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

 

[bookowski]

and in one of the stairs the majority of treads seem to run up to a wall. You may be able to do something here with wall attachment of those treads to stiffen it up a bit. and what are you risers going to be - I don't think it will do too much but if you have solid risers and they are well attached to the treads it may help a bit if any of that softness is local to the tread/stringer attachment points and or localized.

 

[BridgeSmith]

Reading the other responses, I may be in the minority (or alone), but it seems to me that adding something like a channel to both ends of the treads that adequately connects all the treads together would add significant stiffness against torsional deflection of the treads. If I understood the OP correctly, that was the major source of the excessive deflection. Am I envisioning this wrong somehow?

 

[jayrod12]

If you watch the youtube video he linked, you can see quite clearly the torsional movement of the stringer system. Tying the ends of the treads together may improve it, but I still expect there to be significant twisting of the section unless those members were designed to span the entire flight like a typical stair stringer. This would also take away from the visual intent of a single stringer with double cantlevered treads that are all the rage.

 

[BridgeSmith]

I can appreciate not wanting to add edge beams for aesthetic reasons, and if that's the case, then a major rework or replacement may be required to add stiffness.

If edge beams were extended so that they attach to the beam at the top and down to the floor at the bottom, I believe they would add significant stiffness to the system.

 

[jayrod12]

I would agree with that, but I would put my money on that not flying with the owners. Who knows, it's worth pitching the idea.

 

[Tmoose]

How are the treads affixed to the stringers?

I believe The spacers are afixed with single bolts.
How tight are they?
When tightened up, what gets clamped to the stringer ? The aluminum spider or the stainless cylinder ?
Is the stainless cylinder split ?

 

[nomi640]

Hi All,

Thanks for all the input and I appreciate your interest, I am in situation where I am not getting any help from the supplier/manufacturer so I need to rely on our engineering community here.

Yes I need to fix the issue and rework always cost you money, so there is no choice I need to spend money on the fix the ultimate goal is to get the stiffness without sacrificing too much on the style.

Here are some more information you folks requested

- Threads/risers are made of solid bamboo and in my opinion they are strong enough

- Yes, this is very easy fix to bolt/fix upper and bottom threads/riser with the beam both top and bottom but does this will help to add any stiffness since there is not much deflection/movement/ twisting at bottom and top

- Threads are bolted (4 bolts)to the stringer blocks from bottom

- The only response I got from supplier is you need to have ridged connection at top and bottom and you will not any issue but expect 7mm of deflection – in my opinion 7mm is quite big number for the deflection and see and feel the effects

I also uploaded another video on youtube while I had all threads bolted and toured but you see side to side and up and down deflection are more visible while you have all threads on the stringers.

https://www.youtube.com/watch?v=78K25J7bPE8&pbjreload=10

I also attached some photos for your reference to have full picture of the situation

Thanks a lot for all your feedback

 

[mmarlow]

I havent read the entire thread. For a simple fix, what if you blocked between some of the treads and the wall? You could keep the blocking smaller than the treads to make it look as though its not attached to the wall (unless you looked directly between the wall and the tread).

-MMARLOW EIT

 

[Rabbit12]

I'd go after the manufacturer. They seem to have a shitty product.

 

[John2025]

The report calls out 6mm thickness, but they look thinner in the pictures. Did they actually make it in the sizes, material and bolts that they analyzed?

 

[BridgeSmith]

Are all the bolts horizontal through the stringer tightened? I assume they're bolts; it'd be a crappy way to fabricate it, otherwise.

 

[azcats]

I see a pipe 'sleeve' in the calc package but not in the photos. It looks like the photos show only the 'spacer' between the stringers.

Where's that part?

 

[Celt83]

One thing I noticed off the bat was the Stainless Steel Sleeve weighs nearly 3x the weight they "conservatively" used in the analysis, they note they modeled the sleeve as aluminum with the same properties as the spacer.

Did this thing come out in pieces and you had to bolt each stringer piece together, if so any chance the bolts aren't tight enough?

Pretty much agree with others opinions as well.

Open Source Structural Applications:

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

 

[nomi640]

@mmarlow Yes this can be done to main floor staircase, but basement (without landing) don't have any wall beside it
@John2025 you are correct these are not 6mm specially basement one (without landing/ 7inch stringer)
@HotRod10 Yes all bolts are torqued
@azcats I asked same question but Bocani (supplier) said this model doesn't come with these sleeves

 

[nomi640]

@Celt83 Yes these shipped in parts and I torqued each bolt @ 60F.Lb and I think its enough for these type of connection Bocani.com supplier did not specify torque values even I asked several time

 

[Celt83]

I'd press them again for the required bolt torque, their model looks like it relies on the bolts providing a rigid joint at all of these pieces, and a calculation that actually matches the product they sent you based on your reply to azcats.



Open Source Structural Applications:

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

 

[BridgeSmith]

I don't know if I'd count on a torque value to ensure proper tension in the bolts. I'd give them each another 1/4 turn (maybe 1/2 turn on the wider one).

 

[Celt83]

you may have better luck reaching out to the engineering firm that performed the calculations for Bocani, there are several clauses in their calculation package that note a re-check should be preformed if x and your install conditions satisfy most if not all of the x conditions.

Edit:
on second pass the calculations do not appear to match your installed condition at all. Number of stringer sections are different between calc and both installed stair runs.


Open Source Structural Applications:

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

 

[GC_Hopi]

The provided calculations show an deflection of 10.6 mm at the treads so if you add that with the deflection on the other side of the same tread you are at 21.2 mm. That would be noticeable. The only solution is to torque all the connections to the maximum and reinforce the stringer/treads to get rid of any remaining deflections.

 

[Ron247]

I am late getting into this thread but it seems to me that the flanges of these short channels are never really braced against buckling other than each pair to themselves. If you took 2 full length channels of these cross section dimensions and spanned them the same diagonal distance, what do you think the outcome would be with regards to torsion especially if the top and bottom flanges are not really braced.

I am stunned the supplier will not assist you. The FEA does not really tell all the details I would like to see.