What do wall ties really do?

[emphasis303]

Hi

I wanted to know how a double skin block garage with a cavity is stronger then a single skin block wall of the same dimensions which lets say are 3 metres high 5 metres wide and 4 metres long. As realistically, the skins arnt actually touching nor are they secured together apart from with the cavity wall ties which I never thought affected the structure in any way.


Would really like to know

 

[Shytalkz]

Who told you that? A cavity wall is not stronger than the equivalent in solid and this is reflected in design criteria (ie effective thickness for a solid wall is the actual thickness; for cavity walls it's 2/3 of the sum of the actual thickness of both leaves). For both leaves of a cavity to work under lateral loading (wind) there has to be some degree of interaction between the two leaves and that is the purpose of the wall ties.

However, cavity walls are becoming ever less structurally stable as the cavity increases and the wall ties get ever thinner (which is why horizontal spacing for cavities >75mm is reduced to 600mm) and now that, additionally, the returns are not closed by returning the inner leaf, in the temporary condition (ie no load from above and no lateral bracing from floor and roof), they are even more potentially unstable.

Also, the vertical stresses in solid walls are less: in cavity construction, the roof and floor loads are all taken by the inner leaf only (unless you've got a lean-to or bay construction) and the brickwork supports self-weight only; solid walls dissipate the stresses throughout their width.

Nowadays, new-build cavities are 80-85mm to suit BRegs Part L requirements. Further increases in cavity width are inevitable as the regs get ever more onerous, unless someone comes up with more efficient insulation and, once we get to 100mm wide cavities, or greater, the inner leaf thickness will have to be increased markedly to compensate, as well as tie centres being moved ever closer.

Who knows, maybe we'll end up going full circle and building solid walls with internal dry-lining as the de facto method of construction....

 

[Shytalkz]

PS Now what could have possessed you to ask that question at 318am???

 

[Softus]

I think he must have had a dream in which sometime told him that a double skin block garage with a cavity is stronger then a single skin block wall of the same dimensions.

 

[noseall]

"what do wall ties do?"

they are there to keep the walls apart.

 

[Softus]

You could call it "apart tied".

 

[^woody^]

is stronger then a single skin block wall of the same dimensions which lets say are 3 metres high 5 metres wide and 4 metres long.

Do you mean a single skin (ie 100mm) garage of 5x4x3

or

a garage 5x4x3 with a solid walls 300mm thick?

And what do you mean by "stronger"? Able to support more load, impact from a car hitting it or what?

 

[joe-90]

You could call it "apart tied".

A bit of black humour from Softus.

 

[Softus]

Ictually it was blick humour.

 

[emphasis303]

LOL thanks for the replies, I ask bcos I'm going to build a garage and were going to build it single skin, however I was told u can't if the runs r longer than 1.8 metres in which case u have to use piers.

So I decided I would double skin it and build it to house specs in terms of insulation and wanted to know how stronger a double skin building would be against the wind than a single skin.

When I've said stronger i mean against withstanding wind and not falling down.

Also if u cud tell me with out piers how I cud make the building more stable against the wind without piers.


Thank you very much

 

[^woody^]

Expanded metal in the bed joints, and the roof bracing the walls = no piers

 

[2scoops0406]

"what do wall ties do?"

they are there to keep the walls apart.

And what does mortar do? Hold bricks together or apart?

 

[Shytalkz]

is stronger then a single skin block wall of the same dimensions which lets say are 3 metres high 5 metres wide and 4 metres long.

Do you mean a single skin (ie 100mm) garage of 5x4x3

or

a garage 5x4x3 with a solid walls 300mm thick?

And what do you mean by "stronger"? Able to support more load, impact from a car hitting it or what?

In terms of size/thickness, I wasn't being specific: a solid wall of a specific brick or block, is stronger against lateral (wind and stability) loads compared with a cavity wall of two leaves of the same material, in the same mortar, to the same total thickness. A solid wall can support more vertical load for the same allowable compressive strength for the brick/block, as in cavity construction, the majority of load is taken by the inner leaf, so the stresses for the same load are obviously higher, as it has less plan area.

 

[noseall]

"what do wall ties do?"

they are there to keep the walls apart.

And what does mortar do? Hold bricks together or apart?

you've got me there.

my grandad used to say a net (i.e fishing net) was holes tied together with string.

 

[Shytalkz]

LOL thanks for the replies, I ask bcos I'm going to build a garage and were going to build it single skin, however I was told u can't if the runs r longer than 1.8 metres in which case u have to use piers.

So I decided I would double skin it and build it to house specs in terms of insulation and wanted to know how stronger a double skin building would be against the wind than a single skin.

When I've said stronger i mean against withstanding wind and not falling down.

Also if u cud tell me with out piers how I cud make the building more stable against the wind without piers.


Thank you very much

Ah, well two skins of 100 blockwork as cavity construction are clearly stronger than a single block wall 100 thick.

You can omit piers as Woody suggests by using bed joint reinforcement, but you will have to make sure that the roof structure - presumably trusses - is adequately braced, to take wind loads back to the side walls. The usual 100x25 longitudinal bracing and rafter diagonal bracing on their own are not likely to suffice; put additional diagonal bracing on the ceiling members, running between the side walls and the first longitudinal brace running from front to back.