CVEN Bridge Project Report

Truss Issues

ENGG1000: Engineering Design and Innovation

CVEN Bridge Project ReportTRUSS ISSUEsGroup 23

Allen Zhou5020688

Joel Babbage5020581

Emily Hull3459561

Ben Ginnivan5017204

Daniel Miller5016130

Carmen Wang5020172

Submitted 19/05/2014

ENGG1000 Bridge Project ReportTRUSS ISSUES

1. Abstract2. Introduction and Background of Task3. Options and Selection of Preferred Design3.1 Design Options3.2 Design Criteria3.3 Design Evaluation3.4 Design Selection3.4.1 Initial Design Selection3.4.2 Final Design Selection3.5 Sustainable Design4. Modelling and Analysis4.1 Material Analysis4.2 Force Analysis5. Construction Details and Drawings6. Conclusion7. References8. Appendices

1. Abstract

This report details Truss Issues design process through the development and construction of a sustainable and aesthetically pleasing scale bridge model. Functionally, the bridge constructed is required to maintain sufficient strength to endure a 5kg dynamic load and a 7kg static load, while conforming to material constraints. Through testing of materials and supporting structures Truss Issues were able to narrow down three design options into a design that is environmentally and economically sustainable whilst considering the aesthetics and serviceability of the structure. The group chose an arch bridge due to its strength and ability to be constructed from environmentally and economically sustainable materials. This design can be altered for maximum strength and minimum weight due to the arch formations distribution of forces through compression. The bridge was designed, tested and constructed over formal group meetings, which were centred on strong communication and teamwork objectives. This bridge design is a result of creativity, research and experimentation, highlighting the design process and facilitating the growth of collaboration and communication skills.

2. Introduction and Background of Task

Project CVEN01 introduce students to the "profession of Infrastructure Engineering through the studies of engineering design and innovation"1 through the design and construction of an aesthetically pleasing and sustainable model bridge. The strict limitations on materials result in only paper, card, glue, tape and string being permitted to be used for construction. The bridge must maintain sufficient strength to support a five kilogram dynamic load followed by a seven kilogram static load in conjunction with transporting the load safely to the other side of the structure. The bridge is to have a width of 220mm, length of 800mm, 110mm clearance space below the structure and remain as lightweight as possible, staying under 350grams. Through criteria provided by the Faculty of Civil & Environmental Engineering, Truss Issues decided upon a design that we believed would meet all requirements.

Truss Issues regarded a sustainable and safe design to be important factors in design selection. Careful consideration of material strength and structural design ensured that the final product would have high structural integrity against stresses and strains and would not overturn or buckle under force. Issues surrounding serviceability and long-term sustainability were also taken into consideration.

3. Options and Selection of Preferred Design

3.1 Design Options

The team had brainstormed many concepts for potential bridge designs. Further investigation and research allowed us to come to a conclusion on three basic designs to go under scrutiny from the whole team in terms of structural and material evaluation. This brought upon the challenge of combining the teams knowledge and understanding of bridge mechanics to implement the best possible and most adequate solution to the problem.

The three designs that Truss Issues settled upon; Basic Truss Bridge- assembled from paper and cardboard. An Arch Bridge- assembled from paper and cardboard. Under Deck Cable-stayed Bridge- assembled from cardboard and string.

3.2 Design Criteria

The following criteria were focused on to finalise our design: Will it translate to a tiny scale with alternate materials? Will it be under the required weight? Is it possible to build with the time and hardware available? Will it remain stable when interacting with both moving and stationary objects? Are there any clear weak points, which need to be addressed? Will it be able to withstand environmental factors such as weathering? Will it be able to withstand continuous strain? The aesthetics of the design: is the bridge visually pleasing?

Once the each design had been scrutinised through this analytical process the team as a whole was confident that the best possible solution to our knowledge could be found.

3.3 Design Evaluation

The use of the following bullet points provides an objective comparison between the possible designs against the outlined criteria.

3.3.1 Arch Design

Pros: Force Distribution - Force is distributed to ends through the curvature of the arch Low Weight - Bridge components use compression allowing for little glue or tape, giving a much lower overall weight. Simple construction - Pieces can be cut or made and put instantly into place. Allowing simple testing and re-evaluationCons: Weak Point - Centre of bridge is most susceptible to flexing with possible connections failing. Aesthetics - Simple aesthetics not highly appealing Design Constraint - Smooth curve difficult to achieve with available materials3.3.2 Under Deck Cable-Stayed Design

Pros: Low Weight String as a major component allows super-light weight design Force Distribution Forces carried to each end of bridge where there is stable ground support. Aesthetics Aesthetically pleasing due to visual complexity and lack of overbearing componentsCons: Construction Difficult to get precise tension on strings and to anchor the string within the design Balance Design needs extra work to avoid tipping during the dynamic loads movement.

