Why Modern Bridges Cause More Suspension Bridges

Compared with axial tension, bending is an extremely low-efficiency bearing. To a certain extent, improving structural efficiency is to convert bending into tension or pressure as much as possible. If you can do as much as possible to reduce the weight of the structure, it would be even more perfect. The arch structure is an example of transformation into compression, and the suspension bridge is an example of transformation into tension.

A map is the most common beam bridge, completely relying on the bend bearing. This form is very common, almost all of which are subways, elevated and small road bridges. On the right is the stress distribution of its section, with the upper and lower surfaces being large and the middle position being almost zero. In other words, the stress in the entire section is not evenly distributed, but there is a “water bucket effect”. Although there is almost no stress in the middle position, as long as the upper and lower edges reach the limit, the entire section will not be far away from the damage. The stress at the upper and lower edges is the shortest piece of wood in the bucket.

Since the middle section is almost zero, why not omit them? Then, there will be b-beams like this. Those places where the stresses in the middle of the section are small are eliminated and their weight is reduced. Tensile stress is concentrated on the upper and lower edges.

To further expand this trend, that is, to further lattice the original beam structure, remove the small part of the stress, and retain the most basic parts, we have obtained the truss structure of the c map. The d diagram is its approximate internal force distribution, with red tension and blue pressure. Its cross-section distribution is more reasonable, the upper chord member is under pressure, the lower chord member is pulled, and the unused parts are all empty. The famous Nanjing Yangtze River Bridge is such a structural form.

If this optimum trend is to be achieved, then it will reach the e-beam suspension structure. The entire suspension is subjected to the same amount of tension. The tension of the entire suspension is balanced by the anchorage at the support. In fact, this structure is very easy to understand. Think of the e-map as a clothesline with 11 clothes hanging on it. The ends of the clothesline need to be firmly bolted to the wall or pillar.