Mono Stringer Stairs deflection issue 3

[nomi640]

Hello,
I bought modular two mono Stringer stairscase from bocani from Montreal, after installation staircase feel like suspension bridge, both stairs have side to side deflection and also jumping up and down.

48" wide thread have middle Stringer of 15" and 42" stairs consists of mono stringer of 7" span for 15 feet (9 feet ceiling height).

I will really appreciate if someone can help me to reduce the deflections.
I also attached FEA they provided for your reference, also below is video link

https://youtu.be/l9Df8ukuGik

Thanks

 

[nomi640]

I just called wooden stair guy to come and suggest what he can do to fix the issue

He suggested he will do two LVL on both side and connect both LVL from bottom with MDF and close all middle stringer from top with MDF will help

He suggesting to box in this mono stringer sides by LVL and top and bottom with finishing MDF

 

[nomi640]

Does this wooden LVL will work?
I know it will change the look completely but it will be still mono stringer stairscase with much wider beam in middle

 

[BridgeSmith]

The LVL will stiffen it some - how much is the question. Seems like a significant amount of work, though.

Another option for consideration is replacing the bolts through the stringer with larger or higher strength ones with heavy washers to increase the compression force applied to the spacers. Whether that would be effective mostly depends on what is flexing, the connections or the side plates themselves. You'd have to closely observe the stringer while someone bounces on the treads to see whether there are gaps opening between the outside plates and the ends of the spacers.

 

[nomi640]

@HotRod10, Thanks for suggestion I will try this before making big changes.
Thnaks,

 

[Agent666]

Rather than individual plate washers, I'd look at a continuous plate either side with through bolts. Get more benefit in terms of stiffness. Get a structural engineer involved if required to quantify any benefits.

 

[Tmoose]

To start, and only to start, I'd be probing around the end of each spacer with a .002' feeler gage. If the entire face and especially the "outside diameter" of each end of every spacer does not start off in hard contact with the channel flange, any hope of them contributing any stability and torsional stiffness is pretty much lost. If the faces are even slightly convex ( .002"), they might as well be one inch diameter bar standoffs.

My preferred detail with adequately thick components (which by inspection the channels are not) is a relieved center on the contact face, forcing the contact to be at the maximum diameter.
If there is enough meat in the spacers, drilling and tapping a hole axially into each of the eight spacer spokes would allow bolting thru the channel and into the spacer properly. Alternatively, drilling two patterns of eight holes in each channel would allow using grade 8 threaded rods and pretty nuts to clamp the channels to the spacers near the outside diameter. The threaded rods could be hidden under the spacer Ts. This would allow full diameter clamping without dishing the channel webs at each center bolt, as likely will happen with a relieved center, or adding a large diameter ring shim.

What is the depth of the tapped hole in the end of each spacer. How long is the bolt used there?

I tried unsuccessfully to zoom in on the images in the FEA report looking for indications of localized strain/flexing of the channel flanges relative to the channel web when various loads are applied. 90° > 91° > 90° > 89°. And also separation of the flanges everywhere except close to the bolts.
Similarly you could brush a little motor oil or olive oil on the flange faying joints, and watch while somebody walks up and down the stairs. My hunch is some visible "pumping" of the oil would occur, due to the flanges "working" despite being wishfully "bolted together."
The load path of each joint is just miserable in my opinion, and you have over 2 dozen of them.

 

[BridgeSmith]

Good thoughts, Tmoose. I agree with everything you said, I think. I'm not sure I followed where you were proposing to put the oil.

 

[nomi640]

Thanks @Tmoose, I will do all these checka first before making any changes.

 

[Celt83]

@nomi640: I want to press that you either get the engineer of records weigh in on this issue or bring an engineer in. When push comes to shove if the owner ends up displeased with the appearance/performance of this stair saying that I got these fixes/suggestion from some folks on the internet isn't going to fly. This is as much a recommendation to help shield you as it is to get someone local that can actually come out and put eyes on this thing.


Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

1) I also live and practice in Alberta and have extensive experience with performance issues related to mono-stringer systems in wood framed homes. My wife is also a structural engineer who has made a specialty out of designing fancy homes for would be oil barons.

2) In all of the cases that I've seen, for which every one was a hollow tube stringer, the issue was indeed flexibility in the supporting structure. Unfortunately, the issue rarely has to do with the flexural stiffness of the supporting beams. So I'm not surprised that beefing those up didn't help much. The issue is really with the torsional flexibility of the beams and, more critically, the torsional give in the connections. For these systems to perform well, you really do need a high degree of torsional rigidity at the ends. And, since wood is really just spun sugar, that's pretty tough to do. When it's done successfully, the answer is to spread out your fastener group significantly, to the tune of a couple of feet rather than a few inches. As Agent666 intimated, these stair systems simply are not well suited to wood structures.

3) Even at the low connection, I do not share your confidence. That concrete that probably seems very stout to you is likely just a 4" lightly reinforced slab on grade. Again, you want spread out fastener group and, based on the photos supplied, it doesn't appear that you have this. If anything, it looks like your fasteners are inside the stringer. That's not going to cut it.

4) I think that the proprietary stringer system that you've been sold is crap. A mono-stringer depends utterly on it's torsional stiffness and the stringers that have been sold to you have next to none. Some of the flaws include:

a) As everyone has been mentioning here, what you need is a closed, tubular section so that you can utilize St.Venant torsional stiffness. Instead, Bocany has supplied you with an open section that relies on warping torsion and is therefore torsionally flexible.

b) With common members that rely on warping torsion, such as I-beams and channels, you at least have a continuous flange to improve the stiffness of the warping torsion mechanism. Here, because of the the way small channel sections are stacked, you don't even have that.

c) If I understand the calculation package correctly, the channels segments aren't even welded together. Rather, they're flange bolted together which will introduce all manner of additional flexibility that would be extremely difficult to account for.

The system is just terrible from a mechanical standpoint. Terrible. In my mind, the very fact that an FEM report exists for this is evidence that these guys don't know what they're doing. A competent structural engineer develops a reasonable structural scheme and then uses FEM to validate and refine it. That, as opposed to starting off with a grossly flawed scheme and then hoping against hope that an orgy of 4th order differential equation solving somehow makes things right.

5) When I watch the videos, I see two things contributing to the movement. Firstly, there's the torsional flexibility resulting from the use of a crappy, open section. Secondly, I see torsional give in the connections leading to a sort of "slop takeup" phenomenon. You can reinforce this stringer easily enough but that's only going to take care of the first source of flexibility. There's not going to be much that you can do about the connection flexibility. For this reason, I don't recommend reinforcing the stringer. Rather, I think that you viable options are:

a) Throw your stringer away and attach your bamboo treads to a newly acquired 6"x8" HSS stringer with thoughtful connections. This would be oodles cheaper than the reinforcement anyhow.

b) Switch to a stair support scheme that abandons the use of the stringer in torsion. The ideas mentioned above that would accomplish this include: making stiff trusses of the guard rail system, attaching the treads to the walls where possible, and the business with restraining the treads vertically with wires. I'm afraid that your stringers and their connections are ill conceived junk and there's just not much getting around that. The do remind me of giant bicycle chains, however, which is admittedly cool looking.

6) Below I've shown some options for stringer reinforcement that might have legs. That said, as I mentioned above, I don't think that stringer reinforcement alone will solve your issues if the high and low stringer connections are not improved somehow.

 

[BridgeSmith]

To address item #2 of KootK's post, would it be possible to add a wide plate (probably the full width of the treads for aesthetic purposes) bolted to the stringer and screwed or bolted to the wood beam at the ends? At the bottom, you'd probably need a large angle to gain any significant amount of torsional stiffness there.

 

[nomi640]

Hello Thanks all,
Since I am not structural engineer, I may be not understanding all the comments, I will really appreciate if I can get some sketches to understand your recommendations.

