Re: What bridge design supports the most weight?

Lauren: 

Engineers are sometimes called "practical scientists" because we use the 
tools and discoveries of science to make life better for people.  As I'm 
sure you know, the scientific method  consists of four parts:  First, 
observation of some phenomina.  Second, the development of a hypothesis.  
Third, testing the hypothesis by experimentation or observation.  Finally, 
review.

So how do we apply the scientific method to a bridge?  Start by observing 
and researching bridges.  There are many different types:

Arch bridge
Suspension Bridge
Cantilever bridge
continuous beam bridge
Simple beam bridges
Truss bridges

These bridges are built out of different materials:  Concrete, wood, steel 
or stone. Here are a couple of links:
 http://www.howstuffworks.com/bridge4.htm&e=42
 http://www.brantacan.co.uk/truss.htm

In order to apply the scientific method, we must isolate the problem.  
That is, concentrate on only one aspect of the question.  So, instead of 
asking "What bridge design supports the most weight?" you have to, for 
example, ask, "what kind of simply-supported wooden truss bridge of a 
specific span can support a single concentrated load  at the middle?"  
Instead of wondering about all the dozens of different types of bridage, 
you concentrate on finding out about how changing one aspect of a bridge 
design affects how much weight it can support.

I mentioned wooden trusses because there are several different designs of 
truss.  In the 19th century, before methods of calculating stresses in 
bridges had been developed, there were many companies that had patented 
bridge designs that they sold to railroads and local highway departments.  

To test these truss bridges to see what design is strongest, you would 
have to make a model of each, with the length, depth (or height), number 
of panels (diagonal pieces in the middle) all exactly the same, and built 
out of same sized pieces of the same material, fastened together in the 
same way.  Then you would put more and more weight on each bridge, with 
the weight placed on the bridges in exactly the same way, and record how 
much weight each supported, and what part of the bridge broke.  

As for materials, balsa wood is common, but drinking straws might also be 
used.  One bridge building site I found suggested spagetti!   

You don't have to experiment on truss bridges, although that is what a lot 
of science fair entrants do.  You might try experimenting on arch 
bridges:  What carries more weight, a series of short half-cdircle arches 
or a long elliptical arch?  Or compare suspension bridges, one with tall 
towers, one with short ones.  I once had a group of students at my 
daughter's school compare bridges made of fan-folded paper:  One had a 
few, deep folds and one had a lot of smaller folds.  We put pennies on 
each to see which supported more wieght.  (As I recall, it was the one 
with lots of folds.  Why don't you try it?) 

Finally, after you've run a few experiments, have one or more of your 
friends look as what you did.  They should be looking for anything besides 
what you've deliberately changed that might affect the results you get.  
Painful, but necessary.

The heart of the scientific method is to test a series of, bridges or 
whatever, with as many of the details the same as possible;  then change 
one aspect of the design, and test again and again to see what difference 
that one change made.