The bridge designs that most students create can
be described by 3 letters of the alphabet: V, Y and
T. The issue to keep in mind with each bridge type is
that the vertical component of each bridge arm vector
must total half of the load. If not, the bridge will
fall. When you change the angle of the bridge's arms,
the vertical and horizontal forces will also change.
For the bridge to pass its test, the vertical force
must counter the weight of the water bottle and results
in a zero net force on the system.
Depending on its angle, the paper undergoes more or
less stress. The deeper the angle (V), the more vertical
forcethe diagonal arms supply. Designing
a bridge with less steep arms (e.g., the Y design and
even more for the T design), the paper pulls more horizontally
(against itself). To produce the vertical force needed
to support the bottle, the paper strip must pull harder.
It can do this up to a point -- its limit is the tensile
strength of the paper. Once a critical angle is passed,
the paper will give way before it can supply the force
needed to hold up the water bottle.