Meanwhile I got this response from Bocani

"In regards of the stair steel structure, the best is to fix the problem at the source again, which is the endings, and any attempt to fix otherwise would only be to compensate for the issues on installation and on the floor/wall endings"

 

[jayrod12]

Which is more or less what KootK has indicated to you.

It's quite likely that even though the stair stringer is garbage, the best chance for stiffening everything would be to upgrade the connections at the ends of the stringers and to upgrade the connection of the beam at the top of the stringer to whatever supports the beam. It's quite possible that even though the beam seems robust for just standard floor loading, it may be to weak for the loads that are being imparted to it.

 

[KootK]

@nomi640: is there any evidence, anecdotal or otherwise, to suggest that this stair system is performing well in other homes? Frankly, my opinion of the system is so abysmal that I struggle to imagine it working for anything other than, perhaps, the step up out of a sunken living room or something.

My xray vision isn't what it used to be when I was a younger man but nothing about the photo below suggests "torsoinally rigid base connection" to me. Don't for a minute think that I'm siding with your heathen stair supplier however. I think you could embed both ends of the stringer in cooled lava and you'd still have a problem on your hands.

 

[nomi640]

Here is some more thoughts from supplier

I know it can be counter-intuitive, but the very solution is to make sure the endings are secured. The thickness of steel, wether 4.76mm or 6mm, as being simulated and real-life tested with our prototypes, shows that it has a very limited impact. The most important is to make sure the attaching plate and starter block is very well secured on the surfaces, with minimal flexion. That is why we use larger attaching plate, to compensate the installation flaws that may happen. We have sold hundreds of those stairs, without problem in the past. Why suddenly all our system would be bad? Illogical. We have hundreds of happy customers, using everyday our stair system without any kind of problem.

Now, what I suspect in your case is two fold: 1) there is a problem with the endings surface and/or installation 2) your mind is set on an infinite rigidity (0mm flex or torsion). In the first point, I can help only if 2) is realistic. With a central stringer, you cannot get something close to 1-2mm one-way travel, unless it's either very short or massively overbuilt (which means, more or less the look for a normal double stringer). We are limited by physic laws. If your expectations are too high, it's not possible. But if getting something within our acceptable tolerance is ok, then we can surely find a solution. As for now, I am not fully convinced you will ever be 100% satisfied in regards of rigidity, so please confirm what would be the acceptable one-way travel. I will be happy to tell you wether or not it's realistic.

Hope that answer will help your project.

 

[atrizzy]

Fairly condescending response from someone who has no understanding of torsional rigidity. That and they're twisting it around on you saying that it's YOUR problem that you're not happy with the rigidity.

What is the rise/run of your stair?

 

[Rabbit12]

Boy, that response from your supplier makes my heart rate rise. What a prick.

Have they provided any details on what the base and top connections need to look like?

 

[Agent666]

To me the suppliers perception of massively over built is based on probably correctly proportioned stairs. These mono stairs if you do them right do appear pretty heavy, but it is what it is.

To me I'd be aiming for the 1-2mm deflection limit in any specifically designed mono stair. Often if face fixed I'd see quite a wide plate (sometimes as wide as the stairway) with fixings as wide as possible to maximise the torsional stiffness at the supports. Putting a small plate with closely spaced bolts is going to be an order of magnitude more flexible torsionally than a realistic rigid condition as others have noted. I don't think the end supports sre the whole issue here, see kootk's opinion for similar thought processes to mine here (no point in repeating as I think he summed it up perfectly with his opinion).

 

[nomi640]

Here is another response from supplier

"Yes I see on the first video (with standard width stair). The flexion seems acceptable but not the torsion. That is obviously a problem of attaching plate and/or starting block, unless the bolts are not tighten enough which I doubt. Here is some pictures of the custom plates we have in stock, it shows solution for very lenghty stairs (17-18 steps) or unstable (soft) wood floor. Basically, that may help but you should first make sure everything is secured tightly enough. The very fact that you seem to have more of a torsion problem than a flexion problem point out even more to an attaching plate/starter block problem. Something is moving, even not visible to the naked eye, and it causes that torsion"

 

[nomi640]

Any comments?