Bridge Alternatives Analysis: via Artificial Intelligence

Brian Brenner, Professor of the Practice, Tufts University

One of the first steps in designing a new bridge is preparation of a bridge type study. This conceptual design compares different alternatives for a new bridge and provides recommendations on what type to advance in subsequent preliminary and final design. For MassDOT, a bridge type study follows the requirements and format of a “Bridge Type Selection Worksheet.” The requirements for this report are described in the MassDOT LRFD Bridge Design Manual.  

A team of experienced bridge engineers can develop conceptual designs and draft a report, but how about Artificial Intelligence? I was curious, so I asked ChatGPT. This AI engine works by…. I have no idea how it works. Hopefully someone else can provide an explanation. Similar to Google, you can ask ChatGPT questions. Unlike Google which provides a list of references, ChatGPT prepares a more detailed answer to your question, in the form of seemingly coherent sentences and paragraphs. The responses are typically given to you in seconds after posing a question.

To give this a go, I asked ChatGPT the following:

“Prepare a bridge alternatives analysis for a new bridge to be designed and built.  It is to span 60 feet over a small river, carrying two lanes of traffic and two sidewalks.”  

ChatGPT thought about it (assuming AI engines “think”) for a few moments, and then provided the following response:

Project Overview:
The project involves the design and construction of a new bridge spanning a small river with a clear span of 60 feet. The bridge is intended to accommodate two lanes of traffic and two sidewalks, providing a safe and efficient crossing for both vehicles and pedestrians. To ensure the success of this project, a comprehensive alternatives analysis must be conducted to evaluate various design options and select the most suitable bridge type.

Alternative 1: Concrete Girder Bridge

Pros:

• Durability: Concrete is highly durable and resistant to weathering, making it suitable for long-term use.
• Minimal Maintenance: Concrete bridges typically require less maintenance compared to other materials.
• Customizable: Concrete bridges can be designed with various architectural elements to enhance aesthetics.
• Reliable: Proven track record of successful concrete girder bridge construction.

Cons:

• Higher Initial Cost: Concrete bridges may have a higher initial construction cost compared to some alternatives.
• Longer Construction Time: Building concrete bridges can be time-consuming.

Alternative 2: Steel Truss Bridge

Pros:

• High Strength-to-Weight Ratio: Steel truss bridges are known for their strength and can span long distances with relatively lightweight materials.
• Quick Construction: Steel bridges can be assembled relatively quickly, reducing traffic disruption.
• Aesthetically Pleasing: Steel truss bridges can have a visually striking appearance.
• Versatility: Steel truss bridges can accommodate various architectural styles.

Cons:

• Corrosion: Steel bridges require regular maintenance to prevent corrosion.
• Higher Maintenance Costs: Maintenance of steel bridges can be costlier compared to some alternatives.
• Initial Cost: Steel bridges may have a moderate to high initial construction cost.

Alternative 3: Timber Arch Bridge

Pros:

• Aesthetics: Timber arch bridges can offer a natural and rustic appearance, blending well with the environment.
• Environmental Considerations: Timber is a renewable resource, and using it can be environmentally friendly.
• Low Initial Cost: Timber bridges can be cost-effective during the initial construction phase.

Cons:

• Durability: Timber bridges may require more frequent maintenance and have a shorter lifespan compared to other materials.
• Weight Limitations: Timber bridges may have weight limitations, affecting the type of vehicles that can use them.
• Fire Hazard: Timber bridges are susceptible to fire and may require fire-resistant treatments.

Alternative 4: Composite Fiber-Reinforced Bridge

Pros:

• Lightweight: Composite materials are lightweight and have a high strength-to-weight ratio.
• Corrosion Resistance: Composite materials are highly resistant to corrosion.
• Low Maintenance: Composite bridges typically require minimal maintenance.
• Longevity: Composite materials can have a long service life.

Cons:

• Limited Aesthetic Options: Composite bridges may have limited options for architectural enhancements.
• Limited Track Record: Composite bridges may not have as extensive a track record as traditional materials.
• Moderate Initial Cost: While generally cost-effective, composite bridges may have a moderate initial construction cost.

Evaluation and Recommendation:

The choice of bridge type should be based on a comprehensive evaluation of the project's specific requirements and constraints, including budget, construction schedule, maintenance resources, and aesthetic preferences.

Based on the analysis:

• The Concrete Girder Bridge is a reliable and durable option, suitable for a wide range of conditions, but it may have a higher initial cost.
• The Steel Truss Bridge offers strength and aesthetic appeal but requires regular maintenance, and the initial cost can be moderate to high.
• The Timber Arch Bridge is cost-effective initially and has aesthetic appeal but may require more maintenance and has potential weight limitations.
• The Composite Fiber-Reinforced Bridge provides an attractive balance of low maintenance and high durability, though it may have limited aesthetic options.

Ultimately, the choice of bridge type should align with the project's priorities. Considering factors such as budget, maintenance capabilities, and aesthetic preferences, it is recommended to proceed with a detailed engineering and cost analysis for the Composite Fiber-Reinforced Bridge as it offers a promising combination of durability and low maintenance, which can be advantageous for a long-term infrastructure project.

Here are a few comments on ChatGPT’s work:

The “draft” type study is coherent, but the choices of alternatives don’t necessarily make sense in the context of practical bridge design. Also, ChatGPT’s report is pretty generic and does not fit any format requirements that a particular bridge owner such as MassDOT would require.

On the other hand, it may be possible to “train” ChatGPT or AI engines to drill down based on specific requirements and areas of knowledge. So, for example, if I had worded the initial question more tightly and provided specific MassDOT guidelines for how the report was to be formatted, perhaps the response would be closer to the mark.  

In that vein, it may be possible for AI ultimately to provide credible initial drafts of formatted reports.  Actual human engineers would still be needed to provide expertise, review and polish for the final report and recommendations. With this approach, AI might be used to prepare a lot of the initial “leg” work to get the document started, instead of the need to start from scratch each time. Conceptual design, after all, is a synthesis of existing knowledge without providing much detailed engineering.  This is something that AI approaches seemingly can contribute to.  It is not that different from the process of organizing existing bridge data in support of a conceptual design.  

So, it does seem that even with AI, human bridge engineers would still be needed to contribute to the designs. For now.