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TRANSFERRING COLUMN LOAD WITH LEVELING NUTS

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DSPE01

Civil/Environmental
Nov 17, 2010
7
WE ARE RELOCATING EXISTING COLUMNS ON A TRANSIT STRUCTURE. THE EXIST. COLUMNS ARE DIRECTLY BELOW THE END OF THE GIRDERS. THE CONTRACT DOCS HAVE US INSTALLING NEW COLUMNS CONNECTED TO THE GIRDER END FOLLOWED BY REMOVAL OF THE OLD COLUMN. THE HEAVIEST COLUMN DEAD LOAD IS AROUND 275K. ONCE THE NEW COLUMN IS ERECTED, ON THE (4) 1 1/4" ANCHOR RODS, I WOULD LIKE TO USE THE LEVELING NUTS TO TRANSFER PART OF THE LOAD TO THE NEW COLUMNS.
QUESTION
1 - DO YOU THINK TRANSFERRING 50% OF THE DEAD LOAD PRIOR TO CUTTING OUT THE OLD COLUMN IS OKAY (AS THE OLD COLUMN IS CUT, THE BALANCE OF THE LOAD IS NATURALLY TRANSFERRED. ALSO THE EXISTING LOADING IS TOUGH TO FIGURE ACCURATELY.
2 - I AM THINKING THAT TURN OF THE NUT APPROACH IS THE ONLY WAY TO FIGURE OUT HOW MUCH LOAD IS TRANSFEERED (TURN OF THE NUT = PITCH DISTANCE => APPLIED STRAIN => STRESS => FORCE IN NEW COLUMN)?
 
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I am not familiar with the structures you describe.
What is the connection of the existing column to the footings ?
I am guessing something like fully grout supported base plates or something similar.

For my money, supporting anything on leveling nuts temporarily or permanently is looking for trouble. For one thing, many anchor bolt details have features that potentially float unless a large tensile force removes clearance and preloads the assembly.
It also means tightening the upper nuts only clamps the base plate against the leveling nut, not against the grout, or hard shims beneath. The end result is the system sitting on the anchor bolts like a hair brush resting on its bristles with very low lateral stiffness. The anchor bolts also lose the large fatigue resistance inherent with a preloaded, clamped joint.

Rotating machinery, absolutely not.

That opposing nut design is standard practice for Light poles in the parking lot.

I'm sure it sure make truing up the light pole a lot easier, just like the 2 pairs of leveling screws on my builder's level.

 
I picture there is a flange or baseplate welded to the bottom of the column, and the anchor bolt holes are drilled in that plate. I forgot to mention my concern that the plate needs to be sufficiently thick to accept the bending loads from the proposed scheme of being perched on the anchor bolts.
 
I work for the contractor. The replacement columns and base plates are already designed and fabricated per the drawings (3" thick base plates, W36x300 columns). Before the exist. column is cut out, the new column base plate will be grouted solid with non-shrink grout and strength achieved. I would just like to transfer part of the DL using the leveling nuts prior to grouting the base pl. There will be approx. 1 1/2" NSG below the base pl.
 
I have see thousands of column + welded baseplate assemblies, but they've always been in place before my part of the job begins, so forgive my apparently "baseless" fundemental question.

If the anchor bolts/rods are in-place already in the concrete pad or slab, and then the welded column and baseplate are lifted into position over the anchor bolts, it seems that only by using the leveling bolts to hold up the assembly is the only practical way to proceed. If so, then it appears you have to "clamp" the upper nuts down against the leveling nuts underneath - which voids any compression being put on the grout. Since the grout always seems to be "pushed" or shoved under the machine foundation or column baseplate later by hand, how do you ever get compression on the assembled baseplate?

Are you then saying that the column cannot be loaded (additional weight and structural steel members) cannot be added to the columns + baseplate until the grout has dried to sufficient strength under the new baseplate?
 
How are you going to measure how much of the load you have transferred to the new columns? Trying to use the leveling nuts to act as jackscrews bothers me; I can see unequal load distribution between the various nuts under the baseplate. How about adding a temporary jacking column and then all you have to do is snug the new column up under the beam.
 
screwman - thanks. I have 12 columns to replace at 5' o.c. My thoughts were that since rotation of the nut translates into an exact rise along the thread I would therefore know the strain that is being imposed on the column and from that the stress and load transfer. I just think its prudent to know that some load has been transfered to the new column prior to removal of the existing columns. Once I think some of the load is trasfered, we will grout the base plates. Once the grout has achieved strength, we will remove the exist. columns.
 
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