Strange wall design

[wertert]

Hi All.

 

I wanted to ask about the garage wall construction that the structural engineer has come up with.  I've never seen anything like it and i'm trying to understand what advantage it brings over laying flat for example.

 

It's a double skin concrete block all with a 10mm mortar gap between and strip ties every 450mm.   

 

I did ask about using concrete blocks on their bellies instead but that apparently isn't strong enough.  Not sure I believe that.

 

Happy to build it like this if there's a good reason but I failing to see it.  Any ideas ?

 

of the 4 walls built to this design, 2 are rendered and 2 are clad with stone blocks.  ie so design is NOT to give us a fair face inside and out. 

 

Searched a few forums and Google etc.  The only discussion I found,related to this design was about building a KOI pond 😉

 

 

The wall is built off a stepped raft foundation so the 2 skins will be offset by 150mm.  I may do away with the step as finished ground level is actually lower than shown.

 

 

 

 

[Dave Jones]

_{thats a strange one all right.} 

[bassanclan]

It might be something to go with the first floor of the garage

[wertert]

Yes possibly, first floor joists run side to side with a central steel for support but these don't need to be built into the inner skin.

 

 

Overall roof is fairly straight forward with a ridge steel supporting C24 rafters.

 

[bassanclan]

You could build with a wider cavity and therefore insulation, but it looks like its only the gable walls which would benefit from a wider cavity (and the garage area), so far better to build it as the architect has shown

[bassanclan]

Just as an aside, have you modelled the cross section to see the useable head height in the upstair as 5.5m ridge height seems fairly low, especially after insulation etc

Edited May 22, 2023 by bassanclan

[wertert]

1 hour ago, bassanclan said:

Just as an aside, have you modelled the cross section to see the useable head height in the upstair as 5.5m ridge height seems fairly low, especially after insulation etc

Hi Bassanclan.  Thanks for that. Now I'm worried ! What would you expect to see on a building like this ?

 

We have 2m head height in the center.  Only really planning to use this for storage so may not even insulate it.

 

 

 

[bassanclan]

Head height is fine for storage, the stairs and dormers led me to presume it was for a home office/gym etc.

You could have saved a couple of thousand by not having the dormers.

[wertert]

27 minutes ago, bassanclan said:

Head height is fine for storage, the stairs and dormers led me to presume it was for a home office/gym etc.

You could have saved a couple of thousand by not having the dormers.

The wife said they looked sweet ! 😉

 

On your earlier point 

 

3 hours ago, bassanclan said:

You could build with a wider cavity and therefore insulation, but it looks like its only the gable walls which would benefit from a wider cavity (and the garage area), so far better to build it as the architect has shown

 

The architect had originally designed the walls to be in 140mm concrete blocks.  Single skin.  I liked the idea as i'm building it myself and this seemed straight forward.  Once we got the SE involved for building regs they stated that 140mm wasn't strong enough etc etc and came up with the 210 dual skin.  I think this will be tricky to build, I imagine you'd have to build one skin and let it set before laying the second and filling the mortar gap as you go, will make the wall ties interesting as walls are offset.  Grateful to hear any other ideas on how to tackle this.

 

I need to understand why the 210 dual skin is better/stronger etc than laying standard blocks on their bellies.  What is the advantage ? Any SE's out there who have a idea ?

 

 

[Mr Punter]

I would really press the engineer to let you use 140mm blocks.  Most garages are 100mm single skin with a small pier midway.

 

Also there is a lot of work in those dormers for not much more space.  Can you lose them and have roof lights, or are they a planning thing?

[Canski]

If I hadn't seen your engineers drawings I would have built the outer skin to the top of your concrete using either a cut block of a split block ( 120 mm) and a course of concrete brick (80mm) to bring you level with the top of concrete. Then I would have carried on with blocks on their bellies up to joist height. We have done similar thousands of times. There was just one exception where the SE decided that 2 skins upright were stronger than blocks flat. It must be the same engineer.

 

Having said that if you did what I suggest to the top of slab then continue with 2 skins of 100mm with a 10 mm gap at least your ties will line up. Build 2 courses to one skin then2 to the other before laying your ties level across the gap. Filling that gap will be a PITA though and difficult to achieve. 

[ETC]

3 hours ago, wertert said:

Hi Bassanclan.  Thanks for that. Now I'm worried ! What would you expect to see on a building like this ?

 

We have 2m head height in the center.  Only really planning to use this for storage so may not even insulate it.

 

 

 

What a load of sh*t.

 

Build it out of cavity blockwork and attic trusses so that if you decide to put heating in you can insulate the walls at a later date.

 

Get rid of all that steel and why on earth is there a raft?

 

Standard strip foundations and a ground bearing slab. Use Keystone lintels over the openings.

 

Two different sizes for floor joists shown on the drawings and putting a note on to say that the stairs will be designed by a specialist is a complete cop-out!

 

 

[Radian]

5 hours ago, wertert said:

I need to understand why the 210 dual skin is better/stronger etc than laying standard blocks on their bellies.  What is the advantage ? Any SE's out there who have a idea ?

