Steel bending is often subcontracted to companies that specialize in providing curved steel sections sometimes called “rolling houses.” The question arises, which party should supply the steel—the customer or the rolling house? There are several factors to consider.
Where can the steel most easily be procured? Even if the rolling house adds a markup to the cost of the material, it might make sense for it to supply the material if it has good relationships with excellent local suppliers—excellent in terms of quality, service, delivery and value. Customers who are some distance from the rolling house might not have those relationships, and it may be expensive for them to ship the raw material from their local suppliers to the rolling house. Customers might also prefer to have the rolling house handle the logistics of ordering material.
Alternately, at times the rolling house might steer its distant customers to good suppliers close to the rolling house and have the customers buy the material there themselves.
Another consideration is which party is interested to putting up the money early on in a project when it may not be paid for a while. Steel suppliers most often will want to be paid before the customer or rolling house will have been paid.
Furthermore, some customers might want the rolling house to purchase the material to be curved because they want the rolling house to be responsible not only for the service of profile bending but also for any damage to the material that would make it unmarketable.
A final determination of who should provide the material should be the result of discussions between the customer and the rolling house. Rolling houses can quote their services with or without providing material, but considerations such as quick delivery, service, and quality must factor into the determination of best value.
When determining whether a given steel section can be curved without heat, it is useful to determine its section modulus and relate that value to the strength of the steel bending equipment.
Section modulus measures the flexural strength of a given section of steel. More specifically, section modulus is the moment of inertia of the area of the cross section of a structural member divided by the distance from the neutral axis to the farthest point of the section because this is where the material will yield first. The units for section modulus are typically cubic inches / in^3 / in3. The bending moment that it takes to yield that section equals the section modulus times the yield strength.
Various bending equipment has section modulus ratings. For example, a three-roll section bender that can bend a 3 x 3 x 3/8 angle and comparable steel bars, beams, channels, tees, pipe and tube would have a section modulus of approximately 1.2 in3. So given the requirement to bend a certain steel section, if the calculation of the section modulus of a given steel section (either mathematically or by reference to engineering tables) yields a value in cubic inches that equals or is less that the calculated strength of the bending machine, then the machine should be able to curve the section. One caveat, however, is that more power may be needed for tight radius bends.
Three-roll section benders are available with section modulus ratings from 0.4 in3 to 500 in3 and beyond (for special applications.)
After the determination is made as to whether a given section bender can curve a given steel section, it needs to be determined if the section can be bent with the quality requisite for the application. AESS (Architecturally Exposed Structural Steel) concerns might dictate that the steel sections have virtually no distortion or even scratches on the surface of the steel. For example, if the exposed curved steel section appears only a few feet above travelers’ heads as they ride an escalator in an airport, it will have to be near perfect. Similarly, parts for OEMs (original equipment manufacturers) might require curved sections which meet very high dimensional and appearance standards. Even though the tractor will no doubt not be like a shiny new car once it is put into service, the farmer wants to see it that way when he buys it.
Moreover, the improvements in producing steel have been nothing short of extraordinary. According to the American Institute of Steel Construction (AISC), from 1980 until 2009, steel producers have achieved the following:
In 1980 12 man-hours/ton; in 2009 0.5 man-hours/ton
40% higher strength
1/3 the energy
38% reduction in carbon emissions
67% reduction in overall emissions
EPA best industry performance
Curved steel can also contribute to a “green” design. For example, one building has the rounded shape of a “Twinkie” cupcake to steer air to the wind turbines on the roof. Other buildings combine curved steel with fabric to create effective lightweight structures. Lastly, curved structural sections can often eliminate the welding required in a segmented curve.
Each aluminum section, either straight or bent, has grooves machined into the ends of the tube. In the case of the bent aluminum sections, the machining is done after the bending. The grooves are part of a special coupling.
Another special requirement is to pack the curved sections using green lumber because the tube is annealed after being bent. The green lumber does not ignite from the heat of the annealing process.
The bending of beams can contribute to sustainability as is evident in the mile-long walkway at Dos Lagos.
The walkway is made of 213 pieces of curved beams weighing a total of 45 tons of steel formed by multi-radius bending: W6 x 24, W8 x 13, W8 x 35, W6x16, and W8 x 31. Some of the radii are as tight as 4 ft.
The curvy walkway crosses two man-made lakes in Corona California. Once the site of an abandoned silica mine, deep gorges were filled with water separated by a shielded walkway made of curved beams and bamboo poles, to be covered with vines. Curved and green both in color and sustainability, the curvaceous pathway uses three of the most ecologically sound materials: steel (93% recycled), bamboo (quick growing, easy to restore), and plants. And 98% at the frame can be recycled at the end of the life of the project.
