# Structural Strength and Stability

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Structural Strength and Stability
Grade 7 Science: Form and Function Structural Strength and Stability

Learning Goals/Success Criteria
I can explain centre of gravity I can identify factors that help keep structures stable I can identify and explain some specific building construction techniques that help make structures strong and stable I can identify some reasons that cause structural failure

Stability of Structures
How do you keep from falling when you are standing on a moving bus? Discuss Stability: the ability of a structure to remain in or return to a stable, balanced position when forces act on it

Stability 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 concentrated All structures have a centre of gravity. On a human who is standing, this is located deep inside your body just below your belly button.

Stability 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.

Stability of Structures
Which vehicle is more stable? Why?

Stability 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.

Stability 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!

Stability 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!).

Stability 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.

Making Structures Strong
Many structures have common features that have been proven to strengthen structures: Beam: any level structure designed to support a load.

Making 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 beam 2. I-Beams: a beam that is in the shape of a letter I is much stronger and lighter

Making Structures Strong - Beams
3. Corrugation: multiple folds in a material that provide additional strength (think cardboard boxes!)

Making 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.

Making Structures Strong - Beams
Cantilever: a beam supported at only one end. This is a cantilever draw bridge.

Making 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:

Making Structures Strong - Beams
Supporting the beam: How a beam is supported is also important. Tie: Strut: Gusset:

Making 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).

Making 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?

Making Structures Strong – Other Shapes
The Arch: a curved structure used to span a space while supporting a load The curved design transfers compression force downward so they are strong as well as aesthetically pleasing.

Making 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.

The Superdome in New Orleans

Does this dome look familiar?

Structural Failure Despite all of the scientific knowledge available, structural failure sometimes still occur. Documentary on how the NYC twin towers came down structurally: (44 min)

Structural Failure Destroyed in Seconds:
Portugal: (2:30min) In Manilla, Philippines: (2:06)

Structural Failure Of course the famous Tacoma Narrows Bridge Collapse: (4:02) This is the new bridge: (Located in the state of Washington, USA)

Structural Failure Of course, structural failure is not just limited to buildings and bridges....

Collapsed dome in Malaysia.

Structural Failure Definition 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.