Forth Bridge Spatial Reasoning Learning Activity
Learning Objectives:
1. Define parts of a cantilever truss bridge.
2. Use the concept of load path to describe how a load is carried through a structure to its
supports.
3. Explain how a model bridge is proportional to a real physical bridge.
4. Draw a simplified “ball and stick” model of a cantilever truss bridge.
5. Identify the sense of the force (tension or compression) being carried in the elements of a truss.
6. Relate a simplified model of a structure to the real thing.
7. Explain the concept of equilibrium or balance.
8. Estimate forces in a structure, given the forces applied to the structure.
9. Use a computer program to analyze a structure for reactions and internal forces.
Lesson Plan
1. Provide an orientation to the topic using drama, history and photographs. Build suspense. Put
the students in a role (angry/fearful public, engineer “Bouch”, engineer “Fowler”, builders, steel
makers, Queen of England)
2. Provide students with learning objectives
3.
Materials Needed
 Five 2-foot sections of 2x4 with handles cut or attached to them
 Two door hinges
 KNEX “Real Bridge Building” Kit
 Computer with Chrome browser
 Autodesk ForceEffect app for Chrome browser
Structural Modeling Exercise
Use the guidelines provided to sketch a simple model of the living model of the Forth Bridge, using
“joints” where parts connect and “members” like arms, bars and beams. Label the pieces of the bridge
below your sketch as tower, cantilever arm, suspended span, or counterweight. Now consider which of
these members you think is in tension (being pulled), compression (being pushed) and bending (curving
under load). Label the elements with T, C or B on your sketch, accordingly. How sure are you?
Very sure
Sure
Not sure
Very unsure
(Circle one)
The Living Model
Next, let’s have some volunteers actually act as the living model of the bridge and let’s see what sort of
forces they feel.
We need:
1. Five people
a. Two towers (people with similar proportions and good arm strength)
b. Two counterweights (anyone)
c. One load (the lightest person in the classroom)
2. Two chairs (for the towers to sit in)
3. Bar elements (2x4’s cut to length and connected with door hinges)
As we build the living model, let’s discuss what the individuals are feeling, how they support themselves
or the others in the structure, and how this structure might be unstable.
Now that we have built the living model of the Forth Bridge and we have discussed what the individuals
who acted as its parts felt, let’s analyze it! Based on the descriptions from the students acting as parts
of the bridge, sketch the simple model again below changing any labels: tension (T), compression (C) or
bending (B).
How sure are you now?
Very sure
Sure
Not sure
Very unsure
(Circle one)
The Computer Model
How would we know exactly how much force each of these elements carries? Estimate these forces
based on the previous discussions. Label the members of your last sketch:



greater than (>) the weight of half the man in the middle
equal to (=) the weight of half the man in the middle
less than (<) the weight of half the man in the middle
If you were in college, you would take a statics course and use geometry, trigonometry, vector
mathematics, and concepts in physics to analyze this structure. We can take advantage of a software
package called Autodesk ForceEffect to model and analyze the living model to determine exactly what
forces were in it.
Create the simple model in ForceEffect and sketch it again, labelling each member with the force it
carries (e.g. 45 lb (T)).
Assessment:
I correctly identified the members of the human model in tension, compression and bending on the first
try, without seeing the human model, computer model, or KNEX model. YES NO (circle one)
On the second try: YES
NO (circle one)
I correctly estimated the forces in the human model before seeing the results in ForceEffect YES
NO
Explain in words how the forces are carried through human model of the Forth Bridge. First, how is the
load of the man in the middle carried through the structure?
Second, how is the structure supported where it connects to other things, like the ground and
anchorages?