Footings under patio glazing

[SuperPav]

I realised I might have made a bit of an oversight here...

 

We are doing extension (and adding a storey) to our bungalow. To the rear, either side of the kitchen/diner is flanked by masonry walls (on 1.2m deep footings). Across in between these two walls (7.2m span) will be a full wall of glazing floor to ceiling. Internal flooring will be screed over 100mm concrete slab on ~200mm insulation. When doing the schedule of works for the builders, I overlooked the need for footings under the glazing, as I just calculated them under the walls! D'oh!

 

My question is: What/if any foundations are required under the glazed wall (it will most likely be something along the lines of Rationel Auraplus fixed glazing with two sets of patio doors)? I'm hoping it's not necessary to put anything substantial or deep, as the only thing that will be supported is the glazing.

 

What have people done for footings under big aperture spans?

 

Thanks!

Pav

[saveasteading]

 

Tempting to think that you have only to 1. support the glazing, not the house.   2 keep the frost and weather out.

 

If you use a less deep footing, it could move differentially to the main house (which may settle with load and rise and fall seasonally.)

 So I don't think that a slab deepening at the perimeter, or a minimal footing  is appropriate Unless you are bearing on natural sand/gravel. But with 1.2m depth that sounds like pretty ordinary ground.

 

If it was a narrower gap I would suggest building a beam that bears onto the two existing footings. Could be done with bars drilled into the concrete, and a fairly heavily reinforced ins-situ beam. That would also need some calculation.

 

BUT you have 7.2m span so that would need intermediate bases, 1.2m deep.

So by the time you have dug 2 end pits, one or 2 mid pits to 1.2m , and the rest to frost depth , drill ends, reinforce  and prove it to BS, you might as well dig it all out, and use mass fill concrete and then finish to underside as appropriate.

 

I started writing this thinking we can be clever here, and ended thinking, dig it out and mass fill.

 

 

 

 

[SuperPav]

Thanks, I've pretty much resolved myself to asking the builders to just dig out the 7m trench and fill it, over budget before we even started!  I guess I just can't get my head round why a double garage opening is sufficiently accommodated by a thickened edge to the slab without any footings, as surely the loadings on that edge from a vehicle driving in and out would be far greater than from a fixed piece of glazing.

 

With regards to putting beams across the span, there are footings that run nearly central to the span (existing wall there which is being taken down), so I could bear a beam onto it, with resultant split spans being approx 3.2-3.6m either side, would that just be a concrete lintel at a certain depth, with the slab bearing on top of it?

 

Also just realised there's an actual Foundations sub-form on here, oops. Mods, feel free to move if more relevant!

 

 

[saveasteading]

It isn't the weight of the car compared to glazing that is the issue, but that the building should stay still or move consistently. My house is built on 300 footings and moves a lot seasonally, but is made of wood and moves to suit. But it is not ideal.

 

If your garage moves but the door threshold doesn't, then you get a cracked garage floor and no worse.  If the same happens in the house you get cracked glazing, or gaps, and it matters more.

 

This matters especially when building on clay and near trees. Clay shrinks a lot when dry and expands again when wet. At 1.2m deep this is reduced as the ground remains damp, as well as being denser, usually. Buildings go up and down seasonally, but so does the ground around.

 

The beam solution would need a proper Engineer's design, and would cost more than the mass concrete.

Your builders will be much happier with digging a hole and filling it.

[George]

Yeah you need that foundation. It could be made smaller but unless you have unusually large footings you'll often find it's more hassle than it's worth.

 

You need a short section of wall to come up to near FFL for the door cill to sit on and to act as an edge for the underslab insulation and slab edge itself. You could achieve this with formwork etc but it's a much messier detail overall (espeically for DPM/DPC)  and better to have the ~450mm  wall up from the footings.

 

On my 3m bifold opening I stopped the innerleaf blockwork and just have the outerleaf brick for the cill and slab edge. It wasn't a typical detail but close enough. You architect should have this type of detail on a building regs drawing.

 

(structurally it is better for the slab to be isolated from the walls. Unless it's designed as a raft this is the best approach)

Edited April 30, 2021 by George

[SuperPav]

Resurrecting this as about to get cracking on these.

 

Where the foundation is running just for the benefit of glazing (i.e. no structural wall or loads on the foundation), am I best laying out the foundation so that the glazing bears on the inner edge, central, or outer edge? The glazing thickness is approx 100mm.

 

As there is no wall going here, I have a bit of flexibility to get the best detail for thermal efficiency (which I guess means putting the glazing/doors on the inboard edge of the footing, to maximise efficiency from any thermal break/upstand by keeping the footing outside of the thermal envelope?)

 

 

[SuperPav]

Complicating things further...

 

Have just been told by Severn Trent that we can't build some of the extension footings there to the current depth 1200mm as that area is above the sewer (IL ~2000mm). They want the footings taken down to below IL which is going to be fun, I suspect might even hit the water table before then! 

 

It's literally only supporting the floor and glazing (highlighted in yellow). Could do a warm insulated raft (if they allow) but that seems overkill for this section, and I'm not sure how to deal with the external threshold (patio paving/channel drainage) as the insualtion upstand would be in the way?

 

Any creative ideas here? Or am I overthinking it and just need to fork out however much the builders will want to try and dig the footings down to 2m with associated shuttering etc. Really can't be affording this at the moment though...

 

Proposed:

 

 

 

Existing for reference

 

[Mr Punter]

Could the windows be supported off a steel beam with pad foundations?

[SuperPav]

Thanks we could in theory yes, but the problem is I'd need a pad at that corner going down 2m to below the invert level.

And knowing my luck, it'll hit the sewer. Ironically this is in practice much riskier for damaging the sewer, even though ST would probably love the idea.

 

An alternative along similar lines would be some piles, but again, the chances of hitting the sewer are quite high - not fancying them odds. 

[dpmiller]

do you know the exact location on-the-ground of the pipe? the drawing may not be accurate...

[Conor]

I can almost guarantee you that the sewer pipe shown is not where it is in reality. Dig a trial hole where you intend on building and find out.

[SuperPav]

The drawing is based off us doing a CCTV survey and joining the two manhole locations with an aerial photo (taken ourselves). The sewer is most likely not EXACTLY where the line is drawn, but even if its location is slightly off, it will 99% lie within an affected area of that extension shaded in yellow.

 

Wanted to avoid digging down over 2m as the builders doing it with a digger will most likely smash through the sewer, and manually will take forever with shoring the trench etc, but looks like might not have a choice if ST won't accept a raft foundation for that bit - I have asked. I wanted to avoid the raft there since it'll be more likely to result in differential movement to the rest of the house, but the same is probably true of having 2.25m footings on that bit compared to the 1.2m footings elsewhere.

 

[Ferdinand]

At least it isn't an oil pipeline.

 

?

 

 

[SuperPav]

8 minutes ago, Ferdinand said:

At least it isn't an oil pipeline.

 

?

 

 

 

Thanks, I'm not completely sure whether reading that's made me feel better or worse!

[Ferdinand]

5 minutes ago, SuperPav said:

 

Thanks, I'm not completely sure whether reading that's made me feel better or worse!

 

He bought it, and the oil pipeline iirc turned out to be mainly next door.