Rhythmic Excitation in Balcony 1

[SteveGregory]

If the balcony is framed with regular rectangular bays, then Design Guide 11 would apply directly and you should use the walking excitation and the rhythmic analysis.

However, if the balcony follows some type of a curve producing some very irregular bays, then vibration may not be a problem.

In either case, you may want to quiz the architect to find out if they intend to hang or fix some type of video projection equipment to the balcony. This could be a problem and you may need to use a vibration isolation suspension system for the projector.

 

[Prestressed Guy]

You said that this is a stepped balcony. Are the platforms and the risers cast-in-place concrete? If so, this will add a great deal of stiffness to the floor. Are the beams under the risers? If so you can design the beam as an edge beam composite and use the upper slab as the compression element. That is a lot of nearly free additional stiffness.

 

[RWW0002]

@ BTB I'm not exactly sure what all goes on here, but I's assuming alot of moving and dancing. There are Nurses stations throughout the church to hand out cold towels and help those who may pass out.

@ SteveGregory It is a wide arch of 1 row of 35' bays. Any recommendation for at what point can these types of effects be neglected?

@ Haydenese It is a stepped balcony with concrete risers, but even if beam deflection is completely neglected the girders would have to be held to approximately L/1500 to get a nat. freq. of around 7. This seems pretty high to me.

 

[271828]

You had better take this limit state seriously. I have a good example: a few years ago, I worked on a church balcony that had a natural frequency that was too low. During one particular song, the people bounced and clapped at resonance and the vertical vibration was almost enough to cause a panic and stampede. Cameras were pointing back at the crowd and the displacements were large enough to easily be seen. There really wasn't a way to fix the balcony at that point other than hideously expensive exotic options.

The members "looked large" in that case too. The cantilever was larger than yours, but their chords were several hundred pounds per foot and the truss started off about 20 ft deep IIRC.

I'd advise you to proceed as follows. Build a model of the balcony framing. Apply the structural mass as absolutely accurately as you possibly can. Apply a mass corresponding to a 180 lb person in each seat. Use that model to determine the natural frequencies and mode shapes. Apply a sinusoidally varying uniform load (psf of people times the dynamic load factors from Design Guide 11 Chapter 5) for a chunk of the audience which is assumed to be moving synchonously. The size of that chunk is somewhat arbitrary--perhaps 1/4 the crowd in the worst position which is probably near the center, front of the balcony. Use a time history analysis to predict the accelerations near the cantilever tip. Compare to the Chapter 5 limits which I think are pretty generous, over 5%g.

If that's a bit much, then I'd advise hiring a vibration specialist because actual problems are not that unlikely.

 

[IDS]

The Institution of Civil Engineers "Structures and Buildings" Journal had several research papers on this topic around 2004. It would be worth looking those up. I agree with 271828, it isn't an issue to take lightly.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rowingengineer]

I agree with 2.718281828459045 2353602874713526624977572470936, hav many a problem with similar situations, never had a cheap fix.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[dik]

rowing... your 'cheap fix' comment should not be taken too lightly, it can be extremely costly, fixing a vibration problem.

BTW, you have the decimal in the wrong location...

Dik

 

[271828]

LOL.

Actually, I'm thinking of the two worst vibe problems I've seen. Both of them have "sounded" like this one. The engineer was already using large sizes and just couldn't believe that larger sizes were realistic. Thankfully our engineering judgment doesn't betray us very often, but it does in some of these cases.

My other favorite story is from about 10 years ago. One of our principals received a phone call at 9 pm on a Friday night. He had designed a conference center in a city about three hours away, and the architect and a building official were on the line. There had been a Friday night high school event on a longspan floor area and the vibrations almost caused a panic. Usually, displacement amplitudes are very small, but in this case, there was enough movement to visibly move double doors on the floor below presumably due to air pressure changes.

The building official was threatening to close the structure because he assumed a safety problem. Our principal arrived at the architect's office at 1-2 am and they talked for a while and he finally convinced the architect and official that it was only a serviceability issue. We looked at it Monday and told them that they had extremely expensive options or simply not let people jump up and down on the floor. Thankfully they bought that and didn't go call a lawyer. There were just about zero realistic options for fixing the floor at that point. Would've costed a few fortunes.

The error in this case was a natural frequency that was too low. It was designed before DG11 came out, so was designed for strength and deflection only. IIRC, the floor had about a 100 ft clear span and was supported by 7-8 ft deep heavy steel trusses spaced at 8-10 ft. IIRC, fn was around 3 Hz. The kids were jumping on command at about 3 Hz and really got it moving.

 

[B16A2]

The firm I interned with went through this exact same thing with a big baptist church. The engineer designed as best as he could with the project budget and the balconies still hit resonance. Owner sued. Huge ordeal.

Another time, I was sitting in the upper deck of Beaver Stadium in State College PA during a football game when everyone started doing a coordinated jumping cheer, which threw entire bays into resonance that almost threw me to the ground, and they thought it was fun.

Really it depends on the expectations of the people. I think churchgoers are less forgiving.

I suggest you have a sit down with all parties involved and explain the nature of this issue and what it will cost. If they're not willing to spend the money, you need to get this in writing so it does not come back to haunt you.

