Foundations for wide cavity

dfader

Calling on all with experience with building wide cavities. I am planning a build with a wide cavity two story.

A major expense for this build is the additional size of the foundations due to the rule that the foundations have to be three times as wide as the wall and as deep as the wall. This is adding a couple of thousand euro on concrete.

The ground is good so soils capacity is not an issue and standard 900mm wide foundations would be adequate.

Is this extra size of foundation really necessary. Would i need to use a steel cage (top and bottom steel tied together) to avoid the bigger foundations or will bottom steel be sufficient.

I am aware that this is a structural issue and needs input from Engineer but just asking what other people have done in similar circumstances. attached is two options to help illustrate.

Quazzie

I can't see you getting the answer here that you want due to it asking for help on a structural element. The 3T x T rule is a tried and tested rule, and proven to work. You say a couple of thousand on concrete might be wasted now but how much would it cost to fix subsidence under the wall. If you want to minimize a few thousand off the concrete spend a few hundred on an engineer.

archtech

dfader wrote: »

Calling on all with experience with building wide cavities. I am planning a build with a wide cavity two story.

A major expense for this build is the additional size of the foundations due to the rule that the foundations have to be three times as wide as the wall and as deep as the wall. This is adding a couple of thousand euro on concrete.

The ground is good so soils capacity is not an issue and standard 900mm wide foundations would be adequate.

Is this extra size of foundation really necessary. Would i need to use a steel cage (top and bottom steel tied together) to avoid the bigger foundations or will bottom steel be sufficient.

I am aware that this is a structural issue and needs input from Engineer but just asking what other people have done in similar circumstances. attached is two options to help illustrate.

How do you know the ground is of good bearing capacity?

Engage the services of a structural engineer with PI. S/he is best positioned/qualified to advise the most cost effective solution.

dos29

Finished foundations for 200mm cavity recently. Stuck with the regular rule, (3T x T is it? whatevers in the homebond book anyway). That was on VERY solid ground. Never have any doubts that way.
I dont know if it would add a couple of grand though. Maybe my maths is wrong but...
You have an overall width for the large founds of 1390, but 4 x 470 = 1410.
Taking that as the dimension, I'm making out that a mtr cubed of concrete will make up the difference of 17 mtrs of foundations.
At the top end price of €100 per mtr, that couldnt come close to a couple of grand, could it?
Could be wrong though, known to happen...

Poor Uncle Tom

dfader wrote: »

The ground is good so soils capacity is not an issue and standard 900mm wide foundations would be adequate.

Really? then why are you asking here?

dfader wrote: »

I am aware that this is a structural issue and needs input from Engineer but just asking what other people have done in similar circumstances.

I would imagine that others do what their engineers specify.

This is a specific structural enquiry, which needs to be addressed by a structural engineer.

Thread closed.