# How to calculate bridge deformation based on "design"?

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1. Nov 30, 2015

### David Goldman

1. The problem statement, all variables and given/known data
I'm not sure if this belongs here, sorry if it does not. I am trying to find the most amount of mass a bridge can hold before deforming/collapsing. I was trying to factor stress/ultimate tensile strength into it in a way. Is there a way I can calculate ultimate tensile strength and/or stress if I'm creating different designs of bridges using balsa wood? I'm trying to find if a certain design of bridge (truss, suspension, arch, etc.) affects how much load it can carry or how much load a bridge design can carry before collapsing.

2. Relevant equations

Unknown
3. The attempt at a solution
Nothing so far

2. Nov 30, 2015

### David Goldman

Purpose: I want to find the amount of stress or ultimate tensile strength of different designs of bridges.

3. Dec 1, 2015

### Staff: Mentor

Welcome to the PF.

4. Dec 1, 2015

### David Goldman

I have been reading about tensile strength , however, I noticed it was only for materials, not based on different designs. I am currently in my senior year of high school. This is not really explained in my physics text book as it pertains more to engineering. I read about stress, strain, and tensile strength, and I noticed that they only apply to materials? I wanted to see if I could calculate the stress, strain, or tensile strength based on different designs of bridges of the same material (balsa wood) instead of measuring how many weights each design could hold until it collapses. Sadly, I could not find much on this or the equations needed to calculate the, if available. Thanks for the response!

5. Dec 1, 2015

### SteamKing

Staff Emeritus
It's much cheaper (not to mention safer) to test materials in a lab to find out what their ultimate tensile strength is, or their stress v. strain charactertistics.

Doing such testing using complex structures, like bridges for instance, introduces too many unknowns into the process of measuring things like tensile strength. The pieces tested in a strength lab use a simple geometry and have standardized dimensions for things like length, diameter, etc., which makes calculations very simple and reduces the possibility of making errors. By testing only a single piece, it is clear when failure occurs, which may not be so apparent in a complex structure like a bridge.

This article discusses tensile tests and how many engineering materials are tested:

https://en.wikipedia.org/wiki/Tensile_testing

The information derived from such tests is then used to design a particular structure, say a bridge or a skyscraper.

A further word. The strength characteristics of materials like balsa wood are dependent on several factors which may not be present in metals. The direction of the grain relative to the application of the load, the moisture content, and the species of wood all factor into what strength characteristics you will obtain. Metals don't have these limitations.