Structures 1 Technology This presentation will explore: components of structures strengthening structures principle of moments types of levers Structures.

Structures 1Tech

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This presentation will explore:

components of structures strengthening structures principle of moments types of levers

Structures

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Structures 1Tech

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Frame Structures

Examples of structures include cranes, bridges, furniture, buildings and various forms of transport such as automobiles, airplanes, ships and trains.

A crane is an example of a frame structure. Each part can be easily seen and examined.

An automobile has a shell structure, which is more difficult to analyze as the frame is hidden. We will confine ourselves to frame structures in this presentation.

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Structures 1Tech

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Purposes of Structures

Structures must be able to carry loads for which they were designed.

Structures must not collapse in use.

Structures must hold all the various parts in their correct relative position. This is particularly important for structures that hold mechanisms in place.

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Structures 1Tech

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1

A) A wooden table

B) A hot-air balloon

C) A crane

D) A suspension bridge

Which of the following do you think is NOT a frame structure?

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2

Structures must hold all parts in their correct relative positions. Is this true or false?

Answer True or False.

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Joints and Members

Frames consist of joints and members:

Joints can also be reinforced with gusset plates to improve the strength of the joint.

Members may be glued, welded, riveted, bolted or screwed together. Each method has its advantages and disadvantages.

Different types of joints will be used depending on how the structure is to behave and how the loads are to be applied.

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Triangulation

If four members are loosely pinned together, the structure is unstable. If a force is applied, it will move easily.

Adding one more member makes it stable by creating two triangles.

An example of an unstable frame:

Making the frame stable:

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3

Consider this shape. What is the minimum number of members that must be added to make it stable?

Enter your answer and press Send.

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Tension and Compression

Forces in structures are usually either tension or compression forces.

Tension

Compression

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Struts and Ties

If a member is under tension, it is called a tie.

If a member is under compression, it is called a strut.

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A) Strut

B) Tie

Is the member shown in the horizontal position a strut or a tie?

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Pivots, Levers and Moments - The Tower CraneThe crane looks unstable. It looks as if it should fall down. There seems to be too much of the boom on the right-hand side.

In order to understand this better we need to know about pivots, levers, moments and equilibrium.

The boom is therefore balanced.

It doesn’t fall because of a counterbalance weight on the left-hand side. Boom

Counterbalance

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Equilibrium

The pivot point (or fulcrum) is the point at which something rotates.

The weights on the scales are at equal distances from the pivot.

If something is balanced, it is said to be in equilibrium.

The man on the wire has two equal weights at equal distances from the pivot point.

If one of the people on the seesaw moves backward or forward, the balance will tip one way or the other.

The seesaw is no longer in equilibrium. Next >

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5

The center of the wheel on a wheelbarrow is its pivot point. Is this true or false?

Answer True or False.

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Moments

When something is in equilibrium, the moments of a force are balanced.

The moment of a force is the force multiplied by the distance from the pivot (moment = F x d).

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A) Force x extension

B) Force x distance

C) Force / distance

D) Force + distance

Which expression correctly calculates the moment of a force?

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Principle of Moments

The principle of moments says that for there to be equilibrium, the clockwise moment must equal the counterclockwise moment.

Clockwise moment = F2 x d2

Counterclockwise moment = F1 x d1

If F2 x d2 = F1 x d1 there is equilibrium

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A) 1 cm

B) 5 cm

C) 10 cm

D) 20 cm

If a 10N force is arranged 10 cm from a pivot point of a seesaw, what distance would a 20N force need to be from the pivot point in order to balance the seesaw?

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Types of Lever

There are three types of levers:

Examples include:Wheelbarrowmicroswitchbrake pedal

Examples include:Shovelforearm musclesfishing rod

Examples include:seesawcrowbarscissors

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A) Class 1

B) Class 2

C) Class 3

Which class of lever is a pair of tweezers?

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A) They are all class 1 levers

B) They are all class 2 levers

C) They are all class 3 levers

D) They are all double levers

What do scissors, tweezers, nut crackers and tongs all have in common?

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Plane (2D) Structures

Three members, AB, BC and AC support a traffic light:

AB and AC are of equal length

Which members are in compression?

Which members are in tension?

At which point is the load applied?

Where is the pivot point or fulcrum?

Is this in equilibrium?Next >

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10

A) Strut

B) Tie

In the example you have just seen, is the member holding the traffic light to its support a strut or a tie?

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Summary

structures exist to provide support for loads or mechanisms

In this presentation we have seen that:

The most common forces acting on structures are:

tension (in ties)

compression (in struts)

structures can be strengthened by triangulation

there are three type or class of lever

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