Facade attachment to Steel framed buildings 1

[Samwise Gamgee]

I had a quick question about the need to use kickers to avoid torsion from the facade attachment. I have a slab cantilever of 6" over the beam for the slab edge all around the perimeter of the building. AISC design guide specifies that as long as the slab is designed as cantilever over the beam, no torsion will be delivered to the spandrel beam and kickers are not needed.

However if the steel edge member is used to transfer the forces to the spandrel, there will be torsion in the spandrel which needs to be resolved using kickers.(In case the slab is not designed as a cantilever).
My question is when would we even use this case ? Wouldn't it be ideal to just design the slab as a cantilever ? Also how does this apply to roof where we only have facade attached directly to the deck. What drives the need for kickers is what I am trying to understand.

 

[phamENG]

There's not always room to put a cantilever on. Most of the architects I've worked with like to hide columns in the wall when possible, and it is usually possible in one and two story steel buildings. This is especially the case when it's being designed for a purpose. Shell buildings aren't as picky since the architect doesn't know the layout anyway. So when you do that, the spandrel is tight to the exterior face of the building.

Even with the cantilever slab, you could still have torsion in the beam from the facade attachment - just depends on how you attach it.

Braces at the roof are usually needed regardless as you need to brace the bottom flange for uplift in most scenarios.

(By the way - I've never liked "kickers"...I put it in the same category as "footers"...just a pet peeve of mine)

 

[Samwise Gamgee]

Thank you for the response. The way I understand is under uplift, the beam bottom will be in compression and they are not braced at all. That is the reason for bracing them ? With that said , do we need to brace all members bottom flange ?

 

[phamENG]

Yes, uplift will induce compression in the bottom flange. That's precisely why it needs to be braced. For gravity loads (vertical down), the top flange is in compression but is typically braced by the steel deck or by members framing into it. So for uplift when the bottom flange is in compression, you typically need something to brace the flange.

If the bottom flange is in compression, yes - it should be braced unless you've analyzed it and proven that the beam is not subject to lateral torsional buckling or other instabilities without it.

There's also the question of convention. If your design is subject to review by an AHJ, the reviewer may be so used to seeing that brace that they'd flag it if it's not there. Then you cost the project time in delays and your company the cost of your time to prove that it isn't necessary. They're cheap, so unless you're doing some mega structure where excluding them will save tons of steel, it's probably best to just include them.

 

[Samwise Gamgee]

Thank you so much for the update. With that said, what governs the spacing of these kickers to brace the bottom flange ?

 

[phamENG]

Some light reading for you: Bracing for Stability, Yura and Helwig