A4- THOTTAKARA, VARGHESE, ZHU

Background

Engineering Design Lab goals for this term were to learn and to build a bridge with the smallest cost to strength ratio. Therefore, the bridge had to be as cheap as possible while still being able to hold the most amount of weight. To simulate this idea, the bridge had to be built at a smaller scale. To accomplish this goal, various tools were given. Beginning with a computer program called West Point Bridge Design. West Point Bridge Designer allowed maximum force on the bridge until the point where the bridge would break. It seemed as if the failure point was exaggerated. The next resource given was Knex. Using the Knex, a couple small scale bridges were to be built with different constraints. The first one had to be 24'' in span, and the final design had to be 36'' in span with a hollow gap of 2'' high and 3'' wide.

Design Process

In the beginning, our goal was to construct a serviceable bridge that has a low cost to weight ratio while still satisfying all the design constraints. Throughout the term the goals became more personal. We did not just want to build the bridge as part of the competition, but to see how well we could complete the task. It became a personal challenge. We first used WPBD to design a bridge that is strong enough so that the truck can pass through. Then in order to meet our goal, we either took out unnecessary chords and connectors or replace the longer chords with a couple shorter ones to make the bridge as cheap as possible. Then as we were actually building the bridge using Knex pieces, we realized that making the bridge as cheap as possible might lead to bridge that is not as strong as it could be. Meanwhile, we learned Method of Joints(MOJ)—a way to calculate the tension and compression forces acting on each of the chords. That was a very helpful technique because it not only show the forces, but it also allowed us to predict the failure point—which most likely be the chords that have the most forces. Other than MOJ, online Bridge Designer was also helpful since it automatically calculated the forces on every single chord so we can modify the chords that have small forces and make the bridge stronger as a whole. Before we started to pick the design for our final bridge, we looked back to all the individual projects with WPBD and Knex Bridge Designs as a reference. As a group, we brainstormed together on what general shape the bridge design should take and we tried to combine some of the old designs together. That did not work out well, so we ended up making a whole new design using a WPBD and online Bridge Designer. First, we made the design on WPBD to estimate the cost and to make sure that it doesn’t break at any point. Then we used online Bridge Designer to make sure that we met the minimal requirement of chord to connector ratio. On top of that, we can modify the bridge with the calculated forces and replace cheaper chords on the ones that have small forces on them. After all that, our bridge ended up costing $409,000 and we predicted that it would hold about 35 pounds of loads. 

Final Bridge Design

Cost Breakdown                        

Elevation View

Plan View

The final design of our bridge cost $409,000.00 and used 423 pieces. It was a little over 3 feet long and met all the constraints presented.

Testing Results

Video of Bridge Testing At Failure Point

Elevation View After The Testing

Our bridge held much less than we estimated. The load of failure was 30.2 pounds, while we estimated the failure to be at about 35 pounds. The closer we got to our fail point the more the bridge started to bend in the middle. It happened so slowly we did not notice it at first. Then suddenly the middle started coming apart and the bridge snapped in half and collapsed. Our estimated load of failure was 35 pounds, and at the rate that we had been going we thought we were going to make it until the bridge suddenly collapsed.

Conclusion

Our group came into lab week 9 with the hope that our bridge would be able to carry a load of 35 lbs. After a disappointing performance during lab in week 8, where our bridge was only able to hold 17.2 lbs of load, we had confidence that our new modifications would be able to turn things around. When it was our turn to begin applying the load onto our bridge we could already tell that by the amount of sand already in our bucket, we were doing much better than before. After a couple of minutes of applying more sand, our bridge finally snapped right through the middle. The final amount of weight that our bride held was 30.2 lbs. We fell about 5 lbs short of our goal, but it was definitely an improvement from the week before even with the added bridge costs taken into account. The final cost of our bridge was $409,000 with a cost of $13,344 per pound.

