structures |
Escrito por Elisa Calvo Villanueva | ||
Lunes, 15 de Febrero de 2016 18:43 | ||
STRUCTURES
INTRODUCTION
A structure is something that supports an object or a load1. A structure must:
TYPES OF STRUCTURES
There are three types of structures:
WHATEVER THE TYPE OF STRUCTURE IT CAN BE EITHER A NATURAL OR A MANUFACTURED STRUCTURE.
Mass structures Mass structures are solid structures which rely on4 their own weight to resist loads. Examples of mass structures are: roman bridges and aqueducts, walls and domes5 of ancient stone churches.
Frame structures Frame structures are made from many small parts (called members), joined6 together. Electric towers, some bridges (for example suspension bridges7), cranes8, buildings and scaffolding9 are just a few examples. There are three types of frame structure:
The term “triangulation” is used to describe the use of triangles, arranged together to form a frame. This is one of the most economical ways of building structures.
Shell structures Shell structures are made or assembled10 from shaped panels to make one piece. Tin cans, bottles and other food containers are good examples of shell structures, but larger things such as car and aeroplane bodies are examples of more complicated shell structures.
Natural structures Structures are not new. Nature produced the first structures before humans. A tree is a natural structure. It has to carry the weight of its own branches as well as resisting strong winds.
Manufactured structures A manufactured structure is a structure built by human beings11. Many of Nature’s structures have been copied by humans. The shell of a snail and the body of a modern car are both shell structures designed to protect their occupants. When we think of manufactured structures we often think of obvious examples such as bridges, electricity pylons12 and tall buildings, but some of the common structures are quite simple. Most objects resist loads, and most objects are also structures.
In short13, all structures do one or more of these three things:
FORCES ACTING ON STRUCTURES
A successful structure must resist all forces acting on it without collapsing17. Forces can be:
Dynamic forces are usually much greater than static forces and very difficult to predict. These are the most common reason for structural failures18. External forces or loads cause internal stresses19. Not all forces or loads acts in the same way. Forces can: - Bend - Pull - Press - Twist
Depending on the external force or load acting on a structural member, it can be under:
MEMBERS IN A STRUCTURE
The different parts of a frame structure are called members. Different members within a structure have to resist different kinds of forces. It is important to choose the most suitable member for the job. How well a structure member resists the forces acting on it depends on: the material it is made of, its shape and its size. When a member is required to resist tension forces, flat strips26, cables or wires27 can be successfully. However, all these shapes are poor in compression. When compression forces are present, angle girder28, I girder and other similar sections must be used. Any member which has to resist bending is called a beam29. Beams are used in bridges, buildings… where we need to span a gap and carry a load without deflecting30. Beams are made from different sections or shapes and they are easy to remember because they are named after their shapes.
A beam is a piece of material (girder) supported at either end. A cantilever is a beam which is supported at one end only (diving board 31 for instance)
STRUCTURAL CONDITIONS
There are three conditions for a structure to work well:
How to ensure that structures have good structural conditions?
1. TRIANGULATION. If you look at some pictures of familiar frame structures like cranes, electricity pylons or roof supports you may notice that triangulation is used to make them rigid. When forces are applied to a simple four-sided structure, it can be forced out of shape quite easily. A structure which behaves in this way is said to be non-rigid. By adding an extra bar or member, the structure then cannot be forced out of shape, and is said to be rigid. The additional member has formed two triangles in the structure. This is known as triangulation. The triangle is the most rigid frame structure.
2. An alternative to triangulation is to use a gusset plate32. A gusset is simply a piece of material used to brace 33 and join34 the members in a structure. It can be triangular or a corner gusset plate.
3. Support bracket can be used instead of triangulation too.
4. Straps or tensioners are another alternative. They are used to avoid the structure fall down.
KEY WORDS
1 load: carga 2 topple over: volcar, caer 3 shape: forma 4 rely on: depender, confiar 5 domes: bóvedas 6 To join: unir 7 suspensión bridge: puente colgante 8 crane: grúa 9 scaffolding: andamio (también climbing frame) 10 to assemble: montar, unir 11 human beings: seres humanos 12 pylon: poste 13 in short: en resumen 14 to hold upright and steady: mantener vertical y firme 15 to span: cruzar 16 gap: hueco 17 to collapse: derrumbarse, hundirse 18 structural failure: fallo estructural 19 stress: carga, tensión 20 to pull apart: romper en dos, tirando 21 tie: parte de una estructura que resiste esfuerzos de tracción 22 to squash together: aplastar, apiñar 23 strut: parte de una estructura que resiste el esfuerzo de compresión 24 to slide: deslizar 25 to bend: doblar 26 flat strip: lámina plana 27 wire: alambre 28 girder: viga, aquí se refiere a perfiles metálicos con diferentes secciones. 29 beam: viga, travesaño 30 to deflect: desviarse, deformarse, cambiar de dirección 31 diving borrad: trampolín 32 gusset plate: placa de refuerzo, normalmente triangular. 33 to brace: asegurar, reforzar 34 to join: unir
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Última actualización el Viernes, 18 de Marzo de 2016 10:06 |