The picture above shows the top end of a ten-story column, where it’s embedded just below a parapet. My first thought, when seeing something like this, is that I sometimes have trouble getting people to understand the nature of steel embedment in masonry between 1900 and 1940. That photo shows intimate contact all around the wide-flange shape.
More importantly, the whole reason we’re looking at this piece of steel is to determine whether it needs repair. The adjacent spandrel beam did need repair, so we opened up the masonry aorund the column to check on it. There’s obviously rust here, but how bad is it? If you give this a cursory glance, it seems like a lot of the original black paint is still present. If you look more closely, you’ll see that the black is the exposed face of steel where delaminated rust has broken away. This is perhaps easiest to see at the triangle of rust at the bottom of the exposed web: look at its upper edge and you can see the thickness of the rust and a slight gap where the rust is pulling away from the black plane.
I have no idea why the exposed face of unrusted steel is black. It’s probably some random chemical reaction with some component of the paint or mortar. I’m much more interested in the thickness of the loose rust and the remaining thickness of viable steel. In this case, the column had lost about 20 percent of its strength, but it’s very much oversized for the loads on it, and originally had something like twice the capacity needed, so that loss is acceptable. Scraping off the rust, painting the steel, and providing waterproofing will prevent further rusting (for now – some other engineer will specify some work here later this century) so we’re done.
Why was the column so oversized? Because we’re at the top of the buiding, where the loads are low. There’s a minimum size column, rarely ignored for the main structure of a buiding like this, based on the ease of access for field riveting. You’ll find angle and double-angle columns in penthouses and watertank dunnage, but the main columns are rarely smaller than 8 inches deep with a 6-inch flange.
