What are the tolerances for straightness (camber and sweep) for Q355B I-beams per GB/T 706?

GB/T 706-2016 specifies comprehensive geometric tolerances for hot-rolled sections, including straightness, which is critical for erection and fit-up. Straightness deviations are categorized into two types: Camber (curvature in the plane of the web, i.e., vertical bending) and Sweep (curvature perpendicular to the web, i.e., lateral or horizontal bending). The standard sets permissible values based on the beam's length (L) and sometimes its section height (h). Typical tolerance limits are: Camber (Vertical Straightness): The permissible deviation is usually expressed as a maximum arrow value over a specified gauge length or the total length. For example, the tolerance might be L/1000 but not exceeding a certain value like 15mm for beams over a certain length. The specific value depends on the beam's size and the tolerance class (normal or high accuracy). Sweep (Lateral Straightness): This is also tightly controlled. A common tolerance is L/1000 with a maximum absolute value (e.g., 10mm). For wide-flange beams (HW/HM series), the tolerance might be stricter. Local Flatness of Flanges: The standard also limits the deviation of the flange surface from a plane, preventing excessive waviness. Twist: This is the rotation of one end of the section relative to the other about its longitudinal axis. It is particularly important for beams that sit on flat bearing plates. Tolerance is typically specified in degrees or as a height difference over the flange width. The exact numeric values are found in tables within GB/T 706-2016. These tolerances ensure that beams are sufficiently straight for practical construction. For applications requiring exceptional straightness (e.g., for long-span, aesthetically exposed beams), a "high accuracy" class can be specified, which halves the standard tolerances. Fabricators must check these during receipt inspection, and excessive camber or sweep may require cold straightening (jacking), which must be done carefully to avoid damaging the material.