Dos Lagos does indeed show how a curved steel can contribute to a sustainable, environmentally oriented community which is a development of regional significance: 543 acres of a master planned community. Having homes, stores, offices and a park in close proximity to each other enables people to avoid using their cars. The mixed use project has also sequestered over 300 acres of wetlands
Steel plate rolling can be done either on plate rolls—machines that incorporate three or four rollers to form curved shapes—or “bumped” with a radius die on a press brake. Both methods have their strengths and weaknesses. Everything else being equal (same plate thickness, same plate radius, same grade of steel), press brakes can usually “nose” the plate closer to the end of the plate thereby minimizing or eliminating any flatness. Plate rolls can also prebend plate to minimize straight tails but usually not as well as press brakes. At times plate is nosed in a press brake and then rolled in a plate roll to get the best results.
Plate rolls can usually roll 360 degree cylinders better than press brakes can. The ends of the curved cylinders can end up be obstructed by the press brake die or by the machine itself. Plate rolls can roll a cylinder through 360 degrees. This process is sometimes used to “round up” an out-of-round cylinder.
For example, plate rolling is the best method to form a 2-1/2 inch thick plate 40 inches long to a 2 foot 7 inch outside diameter full cylinder.
After being rolled, this cylinder was sent to a machine shop where additional fabrication created a part that is the main hub of a custom milling machine. The quality of the rolling contributed to the success of the machining.
Steel bending is a discipline, and a wise teacher once said that any given discipline has three aspects to it: the subject matter itself, an aesthetic, and a spirit.
For example, mathematics is a discipline about numbers, computation, formulas, etc. But some mathematical solutions are said to be more “elegant” than others, the beauty of which might stem from its being simpler. Beyond an appreciation of why one solution is more attractive than another, there is a spirit to a given discipline that can generate enthusiasm in its students and those who apply the mathematical solutions.
For example, an enthusiasm for automobile design is manifest at car shows; the beauty of some of the designs is undeniable; and, the significant knowledge base in car design and manufacturing is extensive. I have often marveled at how the mathematical solutions to how to design race cars almost always result in beautiful vehicles that start my heart racing—like a Ferrari.
These three components—subject matter, beauty and enthusiasm–can also be identified in the process of bending steel. The discipline of bending steel involves having the right machine, material, bending method and operator—the key inputs into a “fish diagram” often used to troubleshoot manufacturing problems. The subject matter of bending steel is comprised of a vast amount of know-how that has been developed over centuries, actually. So a body of knowledge has been accumulated about the best ways to bend, say, rectangular tubing against the strong axis (x-x) or the “hard way” that results in a smooth, true, arc of the right radius and length.
Those who are specialists in bending steel are perhaps the most appreciative of the beauty of both the process and the result of bending steel. First of all, in many cases, the material to be bent was initially designed not to bend in certain directions. A wide-flange roof beam is designed to remain straight and not sag. The web of the beam is designed to resist bending. The fabricator who not only bends the beam but who does it with no web buckling, no distortion, and no other cosmetic imperfections recognizes and appreciates this special achievement.
Questions about surface finish often arise when there is a requirement for bending stainless tubes for circular stair handrails. Will the helical tube bending process damage the finish? And what if the project requires bending polished stainless tubing? What if the tubing was polished before it was rolled helically? What will it look like?
A recent requirement called for type 304 stainless tube with a 1 ½” outside diameter, a 3/16 wall thickness, and a #4 finish. The customer needed it curved helically to a 16ft inside radius with a 32 deg pitch.
The customer was told that there could be scratches and swirl marks on the face of the tube; scuffs and possible carbon impregnation on the outside of the tube; and possible shipping damage caused by the freight company. The customer appreciated this information and said that he will put in extra fabrication time to touch up or buff out the possible markings. He also mentioned that most polishing tools are easy to use on straight material. And if he were to get pre-polished material and curve it, it would still be a lot easier to fix any imperfections rather than try to polish the tube in the curved stage.
Here are some of the different finishes:
#4 finish is a directional grained finish created by polishing to the equivalent of 150 grit abrasive.
#6 finish is a directional grained finish created by polishing to the equivalent of 240 grit abrasive.
#7 finish is a buffed finish and is mirror like but has some surface imperfections.
#8 finish is mirror-like and has had all surface imperfections removed resulting in a very smooth and reflective finish.