Key Questions to Determining the Minimum Bending Radius of a Given Section of Structural Steel. Written by George F. Wendt

Read here to learn the determining factors of minimum bending radius for steel tubes, pipes, beams, bars, angles, tees and channels.

What is the minimum bending radius for a given structural steel section?

 The factors involved in answering this question are numerous:  1. What is the section to be bent? 2. What is the orientation of the bend e.g. a rectangular tube bent on the x-x axis or the "hard way"? 3. What is the condition of the end of the curved section(s)? 4. What, if any, distortion is acceptable?, and 5. Does the bending process produce a product that meets the specifications of the application not only dimensionally but also as regards changing the chemical and physical properties of the material? 

What is the steel section to be bent?

 Structural steel sections include angles, beams, bars, tees, channels, rail, pipe as well as  square and rectangular tube.  And each of these sections comes in various dimensions.  For example,  there are at least 25 different sizes of 36 inch wide flange beams (from 135# per foot to 848# per foot), there are 9 different sizes of 6 x 6 angle (from 5/16 to 1 inch thick), and there are 4 sizes of  20 x 12 rectangular structural tube (5/16, 3/8, ½ and 5/8 wall).

 Each of these sections may be more or less conducive to bending.  Angles, for example, when bent into a leg-out ring (i.e. with one leg horizontal out from the center of the circle and one leg vertical) tend to want to twist when curved.  Perhaps this is because one leg is being bent as if it were a "hard way" bar (bent on the x-x axis-like a steel washer) while the other leg is being bent like an "easy way" bar (bent on the y-y axis-like a belt).  And the two "bars" are attached at the apex of the angle.  Considering only the cross section of an angle, the material most likely would be most easily bent with the apex in or apex out.  Similar concerns may also apply to all the other sections that are to be bent with each section having its unique forming characteristics.  In general, larger sections have bend radii that are larger than those of smaller sections.

What is the orientation of the bend?

 The bending orientation--bending against the x-x axis or "hard way", against the y-y axis or "easy way," or off axis-also is determinative regarding minimum bending radius.  It is often the case that "hard way" bending is indeed more difficult to bend to a minimum radius, but that is not always the case.  Rectangular tubing, for example, may distort less when it is bent against the strong axis than against the weak axis.

What is the condition of the end of the curved steel section?

 Structural steel sections are often bent with extra material on the ends.  The material is commonly cut to length by a structural steel fabricator in the case of parts for construction or often cut to length by the roller/bender in the case of a part supplied to an equipment manufacturer.  Depending on the bending process, some extra material (pick-up, trim, or grip) may be required on each end.  On the other hand, some sections are bent with straight tangents on the ends, or with a reverse curve, or with another radius as in a multi-radius part.  Alterations to a curve on a metal section may determine the minimum radius of a section.

What, if any, distortion is acceptable?

 Most often the application determines what, if any, distortion is allowed.  If, for example, a large curved tube is covered with metal studs and drywall, more distortion would be allowed as opposed its being exposed as an architectural feature.  In some equipment applications, engineers have designed parts that have what would normally be considered significant distortion, e.g. a tube bent on the y-y axis with severe concavity on the inside wall.  For its application, the section may be stronger because of the concavity.  The amount of distortion allowed (or preferred) will also determine the minimum radius.

Does the bending process produce a product that meets the specifications of the application not only dimensionally but also as regards changing the chemical and physical properties of the material?

 Research has shown that cold bending can increase the tensile and yield of a given section of steel but at times may lessen its ductility.  Hot bending may change the chemical and physical properties of a given metal in ways that can be tested also.  Some applications are more critical than others, for example, seismic concerns for structural steel projects or bridge applications with dynamic loading and significant temperature variations.

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