Historic Structural Detail: Smooth

by Don Friedman on June 21, 2016

Revisiting old debates can be a bizarre experience. Outcomes that we take for granted because the debate was over long before we were born are being argued as if the answers are unknown. That happens, of course, because the answers were not known at the time the debates were taking place. We benefit both from hindsight and from the result of the debate.

What’s this? Pipe encased in concrete? Maybe electrical conduit?

It’s rebar. It’s rebar that has been exposed to view by a probe that removed the concrete cover. There are paired longitudinal bars at the bottom as flexural reinforcement, and the small hooked bars are the beam’s shear reinforcement.

The bar is smooth because of an honest debate on the merits of various rebar shapes that took place about a hundred years ago. Experimentation had shown that sharp edges on embedded metal tended to cause cracks in the concrete, and deformations on the bar often had sharp edges. That splitting problem is exacerbated by the use of weak concrete, but at that time there was no concrete we would consider strong. At the same time, the importance of bond strength was not well understood, in part because of the difficulty in measuring the effect of deformations. Finally, people’s perception of the issue may have been warped by the commercial success of Ransome bar, which has extremely high bond strength.

Modern concrete theory – and, of course, current code – says that bar has to have deformations. However, the International Existing Building Code and the existing-building portions of the International Building Code say, in short, that existing structure that is performing can be assumed to be adequate for the existing loads. The beam shown has been performing and was probed only so that we could try to calculate its capacity.

As odd as those smooth rebars look, they work well enough for the structure they are part of. Worrying about them not meeting current code is not only counter-productive, it also blinds us to the history they represent.

Previous post:

Next post: