INDEX
  1. Warming up
  2. What is a structure
  3. Key concepts in structure design
  4. Classification of structures
  5. Let's work with structures: Truss structures
1. Warming up


Activity 00: After having watched the video, answer the questions below
  1. List the steps presented in the video
  2. Pair each step presented in the video with the following list of Engineering Practices: Defining Problems, Using Mathematics and Computational Thinking, Designing Solutions. 
  3. Describe what these steps mean in your own words.


Activity 01: In order to build your vocabulary about structures match the English words with their Catalan translation. You can look up the words in the online dictionary Wordreference .

Torsió  /  estructura  /  centre de gravetat  / pes  /  càrrega  /  carcassa (closca)  /  biga  /  formigó  /  compressió  /   tensió  (tracció)  /  flexió  /  tall  /  columna  /  força

Frame            :                      
Beam:
Column:
Centre of gravity:
Load:
Force:
Weight:
Concrete:
Shell:
Compression:
Tension:
Bending:
Shearing:
Torsion:
 




1. What is a structure?

Have you ever thought about the role your skeleton plays in your body?. Skeleton is the frame that supports your body’s weight and provides it with stability. Therefore animals have structures in their bodies.
http://www.virginmedia.com/science-nature/amazing-bodies/amazing-facts-about-the-human-body.php?ssid=7

Structures in objects play the same role as skeletons in animals. They  support objects’ weight and provide them with stability. They must also be strong enough to support their own weight. Imagine you build a brick wall around your house, the wall must be able to resist the force of the wind, the weight of the bricks, and the load of a person that decides to climb on top.

We can find examples of structures in buildings, bridges, dams, cars, etc.



2. Key concepts in structure design

Engineers and architects design structures that must be:

  1. Rigid
  2. Stable
  3. Capable of supporting the loads and forces for which they have been designed
Let’s study these three concepts in structure design:

Rigid. It means that the structure does not bend or flex when a force or a load is applied. Therefore the structure preserves the original shape. The question is; how can we design rigid structures?, to answer this question look carefully at the images below: 
Activity 1: Looking at the pictures you’ll have noticed that geometric shapes help engineers to achieve rigid structures, try to find out which geometric shape helps us to achieve rigid structures. Justify your answer.

Activity 2: Build the shapes below and try to find out if they are rigid. What do we need to do to achieve rigidness in all these structures?




But rigidity is not always a good property in structure design. The ones that must support dynamic forces (forces that change their direction and intensity quickly) must be designed to be flexible. This is the case of buildings and bridges built in earthquake regions. These structures need to be flexible in order not to be destroyed in case of an earthquake.


Stable. It means that the structure must maintain its position when forces which tend to tilt it and knock it down are applied. The stability in objects has to do with the centre of gravity.


Activity 3: Draw the shapes below and try to locate their centre of gravity



Looking at the picture below we’ll understand that:

a.    In stable objects the centre of gravity raises when the object is tilted so it’s not easy to knock it down

b.    In unstable objects the centre of gravity lowers when the object is tilted so it’s very easy to knock it down

In neutral objects the centre of gravity stays at the same level even if the object is pushed and it rolls.



The position of the centre of gravity in objects has to do with its stability. The lower the centre of gravity (G) is, the more stable the object is. Moreover, increasing the area of the base the stability also increases. These are the reasons why F1 cars have such low centre of gravity and wide base in order to be fast round the bends.





Activity 4: Looking at the picture below, try to explain which table will topple


Activity 5: Looking at the pictures below try to justify their stability



 
Activity 6: Take a cardboard, draw and cut the shape below. Then, you must think of a method to determine its centre of gravity.


Capable of supporting the loads and forces for which it has been designed

Structures must support loads or they will fall apart. These loads produce forces that act on structures. Therefore, in structure design we need to know what kind of loads and forces will affect them.

