The 160lb is a requirement (the teacher has to walk across the bridge) so it would have to hold 160 lbs across the deck.
Also, this is a high school engineering class, we have gone over reactions (each joint has to evened out at 0N, and each member has to "cancel out" the force of another member), I am possible confused as to why a standard abutment does not provide a horizontal reaction.
Thanks for confirming my problem about the weakness of an underslung design, I started the thread to try and solve this problem. I thought about continuing the truss under the table, but I could not think of a way to attach it under the table, and still have the bridge be "portable." Also, since the span of the bridge will be 3 1/2 feet long, I thought it would be okay if I had 2 trusses, but I haven't gotten to crossing the loads, my textbook did not explain it.
The arch approach was mainly an aesthetic one, but I can see how problems could arise especially towards the middle. On that note, do you have any suggested readings that can may be explain the designs of different bridges vs. the requirements of their members?