Cantilevered balcony in wood framing - General Discussion 6

[SleeplessEngineer]

Hi everyone!
As a new member, I am enjoying and learning a lot from this forum.
I have always been pondering about balcony framing for wood structures and would like to get some expert opinions.
My go-to approaches are:
[ul]
[li]cantilevered PT joist with back span sistered with floor joists ( My preferred approach -just because of not much need to be checked and contractor unlikely to mess up this, but does not work if the cantilever span is more than 3'6")[/li]
[li] cantilevered steel beam ( channels or WFs) with back span (if the cantilever span is higher; while I prefer WF beams, architects prefer channels for some reason)[/li]
[li] moment connected steel beam ( WF or HSS) with column (although I prefer steel beam with backspan, sometimes situation does not work out; for example, not enough back span due to floor opening and balcony inclined to floor framing; this tend to be pretty extensive since I brace the columns with other steel beams. I am not sure and not tried if bracing can be provided just using wood beams.[/li]
[li] moment connected steel beam ( WF or HSS) with girder ( for the occasions when a column can not be provided at the exact point due to window opening and such)[/li]
[/ul]
Are there any better ideas I am missing? I would appreciate if you can share any of your interesting experience.
Also I do have two specific questions:
[ul]
[li] HSS connection design: RISA and RAM doesn't offer HSS connections (other than for the truss purpose). Since HSS are good in torsion, I tend to specify them for moment connected cantilevered balcony (even though they are heavier than equivalent capacity WFs). I specify direct weld and let the detailer design for the loads. As EOR, we don't generally design connection (only do cursory check using software) and don't want put in extra work in checking these. Should I avoid specifying HSS?[/li]
[li] Balcony inclined to interior floor orientation: So I mentioned earlier, I provide moment connected beams in this scenario. But I would love to learn if there are any other way. In one of the project, I chose to provide the floor framing in skewed orientation to allow for the back span of steel beam to go through; the contractor was not happy and complained. At the end, they did something on their own, different from structural and archi layout and finished the project without our blessing. So I stopped going that way. [/li]
[/ul]
Please feel to criticize any of my approach or pick on anything I described here.



Thank you for reading my post! I appreciate your time.

 

[jayrod12]

Except that you're likely to get a whole bunch of people talking about how terrible cantilevered wood balconies are, I also understand that sometimes you've got to do what you've got to do.

I don't really think you're missing much there in terms of options or solutions.

Sometimes for the slopes, we'll just provide the beams at the lowest elevations, and then provide sloped infill framing on top. That way the fabrication and installation of the main structural members is easy, and they just have to be a bit finicky on the infill framing.

 

[Yao1989]

Where I am from we see a lot of balcony beams aligned with wall then cantilever out. See below.

 

[dhengr]

SleeplessEngineer:
You use PT lumber for the deck/balcony beams and joists, but not for the ext. walls or int. fl. framing, and that’s often what gets wet and rots, hidden for a while. Cantilevers through ext. wood walls are a bitch to do properly and to flash and treat the water plane system properly. You should really leave them open on the underside (no enclosed ceilings/soffits without lots of venting and inspection access) so they are easily inspected and so they dry out. Watch out for connection hardware and fasteners and the PT lumber, not a common int. framing concern. Structurally, pay attention to good load path and sensible framing, and then if it works, it works. Decks and balconies fail far more often due to moisture intrusion, structural material rot, and poor connections to the bldg., than they do to basic structural inadequacy. The devil is truly in the details, yours and the builder’s quality of detail and workmanship, etc., slope everything to drain to the outside, flashing, counter-flashing and more water proofing tape and membrane are important.

 

[MIStructE_IRE]

I’ve seen 3 real life examples of this go horribly wrong in my country and then there’s the famous collapse as well.

As above, if you’ve got to do this, then the devil truly is in the detail.

 

[phamENG]

I generally avoid your preferred approach. My preferred approach is to convince the architect it's a bad idea, and it works pretty well. If you don't know why and would like an explanation, just say so and I'm sure any of us would be happy to expand on that one. Then, I go to two beams cantilevering out, preferably steel and preferably rectangular HSS. Why HSS? Because it's easier to flash and there's less chance of getting a leak. Even if it has to be treated wood, fewer penetrations means easier to repair (though it can also mean decreased redundancy, which is a big part of why I prefer steel). And it will need a repair. Even if the flashing is done right, there's a good chance the balcony framing will need to be repaired or replaced decades before the interior framing.

