Use of concrete lintels to simplify extension foundations/avoid trench fill

[David Costelloe]

I understand that in garage conversions, you can avoid new foundations by spanning two walls that are properly supported by existing foundations with two concrete lintels and then build masonry walls on the lintels.

My application is a lightweight extension structure that is heavily glazed to three sides, using three glulam posts and beams to form the guts of the structure.

Rather than traditional foundations, I would like to (1) use three post footings (eg 400mm x 400mm x 250mm pad, 250mm x 250mm x 1400mm column (depth because of trees), with rebar cage within); (2) secure post holders to the columns and; (3) rest or fix the ends of the concrete lintels on the columns.

I have shown a crude sketch below (interim drawings attached in a pdf) with the post footings shown as cylinders. From a site prep perspective, what I have in mind is use an auger to get three holes dug of sufficient depth, perhaps line the bottom with compacted gravel/ concrete, put in place rebar cage, cast the pile, cast the column, backfill the hole, use the columns as the basis of the structure.

The load on the soil is much less than a masonry or steel structures. The lintels are well able to cope with glazing load.

Questions
If I set the lintels into a screed, will that suffice to fix them in place? Can you "clamp them"

How "out there" is this as a concept for a permanent extension eg are building control used to approving such a design? Or is it meat and drink and no big deal (Part L energy aspects are fully covered off in the design).

All advice welcome, thanks in advance

 

[^woody^]

You will need to justify this design to building control via calculations. Soil loading is not just the dead weight of the building, and ground movement needs to be dealt with.

I don't understand the reference to lintels and screed. A screed goes on top of a floor.

 

[David Costelloe]

Thanks Woody. The reference to lintels and screed is basically around how do you "fix" the lintels in place (if indeed you do need to). You have the vertical load on the lintel from the weight of glazing etc which would be borne ultimately on the bearing at the post footings. Whilst the forces on the other axis are generally zero, I'm guessing you would need to consider wind etc trying to push the glazing and lintel out of place. With a lintel in a more typical application to support a wall or above a window, the compressive forces and resultant friction would fix it in place to deal with from suction forces.

Having thought that through a bit more, i think that I can see how to tackle it Towards the house you have friction and the floor stopping movement, so no problem. Going out to the garden you will have friction and whatever we pack the ground next to the lintel with... So my original question was wondering would you ever clamp or pin a lintel to a structure.

I have the live load and dead weight loadings from the roof, posts, glazing and lintels on each post footing and the existing structure's foundations, together with data on load bearing capacity, to show that the foundations are adequate from that aspect. I'm going down 1.8m as that is the level that trench foundations had to be to satisfy building control for a similar shaped structure, build with some masonry. When I'm confident the structure is not "out there", I'll invest in a structural engineer to validate/re do the design. But in essence, it does not sound like a ridiculous concept?

 

[bsr]

It doesn't sound ridiculous in that you are mimicking a type of foundation known as "pile and ring beam".

I believe that typically in these foundations the ring beam is tied into the piles mechanically, for example with steel piles they are left long so they embed in the ring beam.

What BCO would want to see in terms of calculations /justification I have no idea and others are the experts.

 

[^woody^]

Building regulations will state something like "foundations must be adequate". The Approved Document will have some standard strip foundations as examples and the performance of these are known, so follow them and you won't need to justify anything. But if you want to do any other foundation design, then performance is not known so will need to be proved to satisfy the "must be adequate" criteria.

Lintels won't perform for frost resistance at ground level, so need to be encased. Also the lintels will need mechanical fixings to the pads so that the foundation is rigid and acts as one without flexing.

In principle that type of foundation is possible. Whether it's practical or economically viable is another matter.

I would suggest that's it's a waste of time pondering details or loads or whatever, unless you have the engineering knowledge to design this type of foundation. As things might not work out as you think they might. An engineer will want to know why you want this specific foundation with pads and lintels and not a more typical pile and beam. As it stands it seems pointless when other options are available.