Michael Lyons and Mikile Bengtson

Leonard B. Zakim Bunker Hill Bridge Design Contest

Engineering Design Process:

Step 1: Ask

The problem presented in this situation was how to improve the design of the Leonard B. Zakim Bunker Hill Bridge. There were several other designs currently available on the internet. One of them was the Tufts design presented on the contest website. This design showed the roadbed split into fifteen pieces and each one was connected by wooden dowels. The bridge also showed only four wires present on the middle sections and only eight wires present on the outer sections. The bridge itself in Boston, however, has more wires in the middle as well as on the outside. That sentence was even worse. The roadway also seems to be made bit by bit into one large roadway not multi sectional.

Step 2: Imagine

The design, materials, and the methodology used in this bridge too some time to develop. At first we thought that we would use the same design as Tufts did which was a roadway divided into several segments. Dwelling on the roadway we thought the forces the hybrid steel/concrete bridge would be better mimicked the simplistic design composed of two large segments. Basically we believed that the multiplexed highway used in the Tufts design would be too unstable to perfectly mimic the forces present in the actual Leonard B. Zakim Bunker Hill Bridge. We did decide however to keep their idea of the angled side of the roadway to show proper wind flow. The wind heading directly into the side of the roadway would be driven underneath the bridge and the wind above the side of the bridge could flow right on by without much disturbance to the crossing traffic. This was major a cause for concern in our bridge design because of the Tocoma Narrows Bridge disaster in November of 1940. The material for the bridge was also a major consideration. A sturdy material was needed but it could not be excessively heavy to break the towers. Concrete was not a realistic option because of its weight and frailty during transportation. After dwelling on the subject for some time we decided to use plastic since it is a durable light weight material that is easy to work with. Then the problem of how to connect the two halves of the bridge came under consideration. After much deliberation we decided to use the tongue and grove method. This method would allow the two pieces of the bridge to be connected. Moreover the tongue and grove also allow the roadway to maintain a level roadway because the halves can lean on each other for support.

Furthermore, we contemplated on how to construct the towers. The Tufts Bridge has made the bridge completely out of wood; however, we did not consider this to be satisfactory. Wanting to mimic the bridge forces with the most accuracy possible we decided to go with a heavier material, aluminum. This heavier material would be able to support more weight and stabilize the bridge with more ease. The difficult part with aluminum is that we did not want to have one and a half inches of aluminum because it would cause difficulty in the transportation and assembly of the scale model. As a result we decided to use foam to fill the excess space. Then we needed to decide how we were going to attach the roadbed to the towers. The original bridge in Boston probably uses many metallic wires attached to the towers. Deriving from the original bridge we decided to attach them together using copper wire. The copper wire would be attached to the roadbed and towers using eye hooks. This way the weight of the bridge would equalize by pulling in opposite directions when the tower is placed in the center of the half bridge segment.

Step 3: Plan

-See attached plan diagram

Materials:

- Aluminum – 4 sheets – 21” by 10” by .25”

- Copper wire

- Plastic sheets

- 4 sheets – 8” by .75” by 40”

- 1 sheet – 8” by .75” by 41.5”

- 1 sheet – 8” by .75” by 38.5”

- Foam – 4 sheets – 21” by 10” by .5”

- Eye Hooks – 202 eyes hooks

- Instant cement – Quart can

- Bonding agent – One tube

Tool:

- Drill and bits

- Jigsaw

- Hammer

- Clamps

- Table Saw

- Wire cutters

- File

The bridge works because the opposite and opposing force balance out. This occurs because the roadway pulls down on the wires of the copper which then pull down on the towers. Since the towers are placed in the middle of the halves of the bridge then the force of gravity of one half pulling down on the copper wires will equal the other half’s pull. These forces are then balanced out in the tower because the equal forces pull in opposite directions. In the end the towers act as a ground for the entire bridge. Also the two halves of the bridge come together in a tongue and grove formation which allows the two sections of the bridge to connect and rest on each other so the entire structure does not tip over. We will test these forces by construction the bridge and allowing it to free stand for a week while changing the weight in the center of the bridge to accommodate different traffic situations.

Step 4: Create

We created a prototype of the bridge with the plans. The towers were going to be the most difficult piece of the bridge to design so they were cut out first from the aluminum sheets using the jigsaw. Once all four of the Aluminum pieces were cut out they were sanded and filed down to exact measurements. Then the foam pieces were cut to the same measurements as the aluminum sheets. Then two of the foam pieces were sandwiched in between the aluminum sheets. Then the bonding agent was applied and clamped together. After this we started work on the roadbed. We cut the plastic sheet to the exact specification of the drawn up blueprint and separated them into groups where the thirty-eight and a half inch and the forty-one and a half inch of the six sheets were sandwiched in between two of the forty inch plastic sheets. With these two groups representing each half of the bridge they were bonded together using instant cement and then clamped. Once everything had sufficiently dried holes were drilled in the appropriate locations for eye hooks. The eye hooks were then screwed into the drilled holes. Following the installation of the eye hooks the towers are then placed respectively into the center of a half of the roadway. Holding the tower and the roadway perpendicular to each other the copper wires were strung to support the bridge. Subsequently after the wires were strung the two halves of the bridge were connected and the bridge was completed.

The bridge was then tested by placing weights (5lb., 10 lb., 15 lb.) in the center of the roadway for a period of time. The longer the weights stayed on the bridge the more disfigured the bridges became. Basically what happened was that the weights in the center of the bridge created such an imbalance of force in the towers that the bonding agent broke. As a result the towers began to split apart and bend. Due to the results of this test we believe that what didn’t work was the eye hooks in the tower and the copper wire. We also believe that using any type of metal wire or string would only cause more tension issues.

Step 5: Imagine

After much deliberation on the subject we decided that the copper wire could not carry the roadway because no matter how tight you made the wire it would never be at full extension. As a result the extra pull on one side’s wire brought the roadway lower in elevation which created the extra pull to split the tower into several sections. We then decided to switch to using twine instead of copper wire so extra weight would not affect the tension because the twice length would already be at full extension. We also decided to drill hole all the way through the towers because then the weight distribution of the roadway would be evenly distributed throughout tower’s structure instead of focused a certain points.