Week 7: Analysis Process

This past week we have continued to work on our final knex bridge design, we have also been working to complete a truss analysis of a sample bridge and apply the same logic to our knex bridge to improve our design. In the coming week we will continue on the truss analysis and design of our knex bridge. This week we had no major accomplishes. The only issue that I see is related to post A3, I don’t believe that Bridge Designer will allow us to submit what Dr. Mitchell is asking of us.

I believe that the method of joints is sufficient for small scale or low load bridges such as foot bridges across a wet area. I do feel that while it provides good insight about what should be changed in a bridge design it does not take into account all of the forces acting upon the bridge. The method of joints should work great in a situation where effects related to nature are negligible; wind is a major factor in bridge design. The bridge needs to not only stop the wind from pushing it over but it also needs to be able to support the load of cars while wind is acting upon the bridge.

I would like to analyze how a bridge preforms over time with an increasing load on the bridge. The load of the bridge changes each time the bridge is repainted or the bridge deck is surfaced, the load also changes as the vehicles we drive change. A bridge does no good if it only lasts 5-10 years because there is no tolerance for maintenance to be performed.  As  the vehicles we drive change so does the load on the bridge and how the bridge handles on windy days. A bridge of cars is going to handle different on a windy day than a bridge full of tractor trailers since the trailer are going to be more impacted by the wind. I feel that we need a method that incorporates weather related stresses as well as stresses ove time.

Week 6: Analysis Desires

In our week 6 lab we load tested our 2' span bridges. Before the test we made a few minor modifications that helped with our cost. In the load test, our bridge exceeded our expectations. It was able to hold 48 pounds of sand, including the bucket, before ultimately failing. The bridge failed at one of the gusset plates near the middle of the bridge. We had actually predicted that it would fail in this area, so it was not a surprise to us. We plan to keep this in mind in our next design.



For the 2nd bridge design, we must fabricate a 3' span bridge that is hollow in the middle so a roadway can be placed through it. This will pose a little more of a challenge for us, because we will not be able to add cross bracing in the middle of the bridge, something that greatly increases the rigidness of the structure.



One thing that I believe would greatly help in the Knex design process would be the ability to see all of the forces acting on the bridge. If a script was generated that could hook up to a bridge and record the changing forces on the bridge, it would be very difficult to test without again. The ability  to see all of the forces in the bridge allows one to have a better idea of what parts are weak and failing or what parts are holding strong. This is unfathomable for a small scale Knex project though, because a machine capable of all that would take a large sum of time and money to create. This would be unnecessary for a small scale project like this one because of the time and money involved in something like that would make our design too expensive

Week 6: Analysis Desires

This past week the group took our 2' KNEX bridge design and made a few modifications before the load test. The group wanted to try to decrease the cost and still provide the strength that the original design had.  Our bridge managed to hold 48.4lbs which was the highest in the class.  However our cost/breaking weight was not the best in the class.  Our bridge was not the highest costing bridge in the class but it was among the top costs.  In the next test we will try to make modifications to the design to lower the cost without forfeiting the strength.  The bridge failed at the gusset plates.  The group predicted that the gusset plates underneath the load plate would break.  This was a close prediction.  The plates at the edge of the load plate on the underside of the bridge truss pulled apart.  The KNEX bridge structures week points are in the gusset plates dealing with tension.  They tend to pull apart or snap the small connection points.

If possible it would be great to be able to calculate the tensions on each individual connection on a gusset plate.  It is obvious that the plates close to the load plate on the underside of the bridge experience the most tension.  Trig functions may be able to help break down the forces along different cross members.  The fact that all connections are made at either 45degrees or 90degrees should allow for easy calculations.  I am not sure how to account for the decrease in forces toward the end of the bridge or how much force the slight bending of each individual beam takes out of the overall force.  The compression forces do not seem to be a problem.  The beams are much stronger than the gusset plates and are less likely to bow.  It is also important to figure out how the cross members from side to side strengthen the bridge.  Bridges will fail easily by racking from side to side.

The rules for next week has changed some.  The group must complete a 3' bridge with a hollow section 3"x2" for traffic to travel through.  This provides a whole new set of challenges and lots of modifications to do from the original design.  The group will get together and try to use what we have learned from the first load test to create a bridge with a better cost/breaking weight ratio.

