Selective Demo - Concrete Columns

[BubbaJ]

A concrete building will experience selective demolition. The lowest floor will remain intact. Aside from being troublesome to cut the existing columns off flush with the remaining concrete slab, can you see drawbacks to this. What if they come back later and rebuild?

I was considering advising them to leave 5 - 6" of column above the slab, which would be the thickness of the new roof insulation, but this may create a thermal bridge.

Thoughts?

TIA

 

[jayrod12]

What about the development of the column bars? There's likely to be moment present in the top of the column, therefore the bars need to be working almost instantly.

 

[BubbaJ]

That was my other concern. We also don't know the configuration of the beam reinforcing. If the bar is hooked into the column for development length, they'd be cutting that off as well.

They won't like it, but I think they need to leave at least 20" of column if they want to reuse them in the future.

 

[jayrod12]

Agreed

 

[bridgebuster]

Could you use couplers? We just finished plans for a bridge project where there will some select demo of columns and cap beams. We called for couplers at the column tops to hook bars into the new caps.

Also, you need to think about how the shoring will impact the work.

 

[RayGon]

Demolition of the building will reduce the loads on your column. Its your beams and floor slab that you should be checking.

When you demolish the column above the floor, the BM distribution is altered as the stiffness of the upper column is removed. Your column,slab and beam will therefore have to share the BM.

In my experience old buildings are designed conservatively that most beams and slabs are designed with wl2/8 instead of the reduced BM (i.e. wl2/10 or /12) and because your column have a lot less axial load its BM capacity is much more higher due to the interaction curve BM increases with decreasing Nu.

There should not be any concern at all. But as always engineer should do at least some form of investigation specially if demolition is not your cup of tea.