Re: What is the best design for a truss bridge?

Not sure I can answer your question without more requirements.

Truss type bridges use short beams of steel or wood to form triangles.  
Each triangle takes up a portion of the load applied, either in tension 
(pulling) or compression (pushing).  The length of the triangle'S side, the 
number of triangles, and the configuration will all depend on what you are 
trying to accomplish and what kind of material you are using for the 
trusses.

For instance, if you are trying to build a true bridge holding up trucks 
and cars, the weight of the bridge itself may be significant compared to 
the weight of the cars and trucks.  If that is the case, the loading is 
fairly uniform accross the length of the bridge.  I would expect the cross 
section of the optimum bridge to look very similar as you went from one end 
to the other.  You will see many of these type bridges in northern cities 
or where railroad tracks cross rivers or highways.  Many of them were 
designed in the 1800's when railroads were interested in using the least 
amount of steel per bridge.

If you were talking about a toy bridge made of toothpicks and holding up a 
large weight in the center, then the answer would be different.  The weight 
in the center would be much larger than the weight of all of the 
toothpicks, so the bridge has to be stronger in the middle and not as 
strong at the ends. To keep it from bending too far, you would have to make 
it tall rather than wide to stiffen it.  Although I don't talk about truss 
bridges in particular, I have written a Design Booklet about the subject of 
building paper bridges.  You might go to my website and download a .pdf 
copy to look at some of the principles involved.
 HTTP://OURWORLD.COMPUSERVE.COM/HOMEPAGES/JMSALTERNATIVES