No matter where you live, chances are high that you've heard of the name of Golden Gate Bridge. The Golden Gate Bridge floating over the Pacific ocean is one of the longest and tallest suspension bridge in the world.
Your eyes may be drawn to its beautiful suspension cable system. What would happen to the bridge if the cable system was not present? In short, it would be a catastrophe(refer fig 1a & fig 1b).
In this article we will explore physics behind suspension bridges & the mesmerizing engineering facts of the Golden gate bridge. So let's start with its main cable.
The Golden Gate Bridge is a suspension bridge. A highly simplified suspension bridge can be constructed by erecting two towers at both the ends of the ocean and suspending a long cable between the towers as shown in fig 2a. This cable can be approximated as a parabola. Now, let’s attach a concrete road deck with pillars. This clearly provides support to the end of the road deck. When we connect the suspension cables between the main cable and the road deck, the bridge is also supported along its length, so the road deck won’t fail. This is the basic design behind the suspension bridge as shown in fig 2b.
Do you know the distance between two coastlines? That distance is 2.7 kilometres. Before exploring more about the golden gate bridge, let’s first understand why the engineers choose a suspension design for this bridge. Were any other designs viable for this location? Let's check commonly used bridge designs.
Let's construct a conventional beam bridge(refer fig 3a). The road deck is supported by various piers. The presence of these piers blocks the movement of ships underneath, constructing piers 300 feet deep in the water would be extremely costly. Thus, the beam design does not make sense.
Now, let’s consider an arch bridge(refer fig 3b). This would definitely provide passageways for ships. However, to maintain the arch shape, the bridge would need to be extremely high. Such a structure would be quite complex to construct and costly as well. That’s why engineers opted for a suspension design for this site - a bridge that could overcome all the drawbacks we discussed.
Now, let’s get into the design details of the suspension bridge(refer fig 4a). This design has one glaring issue. If we construct the bridge as shown in figure, the towers will bend inward. The main cable is under a huge tensile load. This applies a force on the tower.
When we resolve tensile force, you can see that there is an imbalanced horizontal force acting inward on the tower, which explains why the towers bend. Can you find a solution for this issue? To cancel this horizontal force, we need the same force acting in the opposite direction. The straightforward solution is to extend the main cable and anchor it down to the ground via an anchorage system(refer fig 4b).
However, we can optimize the financial resources needed to construct this bridge with a simple idea. All we need to do is move the towers closer to one another. Now, the length of the unsupported bridge deck is reduced(refer fig 4c). Due to this tension in the cable will be reduced. This will obviously lead to cable with less cross section area.
Do you know the width of the main cable? The width of the main cables are more than half the height of the average human! As a tourist attraction, a piece of this impressive main cable is demonstrated near the Golden Gate Bridge(refer fig 5).
If you construct the bridge with this exact design(refer fig 6a), it will experience a premature death. Can you guess why this would be the case? Connections are the weakest part in any structural system. The direct connection of the steel suspenders with the concrete deck will lead to the formation of cracks on the deck since concrete is brittle in nature(refer fig 6b).
Engineer’s decided to connect the suspenders to a steel structure. Steel to steel connection is always strong. The details of the connection between the steel suspenders and steel deck is shown in PIP of fig 6c. The road deck is placed on this structure. Engineer’s kept the width of the road to 27m to account for current and future traffic demands.
We have developed the design structure of the bridge with steel. Our bridge looks perfect now! But is it ready to support vehicle movement? Not yet. We must first tackle another major engineering challenge: thermal expansion(refer fig 7a). The concrete and associated steel structure will expand or contract based on environmental temperature variations. If we had constructed this bridge as a single piece, during a hot sunny day, the bridge would expand and cause tremendous stress on the tower as well as on the road. Eventually, the bridge would experience damage.
If you have ever visited the Golden Gate Bridge, you may have noticed peculiar connections on the road. These connections, called “finger expansion joints,”(refer fig 7b) were engineers' solution to solve the thermal expansion problem.
Engineers divided the deck into 7 separate pieces. You can see the bridge has 3 cradles(as shown in PIP of fig 8a). The finger expansion joints are installed between the gaps. During an extreme temperature increase, the length of the road deck increases, and these joints move by almost 4 feet! What an elegant solution for a serious issue!
Has the thermal expansion problem been solved? No, there is still a small problem remaining to solve. The thermal expansion of the steel is slightly higher than that of the concrete. This differential expansion can cause trouble for the concrete deck, which is composed of a mixture of concrete and steel bars, but this expansion issue is negligible when the length is small(refer fig 8b). This is why the Golden Gate contains tiny expansion joints every 50 feet.
I hope you have learned the basic physics behind this amazing bridge. In the next article we will explore some amazing construction details and challenges they were faced.
Thanks for reading!