9. structures 9.1. text 1: bending - intef -...

TECHNOLOGIES 1ESO BILINGUAL PROGRAMME

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9. STRUCTURES

9.1. TEXT 1: BENDING

BENDING

Fill in the gaps using the words in the box:

the top of stretched the bottom of beam compressed

In engineering mechanics, bending (also known as flexure) is the stress state

when a structural element tends to be bended (curved) by external forces. Typically a

(1) is subjected to bending. A closet rod under the weight of clothes is an

example of a beam experiencing bending.

Bending produces reactive forces inside a beam as the beam attempts to

accommodate the flexural load: in the case of the beam in Figure 1, the material at the

top of the beam is being (2) while the material at the bottom is being (3) .

Consequently (4) the beam is experiencing compression (compressive

stress) and (5) the beam is experiencing tension (tensile stress). Therefore there

is a point in between where there is no stress as is shown in figure 2.

Normally a beam experiencing bending also is subjected to shearing.

Figure 1 Figure 2

neutral axis


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9.2. BRIDGES

SUSPENSION BRIDGE

N Element Stress

1 Suspension cable (catenary-shaped)

Tensile stress or tension

2 Suspender cable (tie) Tensile stress or tension

3 Foundation Compressive stress or compression

4 Columns, towers or pylons Compressive stress or compression

5 Deck (it is supported by

beams or girders)

Bending and shearing

1 2

4

3

5


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CABLE STAY BRIDGE

pylons or

towers

deck

cables


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9.3. TEXT 2: FOUNDATIONS

Shallow foundations

Shallow foundations are usually embedded a few metres into soil. One common

type is the spread footing which consists of strips or

pads of concrete which extend below the surface and

transfer the weight from walls and columns to the soil

or bedrock.

Another common type is the slab-on-grade

foundation where the weight of the building is transferred to the soil through a

concrete slab placed at the surface. It is like a kind of spread footing foundation but

using only one foot for the whole building.

Deep foundations

Deep foundations are used for structures

when shallow foundations can not provide

adequate capacity. Piles reach the bedrock or

even get into it.

In the following figure you can see the

foundation of the Petronas Towers in Kuala

Lumpur, Malaysia. The bedrock in Kuala Lumpur

is quite deep, so a deeper foundation is needed

to reach it.

Fill in the gaps: There are three types of foundations: __________, __________ and

__________

_______ _________ are used for the least soil capacity.

A concrete slab is used when the soil has little capacity and ________________ is not

enough.


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9.4. EXERCISES

1) Unscramble each of the clue words. Copy the letters in the numbered cells to other cells with the same number to obtain the final word.

2) Are there more steel bars along the bottom or along the top in the next two beams?

3) Find on the Internet several examples of cable stay bridges and suspension bridges. Stick to this page a photo of each one.

load

beam

load

beam


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9.5. PROJECT: THE CONSTRUCTION OF A BRIDGE

Specifications:

1-Firstly you must draw an individual sketch plan. Write down in the drawing the

type of element and the material used in each one. Then, work in pairs to agree the

better design to construct. Decide every single detail for construction.

2-As materials, you can use:

- Paper: make a roll using A4-size sheets (the technique is explained in page

172 of your book). The paper must be used paper: lets help the environment. Dont

waste blank paper that you could use for any other thing as drawing or writing.

- Thread: To use as cables in real bridges.

- White glue to join the elements.

- Card in the joints using little triangular pieces and the adhesive.

You also need clothes pegs to keep together the joints while the glue is drying

and a box to store the project under construction.

3- The bridge must be 15 cm wide. The deck must be 20 cm above the base.

The span must be at least 24 cm.


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4- Design of the joints:

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joint plate made of card

bar made of paper

35

JOINT OF 3 BARS

35

35

joint plate made of card

bar made of paper

JOINT OF 2 BARS


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5- We will use the following formula to evaluate the bridge:

WLR = 25

L: load supported (Kg)

W: weight of the bridge (grams).

The greater the ratio (R) is, the better your mark will be.

The maximum weight for the structure is 260 grams.


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9.6. LISTENING

Using your web browser, watch the Tacoma Narrows Bridge collapse at

http://roble.pntic.mec.es/gibm0000/media.htm. Listen and answer the questions. Copy

the questions and the answers into your notepad.

1. How long did the bridge last?

2. How long was the bridge?

3. How wide was the bridge?

4. What was the wind speed when the bridge began its twisting motion? What

does steady mean in this context?

5. How long did the bridge keep moving before collapsing?

6. Was the bridge rebuilt? What is the difference in the new bridge to be stable?

7. Was there anything inside the car?

8. Who was the man walking along the deck of the bridge?

9.7. VOCABULARY BUILDER

Remember: This vocabulary is not to be learnt by heart. It only tries to help you

to do your homework. You have to get used to using this words to explain general

ideas in English.

