Depth to top of foundation

[MIStructE_IRE]

Typically in a steel frame (data centre in this case) I would have top of pad foundations anywhere from 450mm to max 600mm below slab level.

In this case I have someone telling me they want top of pad 1m below slab level, which is killing my column effective lengths.

What are you guys typically doing?

 

[jayrod12]

Foundation depths are usually dictated by the geotech. Why couldn't you pour a plinth or pier up to whatever elevation you'd prefer?

If someone on the client's end is demanding 1m from floor to t/o foundation, and it's causing your column sizes to change, then tell them that and let them figure out how badly they want the 1m.

 

[MIStructE_IRE]

Thanks!

Geotech has confirmed the foundation formation level, which I’m trenchfilling down to. This is purely related to top of found.

 

[human909]

What is the infill between the top of the foundation and the slab? Surely it could be considered to laterally restrain your column and thus reduce your effectively length.

 

[human909]

I agree that some rational approach should be used to check if either a pedestal, infill or the slab can act as an effective brace. It really depends on the magnitudes of axial loads we are talking about. Also many column base plates in my experience provide non insignificant fixities and assuming a pinned connection could be conservative. (Though this would depend a fair whether the foundation was a raft/ground beam or an isolated pad.)

I'm used to 0-200mm of embedment. With the foundation quite often being poured up to the level of the SOG. The 200mm has never troubled any of my designs as it is normally a negligible increase in effective length.

 

[MIStructE_IRE]

I’ve often thought about using the SOG as a restraint. However I usually have a compressible filler board isolation joint around the column’s below ground encasement. So I’d get a fair degree of movement before its ‘restrained’.

Similarly I’m not sure I’ve ever managed to convince myself that the compacted stone below slab would actually restrain the column. An interesting thesis for someone!

 

[KootK]

Actually, I rescind my previous objections. I'm fighting a wicked cold and have a head full of cotton right now. I'm in no state for any deep thinking.

 

[MIStructE_IRE]

Get well soon Koot! Look after yourself.