Week 9 - Yilei Jiang

       During the last week, we worked on building our three foot span bridge. we didn't change our design a lot because we thought that is the most serviceable way to build. The bridge we had based on the tension and suspension measured by "Method of Joints", also depended on the predicted price. We then tested the bridge using the parameters we will be using in the next week for the official testing of the bridges. It was able to hold decent amount of weight on the first try (about 25 lbs) so we decided to reduce its cost by removing some of the members connecting the two sides of the bridge. We tested it again but the amount of weight it held dropped significantly. So our plan for next week is to re-design the final bridge that can hold decent amount of weight without failing which we will be test during week 9. The only accomplishment this week was to test the same design with different number of members connecting the two sides of the bridge which gave us an idea on how much the connecting pieces contribute to its weight holding capacity.

      We have completed almost 9 weeks of the bridge module which has brought many new things into my perspective. I learned quite a few things from the bridge design tools such as WPBD, Knex, truss analysis etc. that we utilized throughout the term. One of techniques I learned in designing a bridge when our goal is to have the lowest cost is that you can analyze the compression and tension forces using WPBD and try to reach the ratio of 1. This can be done by using different variations throughout the bridge which include changing the member size, material and length etc. while reaching a ‘functioning’ bridge. However, in real world, the bridge would be undergoing a lot of external forces such as wind turbulence etc. The amount of force applied by the vehicles travelling over the bridge would also vary constantly. So to ensure the safety, the bridge would have to be designed such that it can withstand the maximum amount of force. Every members and gusset plates would have to be analyzed in great detail to ensure that they wouldn’t give up under the normally expected force. In this case, the safety would be the first priority, not the overall cost of the bridge.

week9-xue

Last week in class we start work on out 36”inch bridge. The 36”inch bridge has a different rule than then the first one. The 24” inch bridge only have rule on lengths, but the 36” Inch Bridge have limited on high of the bridge. The span of the bridge is also increasing a big amount. That made the second bridge has a lot of different part than the first one. By using the force calculation we find out that the 36” bridge could not be simple as make it longer. It needs a lot of change. We are trying to work the best way thought to make a better bridge. This week in class we are start to text our 36”bridge. The 36” bridge would be a more advance bridge. We have learned a lot of different thing in the class. We also get data from our test information. I am really exciting to see how does our bridge do in the finial competition. I believe this time our bridge would improve.

    This term engineering class I really learn a lot. First I get know different type of the bridge, Especially the truss bridge. We get know about truss bridge, and use it in all our bridge. The first bridge is doing on the west point bridge design. The west point bridge design give an ideally bridge. On the west point bridge design we could test bridge and see the weight that is do on each member. That is very helpful for future design. After the west point bridge design we start our k’net bridge and learn how to calculate the force on each member. How to make a good serviceable bridge in a low cost is our finial goal. In the class we use a lot of thing we use in physic and really life to made the best bridge we can possible make.  

Week 9 - Kyle Hayes

Last week we finished our work on the static of bridge design using the method of joints to solve for the forces in the members. We also worked on finishing up and testing the final design of our bridges, testing numerous small factors and detail to minimize cost and increase strength. Small changes such as changing the length of pieces and changing the gusset type.

What I have learned about bridge designing is that the maximum capacity of the bridge is determined by the maximum pull out force of a gusset and that the tension can be reduced in a member via the force distributing to adjacent members. I learned that the point of failure is at the gussets and usually occurs at the ends as they have to take all the weight to disperse it to the ground. Also I have discovered through testing that having fixed connections are important as free ones cause the bridge to be able to shift and bend and will cause a easy quicker failure. I learned that hollow bars are better to use then solid bars as they have more give and flexibility, and that the most stable shape is the triangle so it is essential to the design of a truss. Finally I learned that the cost of a bridge is directly proportional to the weight of the bridge. There were many other things that I learned but this are some of the most important

This week in class we will be having our in class competition to see what group had the highest strength to cost ratio. All are work and testing comes down to this.

    -   Kyle Hayes

A3 - Yilei

• Free Body Diagram & Calculations

     By using the "Method of Joints", the analysis of the truss bridge is showed below. The bridge has a span of 24", a height of 8" and a load of 15 lb at point C.

• The overall forces can be seen below.

• The  same results of analysis in The Bridge Designer's version. 

       The online Bridge Designer is a program allows to design a virtual truss, and then put a load on it. It will calculate the tensions and compressions of each members itself. 

       When I did this program, I had to use the same scale of all members and angles to correspond the results of my hand analysis.  I made each grid be 2"so that the length should be 12 grids. And the height should be 4 grids. Then I picked up the middle node of the length to put a load on, which was 15 pound in this case. Consequently, I got the most approximating forces equaling the ones calculated by hand.   

