modifying strip footing on 9 storey building

[calculor]

Working on a project, 9 storey building with two way slabs and exterior ICF walls. Foundation wall is poured with the strip footing for exterior walls. Problem is the client wants to change the exterior walls to concrete columns which the strip footing obviously cannot support. Anybody have any suggestions on how to "economically" retrofit the strip footing for the column loads?

 

[271828]

What kind of soil/rock?

 

[271828]

Another Question: Do they want to demo the entire wall and then install new concrete columns? Or...is the idea to chop out big parts of the wall, effectively leaving "columns" behind?

 

[flyby911]

3rd question :

your how question is about the design or execution?

 

[calculor]

First question, the soil varies around the bldg the bearing capacity is 5ksf for half the bldg, 8 ksf at middle, and 10ksf for remainder.

Second question, No, I do not want demo wall I'm thinking of pouring columns tight to adjacent walls that have already been pouring. My issue is primarily with the footing.

 

[hokie66]

Calculor,

If there were intended to be bearing walls all the way up and you are replacing them with columns, then there is obviously already plenty of capacity in the strip footings to carry the load. To distribute the load to the strip footings, you would need to design the floor above to transfer the new column loads onto the already constructed wall.

If that is not an option, then you will have to either 1)demolish part of the wall and strip footing at each column to allow construction of a column footing, or 2) dig under the wall footing and construct the column footing, probably not requiring much in the way of underpinning since there is not much load from above as yet.

 

[dik]

With the varying bearing capacity, the changes could cause significant differential settlement... Can you cut out the lower portion of the wall to provide a stiff spandrel?, provide more frequent column supports and use steel to stiffen the strip footing?

Dik

 

[calculor]

Adding steel to stiffen the footing is not an option as is cutting the lower portion of wall. However, reducing the column spacing and transferring the load to the wall as hokie mentioned may work. My problem is the foundation wall is only 10" thick, which limits the dimension of my column.

 

[hokie66]

How big are the columns above? Are they partly over the wall? You may be able to counter the eccentricity with an equivalent couple in the floor if you have enough depth. Use strut and tie analysis.

 

[msucog]

from the geotech side of it: someone gave you from 5ksf to 10ksf in soil for the bearing capacity across the building foundation? yikes...i hope someone is on top of their game when giving recommendations like that for foundation support on soil. i would think differential settlement would kill recommendations like that even if the soil was good to 10ksf, but that's just me not knowing anything about the site conditions...i would enlist the help of the geotech to evaluate the settlement to see if your structure can tolerate it.

 

[calculor]

Hokie,
I'm looking at 12x24 columns centered over the walls with 24" dimension parallel to the wall. I'm thinking about increasing the thickness of the wall in this area to 12". Placing the columns over the wall I can take of the wall to distribute to column loads more uniformly. Problem with this approach I vaguely recall seeing a new section in CSA A23.3 CL 14 which restricts the amount of wall that be considered part of the column when cast integral. I don't have a copy of the code near but if my recollection is incorrect this is will be the approach I will consider.

 

[dik]

Can you not cut out a portion of wall to install a steel column, letting the wall arch across the opening for the new steel beam/channel?

You have no design live load on the structure and likely a conservative dead load (no finishes, etc.)you can even use a braced beam/channel until the adjacent one is installed and welded for continuity then the brace can be eliminated.

Offer to do a new building for the client <G> and live with th e wall. <G>

In these environs, 10" is the minimum size for a column and you might have difficulty getting this to work. You may have to reconsider some of your constraints for the remedy. Too many restrictions and you may not be able to do it.

Dik

 

[hokie66]

Calculor,
Just so we understand exactly what your situation is, does the 10" wall go up to support the first suspended floor, and then the 12" x 24" columns start on top of that floor? If that is the situation, you don't have a problem. Column is only 1" wider than wall each side. The load should be taken down by the storey high wall as a deep beam to the footing, provided geometry is reasonable and reinforcement in wall is sufficient to provide ties.

Hang on, I may have missed something. Just reread your original post. What is an "ICF wall"? Hate acronyms in the worldwide environment we live in. My advice is all based on the foundation wall being cast in place reinforced concrete.

 

[calculor]

Hokie,
Yes you are correct, the 10" wall will support the 12x24 columns which will start on top of the suspended slab, dowels will be drilled for the columns. I'm still going to increase the wall thickness below the column because the face of the column will be flush with the outside face of wall. The wall is cast in place with one layer of 15M bars at 16" horizontal and vertical on the inside face; and one layer of 10M bars at 16" horizontal and vertical on the outide fac. The wall to footing dowels match the spacing of the wall reinforcement.
An ICF wall is a cast inplace concrete wall that uses the insulation as the formwork.