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

 

[JAE]

The analysis provided seems to include off-center live loads of 600 lbs. (see page 12 of 61).
However, I can't tell if this is an off center load on the edge of the mono-stringer or if the load was applied on the edges of the actual treads (with the treads not included in the model apparently).

With the 600 lb. off-center load, they reported a 10mm deflection - again - not sure if this is a deflection at the edge of the stringer (probably) or at the edge of a tread, which again, wasn't modeled.

They even comment to the extent that - 10mm seems high but it's a 600 lb. load so nevermind.

I don't see any analysis relating to the torsional twist of the stringer, which is sounds from your post that torsional twisting is the main issue ("side-to-side" deflection??)

They also don't appear to have considered any modest lateral loading, which can happen as you walk along the treads.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Agent666]

In my mind it seems like a case of death by FEA, possibly ignoring the real world performance.

The deflections they show in their analysis seem quite high to me, ~10mm under a 600lb load meant to represent the dynamic input as noted on page 12 of the PDF (span unclear but does not look too large). So basically someone going up/down the stairs a 10mm deflection has been deemed acceptable by the report. This seems a bit crazy to me, basically the stair has very little mass so you'd expect this to cause issues and feel like a suspension bridge like you have observed.

Analysis is based on the bottom being fixed for rotation, a condition rarely achieved in practice (especially if it is attached to a timber structure). Report didn't look at vibrations at all, and conclusion notes with further analysis they could make it even lighter taking material out of the sides. Sort of suggests they are missing the point.

Usually these types of stairs use some form of closed section more appropriate to resisting the torsional stresses and providing much more torsional stiffness than is apparent by the person jumping on the stair in the video. These types of sections are obviously much stiffer as well under the bending loads.

Hey but it look cool at least.

How to fix, replace it with something else, augment it with further material (SHS/RHS/Plates) to stiffen the span and increase the torsional stiffness. Perhaps the supplier can provide some guidance, would be interesting to see what they have to say at least.

 

[POPEYES]

Seems like the load wasn’t intended to be eccentric to the stair stringers like is being shown in the video.

 

[MIStructE_IRE]

Zero consideration for lateral loading, vibration or torsion.

Neat looking set of calculations but lacking a lot of real substance for me.

 

[Agent666]

I think the load is meant to be eccentric and its modelled as such, they seem to have added some plate section across the top of the tread supports in the model to represent the eccentricity. The max deflection occurs at the tip of this member. Assumptions noted that the treads are assumed to be infinitely rigid though.

 

[POPEYES]

I was also confused by the “infinitely rigid” assumption. The treads are far from infinitely rigid, even relatively.

 

[Agent666]

Well its 'more' rigid than the structure supporting it. The rotation of the structure seems to be a large contributing factor to the deflection seen out at the tip of the tread. I guess they are trying to isolate the deflection due to this as any type of tread could be adopted by the specifier, and that would be outside the scope of the stair supplier. But definitely something to be wary of, selecting a flexible tread only compounds and deflection/vibration issues.

 

[nomi640]

Supplier (Bocani) is not willing to help, the only response I got from them is their system is good issues are with my installation.
But I added beam for top block and bottom is bolted to concrete floor.
I won't get any deflection on top 2 or bottom 4 threads which translate that both top and bottom are secured properly.
I am hoping to get some suggestions from this group to fix the issue.
Thanks in advance.

 

[JAE]

Suggestion: Usually a mono-stringer is a large tube. That provides you with the highest level of torsional rigidity - and perhaps lateral rigidity.
So anything you can do to add plates, etc. to create a "tube" will help.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

And if they are taking a position that it's not their issue, then at some point you may have to engage another engineer and lawyer to help you directly to either solve the problem or get your supplier "interested" (out of fear).

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Agent666]

So ask them what the issues are with your particular installation, maybe they have a point if bolting has not been tensioned adequately or similar which might be leading to increased deflections or liveliness? Way to stand behind your product.

Based on the analysis they provided the deflections being seen are perhaps to be expected, maybe this is what they term 'good', I'd call it something else mind you at the opposite end of the spectrum.

The system will naturally be stiffer at the ends, so you would expect the deflection and 'liveliness' to be reduced at the two ends.

Can you return it and get a refund, then get a structural engineer to specifically design an alternative thats slightly more robust. If its not fit for purpose surely there is some recourse under your local laws to get a refund.

 

[nomi640]

They were saying I need to have solid beam at top end and solid surface at bottom, I have both (added additional beam at top) bolted bottom in concrete all bolts are torqued to 60 lb/ft,

They are in Montreal I am in Alberta, they told me they won't return it, I may need get them fixed my self, they won't pickup phone or response the email so I am kinda stuck.

I was planning to get tempered glass railing, will glass railings help?

Thanks

 

[Agent666]

I'd be concerned about the movements and its effect on the glass, too much movement combined with some restraint of the glass (such as a continuous top rail) might cause fracture of the glass, its not that forgiving of localised stress concentrations at fixing points, etc.

I wouldn't be reliant on the glass to increase the stiffness.

What about repurposing the two proprietary systems into one flight (one span each side) and do something else for the other flight.

 

[nomi640]

Both mono strings are different, one is 15" wide other is 7",
Any other option to stiffen the structure by bolting continuous tube (square, may be 3"x 3") on both side of the stringer?

 

[jayrod12]

I doubt a 3x3 tube would be enough. You could weld a bunch of plates, but the material required and the welding required would cost significantly more than smacking on some appropriately sized tubes.

 

[Agent666]

Maybe throwing a channel/RHS out on the glass side, use it to mount the glass + attach to that edge of the stair treads? Will address some of the twisting effect if you can get the connections to work.

I don't really feel like throwing some small 3" SHS's on each side is really going to achieve the improvement you are looking for, you are still reliant on the rather flimsy looking proprietary assembly to transfer loads between the add-ons. What ever you put in really needs to take all the load and be stiff enough to do so. If you sandwich the existing stringer with something that can deal with the stiffness aspects then the existing stringer really only becomes a means for attaching the treads.

Another way I sometimes see is some sort of wire/rod going up to the roof/down to the slab at specific intervals to hold the outer edge of the treads in position, sort of acting to prevent the twist.

To put what you are proposing in context based on my experiences with these type of stairs, for a span like proposed I'd expect to see a 150-200mm SHS or RHS tube with nice thick 9mm wall to make it as stiff as possible (torsion and bending), compared with what you have in the proprietary system its a night and day difference in terms of stiffness.

If I was responsible for the design on something like this, you might see me go a 'bit heavy handed' on it. Because well, these mono-stringer stairs exhibit the issues you are seeing and as a designer you want to make sure it works as intended without having any serviceability issues come back to get you. Provided you have reason to do this based on calculations, etc then I have no reservations going with something I know will work vs crossing my fingers and hoping its ok with something that is more economic on material.

 

[bookowski]

How about switching to some kind of neat looking steel railing, designed by someone (engineer) to act as a beam and your treads attach to it? Essentially make it not a mono stringer anymore - one stringer would be the railing but someone could come up with something light/neat that doesn’t look to be obviously suppporting it.

Not to defend the guys that designed this too much but these things are tricky. This is always my fear when I’ve done these or any weird stair. Really do a vibration analysis and your design looks like overkill. But given that they mass produce these they should have tested them, and in your video this is pretty clearly not acceptable.

 

[nomi640]

So conclusion is I need to buy new stairs, there is no way to fix these stairs?

 

[JAE]

Well no. We’ve offered a number of options. Why would you conclude that? The question is - how much money will you pay to minimize deflections?

Check out Eng-Tips Forum's Policies here:
faq731-376