analyzing parking garage for large crowd of people

huangyhg

analyzing parking garage for large crowd of people
i am in charleston sc where the city is about to open a new bridge.  they are having an opening ceremony for the bridge with fireworks and one of our clients has asked us to check the capacity of their parking garage because he is expecting a large crowd of people for the firework display.  the drawings were sealed in 1979 and they show this to be a prestressed structure with a design live load of 50 psf on the typical parking levels.  my first thought was that vehicles weigh more than people but then normally the design live load for lobbies and other large gathering areas is typically 100 psf.  any ideas of a simple analysis to determine that the owner of the building has no need to worry or if he should limit the number of people and/or vehicles in the parking garage?

find your worst case horizontal and vertical   
fbaxley - 100 psf is typical loading for elevated industrial platforms. for the structure to actually be subjected to that load, the people must be standing essentially shoulder-to shoulder. the checks that astructurale has suggested sound reasonable. however many building codes, including ibc, do not allow live load reduction for public assembly live loads of 100 psf, or greater.
i live near charleston and am familiar with the parking garage that you are talking about. perhaps you could check with your client and see if there have been events in the past, such as during the cooper river bridge run or spoleto, when the structure has experienced similar loading.
agree with slideruleera - also - with large groups of people, you also run the risk of music playing and people bouncing up and down in unison, creating extra impact forces.
astrucurale's point on checking the "best view" side is a good one and it also leads right into "unbalanced loading" checks that should be made.
jae's comment about the effect of music on crowds is very good too. the university of wisconsin is noted for a number of things and one is their "5th quarter" after their football games.  the marching band whips the crowd into a frenzy dancing the polka in the stands ().  the precast structure swayed so much that an extensive investigation and reinforcement program was undertaken back in the 80's.
there was a hockey arena problem 20 yrs. ago.
when a goal was scored . . . the crowd all rose up in unison. maybe weight + 2g loading?
your "first thought was that vehicles weigh more than people" is seriously flawed.  
the persons that can fit in a given space weigh more than the vehicles that can fit in that space, even if each person individually weighs less than a car.  a car is as wide as approximately four persons standing shoulder-to-shoulder and as long as 10-20 persons standing front to back.  therefore, you might have 40-80 persons in the space normally occupied by each car.  combined, they would weigh more than a single car.
there was a case in which a bridge "flattened out" when it was closed to traffic and filled with humans for its anniversary celebration.  it did not do this when filled with cars.  engineers had to calculate (while this was happening, because no one thought of it in advance) whether the bridge was safe like that.
the problem of humans moving is unison to music is real.  this may have been a contributing factor to the famous incident in which the internal elevated walkways/balconies in a hotel atrium collapsed during dancing.
you also need to consider the issue of the walls, fences, or guardrails at the edges of each deck, and make sure that when the crowd surges toward the edge, they do not either push anyone off or crush anyone.  there have been several cases in which crowds crushed large numbers of persons to death without knowing it, most famously one at a soccer match in the uk, but also one in ny city at the bottom of a wide staircase leading to a narrow open door to a charity basketball game.  parking garages typically contain vehicles of normal width and going singly and slowly, and are not usually subject to large forces from a crowd, and do not usually need to contain anything narrower than a vehicle going with more force than a single person.
stephenweinstein,
firstly i think you are unnecessarily hard on fbaxley for the original post. it is clear from that post that the difference in loading is understood. i.e. 50 psf to 100 psf.
the remainder of your post is very good and raises points not previously addressed.
i don't know anything about the bridge which 'flattened out' but clearly in that case the owner was not as smart as fbaxley's.
the elevated walkways are the now infamous hyatt hotel (in kansas?)
the uk soccer match was at hillsborough stadium but as you say there have been many other similar situations.
horizontal barriers in a parking garage would be inappropriate to restrain crowds even if they were strong enough as they are designed to take force at bumper level not waist/chest height.
if crowds are to form in this garage, then temporary restraint barriers should be provided. whilst these would impose loading on to the floor (or columns) there is a possibility that they could be positioned back from the edge to aleviate the worst of the floor loading problems.
finally my guess is that fbaxley's client, when faced with the possible problems of loading, will either prohibit the use of the garage as a viewing platform or tightly control the numbers of people who can do so.
jheidt2543,
polka dancers in a frenzy is a hard thing to visualize, even for me, and i live in wisconsin.  but it was true.  
i think it had something to do with beer.

-mike
lots of beer. add 8 lbs. per aveage person in wi. for that. but only at uw madison.
. . . and don't forget to account for the possible deterioration of the existing elements (corrosion, vehicular damage, etc.).