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.