Analysis Process

Last week in class, I worked with the other members in my group to analyze our Knex bridge design using online bridge designer software created by Johns Hopkins University. It was difficult for us to put our exact bridge design into the software because of a special restraint the program called for: "Members + 3 equal twice the nodes." Basically the problem was that our bridge is symmetrical about a single node--not about a single member. This caused our bridge to have an even number of members. When you add 3 to any even number, you come up with an odd number which, when divided by, is a decimal. But for our bridge, we cannot have a fraction of a node, so we needed to change our bridge design in order to use the online design program. The fact that this program wouldn't accept a bridge whose exact center point was a node rather than a member, led me to question our bridge's strength. This week in class, I will discuss with my group mates ways we can modify our current bridge design so that it follows the "members + 3 = 2*nodes" rule.

As far as analysis goes, I do think this method of analysis is efficient for a real bridge if additional factors are considered. On a real bridge, the weight of each bridge component needs to be taken into consideration when determining the tension and compression on chords. The steal beams, metal bolts, thick plates, and even the paint, all add to the total dead weight of a bridge which is a factor significant in determining the forces on each bridge member. If this dead weight is not accounted for, the bridge would fail before reaching its calculated load capacity. After thinking about this, I decided that I'd like to analyze the twisting of truss bridges. One bridge design that my group mates and I tested ended up twisting and snapping. I think it is important to know how strong and, as a result, heavy, a cross beam needs to be so that it is not adding to the bridge's dead weight, nor decreasing the bridge's weight capacity.

Analysis Desires

Last week, my group mates and I tested our 2' bridge in a competition against the rest of the groups in our class. Our bridge would have cost over $305,000 to build full-scale, but it only supported 17.4 pounds of sand during the test. This was the first time we tested one of our bridges using the apparatus, and we were able to get a good view of how our bridge failed. This week we will be testing our 3' bridge which was designed completely differently. We decided to cut costs by using different gusset plates and longer chords. We believe our bridge will be able to support more weight than our last bridge. And even though this bridge is a foot longer than the last, it's nearly $50,000 less expensive.

In designing the bridge using Knex, something that is challenging is having to run several tests and analyze each one to determine where the weaker points are. In West Point Bridge Designer, a table of information was presented after each simulated test. The info included statistics regarding each individual chord's tension and compression forces, and slenderness. I would like to be able to determine the tension and compression forces across the top and bottom of the Knex bridges we've been designing, but the problem is that WPBD bridges normally fail because of weak chords giving out. The pattern I've noticed with the Knex bridges is that the joints, or "Gusset plates," have been failing before the chords. I am not sure how we could calculate the tension and compression forces.

Analysis Desires

When designing a bridge on West Point Bridge Design, it tells us what pieces fail and how much tension and compression is exerted on each piece.  I feel that this information would be really helpful when designing our K’nex bridges.  It is a completely different experience to design a 3D bridge and also including the top and bottom designs, rather than just the side pieces.  We have no idea of how our bridge is going to work until we actually test it.  This makes us put a little more effort into our design, but it also means we might not create the most effective or efficient bridge because we are not able to participate in trial and error type experiments.  If we knew before hand which pieces were most likely to fail, we could design our bridge differently before actually participating in the class competition.  I’m not sure how this could be done, but I know it would be helpful to have the tension and compression numbers known so we could build our bridge to its max potential. 

Last week we tested our bridge in class.  It did much more poorly than we expected it to.  It only held a total of 17 pounds.  It broke right on the edge of the bridge; the piece that was on the horse holding the weight of the bridge.  We did not expect this.  We thought that the reason it would fail would be because the gusset plates would slip apart because they seemed like the weak pieces of the bridge.  We decided that we should scrap our bridge and create a new design.  In class we decided we should go with a square shaped bridge instead of the trapezoid we had originally built.  We think this will help disperse the weight of the bridge off of the end points.  We hope to test our new design in class this week with better results.

K'nex Process

There are many differences between using K’nex pieces to build a bridge and using steel to build a ‘real’ 20’ bridge.  K’nex pieces easily fit together and you have a lot of options when picking ‘gusset plates’.  However, all of the connecting pieces are 45 degree angles, and you are very limited to the types of triangles you build.  In real life, you can make any desired shape out of various materials and connecting pieces.  This is a disadvantage of using K’nex pieces. 

Something our group struggled with was figuring out how best to use the connecting pieces.  We found out early on that the grooved pieces can easily slip and slide apart from each other when enough pressure is exerted on them.  We were just hoping that the designs we had would uphold a significant amount of weight.  The other disadvantage of using K’nex pieces is that there is no way to ‘bolt’ or glue the pieces together as you can do on a real bridge.  The pieces just fit inside of each other and we have to hope that’s enough.  The steel bridge in real life has the potential to be more efficient. 

