The welded connections of rolled HSS members of differing wall thickness and radial geometry must be considered by the construction team prior to the rolling, fabricating and erecting of the HSS members. This is due to the ungoverned consequential cross-sectional distortion of curved HSS members from the rolling/curving process. wThe issue exists in the fit up when trying to make welded connections between two of the same HSS members where one member is curved and the other member is straight or where both members are curved but their wall thickness and radial geometries differ. The cross-sectional distortion of curved members is a constant issue for bender/rollers that differ from process to process and from piece to piece depending on desired geometry and wall thickness. This cross-sectional distortion could be seen as ovality and/or a reduction in diameter in Circular Hollow Section (CHS) and an increase or reduction, known as shrinkage or growth, of the cross-sectional dimensions in Rectangular Hollow Section (RHS) and Square Hollow Section (SHS). The resulting reduction/growth in cross-sectional dimensions happens as a result of an attempt to eliminate localized buckling of curved HSS members, known as concavity and or rippling. Bending companies are able to restrict concavity/rippling to a certain degree through the use of tooling but it is usually the case that the restriction of buckling in combination with the extremely high pressures used to favorably yield the material to achieve the desired geometry, has caused a change in the cross-sectional dimensions of the curved HSS member. This is an issue that is easily overlooked but could wreak havoc on a project if not considered early on in designing and detailing.
(It should be noted that a large majority of bender/rollers in the industry use a 3 roll section bender to achieve the desired curvature in a member. The roller bending method, as it is commonly referred to, is a cold working process and such previously described deformations are a reality of cold forming steel. It should also be noted that other specialty methods, although most of which are proprietary, do exist. One optional method of curving HSS sections is through heat induction bending. This method is known for producing curved HSS members with little to no distortion, but is a much slower process that is usually cost inhibitive on larger projects containing high volumes of curved HSS members.)
The idea concerning welded connections of rolled HSS members occurred to me early in 2016 when I was given the opportunity to work with AISC in presenting to local architects & engineers on Architecturally Exposed Structural Steel (AESS) and AESS spec’s on curved/rolled members. AESS specifications are used to tighten up the tolerances of structural steel fabrication and erection so that the aesthetic appeal of connections, finish and form are all suitable for exposed visibility from a certain predetermined distance. In understanding how the AESS specification impacts issues like steel connections, it dawned on me that quite possibly the solution to the problem of welded connections of curved HSS members lies somewhere within this spec. The AESS specification governs curved steel members in saying that the fabrication tolerances required for AESS specified members should be half the normal tolerances as specified in the Code of Standard Practices (COSP). This arose an awareness in myself, and in looking through the code of standard practices there is nowhere I can find, that details fabrication tolerances of curved steel members; but it should be noted that AISC has made a push to publish tolerances in the new print of the COSP concerning radial geometry, by detailing the permissible variation of the mid-ordinate rise dimension of the chord length over the entire curved steel member. This newly printed tolerance in the COSP is a large step forward in the steel industry as AISC is starting to realize that more and more designs are utilizing curved steel members and in doing so some sort of regulations must be placed upon them. Still, this printed tolerance on the radial geometry of a rolled member does not speak towards the connections of these members; but if an AESS specification of half the fabrication tolerances on curved members could possibly address the fit up issues in welded connections of curved HSS members, we first need to establish what the tolerances are for non AESS members. In order to speak towards this issue on welded connections of HSS members, AISC really needs to touch upon the cross-sectional dimensions of curved members and what the acceptable variation from mill tolerance is. It is my guess that AISC has yet to look into this issue due to a non-standardized method in producing/achieving a curved/rolled member; as there are a varying number of methods that bender/rollers use and the resulting advantages and disadvantages of each method has a direct impact on the resulting cross-sectional deformation. In further seeking any sort of guidance on the connection issue at hand, we refer to section 05120 of the COSP. There we find a detail on the erection of steel members that allows ¾”-1” of open space between the ends of aligned members but this speaks little to the issue of welded connections. All that this section speaks of pertaining to welded connections, is that field welding is to be completed per requirements specified for shop welding. So it seems to me that there is a rather large gray area here that fails to speak directly to the issues detailed above concerning welded connections of curved HSS members.
An example of this connection issue arose in a recent project where straight RHS 18 x 6 were used as purlins to connect curved roof trusses made from RHS 18 x 6 members rolled about their strong axis. The difference in the cross-sectional dimensions between straight and curved members was cause for unsightly sloppy connections, and in other cases pertaining to CHS could be cause for an inability to complete/perform the welded connection as spec’d. It was questioned if the rolled members containing some cross-sectional deformation were curved to AISC standards/tolerances. A request was put into the AISC solution center asking if a tolerance exists in the COSP that specifies cross-sectional dimensions of curved members and the response received was detailed as such:
There is no tolerance for this. It is something that would need to be addressed if it is a concern. Section 10.1.b of the Code of Standard Practice states that tolerances needed for a special case and not already covered in the Code should be specified in the contract. Either the buyer can ask for it or the seller can alert that it is needed.
This response speaks directly to the AESS specifications stated above, in that if detailed connections within a project, like the welded connections of curved HSS members are of concern, that tolerance should be specified in the contract by either buyer or seller; which in many of the same ways is very similar to how AESS specification are applied to a project. But in many instances, such connections are not architecturally exposed. If an AESS specification on certain curved and linear members in a connection could alleviate this connection issue, what is to be done with like connections on non AESS projects? Are all such connections to be specified as AESS in order to alleviate this issue? Specifying AESS on such connections is not the solution unless they are truly architecturally exposed. AESS specifications, although possibly aiding in the fit up of these connections, largely increases the costs tied to the fabrication and erection of these members. With such a gray area that AISC leaves on this subject, the only way to help with this industry-wide issue is to inform the industry that such possibilities exist.
Recently I was given that opportunity in presenting a webinar to the members of the Steel Tube Institute. The webinar titled, Curved HHS Members: Design & Practical Considerations, touched much upon the history of bending/rolling and the processes used to achieve desired geometries in HSS members but more importantly it spoke directly to the issue of distortion and what should be expected when curving/rolling HSS members of varying size and wall thickness to differing geometries. During the webinar, I made sure to speak directly to issues that exist with welded connections of curved HSS members due to cross-sectional deformation. The art of rolling is a highly specialized type of steel fabrication that is still somewhat new to the industry thus a lack of shared knowledge stemming from the bender/roller up through the construction team into the hands of architects and engineers designing and detailing the project. As there is no tolerance that exists on the cross-sectional dimensions of curved HSS members it is our job as bender rollers to educate and inform the steel construction industry of the realities when curving HSS sections and what are the expected outcomes. Welded connections of curved HSS members are certainly possible but the need to consider these connections long before the rolling takes place is of utmost importance in order to avoid complications during fabrication and erection. It is always encouraged to seek early involvement on a project from a bender/roller in order to gain their input which could help in avoiding costly issues with welded connection of curved HSS Members.