【Polymers in the News】The Use of Fiber-Reinforced Polymer Materials in Bridges

On May 27, 1937, the Golden Gate Bridge, spanning the Golden Gate Strait, was officially opened to traffic. Since then, the Golden Gate Bridge has become one of the most famous landmarks in the United States. In 1870, it was realized that there was a need to build a bridge across the Golden Gate Strait to connect San Francisco with Marin County on the other side. However, it was not until half a century later that a structural engineer named Joseph Strauss came up with a viable design. The plan was revised several times, and the suspension bridge design was finally approved; it took another four years before the bridge was completed. When the Golden Gate Bridge was first completed, it was the longest suspension bridge in the world – its main structure was supported solely by steel cables on either side, with no intermediate supports. The entire building is a miracle created by both technology and funding. The construction cost of this bridge back then was around $37 million; today, the cost of building the same bridge could reach one billion dollars. (Schematic diagram of suspension bridge structure. Due to its innovative structure and extraordinary appearance, the bridge is recognized by the international bridge engineering community as a model of beautiful architecture. It is also one of the most photogenic bridges in the world. ) The world’s largest suspension bridge: As a suspension bridge, the weight of the entire bridge deck is distributed to the two piers through the suspension cables, with no supports in between. The idea of suspension bridges dates back to ancient times. Hundreds of years ago, people used ropes to cross valleys or rivers; in modern times, chains have become a more reliable material. The Brooklyn Bridge in New York City, which was put into use in 1883, was the first to employ a wire suspension design and became the standard for suspension bridges thereafter. The two main cables of the Golden Gate Bridge are likely the only parts of the bridge that have not been replaced during its decades of inspection and maintenance. Each main cable is composed of 27,572 steel bars as thick as pencils, and the total length of all these steel bars is nearly 130,000 kilometers. The method of “binding together many strands to form a rope” is used because it is almost impossible to manufacture a single long and thick cable. More importantly, a problem with just one cable can lead to catastrophic consequences. Tens of thousands of fine steel rebar make maintenance and accident rescue easier. As a vital route connecting San Francisco and Marin County, 112,000 vehicles pass over the Golden Gate Bridge every day. Given the unique geographical conditions of the Bay Area, the bridge needs to be capable of withstanding strong winds, flowing water, and potential earthquakes. San Francisco is located at the boundary of two tectonic plates, and plate activity is a factor that must be taken into account in bridge design. To withstand the effects of natural forces, shock absorbers are installed at each end of the bridge to absorb the energy from strong winds and earthquakes. These shock absorbers are specially designed; a rubber coating surrounds a lead core to form cylindrical dampers with a diameter of one meter, turning the forces that could otherwise destroy the bridge into nothing.

Bridge inspection: The construction process of traditional projects comes to an end with its commissioning. However, the Golden Gate Bridge still requires ongoing maintenance after construction is completed. For 80 years, specialized maintenance workers have replaced corroded and damaged parts and repainted the bridge to preserve its original appearance. Maintenance work must be carried out in accordance with strict standards. For example, when the bolts connecting the various parts of a bridge need to be replaced, workers have to remove two bolts at the same time to ensure that the bridge remains unaffected by strong winds or earthquakes. There are also issues related to structural maintenance. The passage of time and changes in temperature can cause the main cables and slings to stretch or contract, requiring regular inspection and re-tensioning. This type of maintenance work is aptly described as “tuning” the bridge, similar to how musicians tune their instruments.

What would be different if the Golden Gate Bridge were rebuilt? Due to the high maintenance costs, some have suggested simply rebuilding the Golden Gate Bridge. Putting aside the political feasibility for now, what kind of design will engineers come up with? Advances in technology have provided lighter materials. Fiber-reinforced polymers (FRP) can replace bulky reinforced concrete. Currently, the self-weight of the bridge accounts for 70% to 80% of its maximum load-bearing capacity. By reducing the weight of the materials, less pulling force is required for the bridge, which in turn lowers the overall construction cost. There are already cases of FRP materials being used in bridge construction, such as the Market Street Bridge in West Virginia. FRP combines glass fibers or carbon fibers with plastic resin; its weight is only 1/4 that of concrete, yet its strength is 5 to 6 times greater than that of concrete. Furthermore, the bridge cables are likely the part that engineers want to replace the most. The steel used in the Golden Gate Bridge is prone to corrosion, and its performance deteriorates over time under the humidity and temperature conditions of the Bay Area. Carbon fiber cables have more stable properties and are now widely used around the world. The new material weighs only 1/4 of that of steel, and can also be used in other parts of bridges, such as traffic lanes. Composite material pavements can reduce the self-weight of bridge decks to 1/5. The significant reduction in weight allows engineers to use cable-stayed bridges instead of suspension bridges. The difference between the two is that a cable-stayed bridge eliminates the main cables, with the deck being connected to the piers directly through stay cables. The first highway cable-stayed bridge to use carbon fiber reinforced polymer (CFRP) was the Stork Bridge in Switzerland, which was opened to traffic in 1996. Cable-stayed bridges can achieve longer spans than suspension bridges, and their support design as well as the structures connecting them to the banks can be further simplified. Building bridge piers in shallower waters closer to the land can also avoid difficult deep-water operations, a problem that plagued the construction of the Golden Gate Bridge back then. The damping system can also be replaced with a new design to achieve better seismic performance. Such an improvement could prevent the tragedy of the Washington Tacoma Narrows Bridge being blown apart by the wind. As a mere speculation, the Golden Gate Bridge is still in good condition at present and could likely serve for another eighty years. California **takes great care of this bridge to ensure it remains undamaged due to normal wear and tear, as well as wind and earthquakes. Even with cheaper options and better design solutions, no one will really try to replace this unparalleled California landmark. Its cultural significance is in no way inferior to its economic value.

What matters is history~~ practical value is secondary~~