# Homework Help: Best bridge type

1. Feb 2, 2016

### nikirock021

1. The problem statement, all variables and given/known data
In class we were asked to build a bridge with the length of 50 cm and the width of 20 cm, maximum weight is 50 g and the test is to place the bridge on two parallel chairs and then place a heavy object in the center. we can build any type of bridge. material: unknown. My question is that what would be the most efficient and easy to build bridge type that can cover all mentioned criteria?
if you have any advice on bridge making please share, it would be a great help. Thank you

2. relevant equations

the modulus equation?

3. attempt at solution

I was thinking of building a cable stayed bridge as it seems to be the lightest as per theory.

Last edited by a moderator: Feb 3, 2016
2. Feb 3, 2016

### Staff: Mentor

What are the rating criteria?
Two cables with a tiny bit of additional material for stiffness would hold a large weight easily - do you need some deck, and does it have to be horizontal?

3. Feb 3, 2016

### nikirock021

The criteria is simply that the bridge has to hold. The material has been updated straws, masking tape, and string

4. Feb 6, 2016

### pongo38

A cable-stayed bridge would need to use the deck as a compression anchor, and it would be difficult to load. A better solution is to use a tied beam with the cables going below the deck. Cables still need to be anchored to the deck, but it's easier to load. You need to know the dimensions of the weights to be used, but a triangular or trapezoidal form is closest to the bending moment diagram and should be the most 'efficient' Think about how to modify this two-dimensional shape so thet it doesn't buckle out of plane. Perhaps you should list all the possible failure modes before you start.

5. Feb 6, 2016

### Staff: Mentor

Ah, good point. Do you get anchor points that can take tension from a string?

6. Feb 6, 2016

### pongo38

Think of winding the string around something that won't slip. There are hazards in all designs but you will best learn from the experience itself.