I think you would need drag struts to transfer the diaphragm force to the lateral resisting elements (shear wall, etc.), in the case of an L or U shaped building.
For a diaphragm with openings, draw a free body diagram through it. You'll probably need some kind of chord element around the opening to make sure the diaphragm isn't overstressed in bending. For shear, look at the force acting on the net area.
Is this then a single story building with a 1.5", 18 to 20 gage steel deck/ open web joist/joist girder or steel beam roof system with no concrete infill?
Or is the steel deck a floor system with composite concrete infill?
If it is the former, it is probably a flexible diaphragm. If it is the latter, the diaphragm is rigid.
The former is easy to analyze the diaphragm as horizontal beam with the appropriate wind or seismic loads applied. Look to the shear diagram of the forces resisted by the diaphragm as the answer to your problem. Add appropriate drag struts as required to tie in the shear wall/resisting element lines, and extra ones around your openings as required. Diaphragm stresses will increase around your openings.
The latter condition will need a rigid diaphragm analysis, but once you know the forces to the vertical lateral resisting elements such as shear walls, you should be able to construct the shear diagram for the diaphragm and add reinforced elements for the openings.