Loads

The weight of the structure. It’s permanently attached to the structure and it includes columns, beams, and other construction materials. This load is called “the dead load”.
www.aulatecnologia.com

The weight of the objects. This load includes the weight of all the things that are on the structure, like people, furniture, cars, etc. It’s called “the live load”. Columns and beams must be designed to support a minimum value of live load above them. The thicker beams and columns are, the less likely they are to bend. But thick beams and columns increase the dead load and the cost of the structure. Nowadays, scientific knowledge and mathematical methods used on computers let engineers calculate the necessary amount of material needed to build any structure. Unfortunately Roman engineers didn’t have computers, for this reason they designed structures with thick beams and columns that have lasted till today. The question is: will modern structures last for 2000 years?


Activity 7: Compare the two pictures above. They belong to different periods of human history. Write a brief summary explaining all the differences you see regarding the dead load and the live load both bridges can support. Justify the differences.

Soft soil. Structure designers must avoid that the soil beneath a structure settles irregularly. When this phenomenon happens the structure sinks and it can collapse. In order to support structures on soft soils we can use heavy concrete columns that sink deep into the earth until they find hard solid soil, thus keeping the structure safe and sound.


 Thermal load. This load produces forces that expand or shrink the structure. When temperature changes, beams and columns change their shape and push or pull on other parts of the structure. In order to avoid the effect of this load, expansion joints and roller joints are used. They provide columns and beams the freedom to expand and contract as the temperature changes.


Earthquake. This dynamic load pushes and pulls horizontally on a structure. Solid walls of reinforced concrete provide structures with earthquake resistance, moreover mobile joints let structures absorb tensions. In the pictures below, we can see how mobile joints work in case of an earthquake


Wind. This load pushes horizontally on the structure. Structures are reinforced by diagonal braces usually made of steel. In order to understand the importance of the loads produced by wind in structures, watch the following video:



Forces

The most important types of forces produced by loads on structures are: 

      • Compression


      • Tension

      • Shear force


      • Bending


      • Torsion


By means of the link below we'll understand how forces work on structures:
http://www.pbs.org/wgbh/buildingbig/lab/forces.html

Activity 8: Using the webpage write a summary about how each type of force acts in structures. Your summary should have the contents below:

  1. Definition
  2. Simple sketch showing how the force works  
  3. Some examples about how the force is applied in real structures
  4. Suitable materials to support the force
Activity 9: Explain the difference between loads and forces, and give some examples of both. 
Activity 10: Explain the types of forces that act on the structures below




3. Classification of structures:

Looking at different structures we can find three basic designs:

·         Mass structures
·         Frame structures
·         Shell structures

Let’s study these three types of structures in detail:

·         Mass structures


These structures are built by piling up materials such as stones, bricks, concrete blocks etc. If you look at a mass structure you’ll notice that it’s made of arranged pieces. But have you ever wondered about the pattern of bricks in a brick wall? The centre of each brick is placed over the ends of two bricks in the row below. But we can find bricks and concrete blocks arranged in other ways:


Examples of these structures are brick walls, ancient buildings, etc. Advantages of these kind of structures are that they are very heavy so they hold firmly in place by their own weight, and it makes no difference if small parts break off. A prove of that is the perfect state constructions such as the Great Wall of China and Roman bridges and aqueducts are in nowadays. A disadvantage is that bricks add a lot of weight to the foundation. Therefore, these structures must be built on firm foundations.

·         Frame structures

They have a skeleton made of a strong material, which supports the weight of the structure.  Depending on the material they are built of they can be light weight structures which can resist loads many times their own weight.



The advantages of these structures are:

  1. They are low weight structures because they don’t need thick walls as the dead and life loads are supported by columns and beams.

  1. Their construction is very fast due to the simple geometry of the main structural elements (columns and beams).
  2. They are very rigid and stable which enables them to resist big dead loads and lateral loads such as the wind.
  3. Flexible utilization of space. As most interior walls are not structural, they may be removed even when the building construction is finished.
  4. Almost any shape can be built.
  5. It’s easy to provide the building with natural light as openings can be placed easily on external walls as they aren’t structural.


