Guardrail attachment over existing stucco

[shacked]

Ok, here we go....I am the E.O.R. for this project, which is a new custom 2 story residence(spec house for a developer). Everything has been approved and built. The developer sold the house but the owners wanted to make some changes. The contractor came to me with this guardrail detail for the 2nd floor balconies.

They want to attach the metal posts through the existing stucco. I dont like this idea and I have never come across this before.

What say you?

Thanks

 

[jayrod12]

I don't think the stucco is the weak point of that detail.

 

[SJBombero]

These details are always a problem since the engineer is limited in the way in which he can definitively justify them. This detail will most likely work, but you would have to use some unorthodox methods to figure that out or directly test it. It doesn't really matter where the lag screws are coming from, they are all entering the rim beam. So the rim beam needs to be restrained for the balustrade moment. If you can justify that the rim is restrained, you are probably safe. However, I do see that the only significant support for the gravity load of the glass panel will be friction against the stucco.


It looks like the guardrail glass is at least 3-1/2" off the edge of the deck edge. I'm sure that wouldn't pass inspection.

 

[Enable]

I'm recently ingested some melatonin, and had a few beers, so take what I'm about to say with the requisite grain of salt

But I don't hate the detail. There is just a lot to consider. First, to SJbombero's comment about the 3-1/2" standoff that would certainly pass code here in Ontario. 100mm max opening for guards in any direction. Stupid? Yes. Code compliant? Also, yes.

The guard load will be resisted by a force couple at the top anchorage in tension (well shear) / base in compression, right? So the stucco is only a concern if it crushes locally (tensile capacity into the wall/wood framing shouldn't be much a concern in pull-out i'd suspect for inward loading). I honestly don't see that as a concern unless we are talking some rather shitty stucco built on top of popcorn insulation (which from the schematic it doesn't look like). Really what needs to be checked is the roll-over capacity of whatever member you are putting the top lags into. Also, even if roll-over is good you need to think durability due to the wood locally compressing / giving over time. I'm in zero state to offer guidance on that front but this thing will feel unsafe, even if it isnt, once those bolts start to wiggle in the wood.

 

[shacked]

Well, I have already considered the tension that is why I did not mention it.

The beam being attached to is a 6x14. There is no way in hell a 200Lb point load applied at the top of the guardrail is going to twist this beam. The beam is a 10ft long 6x14 edge nailed to the 3/4" plywood with intermediate blocking down the length @32" o.c. and secured at each end in a post cap. The dead load of the beam is approximate 15plf...for a total weight of 150lb. You can run these #'s all day long but in this situation this is not the issue. This is new construction, and as the E.O.R. I have verified that the construction has been IAW plans and code. Only way that it would be an issue is if every foot there was a 200lb point load applied on the top of the guard.

I dont like this detail because it could be a potential waterproofing issue as well as a potential failure point. . For example I can guarantee you that if the top rail is pried back and forth enough, like drunk adults banging into it, that eventually the stucco will start to crack around the area. When the 1" thick stucco starts to crack it will eventually create a condition where the lag bolt is free to bend since it is not fully restrained from bending.

 

[Ron]

You are breaching the waterproofing at two locations, which will lead to wood rot. Otherwise, your detail is workable.

 

[jayrod12]

Are you sure that it's ok for the pullout? That's at least 700 lbs of tension in the top or bottom anchor depending on load direction. That's a fair bit of tension for a lag screw. It does not give me warm and fuzzy feeling.

 

[dik]

The main issue with the connection is the penetration of the building envelope as Ron has noted. The 1/2" dia lag screws can easily develop the 700lb with proper embedment. The SDS are also quite strong. The big problem with the lag screws is the width of the material it is going into and the connection of that material to the remaining structure... likely need solid dimensioned lumber like 6x6 or something of that ilk.

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

-Dik

 

[shacked]

Thanks guys. I had a feeling that this was the main issue.

 

[MotorCity]

I don't like:
1. The gap between the rail and edge of balcony
2. The spacer crushing the stucco
3. The plate mounted to the exposed walking surface
4. The cross grain tension and edge distance of the SDS screws
5. The waterproofing issues mentioned above
6. Sheathing on a deck or balcony

 

[shacked]

Ok, thanks everyone that has commented on my original post. Since my post I have gone back and forth with the contractor and it looks like they will not be mounting it directly to the stucco, but now upon further investigation I see another issue with respect to a similar attachment condition that I have proposed.(see attached)

Since there is a 3.5" gap between the post and the mounting surface wouldn't this also create shear between the lag bolts and the wood?
Just want to make sure I am thinking about this condition and the resultant rxns correctly:

Height to top of post = 52", Moment = 200x52" = 10,400"-#
Shear perp to grain, per bolt = 10,400/(3.5*2) = 1,500#
Withdrawl = 10400/5.75" = 1,810#

What do you guys think?

Thanks

 

[jayrod12]

I'm not sure I understand your calculation for the shear. Wouldn't it just be equal to whatever the downward load on the guardrail is?

I'm not keen on this installation at all. Stand-offs, stucco, wood lags in both bending and tension. It all screams of inevitable failure. And that doesn't even bring into the discussion the building envelope issues.

 

[phamENG]

Your lags are already almost all the way through the beam. I'd make them through bolts and end the top one in a hold down fastened to one of the joists.

What does this look like if you look down on the connection? A 2" square tube with a 1/4" plate to offset it won't give you much room for your bolts. Why not weld your plate to the tube, the plate to a mounting/base plate, and then bolt the mounting plate to the beam with a good tension tie into the framing to transfer it into the diaphragm? It'll look a lot nicer, too. Or maybe a steel angle? You'll have to check prying, but that shouldn't be too bad.

 

[shacked]

Jayrod, as I said this is not now attached over stucco. As far as the shear, there is a 200lb point load horizontal at the top of the post. Therefore I believe that the resultant rxns at each of the bolts would consist of shear due to the 3.5" standoff(moment being resolved into couple) as well as withdrawl.

I agree with phams drawing, but the only caveat is that this is already built and the eaves stuccoed so they would have to tear off the stucco and the contractor does not want too. Well too bad, they are going to have to.

Thanks for the confirmation

 

[jayrod12]

A horizontal load does not magically become a vertical shear. Generally, these connections are at 4 feet on-centre for most guardrails, and there's a vertical load on guards. The shear on the screws/bolts would come from that vertical load, not the horizontal. The standoff just causes moment in the connection from the vertical load, otherwise in a free-body diagram, technically the standoff doesn't change a damn thing if you're only looking at the horizontal load.