Bridge Process

During class last week, my group and I tested a three-foot bridge we'd designed over the course of the previous week. This bridge held only 14.2 pounds and its point of failure was caused by weak joints for the cross beams. Under 14.2 pounds, the bridge twisted and its members popped apart. To fix this, we replaced the weak connections with stronger ones. The original connections consisted of a Knex chord laying in the gusset plate joint. These connections were weak because the chords could slide freely through the plates, although being held against them. The new connections were created with two grooved 360-degree gusset plates joining the chords' ends like two puzzle pieces interlocking. This type of connection does not allow for movement and helps minimize the bridge's horizontal displacement. Next week we will test out three-foot bridge and compare our results to the rest of the groups' results to see which group has the best overall bridge.


Now that my group and I have almost completed the bridge design process for the term, I am realizing how much I have learned, specifically about bridge design. In class yesterday, Dr. Mitchell put three rough bridge analyses on the board. All were the same width and comprised of three triangles in the same order, however each was a different height. According to the analyses, a bridge whose height is the taller, or whose triangles are the least obtuse, will have the least amount of compression and tension on its members under a given load. Another important key to designing bridges that I've learned is that joints tend to be the weak spots. Before taking this class, I always thought that chords were the "weakest links" in bridges, but it is truly the "links" which are weaker. In testing the Knex, I have read information about which joints can handle the greatest stress force before failing, but we have never analyzed the force it would take to snap a chord in half. This is because of weak joints.

A3 - WETZEL

Truss Overview:

Free Body Diagram & Calculations:

Replication of Analysis in Bridge Designer:

In order to make the results of the hand analysis correspond to the online Bridge Designer program, some simple scaling was used. While using Bridge Designer, I mentally set each grid square to be 3"x3". Because my bridge constraints set the bridge length to be 36", that meant 18 squares on the grid would correspond to the length of my hand-drawn bridge. The height, needing to be 10", I scaled at about 3.3 squares on the Bridge Designer grid. This enabled me to use Bridge Designer to see if my own calculations were right about this bridge by creating angles similar to the ones I used. As long as the triangles composing my bridges corresponded, the tension and compression forces would also correspond. I was happy to see that the results calculated by Bridge Designer were very closely related to the results I'd calculated myself.


Knex Bridge Designer Analysis:

Just as I've done for the hand and computer analyses of the small bridge, I've now designed the Knex bridge using the Bridge Designer program. In order to scale the draft's calculations to the actual bridge, I will need to measure the actual bridge and set a value for each square on the grid in the program. After we test the bridge and see the weight it fails under, I will be able to scale the real load with the one I've used in the designer to determine the stress on the Knex bridge when it is holding a specific load.

Given the testing information about Knex joints, I might use this analysis as a guideline for the Knex bridge. I can say that, theoretically--and based on the computations of the Bridge Designer--the bridge will be able to hold x amount of weight when there are y joints. But if I optimize the strength of the joints by having more chords meet in a single joint, the bridge should be able to hold more than the program suggests. The testing information about Knex shows that a joint is stronger when more member meet at it. At the same time, the Bridge Designer's unique formula "members + 3 = 2*nodes" leads users to do the same thing: have multiple members meet at a single node. By following this pattern, my group and I should be able to increase the strength of our bridge enough for the cost to strength ratio to benefit from the modifications.

Analysis Process

               When analyzing a bridge that an engineer is designing, many factors play a role that you need to take into consideration.  Bridge Designer is great when you are playing around with K’nex pieces or if you want a general idea of the compression and tension forces, but it would not work for a real bridge.  Bridge Designer does not take into effect the load of the bridge itself, the weather, or any other outside forces acting upon a bridge.  I personally would not trust a bridge that was more carefully thought out and experimented/analyzed on.  I also think you need to apply more than a few trig and physics equations to a bridge design in order to make sure it will be a safe, efficient bridge.  

 

                For our class, I would like to take a look at the gusset plates that we are using.  It is hard to figure out which ones are strongest when you can only test your bridge once in class.  I also wish there were general guidelines to using K’nex pieces, such as ‘the longer the members, the stronger the bridge’ and so forth.  I have no idea whether that is actually true or not, and again, there is not a lot of time for experimentation.  Some choices you can make are common sense, but for someone who is new to bridge designing, there is an awful lot to learn.  I also think it would be helpful to have a way to analyze the differences between a two foot bridge and a three foot bridge.  We are not exactly sure that what we did for our two foot bridge will be efficient when lengthening our bridge.  There are just a lot of unanswered questions. 

                Last week in lab Melissa and I worked with Bridge Designer and tried to calculate the forces on our K’nex bridge three foot design.  However, we could not get this to work because “members+3 did not equal 2*nodes”.  This proved difficult to fix since our bridge was centered about a single node, not a member.  It was frustrating to tamper with the design on the computer and find something that would work out similarly.  In the end, the design is simplified in my A3 post because I was really running out of designs to try that still resembled our bridge.  This week in lab I hope to continue learning about the design process and come up with ways to improve the design we already have. 

A3-O'Callaghan

Free Body Diagram:

Calculations:

Finished Diagram with Forces:

Bridge Designer Analysis of Given Truss:

          Based on the numbers I got through my calculations and then the numbers Bridge Designer gave me, I can honestly say I am a little confused.  My numbers are very similar but not exact, which tells me that I don’t have a different ratio, especially because my top member piece also has a compression of 15 pounds.  My guess is that they used the angle of 60 degrees in Bridge Designer whereas as I used 53.13 degrees and that is why our numbers are slightly different.  I thought I would have to use a scaling technique and match up the ratios, but it has not turned out that way.  

Bridge Designer of Our Bridge:

          Bridge Designer would not let us replicate our actual bridge.  Our bridge does not follow the rule: members +3 =2* nodes.  In order to be able to resemble our bridge somewhat, I made the bridge shorter and it is missing two important members than run vertically on the left sides of the two center squares.  In doing this, I’m not sure how much is different compared to our actual K’nex bridge.  I also don’t know now that these ratios will be correct if we apply them to our own bridge.  I am hoping that these numbers actually somewhat depict how our bridge behaves or else it will not be very helpful.   However, this is a twenty pound load, and if I used the same method that I used for the other bridge, these forces would be pretty accurate.  All of these members are connected in 45 degree angles and I feel like Bridge Designer has an accurate display of what the forces would be on our K’nex bridge if it looked exactly like this.  

         This type of analysis could be helpful when building our K’nex bridge.  I might think about following the rule of member pieces to nodes that Bridge Designer requires.  Also, using the testing information about K’nex joints, I have decided that I definitely want to use as many pieces as possible in a single connector.  It has the highest pull out load and this could potentially increase our cost to load ratio significantly.   This also makes me think that we don’t want to use nodes that have a lot of spots for members to attach to them.  If we did put members everywhere they can attach, our bridge would become extremely expensive.