Fill in the gaps using the words in the box: the top of stretched the bottom of beam compressed

Transcription

1 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. neutral axis Figure 1 Figure 2 65

2 9.2. BRIDGES SUSPENSION BRIDGE Nº Element Stress 1 Suspension cable Tensile stress or tension (catenary-shaped) 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 66

3 CABLE STAY BRIDGE cables deck pylons or towers 67

4 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. 68

5 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? load beam load beam 3) Find on the Internet several examples of cable stay bridges and suspension bridges. Stick to this page a photo of each one. 69

6 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: let s help the environment. Don t 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. 70

7 4- Design of the joints: 35 JOINT OF 2 BARS bar made of paper 35 joint plate made of card JOINT OF 3 BARS 70 bar made of paper 35 joint plate made of card 71

8 5- We will use the following formula to evaluate the bridge: R = 25 L W 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. 72

9 9.6. LISTENING Using your web browser, watch the Tacoma Narrows Bridge collapse at 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 Bending Buckling Cable Cantilever A beam is a structural element that carries load primarily in bending (flexure). Viga 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). Flexión 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 A cable is one, two or more wires bound together, typically in a common protective jacket. Cable A cantilever is a beam anchored at one end and with the other end projecting into space. Voladizo 73

10 English Explanation Spanish Catenary Column Compression Concrete Depth Dome Flat truss Force Foundation Girder Joist Load 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). 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. Compressive stress is the stress where external forces applied to bars or columns tend to shortening (decrease of length). Concrete is a construction material that consists of cement, aggregate (generally gravel and sand) and water. Thickness in a structural element in the same direction of the load, typically vertical. A dome is a common structural element of architecture that looks like the hollow upper half of a sphere. The parallel chord truss or flat truss gets its name from its parallel top and bottom chords. It is often used for floor construction. Force is an influence that may cause a body to accelerate (change speed) or be deformed. 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". 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. 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. 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. Catenaria Columna o pilar Compresión Hormigón Canto Cúpula Celosía Fuerza Cimentación Viga, jácena o perfil metálico. Vigueta Carga 74

11 English Explanation Spanish Pitched truss or common truss Reinforced concrete Shearing Slenderness Span Strength Stress Structure Suspender cable Suspension cable Tension Torsion Truss Vault The pitched truss or common truss is characterized by its triangular shape. It is most often used for roof construction. Reinforced concrete is concrete with reinforcement bars ("rebars") inside. In industrialized countries, nearly all concrete used in construction is reinforced concrete. Shear stress or shearing is a stress state where external forces applied to bars or structural elements tend to. The ratio of the length of a column to the least width is called the slenderness. Span is a section between two intermediate supports, e.g. of a beam or a bridge. 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. Stress is the internal distribution of force per unit area that balances and reacts to external loads applied to a body A structure is a man-made construction used for supporting external forces without being deformed considerably or breaking down. (see also cable) In a suspension Bridge, the suspender cables are between the suspension cables and the suspended deck. In a suspension Bridge, the suspension cables are the catenary-shaped ones Tensile stress (or tension) is the stress state where the bar tends to be stretched. Torsion is the stress resulting of twisting an object due to an applied torque. 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 A covering system made by consecutive semiround/circular arches Cercha Hormigón armado Cortadura o cizalladura Esbeltez Luz Resistencia (mecánica) Esfuerzo o tensión Estructura Tirante Catenaria Tracción Torsión Estructura triangulada Bóveda 75

12 TRIANGULATION b a 1 2 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 hormigón con barras de acero corrugado) in-floor brick (bovedilla) reinforced concrete joist (vigueta de hormigón armado)

13 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 Andrés Vigueta Tablero Viga Tirante Torre Cúpula Bóveda Celosía Zapata Ginés Ibáñez Mompeán