 

For example - If you take a block, lay it flat with the ends supported on a couple of bricks then drop a sledgehammer in the middle it will be much more likely to break than if two upright blocks spanned the gap. How this observation relates to a built wall is another matter. On a weak foundation, yes it might be relevant. But yours is not exactly weak.

[Canski]

3 minutes ago, Radian said:

For example - If you take a block, lay it flat with the ends supported on a couple of bricks then drop a sledgehammer in the middle it will be much more likely to break than if two upright blocks spanned the gap. How this observation relates to a built wall is another matter. On a weak foundation, yes it might be relevant. But yours is not exactly weak.

Oh lordy lordy ( remember that ? ) but .... from your description of the block being laid there is no mortar in the middle. Do the same to an upright block and the result would be the same. No logic there in your comment.

[wertert]

23 hours ago, Canski said:

If I hadn't seen your engineers drawings I would have built the outer skin to the top of your concrete using either a cut block of a split block ( 120 mm) and a course of concrete brick (80mm) to bring you level with the top of concrete. Then I would have carried on with blocks on their bellies up to joist height. We have done similar thousands of times. There was just one exception where the SE decided that 2 skins upright were stronger than blocks flat. It must be the same engineer.

 

Having said that if you did what I suggest to the top of slab then continue with 2 skins of 100mm with a 10 mm gap at least your ties will line up. Build 2 courses to one skin then2 to the other before laying your ties level across the gap. Filling that gap will be a PITA though and difficult to achieve. 

Hey canski,

 

I'm actually planning to do away with the stepped raft and just square it off. ( Subject to SE approval of course ).  

[wertert]

21 hours ago, ETC said:

What a load of sh*t.

 

Build it out of cavity blockwork and attic trusses so that if you decide to put heating in you can insulate the walls at a later date.

 

Get rid of all that steel and why on earth is there a raft?

 

Standard strip foundations and a ground bearing slab. Use Keystone lintels over the openings.

 

Two different sizes for floor joists shown on the drawings and putting a note on to say that the stairs will be designed by a specialist is a complete cop-out!

 

 

Hey ETC.

 

Raft is down to local conditions.  Last garage suffered due to subsidence and the site is slightly sloping. We also have/had trees around and although they've been cut down (4 years) we are suspicion of the ground conditions.  Raft was recommended as the safest choice.  Our neighbours did the same when they built their extension.

 

So do you mean build dual skin cavity blockwork wall and inject insulation later on if we decide or insulate on the inside ? What sort of cavity would you go for ? I actually think that would be easier to build than the weird 210mm design we currently have. 

 

Not sure I understand the attic truss option ? I did ask about this during design.  COuld we have trusses and retain the first floor height or would it be higher ?

 

SE / Architect said we needed the steel due to the spans.  Roof is ~8m long. So not required with trusses. ? The 2 sizes are because the SE superseded the architect recommendations.  so we are looking at

 

 

So we went from 170mm to 195mm.

 

Agree with you on the cop out of the stairs.  They were useless on these. I'm no builder and I have concerns over aspects of the design. Hoping to find some answers in this forum so thanks again for your input.

 

At this point I'm thinking I need to get the whole design peer reviewed.  Any recommendations ? New set of drawings.

 

Haven't gone through building regs as yet.

 

 

[ETC]

1 hour ago, wertert said:

Hey ETC.

 

Raft is down to local conditions.  Last garage suffered due to subsidence and the site is slightly sloping. We also have/had trees around and although they've been cut down (4 years) we are suspicion of the ground conditions.  Raft was recommended as the safest choice.  Our neighbours did the same when they built their extension.

 

So do you mean build dual skin cavity blockwork wall and inject insulation later on if we decide or insulate on the inside ? What sort of cavity would you go for ? I actually think that would be easier to build than the weird 210mm design we currently have. If you build it as a cavity wall you can insulate it by pumping it with insulation at any stage of the build or after the build. In terms of width you’d need to see what u-value you need to achieve and see what a given thickness of insulation/cavity width is required.

 

Not sure I understand the attic truss option ? I did ask about this during design.  COuld we have trusses and retain the first floor height or would it be higher ? Yes. Speak to your local truss designer/supplier.

 

SE / Architect said we needed the steel due to the spans.  Roof is ~8m long. So not required with trusses. ? The 2 sizes are because the SE superseded the architect recommendations.  so we are looking at

 

 

So we went from 170mm to 195mm.

Roof trusses will get rid of the steel at ridge level and at floor level as well as getting rid of the floor joists.

 

Agree with you on the cop out of the stairs.  They were useless on these. I'm no builder and I have concerns over aspects of the design. Hoping to find some answers in this forum so thanks again for your input. Stairs aren’t difficult to design. All you need is the floor to floor dimension - divide by the number of risers needed (maximum of 16). Make sure the rise and going are no more than 220mm (although you cannot have a 220mm rise and a 220mm going) and make sure that 2r+g is between 550mm and 700mm.

