Popolopen Brook Float Bridge Project: Integrating History, Community Service, and Engineering Education

Session 2215

Ronald W. Welch Stephen J. Ressler United States Military Academy

Abstract

This paper describes a one-semester design-build capstone project in which two senior civil engineering students designed a 230-foot pedestrian float bridge for the local state park commission and built one full-scale module of the bridge as a “proof of concept.” The project was a particularly effective learning experience, in that it involved complex technical issues in structural, hydraulic, and construction engineering, substantial real-world constraints associated with a historically significant site, and a strong community service component. The educational benefits of the project included grappling with real-world constraints, solving substantial technical problems, using the worldwide web for research, and coping with construction management difficulties such as limited funding, delayed shipments, and miscommunication. Most important, the students learned how to bring a project from concept through construction. Student assessment data demonstrate that such projects contribute much, not only to students’ learning, but to their motivation, thinking skills, and creativity as well.

I. Introduction

During the American Revolutionary War, the Continental Army built Forts Clinton and Montgomery to guard a strategically important point along the Hudson River near West Point, New York. The British were attempting to gain control of the Hudson Valley, as a means of splitting the American colonies in two. Therefore, Forts Clinton and Montgomery were two very important cornerstones in the defense of this region. Because the two mutually supporting forts were physically separated by Popolopen Brook (Figure 1), it is believed that the American garrison built a wooden pontoon bridge to allow soldiers and materiel to move easily between the two emplacements. Very little information about the physical characteristics of the bridge have survived, except an inferred location determined through an archeological study by Mr. John H. Mead in 1992 (1). (This location is annotated on Figure 1.)

Today Forts Clinton and Montgomery are being restored and opened to the public, in preparation for the 225th Anniversary of the American Revolution. As a result, there is once again a need for people to be able to move easily between the two historic forts. The Revolutionary War bridge is long gone and the nearby highway and railroad bridges are inappropriate for pedestrian traffic, so there is no easy way for tourists to cross Popolopen Brook at the fort locations. The Fort Montgomery Battle Site Association recognized the critical need for a suitable bridge between the two forts but had little money available to advance the project. Thus the association contacted the United States Military Academy (USMA) to request help with designing and possibly building a bridge as a student project. The project appeared to be within the capabilities

of senior civil engineering students and represented a unique opportunity for a culminating Page 7.921.1

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright © 2002, American Society for Engineering Education design experience incorporating real-world considerations of health and safety, constructability, usability/sustainability, historical and environmental sensitivity, economics, and aesthetics.

Pontoon Bridge

Figure 1. Locations of Fort Clinton (left), Fort Montgomery (right), and the pontoon bridge (provided by the Mid-Hudson Historian).

During the spring semester of Academic year 1999-2000, Cadet Heath Nero, a USMA civil engineering student, initiated the project with a comprehensive feasibility study. He worked with the Fort Montgomery Battle Site Association, the Palisades Park Commission, and the State Historical Preservation Office (SHPO) to define the project requirements and to develop a range of feasible solutions. The Battle Site Association initially expressed an interest in attempting to reconstruct an authentic wooden pontoon bridge on the site. However, Nero determined that such a structure would not be approved by the Historic Preservation Office, because lack of detailed information about the original bridge would make a historically accurate reproduction impossible. Thus the bridge would need to be modern in appearance, though still aesthetically suitable for the site. Based on the relatively long required span (232 feet), the conditions on the stream banks, and the flow characteristics of Poplopen Brook, Nero proposed three possible bridge configurations—a timber suspension bridge, a standard timber pile bridge, and a pontoon bridge. Based on a comparative analysis of these alternatives, Nero concluded that a semi- permanent pontoon bridge would best meet the clients’ needs (2). A pontoon bridge would be relatively easy to construct and would be most consistent with the historical setting and the original Revolutionary War structure—though the new bridge would not be a historically accurate reproduction.

II. Bridging the Gap

The following year, during the spring semester of 2000-2001, Cadets Robert Elliot and Bryan Hilton took over the project. Their mission was to do a detailed design and cost estimate for the Page 7.921.2

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright © 2002, American Society for Engineering Education float bridge and to build a single module of that structure as a “proof of concept” for the clients—the Fort Montgomery Battle Site Association and the Palisades Park Commission (3). A New York State grant to develop the battlefield site provided the limited funding necessary for the project, though funds to build a complete bridge were by no means assured.

