Pedestrain Wood Bridge Help
Pedestrain Wood Bridge Help
(OP)
I am a civil engineer, but not a structural engineer. I am trying to help a friend out, but I need a little structural help myself. He wants to build a wooden pedestrian bridge over a small pond in his backyard. This is not for public use. He wants to build a bridge 4' wide and span 34 feet. He wants to glue / nail / screw two 2 x 10 x 18' long pieces of lumber together to make a beam. The lumber will be offset in the middle by 1 foot and reinforced with a metal plate and bolts to make the splice. There will be one combined beam on each side of the bridge. The bridge will be decked with 2 x 6 lumber (like a deck) and will have some sort on handrail attached (no truss or anything like that to help with the weight of the bridge). I have calculated that the proposed beam will not work.
My calculations (rough and with help from internet design tables) shows that he will need five 4 x 16 beams for a 34' span, which seems like overkill, but I know that is an extremely long span for a wood beam.
Could someone be so kind to confirm if the proposed beams will or will not work and if not, what would you recommend.
Thanks
My calculations (rough and with help from internet design tables) shows that he will need five 4 x 16 beams for a 34' span, which seems like overkill, but I know that is an extremely long span for a wood beam.
Could someone be so kind to confirm if the proposed beams will or will not work and if not, what would you recommend.
Thanks






RE: Pedestrain Wood Bridge Help
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pedestrain Wood Bridge Help
Trusjoist (Ilevel.com) makes a large line of wooden I-beams that can easily span 34' By the time you buy all the hardware and put it together - this may be a cheaper route.
Not sure about weather protection - but you can frame it with treated lumber.
RE: Pedestrain Wood Bridge Help
Mike McCann
MMC Engineering
RE: Pedestrain Wood Bridge Help
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Pedestrain Wood Bridge Help
As a professional engineer for over 20 years, I have more than once used my engineering expertise to help a friend. Isn't that what we do as engineers is help people. I have even solved problems for clients for free. What a novel idea....being an engineer that openly helps people.
MiketheEngineer – I will see if I can get a better idea of the loads. Obviously the dead load will be the uniform load of the wood and the handrails. The live load would be approximate to 4 adults, so estimate 800 to 1000 pounds. Thanks.
msquared48 – I have thought about the truss idea and that may be the way to go. I like the idea and I think he would like the old style aesthetics. I may have to look into that one a little more. Thanks.
RE: Pedestrain Wood Bridge Help
You really need to provide more details to increase the value of this free advice by 200%. There needs to be equal thought put into details, how are you going to support this bridge, how are you going to detail this, what is the strength grade assuming you're in the Zimbabwe it is probably F14, and normally you would have to grade the beam as well for quality. Are four people really a loading? You really need to take a look at the standard that would be appropriate. I assume that since you're a civil engineer with 20 + years' experience that you have probably come across a structural engineer or two in your time that you could easily ask them, maybe even create a sketch or two.
and just incase anyone was wondering yes I have had a bad week.
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Pedestrain Wood Bridge Help
I agree that "engineered wood" is your better option...more expensive, but better for your application. Check serviceability (e.g. deflection) as "pedestrian comfort" is the intangible design criterion in these cases.
Now that you've received advice from here, the quid pro quo is that you stick around, participate and give advice in return.
RE...it should already be Friday pm in your area...have an extra pint!
RE: Pedestrain Wood Bridge Help
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Pedestrain Wood Bridge Help
The wood should be pressure treated and you will need a manufacturer to detail your connections.
RE: Pedestrain Wood Bridge Help
"to code" means that someone competent in bridge, or at least structural, engineering should be doing it. I looks like you have some knowledge of structural analysis and calculations but if you need to come here to get assurance that you did it right, that tells me you might be limited in your understanding.
Never a good idea to practice outside your experience and education - even as a gallant knight serving a friend.
I would say that your friend's original 2x10 beam wouldn't work for the span.
Your larger beam sounds more appropriate but without loads and details I can't comment.
RE: Pedestrain Wood Bridge Help
I am glad your friend can afford an structural engineer. Considering you think that only four people will ever be on a 34' bridge at any time, I say STOP giving advice outside of your area of expertise and tell him to hire one.
As for my first comment, I still stand by it. It is very expensive to splice sawn lumber for a 34' clear span. For a clear span bridge, GLB's are one of the better options. Have him go talk to a GLB supplier (or a structural engineer) about the bridge.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pedestrain Wood Bridge Help
Personally I would build two piers on each side 4' appart and have two galvinized steel beams fabricted and delivered. Then your friend can have a field day being handy building a wood deck and railing that will sit on his steel frame.
