Week 24 - Measuring Forces and Designing Structures to Resist Forces

Tab: Exercise 2.4

Lesson 2.4: Designing Structures to Resist Forces and Maintain Stability

ACTIVITY A: Strong Structural Shapes

Required Readings

Science in Action 7
pages 300 to 302

Science Focus 7

pages none

When taking the time and money to build a structure, we ultimately want them to last as long a possible in a safe manner. How can you ensure that a structure is stable?

For thousands of years humans have been building structures. If we look at those structures that have lasted for centuries what are the common characteristics?

When looking at two dimensional structures what is the strongest structure? Is it a rectangle, a square, or a triangle. Read page 301 in Science in Action and try the simple exercise with a drinking straw to assess these different structures.

By gently pushing on the upper corner of each shape, which structure was the strongest?


From what you have learned about strong structural shapes, build a model of a tower using mini-marshmallows and toothpicks. How many stories high can you build?

Build some floors with triangular cross beams and some without. Which is more stable? Which has more deformation?

For those toothpicks used in the vertical and horizontal directions, cut 1 cm off. Leave those toothpicks which will be used as diagonal cross beams at the full length of the toothpick.
Strong Structural Shapes (optional)

Which shape was the strongest: with crossbeams or without?

Which tower could you build taller: with crossbeams or without?

Why do you think that was your result?

What do you think this means for modern architects?

ACTIVITY B: Structural Components

Required Readings

Science in Action 7
pages 302 to 303

Science Focus 7

pages 321 to 324

Bridges and buildings are usually composed of many components which make up the entire structure. Some components are built to add aesthetic value and others to add strength. Lets take a closer look at arches and beams.

Using your textbook and the How Stuff Works Website answer the questions below which talk about key structural components.

Arches can carry very large loads because the force of the load is carried down through the arch. Arches have been constructed for thousands of years. Many of these arches still stand today.

If you would like to build your own arch, watch the video below.

ACTIVITY C: Structural Stress, Fatigue, and Failure

Required Readings

Science in Action 7
pages 303 to 306

Science Focus 7

pages 315 to 318

No structure is perfect. With enough force acting on a structure over time it will begin to fail. Sometimes bad design will speed up the process. In some cases, when certain factors are not considered when building a structure, failure can happen much faster than original designs would have suggested.

This is what happened to the Tacoma Narrows bridge in November 1940. Nicknamed "Galloping Gertie", the Tacoma Narrows bridge was prone to sway in the wind even when it was being constructed. The natural forces of the winds were not considered when the original architectures designed the bridge.

When a combination of external and internal forces act on a structure the stress can weaken it. When a structure under goes repeated stress over time it leads to fatigue.

Metal fatigue is a common source of structures failing. Take for example, a strip of metal that is bent in half and then straightened and bent in half over and over again. Eventually, the seam in which the metal is bent along will become weak and eventually fail.