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Old 10-09-2010, 09:46 AM   #1
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Laminated support beams for 25 foot long footbridge.


My 17 year old son is rebuilding a footbridge (newbie to consruction) and is thinking of replacing the old decayed utility poles used for support with 3 laminated beams spanning a 25 foot wide creek bed. He is needing to know the load bearing capacities of a 30 foot long laminated beam made out of 2 x 10 treated lumber ibolted and glued together. He is planning on using 3 beams to span the creek. He would laminate (4) 2 x 10 together for 30 feet in lenght and would rest them on foundations on each end. He is hoping this would provide enough of a safe bearing load so as not to require a support foundation in the middle of the creek. Then there is the question of using a solid concrete foundation or a combined concrete/block foundation?

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Old 10-09-2010, 10:51 AM   #2
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Laminated support beams for 25 foot long footbridge.


Your best bet is to have a certified structural engineer do your load calculations and drawings. He would also supply the proper material and installation method for your project. I believe you would also require a permit to do your project.

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Old 10-09-2010, 11:17 AM   #3
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Laminated support beams for 25 foot long footbridge.


I assume this is on private/your property. How high is the footbridge? I would consider treated LVLs or even steel. I'm not sure if decaying utility poles is a good idea
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Old 10-09-2010, 11:26 AM   #4
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Laminated support beams for 25 foot long footbridge.


The original plans were drawn up by an architect and he insist that the old poles be replaced. My son has contacted the utility companies and they do not have any poles to let him use and do not know when they will have them. This is for a BSA Eagle project being done for the City Park and Recreation service. He was trying to find a substitute for the utility poles. When finished the bridge will be inspected and needs to be built right. Therefore he needs the the correct load and specifications for laminated wood beams if he uses them. He does not need a "building permit". When going through this process these kids are expected to do as much of the planning of the project as possible and sometimes its hard for them to do. They have to procur all the supplies, labor so on so forth. the bridge is 3 feet high.

Last edited by fifty7flash; 10-09-2010 at 11:33 AM. Reason: ommitted height of bridge
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Old 10-09-2010, 11:40 AM   #5
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Laminated support beams for 25 foot long footbridge.


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Originally Posted by fifty7flash View Post
The original plans were drawn up by an architect and he insist that the old poles be replaced. My son has contacted the utility companies and they do not have any poles to let him use and do not know when they will have them. This is for a BSA Eagle project being done for the City Park and Recreation service. He was trying to find a substitute for the utility poles. When finished the bridge will be inspected and needs to be built right. Therefore he needs the the correct load and specifications for laminated wood beams if he uses them. He does not need a "building permit". When going through this process these kids are expected to do as much of the planning of the project as possible and sometimes its hard for them to do. They have to procur all the supplies, labor so on so forth. the bridge is 3 feet high.
In the past, I have often gone to a lumberyard where they usually have a person that will do takeoffs from blue prints and will specify beam sizes from charts, or will call their lvl vendors who can specify the correct size. It beats having to pay for an engineer
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Old 10-09-2010, 01:10 PM   #6
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Laminated support beams for 25 foot long footbridge.


We have similar projects in my area, and on occasion I have been asked to prepare the plans (at no charge) for the bridge. I have always declined, for the following reasons. Since I am not being paid, I probably cannot develop a legitimate contract that would protect me from future claims (i.e. someone falls off the bridge, claims defective design, the bridge collapses causing injury, I get sued etc.) Just because it is a Boy Scout project, this provides no cover for the designer of the structure.

Additionally, suppose I supply a perfectly good design, but it does not get built in accordance with the plans. This happens all the time, however if there is a problem later, I could be named in the lawsuit even though the bridge was not built according to plan.

In your case, you must be planning to hold up something reasonably heavy, possibly a vehicle, maybe horse traffic, maybe ATV, who knows. Even if you think you can restrict the use of the bridge to pedestrian traffic, what happens if someone takes a snowmobile over the bridge and it falls off, or the bridge breaks. Who holds the liability? Almost certainly the designer of record holds part of the liability.

Call me paranoid. I have spent over thirty years investigating structural failures of one kind or another, testifying in court as to cause and origin. When something goes wrong, the designer is usually the first to be named.

So in your case, you absolutely need a designer, who will develop the loads on the bridge, size the members, design the connections and supports, design the footings, and should ideally have a continuing role during construction to insure that the bridge is built according to plan. If you can find a professional willing to do the work pro bono, good for you. As I said, I would never take such a risk, but there are those who will. Also, I know you stated you do not need a building permit, but realistically a bridge designed for foot or vehicle traffic must be designed according to accepted standards, meaning that there is probably an agency that sets forth standards for public structures (I assume this is public, as the Boy Scouts in my area would never work on a private bridge). Those standards may be State DOT, they may be local highway department, or they could be county.
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Old 10-09-2010, 04:47 PM   #7
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Laminated support beams for 25 foot long footbridge.


Since the poles that the archie spec'd can't be obtained your son should ask the architect for more options. The archie should be able to spec suitable beam designs. Son should also search for other sources of the poles even if they aren't particularly close. Someone will have a truck and trailer.

If the archie is ok with either foundation then it just boils down to what works better from the standpoint of cost, labor skills available and timing. That's for your son to figure out. I would tend toward one monolithic pour if the archie is ok with it and the concrete is affordable because it will likely require fewer organizational issues for your son to work out.

BTW, How come you posted instead of your son? It's his project isn't it? You should certainly be coaching your son through this but make sure he is doing the work and you are just coaching him in how to figure it out.
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Old 10-09-2010, 05:03 PM   #8
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Laminated support beams for 25 foot long footbridge.


Quote:
Originally Posted by fifty7flash View Post
The original plans were drawn up by an architect and he insist that the old poles be replaced. My son has contacted the utility companies and they do not have any poles to let him use and do not know when they will have them. This is for a BSA Eagle project being done for the City Park and Recreation service. He was trying to find a substitute for the utility poles. When finished the bridge will be inspected and needs to be built right. Therefore he needs the the correct load and specifications for laminated wood beams if he uses them. He does not need a "building permit". When going through this process these kids are expected to do as much of the planning of the project as possible and sometimes its hard for them to do. They have to procur all the supplies, labor so on so forth. the bridge is 3 feet high.
They are kids and have no idea how to engineer a beam. Tell your son to call the architect up who drew the plans and have him spec a replacement of the poles since you can't get what was spec'd.

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