How the design of bridges relates to torque

Physics is the fundamental tool used in our lives. However, as you may not notice, the architecture of buildings, the design of the shape of our cars, and many other fields related to engineering are all applied physics. In this case, our topic is how the design of bridges relates to torque.

As you can see from the picture, there are two pillars that support the bridge from falling into the river. The two supports are very crucial as they must provide the same normal force in order to keep the net torque zero. That means the bridge should be firm and secure, not rotate around.

However, things are not so simple as just to keep the bridge secure and not rotate. Bridges are designed to be used by cars and passengers that want to cross the river safely. Therefore, engineers are faced with another challenge which is to know how much force will act on the supports when cars are passing by. We obtained a simple scenario where only one car is crossing the bridge. 

Suppose that the car weighs 1ton (1000 kg), and the bridge weighs 100tons (100000kg) how much force will each of the support exert in order to keep the net torque= 0?

we set point A to be the pivot point
Let's calculate Fb first:

(1000kg*9.8m/s/s*6.0m) + ( 100000kg*9.8m/s/s*10.0m)= Fb*20.0m

Fb= 492940N

Now we can calculate Fa:

Fup=Fdown

492940N+Fa=9800N+980000N

Fa= 496860N

It turns out that each of the support is enduring so much force. Although it seems quite easy to calculate and see the results for this simple scenario, in real world engineers have to consider other factors that will affect the stability of the bridge such as: wind, current of the river, rain or snow that will accumulate on the surface of the bridge, and many other potential situations. Therefore, it is important for engineers to think of good  framework that can better support the bridge.

Through hard work and research, the engineers finally came up with many different types of bridges. For example, a suspension bridge. Suspension bridges use thousands of steel cables to "pull up" the bridge from falling. With the cooperation of the cables and the supports of the bridge, the load capacity of the bridge increases.  A famous and well known suspension bridge is the Golden Gate Suspension Bridge located in San Francisco, California, USA.

USA's Golden Gate Suspension Bridge

Another way to increase the carrying capacity of the bridge is to have multiple bridge piers. Adding more bridge piers means that there are more supports, so the weight of cars and the bridge itself are split and  shared onto each bridge pier, each support doesn't have to exert as much force to support the bridge as compared to only two bridge piers.

China's Qiantang River Bridge

Engineers use the principles of torque to design bridges, it takes countless experiments to eventually come up with a good design. We must study hard to carry on the knowledge of physics and continue to innovate.