Curved beams are used everywhere – in structures for functionality or aesthetic appeal, in circular shell stiffening rings on vessels and for monorails or roof trusses. Beams can be curved vertically (the weak-axis) or horizontally (the strong axis). When curving beams, it is important to consider distortion.
Distortion is any deviation from the original cross-sectional shape that usually occurs in every bent member to some degree. Think about curving a straight piece – the curving process is distorting the original shape to a curved shape. The potential for distortion is dependent on several factors, including the bend radius, the thickness of the material, the dimensions of the beam, bending axis, bending method and initial material geometry imperfections from the mill.
When a steel beam goes through the bending process, it is strained past its yield point which usually causes some level of cross-sectional distortion. The procedure creates inelastic compression stresses in the steel that can cause cross-sectional distortion. Distortion is the product of localized contact forces where the rollers of the machine contact the steel beam.
When rolling a beam the hard way or over the strong axis, complete an inspection both before and after rolling to ensure quality. This includes checking that the straight beam applies to AISC mill tolerances. Check the beam to ensure the flanges are perpendicular to the web using a framing square. With the beam flanges sitting on a level floor, check for gaps between the flanges and the floor. Gaps indicate sweep from the mill.
On very critical radius parts, the inside rolling radius must be maintained and fit to a scribe line. The rolling radius tolerance should be specified by the customer (e.g., +/-1/8” from the scribe line). The minimum length tolerance of the rolled section should be measured along the outside radius surfaces using the theoretical outside arc. Rolled sections must allow 4” minimum extra length for fabrication or field trim. The sweep tolerance is usually specified by the customer (e.g., 1/8” on a 36” section). The flange out of square tolerance should also be specified by the customer, (e.g., +1/8/-0”).
Here is a quick overview of tolerance checks on a hard way rolled beam:
- Radius tolerance (+/-)
- Minimum length, outside arc (+)
- Sweep tolerance with 36” template (+/-)
- Max out-of-square (+1/8)
- Flange Offset (+/-)
- Flange Warpage & Tilt (+/-)