Steel insert in aluminum mullion

[XR250]

I have a situation where a contractor wants to use tempered glass as an infill panel for a residential deck guardrail. The glass comes with a perimeter aluminum frame that then screws to the surrounding structure. The panel will fasten to the deck surface at the base and wood balusters at the sides. At the top, they want to run a 2x2x18ga aluminum extrusion from post to post.
When I check deflection of this top rail with a 200 lb vertical point load on a 6 ft. span, I get 3/4". The glass in the frame does not use a traditional silicone gasket like aluminum storefront but a heat bonded system. The sales rep says it may be able to take 1/4" deflection but does not guarantee that. Anyway, I want to make the top rail as stiff as possible.
I can fit a 1 1/4" ID pipe in there. Steel would be best as it is readily available and stiff. My worry is water getting in and having a galvanic reaction. Any ideas how to mitigate this? Also, the aluminum will have to deflect a bit before the steel engages. The contractor says the 18ga extrusion is the thickest he can get. I'm also thinking about just walking away from this.

 

[phamENG]

I'd either force him to find an extrusion that works on its own, or walk away. There are systems that are designed to work that do this - they should pick one of those.

 

[XR250]

I hear ya. He likes this one as it is easy to get and already painted or maybe anodized.

 

[phamENG]

Could you get him down to a shorter span that works? That might be an option, but still sounds like it could be an issue. What deflection will the manufacturer guarantee? If they won't give you a number, that's probably another reason to run from this one.

 

[SwinnyGG]

Is this attaching to W deck or something with a flange at the bottom of your intended round post?

If it's an arrangement that doesn't require welding you could specify an epoxy coating for the steel. A heavy zinc flake might also work.

 

[XR250]

Could you get him down to a shorter span that works? That might be an option, but still sounds like it could be an issue. What deflection will the manufacturer guarantee? If they won't give you a number, that's probably another reason to run from this one.

These are retrofits of existing rails and he is using the existing post spacing. They won't guarantee anything.

 

[TRAK.Structural]

Can you rely on the semi-circular protrusions on the inside of the aluminum? This looks like where the extrusion would contact and load the steel. Would these little protrusions just deflect/buckle when the load gets too high?

 

[jayrod12]

What about using a 1" aluminum bar? That's got to get it much closer?

 

[XR250]

Turns out the span is only 5'-2" I got this to work with a 1 1/4" sch 40 aluminum pipe. The contractor complained it was more work than they were used to. I told him to shop around for a different engineer.

 

[SE2607]

I would be concerned about galvanic action.

 

[phamENG]

@TRAK.Structural 's concern is very valid here. You'll be yielding those little round tab things on the inside of the extrusion before the load really gets into pipe.

 

[XR250]

The pipe will be tight to boih the tabs and the top of the tube so that should not be an issue (assuming the CAD is correct.).

 

[phamENG]

right, but I don't think those tabs are strong enough. bearing stress between the steel and the aluminum as well as bending stresses in that tab from the eccentricity caused by the curve will cause plastification of the aluminum, allowing it to deflect more than it might otherwise appear before engaging the pipe.

 

[jayrod12]

As long as it protects someone from falling, I don't really care if there's minor deformation of those tabs personally. I liken it to other life safety things like fall arrest. There may need to be repairs after a fall event, but as long as it adequately prevents the person from falling, job complete.

 

[Harbringer80]

As long as it protects someone from falling, I don't really care if there's minor deformation of those tabs personally. I liken it to other life safety things like fall arrest. There may need to be repairs after a fall event, but as long as it adequately prevents the person from falling, job complete.

For this instance I disagree. Of course you have to meet strength but the minimum criteria for this situation is serviceability so the glass doesn't shatter when a couple of people lean on the rail. Additional deformation leads to additional deflections which leads to a phone call about shattered glass.

I agree with phamENG that the pipe will likely not limit deflection as much as you would like due to localized yielding of those tabs.

 

[jayrod12]

For this instance I disagree. Of course you have to meet strength but the minimum criteria for this situation is serviceability so the glass doesn't shatter when a couple of people lean on the rail. Additional deformation leads to additional deflections which leads to a phone call about shattered glass.

I agree with phamENG that the pipe will likely not limit deflection as much as you would like due to localized yielding of those tabs.

I believe the glass breaking is precisely the requirement for a top rail in glass guard applications. In the event the glass breaks while preventing a fall, the top rail is supposed to provide the fall protection.

A couple of people leaning on the guardrail will not cause the level of damage you're envisioning I feel. Only a code level fall protection event is likely to do that.

 

[XR250]

right, but I don't think those tabs are strong enough. bearing stress between the steel and the aluminum as well as bending stresses in that tab from the eccentricity caused by the curve will cause plastification of the aluminum, allowing it to deflect more than it might otherwise appear before engaging the pipe.

BTW, the pipe is now aluminum.
My deflection concern is vertical, not horizontal as I feel the glass is more fragile in that direction and the chance of someone sitting on the rail is a lot higher than actually getting a 200lb horizontal load on the rail.
Honestly, I will be getting some composite action between the frame of the glass and this assembly that i am not taking into account. It gets fastened pretty frequently.
Also, , this glass is tempered and can take like 120 psf wind load so it can probably support 200 lbs vertical.