Presentation on theme: "Structural Strength and Stability"— Presentation transcript:
1Structural Strength and Stability Grade 7 Science: Form and FunctionStructural Strength and Stability
2Learning Goals/Success Criteria I can explain centre of gravityI can identify factors that help keep structures stableI can identify and explain some specific building construction techniques that help make structures strong and stableI can identify some reasons that cause structural failure
3Stability of Structures How do you keep from falling when you are standing on a moving bus? DiscussStability: the ability of a structure to remain in or return to a stable, balanced position when forces act on it
4Stability of Structures Centre of gravity: the point around which an object’s mass is equally balanced in all directions – where the mass seems to be concentratedAll structures have a centreof gravity. On a human who isstanding, this is located deepinside your body just belowyour belly button.
5Stability of Structures Centre of gravity changes every time you move or bend your body in different shapes (Try it: what happens when you stand on one leg? Lean over?).The centre of gravity depends on the shape of the object and how its mass is distributed.
6Stability of Structures Which vehicle is more stable? Why?
7Stability of Structures The car has greater stability because of two features: it has a low centre of gravity (the mass is concentrated lower to the ground) and it has a wide support base (when compared to its height).Conditions for greatest stability: Objects with low centre of gravity and a wide support base tend to be stable.
8Stability of Structures Stability decreases as the centre of gravity rises. If the centre of gravity rises so high that it is no longer over it’s support base, the object will fall over.This is why transport trucks must be careful going around corners!
9Stability of Structures The Leaning Tower of Pisa in Italy was closed for many years due to the tower’s centre of gravity having shifted to a dangerous point of instability. This has since been restored (more on this later!).
10Stability of Structures Did you ever wonder why a tightrope walker uses a long pole? The pole lowers the centre of gravity, thereby contributing to stability.
11Making Structures Strong Many structures have common features that have been proven to strengthen structures:Beam: any level structure designed to support a load.
12Making Structures Strong - Beams There are several ways to make a beam stronger for its purpose:1. material: steel is stronger than wood but heavier – engineer must consider the purpose of the beam2. I-Beams: a beam that is in the shape of a letter I is much stronger and lighter
13Making Structures Strong - Beams 3. Corrugation: multiple folds in a material that provide additional strength (think cardboard boxes!)
14Making Structures Strong - Beams 4. Rebar: steel reinforcing rods are placed in concrete as they can withstand tension much better than concrete on its own. “Reinforced concrete” is able to resist both compression and tension.
15Making Structures Strong - Beams Cantilever: a beam supported at only one end.This is a cantilever draw bridge.
16Making Structures Strong - Beams Cantilevers are useful in spanning great distances without the use of a central support.This cantilever is on the Observation Tower over the American Falls at NF:
17Making Structures Strong - Beams Supporting the beam:How a beam is supported is also important.Tie:Strut:Gusset:
18Making Structures Strong – Other Shapes Designers sometimes want to use shapes other than beams to make structures stronger and more interesting looking. They can do this by adding triangles (trusses) or curves (arches and domes).
19Making Structures Strong – Other Shapes Truss: a network of beams that form triangles. It can be used as a bridge or a cantilever and for many other things.Look at the next few slides. What shape is used is used to strengthen the structure?
23Making Structures Strong – Other Shapes The Arch: a curved structure used to span a space while supporting a loadThe curved design transfers compression force downward so they are strong as well as aesthetically pleasing.
26Making Structures Strong – Other Shapes The Dome: a shell structure that looks like the top half of a sphere. It directs compression downwards in many planes. This is better than an arch which directs compression downwards in one plane.
36Structural FailureDefinition of structural failure: the failure of a structure as a result of the structure, or part of the structure, losing the ability to support a load.There are many reasons why a structure might fail. Read through your text pages 306 – 309 and take notes about the top 4 reasons for structural failure.