# The Bouncing Balcony



## gafftapegreenia

This video has been circulating on the blogs (iSquint and Theatre Safety Blog). It's a video of the balcony of the Fox St. Louis bouncing violently during a Phish concert. 

YouTube - Fox Theatre Balcony @ Phish 6.16.09

Apparently, form the YouTube videos, the same thing happens at the Fox Theatre Detroit. 

YouTube - 1/16/10 Fox Theatre, Detroit Balcony Bouncing Nick Jonas Concert

I've never heard about this till now. How normal is this? Is it "bad" persay? Can these structures handle this pressure? How long till we open the daily electronic newspaper only to see "theatre balcony collapse kills hundreds"?


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## DuckJordan

The problem i see is too many people on that floor let alone all bouncing to a beat. I can bet that the structure was not supposed to flex that much. So my next thing would be to tell the owners of that theater to not place concerts in their space where they expect dancing in the aisles.


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## gafftapegreenia

DuckJordan said:


> The problem i see is too many people on that floor let alone all bouncing to a beat. I can bet that the structure was not supposed to flex that much. So my next thing would be to tell the owners of that theater to not place concerts in their space where they expect dancing in the aisles.



You can tell em, but I'm positive that, at least in Detroit, Olympia Entertainment isn't going to give up large music events in one of its prime venues.


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## Footer

Everything is designed to give. Bridges move that like. So do buildings. Anytime you have a structure, you have two numbers that you deal with. Without getting into the math, when designing a structure you have to deal with how much movement is allowed before the deck moves and then how much movement is allowed before the unit breaks. When I design things onstage, I try to design so that nothing will deflect more then a quarter inch at the maximum. However, when designing larger structure such as balconies, you design so that nothing will deflect but you also design so that if a huge amount of strain is put on the structure it will flex and not break. The fact that the balcony was deflecting is a good thing. Deflection is fine as long as the piece goes back to where it was before when the stress is removed. It all has to do with the modulus of elasticity of the materials used to build the balcony and the structure as a whole. I am willing to bet that when the balcony is full it sits a few inches lower then it does when it is not loaded. The only reason you can see the flexing is because people are bouncing. 

Elastic modulus - Wikipedia, the free encyclopedia


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## derekleffew

Footer said:


> ...I am willing to bet that when the balcony is full it sits a few inches lower then it does when it is not loaded. ...


That might make focusing fixtures (particularly projectors) on the balcony rail annoying.

Still, it's unlikely that the designers in the 1920s of these movie palaces ever anticipated the entire audience jumping up and down in unision. See also Why do soldiers break step in marching over a bridge? | Answerbag and MythBusters Episode 12: Break Step Bridge, Toothbrush Surprise, Rowing Water Skier .


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## MNicolai

Balcony collapses are unfortunately not unheard of. As was the case in the Diamond Horseshoe Casino.

Of particular note is the 1922 Knickerbocker Theatre collapse where snow built up on the roof for 28 hours straight in a storm, causing the roof to collapse onto the balcony, and the balcony then collapsed itself, injuring 133 and killing 96.

In no way do I envy the jobs of structural engineers, but I certainly have a great respect for them. Derek's on the ball; nobody designing that place probably ever planned on a design factor for structural integrity that would've factored in everyone in the balcony jumping up and down at the same time, practically in sync. Hopefully they're taking this seriously and are not going to try to just sweep it under the carpet because even if they didn't have the jumping up and down in sync regularly happening before this, there will be a disturbing amount of people who will want to bounce that balcony up and down just to say that they did, all because they saw this video.

I pity the structural engineer who gets his reputation tarnished should anything happen -- I wouldn't expect a structural engineer to ever think, "OK, so I've got the balcony reinforced to hold the audience -- now I just need to calculate how many forces are present if everyone in the balcony jumps up and down for two hours straight." It's one of those details that usually is an after-thought to the construction process and unless someone specifically told the engineer ahead of time that people might jump up and down a whole lot up there. I could easily see that being a detail that wasn't factored into the design and if it wasn't, it's really on the shoulders of staff there to either keep people from jumping around (good luck), or for them to have it structurally inspected and to have new calcs performed on it.


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## mstaylor

I can remember running spots in the late seventies in a brand new building where the building bounced enough at concerts that it affected our spot hits. I have seen the same deflection in the stands at large venues, football stadiums, doing the same.


