Rhythmic Excitation in Balcony
Rhythmic Excitation in Balcony
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.
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.






RE: Rhythmic Excitation in Balcony
RE: Rhythmic Excitation in Balcony
@ 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.
RE: Rhythmic Excitation in Balcony
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.
RE: Rhythmic Excitation in Balcony
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Rhythmic Excitation in Balcony
ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
RE: Rhythmic Excitation in Balcony
BTW, you have the decimal in the wrong location...
Dik
RE: Rhythmic Excitation in Balcony
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.
RE: Rhythmic Excitation in Balcony
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.
RE: Rhythmic Excitation in Balcony
@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
RE: Rhythmic Excitation in Balcony
RE: Rhythmic Excitation in Balcony
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 www.floorvibe.com
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.
RE: Rhythmic Excitation in Balcony
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rhythmic Excitation in Balcony
RE: Rhythmic Excitation in Balcony
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.
RE: Rhythmic Excitation in Balcony
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.
RE: Rhythmic Excitation in Balcony
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.
RE: Rhythmic Excitation in Balcony
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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.
RE: Rhythmic Excitation in Balcony
RE: Rhythmic Excitation in Balcony
RE: Rhythmic Excitation in Balcony
ajk1, such remedies are anecdotal. I've also heard of people doing that sort of thing, and much more, and the problem either remained the same or got worse. These are definitely into the emergency fix category, not a fix one would plan on ahead of time.
csd72, there's no doubt that increasing damping will decrease vibrations. However, I'd take exception to the 1% increased damping statement because damping estimation is voodoo (no offense to actual voodoo practitioners on the board LOL). I don't think anybody can look at an arrangement of strl and nonstrl elements and say that the damping is 2% or 3%, 4% or 5%, etc. I think there's something else going on in most cases. For example, we use 5% damping with full height partitions. However, what really seems to be happening in a lot of these cases is that the partitions are like shear walls on the slab, adding stiffness and driving the frequency way up. That's too much of a hijack to go further, so I'll stop with that.
I thought you were talking about devices and materials specifically designed to increase damping. I'm very interested in this subject, but have yet to see a solution that looks promising for routine design problems such as this one. The first link is a primer. Second link provides some tips for estimating damping, and there's no doubt that it should be estimated as accurately as possible. Third is a general article on vibrations. Very useful, but not what I'm personally looking for.
To my knowledge, the current exotic options are tuned mass dampers, damping concrete, active control. There are currently problems with each of these approaches.
TMDs get out of tune if the mass or stiffness changes, so they're problematic for applications like this one. I think they have been used in somewhat similar applications, but they shouldn't be considered a primary choice. Also, keep in mind that your client can buy tons and tons of steel for the cost of the analysis alone, plus site visits to tune, re-tune, etc. Again, these fall into the emergency category IMO.
I have heard of a prelim study using damping concrete that was supposed to do a lot, but it didn't do much of anything. Maybe the full study will show that this is an option.
Active control is extremely effective, but would be prohibitively expensive for an application like this one. I forgot the exact number, but I think it's in the large 5-digit cost per bay. I imagine that it would take a multiple shaker system to control a floor with a group of people bouncing. I'm guessing this is a $200k range option. One can buy A LOT of steel for that!
The bottom line is that 99% of the time, one should change member sizes and try to satisfy the Design Guide 11 criteria.
Just my opinions and they're worth what you paid for them!!