The students met with the clients on two separate occasions early in the semester. These two meetings were crucial to clearly defining the scope of the project. The Battle Site Association wanted to open up Forts Clinton and Montgomery to tourists by October 2001 and to span the Popolopen Brook at river level, to maximize accessibility to visitors. The Palisades Park Commission wanted the new bridge to be permanent, to require as little maintenance as possible, and to be aesthetically consistent with this historically sensitive location. Elliot and Hilton quickly validated Nero’s conclusion from the previous year: a float bridge would best meet the clients’ functional requirements, aesthetic/historic preferences, and time constraints. A float bridge would not be truly permanent, but no permanent structure could possibly be in place by October 2001.

After defining the problem, the students began generating ideas about possible bridge configurations. Given the limited time available for construction, they decided that the bridge should be made up of readily available commercial products, rather than being constructed from scratch. They used the Internet and visited numerous local sporting goods stores, in search of boats and floatation systems that might be used as components of a floating pedestrian bridge.

During this initial fact-finding phase of the project, Elliot and Hilton visited the proposed bridge site on several occasions to determine the optimum alignment of the bridge, the bank conditions, and the flow characteristics of Popolopen Brook. The logical place to construct the float bridge is at the narrowest point, exactly where the original pontoon bridge was believed to have been located (Figure 1). Using a Philadelphia rod, the students walked out approximately 25 feet from the shoreline in hip waders and measured a depth of only 5 feet. They also noted tidal variations in flow depth and direction, due to the tidal nature of the Hudson River at this location. The Hudson River flows in a northerly direction at high tide and in a southerly direction at low tide. These flow characteristics proved to be critically important in designing an anchorage system for the bridge.

As the first step in their design process, the students performed a load analysis. The gravity loads were based on their decision to use a four-foot wide walkway, so that two-way pedestrian traffic would be possible. Nero had made a similar assumption in his feasibility study, but Elliot and Hilton thought that Nero’s assumed live load intensity was overly conservative. To verify their suspicion, they conducted a live load test with the help of fellow students. The experiment consisted of drawing a rectangle on the ground the size of one bridge deck module—a 4 foot by 12 foot rectangle. They then placed as many freshmen as could comfortably fit into the rectangle to simulate heavy traffic conditions on the bridge. They computed their fellow students’ total weight and calculated the resulting maximum live load intensity, which was indeed smaller than Nero’s assumed value. Elliot and Hilton used this new experimentally derived loading for all remaining design calculations. The students did not include a snow load, because they determined that the float bridge would need to be taken out of the water each winter. Page 7.921.3

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright © 2002, American Society for Engineering Education As the design developed, the students’ Internet research began to produce results. They found that a wide variety of prefabricated decking products are sold for use in constructing docks and walkways. A number of these appeared to be appropriate for use as deck modules for their pontoon bridge. Elliot and Hilton selected a system consisting of cedar decking with aluminum side rails (Figure 2). This system is strong, light-weight, and presents a reasonably authentic appearance.

Figure 2. Prefabricated Commercial Decking System (EZ-Roll Decking)

Cedar is a highly rot-resistant wood and the decking is available in 4 ft x 4 ft sections that slide in and out of the aluminum rails for easy replacement. Additionally, the frame has leg sockets that extend approximately 5 inches above and below the decking. The upper sockets connect to an aluminum railing system for pedestrian safety. The bottom sockets would normally connect to aluminum posts; however, the students planned to connect to a flotation system at these points.

Through their Internet research, Elliot and Hilton also discovered two suitable devices to use for the bridge floatation system—a flat bottom boat and polyethylene floatation modules. Historically, flat bottom boats have been used as pontoons for floatation. The Landau flat bottom aluminum boat is ideal for a pontoon. It has high buoyancy, low maintenance, and reasonably low cost. Additionally, the appearance of the Landau boat is consistent with the likely appearance of the original wooden pontoons used on early floating bridges (4). The students used Archimedes’s principle to determine the available buoyancy of a standard landau boat; thus they could determine the maximum span length between boats. As part of this analysis, they also needed to consider the possibility that the open-topped boats might be swamped in a heavy rain. Permanently installed floatation in the Landau boat normally would keep it afloat even when swamped; however, the students determined that additional buoyant material would be required to support the added weight of the bridge deck.

