By : Ximena Kitain Hamilton | TUB-ID: 490453 |

ABSTRACT : 

Modelling a Suspension Bridge in Dynamo, varying different parameters in order to assess two high performance criteria.

Suspension bridge

Design Challenge: 

Suspension bridges offer an aesthetic alternative to build a bridge to connect two points. This alternative can be more sustainable than other variants, and the cables supporting the bridge not only allow the bridge to be longer, but also to be lighter in structure. A suspension bridge is a type of bridge where the deck is hung from cables that are supported by towers. The design allows for long spans, making it ideal for crossing wide bodies of water or deep valleys [1], [2].

Effective project management is key to keep costs under control. This includes meticulous planning, scheduling and resource allocation to avoid inefficiencies. Using construction management software can help to track progress and identify potential issues before they become costly problems. Clear communication with team members and stakeholders is also vital to ensure that everyone understands cost objectives and works together to achieve them.

The design challenge I select was to minimize the costs of the bridge, considering five different parameters. Variations in the span, tower height, and material choices affect the total cost of building the suspension bridge. The goal is to achieve the best balance between performance and construction/maintenance costs.

Parametric Model : 

The parametric model of the suspension bridge was developed to systematically explore and optimize the key design parameters while ensuring the bridge meets performance requirements, cost constraints, and safety standards. The model includes adjustable parameters such as length of the bridge, deck width, number of cables, varying the cable sag and tower height, which are used to define the geometrical configuration of the bridge.

To reduce costs, it is possible to reduce the number of cables by optimizing the design for fewer cables without sacrificing load-bearing capacity. However, it is important to ensure that the bridge can still support the desired traffic and environmental loads, so careful structural analysis is necessary. It is possible to explore using stronger materials for fewer cables, reducing overall cable quantity but maintaining strength. 

Design process includes Frame of the bridge, main cables, secondary cables, Girder, deck etc.

Here is a high-resolution image of our parametric model. For a better understanding, Click on the picture and zoom in for more detail.

 

Design parameters :

• Bridge Length: The distance between the two points and the bridge towers is a critical factor. Longer spans generally require more materials and sophisticated designs but reduce the need for intermediate supports, which can lower costs in certain situations.
• Tower Height: Taller towers allow for longer spans but can increase construction costs. The tower design impacts material use and structural integrity. A longer span generally requires taller towers and stronger foundations to support the cables. This increases the cost of the towers and the foundation, which is often one of the more expensive parts of a suspension bridge.
• Number of cables: The number of cables (main cables, suspender cables) affect both structural performance and material costs. It is possible to explore different materials for cables to balance cost and performance. The number of cables directly influences the overall cost, as more cables mean more materials, additional labor to install them, and more complex cable support systems. In a suspension bridge, the main cables and suspender cables are critical to the load bearing capacity.
• Bridge Width: Wider decks can accommodate more traffic (more lanes or lanes for pedestrians and vehicles), but the width also affects the overall material requirements and structural stability. The wider the deck, the more weight it adds, which requires more suspension cables to evenly distribute the load. In some cases, additional reinforcements in the deck and the towers might be needed, to support the extra weight.
• Cable sag: Varying the cable sag (also known as the catenary curve) in a suspension bridge design can have a significant impact on several key performance criteria, including structural efficiency, weight reduction, and cost minimization. The cable sag refers to the vertical displacement or curve of the suspension cables between the towers. A shallower sag may reduce the material requirements for the main cables but may require larger secondary cables and more tower height. A deeper sag provides better load distribution and reduces the height of the towers, but it can lead to higher tension in the main cables, which might increase material costs and the weight of the structure. The optimal solution will depend on factors such as span length, expected load, environmental conditions, material properties, and desired aesthetic considerations. The goal is to find a sag that balances these factors, ensuring that the suspension bridge is both structurally efficient, cost-effective, and lightweight while still being robust enough to handle the expected loads and dynamic forces.

High performance criteria :

• Load Capacity and Structural Strength with focus on minimize costs: this refers to the ability of the bridge to handle traffic loads (vehicles, pedestrians, etc.) and environmental loads (wind, seismic, etc.). This needs to be balanced with the material costs and design complexity.
• Structural Efficiency and Weight Reduction: an increased number of secondary cables can help in achieving a more efficient structural system, reducing the need for larger or heavier main cables. However, this comes with the trade-off of needing more suspension elements. The ideal number balances material costs, weight distribution, and structural integrity. In order to do so, it’s possible to calculate the span to weight ratio, also aiming to minimize the dead weight of the structure while ensuring stability. Using the right number of secondary cables ensures the bridge remains light enough to avoid excessive structural mass but robust enough to handle the expected loads.

Evaluation of the design space

To evaluate the design space of my bridge, I systematically explore the parameters I choose, and their influence on the design. Is also important to notice that there are some constrains on the design. It would be more efficient to have catalogs integrated into the design to explore the different materials to be used.

