Rebar and the Alvord Lake Bridge

Roman Mars: This is 99% Invisible. I’m Roman Mars.

Roman: There’s a well-known literary technique called Chekhov’s Gun, named after that Russian playwright. Basically, if a gun appears in the first act. It better be fired in the following act. There are no unnecessary details. You see a gun, that gun gets used to shoot someone. I have a corollary principle of my own invention and it relates to movies where there is a scene at a construction site, and it is this: If on the screen you see an ugly shaft of exposed rebar, somebody is getting impaled.

Roman: There’s something about rebar that fascinates me. If nothing else because there are very few things that invoke a fear of being skewered. My pathological preoccupation with metal reinforcement bars dovetails nicely with a structure in San Francisco I’ve kind of become obsessed with — a tiny bridge in Golden Gate Park.

William Littman: This is the Alvord Lake Bridge by Ernest Ransome.

Roman: This is William Littman, he teaches Architecture at the California College of the Arts and he was the first person to tip me off to the importance of this humble little structure on the very eastern edge of the park.

William: It’s sort of the entrance to the park from the Haight side, from the kind of hippie slacker Haight area, right at the edge of the park, sort of leading into the children’s playground.

Roman: It’s not a place that most people really want to linger.

Robert Courland: It is a spot for drug dealing and various illicit fascinations.

Roman: And that is Robert Courland author of “Concrete Planet” giving us a lay of the land.

Robert: We’re in the Alvord Lake Bridge, one of the earliest surviving reinforced concrete structures in the world.

Roman: The bridge was constructed in 1889.

William: It may be the least appealing sort of monument of architecture or civil engineering. On one side it’s cracked where the earth is sort of pressing through. It’s covered in mold and lichen. Inside it’s this kind of odd, surrealistic tunnel of stalactites that really kind of looked like some sort of folk art impression of what the surface of Mars might look like.

Roman: I think it looks like the inside of a giant colon.

William: It’s really unappealing to pass through it. It’s not well kept up at all.

Robert: Well, there’s nothing remarkable about it. It is an arched– it’s actually as much a pedestrian tunnel or bridge.

Roman: A pioneering structure in the shape of a dumpy and neglected little bridge.

Robert: But that’s truly sad because it is one of the pioneering buildings in the story of reinforced concrete.

Roman: There are plenty of candidates for the most overlooked, most invisible part of the built world but reinforced concrete has a good claim to be the most invisible of all. Because if it’s made right, you never see the steel skeleton underneath all the concrete structures that you work in, drive over, walk under.

Robert: Reinforced concrete is concrete that is strengthened by the addition of initially iron, and then later, steel to give it tensile strength.

William: The thing about concrete is it’s great in compression, meaning, it stands a lot of pressure in terms of weight but it’s not very good in tension, meaning when you– for the spanning long distances, it sort of can collapse.

Roman: So if you see concrete going high in the air or spanning a long distance, there is metal inside that. And you have this unassuming vanguard of a bridge and its engineer Ernest Ransome, to thank for that.

Robert: Ernest Ransome’s principal claim to fame is that he’s the father of modern reinforced concrete. The experiments done with reinforcing concrete with iron previous to Ernest Ransome, were just one off experiments: A cottage in England, a house in New York, and a rowboat in France. Ernest Ransome experimented with different forms of iron reinforcement until he hit upon what we now call rebar which is short for “reinforcement bar.” His technology was far beyond any of the others who were experimenting with reinforced concrete at that time.

William: A lot of people in Europe and America are playing with putting in bars or metal into concrete at this time. There are many different techniques and everyone’s experimenting. But Ransome’s sort of major innovation is, he takes sort of a square bar that runs through it and he twists it slightly, and that gives it an adhesive quality to the concrete itself and it sort of stays together much better. Ransome was said to have come to this idea when he found a twisted rubber band in his pocket one day, and thought, “Well, that’s what I’m going to do to this iron bar. I’m going to twist it so it just binds to the concrete better.”

Roman: You can see a diagram of this twisted rebar in Ransome’s 1884 patent. By the way, if you’re anything like me, Google patent search is the best way to spend time in front of the computer. I could lose hours jumping from patent to patent. Anyway, this innovation of messing with the bar to help it bond with the surrounding concrete is still used to this day.

Bob Risser: We put deformations on the reinforcing bar so that the concrete will hold on to it.

Roman: That scoring is probably why I find rebar so ugly and unsettling. That’s Bob Risser right there.

Bob: I’m Bob Risser. I am President and CEO of the Concrete Reinforcing Steel Institute.

Roman: I called the CRSI to get the full scope of what reinforced concrete means to the built world.

Bob: Well, without over exaggerating the point, I would say that the significance of reinforced concrete is that modern society is not possible without it.

Robert: Well, as humble as the Alvord Lake Bridge is, it is a direct precursor to the Ingalls Skyscraper built just 15 years later in Cincinnati, Ohio, the world’s first reinforced concrete skyscraper. It also led to bridges, dams, freeways, streets of reinforced concrete.

Bob: I mean, without reinforced concrete, you would only be able to build a series of unconnected asphalt roads.

Robert: It’s a very humble beginning, but it was from here in San Francisco that the reinforced concrete revolution took over the world.

Bob: That’s why we talk about it being the foundation of civilization.

