Suspension Bridges

A suspension bridge, by definition is a bridge where cables ( ropes or chains) are strung across the river (or whatever the obstacle happens to be) and the deck is suspended from these cables. Many aspects of physics such as: tension, shear (hopefully not), compression, torsion, and resonance play very important roles in the success or failure of a suspension bridge.

Tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. For example, this happens on a suspension bridge when the main cables support the bridge from shearing. The main cables are what keep the bridge from falling, they are joined to the ground by bars of steel, titanium, and many other types of strong bars. As you can see, by definition, the suspension bridge must be lofted over the object or body they need to clear and held sturdy by cables. If you take away or downsize/change the material used on the cables, then there is nothing else supporting the bridge. Tension is measured in newtons (pounds-force) and the tension is always parallel to the object it supports. So if the cars that go on your suspension bridge total up to weigh more than the weight or strength of your cables and base, then the cars will capitalize and over weigh the bridge, bringing it down. This is where material and strength of the cables come into play.

The next aspect of a suspension bridge is torsion. Since suspension bridges are supported by cables and are suspended above the ground, high winds are a very huge factor in much larger bridges. High winds apply torsion to many bridges. Torsion is the rotational or twisting force, which has been effectively eliminated in all but the largest suspension bridges. So, wind produces torsion which nearly twists or “rocks” some bridges. This plays a big part in larger suspension bridges that cover huge landscapes, rather than shorter ones. I did not see any torsion in my project I did, but I did witness something similar. When I was building my bridge, one of the cables got cut loose when the car was riding by on the bridge. This was caused by the torsion of an unequal force being placed on my bridge. I later realized that these strings were two different types. One was thick and one was thin. This helped me realize the torsion that occurred on my suspension bridge when the string snapped and twisted, as well as the center of the cardboard. In reality, all suspension bridges have deck-stiffening trusses. These trusses, as you may guess, reduce the aerodynamic reaction of wind on a suspension bridge which creates torsion. Though these trusses reduce the torsion, these alone don’t have a tremendous effect on the torsion that is applied to suspension bridges of incredible length. Many suspension bridge models are tested in wind tunnels to make sure they can withstand the wind tension on it. Aerodynamic truss structures, diagonal suspender cables, and an exaggerated ratio between the depth of the stiffening truss are some of the main ways that civil engineers design their suspension bridges to reduce the effects of torsion.

My next aspect of suspension bridges is compression. Compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers or huge bars of steel, which dissipate or eradicate the compression directly into the earth where they are firmly entrenched. So, compression is basically the downward force applied to the suspension bridge from cars or other objects that go on the bridge. Isaac Newton’s third law of motion is that, for every action there is an equal and opposite reaction. This is the exact force that is being applied to the truss and towers of a suspension bridge. On suspension bridges, the cars going over the bridge is the action. The re-action is the force that is being “compressed” on the base and towers of the suspension bridge which keep it from collapsing. The towers are the only substances on the suspension bridges which support compression. The anchorages, cables, and suspenders are the structures that support tension. As you can see, each structural part of a suspension bridge has its own very important purpose which without, would definitely be the end of a suspension bridge. In my opinion, one of the most important structures of a suspension bridge are the towers. They reduce compression and sometimes act as the trusses which keep the suspension bridge safe from torsion as well.

Another aspect of suspension bridges is resonance. Resonance is a vibration in something caused by an external force that is in harmony with the natural vibration of the original thing and is a force which, unchecked, can be fatal to a bridge. These resonance vibrations travel through suspension bridges like waves and in most cases can completely tear a bridge apart. For example, the Tacoma Narrows Bridge in 1940. The truss of the bridge was not strong enough, this wasn’t the full reason of the collapse but it was a factor. The major factor of the destruction was the high wind and how vulnerable the bridge was on that day. The wind hit it at the perfect angle and at the perfect time that it sent vibrations of immense sizes through the body of the bridge which took it down. To prevent this from happening, dampeners are placed in bridges. These dampeners interrupt resonant sound, which intern, stops vibration completely. The trick is to place these in certain spots of overlapping plates which increase friction to reduce the frequency and strength of a resonant wave.

My final aspect that effects the efficiency and durability of a suspension bridge is shear. Shear is also known as stress, and is quite similar to compression. Shear is also caused by wind (known as wind shear). It is when something is just completely pulled apart by unforeseen elements. For example, wind or water can effect all suspension bridges and could be their demise. This is also, one of the other factors that led to the destruction of the Tacoma Narrows Bridge. Its light weight, design flaws, flat-plated girders, and high length to depth ratio made it very susceptible to wind shear.

With all the new technology we have discovered from the mistakes we have made with past suspension bridges, civil engineers have gathered up a tremendous amount of information which can help us create more stable and durable suspension bridges. Technology has advanced so much that disasters like the Tacoma Narrows Bridge are a thing in the past. Something we will be guaranteed to never see happen again.