Strange wall design

[Gus Potter]

4 hours ago, George said:

Wall 'strength' is a function of the normalised compressive strength (how to account for the different height / thickness - a 215x100 upright is approx twice the strength

To add a bit more to @George's comments.

 

Often folk think that masonry walls can carry loads of load... but sometimes a lot less than you think.

 

A masonry wall well braced with piers / return walls on a good founds does often carry a lot of load. But if the applied load is not in the centre of wall (an eccentric load) this eccentricity introduces a bending effect in the wall top to bottom..and this can significantly reduce the load bearing capacity. Common case is where roof trusses and joists bear / are built into the wall so the load is not applied to the centre of the wall.

 

External walls also carry wind load which introduces another bending effect.. and again this can significantly reduce the load bearing capacity of the wall, especially if the roof is lifting up and you are using the walls to hold the roof down. I do mean significantly.. not just a little!

 

Now the stength of the wall is derived from partly its effective thickness. What you have here is a collar jointed wall.. the codes allow us to take the effective thickness of the collar jointed wall as being the actual thickness = say 215mm. But introduce a cavity and the effective thickness is now 2/3 of the two leafs (100 + 100) = 133mm.. a very big reduction in stength!

 

For the mathematically minded the bending strength of the wall is partly a function of the wall thickness squared so the reduction in strength is exponential... 215^2 = 46225 (collar jointed) compare with 133^2 (cavity wall) = 17689.. now 17689 / 46225 = 38%! of the strength of a collar jointed wall.. oh dear! Maybe your SE has a point here and while it may seen a bit "odd" maybe your SE is saving you a load of cash?

 

For a collar jointed wall you need to build both leaves at the same time and use the correct wall ties specifically designed for collar jointed walls. You don't need to fill the 10mm gap as the ties are thick enough to resist the small amount of bending. But you do need to make a good effort to reduce the eccentricity of loading. See link below for an example of wall ties.

 

https://www.ancon.co.uk/products/masonry-reinforcement/ancon-amr-cj

 

On the practical side if you are cladding the wall in stone often you will get coursed stone. To make life easy you want the coursing to match up with the blockwork say 225mm. But if you lay blocks on the flat you end up with an average mortar bed of 12.5...or some other nasty derivative.. starts to get messy. Also you use more mortar as the blocks are 100mm laid flat but you need to achieve 225mm. A normal bed is 10mm.

 

Please think carefully before changing the SE's design.. best just to ask them first before carting on.

 

 

 

 

 

 

 

 

[wertert]

Hi Gus

 

Thanks for your detailed reply.

Does the width of the cavity have a factor on the strength of a cavity wall overall ? ie 30mm / 50mm / 100mm or will it always be 2/3 ? 

The SE has advised that the collar jointed wall is required for wind lateral stability of the rear wall, not for load bearing.  I think one problem is they never visited site.  I have explained that area is sheltered and the rear wall is ~1m from the boundary wall and neighbours extension.  I do not see wind being a factor here.  I understand they need to design to certain standards.

 

Ariel pic below shows outline of proposed garage with rear wall in red.  Neighbours garage/extension has roof window. ( they didn't use a collar jointed wall! )

We already have the bath stone blocks that will become the outer skin of 2 of the walls.  They are 6 inch thick and either 12 or 14 inches high.    There is no practical way i'm going to get the courses to line up 😉 You can see the stone on pallets above.

Will probably have to use the screw in stainless ties.

 

[AliG]

Can I just stick my oar in with an aside -

 

If you plan to actually put a car in the garage, 2134mm wide doors are extremely narrow. It looks like they are 7ft 6in by 6ft 6in frames. You have a lovely wide garage with very small doors.

 

To get the 2134mm width the doors will also have to be swung back round a full 180degrees, or it will be even narrower.

 

Plenty of cars are give or take 2m wide at the body and 2.1-2.2m wide at the mirrors.

 

It will be so difficult to squeeze a car through the door you will never put the car inside. My parent's old house had 7ft6 up and over doors (Similar sized opening) and it was very tight putting their A3 into it. My SUV would not fit.

 

Looks like you are settled on the wall stuff. I spent a bit of time looking into this a couple of years ago and it seemed that by the time you had built piers which cut into the garage on a single skin building you were as well building a cavity wall which has other benefits.

[wertert]

Hi AliG.

