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Failure Portrait: Not Yet

An office building in midtown has a slight rusting problem…

The column is still performing adequately, still carrying load. Obviously, if it continued to deteriorate that would end at some time, but since the project we’re part of includes both reinforcing the steel and waterproofing it, that unfortunate outcome will not happen.

This level of damage is a good illustration of the way safety factors work in practice in aging buildings. The column’s capacity has been reduced by both gross section loss and by secondary stresses from asymmetric section loss creating some eccentricity in loading and therefore some unplanned bending stresses. At the same time, most columns in most buildings never see their design loads because of live load reduction (the floors carried by the column are, statistically, very unlikely to all be loaded to code levels simultaneously) and because any loads from lateral wind or seismic loading are, statistically, not at their maximum nearly all of the time. So both demand and capacity in this case are less than were assumed in the original design or in a modern back-calculation, and the safety factor (the ratio of capacity to demand) has stayed greater than one. Our repair should get the capacity back to its original planned level, and the cycle starts again.