Masonry

Not Code-Compliant

by Don Friedman on April 24, 2018

 



Helen Rogers, an English structural engineer, has a twitter account full of incredible stairs. I want to focus on one of them for a couple of minutes, a stair in the Blackburn Museum from the 1870s. Two of Ms. Rogers’s pictures are above.

The stair is structurally very simple: each tread is a beam, with a triangular-prism cross-section, cantilevered from the masonry side wall. The sloped underside faces align, creating a continuous sloped plane, while the top and side faces form the treads and risers. This is a fairly common type of stone stair in areas where stone stairs are common, which is to say not the United States. The wrought iron handrail* seems to have very simple mounts to each step that make it not helpful as a way to distribute structural loads between steps.

The stair is in excellent condition, which is always a good indicator that something is performing structurally. It’s quite thin, which makes the performance more attractive. And, it could not be built today in most, if not all, of the US. Technically, there is ashlar masonry (that meets specific ASTM specs) allowed by the IBC, but convincing building officials to allow new structure that consists solely of unreinforced masonry is quite difficult. Our current design culture is so steeped in the idea that ductility and multiple load paths are necessary for safety that a structure like this stair, with a single point of failure for each step and no ductility to speak of, makes most engineers recoil in horror.

This stair is not really suitable for high-seismic-risk areas, because the vertical component of seismic force could really play havoc with the anchorage of the steps. On the other hand, an earthquake that could destroy this stair would bring down the wall it’s cantilevered off of, which means the building would collapse. So I’m not sure how important that issue really is.

In the end, we have a stair that has a long record of performance that suggests it’s structurally adequate but is built in a manner that does not conform to the paradigm used for new structures. What this stair and a lot of other structures show is that other paradigms – ones not represented in the code – can be safe.

 


* I’m assuming it’s wrought iron based on the date and intricacy of the ornament. It could be ¬†steel.

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