Spiral ladder fabrication.

Fabricating steel spiral ladder of 1500mm width * 4200mm elevation & with following details-

1) 26 steps & in between landing.

2 ) Outer radius of ladder 5800 mm.

3)ladder width 1500 mm.

4)Supported on two main beams of rectangular box ( 150mm*350mm)fabricated out of 8mm thick steel plate.

I need a guidance to fabricate the main beams, specially the rolling of 350mm vertical portion of box beam.

Thanks.