100 Year Old Bridge

Austin Highway Then and Now  [Click on the title to access the photo]

I cross this bridge almost every day. I found a photo of this bridge posted on “Back in the Day San Antonio” Facebook page and was told the location. I was intrigued since I recognized the location even after the passage of approximately 100 years. Years ago, I came across a map of San Antonio from 1935 and it indicated this bridge was on the primary road between San Antonio and Austin. In the sepia photo, you can see that a car coming in from Austin would approach the photographer and cross the Salado Creek concrete bridge and continue toward the photographer crossing in front of him and disappearing off the right side of the photo. The photo is pointing northeast. On today’s map, that old road, just off the right of the photo is called Corrine today. Corrine can be seen blending into today’s Harry Wurzbach Road leading further southwest toward Fort Sam Houston Army post. This was the 1935 prime road just outside San Antonio on the way to Austin. Today, there’s the Austin Highway which was the main road sometime after 1935 and continued to be the main road until Interstate Highway 35 was completed bypassing all of this on the way to downtown SA. The Austin Highway “new” bridge is shown in the color photo on the left.

Yesterday, I tried to find the spot where the sepia photo was taken and took a shot or two with my phone. Amazed at the way transportation routes and cameras have changed in a hundred years, it was just as amazing to see the old Salado Creek bridge still in use. Who knows the number of countless floods that have completely submerged this bridge over the decades? And still it survives. Granted, the amount of traffic today is dwarfed by the traffic crossing the “new” Salado Creek bridge on the left side of the color photo, but the old bridge is a testament to what may have been one of the first reinforced concrete bridges in Bexar County still in use. If you take the time to cross the bridge on foot, you can see indications of where old railing or road signs were once attached. You can also see in the middle of the bridge places where the reinforcing steel (re-bars) is now exposed to the surface.

The other day, I was listening to one of my favorite podcasts on my daily 12 mile bike ride that always includes this bridge. The podcast is called “99% Invisible” by Roman Mars. I highly recommend this podcast to anyone interested in obscure and fascinating stories about things that have been forgotten or unappreciated. It’s design-oriented and is my favorite podcast. Last week, I was listening to Roman Mars describing an early example of a reinforced concrete tunnel/bridge at the edge of Golden Gate Park in San Francisco called the Alvord Lake Bridge. It may be the first reinforced concrete structure in America and today looks quite neglected, but still functional. Please go to the 99% Invisible website and read or listen to Episode 81 for the story of the bridge, reinforced concrete, and how an extremely large amount of our country’s infrastructure is neglected reinforced concrete structures that need to be maintained or removed as reinforced concrete does not last forever.

Back to the story on the Alvord Lake Bridge, Roman Mars mentioned that an early innovator in reinforced concrete, E.L. Ransome registered a copyright on twisting bars of steel, now known as “rebar” to improve the adhesion of the concrete to the steel. I learned in architecture school that the elegant success of reinforced concrete is dependent on the duet between concrete and steel. First, they bond very well to each other (and interestingly, even a little bit better if the steel has a thin coat of rust) and most importantly, steel and concrete have the same thermal expansion coefficient. In other words, they shrink in the cold and stretch in the heat at the same length per degree temperature! If they didn’t they would quickly break their bond. I don’t want to give you the impression that rust is great. It’s beneficial only in small amounts and only when the concrete is cast with the steel. Once cast, care must be given to insure moisture does not penetrate the concrete or the steel does not have a connection with the ground. If these things happen, rust can become a problem that can lead to the structure’s destruction.

I noticed the exposed reinforcing steel on the old Salado Creek bridge was twisted. It was not easy to determine whether the steel was a twisted square bar or a twisted steel ribbon, but it seemed to relate to what I had heard on the podcast regarding E.L. Ransome’s patent over a century ago. Today’s reinforcing steel is different. Instead of being twisted, today’s rebar features ribs on round steel to provide a good grip with concrete.  I know there’s not much regarding a “lesson” with this post. I was just struck by realizing the age of this old bridge I ride on every day and being reminded about the historic story of reinforced concrete thanks to 99% Invisible and Roman Mars.