Modern urban bridge-building is the result of an elegant blending of art and science, and the finished product is a welcome complement to its host city’s skyline in addition to being a crucial part of its infrastructure. Types of bridges will differ based on the mechanisms by which they are supported, but there are still some common steps that must take place over the course of construction, regardless of its design.
The Bridge Foundation
The first thing that has to be plotted out and positioned into place are the load-bearing pillars that will form the foundation. These can take a number of forms and sizes, but are usually comprised of a heavy, concrete base with a cage of steel bars running throughout it for additional reinforcement. Into this, the steel pillars that will form the support towers for the entire bridge are mounted. On some projects, these pillars can rise hundreds of feet into the air, and support millions of pounds of weight without buckling.
The placement of these concrete is an incredibly important aspect of construction, and can make a tremendous difference in the overall integrity of the structure. The bedrock beneath the water must be stable enough to keep from shifting over time. When crossing points are chosen for a bridge installation over a long body of water, the firmness and mineral composition of this underlying sediment is one of the factors that go into the selection process.
Since most of the mass of this initial founding will be underwater, there is a special technique for pouring the concrete into place. A steel tube is put into place that’s the diameter of the support pillar, and water is pumped out of the center with electric motors. The steel cage that will act as the reinforcement is then inserted, and buried to a depth of between 30 and 60 feet. At this point, the concrete can be poured, and the foundation will be complete.
Suspension and Framework
The framing of the bridge, and the suspension cable system that will help to support its weight are typically built together, section by section. The flatbed framework that the actual road will lay on top of is too heavy to support itself, otherwise. Over the course of construction, every time a 50 to 100 foot extension of steel girders has been bolted into place, the suspension cable for that section that will permanently connect it to the closest support pillar is installed.
This process is repeated as many times as is necessary to span the distance, and sections are put into position evenly on each side of the support pillars, which allows for equalized weight distribution throughout the entire process. In this sense, the suspension bridge’s skeletal structure will take form from the pillars outward, and meet at roughly the halfway point in between each one.
Paving and Utilities
At this stage, the new suspension bridge is ready to be turned into a seamlessly integrated segment of the state’s roadway infrastructure, and that means adding pavement. The framework that has been previously laid provides a solid surface that the asphalt can be poured onto, and crowned in much the same way that a road on solid ground would be.
Just like a stretch of interstate highway, the bridge will require lighting. Electrical components and wiring are run throughout the hollow sections of the frame, which protects them from both the elements and from the possibility of being vandalized. This includes a particularly large set of beacon lights that will be mounted on top of the suspension towers.
The bridge is essentially ready to be open to public use once all of this is completed. Ladders and elevated walkways that will allow the state’s department of transportation personnel, and other engineers to access any part of the structure in order to perform periodic inspections are installed, and the mayor of the town will host an opening ceremony. A marvel of engineering is ready to serve the community.