Q: My company is repairing the columns of a concrete building facade, and we have encountered areas where the reinforcing steel has corroded so badly that supplemental steel is needed. Do you have any advice for how and where the steel should be added?
- A: We spoke to Jay Paul of Klein & Hoffman Inc., and he had several observations concerning the addition of supplemental steel to building columns.When concrete delaminates or is removed from the column, the dead load is carried by the remainder of the column. Unless the column is unloaded by shoring it above the repair (which is very difficult and expensive), the repair will not carry any of the existing dead load. The designer must be aware of the redistribution of the load in a damaged column and determine if the column is overstressed. If it is, it might be necessary to relieve, or at least partially relieve, loads from the column before repair.Ideally, supplemental vertical bars should be placed inside the column ties (rebar cage). However, this usually is not possible without cutting column ties or removing sound concrete from within the cage. Paul says that for fear of buckling vertical reinforcement he seldom allows a contractor to cut a tie. Even if the column ties are left intact, removing concrete from inside the cage could allow a vertical bar to buckle inward. It also further stresses the loaded column.Whenever possible, Paul prefers to increase the size of the column and place supplemental bars outside the column ties. The bars can be supported laterally with stainless steel hairpin ties that are anchored with epoxy into the rebar cage.If column ties are corroded, it's important to provide alternative lateral support to the vertical bars before removing any portion of the existing ties. Once again, hairpin ties can be installed that engage the vertical bars at the code-required spacing, but no less than the spacing of the existing ties. It is often necessary to build out columns to provide adequate cover over the supplemental ties.