When should I use hollow sections (RHS, SHS, CHS) vs I-beams?

Hollow sections (RHS rectangular, SHS square, CHS circular) have closed cross-sections and excellent torsional resistance — they also look cleaner architecturally. Use them for: columns with biaxial bending (SHS is equally strong in both directions), exposed members where aesthetics matter, torsion-critical members, lateral bracings in both tension and compression. I-beams (IPE, HEB, W) have open cross-sections with more material in the flanges — better for pure bending about the strong axis, easier to connect with bolts (flanges are accessible), typically cheaper per kg. Downsides: poor torsional resistance, prone to lateral-torsional buckling. Rule of thumb: I-beams for floor beams and portal frame rafters, hollow sections for columns and bracings.