In terms of failure mode, our bridge collapsed in the middle as predicted, but the actual members that caused it to fail was unexpected . Our bridge had a consistent pattern throughout the bridge, and each section of the bridge felt equally sturdy. Since the load applied to the bridge was located right at the center of the bridge, the failure mode was expected to be right in the middle of the bridge. What was unusual about the collapsing of our bridge was that it happened unexpectedly. During loading, our bridge only bent slightly, and its position seemed unchanged for a while until it collapsed. After the collapsing of our bridge, we analyzed it and saw that the forces applied on our final design was equally distributed on both sides, and that is why we felt that it would be durable enough to reach our goal of 35 lbs. Unfortunately, the symmetry may be what made it so that we were unable to reach our goal; there needed to be more support in the middle.

Future Work

If we had the opportunity to modify our bridge one more time, we would have more support in the middle of the bridge. Throughout all of the bridges we built this was where the bridge snapped because of the amount of force exerted on the small area. We would also focus on trying to make the bridge cheaper. With the proper modifications we could make it so that the bridge is more cost efficient as well as able to support more weight.

Term Review

Term Review

This term I learned a lot from this class. I learned of the process and mechanics that goes into a large-scale project. Although the course was based on bridge design I learned a lot of lesson that are related to all fields of engineering. For a project like building a bridge nothing can be accomplished without teamwork. Having a group of reliable people around you is important in every aspect. With a group greater thinking and designing is done because there are more ideas to blend together. Design process is another lesson that I learned. Goals need to be set up and the beginning of the project so that you know what you are trying to attain throughout the process of the project. Of course, these goals are going to change and develop as we go through each step and learn new material. With this the final product will turn out well.

The most beneficial lesson learned had to be forensic analysis. As an engineer you have to design and develop structures and procedures that do not fail. So learning this analysis throughout the span of the course allowed us to find the problems and improve the bridge so that the problem was no longer there. This is a lesson that will be relevant throughout the rest of our careers.

The least beneficial aspect for me had to be the planning. I know that planning is an important part in completing and well done project in a timely manner, yet since things are constantly changing I found that planning ahead of time was not really helpful.

I think that this course ran very well. Everyone was engaged and interested in the project, and we were all able to learn something even if the topic of the course was not prevalent to our specific major. One thing that I think could have gone better is the explanation for the set up for the blogs. At first it was a little confusing but after further explanation it was more understandable.
This week in lab we tested our final bridge. This bridge was 3 feet long and was hollow inside with a width of 3 inches and a height of 2 inches. Our final design costed $409,000.00 and held 30.8 pounds. We had estimated that it would hold about 35 pounds but it failed before it could reach that point. The bridge snapped in half with a few pieces scattering. This occurred because a few of the chords slid out of the gusset plates.
Next week we will be wrapping up the final points of bridge design.

Week 10 - Term Review

Before going into class, I did not expect to learn much since I did not learn much in previous two terms. But as this term progresses, I realized that I have learned so many worthwhile knowledge about the whole designing process. There were new learning experiments each week throughout the term. First, we were exposed to two softwares —West Point Bridge Designer and online Bridge Designer. Both of the softwares were helpful in terms of planning the bridge ahead of time before we actually start to build the bridge.  They also gave me a good understanding of how the forces cause impact on each of the pieces in the bridge. The part that I like the most was building the actual bridge using Knex pieces. Some of the notable accomplishments were Truss analysis, modeling using the softwares, and calculate the tension and compression forces using MOJ. 

I enjoyed every second of that class; I especially like how we recorded everything on our blog.  The part that was most beneficial to me was Bridge Designer since we have to at least meet the minimum requirement in order to see the forces acting upon each of the members. Then the part that was least beneficial was the West Point Bridge Designer. Even though it was fun messing around with all the different lengths chords, but it was not that helpful when we actually had to build the bridge. For future references, I think students should spend more time on analyzing and building more bridges using Knex with more constraints.  

Last week in class, we tested our final bridge, recorded the load failure and analyzed the point of failure. Our bridge ended up being $409,000 and it held about 30.2 pounds.  The actual load failure was lower than what we had expected, but it is close enough. Then this week in class, we will be finalizing the project as a whole and discuss what we have learned.