Briefly, an OHG design is tested per SAE (used to be ANSI, then ASME/ANSI) standards two different ways. The OHG members must not deflect more than a given an amount in order to pass the tests - Test #1 a 100 lb harwood block cube of a given size is dropped on top of the OHG the cross & /or longitunal members. The block is dropped on the entire top surface. Test 2# Meausred the integrity of the OHG outer members. This test some times referred to as the Oregon Drop Test by people with gray or no hair, bad backs and retired (like me - I have 87% of my original hair) - the state where this test orginated in the 1970's then later adopetd by ANSI/SAE/OSHA. The test involves a bunk of hardwood lumber of a given l x w x h (I believe it to be 4' x 4' x 8') is dropped from a specific height (none are from the nose bleed section) to generate a given impact force which will vary depending on the trucks basic lift capacity. A very distructive test.
Because of the establsihed deflection limits, the operator clearance of 39" (from depressed seat to the under side of the OHG at any possible seat position) was established.