Flitch plate design (not used much due to the labor involved, and the more economical glue laminated lumber) as I was taught:
I tot = 2 (Iwd) + nIpl
The transfer of the load through the flitch plate should be shared by the two wood members and the steel plate between them.
Flitch plate...
What about the compatibility of spray on fireproofing? I would have thought that the humidity alone would not be good for the bond to steel, then compounded by galvanizing. This seems like a bigger problem to me.
Depending on the length of the opening, the requirement for concealing the lintel, and the wall construction, I have detailed geoff dale's rarely and steve1's more frequently. I have heard of the needle beam procedure, but honestly, it is not done.
If you detail the channels bolted to the wall...