RSJ Box Frame Depth

[LJD1974]

Hi,
My upcoming wraparound extension calls for a (8m long and 305mm square) Steel box frame to support the 1st floor side wall where it is being knocked through, but I'm trying to understand how deep the bottom Steel needs to sit in the ground?
My solid floor will consist of 150mm compacted hardcore, 50mm sand, DPM, 100mm concrete slab, 100mm rigid insulation, and 50mm screed with UFH, so 450mm in all.
I'm assuming that the steel would have to be encased in mesh and concrete, and then sit between the hardcore, and below the 50mm sand layer (so 605mm to 300mm below the FFL). Is this correct?
Would that steel also need concrete foundations of its own?
More for my undersanding and planning than anything right now.
Thanks!

 

[oldbutnotdead]

Your SE should have specified how the steelwork is to be supported. If it's a full box then yes the bottom needs the same support as the wall it is replacing so foundations of some description will be needed. Goalposts more common but ....

 

[LJD1974]

Thanks. It will sit on the existing concrete foundations of the wall it’s replacing, but once cut down to the required depth for the steel, the foundations will be around 300mm deep. Fingers crossed that will be enough, as I wouldn’t want to have to dig those foundations out completely and redo if 400mm was needed for example. :-/

 

[oldbutnotdead]

Unless you're putting an extra floor in, the steelwork is only supporting the same load as previously, provided existing foundations are adequate it'll be fine. Is a full box really needed?

 

[LJD1974]

Full box is what our SE says we need. It’s 8m long and 300x300mm beam, so quite considerable. It’s just supporting one floor above.
Funnily enough, the same size beam will be at the top holding up the house with no support all along it, other than the pillars at either end, so hopefully a 300mm deep concrete foundation for the bottom steel will be just fine.

 

[oldbutnotdead]

Ho hum. I'm no expert but unless your flank walls are made of cheese or the house is on poor ground a full box sounds a bit overkill. But if that's what the engineer says.....

 

[LJD1974]

We are on an old Thames gravel bed, which it pretty much sand and small pebbles, so that may account for it.