WPBD vs. Knex

The similarities between the products of designs from WPBD and Knex are the manner in which you would build the bridges. The designs that we made in WPBD could be some what implemented into the “real” bridge that we will be making with Knex. Using WPBD we were able to learn how to make a good bridge, even if we do end up pushing the limits. WPBD was able to show us the best way to structure a bridge so that it will not collapse. We can use these ideas when we build the “real” bridge using Knex. In my opinion on of the greatest similarities is that using shorter bars gives the most support. Also there is the aspect of the strength to cost ratio.

            The differences between the products of designs from WPBD and Knex is that in WPBD you can push the bridge to the absolute limit, where if we tried that with the Knex the bridge would collapse with the slightest amount of force. Also using the WPBD program there was no chance of the the bars slipping out of the gusset plates, on the other hand with the Knex pieces the gusset plates are not completely secure, it is possible for the bars to slide out. Finally, the exact strength to cost ratio is not the same.

Week 4

This week in lab the task of building a bridge was given. This bridge must span at least two feet and must be able to support weight. These bridges will be made out of Knex. One bit of useful information that was learned is that it is better to use smaller bars because they are less susceptible to bend and therefore are stronger than longer ones. Then we were given an opportunity to “play around with” the Knex pieces. So we tried building bridges, each of ours a different style. The goal or our assignment for next week is to design a bridge with the best “Cost/Failure-Load” ratio. This means that it has to be the cheapest bridge that can hold the most weight. Hopefully, we will all come up with different designs that we can use to make one bridge for the end of the term that works really well.

A1- Varghese

2D image - Drawing Board View

Truck in the Middle of the Bridge

Load Test Results

As seen in the Drawing Board View, my design has a rectangular outline. First, I started out with a Through truss - Pratt template as a base. I figured that a rectangle would be the perfect balance in preventing the bridge from collapsing so I expanded the template. The final bridge cost came out to be approximately $448,370. Once I become more acquainted with the bridge design program and experiment different types of steel and different angles for a better design I expect the cost to decrease by about $100,000. An effective technique I learned was to keep the outer and inner edge of the design to oppose each other, which will cause the bridge to counterbalance. 

A1- Zhu

2D Design Image

Truck In the Middle of the Bridge Image

Load Test Result

My bridge design was based on simple truss bridge. The purpose of designing this bridge was to ensure that the bridge is strong enough so that the standard 225kN truck can drive through. Initially, my design only consisted of carbon steel bars with a size of 140mm and joints connecting the members. The rough cost of that original bridge turned out to be about $386,000,000—which I thought was a nice price. Later I learned from my group members that I might be able to lower the cost by possible substitution of hollow tubes in place of steel bars. I replaced 24 solid bars into hollow tubes, and changed the thickness of the beams. Right now the solid bars have a thickness within the range from 140mm to 55mm, and the hollow tubes have a thickness within the range from 130mm to 80mm. After all the changes were made, the new cost came out to be $268,799.24. I believe that with further time and knowledge, the price could still be reduced by a few thousand dollars. Overall, this bridge design was really challenging but also a very good experience. I never dealt with this kind of designing before, but I am learning bit by bit as I move along. I learned the importance of the interconnected triangles within a truss bridge, and I also learned a few techniques on how to improve a bridge design.





A1- Thottakara

2D Bridge- Drawing Board View

Truck in the Middle of the Bridge

Loading Test Results

My initial bridge design went from $495,340.42 to $324,181.28. My initial bridge design was a Through Truss Howe design that had an upper layer as well. The upper layer was made out of generally larger scalene triangles. This bridge was sturdy and consisted of all thick solid carbon steel rods. In this design the longest rod was 8.94 meters long, the largest bar size is 180 millimeters, the largest compression force/strength ratio was 0.98, and the largest tension force/strength ratio is 0.22.

My final bridge design is a Through Truss Pratt design and also had an upper layer. It started as extremely sturdy with smaller triangles. Then I took thickness off of the bars, and made some of the bars that had 0.00 compression force/strength ratio into hollow tubes. Doing this as well as taking thickness off of them made this design relatively cheap. In this design the longest rod was 5.66 meters long, the largest bar size is 160 millimeters, the largest compression force/strength ratio was 0.90, and the largest tension force/strength ratio is 0.73.

            With further knowledge on the subject I believe that I could at least make this final bridge design about $100,000 cheaper. If I had a better knowledge about the forces, tension, and angular geometry of truss bridges I could make a bridge that is stronger, that does not slope as much and is cheaper than the design I have currently.

From designing this bridge I learned that it is actually a lot more difficult to build a bridge than you would think. The concept of making a strong bridge that is cheap as well is extremely difficult without the proper knowledge. I hope to attain this knowledge by the end of this term and be able to build a bridge that is both strong and cheap.