The other suitable floatation system was a polyethylene floatation module. This system uses a buoyant material and air enclosed in a polyethylene cube-shaped unit. Several companies Page 7.921.4 manufacture these products. Luckily the team was able to find a local vendor that could provide

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright © 2002, American Society for Engineering Education the modules quickly and at a reasonable price. The system is quite flexible. Individual modules can be connected together in a variety of geometries to build docks, walkways, and bridges with varying degrees of buoyancy. The buoyancy of the chosen system was 209 pounds per module.

Once the students had chosen the individual components of their bridge system, they began considering how to connect all the pieces together. To connect the bridge to the shore, one option was to use one of the aluminum decking modules as a ramp to the first pontoon. This option would provide a rigid shore-to-bridge transition and would have the added advantage of continuing the same aluminum railing system used along the remainder of the bridge. The second option is to use polyethylene floatation modules as a ramp. A floating ramp would have the ability to rise and fall with changing water levels—an important consideration, given the tidal nature of Popolopen Brook. A limitation of the floatation modules is that they do not come with a railing system. Thus, to ensure that this system remained a viable design option, the students needed to develop an expedient railing design using PVC pipe – ease of connection through the polyethylene floatation module connection eyelets (Figure 3).

The connection between the decking and the polyethylene floatation system was considered the most vital component of the design. This connection had to be flexible but strong. The deck sections would need to remain stable for pedestrian safety, while each section of the floatation system would need to be capable of moving independently, due to wave action, tides, and variations in the pedestrian loading. Finally, the connections had to be easily assembled and disassembled for maintenance and seasonal removal of the bridge.

The students developed a simple but ingenious connection design, consisting of a Douglas Fir base plate bolted directly to two floatation modules using standard hex bolts. (See Figure 3.) Two L-angles are used to attach each leg socket of the decking module to the wooden base plate. Adjacent deck sections are spaced 3 ½ inches apart at the connection point, to allow differential vertical movement of adjacent pontoons without damaging the railing units.

The strength of this connection is critical. The possible failure modes are bolt shear and tension, bolt bearing and tear-out of the connection tube, shear yield and rupture of the connection tube, bolt bearing and tear-out of the angle, and shear yield and rupture of the angle. The students’ analysis showed that the connection design was adequate for combinations of gravity and lateral loads, except for the possible rupture of the deck tubing from excessive lateral loading caused by high-velocity stream flow. Elliot and Hilton determined that this failure mode could be avoided with the design of a cable anchorage system to resist the lateral loads applied to the bridge.

The connection of the to the flat bottom boat to the decking system posed a different sort of design challenge. The same L-angle connection could be used at the leg socket joint, but a different means of transmitting deck loads from the L-angles to the floor of the boat would be required. After considering several alternatives, the students devised a simple wooden cradle that sits within the boat, longitudinally between the two seats. The cradle is carefully shaped so that it makes contact all along the sides and bottom of the aluminum hull, in order to evenly distribute the loads. On top of the cradle are two 4 inch x 4 inch timbers, which support the two L-angles at each leg socket. Page 7.921.5

Angle Iron

Polyethylene Floatation Bolt Through Module Pin Module

Figure 3. Single bridge module incorporating four different design alternatives.

Having determined the engineering characteristics of the bridge and its connections, Elliot and Hilton still needed to design a cable anchorage system to restrain the bridge laterally. The anchorage system was required to resist fluid pressure caused by the flow of Popolopen Brook in either direction. The system would consist of a pair of main steel cables spanning the river immediately upstream and downstream of the bridge, combined with a series of shorter cables connecting each pontoon to each main cable. The main cables would be anchored to the shore with buried “deadman” foundations.

