Helical rolling, also known as pitched rolling or sloped rolling is one of the more complex processes that bender/roller companies might incorporate into their rolling services. Helical rolling is typically used for curved stair stringers, curved hand rails and helical transfer conveyers. With the increasing advances in bending and rolling machinery, the potential range of radii and pitch dimensions that can be managed for steel members has grown exponentially. Continue reading Estimating Bending and Rolling for Helical Hand Rails
A bollard is one or a series of posts that prevents vehicles from entering certain areas. A bollard can come in many different styles depending on its usage and/or the customer’s preference. One of the most common curved bollard shapes is the flat-back-U design. These would frequently be placed at gas stations to prevent cars from Continue reading Mandrel Pipe Bending of Stainless Steel Bollards
Quite often, companies specializing in the rolling of helical stair stringers see designs for projects which push the boundaries or physical capabilities of the machines. The reasons for this are varied. In some cases, the difficulties began back at the project design with an architect whose concept is something a bit unrealistic. Sometimes, a client has a request that might be feasible, but a change in material makes the job more difficult. Involving project managers with steel bending and rolling experience early on in the design process can help to navigate many of these issues before they become a problem.
Looking at some initial concerns of what is realistic, we can examine some situations with rectangular tube stair stringers. There are two major issues that come up in the rolling of rectangular tube stringers. The first issue is often the size of stringer to be rolled. When rolling a stringer, concavity or deformation of the tube wall is a serious concern. The larger the hollow section of the stringer, the more difficult it can be to lessen that deformation. As well, the added size can pose problems with mechanical limitations of the rolling equipment. The machines which roll stringers helically are often not the same ones which roll the tubes when level. As a result, the configuration of the helical rolling machines may be more suited to rolling smaller tubing, channels, or plates.
Another concern when rolling stringers can be design changes and customer expectations. Design changes such as modifying material used from steel to aluminum for a stringer may seem minor in fabrication, but can have large impacts when rolling metal. The different chemical composition of aluminum means the material will have a different ‘window of opportunity’ than rolling carbon steel. What may be an acceptable pitch and radius for rolling a carbon steel channel stringer, may be too tight to roll in a similar sized aluminum channel stringer. The issue at hand is the amount of physical stress the machine needs to place on the material to reach a specific radius. The varying yield and tensile strengths between aluminum and carbon steel are such that, aluminum can reach a breaking point before steel when attempting to roll to tight radial dimensions.
Discussed in a previous blog from last year, an Ohio state park committee was seeking curved steel 20×4 tube stringers for a monumental outdoor stairway. The problem the committee ran into was that the size tubing the design team had decided upon was not produced in the weathering steel which was an essential component of the design. So the decision was made to construct the tube stringers out of 1/2 inch flat sections 4” and 19” in width to create the walls from readily available material. This would require care and precision by an experienced plate roller in order to insure each section of the built-up rectangular tube would match up to make both inside and outside tube stringers.
The plate roll operators labeled each section in order to assist the fabricator with the assembly of the stringers. Once they arrived at the fabricator, the steel stringers would need to be assembled with slight variations in radius and pitch between the inside and outside tube walls. Ensuring these were matched up properly with the pair of 4 inch wide flats to be welded would require a great deal of work. To assist in this, the fabricator installed spacers between the 19 inch walls to ensure a consistent hollow cavity space in the rectangular tube.
In the end, the fabrication of the helical tube stringer was only one portion of the complexities of this job. The pieces needed to be brought to the jobsite and installed in a location that you could certainly call out of the ordinary. As seen in the photos, the final installed staircase has become a beautiful addition, contrasting with the natural surroundings and leading visitors of the park up for a closer view of the nearby waterfalls.
The end result, is that some jobs that may call for a helical tube stringer which is out of the ordinary, or perhaps beyond the capabilities for traditional rolling techniques. However locating a bender/roller with experience in constructing challenging helical staircases can offer alternatives that may be previously not considered, and result in an equally beautiful project utilizing rolled plate.
Many companies are finding new uses for the helical bending of steel pipe or tubes. Pipe and tubing can be extremely functional in applications which need a helical curve, though rolling pipe helically presents several challenges. Perhaps the most common example used in helical pipe jobs is when a designer calls for handrails on curved staircases. Very common in handrail jobs, bender/rollers will see call outs for 1 ½” schedule 40 pipe, or something very similar in size.
Now, the initial challenge for rolling pipe in a helical fashion is the difference between the plan view dimension radius, and what would be the actual helical radius. If you were to use the plan view, or ‘top-down’, radius and roll the handrails to those dimensions, you would most likely be left with a piece of material that is well out of tolerance for the project. The level radius, and pitched radius can vary a great deal when compared to each other. Essentially, the pitched radius is running diagonally over a longer distance, increasing the radius. Unless you are correctly including the pitch into the rolling calculations, your end results will not match up with the expectations for your customer.
