Old Sandstone Foundations

[Ben_StructEng]

Hi,

I've got a project where my client is wanting to add an additional storey to a single storey house.

The building is from the 1920's in Sydney and is constructed as Sandstone footings (approx 400mm wide solid blocks) and then timber framed floor, with double clay masonry external walls.

The New additions are proposed to be lightvweight timber framed construction.

Seeing as though I am in reading the loads on the structure, I'm looking for a way to appraise the existing foundations and their ability to accommodate the new loads.

Part of the investigation is obviously to have a Geo site classification and to determine the size and depth of footings, but then comes the tricky part....the justificafion of this system to new codes. The current standards require a concrete strip foooting under any masonry walls.

I'm not concerned about the compressive capacity of the sandstone blocks as there is little weathering and in a very good condition, however I'm just not sure where to start in terms of appraising the system without underpinning the whole house, which sounds overkill..

 

[MIStructE_IRE]

In such cases I’ve often worked out the existing load then compared it to the new. If its less than a 6-7% increase I don’t think I’d lose any sleep.

 

[Tomfh]

It all depends on what the footings bear upon. We’ve worked with similar footings where the sandstone masonry bears upon sandstone bedrock. Happy days.

If however they’re founded on poor soils, not so good.

 

[Ben_StructEng]

Thank you both. Couldn't agree more with what you've both said.

I guess the question begs if you're adding more than say 10-15% loading and you're not on rock. If I assume that a Geotech Investigation verifies that the bearing capacity is sufficient, though I'm on a highly reactive clay or similar soil that shrinks and swells, then a sandstone footing, which doesn't have the flexural capacity of a concrete footing, is going to be tricky to justify.

I've not seen any papers on how to appraise such a system (and this also includes clay brick footings) to accommodate new or revised loadings, given various soil conditions.