Forces act in different ways
Forces can act in the following ways on structures or parts of structures:
- tension,
- torsion,
- compression,
- shear, and
- bending.
The different pieces of a frame structure are called sections, elements or members of the structure.
Forces can push, pull and twist
Make six paper tubes by rolling sheets of used writing paper. Use glue or tape to prevent the tubes from unrolling.
Put your hands on both ends of a tube and push them towards each other. When you do this, you exert compression forces on the tube.
Grab a tube at both ends and try to pull it apart. When you do this, you exert tensile forces on the tube. You put the tube under tension.
Put the ends of the tube on two books and press downwards on the middle of the tube. What happens, and what kind of force did you apply to the tube?
Grab a tube at both ends and twist it as shown in this picture. When you do this, you apply torsion.
Figure 12 Join two tubes by putting a match or small stick through them as shown below.
Figure 13
When you try to pull the two paper tubes apart now, you will apply shear forces to the stick.
Find strength in shape
Figure 14 Figure 15 Fold it a third time, so that you have a flat strip that is eight layers thick.
Make two more folded strips like this.
Figure 16 Put the folded strip at the edge of your desk as shown below. Hold it down on the desk with one hand and press down lightly on the outer part of the strip to bend it downwards.
Figure 17 Now fold your paper strip half-open again, and fold it in a new way so that you get a triangular tube as shown below.
First fold like this: Then fold like this:
Figure 18 Figure 19 Figure 20 Put the triangular tube at the edge of your desk as you did with the flat strip in question 2. Hold it down on the desk with one hand and press down lightly on the outer part to bend it downwards.
What was easier to bend, the flat strip or the triangular tube?
Figure 21 The shape that you see when you look straight at one end of part of a structure is called the cross-section or profile.
Make free-hand sketches of the cross-sections of a round tube, a square tube and a triangular tube, in the space below.
Open your triangular tube and fold it again to have a T-profile as shown on the right.
Figure 23 Figure 22 Let your T-shaped section stand upright on your desk as shown here and press downwards at the top. Do not bend it now.
Take one of the flat folded sections you made in question 1. Hold it upright and press downwards at the top like you did for your strip with the T-cross section.
Which strip is stronger when you press down on its end, the flat section or the T-shaped section? Explain why.
A T-shaped section resists compression better than a flat section that has the same length and is made of the same amount of material (paper in this case).
Figure 24 Compare the resistance to compression of T-shaped, square-shaped and round sections, each made from one sheet of A4 paper. Explain your answers.
Figure 25: Metal cross-sections Metal sections that are used to build frame structures are made in a variety of profiles. Some popular profiles are shown below.
H-profile. This profile is often used as upright supports or columns, for example in buildings. It resists compression very well, and it does not bend easily.
I-profile. This profile is used for railway tracks. The broad base provides stability.
U-profile. This is lighter than the H-profile. It is often used to provide horizontal support, for example in shelves. The chassis of a truck is normally made with U-beams.
This profile is often called angle-iron, even if it is made of metal. It has higher bending strength than flat strips. It is light and is often used for cross-bracing in pylons, towers and other structures.
Tube-profile. This is the best profile for resisting torsion.
Using internal cross-bracing to resist twisting
Imagine that you made a frame structure with straight pieces of wood.
Now imagine that you twist this frame structure like the person in the photograph is twisting the towel.
The frame structure could end up looking like this:
To prevent the structure from getting twisted like this, you could add more elements as shown here.
This is called internal cross-bracing.