reduce sway in free standing stair system

[vintage70]

I have a few free standing stairs and platforms that seem to be swaying a little too much for the people using them. These stairs and platforms have been analyzed for loads specified in ASCE-7 and deflections limited to 1/4 inch. I recommended and designed cross bracing to limit the sway. I don't have the ability to predict vibrations and movement caused by people using the stairs. And design bracing to those parameters. I did design the cross braces for tension according to the uniform load on the stairs. How is this determined? Is there a method to design bracing to dampen sway on free standing stairs? Any software I can use?

 

[kslee1000]

What was your criteria in selecting the straingers? I will first check kl/r to get Fa, and design based on it. The in-plane X-bracing will not prevent side sway completely, you may need intermediate vertical support and tranverse x-bracing to stiffen the stair framing.

 

[vintage70]

Stringers analyzed for 100lb/sq ft, deflection L/360 or 1/4 inch. KL/r is 1.8 columns sized @ 5x5x5/16. Owner wants free standing stairs, they are connected to a top floor though. Appears stairs only swap translationally in the plane of the headers not the stringers. Headers are C8s. Bracing limited to plane of platform headers since I need clearance in the stair openings. Can you explain more your idea of the vertical supports.

 

[dougantholz]

I just ran into this with a stair in an industrial warehouse that had grating for treads. What happened there was the channel stringers weren't adequately braced to prevent swaying. We solved it by lacing the underside of the channels with diagonal angles to stiffen the angles and tie the two stringers together.

 

[vintage70]

Yes same here, platforms and treads are grating not concrete filled pans. So you are saying you braced the stringers not the columns. What about bracing the headers to the stringers at the platform sections? I figured by x-bracing the columns and designing the bracing in tension, I would reduce sway laterally with one brace in tension and the cross brace in bending moment. That's why I thought there was some other factors like vibration and resonance from workers going up and down at the same time. The stairs will only be used by all the workers at the same time when their shift changes. So I thought maybe I should be analyzing this some other way, and there might be a computer simulation program I could use. Thanks I'll relay your information. If there is anything else you can share I greatly appreciate it.

 

[dougantholz]

It depends on where your sway is coming from. In my case it was the stairs felt wobbly, but the frame wasn't swaying, it was the stringers themselves. And by locking in the stringers you also do provide some bracing to the platforms. That said a little over bracing doesn't really hurt - unless its coming out of your pocket.

 

[PEinVA]

Usually you can count on your treads and risers acting as a diaphragm between your stringers, in doughantholz case it sounds like that did not happen, so he added the x-braces to stiffen up the elements.

We just got finished designing a hanging stair (fancy finish, etc,) and counted on the stiffness of the concrete filled pans to help with vibration and stiffness. I'm going out tomorrow to view the stair and hope to run up and down and get a good feel if there are vibrational issues. From my vibrational analysis, I don't think there will be, but lets see how it feels.

You have to determine where yuor movement is coming from in order to fix it. If it is the framing of the stair, brace that, if it is the supports of the stair structure, reinforce that.

Let us know either way. These stairs also have little dead load on them compared to concrete filled pans I've usually worked with. Does anyone have any documents how this lack of DL effects the vibrational issues for a typical stair?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[kslee1000]

The vertical supports, actually columns, run from stringer to the ground, one column on each side of the stair, and braced in between. These form a transverse braced frame, which will provide lateral stability in a global sense (thinking a long chennel displaces laterally, remember Iy is much smaller than Ix, by adding intermediate braced frame, the side swing span "L" is reduced).

 

[vintage70]

Thank you all. I think the sway is in the columns. The stairs are 45 feet high. headers and stringers should be providing the transverse bracing. Stringers and headers analyzed for 100 lb/ft load and 1/4" deflection. Columns are tube so I constant. The sway I'm trying to reduce is probably local translational, and not rotational since stairs are secured at top platform.

What I'm trying to find out is while I'm analyzing the stairs to code how can I also look at possible swaying, before the stairs are installed and used and someone complains?

 

[dougantholz]

You can use a 5% rule, where you brace the columns for lateral translation for 5% of the axial load that is in them. You could also use the smallest member that gives you a Kl/r of 150 for compression members and 200 for tension members. This would keep buckling to a minimum and provide more capacity than you probably need.

 

[kslee1000]

Agree with "dougantholz" - intentionally stiffen the stringers, and/or brace the columns.

 

[kslee1000]

Add'l thoughts:

For 45' height, there should be several intermediate flies/landings. the columns shall be provided at least at one of the landing, either a simple braced frame (2 legs), or braced tower frame (4 legs).

 

[vintage70]

I calculated K to be 1.8 and Kl/r = 180. I've been asked to re-evaluate the tension on the cross bracing for the entire stair load on the one brace, not just the uniform load at each platform. Since the columns are already installed I will apply the 5% of load on the bracing. And it looks like they may ask for bracing at every landing. I'll be in touch. Thanks. Greatly appreciated.

 

[Lion06]

Why not analyze it as a footbridge and apply the vibration criteria per AISC.