Observations and results
Did the suspension bridge hold a greater number of coins compared with the beam bridge?
In this activity you should have seen that the suspension bridge was able to hold more coins than the beam bridge by around 150 percent, such as about 310 pennies (or 140 quarters) compared with about 200 pennies (or 90 quarters). When the beam bridge failed, this was likely because the deck straw bent downward as more coins were added until it bent so much that it slipped down between the two towers. As coins were added to the suspension bridge, the cable (that is, the thread) was under tension and reinforced the bridge deck straw, pulling it upward (while compressing the towers) and allowing the bridge to hold more coins. When the suspension bridge eventually failed, the bridge deck straw likely similarly bent into a V-shape, but because it was attached by the thread the straw couldn't fall and instead the cup may have slipped off of the straw.
More to explore
Super Bridge: Build a Bridge, from NOVA Online, WGBH
Mysteries at the Museum: Tacoma Narrows Bridge, from the Travel Channel
Fun, Science Activities for You and Your Family, from Science Buddies
Keeping You in Suspens(ion), from Science Buddies