Short answer: If you can get yourself up there, the rack will support you just fine.
Long answer: Aluminum 6061-T6 has a similar yeild strength to the steel used in most tubing. This means that it will take a permanent set at roughly the same load. It will however deflect (and spring back to original shape) more than steel will under a given load. I ran FEA (Finite Element Analysis) on my model when sizing my tubing. FEA is a tool used by engineers to calculate deflections & stresses in a target part or assembly. You create a mesh of lines and nodes, apply loads and supports, and the program calculates your deflections and stresses at each node. I used the ANSYS 10.0 FEA program and Autodesk Inventor solid modeler to do my analysis. Yeah, it may be a little overkill, but you can expect anything I build to have this level of attention put into it's design.
Basically I set it up so that a 300# load on any one point of the rack bottom would not 1) breach a safety factor of 3 for strength, or 2) deflect more than 1/4" (half the gap between the rack and the roof). I put the load on the center of one of the center tubes, and it easily passed both of those tests. Imagine a 300 lb. guy standing on one foot at the weakest position, and having no issues.
Next for a dynamic test I considered a 600# load spread over the center two tubes and recalculated deflection and stress with the same requirements. I thought this 600# test would reflect 200 pounds strapped in probably the worst case for the rack, and being put through 3G accellerations on the road (ie: rough bumps). It passed as well, as the images below show.
WARNING: The rack can handle a lot of weight, especially when standing still (loading, unloading). However, I'd recommend 150 lbs or less offroad, considering the owners manual rating for the Jeep Liberty roof. Also, driving with a lot of weight on the roof would require serious strapping and could hasten rollovers, handling oddities, and would catch more wind. I will not be held responsible for damage caused to your jeep or people due to overloading the roof.
FEA Mesh
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FEA Forces and Supports
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Deflection under 600# destributed over 2 center bars
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Von Mises stress under 600# destributed over 2 center bars. Von Mises stress is a method of calculating stress when concerned about yield strength.
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