of Note


99% Invisible, a “tiny radio show about design”, produced the episode “Rebar and the Alvord Lake Bridge” earlier this year. In it, host Roman Mars visits the Alvord Lake Bridge at Golden Gate Park in San Francisco, a pioneering structure of reinforced concrete.

Perhaps rebar is the most invisible part of a building, “because if it’s made right, you never see the steel skeleton”. Ernest Ransome, known as the father of modern reinforced concrete, experimented with steel reinforcement until he obtained the right amount of twist in a square bar to better bond the bar to concrete. His 1884 patented design and his innovation led to the possibility of concrete sky scrapers and bridges of the interstate highway system.

Reinforcing in concrete and masonry had been employed before the advent of rebar. In masonry, Marc Brunel designed brick shafts within the Thames River Tunnel (1825) with 1″ wrought iron bolts built integrally with the brick and placed iron hoops within mortar joints. Many subsequent examples of mortar bed reinforcing are known.

It was more than a century later, and even 30 years after Ransome and the Alvord Lake Bridge, that masonry walls were vertically reinforced in the cores. This practice began in India and Japan, areas more prone to earthquakes, then made its way to the US, most notably, California. According to Schneider & Dickey, vertical reinforcement “was an absolutely imperative step…other wise [masonry] would have been codified out of existence.”

Tensile strength provided by a “not smooth” reinforcing bar is an invisible element that has allowed masonry walls to continue to be a premier option for vertical enclosure.

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