To design the system, the students first had to determine the drag force on each pontoon, caused by the worst-case flow condition and maximum gravity loading (when the pontoons are nearly submerged). This drag force was then used to compute the tension in the main cable and the reactions on the deadman supports. The students discovered that the cable tension and reactions could be reduced substantially by increasing the sag in the cable. However, as the sag length increases, so does the length of cable required to span the river. After some trial and error, the students decided on a 20-foot sag and were able to size the various components of the system, based on their computed forces.

After considering many different ideas, Elliot and Hilton ultimately developed four different bridge configurations to propose to the client. These were (1) a pontoon bridge with Page 7.921.6 polyethylene floatation modules as the pontoons, (2) a pontoon bridge with aluminum flat-

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright © 2002, American Society for Engineering Education bottom boats as the pontoons, (3) a float bridge made up of continuous polyethylene floatation modules (with no pontoons), and (4) a pontoon bridge with either polyethylene floatation modules or flat-bottom boat pontoons and polyethylene floatation modules as the shore ramp. The first two options use cedar decking as the ramp connecting to the shore. The last two options use polyethylene floatation modules to provide the ramp over the shore. The students’ cost comparison of the four options is provided in Table 1. Option 3 has the lowest cost, but Elliot and Hilton found this configuration to be functionally flawed. Because the bridge consists of a continuous ribbon of floatation modules from shore to shore, it will not allow floating debris to pass underneath. Considerably higher maintenance cost will result. The students also found Option 2 to be problematic, due to concerns over possible swamping of the flat-bottom boats. As a result, the students decided to recommend either Option 1 or Option 4P. However, given the complexity of the various systems, they were not sure the clients would truly understand all the options.

Table 1. Cost Comparison of the Four Bridge Configuration Options Design Option Description Total Cost* 1 Pontoon bridge with polyethylene floatation system $38,000 2 Pontoon bridge with flat-bottom boats $31,800 3 Float bridge with continuous polyethylene floatation $28,200 (no pontoons) 4 Pontoon bridge with either pontoon system $37,400 (P) (P= Polyethylene, B= Boat) and polyethylene floatation $32,300 (B) system over the shore *Not including the cable anchorage system, labor, or fabrication costs.

In order to have something physical to show the clients, the students decided to build a full-scale one-section module with representative elements of all four of the design proposals (Figure 3). The prototype incorporates the aluminum and cedar decking panel between pontoons, the polyethylene floatation as a pontoon, an aluminum flat-bottom boat as a pontoon, polyethylene floatation modules as the ramp over the shore, fabricated connections between the deck and the floatation materials, and a PVC railing with polyethylene floatation modules (not shown). The prototype is not an exact replica in all aspects, since the prototype deck section is only 8 feet long. The actual bridge would use 12-foot deck sections. The students chose to use the shorter deck section in order to ensure that this “proof of concept” model was transportable and to keep it within budget.

The completed bridge module was presented to representatives of the Battle Site Association and the Palisades Park Commission during final exam week at the end of the spring semester. Elliot and Hilton briefed their clients from the deck of the bridge as it floated in the Hudson River. The clients were extremely pleased with the work, as they took possession of the bridge. All agreed that the students’ physical product had helped them to visualize what the actual structure would be. And the “proof of concept” module, which cost a total of $2600, would also be a valuable tool for raising funds to build the complete structure. Page 7.921.7

In our discussions with Cadets Elliot and Hilton and in our observations of their performance throughout the project, it was abundantly clear that they had learned many aspects of engineering that probably could not have been learned by any other means:

· They learned to deliver a product to the client on time and within a highly constrained budget. · They learned that a huge portion of any real-world project involves no technical engineering at all. Their experience working with the Fort Montgomery Battle Site Association, the Palisades Park Commission, the New York Bridge Authority, the Mid- Hudson Historian, and their project advisors taught them that strong communication skills are vital to the success of a project. · They learned to cope with uncertain and conflicting design requirements. · They learned to deal with constraints like regulatory restrictions, historical preservation, aesthetic and environmental sensitivities, and the political influence on design and construction. · They learned that the engineer’s ethical responsibility to ensure public safety is something real. The prospect of having hundreds of people crossing a bridge they designed caused them to take the task far more seriously than might have been the case for a “paper design.” · They learned to use the Internet as a resource of “off-the-shelf” products that might contribute to their solution. · They learned that the engineer’s role is to serve society, and they drew immense satisfaction from their own service to the local community in this project. · They learned that taking a design from paper to physical product is a lot harder than it looks and that simpler is almost always better. · They learned to manage construction, to include reacting to delayed shipments, missed deliveries, and constructability issues. · They learned that engineers often face design challenges that are not covered in textbooks. There was no reference showing how to connect an aluminum decking system to a polyethylene floatation module. They had to apply fundamental principles in new ways to devise their own solution methodologies.

