Anexternal force is a force that is applied on a structure from something else • Such as walking into the wind or swimming against the waves Theforce we feel everyday is GRAVITY, gravity is the external force acting on everything on the planet
Thedownward force acting on us due to the Earth’s downward pull MASS is the amount of matter that the Earth is pulling down When we leave Earth the gravity is removed and we are able to float because our mass is not being pulled down
Gravity acts on a structure everywhere, but there is an imaginary point where balance is maintained • THAT IS THE CENTER OF GRAVITY Thelocation of the center of gravity determines the stability of a structure
Inorder for us to increase a structures stability we use two main methods; • 1. Increase the Base compared to the height • 2. Place the majority of the mass close to the ground
Stand up and balance on one foot…. • 1. • 2. • 3. CanI balance a meter stick on one finger focusing only on its center of gravity?
Take a ruler and balance it on your finger How did you get it to do that? What do you notice about the ruler? Moveyour finger one centimeter to the left. Now another, and another what happens??
Symmetry is a balanced arrangement of mass that occurs on opposite sides of an object Fora structure to be stable, its mass must be distributed equally around the center of the structures base
When structures are built, the creators consider the load that structure will have to resist • A book shelf resists its weight and the books • A bridge resists gravity, maybe a car, and the materials its made from Load is split into two different types; • Static • Dynamic
Is the weight of the structure itself and the non-moving load it supports Nails, wood, steel, cables are a few examples Even though these parts make up the load, they create a external force because gravity is pulling on these items Static Load is also known as DEAD LOAD
Is the opposite of static load It is an external force that moves or changes with time These can change very quickly such as a sudden gust of wind or an earthquake An example of a dynamic load is cars driving over a bridge, or students walking up stairs • Dynamic is also known as LIVE LOAD
Bridges are prime examples of structures designed to withstand loads Different conditions call for different bridges because all bring their own pros and cons
Is the most common type of bridge used Is flat and is supported at its two ends Longer beam may be supported by addition piers
Isa light weight but strong bridge, made of trusses (triangle-shaped frames) along its sides
Hangs between two ends that hold it up Modern ones have very tall towers on either end that support its main cables that hold up the bridge The main cables are anchored into concrete, and the smaller cables support the roadway
Designed to withstand very heavy weights The dynamic load of people or cars causes each piece of wood or stone to push against the next one This push eventually leads to the end supports embedded into the ground The ground pushes back strengthening the bridge