HSS: hot-formed or cold-formed hollow structural steel. ASTM A500 Grade C: Fy=50 ksi (round), Fy=50 ksi (rectangular). Common sizes: HSS 4x4x1/4, HSS 6x6x3/8, HSS 8x8x1/2. Uses: columns (compact sections resist buckling), braces in moment frames, exposed architectural structure. Advantages: uniform strength in all directions (vs W shapes), clean aesthetic when exposed, efficient in compression. Connections: through-plate, end-plate, gusset plate, slotted. Welding: preheat may be required for heavy walls. Fill with concrete for composite columns (enhanced fire resistance and strength). Cost: $1.50-$3.50/lb ($3,000-$7,000/ton). Corrosion: galvanize for exposed exterior or seal ends to prevent internal corrosion. CSI 05 12 23.