The overhead guard is designed to protect the operator from falling objects, but they are not rated to the truck's capacity. That is, a 5,000 lb. basic chassis capacity truck's overhead guard cannot effectively stop a 5,000lbs. load falling from overhead. The min test weight OEMs need for a 5,000lb lift should be around 2900-3000lbs from what I've seen.

I do not work for OSHA and safety advice is not something you should take at face value, so I would suggest taking a look at the source I have drawn my opinions from: ANSI B56.1 standards. It is my understanding that OSHA's standard was originally adopted from the ANSI B56.1 standards and when 1910.178 discusses overhead guards it refers to the following:

1910.178(a)(2)
All new powered industrial trucks acquired and used by an employer shall meet the design and construction requirements for powered industrial trucks established in the "American National Standard for Powered Industrial Trucks, Part II, *****ANSI B56.1-1969*****", which is incorporated by reference as specified in § 1910.6, except for vehicles intended primarily for earth moving or over-the-road hauling.

For a current source I looked at the B56.1-2005 section 7.29 "Overhead Guard for High Lift Rider Powered Industrial Trucks". There is information in this section on the overhead guard design along with a table for impact test loads for the forklift OEMs.

I hope this helps jay!