Parking Carousel

[SJBombero]

I have been asked to provide a peer review of the attached structure which is a carousel to store vehicles. My feeling is that even though the cars are free to swing in one direction, the mass of those vehicles must still be included in the seismic mass. I imagine that with some dynamic non-linear analysis one might be able to justify a lesser contribution from the swinging mass. However that is beyond the scope of this project at this point. The EOR has neglected the seismic mass of the vehicles per the ASCE 12.7.2 provision for public garages which I believe is incorrect. Has anyone looked more closely at this type of structure that I could reference to support my position?

 

[KootK]

Interesting. I've no direct experience with the seismic design of this kind of structure. However, I do recall being surprised to find out that the books shelved within libraries are often also not considered to be contributing seismic weight. Similar logic.

I feel you though: even though the independently swinging masses probably dampen the system more than anything, it gives me the heebeejeebees.

 

[ryaneng]

the books shelved within libraries are often also not considered to be contributing seismic weight. Similar logic.

This is surprising to me too! I haven't worked on projects involving library stacks, but commentary in the seismic section seems to indicate that you should include masses like this: "If the design of the building includes permanent masses that are normally included in the description of live load, then the weight of these masses should be included in the calculation of W."

 

[TLHS]

Excellent, a chance to reference the chicken seismic mass thread.

Can the cradles provide the amount of displacement they'd need to take under the ground motion before they start smashing into parts of the machinery and each other? I probably agree that they'd dampen fairly significantly though. Could you do a simplified FEA? Just throw in a few masses on pendulums roughly in the fight places and see what happens?

The car in a garage thing never made a lot of sense to me. The friction co-efficient of tires seems like it's high enough to hold the cars in place for design level earthquakes in a lot of areas. There will be rocking along the length of the car for sure, and to a lesser extent side to side. You'd think that'd provide some damping, but it seems not right that it'd fully cancel out the seismic weight.

I've never bothered to read into it though, as it's not something I've needed to incorporate into a design.

 

[hokie66]

I would be more concerned about the mechanical design and insurance than the structural. This one occurred in Melbourne, and definitely not due to seismic activity.

https://9now.nine.com.au/a-current-...hs-later/d3a1b0b3-3e97-4e92-8a8a-ed567b9e6ad3

 

[BridgeSmith]

We include the weight of vehicles as part of the seismic mass of bridge superstructures, so there's that.

OTOH, we are investigating using a suspended weight inside of our high mast lighting towers to mitigate oscillations (due to wind effects, though).

Another possibly applicable analogous design approach is that of gravity retaining walls (such as MSE) under seismic loading. We get a significant reduction in the overall seismic loading due to the time difference between dynamic force applied and the reaction due to the inertial forces. Because the wall mass is moveable, its inertia counteracts the much of the force applied, similar to the library books on the shelves, which will slide around and actually provide a dampening effect.

All that to say, yes, the weight of the vehicles should be considered in the analysis, but don't be surprised if you model them suspended, as they are, the structure performs better with the cars as mass dampers.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[HTURKAK]

IMO, ASCE 7-16 is clear for the definition of Effective Seismic Weight .

C12.7.2 .....When a building vibrates during an earthquake, only that portion of the mass or weight that is physically tied to the
structure needs to be considered as effective. Hence, live loads (e.g., loose furniture, loose equipment, and human occupants)
need not be included. However, certain types of live loads, such as storage loads, may develop inertial forces, particularly where
they are densely packed.

The cars are not rigidly connected to the carousel SFRS..

However, the bucket weights ( in one direction ) shall be added .. ( free to swing in other direction)..


If you consider the car suspension system ( tires, tire air, springs, shock absorbers and linkages ..) the vibration period of the car will be totally different from the SFRS of the carousel and the effect would be negligible..

 

[JLNJ]

The cars are not on true pendulums and it seems to me that some contribution should be considered.

For normal parking garages I think average occupancy and the relatively small mass of the vehicles to the overall mass of the structure comes into play. This is not analogous to the auto-vending machine in the OP’s query.

 

[Deker]

I’d be inclined to include 25% of the vehicle weight in the seismic mass, similar to the way ASCE treats storage live loads.