Last week in class we brought sketches of a bridge design and compared them all to see which one we thought would work best.  In the end, we came up with a new design all together.  We had decided to use as many short members as possible to increase the strength of the bridge.  However, this caused us to use a lot of connecting pieces which really hiked up the price.  We are going to test it in class this week and we predict that it will fail due to the gusset plates sliding apart. 

Knex Process

Last week in class, our group discussed the bridge designs we'd submitted on paper and chose a strategy for building our Knex bridge. After modeling one of the paper drafts which used a number of 3.375" chords, we chose to shorten the average length of the chords and increase the number of joints. Doing so raised the cost of our bridge but also increased its strength greatly. We are going to test the bridge in class this week and pay close attention to the way the bridge collapses. I would like to figure out how to disperse the load weight across the bridge so that the bridge doesn't have a weak point in the future.

There are important differences between designing a bridge using Knex and designing a twenty-foot "real" steal bridge. The most obvious, I believe, is the length to weight ratio of members. Knex are very light plastic pieces which seem to be very strong for their weight. Without having yet compared a Knex bridge's strength-to-weight ratio to that of a steel bridge, I am imagining that the steel bridge would have more dead weight in proportion to its strength. On this note, a steel bridge with the same design as a Knex bridge may not be able to support as much load as a Knex bridge due to the different strength-to-weight ratios. The steel bridge may give out sooner because of this, or it may hold together longer due to the use of better Gusset plates.

While experimenting with the Knex bridge designs, I have learned that the Knex grooved Gusset plates tend to be the weak points in the designs. The smooth plastic pieces easily slide apart under tension, causing the bridge to fail. On a "real" bridge, there would be stronger Gusset plates due to a greater amount of friction between materials and the usage of bolts for additional support. Both of these elements would help strengthen the steel bridge, but neither of these options are available to strengthen the Knex bridges, making the steel bridge potential stronger.

Research Questions

Currently we are working on understanding the different types of bridges and how they work.  As a class we have all made some designs although I’m sure some are better than others, there are a couple of questions I was wondering if Mr. Bhatt could answer for us.

To begin with, after so many years of bridge producing, is there a certain technique or kind of literary work one can read as an engineering to advance in the proper understanding of bridges and their necessary properties?

Additionally,  what kind of specialists would be needed to bring together the perfect combination of a good quality bridge and a good budget demand for the government and/or company offering the capital to build the bridge?

Lastly,  how can one do research from other types of literary works besides books (lets say one wants to read the mentioned on previous journals or newspapers on a certain subject like bridges)?  Like the commentaries that other successful bridge companies might have of certain historical strategy and designs.

- Ana M. Franco

WPBD vs. K'nex

                West Point Bridge Design is a great program to help you get started when first designing bridges.  However, I feel that it is very different from using K’nex pieces to build a ‘real’ bridge.  Both WPBD and using K’nex help you to be creative, learn what works and what doesn’t and you can experiment with different sized trusses on either apparatus.

                Although West Point Bridge Design teaches you the fundamentals of bridge designing, it is a relatively easy process that involves ‘connecting the dots’.  When using K’nex, you cannot simply ‘connect the dots’.  You actually have to measure the lengths and angles of each piece you want to use and try to fit them together.  A design may work out perfectly on West Point, but it may not be possible to build using K’nex pieces.  Your design will have to be flexible to meet the restrictions of the piece lengths, widths, and angles of the ‘gusset plates’.  In West Point, you can have long rods of material that crisscross each other, but when using K’nex, you need to use smaller pieces and put a gusset plate in the middle of the intersection.  In other words, West Point is not a completely realistic design experience that conveys accurate products and results. 

                West Point Bride Design allows you to fairly easy erase something and add something somewhere else.  When using K’nex, you have to break things apart and put them back together to make your design perfect.  Another difference about the two ways to create bridges is that the weight that a bridge can hold is going to be significantly different.  K’nex pieces do not relate to real life loads and are a scaled down version.  Also, the costs of the bridges are different because each K’nex piece has a set cost and you can easily predict the cost of the bridge you are building.  West Point has so many different sizes and materials that just don’t relate to using K’nex pieces.  Overall, using K’nex pieces will be a very different experience than designing a bridge on West Point Bridge Design.
                Last week in Engineering Lab we learned a lot about how to construct research for our bridges.  Our guess speaker was very informative and helpful to the research process.  We also got to experience what it was like playing with K'nex.  We were able to use the class to experiment with the pieces and see what works best. For next week we have to design a bridge and draw a blue print of it.  We will then collaborate our designs and choose which one our team likes best.  In class we will start building our bridge.  I hope to  design a cost efficient bridge that holds a lot of weight.  I also hope that my teammates have some good ideas that we can all combine to make a really efficient bridge.