This varies from country to country, but about 50 PSF should do it. Some codes specify less, in the order of 40 PSF, as a minimum requirement, but nothing forces you to design for minimums.
Well, let's say a car weighs 4000 pounds. That's a heavy one. A carparking space is roughly 8' x 18', so say 144 square feet. 4000/144 = 28 psf. When you consider that the driving lanes are never as full as the parking bays, the loading over the whole area is usually less than 20 psf. Thus 50 psf is a conservative number for parking...provided there is no potential for change of use.
A car itself may be heavier, but it also takes up quite a bit more space. Many studies have gone into this, and 40 PSF tends to be as hight as it gets for normal use car parking. When you have trucks and such, things get a bit heavier. I would think there is also a point load to check for, but I'm not sure off the top of my head.
40 psf is the US standard, per ASCE. Wheel load is 3000 lbs over a 4.5inx4.5in area.
This includes loads for normal passenger vehicles as well as trucks and buses, though a note on the load table states "Garages accommodating trucks and buses shall be designed in accordance with an approved method, which contains provisions for truck and bus loadings."
I typically design for 40 PSF per code and do not allow live load reductions. You don't want to have reductions because it is likely at some point that a car will occur nearly every parking space.
If the parking structure is meant to accommodate HS-20 trucks, then you have to design for HS-20 trucks in areas they could potentially drive. You should be able to find loads for these trucks in AASHTO. I would not assume that 50 PSF would cover it. We design fire lanes for 350 PSF for instance.
The heaviest load on a gas station forecourt is probably the tanker truck. This should be designed as steellion said, for highway loading. 50 PSF is definitely not in the right ballpark.
It certainly seems low. But the lane load per AASHTO is 640 plf. So if a lane is 10 ft., that's 64 psf. So 40 psf for a parking garage (low speed, except if my wife is driving, smaller vehicles due to restricted headroom, etc.) is actually pretty close.
Anyway, if you're worried, put the AASHTO wheel load (32 kips per axle) with it's 1.3 impact factor (conservative for a low speed situation) spread out over it's appropriate area (very completely defined in AASHTO) on various places on your slab and check it out. I find that kind of exercise therapeutic.
I swear parking garages have the shortest usable life of almost any commercial or industrial structure. To "refine" the load from a scant 50 down to skinny 40 psf is a bit too edgy for me.
At first glance those code live loads seem to defy logic, but well explained hokie. I love telling laypeople that a parking garage is designed for less than most office floor live loads. There have been documented problems when parking structures are converted or used for other things, like roof top party spots!