 

[RWW0002]

@271828 I do plan on taking this vibration serious, it is just a tough limit state to accurately analyze and to effectively communicate to an architect/owner. The guidelines and procedure that you have outlined should really help.

@B16A2 Vibration is a strange beast. For some reason people seem to have more tolerance for excessive movement while outdoors. I agree with a serious sit-down if it is warranted, but I want to have my ducks in a row first.

A couple of comments..
1. This is more of a simple span balcony as opposed to cantilevered. The spans vary slightly at different girders, but generally are about 35' with a 1'-8' cantliver. Will this have a large effect the structure's tendency to vibrate?

2. Any guidelines for at what point geometric irregularities will tend to dampen rhythmic excitation? The bays for this particular balcony arch around the perimeter of a semi-circular building( I don't have the drawings in front of me, but lets say that 10 consecutive 35' bays form 1/2 of a circle) . The senior engineer on this project's first impression is that this will limit the vibration to an extent that it will not control. Any thoughts?


Thanks for everyone's help

 

[archeng59]

I have designed balconies for several performing arts centers. I designed the balcony framing (trusses) using STAAD. I used STAAD to calculate a first order natural frequency for the trusses. As long as the natural frequency is 5 hz or greater, you should not have a problem with rogue or rhythmic excitation vibration.

 

[271828]

1. This is more of a simple span balcony as opposed to cantilevered. The spans vary slightly at different girders, but generally are about 35' with a 1'-8' cantliver. Will this have a large effect the structure's tendency to vibrate?

If you have a bunch of what are pretty much normal bays, then you can use Design Guide 11 Chapter 5 directly. If you have the FloorVibe program, it should be very easy. If not, then I'd highly recommend to buy it at

http://www.floorvibe.com

2. Any guidelines for at what point geometric irregularities will tend to dampen rhythmic excitation? The bays for this particular balcony arch around the perimeter of a semi-circular building( I don't have the drawings in front of me, but lets say that 10 consecutive 35' bays form 1/2 of a circle) . The senior engineer on this project's first impression is that this will limit the vibration to an extent that it will not control. Any thoughts?

I've looked at irregular framing vs regular framing (performed FEA simulations) and I don't think there's any indications that these hypotheses are correct. If anything, I think irregularies sometimes result in modes with smaller parts of the floor moving, which causes a lower effective mass and therefore a higher response. The bottom line is that nobody knows, so there's nothing currently there for the designer.

 

[msquared48]

I recommend that we all listen to the iconic tune "Good Vibrations" for the next 15 minutes and come back here reinfused and totally refreshed.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RWW0002]

Thanks for all of your time and comments. I have not had time to spend on this project lately, but when I get back to it I will definately take a close look at the vibration limit state for the balcony. It is good to have a place like this to seek the advice of seasoned professionals like yourselves. Thanks again.

 

[csd72]

RWW0002,

A sketch would be useful to get more appropriate responses for your particular situation.

Have you considered the option of using resilient flooring or some other type of damping? If you have enough damping then the frequency is irrelevant. A good example of this is the shocks on an automobile - anyone who has drivena car with faulty shocks can confirm that the natural frequency of the car suspension is often matched by the road.

 

[a2mfk]

One option is to allow for budgeting of post-vibration dampeners, this is part of the up-front design and construction budgeting and planning. I read an article about an office building with a large cantilever and some other elements that made it unpredictable to model. They ended up not needing it. Of course this is a difficult thing to sell to a client, but the key is to focus on the economics of this approach- you only pay for it if it is necessary. If you can design the weight of the proposed dampeners and the architect can design the access, it may be a feasible alternative.

I'd also consult a vibration specialist, but that is just me. They'd be part of the overall plan if dampeners are needed, etc. or if you can just stiffen or add mass to the structure.

 

[271828]

csd, I very seriously doubt one could get enough damping from such a material to be helpful. Do you have a link to a good candidate product? I've never seen one.

a2mfk, it's almost always better to come up with a structural solution rather than relying on some kind of passive damping like tuned mass dampers or active control. I'd only keep those as a very last ditch emergency solution, and would definitely hire a vibe specialist if I was going to push for such an option.

 

[csd72]

http://www.wisegeek.com/what-is-vibration-damping.htm

http://www.arch.virginia.edu/~km6e/arch721/docs/msc-office-fit-out.pdf

http://www.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n22_eng.pdf

A few references on damping.

It has been shown that damping ratios as low as 1% can make a big difference to the acceptability of the vibration.

 

[ajk1]

I don't know much about this except that it reminded me that about 35 years ago there was a long span joist floor of a new synagogue in my area that had a vibration problem when the people started dancing at the first social event in the new building ...so much so that people started running for the exit. An engineer in our firm told me about it at the time. The remedy I believe that they used was to suspend boxes of sand from the josits, within the ceiling space, and this apparently alleviated the problem sufficiently. I guess the joists could take the added weight. Like I say, I don't know too much about it. Although in that case it seems to me foolish to have used joists, nevertheless it seems to me that it is safest to do these type designs in conjunction with a vebration specialist. His requirements of course may make the floor very expensive, but no one will thank you later for saving construction costs if there is a vibration problem.

 

[graybeach]

I saw a presentation on the dampers installed on the Millenium pedestrian bridge in London. Taylor Devices was the manufacturer. Pretty interesting how they got that under control. Better to address this issue before construction though!