Hi Jamie,
I have worked with multiple state DOT's and private engineering firms working on bridges. I have done bridge inspections, bridge design, bridge rehabilitations, and now craft bridge policy to preserve and maintain my current state's bridge network.
I have focused primarily on vehicle bridges that cars and trucks drive on, rather than the pedestrian bridge of your capstone project. But man similarities are still there.
I will give you my opinion on your questions that you have asked so far:
What are the structural trade-offs between timber and steel?
-This comes down to cost versus durability. Timber is widely used for pedestrian bridges, because timber is a readily available material and has a low up-front cost. However, compared to other bridge materials (like steel and concrete), it is far less durable. A steel or concrete bridge may last 3 times as long as a timber bridge. From a structural standpoint, both concrete and steel can take more load than timber can. So a timber bridge will most likely require more beams compared to a bridge that was made of concrete or steel.
Where are bridge materials sourced from?
-This typically depends on who the project owner is. If it is a government agency, they may have specific requirements of where the materials come from. But in your project description you mentioned that this project was to be built on private property. Is the private property owner also going to own the bridge? If so, where the project material comes from may not be a big deal.
What is the process of purchasing materials for bridges?
- Again, this would depend on who owns the bridge and if they have any requirements for purchasing and testing bridge materials.
What are the secondary costs associated with weatherproofing and maintenance of bridges?
-This is a great question that can often get overlooked. Once the bridge is built, it still needs to be maintained to remain safe. As I mentioned earlier, timber is a less durable material compared to steel and concrete, so a timber bridge would take more time and money to maintain, compared to steel or concrete bridges. From a maintenance standpoint, concrete would be a good choice, as it doesn't need to be painted periodically like steel, and is more durable than timber. Applying a concrete sealer after the bridge is constructed will extend the useful life of a concrete bridge.
How would extreme conditions like flooding influence material choice?
-Another good question. Since this project is in the Shenandoah Valley area, there is a possibility that flooding could occur. Especially since the previous bridge was washed out. It would be nice to know what the previous bridge was, and what material it was made from. Steel typically doesn't do well in wet environments, because bare steel rusts and deteriorates when in contact with water. Timber bridges typically weigh much less than concrete or steel bridges, and therefore have foundations that don't go as deep into the ground. Because of this, timber bridges can "wash out" by water removing the streambed material from around the bridge's foundations, causing the bridge to be swept away by the current. Concrete could be a good choice in a flood prone area, as they typically have larger foundations and deteriorate slower than timber in steel in wet environments.
Just as a caveat, these are my opinions based on my experience. Others may have different, valid opinions.
I hope that helps get you started on your capstone project, and if you have more questions as your project moves along, you can continue to ask here, or you can contact me directly for more information.
Good luck!
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Doug Cantrell P.E., M.ASCE
Professional Engineer
Durham NC
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Original Message:
Sent: 10-29-2020 03:35 PM
From: Jamie Clark
Subject: Bridge design and material selection advice for a capstone project
Hello,
My name is Jamie Clark. I am a senior engineering major at JMU and found this neat discussion board through my ASCE student membership!
My team's current capstone project is redesigning previously washed out bridges over Mossy Creek in the Shenandoah valley. I am on a subteam responsible for superstructure design specifically. Under a budget constraint, the bridge we are designing will likely be constructed out of locally sourced telephone poles, but we are open to other material options besides timber. The bridge will be built on private property and is designed for exclusively pedestrian use and small vehicles like a John Deere tractor.
I am currently searching for any experts, non-experts, and general resources to consult on timber bridges, bridge materials, or general bridge design.
Some of the questions my team currently has about material selection:
What are the structural trade-offs between timber and steel?
Where are bridge materials sourced from?
What is the process of purchasing materials for bridges?
What are the secondary costs associated with weatherproofing and maintenance of bridges?
How would extreme conditions like flooding influence material choice?
Thank you in advance for your responses,
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Jamie Clark S.M.ASCE
Fairfax VA
JMU Engineering
James Madison University, Class of 2021
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