Foundations Part II


It turns out that foundations and footings are not the same things – foundations are the base of the building that transfer the weight of the walls, floors and roof into the ground, and footings are what the foundations sit upon.

This may seem pretty elementary, but it transpired that there were other fundamental gaps in my understanding. In particular I did not understand the difference between force and pressure. I now do, thanks to this chap:

The fellow presenting gives a carefully thought out, lucid and concise explanation of the difference between force and pressure, and why? Because he wants to help his fellow woodworkers correctly glue their mortice and tenon joints.   Isn’t the internet marvellous?  If you are as clueless as me, please take a look, it is very good.

Anyhow, it turns out pressure varies according to the surface area a force (think ‘weight’) is applied to. Think of someone treading on your toes in stilettos vs snowshoes and you will get the idea.

We will eventually have to do some maths regarding foundations, and the units we will be using are Newtons (N) – a unit of force – and kilograms (kg) – a unit of mass.

For those of you who have a hazy memory of their school physics you will recall that mass and weight are not the same thing, and that weight actually has something to do with gravity (a force) that varies according to where you are on the planet.

Luckily we don’t need to know why this is the case, we just need to know that the relationship between kg and newtons is (roughly):

1 kN (1000 newtons) = 100 kg

It will turn out that what matters when considering foundations is pressure, and the unit of pressure we will be using is kN/m2, ie. 1000 newtons per m2. A kN/m2 is also, confusingly, known as the kilopascal (kpa).

If this all seems a bit abstract, I found the easiest way to think about it was in terms of more familiar measures of mass and convert to Newtons using the formula above.

For instance, 1 kN/m2 is roughly equivalent to having a largish (100kg) person standing on a 1 meter square surface.

In old money 1 kN/m2 is about 20 psf.


in the next instalment: why we care about pressure.

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