Getting Over a Bridge:
A Description of a Suspension Bridge
We know that the Golden Gate Bridge in San Francisco, California is a suspension bridge. But what exactly is a suspension bridge and how does it differ from other bridge types.
What are Suspension Bridges?
A suspension bridge is a type of bridge in which the deck in held up by suspension cables that are placed above. It consists of four major components: girders/trusses (deck): suspension cables (both the main cables and those vertically suspended): the main towers: and anchorages (for the cables that are placed on each side of the bridge), figure 1. The main cables are tension members constructed with high-strength steel which is highly efficient for the loads imposed upon it, figure 2. The issue of buckling is also not a concern with these cables. Having these properties, the bridge’s deadweight can be reduced significantly. As a result, longer spans are made possible. Figure 1 shows the anatomy of suspension bridges. A typical suspension bridge will have two-four parallel cables that run along the edges of the deck that it supports. The main cables run from one anchorage to the other across the deck and span over the main/intermediate towers. Vertical suspension cables, also called hangers, run from the main cable to the deck and support the deck.
Construction of a Suspension Bridge
The construction of a suspension bridge is never simple and straight forward. However, there are some phases that can help to keep track of the process of construction, also shown in figure 3. The first phase consists of setting the anchorages into solid ground. It is important that the anchorages be placed on bedrock or other strong soil strata as they support a big proportion of the decks weight. The second phase consists of constructing the foundation for the towers. Usually, the foundation for the towers are placed in water. In this instant, the foundation site should be dammed, and the water must be pumped out. Following the construction of the foundation is the erection of the towers. The height of the towers determines the span length of the bridge; higher towers allow for a longer span. The next phase consists of installing the suspension cables between the two anchors and over the towers in which the pylon saddles are placed to allow the cables to run freely. Lastly, the hangers (vertical suspension cables) are attached from the main cable to the deck.
Figure 3: Showing the six main stages of a suspension bridge construction.
Load Transfer
All structures impose a load on the earth and on each other that must be accounted for to ensure the structural integrity and stability of the structure itself. Therefore, special precautions or measures need to be taken to ensure that the structure does not fail. We first begin by examining the load transfer or load path in a suspension bridge. All components work to uphold the deck. Thus, the load starts at the deck and moves to the hanger (vertical suspension cables), from which it is transferred to the main cable and distributed amongst the towers from which it reached the anchorages and into the ground. For all structures, the load path must end by load being transferred to the ground. Figure 4 shows the forces that exist in a suspensions bridge and can be used to determine the load transfer from each component.
Conclusion:
The construction of a suspension bridge is not as easy as shown. There exist various calculations that must be completed to ensure its stability. However, a general method of constructing suspension bridges is shown. This should not be used as an extensive understanding of suspension bridges.
Work Cited:
- Zhang, W., Qian, K., Tian, G., & Liu, Z. (2019). An analytical algorithm for the pylon saddle pushing stage and distance during the suspension bridge construction. Advances in Structural Engineering, 22(15), 3290–3305. https://doi-org.ccny-proxy1.libr.ccny.cuny.edu/10.1177/1369433219862436
- Lin, W., Yoda, T., Martin, B. T., & Maranda, M. D. (n.d.). Suspension Bridges. Retrieved December 1, 2019, from https://www.sciencedirect.com/topics/engineering/suspension-bridges.
- Li, C., Li, Y., & He, J. (2019). Experimental study on torsional behavior of spatial main cable for a self-anchored suspension bridge. Advances in Structural Engineering, 22(14), 3086–3099. https://doi-org.ccny-proxy1.libr.ccny.cuny.edu/10.1177/1369433219857840
