What Type of Bridge Can Carry the Heaviest Load?

Question

A few weeks ago, I was looking in my science book and I saw an article about bridges made from drinking straws. I wondered if the shape of the bridge would affect its strength, and which type of bridge would be the strongest.

Hypothesis

I think an arch bridge will be stronger than a beam bridge because when you put a load on an arch bridge the bridge pushes outward as it’s loaded.

Materials

To do my project, I needed building materials for the bridges and some sort of weights to load them. I decided to use toothpicks and wood glue to construct the bridges and bricks for weights.

Procedure

I started my project by reading about bridges on the Internet and in the encyclopedia. I learned there are four main types of bridges: arch, beam, suspension, and cable-stayed. I decided not to build suspension and cable-stayed bridges because they’re too complex. I also learned the words shown below.

compression

the act of pressing or pushing

deck

platform extending horizontally that often carries the roadway

engineer

person who uses mathematical and scientific principles to design and construct efficient structures and machines

span

portion of a bridge between two supports

stress

the force acting on a body divided by the body's cross-sectional area. Force per unit area.

tension

the act of stretching or pulling

torsion

the act of twisting

truss

a rigid triangular framework

First, I drew scale plans of the bridges by looking at pictures and drawings of other bridges. Then, I cut toothpicks to size and glued them in place onto copies of the plans. When the glue dried, I peeled the paper off the sides of the bridges and connected them with more toothpicks.

I made two different beam bridges and two different arch bridges to test different styles of each type of bridge. Next, I weighed the bridges and took pictures of them. Three of the bridges weighed 3/8 ounce each. The circular arch bridge weighed 1/2 ounce.

To load the bridges, I put them on top of two benches, spanning a 7-1/2” gap between the benches, then balanced a wooden platform on top of the bridge and stacked one brick at a time on the platform until the bridge broke. Afterward, I weighed the bricks on a bathroom scale to see how much the bridges held.

Results

The circular arch bridge held the most weight. It held 35 pounds or 7 bricks. The angular arch bridge held 15 pounds. The Pratt truss beam bridge and the Warren truss beam bridge each held 5 pounds and collapsed when the second 5-pound brick was added.

Conclusion

My Hypothesis was right.The arch bridges held more than the beam bridges because when you put a load on an arch bridge the bridge pushes outward as it’s loaded.