Catwalk vibration approach

[southard2]

I'm not sure there is a correct answer to this but a little advice would help. I have a metal building where the owner has mezzanines to the far left and right hand sides of the building. In the middle is a 50 foot length of clear space. There are stairs up to the right mezzanine and plans for future stairs up to the left mezzanine. I have a catwalk that spans 50 feet clear from mezzanine to mezzanine. Also at the middle of the catwalk are two other catwalks that span perpendiculary to the primary 50 foot catwalk. These other catwalks are inconsequental except they limit my options as I will explain later. Initially it will be the only access to the left mezzanine so I have designed it for a LL = 100 psf since I believe functionally it will be a corridor at first. The two beams under the catwalk which is only 4 feet wide meet deflection criteria. However when I analyzed it with the footbridge method for vibration it fails miserably and only truly outrageous sized beams will work. Futhermore the current plans are for metal grating welded to the beams to be the floor not concrete or anything like that.

Originally the architect wanted to add a hanger support near midspan with would connect to the bottom of the metal building wind frame above. I didn't want to do this since the perpendicular catwalks attach to beams spanning between metal building columns at the perimeter of the metal building. My thought is that once I had the vertial post the combinations of wind frame, vertical post, and the perpendicular catwalks will when acting together will attempt to restrain the drift movement of the metal building. So I thought it better to keep the systems seperate.

So I have a light walkway with heavy beams and a far span. How should I approach the risk liability that the owner will complain of vibration. Of course I plan on discussing with the architect the need to prepare the owner for the possibility of vibration as people walk across. But on my plans what kind of note should I add? A verbal agreement does not seem to be enough.

Or should I simply go with W24x176 type beams (verus W24x68 or was it W24x62).

Basically the construction they want to use isn't good if you want to limit vibration. They are expecting light catwalk construction. I'm thinking of simply convincing the architect to put the other set of stairs up to the left mezzanine now. That way the catwalk will only be used as an observation deck for training and would probably never have more than 5-8 people up there at a time.

Any advice would be appreciated. The beams are large as it is and the actually deflection will be low even at 100psf loading. Just not sure how to best set the expectations of the owner.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[Lion06]

Does making it an observation deck somehow get you out of the woods for dealing with vibrations? If anything, it's no longer a footbridge and the allowable acceleration is decreased, making your situation worse.

I would talk to the architect and then have a meeting with the owner. Explain the situation, and he can make the decision. This is a serviceability issue, not life-safety, so if the owner is ok with it and makes an informed decision based on all the information, then all is good. Just make sure that whatever his decision, put it in a memo to the architect, copying the owner - something along the lines of "at the direction of the owner, only strength and deflection have been considered........ The owner is aware of the potential for unacceptable vibrations on he catwalk....".

 

[Lion06]

What about making the a functional handrail? Maybe it's a truss where the top chord is a handrail and the bottom chord is at the walking surface. This should make the problem go away, but will be a little more expensive than a traditional beam with a handrail.

 

[paddingtongreen]

I have done that, with the described spans, a truss girder ought to work. Make it high enough to be the guard rail and mount the handrail, midrails and toeplate on the insisde.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msquared48]

Another solution could be to install one or two dampers per catwalk beam on rods attached to the roof structure, if possible. i.e., an attempt to change the response vibration...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[southard2]

Wow, thanks everyone for a lot of good ideas.

Mike, what would you suggest is used for the dampers?

Lion the primary function is as an observation deck. It will be used so that personnel can observe training below. So I am going to write an email today to the architect to see how he wants to approach this issue. Everything is a budget issue these days.

Paddingtongreen, you said you have successfully used truss girders at that those spans. Did you basically max out the depth of the truss then? Do remember how deep you went and what loading you prescribed for the design of the truss girders? I'm asking because when it comes to vibration stuff it would be nice to replicate this to something that has actually worked.

Wow thanks everyone those were all really good ideas. The truss thing had not occured to me but that is a good idea if I must design for vibration.

Thanks,

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[ACtrafficengr]

I bet you could find a pre-engineered, prefab footbridge supplier that could give you an off-the-shelf solution.

 

[271828]

What acceleration limit are you using? What natural frequency are you predicting, and what mass are you using for that calculation?

 

[paddingtongreen]

@southard2, I don't remember what size I used, but you will certainly have different circumstances so you should pick your own.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[271828]

Lion the primary function is as an observation deck. It will be used so that personnel can observe training below.

It is very realistic that people will complain of vibration with this intended use. Given the span and (especially) lack of mass, I would expect the required size of beams to be very much in excess of those required for strength and stiffness design.

Excessive vibration is far more likely to get you into hot water than if you bust your deflection limit. I'm still not quite sure why people think it's reasonable to need bigger sizes for every limit state in the universe EXCEPT vibration, but I'll only spend this paragraph on the soap box.

Three thoughts:

Be careful with your selection of live load mass. Estimate the actual weight of people on the bay, don't just use the recommended numbers from Design Guide 11. There is NO safe side on which to err. Overestimation of live load mass results in lower fn (and thus a lower dynamic load), but also a higher vibrating mass (and thus a lower response for a given dynamic load).

If the area is mostly filled with people, you might consider gambling a little and using a higher damping, say 5% of critical. Chapter 5, for rhythmic activities recommends up to 6%. It's not clear to me why this situation is much different. However, I doubt there is solid research to pin down a damping value. I'd consider 3% as the default and consider that it very well might be 5%, FWIW.

I would limit the peak acceleration to 1.5%g which corresponds to a shopping mall or indoor footbridge. I would not stretch it beyond that because the likely complaining people are standing still, perhaps bored and wishing they were somewhere else, and maybe apt to shoot the bull with the other guys standing around (Hey'd you feel that?! -- now they have something else to use to pass the time: trying to feel vibrations). If the occupants will generally be moving, and not standing around, then the limit might be stretched a little.

 

[271828]

One more thought. I'd actually analyze two situations: (1) fully loaded mezzanine with the appropriate mass and higher damping, and(2) mezzanine with a single observer standing in the worst place with no other people. In this case, use only the structure's mass and 1% damping.

I'm not sure which is the critical case.

 

[steellion]

I agree that 100 PSF live load is appropriate. You can add some hanging rods from the structure above to help control deflection without tying the systems together. A concrete floor would also help immensely. Explain the issue to the architect and owner and weigh your options from there.

 

[SAIL3]

you could try a king/queen post...it would introduce compression into the bms but you may be able to handle it with what you got now....

 

[StructDave]

King/queen post if headrrom allows, yes.
From your description tho, I can't help wondering how you can have an occupied mezz with only one egress route, and the exit travel distance sounds a bit long. Really this is a Q for your architect.