ENGR102 Engineering Design Lab II Winter 2016-2017 Revised 1/29/2017 http://core.coe.drexel.edu/ Page | 1 Copyright Drexel University, 2017 ENGR 102 – Engineering Design Lab II Bridge Module Week 4 Iterative K’NEX Truss Bridge Design Introduction The main objective of this module is to be able to design a K’NEX truss bridge which not only exhibits the lowest cost-to-strength ratio, but also comes as close as possible to the predicted point of failure. To achieve the latter, you will use Visual Analysis to apply the failure analysis methods of Lab 2 to a pre-designed K’NEX truss bridge. For simplicity, the truss bridge will be modeled in 2D with the assumption that the total load is distributed equally amongst the two trusses that make up the sides of the bridge. Once the failure load is determined, you should be able to validate your results via direct loading of your truss bridges and modify your model/simulations accordingly. Objectives By the end of this lab today, you should: • Be able to perform design iteration on a K’NEX truss bridge using Visual Analysis. • Understand the shortcomings of the modeling approach taken here, and understand why simulation results may differ from experimental results. K’NEX Truss Bridge: Design Example Figure 1 shows the example K’NEX truss bridge design we work with in today’s lab. In addition to the images shown in Figure 1, there is also a constructed model available for viewing within the lab. Begin by constructing this bridge in Visual Analysis as a 2D model of the side truss (Figure 1a). Specify the loading and supports as shown in Figure 2. The bridge will be loaded by hanging a bucket from a sheet of plywood that rests on the top and center of the bridge. Depending on how the bridge is constructed, this may create multiple points of contact, spreading the failure load evenly across the bridge. Generally, loading the bridge at one concentrated point verses spreading the load over several points will yield different results, but in this case, the approximation of a point load is close enough. We further assume that each side of the bridge handles the same loading due to symmetry. In other words, the total failure load of one side of the truss is equal to one half of the failure load of the entire structure. Prelab This lab builds on all of the concepts from the previous three labs. We highly recommend that you go back and review your results from these labs and understand the concepts introduced.
Transcript
ENGR102 Engineering Design Lab II Winter 2016-2017
Introduction The main objective of this module is to be able to design a K’NEX truss bridge which not only exhibits the lowest cost-to-strength ratio, but also comes as close as possible to the predicted point of failure. To achieve the latter, you will use Visual Analysis to apply the failure analysis methods of Lab 2 to a pre-designed K’NEX truss bridge. For simplicity, the truss bridge will be modeled in 2D with the assumption that the total load is distributed equally amongst the two trusses that make up the sides of the bridge. Once the failure load is determined, you should be able to validate your results via direct loading of your truss bridges and modify your model/simulations accordingly.
Objectives By the end of this lab today, you should:
• Be able to perform design iteration on a K’NEX truss bridge using Visual Analysis. • Understand the shortcomings of the modeling approach taken here, and understand why
simulation results may differ from experimental results.
K’NEX Truss Bridge: Design Example Figure 1 shows the example K’NEX truss bridge design we work with in today’s lab. In addition to the images shown in Figure 1, there is also a constructed model available for viewing within the lab. Begin by constructing this bridge in Visual Analysis as a 2D model of the side truss (Figure 1a). Specify the loading and supports as shown in Figure 2. The bridge will be loaded by hanging a bucket from a sheet of plywood that rests on the top and center of the bridge. Depending on how the bridge is constructed, this may create multiple points of contact, spreading the failure load evenly across the bridge. Generally, loading the bridge at one concentrated point verses spreading the load over several points will yield different results, but in this case, the approximation of a point load is close enough. We further assume that each side of the bridge handles the same loading due to symmetry. In other words, the total failure load of one side of the truss is equal to one half of the failure load of the entire structure.
Prelab This lab builds on all of the concepts from the previous three labs. We highly recommend that you go back and review your results from these labs and understand the concepts introduced.
ENGR102 Engineering Design Lab II Winter 2016-2017
Figure 1. K'NEX truss bridge example design used in this lab.
Figure 2. Loading configuration for the K'NEX truss bridge example used in this lab. The green arrow is a point load of 1.0 lbs. The left-most connection is pinned (i.e., fixed x-y displacement). The rightmost connection is rolling (i.e., fixed y
displacement).
To calculate the failure load and location for the bridge using visual Analysis, perform the following:
1. Construct one of the bridge’s side trusses in Visual Analysis. 2. Set up the Visual Analysis simulation, applying the loading so that the total loading sums
to one. 3. Run the analysis. As in Labs 2 and 3, the values found at each member are the results of
the method of joints analysis. 4. Using this information, you should be able to determine the load W that would cause the
truss (e.g., Figure 2) to fail, and the first joint to fail. 5. The actual failure load of the bridge is double the value of W found in step 4 (we are only
simulating one of the bridge’s two trusses, and assuming that the loading is applied symmetrically between both sides of the bridge).
ENGR102 Engineering Design Lab II Winter 2016-2017
6. After calculating the failure load and location of the K’NEX bridge of Figures 1 and 2, validate this by constructing the bridge using the K’NEX pieces from your kit and determining the average failure load and location over three trials.
7. Repeat steps 1 – 6 above for a modified truss design. Only vary one parameter to determine its impact on the overall design (e.g. the connection type at failure location, add a second level, etc.).
In your analysis, keep in mind the following:
• Load Symmetry: We are only modeling the truss bridge in 2D, and hence assuming that the load is going to be distributed evenly amongst the two side trusses of the bridge. This only holds exactly if the bridge is perfectly symmetric, and loaded symmetrically.
• Vertical Loading/Inadequate Cross-Sectional Bracing: The loading in Visual Analysis assumes that the external loading acts completely vertical. Of course, any lateral movement of the external load may cause the bridge to experience twisting along its cross section. If this occurs, then failure will occur in the bridge’s cross section and not in the side trusses where we are performing our simulations. The result will be a premature failure of the bridge from what was predicted.
• K’NEX Joint Failure: Some of the K’NEX joint interconnect to allow for connections along all three axes. You should orient these K’NEX joints such that they do not separate during loading (i.e., on members which are in tension). The result will be a premature failure of the bridge.
Closing Remarks At this point, you should have all of the resources necessary to perform design iteration on a K’NEX bridge. If you are unsure of how to proceed, refer to the previous labs and/or contact one of your lab staff members.
Critical Thinking Questions Address the following in your lab notebook.
• How close were you in calculated failure load and location to the measured values? Explain and justify any discrepancies.
• How can the bridge be changed to support a larger load? Use results from the previous weeks to justify your response.
• Observe the results from step 7 above. Explain your findings to your lab staff before leaving.
