Facade Question: Window Wall Strap Anchors

[GallileoFacades]

Hi

I'm looking for some advice for a window wall facade system. A system I'm working on has strap anchors around the sides and top of the frame which tie back to the main supporting building structure. My question specifically is related to at he head of the system as we need to allow for 1" vertical deflection of the structure above the horizontal mullion so the strap anchor is cranked at an angle. I've attached a rough section through this.

As the window wall system will obviously undergo pressure and suction wind loading, I'm envisioning that the strap anchors will go into bending due to the cantilever created by the deflection gap. If I design the straps to resist this bending however, I get really close spacings of the straps around the perimeter of the window wall frame (so much so that it wouldn't be reasonable to adopt this). From what I've found in my online research, straps are generally installed at 12" to 16" o.c and max 4" from corners. I can't find any guidance though on what capacities I should be using for the straps to justify this type of spacing for our project specific wind load.

Can anyone offer any insight on the structural design justification process for these? Or is it based purely on test data of the straps?

Many thanks!

 

[phamENG]

You don't have a deflection gap. Your strap anchor is going to "restrain" the vertical movement of the head by transferring the force into the mullion. Given the shape, the strap will try to flatten out and push your mullion outward against the angle until a) a fastener fails or b) the strap buckles.

Which way do the windows span? Is the aspect ratio such that you can force the loads out to the jambs of the window? If so, you could forgo the fastening at the head and provide a true "deflection gap."

Just an idea - there are a few facade experts on here that probably have better ones.

 

[GallileoFacades]

Thanks phamENG. Yep they're pretty much all my concerns here too but I'm just not sure if I'm missing something fundamental as from what I can see, this is a common detail adopted in tower window wall systems in non-seismic areas.

To respond to your question regarding the span, it's top to bottom as the width is significantly larger than the top of slab to underside of slab height. I believe this is common too for these systems.

 

[phamENG]

Ok, sorry - I thought your concern about bending was from wind pressures.

Since you're already showing an angle on the exterior that would prevent suction pressure from pulling it out of the wall, can you add one on the interior that would prevent positive pressure from pushing it into the building? Then the strap anchor on the head isn't needed, is it?