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Structures

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Justin McInnes

on 11 February 2014

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Transcript of Structures

Structures
Mass (Solid) Structures
- Are formed from a solid piece or combination
of pieces.
- Have little or no space inside.
- Resists forces that act upon it by relying
on its own mass.
- Tend to be the strongest structure but are
usually very heavy and hard to use.


Force
A force is a push or pull that tends to cause an object to change its movement or shape.

Other Considerations
- Aesthetics
- Consumer demand
- Availability of materials
- Cost
- Effect on environment
- Safe disposal

By their
origin
: Natural or Manufactured

or

By their
design
: Mass (solid), Frame, or Shell. These are the 3 basic
structural forms.
What is a structure?
3 things:

1) Definite size

2) Definite shape

3) Serves a purpose
Materials
- The materials used to build structures are evaluated by many properties.

- The purpose of the structure will determine what materials the builder will use.

Effect of Forces
The actual effect of a force on a structure depends on three things:

- The magnitude (size) of the force
- The direction of the force
- The location where the force is applied

Classifying Material Properties
- Brittleness: how easily it breaks
- Hardness
- Plasticity: how easily it is to shape
- Resistance to heat
- Resistance to water
- Compression
- Tensile strength

Magnitude
The bigger the force’s magnitude, the stronger it is and the more effect it will have on a structure.

However, the effect of the force is ultimately dependent on how massive the structure is and what the structure is made of.

Examples
Frame Structures
- Are made up of a specific arrangement of parts fastened together.
- Get their strength from the way that the pieces fit together.
- Are lighter than solid structures because they use less material.
- Easy to design and build.
- However, failure will occur at the joints. More joints = greater
possibility of failure.

Examples
Shell Structures
- Have a solid outer surface and a hollow inner area.
- Shell structures tend to be stronger than frame
structures and are lighter than solid structures.
- Get their strength by the shape of the thin outer
layer of the object.
- Are often used to provide protection (helmet).
- However, a small defect (scratch) could result in
failure.



Examples
Mix and Match Structures
- Combination of two or more of
the basic structural forms.

Examples
All Structures must have…
Structural Strength
: The structure’s ability to hold itself up.

Structural Stability
: The structure’s ability to maintain its position even if it is acted upon by a force.

Function of Structures
- A structure’s function is its use or
purpose.
- Many structures have multiple
functions.
Ex) A car is designed to provide movement and shelter.
- Only when a designer knows all
the functions of a structure, he or
she can start designing it.


Common Function, Different Design
- Structures that have the same function don’t
have to have the same design.
- Examples?
Characteristics of Structures
- Structures can be interpreted and classified according to the materials and components they are made of.
- Structures are also characterized by their aesthetic quality.
-
Aesthetics
refers to the pleasing appearance or effect an object has because of its design.

How Are Structures Classified?
Combining Materials
- Composite
- Layered
- Woven

Composite Materials
-Are made from more than one material.
eg) reinforced concrete

Layered Materials
- Uses a process called lamination to strengthen a structure.
- Lamination: Layers of material are pressed or glued
together.
eg) Plywood, TetraPak

Woven Materials
- Are fibres that have been spun (twisted), looped, or knotted together to make knitted structures.

Joints That Rely on Friction
- Friction is the force that results when the surface of one object moves
against the surface of another object.
- The size of the force of friction is dependent on:
- The weight of the object.
- The type of surface (smooth or rough).
- The force of friction can be used to prevent individual components of a
structure from slipping apart.

Choosing Materials
- Cost
- Appearance
- Environmental Impact
- Energy Efficiency

Joining Structural Components
- The place where structural parts are joined together is called a joint.

- Some joints need to be rigid while others need to be flexible.

Flexibility

- Is the ability of a material to be bent
under force without breaking.
eg) - tall buildings
- trees
Fasteners: Nails, Screws, Rivets, Tacks, and Staples
- Friction between the metal and the material surrounding it holds them together.
- Most common type of joint.
- An advantage is that they can be easily removed (except rivets).

Interlocking Pieces
- Use friction to hold pieces together.
eg) Lego

Ties
- Use string or rope to fasten things
together.

Joints That Rely on Bonding
- Glue, tape, cement, (adhesive joints), and welds (melting
joints) act to bond the surfaces of two materials.

Fixed (Rigid) Joints
- Are rigid to prevent any movement.
eg) Welding, cementing, gluing,
or nailing parts firmly
together.

Mobile (Flexible) Joints
- Are flexible so that parts of the
structure can move as required.
eg) Hinges, knees
Materials in the Human Body
- Bones: Are rigid and hard that form a structural frame that is strong enough to support and protect other parts of the body.
- Bones are connected by ligaments which are strong and flexible.
- Cartilage is found at the ends of bones and it provides a smooth surface which reduces friction for movement.

Direction
The direction in which a force acts also determines what effect the force will have.

Ex) Pushing an object uphill compared to downhill

Location
The location where a force is applied on a structure affects the outcome.

Ex) If you are trying to slide a fridge across the floor, the location of where
you apply the force is very important. If you apply the force too high, the
fridge will be tipped over.