HSS connection design: I find that approach a difficult sell in residential. A large steel building, sure. But most residential jobs if you put that on the drawing the "fabricator" (some guy the GC knows with a welder in his garage) will just stick two pieces of steel together. No shop drawings. No analysis. So if I have one steel beam on a job, I essentially produce a shop drawing for that one beam because it doesn't make much sense to send it to another engineer to design and detail. And welded moment connections between HSS members are pretty simple. Take a look at the AISC examples and work through a couple, then you can build your own spreadsheet or SMath program to knock out the calcs for it in 5 minutes.

Contractors complain. It's what they do. They want to make things as easy and as cheap as possible so they can maximize their profits. Don't blame them for it - none of us get into this industry to run a not-for-profit business. If they want to do it differently, you have a couple of options: be a team player and listen to their idea and try to incorporate it. It may work and they may teach you something, or it may not and you can explain to them why it doesn't work. OR you can just tell them to shut up and do it your way. The course of action taken will likely be up to your supervising engineer as it may involve additional time that isn't in the budget and somebody would have to pay for. But if they complete the job differently than you specified, you know about it, and you don't do anything - you have given it your blessing.

 

[SleeplessEngineer]

Thanks everyone!
@dhengr, you are right I don't know why did we start we specifying PT lumber. But the intent is to get wrapped all around with waterproofing membrane (top, side and bottom of balcony framing). These are not exposed. The reason for providing separate member rather than cantilevering just the floor framing is create level difference.

@phamENG I believe you are referring to water intrusion at penetration. If the balcony framing is protected with membrane all around and sloped rigid insulation for drainage, will this be still an issue? The deck will be overframed on top of the rigid insulation with pedestals to level them.
For the HSS connection, I was referring to HSS beam to HSS column. I was not able to find any example from AISC on this while I searched last time. Almost all of them were HSS column with other members. It is possible I was not using right search terms. I will try again.
Of course, I understand contractor always complain if things are not simple. My intent is get something simpler for everyone so lesser messing up on site and less time spent coordinating. I hope we get good contractors who provide some reasonable ideas of what they want; usually all they ask is making structural elements disappear based on their experience on previous projects where they actually did that and it didn't collapse(yet)!

Thank you for reading my post! I appreciate your time.

 

[phamENG]

A lot of contractors will tell you that some other engineer approved this or that. Maybe they did (and your 'yet' comment is applicable), or maybe they didn't and they're just trying to pull one over on you and see what they can get you to approve. I've encountered a few unscrupulous contractors who tried to do that to me when I was an EIT. Now I like to call their bluff. "Ok, call that engineer and have him prove that it works." They usually grumble and then do it your way. And if not, then the risk is on the other engineer's head. And if they get the other engineer to look at it with you as final approval, it's amazing how fast an unscrupulous engineer will find his scruples when he knows he's being reviewed.

 

[SleeplessEngineer]

Just 2 weeks back, a developer+contractor client asked us if we can do 10' depth balcony without any posts using just wood framing. I responded to him with a long explanation of why it is not possible. He immediately emailed back 'how about if we reduce to 8' depth'. I checked their company's portfolio and they have done lot of big 'beautiful' projects. These are the moments when I wonder if we are behind the latest construction technology and doing outdated designs or others just ignore everything about structural design and provide some fancy schemes. Anyway we avoided that project, I think we might be able to pull off with some knee brace or cable hanger but we did not have time to engineer all these and also the client seemed in super hurry.

Thank you for reading my post! I appreciate your time.

 

[XR250]

A 10' cantilever should be possible with treated glulams, PSL's or LVL's - as long as you have enough depth to work with.

 

[SleeplessEngineer]

Technically yes, the deck are is 10'x21' for 3 stories. Back span available was like 9', so I could use 5.25”x16” PSL plus beams on two edges. But the problem would the tension on interior column would be huge when all the deck is loaded ( I'm thinking of situation when all three story decks are occupied with people looking out an roadside event (for context, in Washington DC) while the interior floor framing is completely unoccupied). But yes you are right, it is not something that cannot be detailed. I just thought for the amount we charge usually, it is too much work.

Thank you for reading my post! I appreciate your time.

 

[XR250]

Sounds like you need to charge more....