Goal: Design and build a model bridge that spans a given distance and holds the maximum load possible using the provided materials. Your bridge must be strong, stable, and efficient!
Materials:
Popsicle sticks
Wood glue
Only the provided materials may be used.
The bridge must be no wider than 10 cm.
Build time: 60 minutes.
Testing will be done by placing weights at the center of the bridge until it fails.
A bridge must resist different types of forces and distribute the load to the supports.
Compression: squishing force
Tension: pulling force
Shear: forces sliding across each other
Bending: caused by the load pushing down on the bridge
Common Bridge Shapes
Triangular Truss
→ strong and stable
Howe Truss
→ good for heavy loads
Arch Bridge → compression only
Load (L) is the total weight the bridge holds. If the bridge holds 5 weights and each weight is 20 g:
L = 5w
where w = weight of one mass L = 5(20) = 100 g
Stress (σ) is the force on a material
divided by the area (A) of the material.
σ = F/A
If a stick has a cross-sectional area of 0.2 cm2 and the force on it is 12 N:
σ = 12/0.2 = 60N/cm2
Stronger designs spread the load across more members, reducing stress.
Your goal is to MAXIMIZE L (the load) before failure!
Example Calculation:
If your bridge holds 8 weights (20 g each) and the span is 30 cm:
L = 8(20) = 160g
Convert to Newtons (1 g ≈ 0.0098 N)
P = 160g×0.0098 ≈ 1.57N
M = 7.5P = 7.5(1.57) ≈ 11.78N· cm
Bending Moment (M) at the center of a simply supported beam with a center load:
where P = load
M = PL/4
L = spanlength
For our bridge, L = 30 cm
Good design Balance of Strength, Stability, and Efficiency!
S0,M = (P(30))/4 = 7.5P(N· cm)
☆ Plan carefully. Build strong. Test fairly. Improve together! ☆