Have you ever wondered why bridges come in so many different shapes? From ancient Roman arches to modern suspension bridges, each design is rooted in precise engineering principles. Today, we explore the fascinating world of arch bridges—specifically, how to build a surprisingly strong one using nothing but spaghetti.

The Inspiration: From Tokyo Bridges to Kitchen Experiments

This investigation began with a televised experiment testing spaghetti bridge models. Later, during a trip to Tokyo, the elegant designs of bridges like Kachidoki Bridge, Nihonbashi Bridge, and Hijiri Bridge—each with arches positioned above or below the roadway—sparked a deeper curiosity about structural mechanics.

Research Goals: Hunting for the Strongest Arch

Our mission was twofold:

1. Curvature matters: Which arch shape bears the most weight?
2. Position is key: Are "deck arch" bridges (arches above the road) or "through arch" bridges (arches below) structurally superior?

Methodology: The Spaghetti Stress Test

Using standardized spaghetti models (2.1mm diameter, 24.8cm length), we constructed eight bridge types:

• Arch positions: Deck arch vs. through arch
• Curvatures: 90° (1/4 circle), 120° (1/3 circle), 150° (5/12 circle), and 180° (1/2 circle) arcs

Each bridge featured three main beams and nine crossbeams. Weight was applied centrally via water-filled bottles until structural failure.

Surprising Results: The 150° Sweet Spot

After initial trials revealed flaws in 180° and 120° deck arches, refined testing showed:

Notably, 150° arches outperformed all others in both configurations. Through arches consistently demonstrated greater strength, likely due to superior force distribution.

Why 150°? The 60° Angle Connection

Further analysis revealed that 150° arches create ~60° angles at their bases—the same angle found in ultra-stable truss bridges. Supplemental cardboard tube tests confirmed:

Engineering Insights

Through arches excel by converting vertical loads into compressive forces along the arch, while deck arches suffer from tension-induced central fractures. The 60° principle—manifested in 150° arches—proves universally robust by forming stable triangular force distributions.

Future Directions

This experiment underscores how subtle geometric choices impact structural integrity. Future research could explore how these principles scale to steel-reinforced bridges or hybrid designs blending multiple arch types.