![]()
MODERN CONCRETE BRIDGES
THE invention of reinforced concrete has placed in the hands of modern bridge engineers a new material in which to work, a material that, to many intents and purposes, is stone masonry, but stone masonry that has the property of offering great resistance to tensile stresses, by virtue of the steel embedded within, which is protected by the surrounding concrete from corrosion.
Furthermore, this material is plastic and can be cast into any desired form, at less expense than similar forms can be cut out of the natural stone. It is evident, therefore, that the new material offers to the bridge engineer and to the architect collaborating in bridge design, a great opportunity. That the engineers and architects have not been oblivious to the opportunity is evidenced by many beautiful structures built of this material in the last two decades, especially those located in the United States, which are, as a rule, more pleasing than the much lighter and apparently attenuated forms generally used abroad.
It was only two decades ago that the chief engineer of one of the greatest American railroad systems was quoted as saying that concrete would not be used by that company, because he did not believe, and no one could make him believe, that man could make as good a building stone as that made by the Creator. But concrete is now a standard building material of that great railroad system as of most others in the construction of bridges for which cut stone was formerly used. Unquestionably, concrete constructions do not possess the same charm as well designed and executed cut stone masonry, a truth that is explained by one writer as due to the presence of the tool marks of the craftsman in the case of cut stone structures and its absence in concrete. The tool marks express to the observer the human labor required to create the object, and give it a human interest.
The greatest architectural defect of concrete, however, is doubtless the lack of color effect in its finished surfaces, and especially the lack of color variety. The uniformity of color and texture of concrete surfaces is monotonous and displeasing.
This loss of the charm of the natural stone wall, however, is balanced by the economy of the material, allowing its use in many places, especially for small bridges, where natural stone could not be used or afforded and where cheap, unsightly steel trusses would formerly have been built.
In all fairness it should be stated, however, that the ugliness of the small steel bridge is due, not to any inherent defect of the material, but to the utter lack of any attention to considerations of beauty on the part of designers, such lack being caused by the former commercialization of the art, practically all designs for small structures, and many for large ones, being made by the fabricating companies, under competitive conditions that precluded any consideration of art or taste. Such a system, while possibly resulting in the greatest economy of first cost, is essentially bad, because it results not only in the total elimination of artistic considerations, but also results in the production of structures that are weak and short-lived, and more expensive in the long run than would be the case if better designs were adopted at perhaps somewhat greater first cost.
Furthermore, a beautiful and pleasingly designed bridge has a certain value to a community not easily expressed in dollars, but which pays dividends in pride in one’s community, a pride which contributes to human happiness and contentment.
Quoting the editor of “The Builder” (Aug. 27, 1926), “The Engineer’s artistic failures occur when he has not interested himself in the appearance of his building and allows himself to be governed blindly by economy.”
In studying these illustrations of concrete bridges, it will be seen that, in order to obtain the most pleasing results, concrete must be treated as a different material than natural stone and that the obvious forms of cut stone masonry should not be imitated in using the plastic material. The earlier examples committed this error extensively, but later designs are better.
One of the first large concrete bridges to be built in this country is The Connecticut Avenue Bridge at Washington, D. C., completed in 1904, after designs by George S. Morison, noted American bridge engineer and designer of many railroad bridges; and built under the direction of W. J. Douglas, engineer, and E. P. Casey, architect. This bridge is 1341 feet long, 120 feet high and 52 feet wide. It contains seven semi-circular arches, five of which have a span of 150 feet. These arches carry six small spandrel arches, also semicircular. The parapet is composed of concrete posts with a bronze railing. The material of which the concrete of this imposing work is made is crushed granite, and exposed surfaces were carefully tooled when cured, exposing the aggregates of the concrete. The quoins of the piers wer...
