UNARCO offers a closed tubular column section and an open C-shape column section for uprights. We are often asked by customers “Which column section is stronger?” and “Why are both sections offered?”
The closed tubular column is a stronger and more efficient column section than the open C-shape. The closed tube geometry is considered a better design because, in addition to flexural buckling and local buckling, the open C-shape section is subject to some buckling modes that the closed tube will not experience as modes of failure. These modes diminish the capacity of the column. Three of these are shown below with some quick sketches to help illustrate.
1.) Torsional-Flexural Buckling:
2.) Non-Symmetric Distortional Buckling:
One flange “in” and one flange “out”
3. Symmetric Distortional Buckling:
Flanges “in” or “out”
The C-shape is much more sensitive to the location of the frame braces than the closed section so the C-shape section usually requires shorter frame brace panels near the lower part of the frame.
The answer to the second question as to why C-shapes are used is that the open columns are less expensive to roll and often a more than adequate design for most standard warehouse rack designs. The closed tube section has to be formed into a tube and seam welded at the end of the rolling process adding more steel to the strip width and an extra manufacturing process. In some cases the tubular design can be calculated to include a lighter gauge material because of its geometry and might be a more cost-effective solution. In other applications, a C-section upright may be more than adequate and a smarter, more economical choice.
In short, no two rack jobs are the same and both open and closed section uprights allow customers to decide which design fits their application best and is the most cost-effective.