Humans have been building bridges for thousands of years. At first, they were built using trial and error. Recently, humans began to use math and physics to create bigger, safer, and more economical structures. Paired with new materials, economic prosperity, and the development of the Bridge Engineering profession, this has led to the creation of some truly magnificent structures.
When the Brooklyn Bridge was completed in 1883, it had the longest main span (distance between vertical supports) in the world. Since then, this feat has been surpassed hundreds of times. In 1937, using the same type of support system, the Golden Gate Bridge was constructed with a main span that is 2.5 times longer than the Brooklyn Bridge. Around 60 years later, the Akashi Kaikyo bridge increased that by 1.5 times.
The five bridges with the longest mains spans in the world are presented in the below table. All of these are suspension bridges, similar to the Golden Gate and Brooklyn bridges.
The theoretical limit on the length of a suspension bridge is governed by the self-weight of the support cable and deck system. Theoretically, the longest possible length of a suspension bridge, using present day materials, is approximately 5 km. This is 2.5 times longer than the longest bridge built to date. However, there are many practical restraints that would make the construction of such a long span nearly impossible.
In the past century, the materials and methods used for construction have developed at a slow pace, and there have been few significant changes in the general form of long span bridges. Some may believe that engineers have reached the peak performance of construction materials, while others may argue that the current base of knowledge is sufficient for the majority of bridges that society needs. So why would engineers continue to push the boundaries of bridge construction?
Because, we are engineers – and that’s what we do!
Engineers are always looking for ways to improve bridges, either by building them faster, cheaper, or safer. However, only a small group focus on a very interesting challenge – how to make them longer. In a recent paper (link to paper), an analytical model was developed using a numerical layout optimization procedure to study the theoretically optimal form that a very long span bridge should have. One of the most interesting concepts that comes out of this paper is the split-pylon form, presented below. This bridge is shown with a main span of 5 km, which is 2.5 times longer than the longest bridge ever built.
This new structural form would be difficult to construct using present day techniques, as it may require significant temporary supports when erecting the split pylons. However, the final optimal form provides a brand-new artistic form for a bridge – similar to the forms that became popular after the development of suspension and cable stay bridges. This balance of engineering optimization and form need only find a balance with economy, to become a truly plausible option.
At the very least, this paper significantly adds to our understanding of how long span bridges can be built using present day materials. It also offers hope to the bridge engineering profession, and opens up many new potential problems, and exciting challenges, that we get to deal with.
Even if a bridge like this never gets built, it will help to inspire others to study the feasibility of new structural forms for long span bridges. And that, is very exciting!