Roofs and pylons

frame structures.Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg1_img1.png
Figure 1: Internal view of a root structure
Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg2_img1.png
Figure 2: Different types of electricity pylons
Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg3_img1.png
Figure 3: Different types of roof trusses
solid objects that consist of one part only. Other objects, like bottles, pots and water tanks, are hollow objects that can also be called shells. We also make objects that consist of different parts that are put together, like chairs, tables and bridges. These objects are called frame structures and it is important to try to make frame structures strong.Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg4_img1.png
Figure 4
  • Use your hand to press down on the cardboard roof. What happens?
  • Your house's roof plates may be very strong and will not bend. But what will happen when a couple of big men sit on the roof to fasten the roof sheets?
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg4_img2.png
    Figure 5
  • Look carefully at each of the sketches. Then write a sentence for each sketch to describe the method to strengthen the roof by bracing it.

    The word "brace" comes from the French word "bras", which means "arm". When you brace a structure, you put in something like an extra arm to make it stronger.

    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg5_img1.png Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg5_img2.png Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg5_img3.png
    Figure 6
    1. Case A:
    2. Case B:
    3. Case C:
  • trusses. Trusses can be made of wood or steel. The different parts of a truss are called members. Each truss has a vertical member in the middle. This is called a king post . Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg5_img4.png
    Figure 7: Incomplete roof trusses
  • What part of the roof trusses shown here will prevent them from tearing apart when the roof sheets are loaded onto them? Mark this part on one of the roof trusses on Figure 8.
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg6_img1.png
    Figure 8
    tie beam. Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg6_img2.png
    Figure 9: The yellow tie beam is under tension.
    Tech1_LG_gr8_ch1_fig11a.tiftension in the tie beam, just like there is tension in a rope you pull.tensile forces.rafters.
  • Write the name of each of the four members next to the member on the diagram in Figure 10. This is called adding labels to the diagram.
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg7_img1.png
    Figure 10
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg7_img2.png
    Figure 11
  • What can you do to strengthen the trusses so that the rafters will not bend when a heavy load is acting on them? Make a sketch here to show your plan.
    sketchspace
  • struts to support the rafters.Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg8_img1.png
    Figure 12
    compressive force.Tech1_LG_gr8_ch1_fig17a.tifTech1_LG_gr8_ch1_fig17b.tifTech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg8_img4.png
    Figure 13: Compressive forces acting on beams

    A force that is able to stretch or pull something apart is called a tensile force. A force that is able to compress or squash something is called a compressive force.

  • Roll a sheet of paper into a tube and twist it like the person in the photograph is twisting the towel. By doing this you apply a torsion force on the paper tube.
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg8_img5.png
    Figure 14
  • Press your two hands together tightly as shown in this photograph. Then rub them against each other.
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg9_img1.png
    Figure 15

    If you put a piece of clay between your hands while you do this, the shape of the clay will change. The force applied by your rubbing hands is called shear force.

  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg9_img2.png
    Figure 16
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg9_img3.png
    Figure 17
  • In each case, say which kind of force is demonstrated in the picture.
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg10_img1.png
    Figure 17
  • Will this roof structure work well?
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg10_img3.png
    Figure 19

    Describe what could go wrong when roof plates or tiles are put on this roof structure.


    Trusses like the ones in the drawings below are called queen-post trusses. The two vertical members are called queen-posts. They are shaded in Figure 20.

    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg11_img1.png
    Figure 20
  • Figure 21 is a drawing of another type of queen-post truss. Shade the queen-posts on the drawing.
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg11_img2.png
    Figure 21
  • On Figure 22, label members under compression with a "C" and members under tension with a "T". Do this for all the members except for the rafters.
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg11_img3.png
    Figure 22
  • Look at the pictures below and on the next two pages. What purposes do these structures serve? Why do we build them?
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg12_img1.png
    Figure 23
  • Why do you think the pylon in Figure 24 is designed the way it is, and not in the way shown in Figure 25?
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg12_img2.png
    Figure 24
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg12_img3.png
    Figure 25
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg13_img1.png
    Figure 26
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg13_img2.png
    Figure 27
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg13_img3.png
    Figure 28
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg13_img4.png
    Figure 29

    The pylons in Figures 26 and 28 are solid structures made from concrete. All the other pylons are steel frame structures.

  • What do you think is indicated by the green parts of these drawings?
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg14_img1.png
    Figure 30: Skewing of a rectangular frame

    You can also use the word skew as a verb:

    You can say the "forces skew the frame".

    Or you can say that the "forces are skewing the frame".

  • What happens to the lengths of the lines AC and BD when the frame skews? Go measure it and find out!
  • triangulate it:

    A support inserted to triangulate a frame can also be called a brace.

    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg14_img2.png
    Figure 31: Simple triangulation of a frame
  • Compare what happens when you apply forces as in case A and case B.
    1. Why does the frame keep its shape in case A but changes shape in case B?
      Hint: Think about the type of forces acting on the blue beam.
    2. How can you improve the design of the frame so that forces cannot make it skew?
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg15_img1.png
    Figure 32: Cross-bracing a frame
    cross-bracing. It is a special kind of triangulation. With cross-bracing, the frame does not skew when forces are applied as in case A or case B.
  • Compare the frame design in Figure 31 with the one in Figure 32.
    1. Can you use steel cables instead of beams for the braces in both of these frame designs? Explain your answer.
    2. If you use steel beams as braces for both frame designs, do you have to use the same thickness beams in both designs? Or can you save material and use thinner braces in one of the designs?
  • Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg16_img1.png
    Figure 33: Skewing
    Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg16_img2.png
    Figure 34: Twisting
    inside the frame structure. The photos below show how a frame structure can be built with cross-bracing on the inside and on the outside. The cross-bracing on the inside is in red, and the cross-bracing on the outside is in dark blue.Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg16_img3.png
    Figure 35: Internal cross-bracing