Embedded guardrail post 1

DETstru · Aug 15, 2017

I sent this diagram to an architect to illustrate my concerns about a detail they proposed for an embedded guardrail post on the exposed rooftop amenity space of a concrete building I'm designing.
The diagram is somewhat to scale. Just wondering if anyone thinks my concerns are valid or if anyone has additional thoughts on it.

[ol 1]
[li]I have concerns about water infiltration through the crack that will inevitably form at the embedded post.[/li]
[li]Getting this to pencil out structurally will be tough. I'll need DBA's or studs welded to the post to anchor it into the curb/stem wall. Then sufficient vertical dowels coming up from the concrete slab to anchor the curb into the slab. (I'm unsure how the development length of those dowels will even work since I'll have a moment directly at the top of the curb, where the dowels are not developed at all)[/li]
[li]And even if I do manage to make it work on paper, building this will be a nightmare. I can already hear my phone ringing with contractor complaints....[/li]
[/ol]

EngineeringEric · Aug 15, 2017

So that is a steel plate embedded in a tiny concrete 'curb'. Why not just attach the plate to the roof?

Water-proofing. Yes, what you have proposed will not work. if this is what they want.... and it is ugly.... Then the roofing membrane needs to go over the curb, and at each post the roofer needs to wrap and seal the membrane to the rectangle penetration. You can see details of products if you look at Carlisle PVC or TPO details, but you are't getting paid for that. Ask the Arch.

I would also make the curb lower since it doesn't take the moment, or even don't use concrete. Stop it at the pavers, or bring it past and clad it to protect it.

thaidavid40 · Aug 16, 2017

If the curb height needs to remain as tall as you have shown it, why not make an inverted saddle, welded to the bottom of the post, and bolted through the curb profile?
Dave

Thaidavid

Ron · Aug 17, 2017

I can't see how you have enough concrete in the curb to resist the moment from the post.

Further, as noted, the waterproofing will be compromised.

I agree with David....saddle it.

JedClampett · Aug 17, 2017

Is this a guardrail (AASHTO, for traffic) or a guard (IBC, 200 lb. horizontal load)?
It seems if it was a guard, it could be bolted to the outside of the curb.

KootK · Aug 17, 2017

I think that you could get your connection strength easily enough by connecting to the adjacent concrete with studs and DBA as you've suggested. It's really the durability and constructability issues that tank the detail in my mind. I'd think it would even be tough to pour and vibrate around the posts at the proportions shown.

DETstr said:
I'm unsure how the development length of those dowels will even work since I'll have a moment directly at the top of the curb, where the dowels are not developed at all

This shouldn't be a problem. With the connection strategy proposed, you wouldn't be delivering a concentrated moment at the top of the wall. Rather, you'd be delivering a concentrated shear which should be able to be calc'd out alright. Your moment turns into local shears, one at the anchor near the top and an opposing one lower down (presumably).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Ingenuity · Aug 18, 2017

DETstru:

For the past 1.5 years I have been involved with a project near identical what you sketch.

Project built about 2 or 3 years ago - "green roof" situation with perimeter rails. At each and every post (4' c/c) there was a vertical crack. The crack formed before the facility opened. With such a close spacing of vertical posts and the narrow curb width there was inadequate reinforcement, especially when you consider that the roof was placed as a previous pour, providing significant restraint for longitudinal shrinkage, hence cracking.

Fast-forward to today, and the building owner (university) and the contractor and the A/E are all in court.

We were involved to look at repair/strengthening techniques. The re-work on the waterproofing was more costly than the curb/post strengthening.

Your concerns are warranted.

DETstru · Aug 18, 2017

JedClampett, It's a guardrail on a rooftop amenity space. Pavers on one side and green roof on the other.
It would be bad news if a car managed to get up there...

Koot, makes sense. I'm going to end up doing an embed plate at the top of the curb and weld the post to it. I'll probably use DBA's and lap them with the vertical dowels coming up from the slab.

Ingenuity, Sorry to hear that happened. It's exactly what I was afraid of and luckily the architect understood as well. We're changing the detail to an embed plate in the top of the curb, which should lessen the crack potential significantly.
Your comment about the slab below restraining the curb is good to keep in mind. I'll have to think about that to make sure the crack locations are controlled while maintaining waterproofing.

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