A majority of mullions take place, with both doors and windows, between the operable frame and fixed side lites. Depending on the size of the operable frame, we are able to transition from the operable door or window into the adjoining side lite with a combo mull. The combo mull is an immediate transition to glass from the back of the operable frame. This is the narrowest solution when mulling.
As sizes increase, it is necessary to add tube steel to stabalize the loading of the operable sash. This "tube mull" helps with the side load of the operable sash and will also help with the wind load of the overall opening.
Some job site requirements exceed the loading capacity of the tube mullion. In the case of more extreme wind load or structural load demands, we work with the builder to develop an attachment method into the structural steel. Each condition can vary and our design development team can help coordinate the effort with your design team.