 

Apologies - those are the old sizes from the planning application.  I should have blanked the numbers out.  We're now planning a pair of 8 ft wide / off the shelf doors 

 

2# Hormann 8’0” x 7’0” framed retractables in Ilkley designs with white powder-coat finishes and standard plastic handles:

 

8ft sound ok ? openings will be 2590mm block to block.  Doors will be up an over retractable.

 

 

I also saw an example where you buy an open frame and add you own woodwork etc.  Could be an option.  

 

Thanks for pointing that out.

 

[wertert]

Hi All

 

Few progress pics.

Laying the bottom layer of 10mm mesh at the moment on 50mm stands.

I'm planning to make up 40cm high chairs with 8mm rebar ( or 6mm ? ) to support the top layer of mesh.

Progress is slow because it keeps raining and i'm trying to keep the trench as dry as possible.  Ground was rock hard when it was dug out but the base of the trench is slightly soft where the rain had got in. 

Anything to worry about ?  Maybe a few inches of type1 or  plastic sheeting would have been a good idea in retrospect. 

 

Trench is a bit messy due to the number large rocks which came out.  I quite want to put in some formwork for the top edge but probably not required 😉

 

I saw this permanent formwork being used on another site and was very impressed;

 

https://cordek.com/products/corgrid#:~:text=Corgrid permanent formwork panels consists,ensure quick and simple installation.

https://www.maxfrank.com/uk-en/products/formwork-technologies/01-permanent-formwork-pecafil/

 

Anyone familiar with it ?

 

I'll probably order the concrete delivery for later in the week. Weather permitting. 

Edited July 10, 2023 by wertert
adding links

[markc]

I would have turned the cut ends of mesh upwards to give you something to tie uprights into and to keep steel away from sides of trench to maintain a decent concrete cover.

[saveasteading]

You are fine to pour on that. The base and sides are hard. Just scoop out any really sloppy mud last thing to stop it mixing with the concrete.

 

Cordek hads its place, but it is a problem solver, not for standard use.

 

I used to try to be clever, pushing in starter bars, but now prefer to work with the control of the nice hard concrete base. Stuff has improved so much.   Drills are so good, and threaded rod with epoxy is now cheap and reliable.

[wertert]

3 hours ago, markc said:

I would have turned the cut ends of mesh upwards to give you something to tie uprights into and to keep steel away from sides of trench to maintain a decent concrete cover.

Hi Markc - good point.  I will cut the ends off where the mesh is closer than 40-50mm to the side of the trench.  40-50mm should be good cover. 

Planning to make up chairs like these

to support the top deck of mesh.  Will need approx 2.5m of 8mm rebar per chair.  Shouldn't be too hard to bend.

 

 

[markc]

@wertert either cut off or slide a ring spanner over the end and bend up a bit

[wertert]

Hi All

 

Hope all is well.  Thought I'd put up some progress pics.

We had all that rain so my trench ended up as a bit of a mess.  Mesh was sinking into the soft bottom. 

I ended up scraping out the soft stuff and dumping in a few inches of scalpings which I whacked to give a firm base then started again.

 

 

 

BTW - laser level.  Best thing I've bought recently.

[wertert]

Next we had the concrete. RC35 which I overordered by quite alot !

 

Now working on the hollow block wall

 

 

Hope to crack on this weekend.

[wertert]

Hi All

 

I'd like to raise a question which concerns the design of these walls once the hollow blocks are in/filled.  You may have read further back that I intend to go with cavity walls with the hollows forming the outer skin. The idea is I will switch to traditional concrete blocks after 5 or 6 courses of hollows.

 

Two options as far as I can see.  Do i build the external block wall on the center line of the hollows or flush with the edge ?

 

I prefer the right hand / flush option as we don't get a step but my concern is the wall isn't balanced ie The center of gravity is over to the right.  Would this cause a problem ?  The wall will be a gable end at 5.5m at the highest point and about 7m long.  That's alot of weight off center if you see what I mean.

 

Wall skins will be tied together - probably 450v / 450h.  Obviously the flush design will need longer ties.

 

 

 

Thanks All

[saveasteading]

Structurally I don't think it matters enough to worry much. It's a garden wall.

yes, centrally balanced is better for a couple of reasons, but I would do whatever you wish.

 

It's personal but I prefer the step for appearance and because you could grow pots of strawberries or other on there.

 

You are right about ties. Not only longer, but much more robust or they will simply collapse under horizontal load.