In some cases, a ‘close-enough’ radius might work for a handrail over a shorter distance. If the radius is large enough, a level radius can be calculated which is functional. But other projects require a greater understanding of how to roll material in a true, helical manner. In these situations, a skilled operator can make the necessary adjustments to keep the pipe rolled on a consistent radius. Even in situations where the pipe size is large or the radius is very tight. One such recent example was involved a project of 6” pipe rolled helically to several different radial dimensions less than 36 inches, with varying pitch. Production staff would need to verify the dimensions of rolling consistently to ensure these stayed as close as possible.
Even once all material is rolled and completed, there can still remain one often overlooked challenge involved in helical pipe bending. The necessities for shipping helical pipe require some forethought due to the awkward dimensions. Helical pipe won’t always bundle as nicely as an item rolled flat and level. For this reason, helical rolling of structural shapes often involves scheduling shipping via flatbed versus loading into common carrier shipments. Unless the items are small enough to be boxed and placed on pallets, the items will often not store properly in the back of standard box trucks.
A helical strake is a spiraling strip often designed by engineers to give structural support to large metal cylinders such as smoke stacks. The process to make the strakes requires an initial set up of dies and tooling to roll the parts. A flat steel bar is rolled to a specified diameter or radius. At the beginning of the roll, the diameter and radius are gradually increased to attain the desired degree. At the start and end of the strip, there is a segment that is not rolled to the correct radius or diameter. But, after adjustments, they can be within 1/16 of an inch in accuracy.
When each helical strake is made, the initial pieces rolled are out of tolerance and are not usable for the smoke stacks. Luckily for a furniture designer in Chicago, he saw these strakes and was enamored with the shape, the way light bounced off the flat surfaces, and the uniqueness of each strip. These pieces were sold to this local artist who created a one of a kind table. The creator of the table loved the fact that all the pieces could be rotated in any direction to make this unique shape for the table base. He loved the shadows that were created on the floor from the light above. Sitting in the room, from any chair or sofa gave a unique view.
Looking narrow and almost surfboard shaped from the top, yet full and curvaceous through the middle. As one moves around the room, the table has a unique look, and the light makes the table seem to shine as it reflects off the strake. The strakes each have their own twisted form, and curve at different points on the flat bar. The strakes are interesting and eye catching not only for structural reasons, but to the artist as well.
Often, in the rolling and bending industry, we field requests asking about the minimum radius to which we can roll or bend a piece. In some instances, an estimator can quickly say yes or no based on prior knowledge and bending experience. There are many factors to take into account when determining the minimum radius such as the material composition, shape, and size. A 2” carbon steel pipe will have a different minimum radius from a thin wall 2” aluminum tube.
In some instances, a rough guide for minimum radius bending would be to use a multiple of the piece diameter. For example, on a 2 inch pipe elbow, common bend measurements may be referred to as 2D, 3D, or 5D. In these cases, the 2D would reference a centerline bend radius of two times the diameter. 2D = 4 inches, 3D = 6 inches, and 5D = 10 inches for a 2 inch diameter pipe. This type of bending is usually performed on a rotary draw bender for speed and precision, which requires specific tooling built for each bend. So, even though it may be possible to give a piece a 2D or 3D bend, a 3.5D bend may be more difficult because the tooling is not on hand. For tight rotary draw bending, maintaining a bend radius that is a whole number multiple may increase the capability of the steel bender to meet your needs. While it may not be a minimum for every bender, a 3D bend radius is a commonly used starting point for minimum radius bends.
As a piece size gets larger, pre-made tooling for tight bends will be less common. In these cases, the minimum radius will likely need to increase beyond a 3D size. Another important factor to consider when looking at pipe or tubing is the wall thickness of the piece to be bent. Increasing the wall thickness on a tight radius bend generally improves the final quality of a rolled tube as well as lowers the minimum radius a piece can be rolled with minimal distortion issues.
Material used can also change the minimum bend radius. One previous job which provides a good example of this used some 8 x 4 aluminum tubing rolled the hardway to a 16 foot radius. The same tubing rolled in carbon steel could be pulled much tighter. However, the aluminum tubing began to crack, possibly due to age-hardening coupled with the tight radius. To solve the issue, the material was sent out to be stress-relieved before rolling was continued. In the end, this caused a much improved final product for the customer with no additional material cracking issues.
As you can see, determining the minimum bend radius for a piece has many factors to take into account. Available tooling, tube size, material thickness and composition are all components that can add into what the minimum bend radius is for a piece of metal. In these situations, locating a bender/roller with the prior experience in handling these tough and tight bending jobs can be very helpful.
A state park committee in Logan, Ohio, had a vision of a monumental tube-steel, circular stair way with an intermediate platform for viewing one of their many spectacular waterfalls. They wanted to use self-weathering steel for its aesthetic and sustainable advantages over conventional A500 tubing.
