Question: I would like to inquire about the feasibility of using a 12”x12” structural tube to encase an existing concrete column. Is it possible to use two halves of a tubular section and weld in the field? If so, what are the pros and cons?

Answer: It is possible to encase a reinforced concrete column with a square hollow structural section (HSS). Presumably your existing concrete column is also square, and somewhat smaller in dimensions. You could not expect the HSS casing to fit snugly onto the existing concrete column because the inside hole of an HSS is not square, due to the rounded corners, which have an inside radius of approximately t (the design thickness of the HSS). Thus, the HSS will need to be “oversized.” The HSS will then have to be cut longitudinally into two C-shaped sections and backing shop-welded along the entire length to the inside edges of two tips of one of the C-shapes. Then the two Cs can be put around the concrete column and tacked together at the exposed backing. Then, field groove welding along the full length of the column, on two sides would take place. Finally, the gap between the HSS and the concrete column should be filled with a non-shrink concrete grout to achieve integral action between the HSS and the original concrete column. To install this grout, pumping from the bottom via a pre-installed access hole in the steel casing would be optimal (as is done for concrete-filled HSS columns). If you are looking for eventual behavior as a composite column, bear in mind that there is little confinement pressure exerted on concrete infill within a square (or rectangular) HSS composite column. For a circular HSS composite column, on the other hand, it is possible to achieve considerable concrete confinement action – and thus there has been rehabilitation of circular reinforced concrete members using this steel encasement technique described above.