Bridge design homework problem

[Marghk]

I need a bit of help here. I have an upcoming assessment towards bridge design. The task is to create and construct a bridge made out of dried spaghetti sticks. The bridge is to be as light as possible while holding the most weight.

I currently have 1 main design I'm leaning towards, but I'm unsure that it would be successful. I don't want to construct the bridge to find out it collapses on me.

http://img172.imageshack.us/img172/9060/bridgeif7.png

I believe that the triangle, upside down would serve me better. Though I'm still unsure about the whole design of it. I just basically want to know if this would be an idealistic plan to making a bridge, or if any other designs could easily top mine.

The KEY guidelines are below, in which the bridge must follow.

Key Guidelines:
- The span of the bridge cannot exceed 648mm
- The roadway (width) of the bridge must equal roughly 80mm
- The bridge must NOT exceed 1000g

If anyone can offer advice about my plan, it would be highly appreciated.

 

[denverdoc]

why upside down? I'm not a CE, but I'm guessing spaghetti is stronger in tension than compression. And the center truss is under compression where buckling failure might be an issue. Now if you stick a big bundle of strands here, might do fine. This is the wrong thread to post such a problem but caught my eye.

 

[AngeloG]

That's a good basic and simple design; however, if you want to elaborate on it and make it hold up well. Take a page from the Romans and create arches (smaller triangles).

 

[civil_dude]

The problem I see with this is the end reactions/support. I would consider a bow truss, not inverted. The thing to remember in trusses is that the further away the top chord is from the bottom chord, the more resistance you will have to bending stresses. Also, to prevent buckling of the compressive members, use short or built up pieces.