Roman: It’s also a foundation of modern architecture. It made possible forms that were never possible before, like Frank Lloyd Wright’s Guggenheim Museum and more recently Jeanne Gang’s Oakwood Tower in Chicago.

Bob: It’s designed to mimic flowing water. And the– eighty-some floors, there are no two floors that are the same. There are no two balconies that are the same.

Roman: But the problem with reinforced concrete especially if the rebar isn’t covered with enough concrete and is exposed to water and salt, is that the steel inside can rust.

Robert: And as it rusts, it expands. It expands to almost four-fold its original diameter, destroying the concrete around it while the steel itself is being destroyed by the rust and corrosion.

Roman: And when that happens, which it will eventually, reinforced concrete doesn’t last the 1000 years that Ernest Ransome and the early reinforced concrete proponents thought that it would.

Bob: For many, many years, the design life was about 50 years. The entire interstate system was built under the assumption of a 50-year design. These days, our organization and others are working with the federal government and state agencies to try to look at 75 and even 100-year designs.

Roman: Modern reinforced concrete frames encased inside of a building superstructure with normal maintenance will last a lot, lot longer. So don’t worry, the Burj Khalifa, the tallest tower in the world and the tallest reinforced concrete structure is not coming down any time soon. But the clock is ticking for most of the reinforced concrete infrastructure that was put up in the middle of the 20th century in the US.

Robert: People have to realize that all this that they see around them will eventually have to be, with a few exceptions, will have to be torn down and replaced because we built with steel reinforced concrete, and the cost of that, it will be trillions of dollars, an unbelievable amount of money.

Roman: Seriously, the stuff that wasn’t properly maintained is coming down, and as you can imagine, a lot of our infrastructure was not properly maintained.

Bob: Particularly with public agencies where, very frankly, the method has been, and in some cases still is, a reactionary policy rather than a well thought out maintenance routine.

Roman: Even though concrete has the illusion of permanence, it is not that way at all.

Bob: You don’t just build it and forget it, you have to account for going back and taking care of it like you would anything else.

Roman: Ernest Ransome left San Francisco soon after he completed the Alvord Lake Bridge. In his book, Reinforced Concrete Buildings, published in 1912, which is not the most scintillating of texts…. But in it, you can detect a tinge of bitterness in Ransome’s words as he describes how his twisted rebar was “laughed down” by the technical society in California. He left for the East, thinking that his revolution of reinforced concrete would have a better chance out there. He left thinking that no one here would fully appreciate his Alvord Lake Bridge, that no one here would appreciate this literal bridge to the modern world. And looking at it today, I’m sad to say, that he was right.

Roman: 99% Invisible is Sam Greenspan and me, Roman Mars. We are a project of 91.7 local public radio KALW in San Francisco and the American Institute of Architects in San Francisco.

Comments (10)


  1. SD

    I had to rewind twice to make sure I heard it right, or didn’t dream it, as nobody else—listener or broadcaster—apparently caught it. A reinforced concrete rowboat??!??!? What’s up with that? If not a full episode, might you catch us up on that with a blog post? Enquiring minds (at least this one) want to know more about that concrete rowboat.

  2. I enjoyed hearing this on KALW this morning, and finding this blog afterwards. I’ll check it out as a possible place to scan in 3D since it’s close by.

  3. Some verifiable facts about reinforced concrete.
    Only 5-10 percent of the water in concrete goes into the chemical change that turns concrete hard. The rest of the water, about 90 percent, is called “placement water” and is only there to help concrete pour into place.
    It takes many decades for this water to leave the concrete, in which time it’s replenished by rain.
    Concrete does not “Dry”.
    All iron rebar in concrete rusts. The containment of concrete around the rebar must be strong enough to contain the pressure created by the expansion of the iron in the rust process, and when the pressure gets high enough the rusting stops.
    If the concrete around the rebar is not thick enough or strong enough you get spalling, which allows water deeper into the concrete.

    COncrete must by an inch and a half to two inched thick to have the strength to stop the expanding rust. When you see spalling you will usually see that the rebar is too close to the surface.
    Mile and miles of Florida causways were rebuilt with epoxy coated rebar, but it was discovered that small cracks or damage to the epoxy created essentially a battery and corrosion was accelerated.

    “Concrete Construction.Net”
    “Here’s what happens in the bridge concrete. When it is new, the very alkaline environment (high pH) in concrete creates a “passivating” layer around the steel reinforcing bars that prevents them from corroding. At the very high pH of the concrete, the steel cannot corrode or rust. Over time, chloride from deicing salts and carbonation from atmospheric CO2 can penetrate the concrete and destroy the passivating layer. When water and oxygen also get to the reinforcement, the steel begins to corrode or oxidize. The iron in the steel reacts with the water and is oxidized to iron oxide (rust) which has greater volume than the steel. The force of this expansion is greater than the tensile strength of the concrete and so it must crack.”

    Fascinating topic.
    Thanks for the report on Alvord Lake Bridge.

  4. I have been giving Haight Ashbury walking tours since 1992 and always mention the Alvord Lake Bridge. It is right at the end of Haight St.
    It survived both the 1906 and 1989 earthquakes quite well. Kezar Drive, the road over it, connects 2 of San Francisco’s major east-west thoroughfares, and it is not an exaggeration to say that millions of of cars have crossed it since it was built.

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