1. Background
The projects this term were
centered on modeling bridges in different formats. Large projects like bridges,
highways, planes etc. are expensive and require a lot of time to build. Therefore,
these real world projects cannot be built and tested to see how well they
function. Engineers use an alternative form of modeling the projects, in this
case a bridge, to help determine its behavior under normal circumstances. So we
worked on mainly two different types of bridge models- WPBD and Knex in order
to come up with a well functioning bridge designs. Both of these models shared
the parameter of having the cheapest bridge cost. WPBD bridge model was focused
on having the cheapest functioning bridge, while Knex bridge model was focused
on being able to hold the maximum amount of weight while minimizing the overall
cost.
2. The Design Process
When we began designing the Knex Bridge,
our goal was to keep the overall cost of the bridge low. It would certainly
hold less weight compared to a bridge which utilizes more structural members in
its design. But the fact that the success of the bridge would be measured by
dividing the weight it holds until failure by the cost of the bridge, we were
determined to get a good ratio.
While brainstorming and testing
different ideas for the bridge design, we came to realize that the bridge with
the minimal amount of members, hence the lowest cost, was very weak in terms of
the force it was able to withstand. We noticed that adding extra members to the
bridge certainly raised the cost, but also that it was able to hold a much
greater amount of force before its failure. So we decided to make a stronger
bridge where main objective for the bridge was to hold the maximum amount of
weight.
All the members of the team
brought to the table their experiences from designing the WPBD bridges as well
as our individual Knex bridges. These individual projects enhanced our
knowledge on the bridges by observing the results of different designs as they
were tested. One of the most important things we learned from both of these
projects is that the bridges with a lower height seemed to collapse more easily
than a relatively taller bridge. This was a major idea we used for our final
Knex bridge design. We observed that most of the bridges failed at the
connections of members and gusset plates which was also taken in account when
designing the final bridge. We also learned the method of truss analysis, which
further helped us analyze the way in which the forces would be distributed
among the members of the bridge. We also used the online truss analysis program
as a design tool when creating the new design. This helped us determine the
amount of force that would be acting on the members of our proposed design when
a certain amount of load was applied to it. The data provided by this analysis
was compared to the data on the pull-out forces for the gusset plates in order
to determine the stability of the bridge.
As mentioned above, the final
design of our Knex bridge was centered around having a strong design without
putting too much consideration on the cost of the bridge. We also focused on
having a taller bridge which would be more stable and as a result wouldn’t
collapse as easily as a shorter bridge. Another thing which stood out from the
data provided about the pull-out forces was that the average pull-out force of
the members increased with the number of members connected to the gusset plate.
So we decided on utilizing more members at each of the gusset plates in order
to increase the amount of force they can withstand before breaking at the
joints.
We build a bridge incorporating
these ideas and tested it in class before the final competition to see its
performance and make necessary changes. It was able to hold about 25 pounds,
but we weren’t satisfied with the weigh to cost ratio. For the final design, we
decided to change it a bit by reducing the amount of members used in connecting
the two sides of the bridge in order to reduce the cost, assuming it wouldn’t
have a major effect on the stability of the bridge. This was the only
modification we made to the bridge.
Even though we significantly reduced
the members connecting the two sides, we did not expect a major decrease in the
amount of weight the bridge would support prior to failing. However, since we
reduced the support that would hold the two sides sturdy, we expected the
bridge to hold about 5 pounds less than it did before, resulting in the bridge
supporting about 20 pounds before it collapses.
3. Description of the final bridge
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Figure 1: Bill of Cost
The final bridge used in week 9 for the competition turned
out to be $248,000.
dd
Figure 2: Final Bridge Elevation
Figure 3: Final Bridge After Testing
4. Testing Results
Although our
bridge design did not necessarily measure up to our standards, it did hold a
respectable amount of weight. When the bridge finally failed and collapsed, the
net weight applied was 16.5lbs. When designing the bridge, the group did expect
to hold well over 20 pounds, but this expectation was flawed on a few levels.
Primarily, our shorter bridge was barely able to fulfill this expectation. The
shorter bridge was much smaller, utilizing a similar design but with
significantly stronger, shorter cords. Upon meeting our new requirement of a
36” bridge span, the use of longer cords to hopefully lower the cost meant a
sacrifice in overall strength.
The expectations
of the failure weight may have been significantly off, but the predicted
location of failure was absolutely correct. Upon inspection of the Knex pieces,
it was a unanimous decision that the cord pieces would certainly not be the
first to fail. These cords were ridiculously strong, and could barely be bent
by any force we could come up. Through testing the original, smaller bridge and
our larger bridge, it was obvious that the gusset plates would be the first to
fail. Not only did the cords frequently snap out during testing, but it was
obvious that these gusset plates, once bent, never returned to their original
strength. Because it was hard to tell which gusset plate needed replacing, not
many were replaced. It was unrealistic to coinsider replacing every gusset
plate on the bridge, and this led to the ultimate failure of the bridge.
5. Conclusions
The bridge
did not behave as we had predicted in terms of the load and failure. We
expected the bridge to hold much more then twenty-five pounds. Because we were
more focused on having a strong design and made money a secondary priority, the
cost of the bridge was expensive and this in-turn hurt our cost to weight
ratio. The point at which the bridge collapsed was not entirely because of the
load on the bridge, it was more so the durability of the pieces we used. The
white 360 degree gusset plate pieces we used were used on multiple occasions by
many groups, hence why the pieces became less durable, thus leading the bridge
to collapse due to a single piece breaking when the tested. The Bridge
essentially collapsed at only one point while the rest of the bridge remained
intact. The bridge design and the load distribution was not an issue, but the
pieces we used were worn out and this evidently lead to our bridge not being
able to reach its true potential.
6. Future Work
With the knowledge we know now after testing
our bridge multiple times and assessing the reasons for its failure, as a group
we realize that there are various modifications that can be made to improve the
bridge design. For one, we should have made sure that the pieces were not over
used or bent as this causes the bridge to collapse due to only one piece that
was overused and not useable. In addition our bridge had some pieces on top and
on the sides that were not needed, instead of placing these pieces on the left
and right most ends or at the bottom of the bridge, it would have been more
beneficial to place them between the left and right faces of the structure,
thus making the middle part of the bridge stronger. By making the middle part
of the bridge stronger, the amount of weight the bridge can hold will increase
because most of the load is held by the middle of the bridge. Increasing the
number of load bearing members in the middle portion of the bridge will
decrease the tension on each of these members. Looking back at a project, there
are always things one realizes they could have done or should have done to
further improve their work. The bridge we constructed was a good one and over
all we were happy with its performance.
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