in front of the grade beam, and let it be the new wall, but it would have the surcharge on it from the building column, so not a good idea.

What have you done in the past? The existing grade beam also will need to be lowered, or re poured.

thanks.
I can do a sketch if needed.

 

[msquared48]

If you have any drawings you could post, plans, sections, it would be helpful.

Also, I assume that you mean loading docks for trucks and not boat docks...

If so, then you only have to lower the footing by 4 feet or so, correct?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ztengguy]

Correct, grade outside at soil height right now, and want to make truck docks out of it. Got to lower the footing down to 30" below outside grade of new dock (30" frost depth).

I am meeting with the contractor soon, will post a sketch of what we have going on tomorrow if possible.

 

[JAE]

Typically you would provide either two temporary shoring columns or scaffolds on either side of the beam with temporary cross beams.

Sometimes this also requires some steel angle diagonal braces to keep the shored beam from twisting/buckling due to the new bottom side support.

Depending on your conditions, the beam may have to cantilever out off the shoring cross beams.

The existing column would then simply hang from the end of the existing beam above, allowing excavation and re-construction of the new foundation.

 

[dhengr]

Could you just pressure grout around and down deep enough to improve the soil conditions so that the existing found. could be made to work? Then just excavate straight down, maybe trimming the col. ftg. too, and pour a new closure wall and ext. portion of a col. ftg., under the existing wall and ftg. You might also just crib under the existing found. walls, on either side of the excavation area for the new ftg. Of course, these options depend on the make-up of the existing found. walls, soil conditions, and min. roof loads, and maybe some temp. interior bracing. Some bldg. plans and ext. wall sections would be helpful in understanding what you’re dealing with.

 

[ztengguy]

Heres a sketch of whats going on. Its spread footings and grade beams, looks to be monolithic pours. The spread footings at the grade beams are down about 3'-0" from top of slab, and then a pier up to the column base. My thought and (the contractors) was to extend out a bit with a new wall down to frost depth. At the grade beam, this would be doweled into the existing grade beam. At the spread footings, I would have them excavate under and pour concrete down under the footing.

My thoughts were they could do a section of this wall at the spread footings first, using the existing grade beam to support the column, (along with belts and suspenders) why its excavated under and poured. They could do a couple feet beyond the width of the column spread footing each location, then come back and do the wall between at the grade beams. This way, the building is supported, and they can more than likely do each one in a days work. (just do it real quick, it will be ok, not really, but get the idea).

Final design the spread footing would be sort of like a pedestal of sorts that starts at a certain point in the wall height. I had thought about using teh exterior pad to help brace the wall, but not a good idea I dont think.

Depending on the soil, it may even stand up under the footings as they excavate. Not sure yet, got to get the soil report back.

What do you guys think?

 

[ztengguy]

anyone have any thoughts on this? Thanks, sorry for the bump!

 

[TXStructural]

Do not excavate under the existing if at all possible. If the soil is cohesive, then you may be able to excavate and construct (quickly as you say - which is not entirely wrong, since loading can be withheld until it is completed and you would have to avoid rain, etc.)

Your sketch makes me think that this is a pre-engineered metal building (the non-prismatic column), so as long as the building is very light weight and there is no outward thrust on the columns, I think your design should be fine. Also, you may be able to cut away the part of the existing footing which extends toward the new excavation, since you will have minimized loading on the footing (no snow on the roof, and no storage or traffic on the slab, etc.)

I would probably do something more monolithic instead of trying to finesse it. Concrete and rebar are cheap. Also, consider that the least troublesome way to backfill under the footing will probably be flowable fill, controlled low-strength concrete, or lean cement concrete (f'c slightly higher than bearing capacity of the soil). This allows them to simply fill the hole out of the back of a concrete truck with minimal consolidation and no compaction.

If the soil requires it, consider drilling a soldier pile wall to hold the soil back and form the structure of the new wall. I doubt this would be needed, but in cohesionless soils it could be an option.

 

[ztengguy]

Thanks. Attached is a typical bay. It is a metal building, and the wall is a endwall, so wind columns only, with gravity load.(was designed for future expansion, but no frame columns installed) I have not run a design on the wall, but getting it to work without extending the heel under existing might be challenging, since the surcharge from the wind column footing does add lateral load in my opinion (or if i connect to the wall, am I just transfering load down the wall stem). Still got to get my head around that.

I dont have info on the soil, but hopefully it will stand up, and they can work without shoring. The reduced footing size and why they build the other wall might be ok, again worried about pressure pushing out why exposed (asking soil to stand straight up with pressure on it).

The grade wall is only 8" wide, so using that to support the column footings why they excavate and pour helps a bit, but I doubt enough hold all the load.

Thanks