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# Structures and Forces

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## Jinming Wang

on 22 January 2014

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#### Transcript of Structures and Forces

Structures and Forces
AP 7
Look around you, what do you see? Structures are everywhere; they can be natural or man-made. They can be as delicate as a spider web or as strong as a skyscraper. How can we define what a structure is?
Structure

Structural Strength

Structural Stability

Force
Anything that provides support

The ability of a structure to hold both its own weight and any additional loads added on it
The ability of a structure to keep its shape even when it is being acted on by a force. It is difficult to push over
A push or pull that makes an object move or stop
There are 3 basic structural shapes: Solid, Frame, Shell
solid
frame
shell
A structure made from a solid piece of a strong material
A structure consisting of a ridged organizations of parts fastened together
A structure with a solid surface an a hollow inside
Nature: stone
Human: Cement
Nature: skeletons
Human: Picture Frame
Nature: egg
Human: pipes
Why do we build structures?
Function:
The purpose or use of a structure
Example:
Cup: to hold liquids
Knife: to cut
Many times, a structure is built and it can do more than one thing. This is called multiple functions.
Example:
Inukshuk: land mark, greeting, direction, look pretty
Airplane: fly, transport, shelter, travel
Aesthetics:
The pleasing appearance or effect that an object has because of its design
Common functions different designs
Sometimes a structure can have the same function but be built very differently; for example, our homes. Depending on where we live, a person's home can drastically vary.
Forces
A force is a push or pull. The unit that force is measured in is called Newtons (N).The force is found by numerous mathematical formulas, however we are going to concentrate on 2.

F=ma
F=mg
Mass
is the weight of the object in
kg
Acceleration
is how fast the object in going measured in
m/s
2
Example:
Brian is pulling a wagon that weights 2 kg and is filled with 7 kg of rocks. If he wants to pull this wagon with an acceleration of 10 m/s, what is the force that he must apply to the wagon?
m=7+2=9 kg
a=10 m/s
F=?
2
2
F=ma
F=9x10
F=90 N
** when using the formula F=mg, gravity is 9.81 m/s. In most cases, you are expected to memorize that number
How forces affect structures' stability is based on 3 things: Magnitude, Direction, and Location.
Magnitude:

Direction:

Location:
how strong or powerful the force is

which way the force is being applied

where the force is being applied
What is a spring scale?
A spring scale is a device that is used to measure how much force is applied to an object.
External Forces
Forces that act on the outside of a stricture are called external forces. Since gravity is a constant on every structure, we know that it plays a role on how stable a structure is. The mass or amount of matter the object has will play a huge role in what the forces of gravity will be.
Center of Gravity
Gravity does not act on every part of the structure with the same force unless the object is perfectly flat. However, as you know, objects tend to not be flat against the ground. Gravity plays a huge role on how stable a structure is. Depending on where gravity acts on the object will affect if the structure can withstand the external forces.
Center of Gravity
usually the most central part of the object
A
B
B is more stable because it has a bigger base, therefore, it has a lower center of gravity.
**Generally, the lower the center of gravity is, the more stable an object is
2
The symmetry of an object also plays a role in how stable a structure is.
Symmetry
The balanced arrangement of mass on the opposite sides of a line/plane or around a center/axis
A load is an external force that is applied to a structure. There are 2 main types of loads: static load and dynamic load.

