Verified answer

Just by looking at it, I don't think it is a very strong design. The fact that no 'real world' truss bridges look like this should probably tell you something. Meanwhile, here is a link to a truss design web java applet, which lets you design an arbitrary truss frame of your own choosing and then calculate all the forces in the truss members. This would be the secret weapon I would use if I were in high school doing some kind of bridge design contest, which I see here on Y!A a lot.

PS: Chances are, your teacher is not even aware of this truss design program, so you should be able to use it to make a totally amazing bridge. Hint: After your initial design, plugging in to the applet and seeing where the strongest tensile and compressive forces are, you can then 'sister' those weaker members with an adjoining (side by side) member, making that weak link be twice as strong without having to start over with a whole new design, etc. Of course, CAD (computer aided design) is the method used by real civil/structural engineers to see if their design is any good before actually building it.

PS2: I plugged in your truss design to the applet, applied a (more or less) uniform load to the span, and it looks like it fails miserably due to the extreme compressive forces acting along the sloping member(s) from the peak down to the supports. A good bridge design would distribute the forces much more evenly than this one does.

Hope this is helpful. Enjoy!

Strongest Truss Bridge Design

Is this for a school project? I had to make one out of balsa wood and popsicle sticks arranged in triangular forms. I used the upper truss over the bridge. It weighed nearly nothing yet it held 120 lbs until the balsa wood base snapped!