 ·         Shell structures

Before we deal with this kind of structure let’s watch the following video:

Activity 11: After having watched the video, try to justify if the sentences below are true or false:

a) Shell structures are copied by humans from nature. The shell of a car protects its occupants as a shell of an egg protects the egg.

b) Changing the shape of a material by adding a curve makes it stronger. A clue, take a piece of paper and try it!.

c) Shell structures are assembled to make one piece.

d) Tin cans and bottles are good examples of shell structures.

e) Cars and aeroplanes combine both, frame and shell structures.

f) An egg in vertical position is stronger than if we place it horizontally (justify your answer with a sketch showing how the forces act on the egg’s surface).

g) Shell structures are made of sheets of materials such as concrete, steel, aluminium, etc. (which make them light weight structures).

h) In order to make shell structures stronger, we can add ribs into them.

i) Curves allowed Bilbao Guggenheim Museum’s designers to build a building with thin walls and huge open gallery spaces for exhibitions.

j) Double curved dams save almost one half of the materials used in simple curved dams.

K) Shell structures are not good at resisting dynamic forces.
4. Let's work with structures: Truss structures
It's time to apply your knowledge about structures by designing simple structures. We'll deal with an example of frame structure: truss structures. These frame structures  are built by trusses (connected elements forming triangular shapes). The connected elements are stressed from tension or compression in case of static loads. But, in case of dynamic loads they can be stressed from both (tension and compression ). This year we'll focus on static loads. Many bridges are built using trusses because it's an economical solution to save materials.


You'll learn to design truss structures by using the program LinPro (Static and dynamic analysis of plane frames). The steps you must follow to design structures are explained in the video below.


Activity 12: Using the program LinPro analyze the structures below. Then write and draw  your conclusions in your notebook :

  • Sketch of the structure
  • Reactions in supports
  • Type of force in each element (tension or compression) and it's value 





Activity 13: Identify the type of force (tension or compression) applied on each element in the structures below


Activity 14: Let's build our structure



Final activity: In order to sum up all we have learned about structures complete the text with the words below. Some words may be used twice.

Frame  /   bending  /   area of the base  /   mass  /   triangle  /   stable objects   /   lower    /    center of gravity   /   forces  /   shell   /   unstable objects   /   more  /   dynamic forces   /   skeletons   /   dead load   /     weight   /   torsion   /   thermal load   /   stability     /    curve   /     neutral objects   /    rigid    /   stable   /   live load

Structures in objects play the same role as ___________in animals. They  support objects’ ___________ and provide them with ___________. They must also be strong enough to support their own weight.

Engineers and architects design structures that must be: ___________, ____________ and capable of supporting the loads and forces for which they have been designed.

The shape that helps us to achieve rigid structures is the ___________ .

A structure is ___________ when it’s able to maintain its position when forces which tend to tilt it and knock it down are applied. The stability in objects has to do with the _____________. In _____________the centre of gravity stays at the same level even if the object is pushed and it rolls. In ______________the centre of gravity raises when the object is tilted so it’s not easy to knock it down. In _____________ the centre of gravity lowers when the object is tilted so it’s very easy to knock it down. The ________ the centre of gravity (G) is, the ____________ stable the object is. Moreover, increasing the ______________ the stability also increases.

Loads produce ___________ that act on structures. The weight of the structure it’s permanently attached to the structure and it includes columns, beams, and other construction materials. This load is called _______________. The weight of all the things that are on the structure, like people, furniture, cars, etc. is called ______________. The load that produces forces that expand or shrink the structure when temperature changes is called _____________ . When a straight material becomes curved, a ____________ force is acting. ____________ is an action that twists a material.

____________ structures are built by piling up materials such as stones, bricks, concrete blocks etc. _____________ structures have a skeleton made of a strong material, which supports the weight of the structure. Changing the shape of a material by adding a ____________makes it stronger. Tin cans and bottles are good examples of ______________structures. Cars and aeroplanes combine ____________ and _____________ structures. Shell structures are not good at resisting _____________. 


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