The model is also basic since is not calculating any Loads, its more about the physical part of the bridge, the weights and its volume, rather than use it to prove the static of the bridge itself.

Another aspect missing is the environmental considerations, which are essential for further consideration on the project, in order to keep the integrity of it.

With the model I created, it is possible to vary the different parameters I mentioned before, and to see which variation is the more efficient. The calculations focus on the costs of the towers after obtaining their volume. It also gives the price of the Secondary cables based on the costs of one kg Steel per meter.

Not only a consideration on the short term is important, since using better materials can be better on the long term, in terms of maintenance and possible exchange of parts of the bridge.

The task was mostly to consider how altering the design parameters can affect performance. For example, try different combinations of span lengths, tower heights, and materials (steel, composite, or concrete). By using different materials, I seek to balance cost with performance and durability. Also important is to look at different geometrical configurations, such as variations in tower shape, and number of cables.

https://ws-stahl.de/service/gewichtstabellen/

 

Design Alternatives:

1.  Reduce the Size of the Towers (Optimize Tower Design)

One way to reduce the overall cost and weight of the suspension bridge is to reduce the size (volume) of the towers. By using lighter materials or more efficient designs, such as using less concrete, the material costs and the weight of the towers can be reduced. It is possible to design the towers to use the least amount of concrete while still meeting the structural requirements. Consider using high-strength concrete or even composite materials to reduce weight without compromising strength.

A smaller tower will reduce the overall weight of the bridge, leading to potential savings in the structural load the cables and foundation must support. The problem that one might encounter is the fact that reducing tower size might require more precise engineering to ensure the tower can still handle the load and provide the necessary stability.

2. Reduce the Number of Secondary Cables (Optimize Cable Configuration)

Secondary cables are used to distribute the load between the main cable and the deck. By optimizing the number and positioning of secondary cables, you can reduce the overall weight of the bridge and, in turn, the cost of the cables.

The problem to face is that reducing the number of secondary cables could lead to uneven load distribution, which might affect the overall stability of the bridge. Also, the reduced number of cables might require more advanced design solutions to compensate for the load.

3. Use Lighter and More Efficient Materials for the Deck and Cables

Using lighter, stronger materials for the bridge deck and cables can reduce both weight and cost. For example, instead of traditional steel cables, you could use carbon fiber cables or high-strength alloys for the primary and secondary cables. Similarly, the deck can be made with lightweight concrete or composite materials. Lighter materials will not only reduce the weight of the bridge but also reduce the amount of concrete used in the towers (if the deck itself is lighter).

Important to take into account would be that advanced materials like carbon fiber or composite materials may be more expensive upfront compared to traditional steel and concrete. Also, Some newer materials might require more specialized installation techniques, which could add to the cost in terms of labor and time.

Conclusion :

Recommend the most suitable design for the project based on a balanced assessment of cost, performance, and sustainability, aligning with the project’s goals and constraints. Is important to take into account the total lifecycle cost (construction + maintenance). Sometimes, investing in more durable materials or slightly more cables might save maintenance costs in the long run, leading to better overall cost-effectiveness.

The parametric model was used to generate multiple design alternatives by adjusting the key parameters. For instance, a design with a shorter span and fewer cables might be more cost-effective, while a design with longer spans and more cables provided enhanced stability and performance but at a higher initial cost. A final decision on the design alternative needs to be based on balancing the required performance (load-bearing capacity, safety), the budget constraints, and long-term maintenance costs. The model allows for quick iteration and adjustment to find the best design solution on a basic scale, its lacking complexity to make more calculations, but it is a good base for developing more advanced calculations.

In conclusion, the model allows for an iterative and flexible approach to design, ensuring that all critical aspects are considered and optimized before finalizing the bridge’s specifications. Another alteration might be narrowing the deck, to limit the cost, reducing the materials needed for the deck and cable systems. However, this would reduce the traffic capacity of the bridge, which could affect its overall utility and benefits. Prioritize the required traffic capacity and balance it with your budget. This and more alternatives could be explored in a further investigation.

References

[1]   P. D. s. t. M. S. e. al., “Vorlesungsskript : Brückenbau II – Seile,” p. 121, 2017.

[2]  D. B. Yanev, “SUSPENSION BRIDGE CABLES: 200 YEARS OF EMPIRICISM, ANALYSIS AND MANAGEMENT,” 2009.

[3] https://www.diva-portal.org/smash/get/diva2:431643/FULLTEXT01.pdf

[4] https://ws-stahl.de/service/gewichtstabellen/

[5] https://www.listando.de/p/was-kostet-beton/

[6] https://www.stahlpreise.eu/2024/04/aktuelle-stahlpreise-s355-grobblech-je-tonne-1000-kg.html  

 

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