Parapet Windposts with timber joist / posi-joist roof.

[liam1369]

Hi All,

I was wondering if anyone has any experience with parapet posts that are contained within the cavity wall of a wall where the roof is timber joist or timber posi-joists.

I am struggling to justify the base moment and push-pull forces generated owing to the parapet wind post being treated as a fixed cantilever.

I queried this with Ancon who said they can't help. It is unusual as I have seen these built often without any windposts without ill effects.

I am proposing to fix the base of the parapet post to concrete padstones / concrete dense blocks. Concrete anchors can then be used and then the base moment can be treated as a push-pull over a length of around 220-250mm.

The tie force can then be resisted by the straps and deck. With the push being put into the flange of the posi-joists and into the floor plate in general. However, it is proving hard to justify.

Do you have any experience with this detail? - Or do you have any succinct references to details or methodologies or useful guides to this. I am really struggling with this one.

Thank you for your help or the communities help.

Liam

 

[XR250]

Why not make it a reinforced masonry parapet? (I assume that is masonry you are showing)

 

[liam1369]

Hi XR250, yes 1100mm high masonry cavity parapet.

What do you mean reinforced masonry? - Bedjoint reinforcement? - There are movement joints at typical 6-8m centres to both leave.

 

[GeorgeTheCivilEngineer]

Reinforced masonry isn't very common in the UK but it is an option. But it is unlikely to work nicely with the cavity wall below.

You might find bed joint reinforcement works if the returns aren't spaced too far apart.

I think the detail you have shown will be hard to make work. You would need to extend the post to the floor below and design as a propped cantilever. That in combination with the bed joint reinforcement should work. You will also be able to increase the spacing of the movement joints if you have reinforcement, although you will need to seek specialist advice from the brick manufacturer.

Is insulation going on top of the joists? The wall may be another 200mm taller than you section as you need the 1100mm for edge protection.

 

[liam1369]

Hi George,

Very good points, the brickworkdevelopment association recommends a slip plane to parapets, but this seems to be too onerous.

The width of parapet is 18m to the returns, therefore, will definitely need at least one movement joint with windposts either side of the MJ.

Will have a look at the ones from the floor below, these might then be 4m+ so contractor will likely winge.

There doesn't seem to be too much guidance on this.

 

[GeorgeTheCivilEngineer]

If you went for a windpost built into the inner leaf then it'd act as a movement joint as well. A windpost down to the floor below will be fairly expensive but I suspect other solutions won't be as straightforward.

A cavity tray will be needed anyway as part of building regulations so the slip plane isn't onerous.

 

[XR250]

Y'all build shit weird on that side of the world

 

[GeorgeTheCivilEngineer]

The ground isn't wibbly wobbly so we just stack things up.

 

[liam1369]

Thanks George,

I will look at running it and oversailing it so it acts as a propped cantilever.

I think this is the best way forward for this scenario.

Kind regards,

Liam