The weight of the structure and non-moving load it supports
The external load that moves/changes on an object
Wind: dynamic
Truck: Dynamic
Weight of bridge: Static
Bridges
There are 6 main types of bridges. When an engineer is commissioned to build a bridge, there are 2 key things that they need to focus on to ensure that the bridge is safe.
1. What is the bridge crossing? Example. rivers, other bridges
2. What kinds of loads will the bridge be supporting? Example. people, animals, cars
What are the 6 types of bridges?
Beam or Girder, Truss, Suspension, Arch, Cantilever, and Movable
Beam or Girder
Flat bridge that is supported at both ends
most common
Truss
Strong and lightweight
Suspension
Hangs between 2 ends
modern: 2 tall towers
simple: 2 trees
Arch
keystone
Cantilever
Section of the bridge is suspended
Movable
section of the bridge moves
Some engineers will use a combination of each bridge to ensure that it will be strong enough to withstand all the forces.
Performance Requirements
The conditions that a structure must meet to ensure it is safe to use
It is also important to look at a structure load performance to aid in comparing bridges
Performance requirements:
Mass the bridge can support
_______________________
Mass of the actual bridge
Internal Forces
Internal forces: A force that 1 part of a structure exerts of other parts of the same structure.
Acting within the structure
There are 4 main types of internal forces that act in a structure:
Compression
Tension
Shear
Torsion
How does a structure stay standing if these forces are working to pull it apart?
Complementary Forces
When different kinds of internal forces act on a structure at the same time, usually in opposite directions
There are many structures that were built many years ago that are still standing today. This is because they utilize durable materials and shapes to ensure the structure cam withstand all the external and internal forces.
Some shapes are definitely stronger than other shapes:
Square Rectangle Triangle
compress easily
compress easily
does not compress easily
There are several components that are combined to make up a structure
Look around you. What do you see? Structures are everywhere. They can be natural or man made. They can be as delicate as a spider web or as strong as a skyscraper. How can we define what a structure is?
Structure
Structural Strength
Structural Stability
Force
anything that provides support
The ability of a structure to hold both its own weight and any additional loads added on it
The ability of a structure to keep its own shape even when it is being acted on by a force. It is difficult to push over
A push or pull that makes an object move or stop
** from this, we conclude that triangles are the strongest shapes. Many structures have multiple triangles built in to reinforce the structure's strength
Arch, Beams (I-beam, flat beam, box beam), Cantilever, Brace, Joint, Truss, Column
Arch
Common shape
Force is carried down the arch to the foundation
Beam
a flat surface
supported at both ends
I-beam
beam that has less mass
shaped like a I
Girder
long beam that is hollow
rectangular prism
Cantilever
A beam supported only on 1 end
Truss
a framework of beams joint together
usually triangles
Column
a solid structure
can stand by itself
Brace
a reinforcement used in architecture
Joint
where structural parts are joined
can be flexible/ridged
There are many reasons that a structure can fail
Structural stress

Structural fatigue

Structural failure
too great a combination of external and internal forces acting on a structure can weaken it
a permanent change in a structure caused by internal forces
occurs when a structure can no longer stand up to the force acting on it
How can we make a structure stand the test of time?
Some key components of materials that is important to consider
Brittleness
Ductility
Hardness
Plasticity
Resistance to heat and/or water
Compression
Tension

other considerations
Aesthetics
Consumer demand
Availability
Cost
Effect on the environment
Disposal of waste
what is the difference between buckling, deformation and flexibility?
Buckling
Deformation
Flexibility
Sudden failure of a structure
A change of shape in a structure because the material is unable to resist the load acting on it
The ability of a material to bend under force without breaking
How can we make material stronger?
Cost is a huge concern for many companies. Sometimes it is cheaper to modify the materials to make it more suitable for your needs.
Corrugation

Lamination
The process of forming a material into wave-like ridges or folds.
The process of gluing layers of a material together to create a strong bond.
Sometimes, the most suitable materials are not available or practical to use in different situations. Therefore, it may be better to just utilize a different arrangement of the materials to maximize the durability.
How do engineers, architects and contractors account for environmental factors? They look at 3 main categories: Climate Change, Terrain Conditions, and Natural Disasters
Climate Conditions
can the structure withstand harsh conditions?
example: snow, rain, hot vs. cold
Terrain Conditions
type of soil
example: hilly, rocky, soft
some buildings can sink in the ground if it is too soft
Risk of Natural Disaster
Structure must be able to withstand:
earthquakes
avalanches
tsunami
floods
landslides
just choosing a material for the structure is important, but how it is fastened is equally important
Joint: The place at which structural parts are joined together
There are 2 different categories that joints can fall into:
Ridged or fixed joints Flexible or movable joints
A joint that does not move
Prevent movement
A joint that does move
Allows movement
**It must withstand the forces acting on it
A key concept that must always be taken into consideration is
FRICTION
Friction:
Force that results when the surface of one object moves against the surface of another object
KEY RULES:
rougher the surface=higher frictional force
smoother the surface=lower frictional force

some joints utilize friction to their benefit
Nails
Screws
Rivets
Tacks
Staples
Interlocking pieces
Mass
The nail is held into place because of friction that acts on the sides of the nail from the wood
The rivet is held into place because of friction that acts on the sides of the rivet from the wood
The tack is held into place because of friction that acts on the sides of the wood
2 surfaces are rubbing against each other and held into place because of friction
The screw is held into place because of friction that acts on the side of the screw and ramp from the wood (Stronger than nails)
The staple is held into place because of friction that acts on the sides of the staple (folds back)
Weight of the bricks presses on the bricks below it, causing friction
Evaluating a Design
Constant improvement is what we humans strive for. By evaluating a structure's stability, it will help is build structures that will hopefully last generations!
Safety Cost Environmental Impact Benefits
is there a safety hazard?
who could be affected by it?

build
operate
maintain

how can we limit environmental harm?
is there a safe way to get rid to waste?

who will enjoy it?
what does the structure do?

Thank You And Have A Good Day!
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