Cantilevered Handrails on Composite Steel Decks 2

[nategemb]

I have often designed concrete-over-steel composite decks for floors. Recently, we have been receiving requests to attach handrails to the concrete, typically using expansion anchors or embed plates with headed studs. This creates a cantilever condition, which puts a portion of the concrete in tension. Since the concrete only has minimal reinforcement (welded wire fabric) and does not bond strongly to the metal deck, I do not think this is a valid connection in composite deck and would rather see these anchored to steel structure below the deck. Does anyone have any input on how they typically deal with this condition?

 

[KootK]

Shhh... some things are better not openly discussed.

1) I've seen this detailed with through bolts and an anchor plate on the underside. That's not perfect either in many cases but can be an improvement.

2) If the concrete is thick enough and the opportunity to do so still exists, you might put some small diameter rebar in the slab. Technically, you probably need top and bottom bar.

3) If you can make the concrete anchorage check work without reinforcing, you might be able to argue that you can move the moment into the concrete deck using unreinforced concrete principles as well. This is probably a long shot.

4) Yeah, I would also like to fasten the had rails to serious steel structure below.

What kind of deck and slab thicknesses are you typically using?

 

[nategemb]

Haha. I think this is one that is often left unaddressed and we'd rather not know about. One of the many things that in real life often doesn't result in failure but we can never get to calculate out.

It would be nice if we could add rebar, and in the future that will be a talking point during project kick-off on projects where we anticipate this condition. On this job, the concrete is poured. 6 1/2" total slab depth on 2 inch W-deck. I think we will end up trying to grab steel below the deck on this one.

Thanks for the reply!

 

[KootK]

Yeah, if guardrail loads actually came to pass out in the wild, there would be folks plummeting to their deaths left right and center. A lot of that work goes delegated design and winds up in the hands of folks that are basically making a living out of not looking too closely at the anchorage problems (or being unaware of them).

4.5" CIP above deck isn't too shabby. You might make a go of that in plain concrete anchorage.

 

[OldDawgNewTricks]

I personally know a young man who is permanently paralyzed from the waist down due to a guardrail failure. His best friend was killed in the same incident. I don't think the railing was attached to a composite metal deck slab in that case, but the same principle applies. Provide a proper load path and be able to sleep at night.

 

[bones206]

I was installing anchors for something (maybe a guardrail, can't remember) many years ago. Upon torqueing the first expansion anchor, I cracked off the edge of the slab. Like the ENTIRE EDGE of the slab along the length of the stairwell opening. It will always stick in my head when designing things near slab edges. Fracture mechanics are just inherently less stable, predictable at edges. The lateral force induced by expansion anchors in particular is something to avoid.

 

[KootK]

In my opinion, if we really want guards to be able to do what we say they should be able to do, they have to be field tested. Market forces and edge anchorage reliability being what they are, I just don't see any other realistic way to get it done. Sometimes I wonder how well code officials know the reality of this space.

I used to dabble in glass and have seen a lot of glass guard on wood balcony details. I doubt there's one that would survive load testing.

 

[jayrod12]

We have the glass guards on our deck at the lake, fastened down to the wood framing. Granted I blocked the crap out of that last joist space and put double edge and end joists so it's fairly robust, but I still question whether it would truly take a guardrail design load event. I even made the installers provide longer fasteners because they were only going to put in 2" long lag screws, i.e. barely long enough to make it through the decking, much less into the meat below.

Luckily, the fall height really is likely to only result in some bumps and bruises, maybe a broken wrist if really bad luck. So the stakes are low in my case.

 

[Enable]

One nice thing about my industry (restoration) is that it is standard practice, and included right in the tender, to have field load tests on pretty much any railing job. I cannot recall the last balcony restoration project where we did not field load test the guards; and these are generally guards without all the anchorage issues you have with concrete on steel decks.

In my experience testing these assemblies, both in the field and in the lab, a non-trivial amount fail in ultimate (baseplate anchorage issues) but most fail in serviceability (non-recoverable deflection, etc). But again, that's in solidly reinforced concrete or thick SOGs. I completely agree that anchorage to lightly reinforced, thin slabs, is an absolute pipe dream if you really want code loads to be resisted.

 

[phamENG]

glass guard on wood balcony details

I'll be designing one of these later this week. I'm not excited about it.

 

[KootK]

I'm sure that yours will be amazing. Continuous top rail?

 

[phamENG]

Not sure yet. All I've gotten are pictures of a catalog showing 4 different arrangements and a caption that says "this one". Lots to unravel. If I come up with something earth shattering I'll post it.