1ebil Structures Presentationtacomabridge 110331123225 Phpapp02

7/27/2019 1ebil Structures Presentationtacomabridge 110331123225 Phpapp02

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STRUCTURES

Technologies1st year ESO

(Adapted from a presentation by Manuel Suarez Alvite)


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STRUCTURES

You are going to watch a video about a bridge over Tacoma Narrows. After thatyou are going to work for five minutes in groups. Write down the answers and

after that the head of the group will read them.

1.- How long was the bridge in use before collapsing?

2.- What happened to the car on the bridge?

3.- After the collapse of the bridge, what did the authorities do to solve the

problem of crossing the Tacoma Narrows?

http://www.youtube.com/watch?v=3mclp9QmCGs
http://www.youtube.com/watch?v=3mclp9QmCGshttp://www.youtube.com/watch?v=3mclp9QmCGs


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STRUCTURES

How can we define a structure?

A structure is an arrangement of designed components that provides

strength to a built device such as a building, bridge, dam, car or airplane.

There are also natural structures although we will focus on manufactured

structures.

A structure must

be capable of carrying the load it was designed for without failing, and support the load or object in the correct position.


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STRUCTURE REQUIREMENTSThe main requirements for a structure to work properly are the following:

Safety

Strength: The structure is designed to be strong enough to support loads due

to its own weight, human activity, and the environment (such as wind, snow,

earthquakes, ice, or floods). The structure will have strength if it has:

Stability: it must be in the right place and not fall.

Rigidity: it must not be deformed when in use.

Resistance: it must support all the efforts and forces applied to it.

Durability: The materials and details of construction must last. That means that

the structure will not corrode, deteriorate, or break under the effects of

weathering and normal usage during its lifetime. Economy: it must not exceed the budget.

Aesthetics


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The Tacoma Narrows Bridge was a bridge built in the state of Washington, across the

Tacoma Narrows in 1940. At that time, it was a suspension bridge with a revolutionary

design, but four months after opening, a storm caused it to collapse.

Throughout the early morning of November 7th, the central span started to undulate in

winds of 55 to 75 km per hour. At about 10:30 A.M., the central span floor panel dropped into

the water 59 meters below, and the remainder of the bridge kept twisting. At 11:09 A.M., the

remaining sections fell down into the water. The bridge was not strong enough to support the

oscillation caused by the windstorm.The collapse of Tacoma Narrows Bridge did not result in any casualties. The bridge was

blocked a few days before it collapsed. However, this design failure brought many engineers

worldwide to the realization that they did not have enough understanding of building a

suspension bridge.

After the collapse, the engineers started to investigate ways to aid the strength of a

suspension bridge. The official investigation into the collapse recommended the use of wind-

tunnel tests to aid in the design of the second Tacoma Narrows Bridge. Also, new

mathematical theories arose from these studies.

The insurance agent who had written an $800,000 policy on the bridge for the state was

so confident of the bridges integrity that he pocketed the $70,000 premium instead of

forwarding it to the insurance company. He went to jail for two years. Other insurance policies

covered most of the bridges cost.

THE TACOMA NARROWS BRIDGE COLLAPSE


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STRUCTURES: Activity

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What are the requirements that the bridge did not fulfill? (Individually in yournotebook)

Safety: it was safe before the problems started and when they realized aboutthem, the bridge was closed. In fact, nobody was injured.

Strength: it was not strong enough to support the storm. Why?

Rigidity: it wasnt deformed when it was in use by carsResistance: it had resistance to the usual forces, but not to the unexpectedones that appeared when the bridge started to undulate.Stability: it wasnt stable when the wind was between 55 and 75 km perhour.

Durability: parts of the structure deteriorated in several hours under the big

efforts

Economy: this part was not a problem and the people in charge could obtain agood price for the insurance.

Aesthetics: depends on how aesthetic you find this bridge, but it was designedto be beautiful.


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STRUCTURES: Forces

Asuccessfulstructure must be able to withstand all the forces it will experience

without collapsing. An understanding of the kinds of forces which can act on astructure is important to a designer.

TENSION FORCES

Forces which cancause a member tostretch.

COMPRESSIONFORCES

Forces which cancause a member to besquashed or

buckled.


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STRUCTURES: Forces

TORSIONAL FORCES

When a turning force (or

torque) is applied to amember, it may twist.

