If the attic has the potential to become a regular living space talk with a contractor about adding a fixed stairway.
Attic wieght limit.
Most trusses are designed for only 10 lbs psf live load for storage unless they are attic trusses like drooplug mentioned.
This is because the ladder is simply not strong enough to support any more weight and you are putting unreasonable pressure on the attic access ladders every time you break the weight limit.
Is a safe estimate.
Observing the weight rating.
Access to attic storage.
A typical load is 5 and 10 pounds per square foot.
The dead load on the bottom chord of a truss varies with the weight of materials attached to it such as drywall on the ceiling.
Make sure items are spaced so that the weight is evenly distributed over the chords.
Of stuff on it.
If the total weight exceeds the load limit of the truss then the truss is compromised.
It is vital to observe the weight rating and make sure that you comply with it at all times.
If you want to use your attic on a regular basis or to store large items you may need to enlarge the access opening and install an access ladder or drop down stairs.
Modern standards call for around 12 to 16 inches of insulation in the attic and your long term utility bills would probably appreciate it.
So if your ceiling is unfinished you have some excess carrying capacity up there.
With an evenly distributed live load of 30 psf which the tables show the floor is able to support the total weight on the floor would be about 3 360 pounds.
If you care about this you will need to add a second layer of floor joists over the originals perpendicular is best to limit thermal bridging insulate then lay floor over that.
As larry said don t overdo it.
All weight hanging from the truss has to be added to the existing dead load.
Increasing the total weight on the floor to 4 480 pounds however results in a live load of 40 psf which is beyond the floor s load capacity.
To simplify you can place 20 pounds of dead load per square foot on your subfloor as long it s supported mid span.
Your allowed weight and span for a 2 x8 x12 2btr hem fir based on the info you have provided is 20 psf of dead load and 30 psf of live load with minimum bearing length of 0 68.