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## museav

Consider that structural design is greatly defined by well established criteria and codes. Standard allowable maximum deflection is L/360 so with something like a 50' to 100' span that's 1.67" to 3.3" at the midspan and that is more a comfort than structural criteria. And for a floor beam that may be the allowable deflection for just the live load with additional deflection allowed from the dead load or the structure by itself. So the deflection itself may not be as much concern as are the shear and moment where the balcony connects to the vertical load bearing structure resulting from the synchronized dynamic loading.

It was probably 27 years ago when I was in the main balcony at the Elliott Hall of Music (capacity 6005) for a concert and noted one of my Structural Engineering professors attaching strain gages to the balcony structure. Apparently they were attempting to monitor and record the movement of and stresses on the balcony during some concerts. I asked him to give me some advance notice if we should exit the balcony quickly but apparently the results were well within any limits and showed no problems.


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## ruinexplorer

At a venue that I used to work in (designed by Frank Lloyd Wright), they had to stop a Bruce Springsteen concert due to the deflection in the lower balcony. The plaster even cracked on the underside. The balcony was only supported on the sides and not the back, unlike the upper balcony. After that, they cancelled all rock concerts for over a decade (well, didn't allow them either). To this date, they only allow certain acts to perform there, basically ones that will not get the audience up and jumping. The last time The Boss was allowed to perform there was an accoustical concert. He enjoyed that immensely.


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## museav

In April, 2008 there was a church holding a concert. The people in charge apparently thought that since the rated overall occupancy for the room was not being violated that it was okay for all of the attendees to be jumping up and down in a limited flat floor area in front of the stage. The result was part of the worship space floor collapsing and dropping about 60 people 10-15 feet to the floor below. That failure also apparently caused some pipes to burst and a tech tower to fall in the collapsed floor area, worsening the situation. Luckily, I don't believe anyone was killed, however over 40 were injured.

Just highlighting the point made that structures are designed for specific assumed conditions and the Structural Engineers have no control over those conditions being maintained in every use. Provided that the assumed conditions were commensurate with the planned use, the responsibility to work within those conditions falls onto the venue operators.


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## gafftaper

Three random stories:

My local Target store is on the 3rd and 4th floor of a building with an attached parking garage. The parking garage has express entrance and exit ramps that allow you to drive directly to and from the 3rd and 4th floor. If you are on the side of the store where the ramps are, you can feel the building wobble when heavier vehicles go up and down the ramps. Being in an earthquake zone I was a little freaked out the first time I felt it because it felt just like a subtle little 4.0 quake. Since it's a new building I assume it was designed to flex just like this on purpose. 

Back in 1995 the Seattle Mariners Baseball team had their first playoff run in the history of the team. They played in the concrete mausoleum known as the Kingdome. When that place was full of 70,000 people jumping up and down you could feel the floor flex in the upper decks. We are talking about one foot+ thick of steel reinforced concrete. The power of thousands of people jumping up and down is an amazing thing.

The 76 floor Columbia Center here in Seattle is supposedly one of the safest places to be in town if there is an earth quake because the building is designed to sway and flex to dissipate the force of a quake. On a really windy day the building gently sways back and forth a distance of over 10 feet. There is a very exclusive restaurant on the top floor that they close in storms because the toilets start spilling and the food won't stay on the tables. Drifting a little off topic. Every year they host a competition there where firefighters race up 69 floors in full gear. Going all the way off topic... The ladies restroom in the restaurant has glass walls and the toilet stalls are designed so that you face outward while on the throne. They used to actually close the restroom a few minutes each evening so that the men could get a tour. It was named the best restroom in America a few years ago.


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## museav

When they designed Water Tower Place in Chicago apparently one of the design considerations was to avoid blocking any views from the residential portions of the nighboring John Hancock Center. This extended to the point of assessing the potential movement of both buildings due to expected thermal expansion and contraction (e.g. when the sun shines on one side of a building the building materials on that side expand) and wind loads. They had to make sure the views were not normally affected if the two buildings moved toward each other.

The design issues in such buildings can get very interesting. I was involved in the design of a couple of approximately 50 story buildings here and you have to start thinking about things like how much a 500'+ high hot water riser expands once actually filled with hot water and how much a 500'+ chilled water riser contracts when filled with chilled water. Not to mention how much the water in those pipes weighs and the pressures that creates. There are just so many things that kind of vertical scale starts to affect.


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## gafftaper

I've often wondered how engineers prevent a toilet flushed on the top floor from reaching dangerous speeds on it's way down. I assume that it get's slowed down by being diverted in and out of temporary holding tanks along the way.


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## mstaylor

I assume the stack is a wet/dry stack so it is not dropping water huge distances, it is actually only displacing a small amount of water.


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