Studiot said:
How would this design fit on a table?
Is it supposed to be free standing on the table or to span between two tables or what?
Have you considered the horizontal and vertical reactions at the supports?
I think they have given you a clue here as you state the max grade is 10%
A simply supported beam has only vertical reactions.
No horizontal abutment reactions or moments.
An arch has no abutment moments but does have both horizontal and vertical abutment reactions.
How are you to fabricate this bridge?
Is it to be made from sheets of cardboard, cut to shape?
Glue? Pin joints?
I know you said you cannot use a suspension design, but can you use string to prestress the beam?
If you want to use a truss, and the above fabrication methods are available I would suggest cutting the cardboard into rectangles and rolling these into longish cylinders and gluing the joins. Also change the truss design so the compression members are much shorter and/or fatter than the tension ones, otherwise the cardboard will buckle.
We can look again at the abutment question when you clarify what the bridge is to stand on, as your current idea will not work well.
Thank you for your comments. This is only my second bridge and it is not clear to me that I may have no idea what I am doing when it comes to design.
First, it is to span between two tables.
Secondly, I have not considered the reactions at the supports. I am most likely not imagining the flow of pressure throughout the system. Now, the arch abutments in West Point Bridge Designer 2010 are vertical, but provide horizontal reactions?
Here is a reference picture:
http://img109.imageshack.us/img109/7074/abutments.png
However, seeing as how I will simple place the bridge onto the two tables, like so,
http://img704.imageshack.us/img704/4706/95915755.png
How should I design the abutments (highlighted in blue),
or rather,
I don't understand if this would suffice for 160lbs.
This bridge will be made from cardboard, and can be joined by any means possible. I believe the beam can be prestressed, but to be honest, I have no idea what you mean by using a string to prestress the beam.
Lastly, thank you for the idea. To roll up the cardboard, assuming the length of the card board is how long the member should be, how much would you recommend the width /the diameter of the member?