Bridges and culverts can be great indicators of when a highway was built and by whom it was built. In California, there are a few different styles of railing which can be used to put a relative date on a particular bridge when they don’t have date stamps on them. Other indicators may be used when the bridge rail has been modified or even replaced by modern rail. Styling elements on the superstructure vary depending on age. Older bridges tend to be more ornate than more modern structures. This page will serve as a guide to these styles and help to increase understanding of what goes into the design of these bridges.
The type of bridge built is something that is more dependent on location rather than age, though that can make a difference to an extent as well. Older bridges varied quite a bit in their design, with steel truss and concrete arch spans being more common than today. Bridge types vary quite a bit even today, but tend to conform to a few simple concrete designs.
Let’s start with location for a bridge. Modern bridges are built to conform to the roadway alignment, to put it simply. When a roadway alignment is chosen, it is based upon many factors, but when a bridge is needed for the roadway, it is built as a part of that alignment. In contrast, earlier bridges were built where it was easier to construct one and the roadway alignment was seemingly an afterthought, at least in appearance. This resulted in roadways which made sharp turns at the ends of bridges, sometimes even right angles. By the mid-1930’s, this practice was largely abandoned in favor of the more modern method of bridge placement. When older bridges are replaced, the roadway alignment is also modified, sometimes because of “stage construction” where the new bridge is built adjacent to the old one in pieces or when the old alignment was too curvy and needed straightening.
Now, as to bridge types, that is an interesting subject. Bridge types used to be more dependent on the lesser strength of materials and as a result, were shorter or had multiple spans. Steel truss bridges were more common for longer bridges as they had a clear path underneath for whatever needed to pass underneath. Steel truss bridges also contain a few clues to their age. Lattice girders are easily recognizable on older bridges where steel I-beam girders mark newer structures. Older steel bridges also had rivets where newer ones have bolts. Very old steel bridges are much thinner than new bridges as well.
Concrete bridges, such as arch and girder bridges are a bit more difficult to date when the date stamp is missing. Starting with arch bridges, there are some things to look out for. Older bridges are usually more ornate, with more detailing around the vertical columns connecting the arch ribs and the piers at the base of the arches. Some of these bridges, mostly pre-1933, will have the date stamp at the top of the central arch, if it is a multiple arch bridge. From about 1940 to 1950, arch bridges still retained some detailing but tended to be much less ornate. After 1950, most of the detailing was gone, with the bridges being much cleaner and smoother in appearance.
Concrete girder bridges are another story, however. From their beginning, they tended to lack any detailing beyond the bridge rail. Bridges from about 1928 to 1949 did have a noticeable arch to their girders, which was needed at the time for additional strength. Modern construction methods, such as prestressed girders, negated the need for these arches. Bridges earlier than 1930 had far more girders than later bridges also due to the weaker materials used. Prior to about 1935, some bridges had date stamps on the girders, similar to the way arch bridges had stamps on them.
How do you use bridges as an indicator? The first, and easiest, method is through the railing. Bridge railing, in California, has distinct styles for different eras.
Early 1900’s to 1920’s bridges varied greatly in railing style. Some early bridges have a solid concrete rail where others have pipe railing. These variations in style stem from the differences that each county had in their bridges. Back then, the counties generally constructed bridges along State Highways. Each county had a different “standard” style, so each one is fairly unique.
1910’s to early-1920’s railing appeared similar to wooden railing in most cases, but was all concrete. It consisted of narrow posts with square cross beams that were rotated about 45 degrees. This style was in heavy use in California and is a great find when the railing is still intact. In wetter or snowier regions, the railing tended to spall and break apart more easily as the internal rebar rusted.
The mid-1920’s to early 1950’s brought a standardization to bridge railing throughout the state. When concrete was used, the most common style was known as “Mission” style, which consisted of small arches repeated along the railing. While this style was used earlier, older designs tended to either be thinner or have smaller openings.
In the late 1950’s, a modified version of the “Mission” style railing came out. This new style had openings with more rectangular openings with rounded edges. It didn’t have a very long run, however. Judging from known bridge dates, it ran from about 1955 to 1960.
Now, with metal railing, there have been a few more styles.
Late 1930’s to late 1940’s, steel railing had a more squared style, with flat tops. This changed in the 1950’s to a similar style but with a rounded top rail. This change was for safety, as people had been walking on top of the older rail and falling off bridges.
By the 1960’s, metal railing took on a very different look. Experimented with as early as 1951, then known as “skid rail”, a tubular rail became prominent. There were a few variations of this style, mostly due to where it was being used. A “double” rail, with two rows of tubes stacked on top of each other, was common where there was a lot of pedestrian traffic.
In the 1970’s, railing became flatter with a double rail used on some freeway connector ramps and, like in the 1960’s, bridges with more pedestrian traffic. By the later 1970’s, new bridges lost their metal railing almost entirely and had simple concrete rail similar to median barriers. The earliest use I have seen for this style dates to 1968.
Wood rail had few variations over the years. It was most commonly used in the 1930’s and 1940’s in an effort to save resources and costs during the war years as well as during the Depression. The main style used a single row of cross beams. It varied as to why or when a double beam configuration was used.
With all the different styles of bridges and railings, as well as outliers from the standard, it can be difficult at times to put an age on a bridge, without a proper bridge log. Hopefully, with this guide as a resource, bridges can be dated with more accuracy, at least within a few years of the actual date. This guide is incomplete, but will be augmented as additional information and photos are acquired.