Bridge Design Report - Katie Wisdom Online Portfolio



University of IdahoBridge Design ReportGroup D: Molly McGee, Kelsey Benscoter, Katie Wisdom, Spencer JohnsonExecutive Summary:Over the last several weeks, our team has been working to create a program in Excel VBA that would analyze the forces experienced in a truss. The finished program was able to draw the truss from a set of input coordinates, determine whether a member was in tension or compression, and solve for the magnitude of force experienced in each member.Our goal was to design a truss bridge made from manila folders that would hold a minimum load of 15 kilograms (33.1 pounds). We began by making several different types of tension and compression members and then testing the strength in each member. Strips of manila folder ranging from 4 mm to 8mm wide were used for tension members while compression members consisted of manila folders folded into either 6x10 mm or 10x10 mm rectangular tubes varying in length. The results of these tests provided the data necessary to determine which types of members would need to be used in the truss to support at least the minimum load. It took two weeks for our team to complete the bridge. On April 3, 2012, we tested our bridge by placing quarters on the joints to allowing loading to be distributed only over the joints. We then stacked reams of printer paper, text books, and, in our case, a bucket of sand on the bridge until it failed at 51.5 kilograms (113.5 pounds), more than three times the minimum load. Our bridge had an overall load to weight ratio of almost 600 placing our bridge 5th overall among our class of 11 teams.The following report includes the results of our compression and tension member tests, our final design, and design justification.Bridge Design:The basis for our bridge design came from the packet “Build a Model of a Truss Bridge” provided to us via our course website. For the purpose of our project, our bridge was scaled down from a 12 joint to an 8 joint truss bridge. Prior to any paper modeling of our bridge, we worked on creating a computer model of a truss bridge, so as to help in the analysis of our members during our testing and design phases. The final design and dimensioning of our bridge can be found in Figure 1 below. Our bridge had a height and length slightly larger than that given in the packet due to the member types we chose. Figure SEQ Figure \* ARABIC 1: Right Side View of Truss BridgeOur member choices for each portion of the bridge can be found below in Table 1. Table SEQ Table \* ARABIC 1: Truss Members Used in DesignFor the top cross bracing of the bridge, we again used the packet as our guide. The final design and dimesioning of the cross bracing can be found in Figure 2 on the following page. The members with red center lines are the 6 mm wide tension members whereas the members without the center lines are the 6x6x100 mm compression members. For the bottom cross bracing, we utilized 6x10 mm compression bars located at each of the joints.Figure SEQ Figure \* ARABIC 2: Top View of Cross BracingJustification of Design:We utilized the Excel template created in class to determine the anticipated loadings in each member of the truss, using a safety factor of 2. We chose to design for a safety factor of 2 in an attempt to create a bridge that could exceed the minimum load requirement without over-designing. Our anticipated loadings can be found below in Table 2. Table SEQ Table \* ARABIC 2: Member Forces as Calculated Using ExcelA range of data was found for several different types of members (compression members varying in length for both 6x10 mm and 10x10 mm types and tension members varying in thickness) through experimental testing. We graphed the data from all lab groups for the compression and tension strength tests and found lines of best fit for both (see Figure 3 and Figure 4). We used the strength values corresponding to the lines of best fit for each size member to determine which members would best satisfy our anticipated loads. These strength values can be found below in Table 3.3181350-1905 47625-1905Figure SEQ Figure \* ARABIC 3: Testing data for compression members.Figure SEQ Figure \* ARABIC 4: Testing data for tension members.Table SEQ Table \* ARABIC 3: Member Strengths by TypeIn designing our cross-bracings, we attempted to minimize weight; since in an ideal loading situation, they would all be zero force members. We selected the smallest width tested (6 mm) for our diagonal bracing tension members since we did not have data beyond that value. We utilized 6x6 mm compression members for our top lateral bracing members and 10x6 mm compression members on the bottom. We chose compression members at the suggestion of the design packet and used 6x6 mm members on top to minimize weight, but chose to reinforce the bottom with the easier to assemble 6x10 mm members. We used similar logic in our choice for the vertical zero force members in the truss pieces, choosing a 6x10 mm compression member to reduce weight and hold small amounts of tension or compression forces due to building imperfections. Additional measures taken to increase the bridge’s strength included the reinforcement of gussets deemed too small to adequately hold members together and the wrapping of the ends of all compression members, including those that were doubled up. We also included additional cross bracings at the top of the ends of our bridge to increase strength and placed large support gussets laterally on the bottom of the bridge to reinforce the ends, which were ultimately the only element of the bridge making contact with the table surface. Though these measures increased the weight of our final product, they also increased the value of our maximum load. Findings and Conclusions:Our bridge ended up being the heaviest bridge at 3.027 oz. It also supported and failed at the highest weights, 111.5 and 113.36 lb, respectively. A summary of the same values for the other bridges can be found below in Table 4Table SEQ Table \* ARABIC 4: Bridge StatisticsWe ranked each group’s ratio of bridge weight to supported weight in Table 5. We have the fifth best ranking in the class and the third best in our section. The mean and standard deviation of the supported/Bridge Weight ratio were calculated and can also be found in Table 5. We used this to get the probability of a random bridge failing at a smaller load than our bridge, which is 61.2%. This means that we are in the top 40th percentile for the Supported Weight/Bridge Weight ratio (See Table 5 below). Table SEQ Table \* ARABIC 5: Supported Weight/Bridge Weight Ratio by GroupThere appears to be a correlation between bridge weight and supported and failed weights (see REF _Ref321948932 \h Figure 5). In general, bridges that weighed more tended to support more weight and fail at a higher weight. This is because a heavier bridge generally has stronger members. Figure 5: Bridge Weight vs. Supported/Failed WeightsAlso, the data points for our bridge are labeled on the chart. You can see that both the supported and failed data points for our bridge are above their respective trend lines. This shows that our supported/BW and our failed/BW ratios are both above average. We believe that the reason we were able to be above average on both these ratio categories is due to our good design and our construction quality. Though we cannot say for certain due to a lack of video showing the actual failing of our bridge, we believe that failure was caused by torsion in the truss, due to assembly errors. Had the truss been assembled with the single tension members on the outside on both sides, we believe that our bridge would have held additional weight before failing. ................
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