3.3.3 Under Deck Truss Design

Pros: Stable/Solid Design is made to withstand many forces such as compression and torsion Aesthetics When made with precision would be aesthetically pleasing due to symmetry and shapes Construction Repeated pattern allows for mass production, easy replacement and perfection of straight componentsCons: Weight Solidity and strong joints add a lot of weight along with large number of components. Joints Large number of joints gives more possible breaking points if constructed unwell due to the difficult assembly.

3.4 Design Selection

3.4.1 Initial Design Selection

Early in the teams project evaluation Truss Issues agreed that a paper truss design was the most practical option in terms of qualitative analysis. With this in mind we made a small section of our truss design, which showed us results of an excessively heavy section of bridge. We also learnt that using paper alone was impractical as it is unreliable and once deformed cannot be used. During our original design and construction of the truss design we also did not take into account the stability of the structure, which arose as another issue. After this attempt we took the new information to refine our design criteria and move onto more practical options. The group focused on further designs, but as we researched further and built small components, they were often realised to be inefficient or unable to satisfy the requirements.

3.4.2 Final Design Selection

For the final selection Truss Issues had to re-evaluate the designs and reconsider some of the strengths and weaknesses of each design after constructing built prototypes of the differing design concepts. Each design was discussed and the group deemed the arch bridge design would suffice for the task at hand and be under the required weight restriction. It was clear to the group that the main goal of the project was to firstly construct a bridge that could carry a 5kg dynamic load as well as a 7kg static load whilst having a mass less than 350g these two criteria carrying the heaviest weightings in terms of marks allocated. However, Truss Issues also came to the conclusion that aesthetics of the bridge is another factor that most definitely needs to be taken into consideration. It was also noted that a visually pleasing design would reflect a structure that has been thoroughly planned and as result being structurally sound. Truss Issues concluded after an analysis of each initial design the best option was to advance with the arch bridge. The group was united with its decision and believes that through the extensive analysis of each design the correct and most adequate design was selected. Upon completion of a test structure it became clear that this design was superior to the others we had discussed as it held weights greater than necessary with ease. This allowed us to continue the project with this design and focus on increasing the efficient use of weight within our design.

4. Modeling and Analysis

4.1 Material Analysis

String:Through re-evaluation of our original design, we discovered small issues with flaring of the side components. We overcame this by placing string at the correct tension between the components, holding them in place. Truss Issues both researched and tested different types of string. Throughout testing, hemp string often frayed making it difficult to work with and weakening it. After discovering braided nylon string, we found that it had the greatest strength and quality for our needs. Glue:Glue is a key component of our bridge as it holds together each section. Through a small amount of research, we believed hot glue would be the strongest glue to use. We weighed each stick of hot glue and discovered they were 5 grams each, therefore we knew we had to use it minimally to retain a low weight. Paper & Tape:Both paper and tape were originally planned to constitute the majority of our bridge however through many prototypes and testing structures we discovered that the previously mentioned materials were far superior overall. Despite papers low weight, to be useful it has to be used in great quantities which make it heavy altogether. Tape also loses stickiness and is more difficult to work with than glue.

4.2 Force Analysis

Shear force:Shear force occurs when unaligned forces push one part of a structure in one direction and a different part in the opposite direction. This is compared to compression forces where the forces are aligned. A small diagram is provided to illustrate how this force essentially occurs.

This shear force is present within the bridge, underneath the deck. Below the deck a system of small cardboard strips has been implemented in order to support the edges of the deck. In this system small strips of cardboard have been glued to the walls of the bridge beneath the deck, providing a place for the deck to rest on. This can be seen within the graphic below showing the inside of the bridge looking up at the deck. Specifically the shear force occurs between the side of the bridge and the cardboard strip as load on the bridges deck forces the strip down while the cardboard side remains stationary. Overcoming this shear was important for the structural integrity of the bridge, holding the loaded deck in place. This was done by using strong hot glue to bind the pieces together but most importantly the number of small cardboard strips was increased in order to evenly distribute weight, reducing the shear force on each strip.

Figure 1: Tab inserted to support deck on prototype.

Torsion force:Torsion is described as the twisting of an object due to opposing moments along the same axis. This force will occur typically in a suspension bridge, as although the bridge is suspended the cables will not prevent the twisting and oscillation of the bridge, with deck stiffeners used to minimise the torsional oscillation on the bridge. However this force is only significant in suspension type bridges as the vibrations within cables (due to external environmental factors such as wind) can travel through the bridge forming a regular oscillation that can lead to these torsional forces. Torsional forces within the arch bridge design are limited due to rigidity of the components of the bridge as a whole, meaning no additional planning was required in order to effectively manage and overcome potential problems. Compression force:Cardboard has a high resistance to compression forces when force is applied parallel to the corrugation inside. Knowing this, we designed our bridge to take advantage of this strength. Arch bridges consist of largely compression forces making it a great design for our material constraints. Through testing we discovered that our simple arch design, taking advantage of the cardboards resistance to compression, allowed us to easily withstand the 7 kilogram static load and although the 5 kilogram load is difficult to replicate, we believe our design should pass that test. The weakness in cardboard is any area with a crease or fold. To overcome this, we gained the highest quality cardboard we could and placed a supporting strip of cardboard in areas, which could not withstand the compression otherwise. Figure 2 shows the dispersion of the compression force throughout an arch design and the role that a keystone has in connecting each side of the arch and sending the forces down each side of the arch.

