Bridge Construction and Failure 1
Bridge Construction and Failure
Nathan Eddy
Sarah Gussenhoven
Keith McCabe
Spencer Payne
19 April 2012
Bridge Construction and Failure 1
As an engineer designing a bridge there are many components and parameters that have
to be met before building it. In order to construct a bridge there are multiple test and
calculations. These tests and calculations are to determine the material the bridge will be
constructed of, which members will be tension and compression members and the design of the
bridge that will be safe for vehicles to cross. The calculations also determine the load that each
member will hold. There is also a budget that has to be considered when designing the bridge.
The cost should be minimal, but the bridge also has to meet all safety requirements. After all of
this data a constructing the bridge it should be stable to maintain the load. However, there can
still be failures due to the design of the bridge. After constructing our bridge and testing it we
found that there were problems that lead to a bridge failure. To understand why the failure
occurred we will analyze design chose, the different calculations made and the choices of the
different members.
The first step of the design process was to decide how strong each member would be. The
method of how we selected the types of members that we would use for each compression and
tension member came from results that we generated from the structural testing lab. From there
we input our data of what members we were going to use into the truss’ structural analysis tool
along with the minimum weight requirements. This gave us our forces on each member with the
given lengths and areas of our members. Our group intended on building the lightest bridge
possible so our factor of safety was only slightly greater than one. With all of the data from the
testing lab, structural analysis, and the models of strength of each member, we developed a
design for a bridge that would support the minimum load while maintaining a light weight. We
constructed our bridge using 7 compression members and 6 tension members. Through the
member testing lab, we discovered the load decreased as the length of the member increased and
Bridge Construction and Failure 1
the members with the square cross-section were able to withstand a greater load than the
rectangular cross-section. Knowing this, we made members 10 mm by 10 mm for all of our
compression members; which we found each single compression member should be able to
withstand a load of 40 to 55 Newtons. We made our tension members 8 mm wide with the
results from the testing lab showing that they could withstand a load of around 25 Newtons. The
wider the tension members were made, the greater load they could take, which is why we
sacrificed some weight to guarantee the tension members would hold the needed load. All of this
information was then input into the VBA program which gave us the given loads on each
member if the force was applied directly on top of our bridge. For the zero-force member, we
decided to use a compression member. We assumed that if the load was distributed unevenly due
to a miscalculation with the construction of a member, the compression member acting as the
zero-force member would provide added support. After determining the members and applying
the data into our VBA program, we moved onto the construction of our bridge. The main
variable of the strength the bridge was the construction phase.
Members
Amount
Needed
Size
Tension
6
8mm
Compression
6
10x10 mm
Zero Force
1
10x10 mm
The design for the bridge originally came from the template of the truss bridge given to
us in the class hand out. This model of the bridge is called a Howe truss. This simple yet
elegant design is comprised of vertical supports with diagonal members to connect and maintain
Bridge Construction and Failure 1
structural integrity. This bridge was designed to have nearly equal tension and compression
members, which as we found out later was what we ended up using on our bridge design. We
began designing our bridge by using the VBA Excel program, which we have worked on in lab
for several months, to see what members we would need to construct our bridge if the load was
to be applied directly on top of the bridge. The goal was for the bridge to support fifteen
kilograms, so we chose to design the bridge to withstand this load. After we had entered in the
coordinates of the trusses from the example that our teacher provided to us for the original design
we told the computer to apply the load directly on top of our bridge. And as we had predicted
the bridge was to have even number of compression members as tension members, and one zero
force member located in the middle of the bridge. After deliberation and looking over the data
we decided that the best thing to do for zero force member was to assign it a compression
member for extra safety. The next step in the designing process was to go to the drawing board
to create a template for our construction. So with a list of members we wanted to use and the
template of the drawing in our instruction packet provided to us, we went to the AutoCAD
software and started drawing the dimensions. This process was pretty straightforward; we just
followed the design on the paper, minus one section on the end because our bridge didn’t need to
be that long. We did run into some issues though. One problem was designing the diagonal
compression members. These needed to be directly diagonal to the center of each beam,
meaning that the center of the diagonal beam was to match up directly in the center of where the
vertical compression member met the bottom of the bridge. This problem was over come by
drawing a wire frame of the middle of each beam and then drawing the thickness of the beam in
later. In some section we had to stray a little from the design that was provided. If you glance at
the picture of the bridge we drew you will notice it is a little longer than other group might be.
Bridge Construction and Failure 1
This is because we noticed that the design in the hand out was from the center of each beam, so
as a group we decided that we would try and stay true to the handout design, thus making the top
and bottom of our bridge slightly longer then others. Over all our design for the bridge seemed
to work out for us. We had a perfectly straight template to start construction with.
Through this bridge building process, we learned that construction plays just as large of a role in
the design process as theory. We were positive that at least, we would be able to support the
minimum as well as keep our bridge reasonably light weight through our simple design.
However, a lack of symmetry in our construction made that impossible.
The design approach we took was one of simplicity. We basically only used member
sizes that we had personally tested so that we could get a more precise answer when comparing
our needed loads with the data we collected. We thought that this would provide a more
reasonable idea of how our bridge would do when we tested it. In hindsight, however, I think it
would be beneficial to reconsider that approach and maybe double up on the sixteen centimeter
compression members because they were the most likely to fail. Also, it is hard to say whether
the decision to use one twenty centimeter compression member as the top flat beam instead of
two was a good idea. Weary of the fact that the longer a compression member is, the less likely
it would be to fail, we thought that the risk of construction error and the weakness of the gussets
would overweigh this because, the flat top members did not experience too much force.
