Open steel columns on construction site exposed to heavy rain?
It sounds like you have 6" square tubing columns which are welded at the base to embedded plates in the footings. That there is no top plate thus allowing rainwater into the tubes.

Standard practice is to drill a "weep hole" just above the weld at the base to allow for drainage. Ordinarily, the weep hole works to allow normal condensation to drain, but in your case it will relieve the unexpected rainwater as well. Someone should have temporarily capped those open ends.

I would probably make a single hole 5/16"ø or 3/8"ø on a 6" tube. Usually these are done prior to erection at the fabrication shop at the top and bottom of each tubular column, but easily accomplished in the field.

If they are not already painted and the ironworkers are still on site, ask them to heat up the bottoms of the tubes to evaporate any collected water after the weep holes are drilled - afterall, it was their responsibility to protect the ironwork.