Super Structures by Denny Mark

Author:Denny, Mark [Denny, Mark]
Language: eng
Format: epub
Publisher: The Johns Hopkins University Press
Published: 2010-06-04T16:00:00+00:00

FIGURE 5.7. The High Bridge, Manhattan, an open spandrel deck arch bridge. Thanks to John R. Plate for this image.

FIGURE 5.8. A half parabola (dashed line) and a half catenary (bold line), with parameters chosen so that both curves start and finish at the same points.

All this geometry just emphasizes the role that mathematics plays in the physics of bridge funicular shapes. However, modern bridge-building materials are strong enough in tension and compression that the bridge arches do not have to be funicular. When we get to suspension bridges, the story will be different.

Trestle and Box Girder Bridges

There are three primary load-carrying systems for bridges. Loads can be borne by arches in compression, as we have seen. Loads can be suspended from cables or stays in tension, as we will see later in this chapter. Here, I will explain the third-and in some ways intermediary-option: loads can be transmitted through beams or trusses that withstand both compressive and tensile stresses. Apart from the short and simple beam bridges and truss bridges of earlier chapters, there are two significant variants that deserve mention here: the trestle bridge so characteristic of nineteenthcentury America and the box girder bridge of modern Europe. 8

Trestle bridges consist of a lot of short spans that are supported by splayed vertical members. They are normally constructed for railroad use, as is the bridge shown in figure 5.9 . The old wooden bridges thrown up rapidly may have lasted only a short time, but many of the more carefully constructed wooden trestle bridges of the late nineteenth and early twentieth centuries are still with us today. These timber trestles were built from creosoted logs bolted together to form a complex three-dimensional truss structure. If the foundation was firm and the bridge engineers knew what they were doing, such bridges were strong and robust. They easily supported the weight of a locomotive (though we will look at some of the issues that arise from dynamical loading later in this chapter) and were not susceptible to wind loading because of their open scaffold structure. A significant advantage of trestle bridges is that they require no centering: as the bridge is built, each section can support its own weight. As you may imagine, these bridges were inexpensive and relatively quick to build.

Trestle bridges-some of them very long-are still built today, though of steel rather than timber. They are not elegant and will win no awards for architectural style or span length, but they are functional, light, and sturdy. 9 In Western Canada, where I live, many creeks and coulees were spanned by CPR (Canadian Pacific Railroad) engineers with steel trestle bridges in the first decade of the twentieth century. The biggest is the High Level Bridge near Lethbridge, Alberta, which consists of 33 steel truss towers supporting a railroad deck more than a mile long and 300 feet above the valley floor. This bridge was built in 2 years (completed in 1909) and is still used today.

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