English Explanation Spanish

Beam A beam is a structural element that carries load primarily in bending (flexure). Viga

Bending

Bending (also known as flexure) is the stress state where the bar or structural element tends to be bended (curved). Typically one side is stretched (tension) and the other side is compressed (compression). Flexin

Buckling

In engineering, buckling is a sudden failure of a structural member that is subjected to high compressive stress. This failure happens in elastic materials and slender elements. Pandeo

Cable A cable is one, two or more wires bound together, typically in a common protective jacket. Cable

Cantilever A cantilever is a beam anchored at one end and with the other end projecting into space. Voladizo


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Catenary

The catenary is the shape of a hanging flexible chain or cable when supported at its ends and subjected to gravitational force (its own weight). Catenaria

Column

A column in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below.

Columna o pilar

Compression

Compressive stress is the stress where

external forces applied to bars or columns tend to shortening (decrease of length). Compresin

Concrete

Concrete is a construction material that consists of cement, aggregate (generally gravel and sand) and water. Hormign

Depth Thickness in a structural element in the same direction of the load, typically vertical. Canto

Dome

A dome is a common structural element of

architecture that looks like the hollow upper half of a sphere. Cpula

Flat truss

The parallel chord truss or flat truss gets its name from its parallel top and bottom chords. It is often used for floor construction. Celosa

Force Force is an influence that may cause a body to

accelerate (change speed) or be deformed. Fuerza

Foundation

A foundation is a structure that transfers loads

to the ground. Foundations are generally broken into two categories: shallow foundations and deep foundations. E.g., "The Leaning Tower of Pisa leans due to a faulty foundation". Cimentacin

Girder

A girder is a large support beam used in construction, normally made of iron or steel. Girder is the term used to name the main horizontal support of a structure and usually supports smaller beams.

Viga, jcena o perfil metlico.

Joist

A joist, in architecture and engineering, is one of the horizontal supporting members that run from beam to beam, to support a ceiling, roof (or floor). It may be made of wood, steel or concrete. Typically a beam is bigger than a joist. Vigueta

Load

Load is a group of forces applied to a

component of a structure. There are two types of loads: permanent loads (or dead loads) and live loads. Live loads are temporary and include snow, wind, earthquake, occupancy loads or water pressures in tanks or dams. Carga


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Pitched truss or common truss

The pitched truss or common truss is characterized by its triangular shape. It is most often used for roof construction. Cercha

Reinforced concrete

Reinforced concrete is concrete with reinforcement bars ("rebars") inside. In industrialized countries, nearly all concrete used in construction is reinforced concrete.

Hormign armado

Shearing

Shear stress or shearing is a stress state where external forces applied to bars or structural elements tend to.

Cortadura o cizalladura

Slenderness The ratio of the length of a column to the least width is called the slenderness. Esbeltez

Span Span is a section between two intermediate supports, e.g. of a beam or a bridge. Luz

Strength

Strength is the limit of a stress state. Therefore compressive strength, tensile strength and shear strength are the terms used to name the limit states of compressive stress, tensile stress and shear stress respectively.

Resistencia (mecnica)

Stress

Stress is the internal distribution of force per unit area that balances and reacts to external loads applied to a body

Esfuerzo o tensin

Structure

A structure is a man-made construction used for supporting external forces without being deformed considerably or breaking down. Estructura

Suspender cable

(see also cable) In a suspension Bridge, the suspender cables are between the suspension cables and the suspended deck. Tirante

Suspension cable

In a suspension Bridge, the suspension cables are the catenary-shaped ones Catenaria

Tension Tensile stress (or tension) is the stress state where the bar tends to be stretched. Traccin

Torsion Torsion is the stress resulting of twisting an object due to an applied torque. Torsin

Truss

A structure consisting of straight slender members inter-connected at joints into triangular units. It combines strength with economy of materials and can therefore be relatively

Estructura triangulada

Vault A covering system made by consecutive semi-round/circular arches Bveda

TRIANGULATION

When a force is applied from a, the structure keeps its shape because bar 1 is doing a

tensile stress.

When a force is applied from b, the structure keeps its shape because bar 2 is doing a

tensile stress.

If we only use one bar (1), it must be very thick to avoid buckling under compression

when pushing from b.

By using two diagonals, we save material and the structure is cheaper.

CONCRETE SLAB

A concrete slab is a structural element made of reinforced concrete, typically between 30

and 50 centimetres thick. It is used to construct floors and ceilings.

concrete layer with rebars

(capa de hormign con barras de acero corrugado)

reinforced concrete joist

(vigueta de hormign armado)

in-floor brick

(bovedilla)

1 2

a b

STRUCTURAL ELEMENTS

Label the structural elements matching the numbers with each item from each box.

Write down the type of stress for numbers 1, 2, 3, 4, 10, 11, 12 and 13:

Pitched truss Beam Suspender cable Deck Pylon

Flat truss Joist St. Andrews cross Column Vault

Dome Footing Suspension cable

Catenaria Cercha Columna Cruz de San Andrs

Vigueta Tablero Viga Tirante Torre

Cpula Bveda Celosa Zapata

1

2

3

4

5

Gins Ibez Mompen

6

7

8

9

10 11

12 13

9. structures 9.1. text 1: bending - intef -...

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