Column on RC slab - solution?

[greznik91]

I'm dealing with an existing building. They will remove old roof covering and replace it with new one which is much heavier (self weight is 3x larger than before). The problem is that the roof is supported on RC columns that are supported on RC slab that is only 150 mm thick. Columns are not that small (200 x 400 mm) bit punching shear control of a slab shows that slab is not sufficient. So I have 2 options as far as I can see:

1) To make existing clomns bigger in cross section dimensions (so I get larger contact area and punching shear is no longer a problem). To anchor new reinf. cage to a column and in to a slab and then pour the concrete in... But I dont think there is a guarentee that this will actually work?

2) to make another support - columns on top of a wall that is very close to existing column (see attached image). But in this case my load path to foundation bellow a wall is pretty direct which Im not a fan of. But on the other hand I think since existing column is already so close to an edge of a slab (and a wall bellow) I dont think it makes much of a difference.

Thoughts?

IMAGE:

https://imgur.com/a/8Qit5

 

[dik]

Depending on the load and the soil, can you sawcut a 2'x2' square in the SOG and excavate the soil to a depth of 8" or 10" and cast a new footing?

Dik

 

[greznik91]

No known data for the soil...
Load from column is 140 kN.

 

[SLTA]

Get the soil data, cut out the slab, and pour the required footing for the new loads. This is the most common way of dealing with this issue. Taking new load to existing walls can be tricky, as you have to connect it properly or you have a hinge. Any lateral load in the column also goes into the wall, which has to be checked for that.

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