In many cases, it is preferable to be able to open a large area on the side of a building or warehouse, in which case, mullions are used. Mullions are steel structures with side guides fitted each side to allow the shutters to close into them.
Sliding centre mullions
Are designed to slide out of the way when the doors are open.
Are designed to be lifted up against the ceiling and secured horizontally via a manual or electronic winch.