STRUCTURES

INTRODUCTION

A structure is something that supports an object or a load1.

A structure must:

• be strong enough to support its own weight and any load that is put on it.
• be stable (not topple over2 easily when a force acts on it).
• resist without losing more shape3 than expected.

TYPES OF STRUCTURES

There are three types of structures:

• Mass structures (estructuras masivas)
• Frame structures (estructuras de ensambladas: trianguladas, entramadas y colgantes)
• Shell structures (estructuras de carcasa o laminares)

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:

1. Triangular structures. These are structures made of metal or wooden beams joined together to form triangles. The triangle is the only geometric figure that doesn’t deform when subjected to forces. If pressure is applied to a square-shaped beam structure, it is easily deformed into the shape of a rhomboid, while a triangular structure retains its shape under pressure.
2. Post and beams structures. These types of structure are composed of vertical and horizontal shafts (posts and beams). The posts and beams are firmly joined to produce three-dimensional shapes. Modern buildings housing and office blocks are frame structures. The most commonly used materials in frame structures are steel, reinforced concrete and to a lesser extent, wood.
3. Suspended structures. This type of structure uses cables, called suspenders which the structure hangs from. Cables are used to fasten bridges, marquees, antennas and towers. There are many kinds of suspended structures which can cover large spaces like stadiums and pavilions, without the need for columns or pillars.

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:

• To support to hold something upright and steady14
• To span15 a distance or to reach across a gap16
• To contain or to protect something

FORCES ACTING ON STRUCTURES

A successful structure must resist all forces acting on it without collapsing17.

Forces can be:

• Static forces: structure weight and any load permanently attached to it.
• Dynamic forces: produced by the wind, sea waves, vehicles, people, etc.

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

• STRESS: It is the internal tension which all bodies experience when one or more forces are applied to them. There are five types of stress:

Depending on the external force or load acting on a structural member, it can be under:

• Tension (tracción): is the name given to a force that tries to pull something apart20. A structural member in tension is called a tie21. A tie resists tensile stresses.

• Compression (compresión): is the name given to a force that tries to squash22 something together. A structural member in compression is called a strut23. A strut resists compressive stresses.

• Torsion (torsión): is the name given to a turning or a twisting force.

• Shear (corte o cizalladura): a shear force is created where two opposite forces try to cut something in two. That means, one part of the structure can be forced to slide 24 over another.

• Bending (flexión): forces which act at an angle to a member, tend to make it bend25.

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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)

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STRUCTURAL CONDITIONS

There are three conditions for a structure to work well:

1. Stability: It is the capacity of a structure to remain upright and not fall over. A structure’s centre of gravity must be centred over its base and close to the ground for it to be more stable.
2. Resistance: It is the capacity of a structure to bear the tensions that it’s subjected to without breaking. A structure’s resistance depends on its shape and the material used to build it.
3. Rigidity: All bodies deform slightly when a force is applied to them, but this deformation must not prevent the structure from fulfilling its function. To prevent deforming, we can use triangular structures or add cross-bracing using crossed cables.

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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