IV. Assessment

In addition to our many discussions and direct observations of the students, we were able to assess the effectiveness of this project through our institution’s course-end feedback system. This system is administered entirely over the worldwide web and features a small number of USMA-standard survey questions, supplemented by department-specific and course-specific questions of our own choosing. Students respond to these questions using a scale of 1 (strongly disagree) to 5 (strongly agree). For the USMA-standard questions, this system allows us to compare our own students’ survey responses to those of all other students at the institution. More importantly, the inclusion of course-specific questions allows us to survey our students about their achievement of specific course objectives. Page 7.921.8

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright © 2002, American Society for Engineering Education Table 2 shows relevant results from our course-end feedback for the spring semester of 2001. These data are for seven students who were participating in three different projects we supervised: Popolopen Brook Float Bridge, Renovation of Walden Humane Society (redesign in effort to gain funding from towns the Society served), and West Point Lower Post Recreational Center (provide design to assist community leadership in future decision

Table 2: Course-End Feedback Responses Q.# USMA-Standard Questions Project USMA Average Average A1 Students responsible for own learning 4.88 4.47 A5 Fellow students contributed to learning 4.77 3.98 A6 Motivation to learn increased 4.88 3.91 B1 Instructor stimulated my thinking 4.66 4.20 B2 Critical thinking ability increased 4.88 4.02 Department-Specific Questions Project Dept Average Average C5 Understood the importance and practical 4.66 4.51 significance of this course Course Objectives Project Course Average Average D1 I can apply the engineering thought 4.44 4.41 process to solve a complex, real world problem D2 I can develop a creative solution 4.55 4.45 D3 I can acquire information and learn new 4.55 4.37 concepts on my own making). The course-end feedback system is almost entirely anonymous; thus it is not possible for us to extract the numerical survey results on a project by project basis. However, free-text comments from students usually provide enough detail to not single out a particular student, but to single out a particular project. The three projects were very similar in nature—all real-world design projects for real-world clients in the local community. The entire list of projects offered during this one semester is provided in Table 3 (5-6).

The data show that a number of critically important learning outcomes were achieved for our three projects.

· Our students developed as self-directed learners (Questions A1, B1, and D2). · They contributed effectively to each other’s learning—a suggestion that they exercised their teaming skills during the project (Question A5). · They developed critical thinking skills (Question B2), engineering design abilities (Question D1), and creativity (Question D2). · They understood the practical, real-world significance of their work (Question C5).

Some of the free text comments for the Popolopen Brook Float Bridge project were: Page 7.921.9

o I am glad I picked the Popolopen Bridge as my project – what a learning experience. I really liked the real life application. I definitely spent a lot of time on the project, but it was all well worth it. o The guidance given by LTC Welch and COL Ressler was extremely helpful in stimulating my thinking to new concepts.

· Most important things learned:

o Time management and connection design. o Procurement process and applying theory toward a REAL world problem.

· Strengths of this independent study course:

o Reinforced design process with a real project, critical thinking, time management skills, and people skills. o Work at own pace, open to new ideas, and consider all types of designs.