Figure 2: Force dispersion within an arch bridge with two concentrated forces (Graham Dean, 2014) Tension force:Tensional force in archbridges, on the other hand is virtually negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch. However, the members of Truss Issues noticed that when a load was place upon the bridge in testing the arch began to flare out. The team identified this as an issue and came to the conclusion that using string tied from one arch to the other would limit the cardboard from flaring. Flaring is a significant concern as it is applying a force against the compressive nature of the cardboard, perpendicular to the flutes. Another added benefit to the high tensile strength of string is that it is very light weight, whilst increasing the structural integrity of the bridge greatly, this can be seen in Figure 3 and Figure 4. The string keeps the cardboard straight, which allows compressive nature of cardboard to be utilised.

Figure 3: Under view of string used to limit the flaring.

Figure 4: Top View of String used to limit the flaring.5. Construction Details and Drawings

The arch bridge was primarily constructed with corrugated cardboard with some braided nylon line as tension supports for the sides. More details on materials can be found in the materials analysis section. Due to limited resources, only cardboard of poor quality could be sourced. These had creases and damaged surfaces. Truss issues also had limited access to tools. The only tools used were a hot glue gun, scissors, ruler, pencil and box-cutter blade.

To construct the bridge, a large sheet of corrugated cardboard was cut into 3 rectangles: one of 860mmx200mm and two of 850mmx155mm. The 860mmx200mm was cut so that the axis of corrugation would run along the length of the bridge. The two 850mmx155mm were cut so that the corrugation was along the width. These corrugations are displayed in Figure 5 as reference. Using a thin and flexible wooden board, a smooth curve was made and placed along their length so that each end was marked 25mm in from in from these two pieces of cardboard and traced and cut. Circular holes of radius 27mm, 21mm and 16mm were cut according to Figure 6 to reduce weight, called lightening holes. Two pieces of rectangular cardboard size 140mmx200mm and one of 70mmx200mm were prepared, corrugation along the width. Four more small rectangles were cut out of size 50mmx2mm. Using the hot glue gun, the pieces were assembled with minimum glue: our bridge used 15 grams worth. The deck was glued on a slight arch as depicted in the CAD drawings. Two nylon strings of length longer than 250mm were tied between the two arches to provide a tension enough to keep the cardboard from deforming outwards under compression. A basic overhand knot was fastened down on a small amount of hot glue and then more overhand knots on top of that for security. The CAD drawings of the front, side, top and isometric views are included in the appendix.

Figure 5: The configurations and axis of the cardboard.

Figure 6: Side view of bridge.

In the construction phase, many issues arose surrounding the precision and accuracy of cutting and placement of cardboard components. It was very difficult to cut cardboard to the right size with straight edges. Also troublesome was the controlling the mass of the bridge. The prototype weighed 380g without any lightening holes and with excessive amounts of hot glue. In revised models, lightening holes were cut out in numerous structurally stable areas to reduce weight as well as control the amount of hot glue used. This brought the bridge down to a total weight of 350g making it just within the restrictions. Building more models with different types of cardboard pieces, such as double flute corrugated cardboard, yielded varied results in total mass and aesthetic view. It was even more difficult to cut straight edges on double flute compared to single.

6. Conclusion

The design process has proven to play a vital role in aiding Truss Issues in the research, experimentation, testing and construction of a bridge design. It has allowed the assessment of various designs and has lead to the selection of a final design solution, commonly agreed upon by the group. Truss Issues successfully constructed an arch bridge, ready for final testing and within the guidelines specified by the project.Initial plans were to purse an under-deck cable stayed bridge however testing prototypes and further experimentation lead to the realisation that a different approach may be needed. Hence an improved design was developed. With further research and experimentation with materials, various modifications were applied to the design including the addition of holes to reduce the weight of the structure. This design has been constructed with aesthetics and the restrictions kept in mind as well as the goals that the final product can withstand the forces placed upon itTeamwork and time management were also important factors that Truss Issues successfully incorporated into the project. Early on, methods of communication were established to allow frequent contact, roles and responsibilities were distributed evenly. Meetings were organised on a two per week basis with specified agendas for each meeting, this allowed for effective group work. Communication was fundamental; therefore methods of communication including social media, email and document sharing sites along with the face to face meetings reduced conflicts and increased efficiency. Similarly, issues regarding time management were reduced through specifying activities in a time plan.

Over the course of the project, Truss Issues has demonstrated extensive knowledge of the design process and teamwork skills in the construction of an arch bridge. The final design is one the group takes pride in, fitting expectations of elegance and functionality.

7. References

8. Appendices

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