October 27, 2017
Kevin R. Kline, PE, District Executive PennDOT Engineering District 2-0 1924 Daisy Street - P.O. Box 342 Clearfield County, PA 16830
Dear Mr. Kline:
Reference. PennDOT Engineering District 2-0, Statement of Work, subj: Concept Design for Vehicle Bridge over Spring Creek along Puddintown Road in College Township, Centre County, PA, dated September 1, 2017.
Statement of Problem. The Spring Creek bridge, an integral part of College Township, was destroyed by a flood and has significantly altered the daily routines for all community members.
Objective. To create a new design for the now-destroyed Spring Creek bridge.
Design Criteria Using the bridge designer software, undergo two phases of design for both a Howe bridge and a Warren bridge: one to test economic efficiency and the other for structural efficiency. After achieving optimal specifications, construct both bridges out of popsicle sticks and test them to failure.
Technical Approach.
Phase 1: Economic Efficiency. For both the Howe and Warren bridges, design solutions that fit the constraints of $150,000 to $300,000 while stably holding the bridge’s own weight and a dead load.
Phase 2: Structural Efficiency. Minimize the compressive and tension forces acting on the Howe and Warren designs while keeping them stable and in the cost constraints.
Results.
Phase 1: Economic Efficiency. After much trial and error, we came to a design that was able to support its dead weight, and the weight required. Although this met all design constraints, the economics of the trusses were far too great. We then took to research to try to understand the types of materials and how they would react under tension or compression. After understanding these concepts, we applied them to our design and produced stronger, more cost-efficient trusses. The Howe truss was made up of mostly carbon steel bars and tubes
1 | Page Fall 2017 costing a grand total of $234,760.68. Unlike the Howe, the Warren truss was made up mostly of High-strength Low Alloy Steel bars and tubes with a grand total of $291,392.27. Phase 2: Structural Efficiency. Testing revealed major structural deficiencies that we had not noticed in the rendering of the software. The Howe did not have a cord failure, it because of shift in the lower cord during the gluing process. This caused the two sides of the bridge to be uneven therefore the weight pulled the bridge to fall through the gap. The bridge weighed .194 lbs. and the weight at failure was 43.5 lbs. The Warren truss failed due to a glue failure. The glue had not completely settled so when the load was applied, the glue joints were the first to fail at 36 lbs. The Warren weighed in at .145 lbs.
Best Solution. We considered many factors and in the decision of what the best solution for this project would be. After long deliberation, we concluded that the Howe bridge was the best option given the design constraints and other constraints. (i) The Economic Efficiency
The Howe bridge ($234,760.68) cost significantly less than the Warren Bridge ($291,392.27). Overall it cost $56,631.59 less to construct the Howe bridge.
(ii) The Structural Efficiency
The structural efficiency (mass at failure divided by the mass of the bridge) for the Howe truss is 224.22 and for the Warren truss is 248.27. Based on the 8 design teams that summited their data for the Howe and trust bridge, we see that the geomean for Warren is 265 and for Howe is 244. The highest for the Howe is by design team 3 at 366 and Warren by team 2 at 361. The minimum for the Howe is by team 7 at 113, and for the Warren by team 4 at 206. All this information is organized in tables 7 and 8.
(iii)The Design Efficiency
When it came to Design efficacy the Howe truss had a lower design efficiency at $1,053 per unit of structural efficiency while the Warren came in at $1,175 per unit of structural efficiency.
(iv)The Constructability
The material cost for producing the Howe was $48,833 compared to the Warren at $126,042. Connection cost was also an integral part of the design and cost calculation and the Howe was cheaper in this aspect as well, $16,000 compared to $16,800 for the Warren. The final variable was product cost that we had to take into account for the Howe truss it came in at $15,000 and $7,000 for the Warren. This is the only category where the Warren was cheaper that the Howe. Therefore, the Howe was $56,631 cheaper to produce.
Conclusions and Recommendations. In conclusion, we have decided that the best truss system to replace the Spring creek bridge is the Howe because of the following factors. In prototype analysis, the Howe held a higher load at failure (43.5lbs > 36lbs). The overall cost for production of this truss is significantly cheaper when compared to the cost of the Warren, even
2 | Page Fall 2017 though its natural design is much more complicated. And finally, that the Howe has a lower cost per unit of structural efficiency ($1,503 < $1,175). These are the factors that led us to choose the Howe as the best possible solution.