Table 3. Civil Engineering Projects Spring 2001 Community Service Project Clients West Point Lower Area Recreational USMA Housing Office Complex Walden Humane Society Renovation Walden Humane Society Popolopen Brook Float Bridge Fort Montgomery Battlefield Site Ass. Ground Water Study and Aquifer Model USGS and Town of Gardiner, NY Structural Evaluation of Church Bell Tower St. George’s Church, Newburgh, NY Ice Jam Prediction Investigation Cold Regions Research & Engineering Lab Indoor Obstacle Course Load Testing Department of Physical Education Project Wrench Department of Civil and Mechanical Eng. USMA Parking Analysis Department of Civil and Mechanical Eng Rugby Facility Design AOG, Army Rugby Team Competition Project Clients AISC Steel Bridge Competition AOG ASCE Concrete Canoe Competition AOG ASCE National Timber Bridge Competition AOG and PDJ Components Research Project Clients Prioritizing Repair Projects -Locks and Dams Construction Engineering Research Lab Carbon Fiber Reinforced Plastics Army Research Laboratory Modal Analysis of Blast Plates Army Research Laboratory Watershed and Reservoir Study Waterways Experiment Station Auger Pilings Design for LAMS Natick Labs Mine Vehicle Army Research Laboratory

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The true indication of this project’s success was the clients’ smiling faces as they posed for pictures while standing on the one-section module floating in the Hudson River (Figure 4). Though not in the photo, Rob Elliot and Bryan Hilton were smiling too. This project provided them with a unique opportunity to apply their undergraduate engineering and leadership skills toward the solution of a demanding, highly constrained real-world project. The project (and the project advisors) demanded much—time, energy, technical expertise, interpersonal skills, and creativity. But the project gave much in return— reinforcement of numerous engineering skills and satisfaction in a job well done and in providing an invaluable service to the community. Elliot and Hilton will be better engineers for having had this experience. And, by extension, other undergraduate engineering students will be better engineers if they are able to engage in similar experiences. Yes, faculty must establish the list of prospective projects, but they do not need to be the client. Their mission is to occasionally assist the student in considering all aspects of the design process, unleash their creativity, etc., and in the end grade the final product. An additional unexpected bonus is that almost every faculty member learns something unique during the project. We personally have managed over 15 student projects and never once completed a project without learning a new concept that makes us better engineers and educators. We strongly endorse these sorts of client-based design-build projects as highly effective capstone experiences for all students.

Figure 4. The Clients floating on the “proof of concept” module in the Hudson River.

1. Mead, J.H., “An Archeological Report on Fort Montgomery State Historic Site, Town of Highlands, Orange

County, New York,” Prepared for the NY State Office of Parks, Recreation and Historic Preservation, Bureau of Page 7.921.11 Historic Sites, Peebles Island, Waterford, NY, and the Palisades Interstate Park Commission, Bear Mountain, NY, 1992.

2. Nero, H., Welch, R.W., and Ressler, S.J., Technical Report: Popolopen Brook Bridge – A Work in Progress, Department of Civil and Mechanical Engineering, West Point, NY, May 2000.

3. Elliot, R., Hilton, B., Welch, R.W., and Ressler, S.J., Technical Report: Popolopen Brook Float Bridge – A Temporary Solution, Department of Civil and Mechanical Engineering, West Point, NY, May 2001.

4. Welch, R.W., “Pontoon Bridging During the Battle of Fredericksburg,” Paper submitted as a Historical Study for the History Office, U.S. Army Engineer School, November 1985.

5. Welch, R.W., “Client-Based Service Projects – A Mark of Excellence For Any Program,” Proceedings of the ASEE Zone I 2002 Conference, West Point, NY, 6 April 2002, Session C.

6. Rabb, R., and Welch, R.W., “Projects Day: Completion of the Engineering Capstone Design,” Proceedings of the ASEE 2002 National Conference and Exposition, Montreal, Canada, June 2002, Session 2425.

RONALD W. WELCH Lieutenant Colonel Ronald W. Welch is an Assistant Professor and Director, Structures Group at the United States Military Academy (USMA). He is a registered Professional Engineer in Virginia. LTC Welch received a BS degree in Mechanical Engineering from USMA in 1982 and MS and Ph.D. degrees in Civil Engineering from the University of Illinois at Urbana-Champaign in 1990 and 1999, respectively.

STEPHEN J. RESSLER Colonel Stephen J. Ressler is Professor and Deputy Head of the Department of Civil and Mechanical Engineering at the United States Military Academy at West Point, NY. He earned a B.S. degree in Civil Engineering from the United States Military Academy in 1979 and M.S. and Ph.D. degrees in Civil Engineering from Lehigh University in 1989 and 1991, respectively. He is a registered Professional Engineer in Virginia.

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