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FORCES ACTING ON STRUCTURES
Transcript of FORCES ACTING ON STRUCTURES
A force is any push or pull that causes a structure to undergo a change. You describe forces in magnitude, direction and point and plane of application. Magnitude is the strength of the force. Direction is where the force is coming from and point and plane of application is where the force hits the object. Forces can be divided into two main categories. Internal forces, which happen inside a structure, and external forces, which occur on the exterior.
Internal forces are forces that occur inside a structure. It happens when one part of the structure acts on another part. Examples of internal forces are tension, torsion, compression and shear. Each of these forces are able to damage structures if they are not built properly.
What are forces?
Tension is an internal force, caused by stretching of an object. It is usually caused by a pulling force at the ends of a structure, such as a bridge or a wire. When a structure experiences tension, it lengthens and gains potential energy. Should the structure be slackened, it will snap back to its relaxed length. If a structure is stretched too far and experiences too much tension, it could detach and break apart.
An example of how tension plays a role in our everyday life is when hydro workers set up electrical lines. When they set up the lines, they make sure to let them drape down and relax. This is so in the winter time, when the cold temperatures cause the wire to contract, the wire won't snap from tension. If the electrical lines were originally set up with little or no slack, the wire would experience too much tension as it contracts and it would snap, causing a power outage for many homes.
Torsion is an internal force that results in the twisting of a structure. It is caused by a rotating motion at both ends of a structure, with one end rotating one way while the other end rotates either in a different direction or with different speed. Just like tension, structures gain potential energy when they are twisted and have a tendency to spring back to its original, relaxed form after being released. A structure experiencing too much torsion could snap and break apart.
We can find examples of torsion everywhere we look. Many designs on everyday objects have a spiral pattern on it. The spiral pattern is achieved by heating the object until it is soft, and then twisting it, which causes torsion. Also, in many sports, athletes will encounter torsion in their own body, such as in figure skating.
If structures are not properly built to withstand torsion, they can easily fail, due to factors like wind. In 1940, the Tacoma Bridge collapsed due to the strong winds, causing the bridge to twist and finally break.
When a structure experiences compression, it is being compressed, or squeezed together. Unlike tension, where both ends of the structure is being pulled outward, compression involves both ends of the structure being pushed inward. Compression can be uni axial, where compression occurs in only one direction, shortening the length, or bi axial, where the structure experiences compression on all sides, decreasing its volume.
If a structure is compressed too far, it may give away and fail.
Compression is a part of our everyday life. You compress a lemon to get the juice out. Oxygen is compressed and stored at high pressure inside an oxygen tank. Even as you read this, you are being compressed by the atmosphere at roughly 15 pounds per square inch!
When a structure experiences shear, it means that one part of the structure is being pushed one way, while the other part is pushed in the opposite direction. Shear can be a caused by natural factors such as the wind or earthquakes, or by human factors, such as when you spread butter on your bread.
Buildings have to be designed to be able to resist shear, otherwise they would quickly collapse. When wind blows on a house or a tall skyscraper, the wind causes the one part of the building to move one way, while the foundation of the building holds it in place. This causes a shearing force because the two parts of the building are being pulled in opposite directions. As a result, the International Building Code states that all buildings must be braced by with shear walls, which are designed to withstand shear are help protect the building.
External forces are forces that occur on the outside of a structure. Examples of external forces would be gravity, load, and symmetry. Just like internal forces, external forces are able to cause devastating damage to structures.
Load is an external force that weighs down a structure. The load is composed of the static load and the dynamic load. The static load is the load that is always there and always acted upon by gravity. The dynamic load is the load that changes. For example, on a bridge, the over passing vehicles and people are considered the dynamic load because it is always changing. The bridge itself is the static load because it is always there, being acted upon by gravity and won't change.
Depending on the structure, load can also cause compression and compression. If a structure is carrying more load that it is designed to carry, it could collapse and become a structural failure.
Symmetry is the balance of structural load on a structure. When a structure does not have symmetry, it can easily topple over and break apart. Symmetry is important to consider when building structures because the weight of the structure needs to be balanced out evenly. If all of the structural load is focused on one part of the structure, then the structure does not have symmetry.
An example of the importance of symmetry is when the Yarmouth Bridge collapsed on May 2, 1845. Many people crowded onto the bridge to see a circus clown go down the river in a barrel pulled by geese. As the clown passed underneath the bridge, the spectators moved to the other side of bridge. This sudden shift of load caused the bridge to lose its symmetry and collapse
Gravity is the attraction between matter. Living on the Earth's surface, we are able to feel Earth's gravity pulling us towards it. Anything that has mass has a gravitational pull and the strength of the gravitational pull depends on the amount of mass. Gravity affects structures because they are constantly being pulled by the Earth. As a result, structures have to be able to support themselves, otherwise they would crumble under their own weight.
The amount of gravity in a place can affect how high structures go. For example, on Earth, the highest structure is Mount Everest, extending 8848 meters off of the ground. On Mars however, where the gravity is much weaker, Olympus Mons, a dormant shield volcano, climbs an amazing 22 kilometers above Mars surface. This is due to the low gravitational pull on Mars, allowing structures to each extraordinary heights.
By: Kevin Wang 7G