However, as stated above, we will never know whether this design approach was beneficial
because it was not the top, twenty centimeter members that failed. Finally, our decision to use a
compression member for the zero-force member that is vertical in the middle of the truss was, to
our belief a good idea. By doing this, we accounted for the probability that the weight might
shift during the loading and change this once useless member into a valuable support. I say this
Bridge Construction and Failure 1
because, during our bridge testing, the zero-force member was one of the members to fail
making it obvious that the weight had indeed shifted and the zero-force member had become
loaded.
The construction process was tedious; however, it was invaluable to the construction of
this bridge. This fact became blatantly obvious when our bridge failed because it did not rest
evenly. For our construction, we decided to be safe and not score our compression members in
order to make them easier to fold. The reasoning behind this decision was that we found they
were more likely to fail at the scored edges when in compression from our original member
testing. The fact that we did not score these members as we did in our member testing made us
sure that our bridge would exceed the data we calculated and that our bridge would far outlast
the minimum weight requirement. We believe this was true after we tested our bridge because
even the 16 centimeter compression members, which were the most likely to fail, showed no
signs of deformation after our bridge failed. Another decision we made to enhance our bridges
strength was to use rubber cement instead of the Elmer’s that everyone else used. This was risky
because we had no data that showed whether or not rubber cement was stronger than the normal
glue. In examining our bridge after it failed convinced us that it did ok. It is hard to say because
we did not reach a load comparable to many other groups, all of the glued connections and
compression members seemed to hold together, they simply bent under the load, and they did not
split. Finally, the major error we ran into in our construction process was that on one side of the
bridge, there was offset between where all of the bottom gussets would rest on a flat surface.
This occurred because we did not see that the bottom of one of our compression members and its
gusset did not line up until after we had pulled the bridge off of the page. We were out of time
and would have had to go back and redo the whole thing so we decided to simply overlook it.
Bridge Construction and Failure 1
We figured that it would not make a huge difference. However, as we put the bridge finally
together we realized that we had a noticeable offset between the two sides and all we could do
was hope that it would correct itself under an even load. Unfortunately it did not.
In conclusion, we believe that our methods were sound in constructing and designing our
bridge. However, if we could go back, it would have been extremely beneficial to redo the side
we messed up on, even if that meant a lot more work and much more time spent in construction
than the labs we were given allotted, and not underestimate that damage that an unevenly loaded
truss bridge can undertake. However, we think that this bridge building assignment was a
valuable learning lesson to see all the work that goes into designing even a simple truss bridge
built of paper, as well as all of the factors an engineer needs to consider.
Bridge Construction and Failure 1
Failure Justification and Conclusion
After the testing phase of this group project, we were not satisfied with how our bridge
performed overall. This led us to investigate why our bridge in fact failed before the designed
weight load. From close observations of the bridge we noticed one side of the bridge was 1mm
off. This design flaw proved to be devastating to the overall performance of the bridge. When
we applied the load directly to the top we had originally intended the load to be applied at a 270
degree angle (directly downwards), thus eliminating any x component forces, but when the sides
of our bridge were uneven it changed the angle from 270 degrees to 275 degrees thus creating
both x and y component forces. The creation of these undesired component forces was
disastrous for one simple reason, when we designed each member, compression and tension; we
designed them so that they would carry only one type of load. With the new x component force
being applied to each compression member they folded. If you watch the video of our bridge
you will notice the first member to fail was indeed a compression member.
Now that the bridge was failing due to the onset of new forces we will examine into more
detail why this was. After watching the video of our bridge failing we determined that the
shorter side was the side that failed first. Why was this? Well with the uneven application and
redirection of the load forces created a moment around the centroid axis, causing the shorter side
to buckle at the very top of its compression members and started the moment, which in turn
caused the taller side to fail. But why did the members on the shorter side fail? We sought this
answer through analysis of our Excel VBA program. To do this we used the inverse of the
coefficient matrix and multiplied it by the forces being applied to each to determine the internal
load each member would support under the weight given. We found from the video that member
L12 was the first to fail. At first we assumed that our bridge was built perfectly and that both of
Bridge Construction and Failure 1
the sides were perfectly identical. When we applied the weight of our desired safety factor, a
weight of 30kg, we found that that member had an internal force of 21.1712lbs of compression,
but if we subjected that same member to just the minimum load of 15kg the load dropped in half,
as expected. This means that we designed our bridge to support at max a force of 21.1712lbs of
compression and to hold a minimum of 10.5792lbs of compression. Next we moved to a
simulation of what would happen if the bridge sides were not symmetrical and even. To do this
evaluation we had to make some assumptions. One of our first assumptions was the forces acting
on either side of the bridge could not have been the same, that in fact the side that was the
designed height held more of the weight and the taller side held less weight. The second
assumption was that once we loaded the bridge to the weight it failed at, 20.75lbs, if failed at that
exact instant; holding that exact weight for a split second. We assumed that the shorter side of
the bridge held 80% of the weight, so in other words if the member held anything over 80% of
the weight it was doomed to fail. In this case we ran that percentage through the Excel VBA and
it showed that indeed the member failed because the force shown in the member was over
10.579lbs causing that one member to fail in turn causing the rest of the bridge to subsequently
fail.