Unusual foundation - are underpinning and a steel frame needed?

sjb266

Hi,

I am looking for advice not so much to obtain an expert answer on here for my issue, as to ask who is the expert to get the expert answer from.

The issue is with what I understand to be a steel-framed barn (i.e. the roof is supported by a steel frame, not by the walls) which is clad with stone walls. It was converted by a previous owner to include a first-floor flat. Unfortunately, we've been told that this flat is not structurally safe. We want to convert the whole barn into a two-bed two-storey property, but have been told that we need to both underpin the walls (which no-one will quote for, because there's some 40+metres of perimeter and they're worried about finding issues that would raise the cost) and put in a new steel frame to support the roof.

The current structure is, we believe, as follows:

- a steel frame consisting of one pole at each corner and two more poles on each of the two long sides (eight total) which supports the roof. Unfortunately, at some point someone cut through the four corner poles, and they no longer support the roof. Also this is a steel frame for a barn, so it's not suitable for a residential property.

- a solid wall around the perimeter, consisting of breeze blocks on the inside and stone on the outside. This wall would not originally have supported the roof (but currently does so due to half of the poles being cut).

- the foundation under the wall, which appears to be a concrete slab which also extends to the floor of the barn. The thickness of this slab seems to about 8 inches under the wall, but may get thinner where it is just the floor. We understand that the previous owners carried out HGV repairs in this barn, and so the floor is meant to be very strong.

As far as I can tell, the concrete is laid on some sort of stone mixed with a grey gritty substance (about 6-8inch depth). This in turn is laid on stones mixed with a clay-coloured substance. I don't know how deep this mix of stones is; the people who dug the hole to investigate the foundation did not dig deep enough to get through this layer. I don't know what the stones have been mixed with.

The hole dug to look at the foundation is now partially filled with water, and both the clay-coloured and gritty-grey stuff is friable at the edges where it is exposed. However, where the stones et al have been removed from the hole and laid in a pile, the clay-coloured stuff is dry and hard. Smaller pieces may still crumble, but larger pieces are quite hard. The gritty stuff feels and looks much more like lots of little stones, and much less like a gritty soil.

What is concerning me is that the people who came to dig the hole and comment on the foundation (a) do not appear to believe that the stones et al are part of a foundation, and thus consider the foundation to be nothing more than the concrete slab; and (b) didn't in any case dig a hole where the steel poles are, to see what sort of foundation is under the main weight-bearing part of the structure.

What confuses me is that we have been told that the property needs to be both completely underpinned and have a new steel frame. This seems like overkill. If the walls are underpinned to the point of being able to bear a roof, why is a steel frame needed? If a steel frame is put in to support the roof, why do the walls need underpinning when they've stood for a very long time on their existing foundations?

I'm concerned that the people who dug the hole don't know what they're looking at. I don't know what I'm looking at either! Possibly a rubble-trench foundation? Or a raft foundation? These are things I've discovered by google, not things mentioned by the hole-diggers, but I don't know if that's because a) the hole-diggers don't know what these things are; or b) the hole-diggers know that that's not what is going on here. I feel like maybe they were expecting to see a trench-fill foundation (a big trench full of concrete?) and aren't happy with anything else, hence wanting to underpin by digging out what's there and replacing it with concrete.

I feel a need for a second professional opinion but I don't know where to go to get it.

Various photos of the hole, trying to show the differnet layers of concrete; stone mixed with grey gritty stuff; and stone mixed with clay-coloured stuff.

The stone and other material dug out of the hole, with my hand for scale.

Section62

sjb266 said:

What confuses me is that we have been told that the property needs to be both completely underpinned and have a new steel frame. This seems like overkill. If the walls are underpinned to the point of being able to bear a roof, why is a steel frame needed? If a steel frame is put in to support the roof, why do the walls need underpinning when they've stood for a very long time on their existing foundations?

A key issue is whether there is any suitable structure to transfer the roof loads to the wall, and the stability of the walls themselves.

It isn't uncommon with agricultural-type buildings for blockwork infill panels to get their lateral stability from the steel (or concrete) frame of the building.  The wall foundation is sufficient to carry the vertical load of the block wall, but not the roof loads (hence the need for underpinning).

