Cantilever stairs

[Joel Yzerman]

Hi, I recently installed these , design was submitted to my engineer. We are fighting small vibrations on the treads.

I'm wondering if I can use a vibration app on my phone to see if it's unacceptable.

I realized that I need more posts to weld my stringer to in order to stop vibrations, issue is I can't fit anymore in the wall , there is an seismic x brace in the way.

What can I add to make this work ?

Stringer is c12x25/ treads are c10x15.3

I'm new here. Can I add Multiple pictures?

 

[Joel Yzerman]

I've heard the average step frequency can vary from 2-5 Hz depending on weight, going down or up.

I read that the natural frequency of the stair can't be the same range of Hz.

Too bad I couldn't wire a speaker under each tread hooked. To a device like noise cancelling heads phones lol probably would be too large like a subwoofer or something haha.

I have a few acceleration/ Hz vibration apps on my phone but I'm lost as to the calculations, way beyond me.

 

[Joel Yzerman]

Not a great pic , it's all I have for now until I return.

 

[LittleInch]

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Could you reduce the tread length?

But basically they are long slender cantilevers with variable force and weight being applied.

I would just call it a feature of the staircase and build the rest of the house. What's the beef from the engineer?
If client is happy, builder is happy and you ecstatic about them, what's the issue here?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Joel Yzerman]

I think the engineer just maybe isn't sure , or just personally doesn't like it, I'm having him back end of week I'll adding a few things , it should pass.

 

[LittleInch]

"or just personally doesn't like it"

I would go with that one myself, but not sure what he has to "pass" or "sign off"?

They clearly won't collapse so why does he or she need to "pass" them? Pass to what code or standard?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[XR250]

I have designed a few of these and I've always put in my contract that the owner may not be satisfied with the level of vibration.
They all seem to have been pretty satisfied though.

 

[271828]

Pass to what code or standard?

The AISC Specification chapter on Serviceability says vibe must be considered, so there's that.

 

[LittleInch]

Exactly - considered. Not meet some mysterious limit which means nothing. Might not be to the engineers liking, but if the client is ok with it...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dik]

I thought the OP was incorrect... it looked like an HSS with the corner being 'rounded'.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Joel Yzerman]

I'm in Vancouver b.c. Canada . Here the home inspection must be accompanied by a schedule CB by the engineer. So when he reviews and stamps the schedule I give the copy to the builder which he submits to the home inspector for final.

The builder needs the final to invoice the client. So it's all roped together lol the home owners can still move in just on paper the house hasn't passed final lol I guess legally it's a mess.

 

[LittleInch]

What's a schedule CB?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[structSU10]

I don't really understand the process here. It sounds like you are randomly welding things to this stair and hoping for the engineer to say OK. Did this engineer design the stair originally? Are they the EOR and this was supposed to be a delegated design? I would think the vibration of this thing would be treating the tread as a cantilver from a spring support which would be the torsional stiffness of the stringer. I would look at the softest area and run an analysis. As others have said this stringer should have been a tube from day 1, but the added verticals will help reduce the torsional span by some amount that will likely help.

In general though vibration for something like this is perceptibly and comfort, not life safety. If the client says OK, and maybe can provide that OK in writing, this should be a done deal.

 

[BulbTheBuilder]

I've heard the average step frequency can vary from 2-5 Hz depending on weight, going down or up.

Yes, and it is often checked against a 4.6%g for a rapid descent. That's when the worst vibrations are felt. If you can get the frequency of your stair to be +5Hz you should be fine. A normal descent is around 1.7%g so if you are getting vibrations from a normal descent then that's a major problem. You can't always satisfy the 4.65%g so you can decide to go with maybe 3 or 4, at least people can walk on the stairs while ignoring vibrations when someone runs down.

From the look of things, the options I see are boxing them and adding handrails to dampen your stair.

I had a couple of articles on vibrations in stair design I may have to dig into my files but if my memory serves me right you can get about 2% damping from the handrails (depending on the type of handrails used). Another thing which might ruin your design but adding z bent plates connecting the treads. To me that just extra cost and labor for no "concrete" reason (See what I did there.. concrete )

 

[BulbTheBuilder]

I thought the OP was incorrect... it looked like an HSS with the corner being 'rounded'.

It sure did! I wasn't sure myself, till OP said it twice that when I felt convinced.