Are 750mm strip foundations normal?

[SBMS]

Standard brick and block on dense clay, so nothing unusual. SE has specified 750mm wide foundations. There are a few things that I think are over specified by the SE but wondered if anyone else had experience of 750mm strips?

[Eric]

I think it’s fairly normal, we just had to do 700mm wide for timber frame kit.

[joe90]

Yes S.E.s tend to over egg things to be on the safe side, my founds are 700mm on dense clay specced by the BC.

[Gus Potter]

Hiya.

 

Here are a few thoughts and for all.

 

There are quite a few different types of foundations. For example some are designed to interact with say a concrete slab (like a raft) and we call these reinforced foundations like a reinforced concrete beam you may see when you look up at the ceiling of a multi storey car park.

 

Some are designed when you may be on variable ground or ground that may have a significant soft spot in it.. say over an old well or basement. The reinforced concrete spans over the soft bits. The foundations are designed to act like beams.

 

Under the ground the layers of soil often don't lie flat.. and are often tilted, but your building (hopefully) is level so the foundations need to span over the transition in the soil layers which settle by different amounts. Here you often look at reinforced foundations.. like a concrete beam. Reinforced concrete has a lot more steel in it and the codes specify a minimum amount of reinforcement which is a bit of a bug bear for folk that are doing ICF basements.. but hey ho.

 

One key to this is to recognise that concrete is not good at carrying tension forces but very good at carrying compression. You put steel in to carry the tension forces.

 

Now you can also have simple concrete strip foundations. These work in a different way. You often see a light weight steel mesh specified, say an A142 mesh. This mesh is not there to act as reinforcement like a beam in the carpark but only to mitigate cracking as the concrete cures and does what it does. It's easy to mix up the two different concepts.

 

Spread (strip) foundations are designed to act in compression only. The width of your strip foundations is generally governed by the overall thickness of the wall and the thickness of the foundations. The concept is that the load from the wall speads out at 45 deg through the depth of the found from the outer edge of the walls.. thus the concrete does not experience tension forces. Historically this was achieved by using corbelled brick founds or big flat stones.

 

The diagram below hopefully helps understanding.

 

 

[nod]

No 

It’s not normal 

Typical would be 600 

We have  13 meters of plinth wall In those areas I’ll use a 750 bucket 

[Temp]

54 minutes ago, SBMS said:

Standard brick and block on dense clay, so nothing unusual. SE has specified 750mm wide foundations. There are a few things that I think are over specified by the SE but wondered if anyone else had experience of 750mm strips?

 

The width is normally determined by the load bearing capability of the ground. Did you have a soil condition report done?

[Gus Potter]

Nod..

 

I think this needs some qualification from you as could be confusing to new BH members.

 

For me it's about making sure that folk do things in a safe way and don't encounter problems later if someone asks for verification that the founds are ok.

 

Yes there are many practicalities when you get on site.. like bucket size. Often to change buckets and try and dig founds of different widths costs more in time and increases the setting out complexity.. ground is mucky stuff and the spray painting lines say can get lost in the heat / excitement of excavating and getting started.

 

Yes on a large commercial job with all the technical/ setting out back up varying found types are doable. You don't want to be digging for Britain (you'll get the sack as a designer) but we are talking self build / local builders/  diy here and what is practical in this context.

 

 

 

 

Edited February 3, 2022 by Gus Potter
typo as left handed

[JohnMo]

Your all lucky, our front foundation width was closer to 2m wide and reinforced.  All the fun you have an a sand hill.

[Gus Potter]

That site looks sweet! Sets a bench mark. But to burst your bubble no protective mushrooms on the top of the exposed rebar. On the hole take that as a compliment.

 

For all. When you are digging strip foundations say in clay don't run round with a spade smoothing the sides.. you want them a bit rough so the concrete gets an extra key down the sides.. wierd but..

[saveasteading]

20 hours ago, Gus Potter said:

do things in a safe way

It is simplistic to assume that forces go at 45 degrees, but it is a sensible assumption.

If the base shown here was 150 thick then it would also be narrower, so less strength both ways.

If it was built the same width as the wall then the forces would act on a smaller area of ground, and may settle.

 

There are many assumptions in building, and most are  based on common sense and the experience of centuries.

If the Engineer says 750 wide then by all means ask why, but then do it....there are many good reasons....loads, ground conditions, ground strength.

 

If you pay your Engineer more, then he may allow time to reanalyse and may save you a little concrete.

Or if you do a more extensive ground survey.

A little extra concrete (£100 worth?) is probably the optimum.

 

 

[SBMS]

Thanks all - makes sense. Just wanted to make sure these were normal, good to hear they are. 

[Dave Jones]

On 03/02/2022 at 22:01, SBMS said:

Standard brick and block on dense clay, so nothing unusual. SE has specified 750mm wide foundations. There are a few things that I think are over specified by the SE but wondered if anyone else had experience of 750mm strips?

 

and larger, with ever increasing cavity sizes need leeway to get the building on the footing and square as digging is never an exact science.