Spreader Base Plate Below Columns

[BD26]

Hello All,

I am currently working on a project where the client is looking to construct a mezzanine level within an existing portal frame structure. This shall be a fairly heavily loaded mezzanine as there is plenty of plant to be stored on it.

We have been searching the council archives for historical drawings to inform us of the existing foundation arrangements however there are none on record. We are currently carrying out non-intrusive survey works to scan the slab to try and ascertain the existing foundation solution. The ground conditions are pretty bad and from SI we are expecting a piled arrangement with a suspended slab, possibly a piled raft.

Now most of the checks from the column loads are pretty straight forward, we can back check the slab/foundation capacities if we get a fairly accurate survey. The main problem I have is bearing pressure below the columns. I am expecting to have to spread the column load over a decent area in order to justify supporting the column from the slab as punching shear will likely govern.

How would I justify the thickness of the steel plates below the columns to assume that the load is being spread over the full area, say if the base plates had to be 600mm x 600mm square for punching shear?

I am working to Eurocodes. I have seen methods where you can base the plate thickness on the moment capacity of the spreader and assuming a cantilever using the intended bearing pressure. Would this be the correct method, as it does not seem to take into account the stiffness of the plate itself?

Thanks.

 

[KootK]

1) the stiffness question is tough without getting into FEM.

2) a start is designing the base plate to its elastic failure limit rather than its plastic failure limit

3) an old rule of thumb that I've heard but not seen justified is to cantilever the plate no more that 5 x thickness from any stiff vertical plate elements.

4) I do think that plate stiffness is a valid concern and I'd probably be considering stiffeners in conjunction with the 5t business.

 

[r13]

Can you provide a raised pedestal to spread the load?

 

[BD26]

Thanks for getting back to me.

Yeah kootk I had FEM in mind to analyse this but was wondering of there was a way to simplify it by hand.I will have a look at the 5t rule tomorrow in the office and see what I end up with. If it is unrealistic then FEM might be my best bet.

Retired13- We are tight with space and columns are positioned within partitions so client would not be willing to use pedestals. Thanks