Typically the bay length and span of agricultural buildings is as large as possible (the fewest number of columns gives maximum flexibility for different uses) and the size of these is often far more than could be achieved by a free-standing masonry wall of equivalent thickness. In effect the steel frame is probably helping to stop the walls falling over.  If part of the steel frame has been removed then it would be a legitimate concern that the remaining part may not be sufficient to give the walls (and roof) the stability they need to safely carry the loads imposed on them.

You also need to factor in what is acceptable for an agricultural building vs domestic residential one.  If an agricultural wall moves a bit and possibly develops some small cracks then it probably won't be a concern.  The same happening to a residential building would set alarm bells ringing.

The detail would be worked through by a structural engineer - do you have one working on your project already?

stuart45

Were the hole diggers engineers?

sjb266

Thank you Section62
If I've understood you correctly, the steel frame is an important part of the structural integrity of the walls, and thus is necessary for the walls regardless of a separate question of the adequacy of the foundations. Does that sound right?

Do you know anything about typical foundations for a barn like this, and whether they usually need underpinning? My big hope is that the foundations are actually strong enough but haven't been recognised as such if they perhaps don't look like more modern ones. But I think perhaps that's wishful thinking on my part, in horror at the sheer amount of money needed to underpin and to deal with new steel frames.

There is a structural engineer involved. The issue perhaps is my own lack of understanding, which has lead me to want clarity and confidence in what we're being told before spending large sums of money.

sjb266

stuart45 said:

Were the hole diggers engineers?

The hole diggers were builders.
A structural engineer has seen the hole.
The construction internet is perhaps as misleading as medical internet for sending ignoramuses like me on wild goose chases. But the sheer cost of underpinning, plus needing a new steel frame as well, makes me want to be very confident that the underpinning is necessary and correct.

mexican_dave

Hi OP, at the start of your post you wanted to know which expert to go to. Now I understand that you have had builders, and a Structural Engineer turned-up at some point, the big question is was the Structural Engineer advising you or advising the builder? I suspect the latter, if you did not "instruct" the engineer then he wasn't advising you or attending to YOUR needs, but the builder, who to be honest wasn't that interested in your project.
You need to be clear on that, but yes you want a Structural Engineer to assess the viability of what you propose and very probably an Architect to design your house. I hope you will get Planning Permission!

Section62

sjb266 said:

If I've understood you correctly, the steel frame is an important part of the structural integrity of the walls, and thus is necessary for the walls regardless of a separate question of the adequacy of the foundations. Does that sound right?

Broadly, yes.  Your building might be different, but in general you would expect the steel frame to be used to give the infill walls lateral stability.

sjb266 said:

Do you know anything about typical foundations for a barn like this, and whether they usually need underpinning?

The problem here is 'typical'.  Agricultural buildings are often built (at least in part) by farmers and/or their workers. So there is a great variety of methods used... possibly differences within the one building.

But as a generalisation I'd expect to find each column of the steel frame to be on a foundation consisting of a block of concrete roughly 1mX1mX1m.  The walls might be on a strip foundation, but more commonly (in my experience) straight on top of the floor slab, which might be made a bit thicker around the perimeter of the building for additional strength.

sjb266 said:

My big hope is that the foundations are actually strong enough but haven't been recognised as such if they perhaps don't look like more modern ones. But I think perhaps that's wishful thinking on my part, in horror at the sheer amount of money needed to underpin and to deal with new steel frames.

I think where you may have most difficulty is if the walls are built on the concrete floor slab.  As part of the conversion you will be expected to have an insulated floor and barriers/membranes to keep damp out.  Typically agricultural buildings aren't built to be damp proof - hence it is common for the floor slab to be continuous to the outside of the building and have the walls built on top of it.  To achieve a 'break' between the interior floor slab and the outside, you may have to have the floor slab cut, and it could be this that leaves the walls with less support than they currently have and need.

Really you need to talk this through with your SE who should by now have a good understanding of how the structure has been built and what alterations need to be done.

Subject to planning restrictions you may also want to consider other options - for example removing the block/stone walls (partially/completely) and replace them with a lightweight cladding.  E.g. corrugated steel sheeting or timber weatherboards. The cladding could be supported by the steel frame (if strong enough) and possibly reduce the load on the wall foundations to a level that doesn't require the underpinning to be done.  Explore the possibilities rather than focus on what may be the worst case.

stuart45

The engineer you had to inspect the foundation might only have been instructed to decide whether or not the foundation was suitable for the build. 
You now need an engineer to design the new foundation so a builder can price the job. This would involve deeper test holes.
Do you know why the corner columns were cut?