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Shoring a column
2

Shoring a column

Shoring a column

(OP)
Got a situation where a column needs to be shored to replace a footing. Plan was some channels each side of flange, all welded up as needed. What else do I need to look out for? Obviously check the channels for bending, bearing, etc. Any issues with column? Stiffeners needed?

RE: Shoring a column

Even if you know the (expected) loads, be a lot more conservative with the temporary design than you would be for a new structure.

Post a dimensioned sketch, preferably drawn to scale.

RE: Shoring a column

I'm with SRE. What you do depends on the loads and consequences of failure. I'm much more cavalier with replacing garage columns than I am those supporting an occupied high-rise; but both can be and are replaced under load.

If you could explain the plan view plus loading that would help. Also, are we talking a concrete column or a steel column?

RE: Shoring a column

Depends on the loads and reactions at the column. Any uplift or part of a moment or braced frame with large shear / horizontal reactions? I would be concerned with stability if column is part of LFRS.

Would you just support your channels/shoring on cribbing down to earth or an existing slab? I could see a challenge with providing enough clearance during excavation and construction of the new foundation. Do you need enough room for a whole new spread footing and pier? A photo or sketch would help. I've designed shoring to temporarily support and replace sections of damaged steel columns but not their footings.

RE: Shoring a column

2
Make sure your temporary shoring is VERY laterally braced.

RE: Shoring a column

(OP)
Here is my idea. 20K DL, 10K LL No uplift, not part of a frame or anything. new footing interior, so just below slab. More than likely 12" thick. About a 500 sq trib to the column, bar joists, wood floors. Floor and roof above supported. Figure its not going to snow so min roof live load.

(2)-C12x20.7 channels on the flanges. Column is a W8x24.

Will crib w/ 6x6's each end. One end on a slab, other on excavated soil. Figure about 3'-6" square crib.

Will have them tie the channels together at the ends, weld to a bearing plate too.

RE: Shoring a column

Need to check the C flanges for crippling, the C webs for shear buckling.
Maybe a series of vertical stiffeners for the C’s.
And need more lateral support to keep the C’s from overall lateral buckling.

RE: Shoring a column

It seems light to me. But I have not run the numbers.
FYI in bridge construction our jacking designs are the max of factored D+L or unfactored 2*D. whichever is greater. Loads in construction can be hard to predict.

I have lifted a bridge and it was almost 1.6 times what the estimated dead load was (in theory)

when in doubt...make it stout.

RE: Shoring a column

@ JStructsteel (Structural)(OP),

The sizes seems OK. I have a point, you need to transfer the load to the set up with using jack at one side , or wedges to control the deflection of the syst.
The following excerpt from FOUNDATIONS OF STRUCTURES ( BY DUNHAM , 1950,President TRUMAN era. )

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

RE: Shoring a column

(OP)
Thanks all. I added some stiffners and tie plates across the channels. Added a note to pre-load to the estimated deflection.

Thoughts?

RE: Shoring a column

JStructsteel - The beams will deflect, the cribbing will compress when loaded... top of the new footing will have to be somewhat lower than the existing. Will this lowering of the column cause temporary or permanent problems with floor and roof above?

This project is in the "grey-area" that is a blend of structural and geotechnical. Geotechnical has been ignored.

1) Ten feet long beams are too short for a (theoretical) 10'0" center-to-center span.
First, there needs to be a construction tolerance on the c-to-c span, say, +3".
Second, beams whose length is equal to the c-to-c span put unnecessary eccentric point loading on the cribbing. From the drawing, a beam length of about 12' would be a better choice.

2) Size and depth (below ground) of the existing & new footing are not given. The cribbing will put significant distributed surcharge load on the soil. This loading may be too close to the excavation, resulting in unexpected soil settlement under the cribbing, or worse.



RE: Shoring a column

(OP)
Once shored, the contractor is going to pour new footing up to the existing column. It will be poured down to existing soil. My thought is that the soil is already consolidated below the footing.

Not sure about the eccentricity, I am showing them at the center of the cribbing. Layout of cribbing location given by contractor, one side is going to be at same elevation as bottom of footing, other end on slab.

New top of footing will be 4-6" below grade. Footing thickness 1'-4"

RE: Shoring a column

Quote (JStructsteel)

1) Once shored, the contractor is going to pour new footing up to the existing column. It will be poured down to existing soil.

2) My thought is that the soil is already consolidated below the footing.

3) Not sure about the eccentricity, I am showing them at the center of the cribbing.