Unfortunately, a significant design oversight occurred when it was revealed during the bid process that mills do not produce a 20x4x1/2 structural tube in weathering A588 Corten material. Rather than reverting “back to the drawing board” and dragging the design team over the coals, the committee found a local, ornamental steel fabricator who alone was willing to take on the challenge. The fabricator’s solution was to pair up with a skilled bender/roller to fabricate from Corten plate four-piece, built-up rectangular tubes curved helically to serve as the stair stringers.
With the top and bottom, 4in flat section members identical, rolling these mill bars the hard-way (against the strong axis) helically to match the 19in side walls at a right angle was not easy. Highly skilled and trained machine operators took frequent measurements to insure a precise radius-to-pitch fit.
To complete the next (2) members of this built-up tube and limited to 20 foot lengths, (8) 19in side walls were rolled to 4 individual, inside and outside radii, with (8) varying, rise/run pitch degrees. Separated by 3 inches, the inner wall will have a different radius and pitch from the outer wall to maintain a parallel rise.
With a jig-saw puzzle of loose, rolled, identified arc lengths and “miles” of welding and grinding ahead, the fabricator cut a series of steel spacers secured to the inner surfaces to maintain a consistent 19 x 3 cavity dimension.
Progress photos indicate the roller/bender supplier exercised their engineering talents to transform (4) plates into a single helical, steel stair stringer.
The superior corrosive resistance of Corten over regular carbon steel will allow this self-weathering, curved staircase to naturally conform to the surrounding environment. Furthermore, maintenance costs will be significantly reduced because the material does not need to be painted.
Teaming up with the right fabricator and bender/roller can make an enormous difference when you’re building a challenging monumental, curved staircase.
What can a community or chamber of commerce do to beautify a viaduct when it runs under a railroad line? Not much. The crumbling faces can rarely be modified because they are owned by the railroad. Don’t even think about anchoring nicely finished panels to the scummy inside walls…those also belong to the railroad. The newest solution in the village of Oak Park, Ill., is to anchor an arched pedestrian walkway directly to the sidewalk. Rather than use a traditional U-shaped tunnel, the architect wanted a more organic looking arch. The fabricator turned to us for the multi-radius rolling of structural steel which gives a more organic look.
As it turns out, curved steel above this viaduct also contributed to beautify a train station. The station is part of a multi-modal transportation hub where elevated electric trains run (part of Chicago’s famous “el” lines), where diesel commuter trains run from Chicago to the western suburbs, and where taxicabs and buses meet in Oak Park (the first suburb due west of downtown Chicago.) (We were so excited to be “workin’ on the railroad” that we took a family team photo.)
If you took the el train from Oak Park to downtown Chicago, you could disembark from the elevated tracks anywhere in the Chicago loop (so called because the train loops around the downtown area before heading back out). If you took the commuter train from Oak Park to downtown, you could disembark underground somewhere near Michigan Avenue and Randolph Street, underneath such famous landmarks as One Prudential Plaza and Two Prudential Plaza. Here you would find underground heated walkways to take you to various parts of the city as well as to other means of transportation.
These walkways are also beautified with curved steel. In this case, stainless steel sheet polished to a mirror finish adorns the walls of the walkways. Needless to say, great care was taken when rolling the polished sheets to avoid any scratching.
Steel circular stairs (sometimes called “spiral stairs”) commonly incorporate “stringers,” structural members that support the treads, side walls (if any), and handrails. These stair stringers can be made of plate, channel, beam and tube (round, square and rectangular). These steel sections are most often curved into a helix but are sometimes formed into more complex compound bends.
For example, some staircases require an elliptical shape. Others incorporate a narrowing or widening of the treads which changes the geometry of a simple helix. Some circular staircases have landings at the top and/or bottom of the stairs was well as in between. In every case, the accurate fabrication of the stringers is critical for the fit up of the staircase.
Another type of stringer is the box plate stringer. Similar to how a built-up beam girder is comprised of three plates welded together to form the web and two flanges, so too a box plate stringer is comprised of four plates welded together. But here all 4 plates must be rolled into a helix. And these plates have to match up perfectly to help create a staircase that is plumb and otherwise dimensionally accurate.
If we think of the box stringer as it will be installed, we can refer to a top plate, a bottom plate, a plate on the inside radius, and a plate on the outside radius. The top and bottom plates could be called bar rings rolled the “hard way’ helically, i.e. a compound bend of a bar against the strong axis. The inside and outside bars could be called bar rings rolled the “easy way” helically, i.e. a compound bend of a bar against the weak axis.
Whatever these components are called, they have to fit together and work with the rest of the staircase parts.
In the picture below you can see that the 1/2in beveled plates have been tack welded together indicating that they fit up together. To confirm accuracy in construction, there needs to be careful measuring of the radius, pitch, and orientation. In this case, the inside plate has an inside radius of 71.5in with a 27.83 degree pitch; the outside plate has an inside radius of 79in with a 25.85 degree pitch; the top and bottom plates are rolled the “hard way,” and are fit up to make a 16 x 8 box; lastly the orientation is “walk up, turn left.”