RHS Vertical Bracing
We highly recommend using RHS vertical section within the door when the door is over 2700mm high and 37mm thick or less.
It involves using a rectangular section of steel the width of the core, and this s is placed into the core of the door whilst it is being assembled.
While it does add more weight to doors, we do find this to provide additional rigidity where there are close clearances i.e. sliding doors.