Design Process
The goals set by the group were exactly that of the design parameters,
to create a working bridge of three feet of length with the lowest cost to
weight efficiency. Throughout the
course, the design of the bridge changed a few times but the goal was set in
stone since the beginning. While working
with WPBD, Truss Analysis and bridge designing, the group learned quickly as
too what would work or not. Each of
those three played an integral part in the design of the final project for the
group and it was quickly learned that in order to completely understand what goes
on when there's a bridge with a dead load (not moving) set upon it there needs
to be practice with the computer programs as well as the actual representation.
Before the class began to designing truss bridge with K’nex
pieces, they have started to work on a stimulating tool called West Point
Bridge Designer. The WPBD gave a very
in-depth understanding on the breaking point of different members of a bridge
with variable thicknesses, lengths and materials. The designer could design a bridge from lateral side of
view following specific constraints. Designer could examine the loading of the
bridge a 3-D animation truck test. If the truck has safety pass the bridge, the
bridge could be considered as a successful one. After each loading test, the
compression force and tension force would be shown in a “box”. The cost of the
bridge would also be calculated according to the size, material of members and
joints. These data can greatly help the designer to improve the design of the
truss bridge. The WPBD gave the group a
good beginning of designing a truss bridge. The group has learned how to decrease the cost
of the bridge and how to find out an optimal design. The experience that we
have learned from the WPBD helped the group to design the Knex Bridge.
The group
member individually designed three different Knex bridges in A-2. Group members used the Auto-CAD to design the
bridge in elevation view and plan view. This process helped the group to have
an idea of how to develop a bridge from two dimensions to three dimensions.
The Truss Analyses performed in and out of class were also
very helpful in that they incorporated trigonometric calculations in order to
find the load distribution on each member of the bridge. This exercise helped the group to
estimate the failing point of the bridge. The online Bridge Designer tool
inspired the group that the proportions of the triangle greatly design the loading
of the bridge. According the calculation result of the bridge, the group
decided to keep the compact design of bridge.
Building the actual
KNEX bridges gave the group a real life visual performance of the bridge
designs. By testing the Knex model in reality, the group met some problems that
would not happen in idealized stimulating tool. The twisting of bridge happened
in the loading test and had negative impact on the bridge’s performance. The
group decided to add more horizontal chords to prevent twisting of the bridge.
The final design was determined by the way the two foot
design handled the weight-test. The
three foot bridge was built with the same side designs as that of the two foot
design. The two foot bridge design was a
real extravagant piece of art, and due to the white and blue pieces primarily
chosen for the members it was so dubbed, 'The Love Train.' After the first test of the two foot bridge,
it held roughly 17 pounds, but after the addition of two more pieces and
exchanging two gusset plates, the bridge held 47 pounds. The design of the three foot span changed two
times during its design but that was after the top-middle of the span broke due
to the weakness of the joints. The
changes only made a 1-3 pound difference but any increase of weight is
accepted. The predicted load at failure
for the three foot bridge was roughly 34 pounds which was significantly lower
than the two foot span's 47 held pounds. It might due to the increasing span of the bridge.
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