Bracket connecting sunshades to curtain wall mullion

[GastonLagaffe]

I have been tasked with designing sunshades for an aluminum curtain wall. While everything seemed fine at first, some last minute changes now have me questioning my judgement and I'd like to ask your opinion on this.

The sunshades are connected to an aluminum bracket that screws into the curtain wall mullion's screw chase. The screws are 1/4" dia. stainless steel. I've calculated the capacity (factored) of the screws, governed by pullout from the chase, as 335 lb using the ADM. My concern is due to some detailing requirements, I can't have the bracket be nice and symmetrical and I'm left with this:

The 400lb load is also factored. My concern is that the interaction with the rubber gaskets, the glass, and the mullion may lead to some unconservative assumptions. I'm picturing the one screw at the top being ripped out before any of the other ones are engaged in tension and having the entire bracket basically zip away.

Or am I overthinking this and taking my neutral axis at the center of my 4 screws is fine?

Thanks and I hope this makes sense. I look forward to hearing your thoughts on this! As a long time lurker but only a recent participant to this community I feel very thankful to be able to post here

 

[phamENG]

I'd start with a strain compatibility check. Develop a triangular bearing stress distribution between the two pieces of aluminum, using the maximum bearing stress permitted as the max value of the bearing stress. Then use some algebra and statics to determine tension loads in each screw that, when plotted, will fall on the same line as the compressive stress distribution. (Summation of moments will involve the tension forces from however many screws fall within the tension zone and the centroid of the bearing stress).

 

[BAretired]

Your diagram says 1.5" typ for screw spacing, but they don't appear typical. The middle space appears larger than the others. Taking the neutral axis at the c.g. of the screws doesn't help because the force in the upper screw will exceed 335# pullout value.

Screw tension will vary linearly from minimum at the bottom to maximum at the top. Compression will occur in a small area at the bottom of the channel. Even then, the maximum tension exceeds the indicated pullout value.


BA

 

[NorthCivil]

what is the height of that upside down L bracket? that is fixed to the mullion.

 

[dik]

careful you don't damage the building envelope.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dhengr]

GastonLagaffe:
Don’t forget, the mullion must take your bracket load back into the structural framing. All this without hurting the water/air tightness, the glass support and mullion deflection, etc. etc. You might want to hash this over with the company that designed/manufactured/installed the window system/exterior cladding system. They may have something to say about this.

 

[jjl317]

Not sure how much flexibility you have in the layout, but the typical sunshade connections I have seen would have the outrigger components centered vertically on the attachment bracket, with (2) screws at a minimum spacing 3D near the top and bottom. Then the spacing between the fasteners can be increased, reducing the applied tension, without increasing the material requirements.

Are the screws threaded into the chase only? If so, while your exact configuration is unclear, I would definitely be concerned with the vertical slip down the chase. Not sure which version of the ADM you are using, but I believe that the older versions (ie 2010) don't include enough information to fully develop the allowable capacity of connections to the screw chase, and the AAMA TIR-A9 "Design Guide for Metal Cladding Fasteners" can be used.

 

[GastonLagaffe]

Thank you all for your replies! Much appreciated!

First of all I apologize as I made some mistakes in my original drawing - there is a lesson learned there on my side about making drawings too quickly in Paint... The vertical load is actually only 200 lb and the screw spacing is 2", not 1.5".

phamENG - interesting approach with the strain compatibility. My concern with that would be that the bracket may not be rigid enough relative to the mullion assembly for that case to be true. Would there be perhaps a rule of thumb to goo with that assumption?

dik and dhengr - the integrity of the envelope is 100% a concern here. I found the deflection in the mullions caused by the sunshade loads to be about L/900 (at ultimate). We're at less than 5% capacity. In my mind and my boss's, that was enough to consider the loads +/- negligible. Or could we be missing something big? The envelope designer will indeed have a final say in this, which is reassuring in a way, but we still don't want leaks no matter what

jjl317 - What you're suggesting was the first solution I had in mind, but unfortunately there's interference from that at the top edge of the curtain wall, hence the L-bracket. If there are other solutions I would be thrilled to see them! The ADM 2020 does include new provisions for screws in chases which is what I used. For the actual vertical slip, I figured that this would mostly be carried by friction, so I calculated an additional tensile load in the screws to generate the friction required. Since there is a rubber gasket it doesn't take too much to generate enough friction.

My main concern with this post was that since the moment load is applied right at the top, that the screws would just zip out. But it seems like that doesn't worry you guys too much, which is reassuring.

Sorry if my post is too long - just want to make sure I'm covering all bases here Thanks!

 

[jjl317]

Just to be clear, the concept I provided was to try and eliminate the concern by reducing the tension - in part because I have the same concern you mentioned on how the loads might be applied to the screws - especially if the channel component is not especially stiff. Not sure if the reduced loads plus increased spacing resulted in an acceptable design. Plus, if you are not the curtainwall designer, I would consider contacting them to see if they might have existing solutions for attaching the sunshade to their system. Here in the US, many standard curtainwall system has specific extrusions to address these types of connections.

And while I haven't yet seen the new ADM provisions for dealing with screw chases, and I am not exactly sure what this connection looks like, I would personally confirm that the vertical slip loads could be transferred to the mullion without any friction from gaskets, by the AAMA slip criteria. I have always found the values from this approach to be much lower than I would typically expect.

 

[GastonLagaffe]

Thanks, jjl! You're right, I will contact the designer and see what they typically do. And for letting me know about the AAMA slip criteria - I wasn't aware of it. Will go through that check. This is great!

 

[dik]

You will likely void any warantees, you had.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[GastonLagaffe]

Thought you guys might like an update on this - we wanted to have further certainty so we decided to do a few quick tests to validate our assumptions. Perks of having a press at the office We were also able to modify the bracket to have the moment applied closer to center.

Our "pry" test had a max moment load about 3x higher than factored loads, which is reassuring. Not a perfect test but enough to get a pretty good feel of the behaviour.

jjl317 - we also did a test on shear in the direction of the screw chases. 4x 1/4" screws held up to almost 5,000 lb before we had to stop because the pressure plate was deforming too much. No sliding at all.

 

[jjl317]

Thanks - I appreciate the follow-up - always good to see real performance vs analysis theory (and I have to admit I am jealous of the access to test easily)