gbabob

Usually, this is a requirement of insurance companies in industrial settings and usually they require a signed and sealed letter from a licensed structural engineer.  You are asking for engineering services (not tips) on this site.

I would suggest you hire a structural engineer to fully determine the capacity of this roof and the basket as the data that you could provide on this site wouldn't provide all the information needed to fully calculate the capacity, or understand all the parameters, of this problem.

Please hire an engineer. There are too many questions that we could be asking and too little information provided.

For example, how can we determine the load capacity of a manlift basket if we don't even know anything about the manlift itself?

PEinc is 100% correct. Thereore, I am giving him a star.

I'm sorry, I didn't mean for anyone to do this for me. I just wondering if there was a formula for coming up with the figures.

The roof was designed for a definitive load, most likely a uniform load like you say (PSF).  ASCE 7 and Model Building Codes like IBC, and UBC etc., contain the loadings that buildings including their roofs must support.  Your roof was designed to support its own self weight, maybe snow, a roof live load, and weight of any eqipment on the roof.  There may be some other loads like wind or seismic that govern the design of the roof as a diaphragm.  This is one solution method.

Another method is to determine how much weight a 2 x 6 at 24" O.C. over a span of 11.5' is allowed to support in PSF.  In order to answer this question, need to know the species and grade (visually or machine graded) of the wood, or one has to assume a low grade and low strength species.  I would use the National Design Specification for Wood Construction to answer this question.  Would need to check to see what governs the strenth--is it the joists, the joist connections, the plywood sheathing, etc.  

continuing vanscoyoc's list:

....plywood sheathing, support beam capacities, support wall (stud) capacities, lateral stability of the office, code compliance issues (guardrail, stair, etc.), joist shear capacity, etc.

There isn't a "formula" that serves to replace the appropriate practice of engineering.

Right, I think the first method is much easier.  Make the assumption the design was done correctly initially using established required loads by Code, and add them up on a PSF basis.  That will tell you the capacity, at least for vertical loads.

If you're trying to find out if you can add more weight to the roof, such as roof equipment, then you're back to checking ALL the members and connections in its load path down to the foundation for adequacy.   

I wouldn't just assume that it is correct in the first place.  If this guy is being asked to evaluate its load carrying capacity, and he has no experience with this, what are the chances that whomever did the original design was another inexperienced engineer for this application?  Check the whole thing for its actual capacity instead of referring to minimum loads in building codes and working backwards to a uniform load and then uniform pressure.

I would check all the failure modes, e.g. shear, bending, delfection, axial, interaction of axial and bending, headers, jambs, connections, etc. and the load capacity will reveal itself in your results.  Try to base that on the in situ conditions as much as possible also since you don't know that the builder followed the plans, if you even have plans to look at.

I think you should hire someone, pay him or her the couple hundred dollars to write you a signed and sealed letter telling you what you need to know and go home and sleep well at night.

As for the manlift basket. OSHA requires that the manlift basket be rated by a PE. The risk of doing the design calcs wrong and overloading the basket is not worth it. It also has to be tested annually by qualified person. and after every modification or repair.

Thanks, Lutfi.  I didn't have to work as hard as everyone else did for that one; but I also didn't think it needed a detailed answer.

I may be wrong but it sounds like someone wants to store some materials on the roof of this office enclosure (shed?)which is INSIDE an existing warehouse building.  Very risky.  Many unknowns.  Must be seen.  I'd be surprised if anyone ever really engineered the office before it was built.  This office may be nothing more than partition walls with a light duty roof to retain heat and air conditioning and to block out noise and dust.  Sorry, gbabob.

Thanks for all your responces. We're going to get a PE and do it right.