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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 that you

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 collapsing of the bridge, what the authorities did to solve the

problem of crossing the Tacoma Narrows?

http://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 they were 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:

• The safety of the structure is achieved by following the rules of design contained

in standards and codes, as well as through strict quality control.

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

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

earthquakes, ice, or floods). The structure will have the 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.

• The design should provide an economical structure within the constraints of all

other requirements.

• A structural design takes into consideration the community's aesthetic

sensibilities.

The Tacoma Narrows Bridge was a bridge built in the state of Washington, across the Tacoma Narrows 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 bridge’s 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 bridge’s cost.

THE TACOMA NARROWS BRIDGE COLLAPSE

STRUCTURES: Activity

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

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

Strength: it was not strong enough to support the storm. Why?Rigidity: OKResistance: it had resistance to the usual forces, but not to the ones that appeared when the dridge started to undulate.Stability: it wasn’t stable when the wind was between 55 and 75 km per hour.

Durability: parts of the structure deteriorated in several hours.

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

Aesthetics: depends on how aesthetic you find this bridge, but it was designed with the aim of being beautiful.

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STRUCTURES: Forces

A “successful” structure must be able to withstand all the forces it will experience without collapsing. An understanding of the kinds of forces which can act on a structure is important to a designer.

TENSION FORCES

Forces which can cause a member to “stretch”.

COMPRESSION FORCES

Forces which can cause a member to be “squashed” or buckled.

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STRUCTURES: Forces

TORSIONAL FORCES

When a turning force (or torque) is applied to a member, it may twist.

BENDING FORCES

They are forces which act at an angle to a member and tend to make it bend.

SHEAR FORCES

They act “across” a material in such a way that one part of the structure can be forced to slide over anoder

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STRUCTURES: Activity

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

Activity

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

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

Types of structures

1. MASS STRUCTURES

1. Mass structures

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

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

Advantages: They are hold in place by its own weight losing small parts often has little effect on the overall strength of the structure.

Types of structures

2. FRAME STRUCTURES

2. Frame Structures

They have a skeleton of strong materials, which is then filled and covered with other

materials, supporting the overall structure. Most of the inside part of the structure is

empty space.

Buildings made of steel beams and colomns are an example of frame structures. Other

example is when the structure is made of reinforced concrete.

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 but also

bending, and other direct tensile actions.

Types of structures

2. FRAME STRUCTURES

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.

When forces are applied to a simple four-sided structure, it can be forced out of shape. A structure which behaves in this way is said to be non-rigid

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

The effect is known as triangulation

Types of structures

3. TRUSSES

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 a structutre.

Types of structures

3. TRUSSES

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

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. In these bridges, cable suspended via towers hold up the road deck.

Types of structures

6. VAULTED STRUCTURES6. Vaulted structuresVaulted structures are those formed with arches. They are usually used to provide a big space with 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 on the different cultures.

Types of structures

7. GEODESIC STRUCTURES7. Geodesic structuresGeodesic structures (usuaslly called geodesic domes) are a kind of shell structures in which the shell is formed by polygons (usually triangles).

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 Gusset Arch Vault Pillar

Homework: pictures of structures1. Take a walk for your neighborhood and take two pictures of objects or buildings

with different type of structure (mass, frame, truss, shell, suspension, vault or geodesic structures). Another possibility is to take the picture of objects you have at home.

2. For each of the pictures, write down a short text telling: Your full name Date in which you took the picture. Place where the picture was taken. Type of structure in the object/building A sentence explaining what object or building is in the picture. For example, “It is a

picture of the Golden Gate Bridge in 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 your

teacher. Make a file in the computer with a word processor (OpenOffice Writer or Microsoft Word)

with the pictures and the text required. The file name must be your full name followed of “structures homework” and hand the document in to your teacher and:

Send the file by email to the address algunastecnocosas@gmail.com Save the file to a Pen Drive and give it to your teacher when you are in Computers

room.

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