Newton
The standard unit for measuring force is called the newton (N).

One newton is the amount of force needed to hold up a mass of 100 g.

To hold up a 1 kg book, it would take 10 N.
1 kg = 1000g
1000g ÷ 100 = 10 N

External Forces
An external force is a force that is applied on a structure by something else.

To remain standing, structures have to resist the external forces acting on them.

Ex) The force of gravity and wind.

Gravity
Gravity is an external force that pulls everything on Earth towards the ground.

The size of the gravitational force on an object is determined by how much mass that object has.

Mass is the amount of matter in an object.

The more mass an object has, the greater the gravitational force.

Center of Gravity
Gravity acts on all parts of a structure.

There is an imaginary point in every structure where the downward force of gravity is acting upon it.

This point is called the center of gravity.

When a structure is supported at its center of gravity, it will stay balanced.

Therefore, the location of the center of gravity determines a structure’s stability.

Symmetry
Symmetry is the balanced arrangement of mass to make a structure stable.

There is the same amount of weight on each side.

This results in the center of gravity being over the base of support which is necessary for an object to be balanced.

Stable vs. Unstable
Generally, structures with a low center of gravity (most of their mass is near the ground), and a wide base of support are stable.

Ex) Football lineman

A high center of gravity and a narrow base of support will result in structures being unstable.

Load
A load is an external force on a structure.

There are two types of loads:
- Static load
- Dynamic load

Static Load
The static load is the weight of the structure and the non-moving load it supports.

Static means not moving or changing.

Examples?
Dynamic Load
A dynamic load is an external force that moves or changes with time.

Is also called live load.

Examples?

Bridges
Engineers use two conditions to decide which type of bridge should be built.
- What the bridge is crossing (land or water)?
- What kinds of loads the bridge will be supporting?

4 types of bridges
- Beam bridge - Suspension bridge
- Truss bridge - Arch bridge

Beam Bridge
Most common bridge.

Is flat and supported at its two ends.

A longer beam bridge may be supported
by additional parts.

Truss Bridge
Lightweight but strong.

Made out of trusses (triangle-shaped frames)

eg) High and low level bridges

Suspension Bridge
Has tall towers on either end that support
the main cables holding up the bridge.

The main cables are anchored in concrete
at each end.

Smaller cables, that support the roadway,
are suspended from the main cables.

Arch Bridge
Built to withstand heavy loads.

The dynamic loads cause each piece of stone
or steel to push against the piece next to it.

This push is eventually transferred to the end supports, which are fixed in the ground, and the ground pushes back which allows the bridge to support the load.

Internal Forces
Is a force that one part of a structure exerts on other parts of the same structure.

Are forces that act within a structure.

Are classified by the direction in which they act within an object.

There are 4 internal forces
- Compression - Tension
- Shear - Torsion
Compression
Is a force that acts to squeeze an object.

Structures with parts that must resist compression include chairs, tables, desks, walls, and support columns.

Tension
Is a force that acts to stretch and pull apart something.

It can cause lengthening and possibly snapping.

Structures that must resist tension are a ski lift and a suspension bridge.

Shear
Is a force that acts to push parts that are in contact with each other in opposite directions.


Torsion
Twists a material by turning the ends in opposite directions.

Combining Materials
Concrete is very strong under compression but weak under tension.

Steel is strong under tension, but weak under compression.

Builders combine concrete and rebar (steel bars) to make a structure that can resist compression and tension.

Very important when building sidewalks and buildings.

Complementary Forces
When different kinds of internal forces act on a structure at the same time, they are called complementary forces.

Example: Bending

Structural Stress
Structural stress is the effect of forces acting on a structure.

A strong stable structure is designed and built to withstand stress without damage.

However, repeated abnormal use of the structure could cause structural fatigue.

Structural Fatigue
This is a permanent change in a structure caused by external and/or internal forces.

eg) cracks or bending

Structural Failure
Happens when a structure can no longer perform its function due to the forces acting on it.

Structural Components
Beams
A simple beam is a flat structure that is supported at each end.

Changing the shape of a beam can increase strength.

The shape of an I-beam gives it strength and less mass.

Box-beams are long rectangular beams that are hollow.

Columns
Is a solid structure that can stand by itself.

Used to support beams.

A truss is a framework of beams joined together usually in the form of triangles.

A cantilever is a beam only supported at one end.

Beams Cont.
Margin of Safety
Since it's impossible to make everything perfectly safe, designers work with a margin of safety.

This refers to the limits in which a structure’s performance is felt to be acceptable.

eg) Speed limits, load requirements.


Strengthening Materials
4 ways in which materials can be strengthened.

- Corrugation
- Lamination
- Strengthening arrangements
- Changing methods of fastening

Corrugation
Is the process of forming a material into wave-like ridges or folds.

These wave-like ridges form triangle-like shapes which is the strongest structural shape.

eg) Cardboard.

Strengthening Component Arrangements
Adding supports such as “ties” and “struts” will provide more support.

A tie resists tension and a strut resists compression.

In what ways could we change the method of fastening?
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