Existing Column Removal

[spieng89]

I have 1-story structure where I am adding a sub-level below the roof. In order to support the increased load at this level, I am cutting existing columns mid span and adding beefier columns below to support the sub-level and new foundations. The existing columns will rest on transfer base plates sitting atop of the new column below. Essentially a column splice. Done this a few times, no issues but never a large amount of columns at one time.

My question is how many of these columns would you allow a contractor to cut/shore at one time. Of course the contractor wants to cut/shore all the columns at one time. None of these columns are a part of the lateral system, gravity columns only. There are about 30 columns in an area.

I can't find structural reason why they couldn't do this, they'd complete the install of new columns before 180 days (which is the time limit I use based on IBC section 3103 to consider a structural condition temporary), nothing in OSHA barring this. Only my gut feeling of not wanting a good portion of the roof being supported on temporary shoring. Another engineer is responsible for the shoring design.

I'm thinking of telling them to shore every other column for no other reason of sleeping well at night. But would like to have a more robust reasoning behind the decision if there is pushback. Any thoughts?

 

[KootK]

1) I also can't think of a good reason to preclude doing everything at once so long as, like you said, everything is shored properly. Of course, by "shored properly" I think that I'd want the shoring itself to have some lateral load resisting capability. The shoring systems are unlikely to be as plumb as the columns themselves so you're likely to have more gravity induced P-Delta instability in the shored system than the un-shored one. I think that even this effect would be minor but, at the same time, I see no reason to take chances with it.

2) What does surprise me here is your approach to strengthening. I would normally have thought one of these approaches to be less invasive:

a) Take advantage of the fact that the new sublevel probably braces the columns laterally mid-height and, therefore, should substantially increase their carrying capacity without reinforcement. Combine this with footing reinforcement if necessary.

b) Reinforce the existing columns with channels etc to get the job done without removing them. Combine this with footing reinforcement if necessary.

That said, I certainly do like the outright footing replacement from a mechanical perspective. Footing enlargement is always a bit dubious in my opinon.

 

[phamENG]

Not something I'd fight them on. Make sure the shoring design is a good one, and let them do their thing. If you start dictating the procedure, you start straying into dictating the contractor's means and methods. If something goes wrong (like the contractor running into one of the unshored columns with something), what happens when they say "well we wanted to shore all of them - that one was only unshored because the engineer told us to do it that way." Oops.

 

[oldestguy]

Reminds me of the "famous" Frank L. Wright". He just removed the offending column. Sagging roof didn't bother him. It was his own building.

 

[phamENG]

Sagging roof didn't bother him.

Quick way to add a water feature to a building!

 

[dik]

Can you plate across the toe of the flanges to make a 'box' shape... don't need to go full height... just the middle 90%-95% or so... assuming W Sections... don't have to worry about buckling at the ends... so can use phi*As*fy... did that for some large heavily loaded columns for an auto plant in Windsor... columns were 80' high...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[spieng89]

Thanks everyone for the feedback. Makes sense not to intrude on their means and methods. One less item on my plate.

@kootk on other similar jobs I've used channel reinforcement and other methods to strengthen the column. On this particular job, the weight of the sub level is substantial and seismic ended up controlling the design and we couldn't use brscing; only moment frames. Actually made it easier to design, as you said in your post, designing footing extensions is not an exact science haha. I try to avoid it when possible.

 

[r13]

....they'd complete the install of new columns before 180 days

I don't think this is a good idea. The contractor shall order and get the material to site, then start the cutting, for which the unsupport duration shall be kept to the minimum.

 

[KootK]

On this particular job, the weight of the sub level is substantial and seismic ended up controlling the design and we couldn't use brscing; only moment frames.

Interesting. So your gravity columns have to ride along for the same drift that the moment frames do then? I could see that changing the game.