Week 6:Analysis Desires

This week we made a few minor modifications to our bridge as we had thought about ways to improve our design before testing. This week in lab we tested our bridge to see how much weight it would hold and ours held 48.4lbs of sand, the most in the class, the cost associated with building our bridge was also amongst the highest in the class which results in a poor cost to strength ratio. This coming week we will refine our design and begin to work toward a working three foot bridge. With the success of our first bridge we have a good starting point for the second design, we just need to work on reducing the cost. As a team we are facing no issues and as an individual I am happy with our progress in the class.

With our bridge constructed using Knex it would be nice to be able to know the forces that each beam is experiencing, specifically the compression of the members that causes them to bow out. Another quantity that would also be useful to us is the force that each joint of the gusset plate is experiencing and what they are capable of handling. When the bridge fails the beams are not breaking, in some cases they are bowing out however the beam is not the point of failure, the gusset plate is the point of failure. Every bridge fails at its weakest point and in the case of Knex the weakest link of the bridge is the gusset plate, in order to determine the load that a bridge can handle we need to know the forces acting on the gusset plate. In order to calculate the force that each gusset plate is experiencing I think that we need to break the force down into its components via trig functions and add the forces but I am not sure how to take into account that the load is being spread across the structure and is not being focused to one particular point.

Week 5: Knex Process

In our week 5 lab we spent the whole class building and modifying a test bridge, looking for the strongest, yet most cost effective design. We tweaked and modified our design multiple times before we came to a design we are content with, and we are excited to test it. In our next week of lab we plan to break this bridge and then analyze what area of the design was first to fail. This will then allow us to better our design even further for the future.

There are definitely some major differences between designing a bridge with Knex compared to designing one with real bridge materials. A few of the biggest differences would be the cost of the materials obviously. Steel costs much more than plastic. Another difference between Knex and a real bridge is the ability to vary the length, size, and strength of the materials. While with the Knex pieces are set at a pre cut length, steel can be cut, bent, and modified to fit any design imaginable. One similarity between Knex and a real bridge is that the principles of truss bridge design stay close to the same, hence the reason we are using them in this course.

Week 5: KNEX Process

This past week the group began constructing different KNEX trusses systems.  The group worked together to create several designs to test and learn from in order to create a better design.  These KNEX bridges are all 2'.  The final design will have to be 3'.  The group has not had any problems designing a strong bridge but the cost is on the high side in order to achieve a good cost/load ratio.  The group will look at ways to lower this ratio in the next weeks.

My views on the differences and similarities of the KNEX and WPBD have not really changed.  KNEX has the disadvantage of only 45degree increments and WPBD doesn't account for the horizontal forces applied to a bridge.  They both are great tools at looking into truss systems for bridges and testing them.

There are many differences between a KNEX bridge and a 20' steel bridge.  The different number of materials and different design possibilities are endless for a 20' steel bridge.  The KNEX bridges are limited to the few pieces given and to the 45degree joint increments.  The way the steel bridge joints are fastened at the gusset plates have huge variations in options as well.  The KNEX bridge does have a few advantages though.  The KNEX bridge can be tested and rebuilt without any cost.  A 20' steel bridge costs a lot of money to make and it must be right the first time.

Week 5 Knex Process

This week in lab we shared our designs with our team members and we decided upon the best bridge design and we constructed our design. This coming week we plan to test our design and to make improvements with the pieces that we have. Each of us have also agreed to look into other designs and think of ways that we make improvements to our design for the class competition. Major accomplishments this week include designing a bridge to test and fully assembling our design.  The biggest issue we face is quite minor and that is agreeing on who will be responsible for the bridge.

My views on the similarities between West Point Bridge Designer and Knex have not really changed after working with Knex for a second week. I still feel that they are similar in that they can both be used to replicate a bridge. The difference between the two is that you are limited in the options that each has to offer, I also believe that they differ in the forces that they take into account, West Point Bridge Designer is solely based on tension and compression whereas Knex allows us to view forces that act horizontally on the bridge.

The differences between working with Knex and a real bridge are immense. The shear number of options for materials on a full size bridge is overwhelming such choices that have to be made are the strength of the concrete used for the footings, what material will be used in the webbing, how the materials will be fastened together whether it be welding, riveting, or bolting, then you have a decision with what material will be used to cover the driving deck of the bridge. I think that Knex are able to give us a good idea at how to go about designing a large scale bridge but I feel as if there are factors that are still out of our league to discuss. The possibilities are endless when making a bridge out of steel because the steel can be bent, cut or shaped however is desired, with Knex we are limited in what we can do because of beam length and the angles that the gusset plates allow us to use.