Perfect example of the spring action of prestressed members.
I feel similar movements in my car as I sit on an overpass (constructed of pretensioned concrete I beams) waiting for the light to change while traffic is passing to either side of me. I don't even have to play music to rock out!
It is a little scary, especially when there are comments as follows:
"The average passenger vehicle weighs approximately 4,000 pounds, which is equivalent to 26 people that weigh around 150 pounds each. For clarity, that's 26 people per parking stall. If the parking garage can withstand the weight of hundreds of parked and moving vehicles (and their passengers), I don't think that people dancing on the top floor will cause a structural problem."
This guy would be shocked to find that parking garages are designed for 40psf which was recently reduced from 50psf.
Years ago my now wife was living in an apartment next to a major university in a large metropolitan area. She lived on the first floor with a bunch of college kids living above. They used to have dance parties above and you could literally see the floor deflect up and down as they would jumped in unison. Plaster was falling from the ceiling. Had something happened I'm sure their parents would have sued the building owner, city inspector, contractor and anyone else involved. Our future leaders hard at work.... ugh.
And also - now that I think about it - perhaps the failure mode would be ductile (flexural limit state where the tendons went inelastic)...it would be interesting to know the before and after camber in the floor.
Alternatively, look at SteelPE's 26 people/4000 lb per stall. A very tight stall is 9'X20', or 180 sq. ft., & if it's designed at 50 psf (ours is still that) it's 9000 lbs. Live load is live load, whether cars or people, so according to the design you can't put enough people on it to overload it.
Of course, we don't know what the design really was, do we??? Was the designer competent?
I agree regarding the harmonics. The live load can be inconsequential when you are oscillating near the natural frequency in the absence of damping.
There is a pretty substantial cable suspension pedestrian bridge near me. Just from shaking the cables lightly I am pretty convinced that one or two people could bring it down if they wanted to.
Nah! Impact factor of 2 is good...just tune the cables to play something good....maybe something from the 60's or 70's...Clapton, the Byrds...trick is to get a 12-string sound from it.
Just so that we are clear, that wasn't my load of 26 people per stall. That was a comment made in the video by someone who said that the structure should easily be able to take the load of the people because the structure is designed to support vehicles.
[shameless school rivalry]
How many ASU girls does it take to bring down a parking garage?
[/shameless school rivalry]
Also saw this in the comments:
"Just FYI, we did this last year too. After the intensive bouncing, the facilities manager double-checked with a few engineers and people who had been responsible for the construction of the parking structure and got the OK to do it again this year."
Those engineers are "braver" than me; I would never sanction such a thing. For those who haven't seen it this video of a wedding party in Israel a few years back ought to be a wake-up call.
So a student said a facility manager said that he/she double-checked with a few engineers, and people (contractor's apparently), who said that the original design was fine for anything a mob of inebriated teens could dish out. How about a sealed report?
Seriously tho to Ron or Mike or some of you guys with experience in PT component manufacture/testing/approval...Are these panels ever tested to failure under heavy dynamic loads (light load at high amplitude near the natural frequency)? My initial thought was that this type of loading was obviously damaging, but it may not be as bad as it looks if the load never exceeds the cracking moment or causes a stress reversal of the pretensioned member.
In our city there are two parking structures in the downtown area. Every fourth of July the top floors are filled with people watching the fire works. This of course is on top of all the parking spots full. I am hoping that the structures are designed for 100 psf at that level. Perhaps the codes should consider a change to top floor parking.
Honestly, this is one of the loadings that's always worried me. The problem is that "cars are heavier than people" is a thing that's in people's heads, so the layman is always going to think that something designed for cars is going to be strong enough for however many people you can stuff in there. Parking areas are also large open areas that could lend themselves fairly easily to being cheap places to host large gatherings.
However, we don't get a lot of parking structures that fall down because of occupancy loading, so I can't really argue that this is a real problem, but I've spent a bit of time worrying about this on several occasions.
![[jester]](/data/assets/smilies/jester.gif "[jester] [jester]")