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.

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.