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.