Stair Vibration for Floating Landing
Stair Vibration for Floating Landing
(OP)
I am attempting to design a stairway meant for an open lobby with a floating unsupported landing. I have been asked to perform a vibration analysis for human perception and am unsure how to proceed. Reviewing design guide 11 seems to pertain to floor slab systems, and I cannot think of a proper way to apply it to this case.
Does anyone have any suggestions on how to proceed or a place I can look for additional information?
Does anyone have any suggestions on how to proceed or a place I can look for additional information?






RE: Stair Vibration for Floating Landing
What is the landing being supported on? I know I've checked the supporting floor system in this case. As well as the stringers as a beam from floor to hanging support.
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
RE: Stair Vibration for Floating Landing
RE: Stair Vibration for Floating Landing
RE: Stair Vibration for Floating Landing
Are you in the Philadelphia area?
I would probably do a dynamic analysis in something like RAM Advanse to get the frequency of the stair. Using that, I would go into DG # 11 and use the equations for accelerations.
RE: Stair Vibration for Floating Landing
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RE: Stair Vibration for Floating Landing
I would caution you about this situation. Be conservative. You'd be surprised how ineffective what little damping that is present in a staircase can be... In my experience the behaviour is more akin to a beam equivalent to the length of the staircase (the hypotenuse) running horizontally.
You're going to need sufficient stiffness and weight in the staircase proper. If your weight of your staircase is less than ten times the weight of a single user, that's not good. If your span/depth ration is more than twenty for timber, or more than fifteen for concrete, you need to start being very careful to ensure vibration is satisfied.
Good luck, and let us know how it goes,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Stair Vibration for Floating Landing
Other than that, make sure the stringers are sturdy, and deflections (both ways) are much tighter than stairs with supports in the mid-landing, it may help.
RE: Stair Vibration for Floating Landing
First off, the loading is completely different. On a floor, people walk between 1.6 Hz and 2.2 Hz. Average people can descend a stair at 3-3.5 Hz, and very slightly athletic people can descend at 4-4.5 Hz.
Secondly, the forces are a lot higher for stairs than floors, whether we're talking ascending or descending. The DG11 forcing function is way too low.
There was a paper presented at the Arch. Engineering Conference last fall by Davis and Murray on the subject of stair vibrations. If you can get your hands on the proceedings, you can see the method they suggest, based on tests run on a monumental stair. They give recommendations for harmonic loads.
Using their harmonic loads, do a time history analysis or frequency domain analysis to predict the response.
Next is figuring out what limit to use. Somewhere between 1.5%g and 2%g, peak is reasonable, if not a bit conservative.
Finally, there's one huge issue with some stairs that's totally different from floors: crowd synchronization. If 10 people walk on a floor, they won't sync their steps, so the response to their combined loading won't be more than that of a single person. They don't sync because their stride lengths are different. On a stair, the stride lengths are fixed, so a group syncs up and causes a lot higher acceleration than does a single person. An amplification of 3.0 is sometimes recommended in Europe for this case. The problem is that your stair won't have a snowball's chance in you know where of meeting 1.5%g-2%g, so you'll have to use judgment to see if you want to consider groups for your stair.
RE: Stair Vibration for Floating Landing
And yet another thing. In some cases, you'll need to consider lateral vibrations. Some monumental stairs have very little lateral stiffness, so someone could get it moving pretty easily. This is worse if the stair is a bit unsymmetrical so that the main vertical mode also has a lateral component.
And yet yet another thing. Watch out for handrail lateral movement. This can be severe enough to catch people's attention. To my knowledge, nobody has ever set a limit on displacement (would seem to be the appropriate parameter) for this. Of course, if the guardrail is put into the analysis, it's displacement can be predicted.
RE: Stair Vibration for Floating Landing
RE: Stair Vibration for Floating Landing
RE: Stair Vibration for Floating Landing
http://www.proceedings.com/04137.html
I have heard that a more detailed journal paper will be published in the ASCE Journal of Architectural Engineering sometime in the near future. If you can wait, it'll probably be better and cheaper.
RE: Stair Vibration for Floating Landing
RE: Stair Vibration for Floating Landing
RE: Stair Vibration for Floating Landing
Can someone provide a link to this or any other similar refs?