BENDING FORCES

They are forces which

tend to make the objectbend.

SHEAR FORCES

They act acrossa material

in such a way that one partof the structure can beforced to slide over anoder


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Draw the following swing and write the name of the forces that are acting on it


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Activity

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The bracket holding up the hanging basket is made of steel and fixed to the wall with ascrew. Write the forces exerted on the following points

What forces are acting on the screw used to fix the bracket to the wall?


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Types of structures

1. MASS STRUCTURES

1. Mass structures

They can be made by, piling up or forming similar materials into a particular shapeor design.

Sand castles, dams and brick walls are manufactured mass structures.

Advantages: They are hold in place by their own weight. Losing small parts oftenhas little effect on the overall strength of the structure.


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Concrete is a mixture of cement and stone

aggregate and mixed with a small amount of

water. When steel bars are placed in concrete,

the composite material is called reinforced

concrete. It resists compression and bending.

Types of structures

2. FRAME STRUCTURES


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Types of structures

3. TRUSSES

3. Trusses

Trusses are frame structures formed by triangles.

The triangle is the most rigid frame structure and many complex structures are

based on triangles. The technique used in these type of structures is called

triangulation.


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When forces are applied to a simple

four-sided structure, it can be forcedout of shape. A structure whichbehaves in this way is said to be non-rigid

By adding an extra bar the structurecan no longer be forced out of shape,and is said to be rigid.

The effect is known as triangulation

Types of structures

3. TRUSSES


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Alternatively, a frame structure can be made rigid by the use of gusset plates.A GUSSET is a piece of material used to brace and join the members in astructure.

Types of structures

3. TRUSSES


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Types of structures

4. SHELL STRUCTURES

4. Shell Structures

Structures, which keep their shape and support loads, even without a frame, or

solid mass material inside, are called shell structures. These structures use an

outer layer of material to provide their strength and rigidity. The shape of a shell

structure spreads forces throughout the whole structure, which means every part

of the structure supports only a small part of the load, giving it its strength.

Examples include: egg cartons, water containers, gas tanks and cars


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Types of structures

5. SUSPENSION STRUCTURES5. Suspension structures

Suspension structures are those with horizontal planes (road decks, roofs, and even floors)supported by cables (hangers). Suspension bridges are good examples of these structures. Inthese bridges, cables suspended via towers hold up the road deck.


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Types of structures

6. VAULTED STRUCTURES6. Vaulted structures

Vaulted structures are those formed with arches. They are usually used to provide a big spacewith a ceiling or roof. The weight of the ceiling is conducted through the vault into the pillars(or columns) that are supporting it, and from those pillars into the foundation of the building.

Different shapes in the vaulted structures have been used through history and depending onthe different cultures.


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Types of structures

7. GEODESIC STRUCTURES7. Geodesic structures

Geodesic structures (usuaslly called geodesic domes) are a kind of shell structures in which theshell is formed by polygons (usually triangles).


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STRUCTURES: Glosary

Look up in the dictionary the meaning of the following words and then,

write a sentence using them (with the full meaning!)

Arrangement

Dam

Span

Pier

To stiffen

Manufactured structure

Safety

Strength

Durability Load

Tension forces

Compression forces

Torsional forces

Bending forces

Shear forces

To undulate

Casualties

To stretch

To squash

To buckle

Truss

Arch Vault

Pillar


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Homework: pictures of structures

1. Take a walk around your neighborhood and take two pictures of objects orbuildings with different types of structure (mass, frame, truss, shell, suspension,vault or geodesic structures). Another possibility is to take the picture of objectsyou have at home.

2. For each of the pictures, write down a short text telling:

Your full name

Date and place in which you took the picture.

Type of structure in the object/building A sentence explaining the object or building is in the picture. For example, It is a

picture of the Golden Gate Bridgein San Francisco or It is my desk in my room.

3. Hand in this homework in one of the following ways:

Print the pictures, write the required text on their back and hand them in to yourteacher.

Make a file in the computer with a word processor (OpenOffice Writer or MicrosoftWord) with the pictures and the text required. The file name must be your full namefollowed by structures homework and hand the document in to your teacher and:

Send the file by email to the address [email protected]

Save the file to a Pen Drive and give it to your teacher when you are in Computerlab.

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mailto:[email protected]:[email protected]

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1ebil Structures Presentationtacomabridge 110331123225 Phpapp02