I received the following comments from a nationally-known parking garage consulting engineer:
With respect to allowing people load on the top garage level: Normally, the roof top of the garage is not designed for such a live load. The owner should not allow such an event to take place and the fire marshal would be very upset.
With respect to designing garages for a greater people load on the top floor: You really can't because that would be considered a different use (mixed use) and you will be unable to get a building permit as a garage. It would require more stairs, fire sprinklers, etc.
The additional loading isn't a code requirement, nor it is standard practice, nor is there a history of failures despite it not being included in current structures. It'd be pretty hard to justify the extra cost.
Sometimes precast parking garages collapse under snow loads. That is, a snow plough operator piles up snow in a concentrated area exceeding the capacity of an individual double-T or even a whole bay.
I am in complete agreement with PEinc's comment. I think it is unconscionable that the roof of a parking garage is being used for dancing if it has not been designed for it in all respects and to all requirements of the Code. As I have been involved with design of parking garages for a number of decades, I have always asked myself before each design whether the roof might be used as a viewing area by the public (to watch a parade, or the like) in which case it should be designed for 100 psf minimum live load. In any event the roof should be designed to accommodate the weight of snow piling (usually >> 100 psf) in selected locations, and the weight of the snow plough and pile of snow it is pushing in front of it, I think. The old Toronto Code (in effect until 1975 when the provincial Code superseded it) required parking garage floors to be designed for a live load of 75 psf - more than needed for a typical floor of a garage, but perhaps a good idea for the minimum live load of a roof.
I was in San Francisco recently and heard the tale of how they closed the bridge to traffic and allowed 300,000 pedestrians to walk on the bridge at once during the bridge's 50th anniversary. It deflected by about 13 feet in the middle and experienced stresses likely beyond anything it had experience before.
Yes, densely packed people are heavier than cars, but 100 psf is pretty hard to achieve in any case. Look in the mirror & tell me you can imagine being in a 1000 sq. ft room with 499 people getting the same 1'X2' space as you. It ain't gonna happen & you know it. The 100 psf is to allow for the effects of dancing, stomping, whatever, without introducing yet another factor, because face it: if there are enough people to make 100 psf they sure aren't going to be able to produce an impact of 2 or 20 or anything. They'll be happy to breathe.
Since this was in Arizona the most likely snow load is 0psf. I am interested is seeing how you figure out how much snow load would be piled in the the designated areas? I have always wondered how this was done. I have been asked to look into this before but the projects have always fell through.
We have detremined the density of ploughed piled dense wet snow by actual measurements on site in downtown Toronto.
We limit the pile height and design the designated area for the corresponding load. Usually > 200 psf.
This is certainly not ideal for a number of reasons, including enforcement of pile area and pile height (say 5 feet), but we have seen garages where no snow load piling was designed for and yet we saw large snow pile with no visible distress.
In most garages the snow is removed by ploughing it into a snow melter (for larger garages like the 10,000 car garage we designed), or for smaller garages it is ploughed over the edge at barriers that open to allow the snow to be ploughed over the side Or down a shaft), or other snow removal methods.
Too bad that Arizona gets no snow. There is nothing more beautiful than to watch a raging snow storm (while looking out the living room window) or out walking thru a blinding storm.
That part (Phoenix Metro area) of Arizona gets no snow. Other parts can get considerable snowfall. I've worked on projects in the Flagstaff area where snow load was certainly a consideration. Lots of climatic differences in our state. I'll agree that snowstorms are gorgeous, but as my father-in-law always says, "You don't have to shovel sunshine."
