钢材表面积计算
理论几何表面积和估算涂装面积，开口截面与空心截面分开计算。
方法与假定
External area = Pext × L × N ; Internal area = Pint × L × N ; Coating area = (Pext + k·Pint) × L × N × (1 + allowance) + end areas
Pext / Pint = external / internal coated perimeter (m²/m), k = 1 if the inside is coated (e.g. hot-dip galvanizing) else 0
Theoretical geometric area. Practical treated area depends on the coating system: hollow sections are normally painted externally only, while hot-dip galvanizing coats inside and outside. Add allowances for connections, stiffeners and cleats.