Designers, architects, owners, engineers, structural steel fabricators – anyone who is interested in using curved steel shapes in their buildings, equipment or other structures can benefit from entering into a dialogue with companies that specialize in curving steel.
While there are copius manuals and guides for what pipe can be bent successfully on rotary draw benders, there is a shortage of materials regarding what can be bent through other methods. Whether it’s three-roll bending, ram bending, heat induction bending, or plate rolling, the best way to explore capacities is to contact the experts, companies that are sometimes referred to as Benders/Rollers.
Typically, these companies keep detailed records of what they have done, how the bending turned out, and how long the bending took. Depending on the thoroughness and complexity of the record keeping, a project engineer can answer questions regarding the limits of bending a particular steel section of plate while on the telephone with the individual seeking the information.
For example, a structural steel fabricator wanted to know if a W27 x 102 beam could be bent the hard way (against the strong axis) to a 40ft outside radius. The project engineer answered that it probably would be difficult to do. Upon looking up the company’s records, however, he found that the machine operators in the plant had successfully curved three previous jobs very similar to the one in question. A quick response to the inquiry can speed the development of an entire project, e.g. curved roof beams for a wavy structure.
Records of actual jobs are invaluable. It is not often the case that one can interpolate between section sizes and radii to determine if the curving can be done successfully. Furthermore, different Roller/Benders have different machines, different processes, different machine operators, and different material. Consequently, all these variables contribute to answering a common question: Can this steel section or steel plate be rolled to this radius?
When performing steel plate rolling and bending of higher strength steels, you will have two main considerations:
1. The tonnage/capacity of the machine
2. The spring back of the material after pressure is released
The tonnage is fairly linear in proportion with the yield strength. So if the yield strength doubles, so does the tonnage required to bend it. Most bending and rolling machines are rated using a “standard” of mild steel A36 or 1018 with yield strength around 36,000 psi.
Here are some typical grades specified for bending and rolling:
Grade TYPICAL Yield strength Tonnage Multiplier
A572Gr.50 50,000psi x 1.38
A588 Corten 50,000psi x 1.38
A516Gr.70 38,000 psi x 1.19
AR400 100,000psi x 2.78
Note that these are typical or even minimum properties.
You will have to look at the MTR (Material Test Report) from the producing mill to verify the actual properties of the metal you are trying to bend.
The spring back of the material is a more complicated formula, but in general:
1. The higher the yield strength, the more that the material springs back
2. The larger the forming radius, the more the spring back angle
For example, when you bend ¼” plate to a ¼” radius 90 degrees,
A36 will springback is only 0.25 degrees.
Gr50 will springback still is only 0.35 degrees.
But if the radius is larger, say 2”, then
A36 will springback 2.58 degrees.
Gr50 will springback 3.58 degrees.
I have frequently lectured and written about structural steel bending for architects, engineers and structural steel fabricators as well as for undergraduate and graduate school engineering students. In the dialogue that followed the presentations, I have been asked a number of questions.
Some of these questions are best answered by the rolling houses (companies that specialize in curving structural steel), others best answered by structural steel fabricators, others best answered by architects and engineers, and still others best answered by a combination of the above groups.
The following list is a compilation of the major questions that arise and what Chicago Metal Rolled Products has blogged about that topic:
There is not a mathematical formula for determining the minimum bending radius of steel sections. To better explain this, lets look at bar bending. Steel is curved using a cold-roll bending process. Steel sections are put into a section bender (also called an “angle roll”) with a three or four roll configuration. Rollers put force against the feed path to force the metal stock into a predetermined curved path. There is force applied by the rolling machine (stress) which in turn changes the structural steel into a curved shape (strain). If you look at the diagram below, steel is very ductile, and after a very brief elastic state the material goes into plastic deformation. The goal is to reach the desired radius without reaching the necking or ultimately the fracture point.
Looking back at the diagram above, the goal is to maximize the elongation limit in the bar bending process which is done by going with steel that is more ductile. A case study was done recently on bending 2.25 inch diameter round bar to a 3 inch inside radius. Two different material grades were used. The first was AISI 1144 which does not have a lot of ductility. The reason for this is 1144 is a free-machining material that has sulfur added to it to make it more brittle. It is made this way so when the cutting tool hits the sulfide inclusions, the material chips away easier. When bending 1144 to a 3 inch inside radius, the material was not able to withstand the stress and fractured. The second material type was Alloy 4140 HR which has a typical elongation of 25% vs. 1144 which has a typical elongation of only 10%. The 3 inch inside radius was successful without fracture using 4140. This case study better explains that the minimum radius is not the same for different material grades.
Probably the most common question we get from new customers is, What is your capacity for bending, particularly for the bending of beams. We have a quick and easy answer: W44x290# beams the “hard way” (x-x axis) and the “easy way” (y-y axis) as well as W36 x 848# the “easy way”—all on the world’s largest beam bender. But a more comprehensive answer would include information on all of the following:
The size of the section. Some lighter sections are sometimes more difficult to bend than heavier ones.
The radius, particularly if the radius is relatively tight.
The radius tolerances for the specified part.
The amount of deformation, if any, that is allowed for the application.
The amount of excess material required at each end, if any, for trim.
Most bending machines do not start bending a beam until a certain amount of straight section has moved through the bending process.
Whether there are straight tangents or not.
What is the maximum length that can be curved within the confines of a given plant, and that can be loaded and shipped on a truck.
Recently, we have seen a case relating to maximum capacity involving rather heavy beams rolled on the strong axis (“hard way”) to a tight radius. Although Chicago Metal Rolled Products had successfully curved sample parts for this project, it was not successful in its bid for the whole job.
The company that won the contract bent the beams but its bending process caused up to 5/8 inch buckling or crippling of the web. It has yet to be determined if this deformation will be acceptable in its application. So, this company in some sense has the capacity to bend these big beams, but not really because the distortion will probably not be acceptable.
Have you run across this issue of someone saying they could bend some steel, but it turned out that the quality was not what you expected?