 

At this point I'm thinking I need to get the whole design peer reviewed.  Any recommendations ? New set of drawings. If it’s what you want crack on. If not either get them amended by your current designer or appoint a different designer. It’s really not that difficult a design/drawing to do. Could do better myself!

 

Haven't gone through building regs as yet.

 

 

 

[Radian]

22 hours ago, Canski said:

No logic there in your comment.

 

23 hours ago, Radian said:

How this observation relates to a built wall is another matter.

 

The logic in my comment related to a single concrete block taken in isolation. It's an example that could possibly explain the (possibly dubious) thinking behind the SE's requirement. I don't credit or discredit it as a valid reason for the requirement. However, I would point out that there is virtually zero deflective strength in a mortar joint and viewed as a beam supported at two ends, a concrete block set vertically would take more load before fracturing than the same block laid flat. As I said before, it may be more a function of the conditions of the foundation that the SE is basing these requirements on. I just don't know.

[JonBATConsult]

On 22/05/2023 at 16:44, wertert said:

The wife said they looked sweet ! 😉

 

On your earlier point 

 

 

The architect had originally designed the walls to be in 140mm concrete blocks.  Single skin.  I liked the idea as i'm building it myself and this seemed straight forward.  Once we got the SE involved for building regs they stated that 140mm wasn't strong enough etc etc and came up with the 210 dual skin.  I think this will be tricky to build, I imagine you'd have to build one skin and let it set before laying the second and filling the mortar gap as you go, will make the wall ties interesting as walls are offset.  Grateful to hear any other ideas on how to tackle this.

 

I need to understand why the 210 dual skin is better/stronger etc than laying standard blocks on their bellies.  What is the advantage ? Any SE's out there who have a idea ?

 

On 22/05/2023 at 16:44, wertert said:

 

Surely just use 215 wide block or blocks laid flat single blocks or hollows or discuss windframes. Im sure he is doing this for a very valid reason but important to have a bit of back and forth in design development with se

[wertert]

Hi All

 

Picking up this thread again.  I'm looking into redesigning the garage to include a lot of suggestions above, including traditional cavity walls.  I have a question around using the existing hollow block walls as the outer skin.

 

Not the best picture but gives you an idea of the existing hollow block wall

 

 

6 courses, Reinforced with 12mm bars, Filled with concrete etc.  Hollow block wall is technically retaining but the ground is very solid and I would say minimal side load on the wall.

 

Could I continue this wall up in normal concrete blocks ( another 6/7 courses ) ? This would then  become the outer skin of a cavity wall ? ie we would build the inner skin in front of what you're seeing above and tie to the hollows.  We would use screw ties into the hollow blocks and traditional ties normal block to normal block. 50mm cavity.

 

 

Attic trusses would then sit on the inner skin/roof plate.

 

We would expect to see a step in the block work where we change from the hollows to the normal blocks but other than that I can't see a problem with it.  Planning to render the outside and possibly dry line inside.

 

Great to hear the forum's thoughts on this.

 

 

[saveasteading]

On 22/05/2023 at 23:34, Radian said:

it will be much more likely to break than if two upright blocks

Just seen this from way back. Yes because it will be 100 thick whereas the others are 200.  2 blocks laid flat is just as strong, esp when mortared together.

[saveasteading]

2 hours ago, wertert said:

Great to hear the forum's thoughts on this.

 

It's OK by me and the reinforced hollows will resist any pushing from the bank.

 

Another thing just noticed.. if I was the bco I'd say you have a choice. Dormers and build it as an occupied space, with insulation and headroom, because that is surely your intention.

Or no dormers.

 

 

 

[George]

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 than when laid flat), mortar strength and bond pattern.  A simple way to think of this effect is that a wall with blocks laid flat is weaker because mortar makes up more of the wall than when the blocks are upright. (Mortar being between 4 - 6MPa typically).

 

This is part of the reason why blockwork of 3.6MPa strength have as much load bearing capacity as clay bricks which are 20 - 50MPa. 

 

Not sure I buy it as a significant problem in this situation but that is the technical reasoning. 

[wertert]

Hey George - Thanks for your input. 

What do you think of my last post from today ? I'm proposing to forget the laying flat idea from a few weeks back and build off the hollow blocks with traditional concrete blocks laid upright ie maximum strength as you mention above.  This would then form the outer skin of a traditional cavity wall.

[George]

3 hours ago, wertert said:

Hey George - Thanks for your input. 

What do you think of my last post from today ? I'm proposing to forget the laying flat idea from a few weeks back and build off the hollow blocks with traditional concrete blocks laid upright ie maximum strength as you mention above.  This would then form the outer skin of a traditional cavity wall.

It could work - I'd want to know more about the foundation of the retaining wall and have the newly formed cavity to be drained. 

 

What you don't want to have happen is the weight of the new innerleaf wall on the retaining wall foundation causing it to rotate forward. It shouldn't happen (especially if the retaining wall has a large heel), but that is what I'd check for. 

 

Vertical strength wise it should be OK.