Pad foundation on top of existing foundation

[iwanttodiy]

Hi guys,

Has anyone come across this? And is it structurally stable? My house is a 1900ish house, and the existing foundations have held this house as well as many others in the area for coming up 100yrs. The soil is clay, no trees.

My current foundations are 700mm below ground level and are 200mm thick and we assume it is 600mm.

The structural engineer has specified a pad foundation for columns at 1200x1000x500. This puts the pad right on top of the existing foundations. The SE has provided all the calcs, and to be checked by BCO.

I suppose my question is, is this standard practice to sit foundations on top of foundations? Surely there’s a risk of it pushing the existing foundations down?

Would it not be safer to just break the existing foundations and do a full 1m foundation pad?

As work is starting next week, I thought I’ll just double check everything to give me peace of mind

 

[noseall]

You would typically dig beyond the existing foundations and wrap yours around the existing.

 

[wessex101]

I would suggest that means the new concrete pad needs to be 1200mm long x 1000mm wide x 500mm thick and should be set at the same depth as the new foundations.

 

[tony1851]

1200 x 1000? Is this a house or a 10-storey block of flats?

 

[iwanttodiy]

This is 1 of 2 padstones for holding a goal post frame consisting of 2 columns and a 6m RSJ, for a house

 

[iwanttodiy]

Have actually gone to the builder to go 1m deep for the padstones to preempt any objections from building control.

Also, gives me peace of mind, that the columns are sitting on something that is higher spec’ed

 

[^woody^]

Where are the drains?

 

[iwanttodiy]

No drains near the pads.

Or do you mean the make shift air duct that looks like a drain? (Decided to not do a solid concrete floor, and instead do a timber suspended floor so it matches the rest of the house, and can have free flowing air)

 

[^woody^]

No shared drains along the back within 3m of any new foundations?

 

[iwanttodiy]

No, lucky for my semi the drains go from the side of each of the detached houses and joins in a manhole in the middle and down to the main road. No drains in the back.

 

[iwanttodiy]

A question for you guys who have seen or installed pad foundations.

What’s the accuracy threshold you are generally happy with? We are sitting a 203mm column on a 350mm bearing plate on the 1200x950x500 reinforced pad. Engineer has specified its eccentric by 200mm from the centre away from the load, see attached. Obviously when on site, it’s best endeavours thing when it comes to precision, due to different varying factors.

On consultation with the engineer, we are asked to keep it as close as possible to the mm. On a whim, he’s mentioned +-100mm but that’s then not his responsibility if problems arise, though unlikely at that threshold. What experiences you have had?

 

[^woody^]

More accurate than those drawings would be something to aim for.

 

[freddiemercurystwin]

Gonna end up with a nice cold bridge on that column at the bottom.

 

[iwanttodiy]

Gonna end up with a nice cold bridge on that column at the bottom.

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yes, figured that much. engineer didn't mention anything about thermal bridging. None of the steels he has specified had any thermal considerations.

I tried to locate some steel thermal breaks, but seems they are still very specialist. I was just going to wrap it up in PUR insulation and either an aerogel blanket on the outside. maybe put a Tyvek avcl layer on the inside​

 

[iwanttodiy]

More accurate than those drawings would be something to aim for.

Guys, are there any structural engineers who can help me confirm the maximum bearing for a column on a bearing plate on a reinforced pad foundation (1200x1000x800) created on C35 with 10mm mesh on top and bottom. Unfortunately they are being poured today...

My SE says it needs to be exactly where he designed. Worst case +-100mm (he keeps changing this tolerance). This pretty much puts the column in the centre of Y-axis and -200 on X-axis.

My contractors have excavated the pads ready for the pour, however I just checked them and they are way off the intended positioning justifying that minimum 100mm bearing is all that is needed, and that is normal. I understand for a standard pad stone and RSJ that is all that’s needed, but a pad foundation with a column is a bit more sensitive? The pad will be tied into the existing foundations (with more mesh) and poured at the same time as the rest of the foundations. We have also increased the side of the foundations from 500 to 800.