4) Layout of cribbing location given by contractor, one side is going to be at same elevation as bottom of footing, other end on slab. New top of footing will be 4-6" below grade. Footing thickness 1'-4"

1) The beams will have deflected and the cribbing compressed...top of the new (full depth) footing will be lower than the existing.

2) Agreed... now... how about after disturbance caused by removing the existing footing?

3) I am sure... but it's your project.

4) Redraw the sketch to scale, both vertically and horizontally, and post it here. Be sure to show the size of the existing footing located in an (oversized) excavated hole that is 5' 10" deep.

RE: Shoring a column

(OP)
The new top of footing can be at bottom of baseplate, or a bit lower, then they could grout.

I guess I am not sure about the eccentricity part. Are you saying they will load the edge of the cribbing?

RE: Shoring a column

(OP)
Here is a picture. There really wasnt a footing. It was more slab bearing than a footing. They cut the slab to do new pipe work.

RE: Shoring a column

JStructsteel - Grouting can and should be done, but unless the column is jacked (raised) to do the grouting, the column will be lower than it is now. The amount lower will probably be small, say, << 1 in, and this may be acceptable. However, you (engineer) make that decision, it is not something to just ignore or "pretend" is not a potential problem (say, roof ponding).

Eccentric loading, yes... more or less, the assumed point load from the beam will be off-center. Again, this could be minor... but it is easily avoided by using longer beams. Don't take any risks that are unnecessary. Also, eccentricity may NOT be minor if the cribbing is not level... a real possibility.

Redraw, to scale (horizontal & vertical), one thing for sure, the first two sketches are clearly inaccurate. We need to see exactly what you are dealing with... a picture is worth a thousand words.

RE: Shoring a column

(OP)
Here is the to scale footing

RE: Shoring a column

Show the existing ground line and top of concrete slab on this drawing.

What about the "top of the footing will be 4' 6" below grade" plus 1'4" thick?
That must be the new footing?
Draw a separate scale drawing showing the new footing. There may be significant (geotechnical) "challenges" to overcome in that case.

RE: Shoring a column

(OP)
Four to six inches below grade.(4"-6") not 4'-6" below grade. my bad.

RE: Shoring a column

Strap the channels together with welded plates to make them boxy so they do not roll sideways.

www.PeirceEngineering.com

RE: Shoring a column

(OP)
PE inc its hard to see, but I have some plates top and bottom welded to the channels.

RE: Shoring a column

Ok, here is my understanding... it does not make total sense to me, but my best interpretation:



Blue - Is the bottom of the cribbing below grade?

Orange - Some soil (wider than the perimeter of the footing) will be disturbed during demolition... how much is an educated guess.

Red - If an eccentric load were applied to the cribbing, note that contact pressure between cribbing and soil is highest at the worst possible place... closest to the excavation. Avoid eccentric loading, as much as possible.

Green - Nine inches is a ridiculously tight clearance. The excavation is shallow, but the risk of heavy equipment accidentally impacting heavily loaded, weak (Y-axis) beams is high. I would want that distance to be at least 3' instead of 9"... of course there are consequences:

The selected beams that passed for 10' span, likely fail for 14.5' span. Revisit beam size and bracing, do what it takes to increase Y-axis strength.

There is probably more, but that is what I see with my assumptions... comments?

RE: Shoring a column

Unless there's huge space restraints I'd assume at least 6" of over-excavation below the footing (potentially 12") and a handful of inches to the side, and then draw a load influence area from the cribbing to see if they interact.

The extra beam length to buy a couple of feet is way cheaper than the stress involved if you need to try to qualify it after the fact for dealing with field requirements.

I've basically never ended up thanking myself for being aggressive on shoring work. I often tell myself initially that what I've got on paper looks excessive and then talk myself into bringing things down. In the field, though, it rarely actually looks excessive.

My question is whether there's any reason to be concerned about lateral restraint on this column. Does what you've got provide enough restraint into the page for whatever buckling conditions may be expected? The normal assumption is that the base is stiffly restrained in the lateral directions. Here you've got reduced friction and weak axis bending in the channels. It's likely okay, but it depends on the geometry of everything. Do you want to run a tie to another column into the page?

RE: Shoring a column

(OP)
Update: Contractor completed the new footing. He reported they had no issues with movement, sliding soil, etc. They leveled out the soil to be at the same elevation as the footing. They were able to jack up the existing column to where it was previously.
Thanks for your help

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