RHS

Rectangular hollow section (RHS) is a structural steel member with a closed rectangular cross-section, typically square or oblong. The tube is welded along its length, with the seam usually on one face. RHS is made by rolling flat steel plate, folding it into shape, and welding the edges together under tension or through high-frequency induction. Common dimensions range from 40 × 20 mm to 400 × 300 mm, with wall thicknesses between 1.5 and 12 mm depending on grade and application.

RHS is favored in construction and fabrication because its closed box form provides high torsional rigidity and bending strength relative to weight and material cost. A rectangular tube resists twisting far better than an equivalent open profile like a channel or angle, making it ideal for columns, beams, roof trusses, and crane booms. The broad flat faces also accept bolts, welds, and connection plates easily. Galvanized RHS is used in exposed outdoor work; untreated mild steel RHS is standard for internal frames and structures that will be painted or embedded.

Square hollow section (SHS) is a subset of RHS where both dimensions are equal. RHS itself includes all rectangular variants, including long-span tubes where one dimension exceeds the other by a significant ratio. European and North American standards define RHS grades: S235 and S355 are common structural grades with yield strengths around 235 and 355 megapascals respectively. Grade 450 RHS exists but is less common and more expensive. Chinese and Indian mills produce RHS at lower cost but with looser tolerances; UK and European mills maintain tighter dimensional control.

The primary failure mode in RHS under load is local buckling of the walls if the section is too thin relative to its span and applied stress. Design codes account for this through width-to-thickness ratios. Corrosion is the main service hazard; unprotected RHS exposed to moisture will pit and rust from the inside outward, weakening the section invisibly before external corrosion becomes obvious. Welded seams are stress concentrations and potential failure points if workmanship is poor or if the weld is subject to fatigue loading.

RHS pricing is driven by steel commodity prices, wall thickness, and mill location. Thinner-walled tubes (2 to 4 mm) are cheaper per unit weight but more prone to handling damage and local buckling. Heavy-wall RHS (8 mm and above) costs more but is necessary for heavily loaded columns or when impact resistance is required. Stock availability varies widely; 150 × 100 mm and 200 × 100 mm RHS are usually held by merchants, but non-standard sizes must be ordered from mills with lead times of several weeks.