However, because a bridge has a considerably more stiff structure, the torsional effects are significantly more severe than those of a wrung dish towel. Torsion is zero at the center of a bridge, but it is greatest on the edge of the item it is acting on. Shear is a force that pushes an object's ends in opposite directions. It can be either tensile or compressive, depending on how it is applied.
Thus, shear is much more severe in a bridge than torsion because every aspect of the structure will experience some degree of shearing. Even the joints between the stones will shear against each other.
The most common type of joint used in bridge construction is called a mortise and tenon joint. The pieces of wood being joined are shaped like teeth with a gap in between them. The mortiser tool is used to drill holes through both pieces of wood, then lever one piece of wood away from the other to allow for insertion of the tenon tool, which expands the end of the hole out further. The tenon is then pulled back into the hole, locking it into place.
This joint suffers greatly from shear stress because the forces pushing up on one side of the joint will also push down on the other, causing the joint to fail. A simple way to prevent this kind of failure is by using a dovetail joint instead. This joint consists of two parallel strips of wood with notches cut into them.
Torsion. A force that twists. Arch. A bridge that is extremely strong in compression. Bending stress will cause an arch to twist.
An arch bridge is one where the main support is arches. The word "arch" comes from the Greek word archon, which means "ruler." Thus, an arch bridge is one that has rulers, such as large pillars, supporting it instead of beams or trusses.
The twisting of an arch bridge is when the top chords of the arches do not lie in a straight line but are at an angle to each other. This happens because the tension of the cables acting on the tops of the arches tries to make them lie flat, but since they aren't horizontal, they'll tend to curve away from each other.
The effect of the twisting of an arch bridge is that it puts stress on the joints between the arches and the piers (the base plates under the arches). If these joints are not strong enough, they may break under the pressure created by the weight of the vehicle crossing the bridge.
Arch bridges were most common before the advent of the beam bridge.
With this information, they use aerodynamic truss systems and diagonal suspender cables to reduce the impacts of torsion. Shear stress is caused when two attached structures (or two portions of a single structure) are pushed in opposing directions. Shear force may physically shred bridge materials in half if allowed uncontrolled. It is important to note that bridges must be designed with adequate shear strength in order to avoid damage to or collapse of the structure.
The three main types of forces on bridges are weight, wind, and people. All bridges must be strong enough to support their own weight as well as the load placed upon them by any vehicles crossing over them. The amount of weight a bridge can bear without failure is called its capacity. The maximum load a bridge can safely carry depends on how much stress it receives from the forces acting upon it.
Weight causes stresses throughout a bridge. As a vehicle crosses a bridge, its weight adds new loads to some parts of the structure and removes them from others. This repeated loading and unloading causes stress to build up in bridge components. Over time, this may cause damage or destruction of the material itself. For example, heavy trucks driving over an unpainted wooden bridge will wear away at the wood's surface. This will eventually lead to structural failure if not repaired.
Wind causes stresses to develop in bridge elements due to the force of wind acting upon them.
Torsion, for example, is a major challenge for engineers who construct suspension bridges. When severe winds drive the hanging highway to swirl and twist like a rolling wave, this is what happens. As we'll see on the following page, Washington's Tacoma Narrows Bridge was damaged by torsion, which was produced by another significant physical force.
Suspension bridges are twisted machines. If they were not, then when wind blows across their surface they would make a constant uniform motion up and down or from side to side. But because of their twisting structure, wind forces them to spin like a wheel. This makes life difficult for the bridge designer and builder. Torsion is one of many problems that must be overcome in order to create a functional suspension bridge.
There are two ways that torsion can damage a bridge: 1 if not corrected, it will cause the deck to sag over time, eventually leading to collapse; 2 any movement of one section of the bridge relative to another will increase the amount of torque applied to the structure, which could lead to failure at some point in the future.
The first recorded instance of a suspension bridge being damaged by torsion occurred in 1872 when the Tacoma Narrows Bridge collapsed due to excessive weaving in the face of strong winds. The narrows are a constriction between Puget Sound and the Pacific Ocean near downtown Tacoma, Washington.