Respectfully,
Omar Aboelenin Sade Langa Engineering Student Engineering Student EDSGN100 Section 001 EDSGN100 Section 001 Design: Team 5 Design: Team 5 Design Team: Penn Engineers Design Team: Penn Engineers College of Engineering College of Engineering Penn State University Penn State University
Tommy Kiefer Mike Papiernik Engineering Student Engineering Student EDSGN100 Section 001 EDSGN100 Section 001 Design: Team 5 Design: Team 5 Design Team: Penn Engineers Design Team: Penn Engineers College of Engineering College of Engineering Penn State University Penn State University
3 | Page Fall 2017 ATTACHMENT 1 Phase 1: Economic Efficiency
Howe Truss. The group had divided the work to Omar and Tommy to be responsible for the Howe truss, there was also insight from the other members of the group when new ideas would arise. The Howe truss can be viewed on figure 1, while the load results can be viewed on table 7. After research and trial and error we began to see what member were under tension. We understood that we could save money and actually increase the structural integrity of the bridge by replacing the solid bars into hollow tubes. Not only did this save us money, as a concept but it led to cost cutting in different aspects. We later cut more cost by decreasing member sizes and other extremities. With half of the truss being in tension, the other half was in compression. We saw this and learned that the best way to deal with compression was with solid bars, but in most cases, the sizes we were using (the default by the simulator) were entirely too big. We also use this knowledge to further more cut the cost of the bridge. The entire truss was made out of carbon steel which was one of the cheapest alternatives. There is a possibly that we have left money still on the table with the material type. Other alternatives were priceyer but a combination of many could have saved money in the long run.
Warren Truss. After experimenting with the Warren’s members, we came to a conclusion that met all the constraints and still fit the budget easily. The truss can be viewed in figure 5. The Warren truss used significantly less type of materials and sizes. We used entirely High-strength Alloy Steel bars and tubes, and only 7 sizes where the Howe 15 different sizes and types. We chose High Alloy steel because in our constraints it was the best suited to fit out needs. A little pricey but because the Warren does not have many members we could afford to sacrifice some of the cost for stronger, more light-weight members. The total cost of the Warren truss $291,392.27.
4 | Page Fall 2017 ATTACHMENT 2 Phase 2: Structural Efficiency
Howe Truss. The Howe truss was created using wooden popsicle sticks, glue, and hot glue. It can be viewed in figure 1.
Prototype Bridge. How bridge weighed in at .194 lbs and had a unique design compared to the rest of the bridges in the fact that it had so many supporting members. You can see the prototype in figure 3.
Load Testing. When looking at the data we collected we saw that the truss held a lot more than what we expected it to (15lbs), it realistically held 36 lbs at failure. It scored among the average in the class, but there was definitely small fixes we could have made to increase the weight held dramatically. The results of the structural efficiencies are in table 7.
Forensic Analysis. When looking at what caused the failure we saw not damage to the members themselves, what we did see was that there was movement while gluing causing the bridge to twist and turn during the design tests and therefore causing failure before the sticks were breached. In figure 4 you can view the damaged Howe truss.
Results. The Howe truss of team 5 ranked competitively among the rest of the groups in the class, and like previously mentioned, small changes could have produced huge results. The data compared against the rest of the class can be found in table 7.
Warren Truss. The warren was made out of 60 wooden sticks, white glue, and hot glue. The Warren also scored in the middle of the pack compared to the rest of the class.
Prototype Bridge Elmer’s glue was used to join the members together and to create two separate trusses. They were later connected by 8 popsicle sticks that ran horizontally connecting the two separate parts, in this instance, hot glue was used instead of elders for a stronger hold and a quicker drying time. An image of the Warren truss is attached in figure 5.
Load Testing. We saw that the bridge actually held a weight of 36lbs which is very impressive considering that it was one of the lightest bridges in the class at .145 lbs. We saw the bridge stand more stable and did not twist as much as the other truss in our group, the Howe. The load at bridge failure is attached and can be referenced in table 8.
Forensic Analysis. We saw the same type of failure as the Howe, in the perspective that the physical sticks did not break, it was the hot glue joints. The joints were not given much time to cement therefore they gave out before the popsicle sticks did, causing premature failure, nit failure nonetheless. The image of the aftermath of the warren bridge after failure is labeled as figure 6.
Results. The Warren did very well compared to the rest of the class, and that’s not taking in account its weight, being one of the featherweights in the pool, it performed very well. The class layout among warren trusses is seen in table 8.
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Enclosures. Tables Nos. 1 through 8 and Figure Nos. 1 through 8 are attached.
6 | Page Fall 2017 TABLES
7 | Page Fall 2017 Table 1: Howe Cost Calculations
8 | Page Fall 2017 Table 2: Howe Members
9 | Page Fall 2017 Table 3: Member Detail Graph
10 | Page Fall 2017 Table 4: Warren Cost Calculations
11 | Page Fall 2017 Table 5: Warren Member Details
12 | Page Fall 2017 Table 6: Member Detail Graph
13 | Page Fall 2017 Table 7: Load at bridge failure date & SE for all Howe trusses
14 | Page Fall 2017 Table 8: Load at bridge failure & SE for all Warren trusses
15 | Page Fall 2017 FIGURES
16 | Page Fall 2017 Figure 1: CAD Howe Bridge
17 | Page Fall 2017 Figure 2: CAD Warren Bridge
18 | Page Fall 2017 Figure 3: Howe truss before testing
19 | Page Fall 2017 Figure 4: Howe truss bridge after failure
20 | Page Fall 2017 Figure 5: Warren Truss before Load testing
21 | Page Fall 2017 Figure 6: Warren Bridge after load testing
22 | Page Fall 2017 Figure 7: Class Structural efficiency comparison (Howe truss)
23 | Page Fall 2017 Figure 8: Class Structural Efficiency Comparison (Warren Truss)
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