Introduction
Engineering 103 was comprised of exploring and experiencing the design and modeling process for building bridges. This process was followed by a sequence of designing,
building, and testing through the use of a variety of tools such as the
computer software West Point Bridge Designer 2012, model materials such as K’nex
tools, and innovative in-class testing of the finished versions of the various bridge
designs. The objective was to understand
how the bridge would, in theory, behave when the real-world bridge project was completed,
as well as the effectiveness of each group's bridge and the groups' abilities to work in teams
efficiently.
Each group entered their design into the class competition. The different
models were tested in comparison to each other which resulted in determining a
model that satisfied the best cost to weight ratio. As the experimental process followed an in-depth analysis, many steps were executed by each team. The teams went through several phases which
included planning, documenting, computer modeling, static analysis and finally, physical modeling. Each failure, as well
as each success, was observed and analyzed in order to gain more knowledge of
the module through blog documentation.
The Design Process
Throughout the course, the
students learned what happens behind the scenes when a bridge is being
designed. When the class was first
introduced to the topic of bridge designing, group seven had the obvious goals of
designing and building the most efficient bridge possible. This was determined by who had the lowest
cost to weight ratio, or the bridge that could hold the most weight relative to
its cost. The instructor had many ideas
as to how to help the students understand how to complete this goal.
Before the class could start designing
their own bridges with K’nex pieces, they were instructed to learn about bridge
designs through their own research and to use a bridge designing application,
West Point Bridge Designer 2012. This
program let the students design their own truss bridges from a lateral point of
view. The students had the option of
using different types of materials as well as different lengths and thicknesses
of these materials. After the design was
complete, the program then took the design and turned it into a 3D animation,
demonstrating a truck driving over the bridge.
If the truck made it all the way across the bridge, the bridge was
successful. However, if the truck failed
to clear the bridge, it meant that the bridge design failed and at least one
member of the bridge had too much compression or tension acting on or against
it. The program also showed how much
tension and compression was on each member and calculated the cost of the
bridge based on the member size, length, slenderness, cross section, and
material type. West Point Bridge Designer
2012 really helped the students to learn which designs were most efficient and
which designs dispersed the weight of the truck most evenly. After playing and experimenting with WPBD,
the group decided that a deck truss would work best. This is when the truss sits atop the
bridge. This was picked because it was
most common out of the group’s designs and the group felt it would be easiest to
build out of K’nex pieces.
The instructor also required the
students to complete an exercise where they had to figure out the tension and
compression on each member of their bridge.
This gave the group an estimate of how their bridge would fail and what
changes they should make, if any. Group seven decided that they should add more support pieces to their bridge and try to
add some middle cross section pieces, rather than just connecting the two sides
of the bridge together on the top and bottom.
At this point the bridge was composed of nine cubes lying side by side. In the three middle cubes, there was an X
shape along the two vertically standing edges of the box with a gusset plate at
the middle of the X. A K’nex piece was used to connect the X’s front to back to keep the bridge from twisting. This can be seen in Figure 1.
Figure 1. Bridge before Modification
This was the one problem that
was noticed with the bridge design – the bridge would twist in opposite
directions when weight was placed on top of it.
This was taken into careful consideration when the group redesigned
their bridge. Up to this point, this was
the final design chosen for the three foot long bridge, but could only hold a maximum
of thirteen pounds. Only after testing
the bridge which resulted in this outcome did the design change once more. The bridge as designed was not very efficient
and the group hoped to increase the amount of weight their bridge was able to
support. After the bridge was modified one more time,
the group predicted that the bridge would be able to support seventeen
pounds.
The Final Design
Figure 2. Drawing of Final Bridge Design
After designing, testing, and
analyzing the three-foot bridge in numerous ways, the design was ready to be
finalized. The final design for the bridge consisted of nine squares, making up
the length of the bridge. The plan for the final design bridge can be seen above in Figure 2. Each square had an X through it.
In other words, each square was composed of four right triangles, their right
angles at the center of the square. To give the bridge width, enabling for
vehicles to travel across, 3.375” chords were extended to and from the corners
of each square, respectively, between the two side trusses of the bridge.
Figure 3. Final Bridge Design
Having learned that the weak
joints were those composed of a chord clipped into the gusset plate and lying
perpendicular to the gusset plate, all but eight of these joints were replaced.
The old joints allowed sliding of the chords which were responsible for
preventing twisting of the bridge, causing the bridge to twist under light
loads. The new, stronger joints were composed of two interlocked
three-hundred-sixty degree grooved gusset plates with chords snapped securely
into their sockets for better support against twisting of the bridge. By
analyzing the way the bridge twisted as the weight of its load was increased,
the best set of joints to leave unchanged was determined. Because the price of
each new joint was quadruple the price of each old joint, eight old joints were
carefully chosen to remain intact, saving $24,000 from being added to the cost
of the bridge. The final design is pictured above in Figure 3.
Overall, the bridge was priced
at $409,500 and was comprised of two-hundred-sixty-five parts, as calculated on the spreadsheet in Figure 4. During the competition, the bridge collapsed under the weight of 14.7 pounds after undergoing too much tension, as seen in the images below, Figures 5 and 6.
Figure 4. Bill of Truss Materials
Figures 5 and 6. Breaking Point
Final Results
The bridge was designed as a
deck truss made up of equal size cubes in a line. When weight was placed in the bucket, the
bridge had a tendency to wiggle back and forth.
In other words, the top level of the bridge would move back and forth
while the bottom stayed still. This
resulted in more and more twisting as the bucket of sand got heavier. After the bridge reached its maximum load
weight, 14.7 pounds, the bridge twisted so much that a member from the top
level snapped out of the gusset plate that it was connected to.
The group tried to prevent this
twisting from occurring. After adding in
the X shapes all along the bridge, the front and back pieces of the bridge were
able to be connected in three layers top to bottom instead of just two. The group thought this would support more
weight because the bridge wouldn’t be able to twist as far if more pieces were
holding it together. However, this
really did not help at all since the bridge repeatedly broke in the same place
and only held half a pound more. This
was insignificant compared to the cost of all the members we had to add to
create the X shape design.
Conclusion
As
engineers of all fields, the class gained valuable experience by
undergoing the bridge design process. One did not need to be a major
in civil engineering to learn that there is much testing, analyzing,
and teamwork required for the final product of an engineering design
process to be successful. Although the bridge designed by group seven
was not successful compared to other bridges in the competition, it
was successful in putting the group through a small-scale engineering
project that required them to understand all of the major—and some
of the minor—steps in any engineering design process. The
strategies, plans, elevations, calculations, computer testing,
physical modeling, and physical testing are all aspects of
engineering projects that the group members will grow to be very
familiar with, even as mechanical and electrical engineers.
Future Work
Based on the final product of this project, one of the changes that could be applied in the future would be to change the way the K'nex grooved gusset plates are joined to each other. During the design process, the group learned that grooved gusset plates tend to slip apart under tension. Therefore, whatever pieces join the chords of the bridge together
need to be very strong in order to successfully sustain the amount of weight and force
distribution on the bridge.
Additionally more care should be given to the variety, magnitude, and
location of the possible angles of the structures that form the bridge. There might be new undiscovered advantages if there were more angles available for use in the bridge structure, but K’nex pieces limited which joint angles could be used.