• The results of analysis of our two foot span bridge in Bridge Designer program.

        Might because of our design, it was hard to computed via the Bridge Designer program. Our two foot span bridge has a bottom truss. So when I completed the adding process and pressed the calculate button, there's an error came out. It said that we had to followed stable structure is M + 3 = 2*N , where M is the number of members and N is the number of nodes. The simulation will not calculate forces unless this condition is met. Thus, we changed our K'Nex design a lot. Besides, we did looked the K'Nex joint test page and it showed the tension would increase if the bridge is symmetrical. So we did the bridge symmetrically.

        That type analysis of the K'Nex truss bridge can find out the strong and weak parts of the bridge by using the average strength of each joint. So we can change our design to reach the proper number. That can make the bridge more stronger.    

week 8 -Yilei Jiang

       I learnt the "Method of Joints" during last lab which is s a way to find unknown forces in a truss structure. This method also would be used in completion of the A-3 assignment.The method of joints consists of satisfying the equilibrium equations for forces acting on each joint. In the lab, we also discussed some ideas of our next three foot span bridge. We agreed that we would use the new method we learnt to calculate our new design to get the most serviceable design.  


       After the amounts of calculating, I think this method of analysis, the "Method of Joints," is not sufficient for a real bridge for some reasons.  First, a real bridge should be tested and analyzed in many more ways than just testing a downward force on a connection joint. Even this method shows the tension and suspension of every members, a real bridge could not be built of this simple databases. Secondly, A real bridge is not only hold its own weight but also deals with dynamic loads and the things like side pushing from the side and even up drafts from underneath. "Method of Joints" does not show that side.  
    
      I am not saying that the "Method of Joints" is completely useless because it at least shows the force when a truss bridge is under a special circumstance, which is  in equilibrium. If a truss is in equilibrium, then each of its joints must be in equilibrium. That's how this method works. This method is one of many methods that are used to analyze bridges. Of course there should have more different calculation for a real bridge. 
      
      There's one thing I'd like to further analyze, which is the accurate breaking point of the K'Nex gussets when they are   experiencing forces. I think that this information would be useful at our next assignment, which is designing a three foot span bridge.
      
      For next week, we will working on our new bridge and complete the comp2.

week8 - xue bai

Last week in class we start work on the basic calculation of the force that on each member. We learn how to use free body diagram and trigonometry to get the force that is one each member, but it need a lot of calculation. The bridge design is the next thing we learn that could help calculation the force. It was very quick and useful. It is better the calculation by hand. I believe we can do a better job on our second bridge.

Next week we would start to work on our second bridge. By the experience we had on our first bridge and the ability of analyze the tension and compression of each member we can design our second bridge in a better way and made it more effective.

The ability of analyze is very helpful but it also have a lot of limited. Analyze we learn just calculate the force form one side but not all the side. The structure in the middle also can be affecting the finial load. In the first test our bridge is fall down because of the twister. The bridge design did not count the chance of bridge twister. The other part that is also very important is the joins. K’net has a very weak join. It fall apart very easily even the member can support the loads the joins could fall apart and made the bridge fall down. It better to know the limit for the join and how much force is do on it. That could help a lot.  

A3 - XUE

 Free Body Diagram

Angle Calculations

Calculations

Replication of Analysis in Bridge Designer

In order to make sure results of the hand analysis correspond to online Bridge Designer I use every square as a two time two square. The length of the bridge is 36’’. That meant that I use 18 little squares as my base and my high is 10’’. That made my bridge has 5 squares high. So my hand analysis has same angle as the online bridge. Same angle is very important in the calculation. That make sure I have a correct number in my online bridge designer. The online bridge designer also is a tool to make sure my calculation is right. My number of calculation is mostly match to the online one, so my calculation should be right.

We change our bridge a lot to follow the ruler of member add 3 equal twice the nodes. The online Bridge Designer cans only calculation particular member and nodes. That made the calculation it give did not match the number the number we get in the text. But I try my best to get it as close as possible. I put 35 pounds load on the bridge. It should that some member get a lot of forces in other hand some member did not get any of the force. I do not know is that number come out because my changer of the structure or our bridge have this problem in the first time. However get to know force on each member is very helpful in future design. We can improve our design and made it became more effective by that way. Such as cut up the member in the place that have less force or do not have force at all. Add more members to the place, which has a lot of force. In the picture shows that middle has a lot of force. We may add some member in the middle. The online bridge design was very helpful. I hop we can made a better bridge next time.