Gone off piste, need some footings design help.

[epsilonGreedy]

My foundation concrete was poured last Thursday & Friday and I now have a pair of brickies lined up to build my footings. They can start asap because another job is on hold for them.

 

The problem is that I do not know what block types I should order for the garage footings where I have strayed away from my building control diagrams. I got my JCB man to dig an extra internal wall trench for my garage. What I have done here is jumped the gun and made a provision for a future planning amendment that would divide my double garage into a large single garage with a workshop and store to one side.

 

My present building control diagram shows a solid 215 double skin brick/block wall all around. I plan to upgrade this to a basic default 50mm cavity wall and widen the cavity to 100 or 150 around the workshop. The floor of the workshop would be beam & block to the same standard as my main house. The remainder of the garage floor would be laid as a solid concrete slab.

 

Should the footings of the internal wall be the same standard as a main house external wall or can I economize? As shown in my hand drawn diagram could I sit the beams on a single skin of 140mm footing blocks and then build both skins of the internal cavity wall direct off the beam & block floor? I am concerned about designing in a conduit for rising damp if I do this.

 

The internal cavity wall would just be thermalites to 2.4m in height with no weight above.

 

Diagram Notes:

 

1. The first diagram is a plan view showing the position of the new foundation trench and a basic plan for the cavity walls.
2. The "T" blocks in the second diagram are assumed trench blocks.
3. All poured concrete is 600mm x 600mm and well in excess of the structural minimums for the ground conditions.

 

Edited July 16, 2018 by epsilonGreedy

[Declan52]

For the main part just use standard 300mm concrete blocks. This will allow you to do 100/100/100 type setup. 

For the internal wall in the garage would stud work not by easier to do down the line. 

If you really want the wall to be block then just a course or 2 of standard 4inch block on their flat and the final course on it's edge. No need to use the 300mm blocks here. 

[epsilonGreedy]

9 minutes ago, Declan52 said:

For the main part just use standard 300mm concrete blocks.

 

 

Although not central to my main question, are such "standard 300mm concrete blocks" different to trench blocks which I thought were always a lightweight construction? 

[Declan52]

You can get blocks in all sorts of different sizes. Some people call them trench blocks or Foundation blocks doesn't really matter that much what they are called.

To give you a 100/100/100 type setup above it's easier to use a block that is 300mm wide. However just to confuse things even more they can be 350mm long or 275mm if you go for 140mm deep blocks. 

These blocks don't need to be lightweight, ordinary concrete blocks work fine here.You just use a lightweight block when you start your cavity on the last course. But as there are plenty of block suppliers through out NI we are spoilt. What type you end up using will be more or less what your local merchant can supply.

[epsilonGreedy]

20 minutes ago, Declan52 said:

If you really want the wall to be block then just a course or 2 of standard 4inch block on their flat and the final course on it's edge. No need to use the 300mm blocks here.

 

 

OK here is my read of this...

 

Start the footing blockwork of the internal wall with basic standard concrete blocks on their side i.e. the footing wall will be 215mm wide. Then play with the block height maths to bring the footing height up to the right level to carry the beams on a final block laid the normal way up? This design removes the need for a cavity footing wall below the internal wall.

 

If I sit the beams on a dpc and the adjacent solid garage floor slab is poured over a membrane bought up the side of this footing wall there will not be a path for rising damp.

[Declan52]

Will the beams not go front to back, the shortest span??

Build X amount of courses with a block on it's flat, 215mm, and then the last course on it's edge will be in-between 2 beams. Depending on how deep the concrete for your floor will be you might get 2 blocks high built here and then it will be easier to cut out and continue upwards when the time comes. 

You can cut strips of insulation and have them on top of the wall so it's only covered in 50mm of the finished floor concrete. Will be much easier chipped out this way.