Where in the Uniform Building Code (California) can I find the required bearing length on each side of the open excavation for a 42" x 0.5" wall steel casing pipe, containing a 30" steel waterline?
I'm not sure exactly what you're looking for here, but I believe AWWA Manual M11 has some information that might be of help you with regard to the design of steel piping as beams (perhaps the interior pipeline and anything expected in the annulus could be handled as a distributed load, or point loads at any interior supports?). Otherwise, you may also want to contact whoever is responsible for the "Uniform Building Code (California)" to see if they have anything applicable to your situation.