RE: Pedestrain Wood Bridge Help
Jae and Ron – thanks for the input. You are correct, structures are not my discipline and why I have asked for advice. I did a little earlier in my career but even then it was steel and concrete and not wood. We do want to build it to code and to last. I agree, I thank the 2x10 will be inefficient in this case. I think we may look at the truss or engineered beam. Thanks for the help
For clarification, this is not my first time on the forum. I forgot my login information and had to open a new account. I have been on and off this forum for probably 10 years. I quit being active because I got tired of seeing people ask for advice and help and receive answers of Do it yourself, look it up, hire someone, don't ask for free help, etc...I looked at the overall forum last night and I see that is still not that uncommon. And yes, I am on other technical forums as well.
Thanks for everyone's input.
RE: Pedestrain Wood Bridge Help
You yourself posted here looking for a more experienced engineer to give you some assurance that a beam you designed was "OK".
So in fact you don't really know how to design wood very well and you were looking for a warm fuzzy , or an actual design, to send you on your way to helping your friend. And this was going to be you engineering something based on website assurances in an area of engineering you weren't good at.
Eng-Tips is a site defined as "INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS" In MOST posts that is the case. In some the questions posed are not by engineers and many times aren't very intelligent.
The engineers primary directive is to protect the public welfare and safety. I personally don't see how us giving you a warm fuzzy does that.
RE: Pedestrain Wood Bridge Help
Wide flange beams, cut to length without fabrication time on them are somewhat cheap.
I agree that sometimes some people jump the gun and say hire a engineer. Sometimes some want to protect the profession with a union attitude.
RE: Pedestrain Wood Bridge Help
"(W12x26 Seems to work)"
This size does not come close to working as an unbraced beam. What assumptions for bracing are you using and how do you plan to achieve it? And why are you willing to give this disinformation to someone with so little structural background?
"The union guy"
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pedestrain Wood Bridge Help
Still need to size the chord members (what kind of pressure treated lumber do you have available?) and design the connections (Galv steel plate and screws?).
The biggest concern is the buckling of the top chord as well as resisting the rail loads.
Just a concept but I had fun putting it together.
RE: Pedestrain Wood Bridge Help
RE: Pedestrain Wood Bridge Help
I am not going to (due to "union rules") design a 34' bridge on this forum. The point I am trying to make is that there are many things that need to be considered in designing a 34' bridge. As the original question is from someone with little structural engineering knowledge, any information you want to give should have it's assumptions clearly defined, especially if you are not going to clearly state that a qualified structural engineer needs to review your help.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pedestrain Wood Bridge Help
My assumptions for all of my designs/information on Engtips is that users of that information aren't stupid enough to think that some random guy on an anonymous website is going to take any liability what so ever for said information.
Please note, in order for the anonymous part to work, I have not provided my name, licensure status or area where I live under my posts.
As for my "Union" rules, I have one. - I hate union rules.
RE: Pedestrain Wood Bridge Help
Sorry for confusing you and others.
RE: Pedestrain Wood Bridge Help
This is not an "anonymous website" as far as I can tell it is a respected engineering one.
My assumption is that at some point it is going to become the responsibility of websites to provide e-mail addresses, etc. of the people who post comments on their website to any concern party. Also all the electronic trails that are created when e-mailing/posting information. As time goes by it will become easier and easier to track down this information.
You might want to reconsider your assumption of how anonymous you are on this and other forums.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pedestrain Wood Bridge Help
You seemed to want to use wood of some sort since it is usually much easier for the average handyman to work with. So I suggested wood I-beams with treated lumber over lay ( enclose it and paint or stain it)
As for loads - 4 people seems light. If you have a daughter - guess where she will want her wedding pictures - ON the FREAKING bridge with like 19 bridesmaids and groomsman!!
I would prefer steel - but......
Also the Fire Department may want something to hold their 8,000,000 lb firetruck.. Seen it happen many times more than once!!!!!
RE: Pedestrain Wood Bridge Help
TPWGM... You did give some info. on your background, so no fault there, but go back and read your OP. The problem info. you give and the details you suggested were left so wanting as to amplify your statement that you were not a Structural Engineer, and it did this in a fairly negative light, and this was not the reader's fault. You didn't give it enough engineering thought in the first place, and in retrospect I'll bet you know that. Without your showing some deeper thought for loading; if it's 4' wide they will drive a riding mower or a four-wheeler over it; code compliance, hand rails to meet wooden deck criteria, public safety, his yard or not the whole neighborhood will be out their to watch the fireworks over the pond; splices at max. moment point on the beams, etc. etc., what were you thinking, you left yourself wide open. And, as an engineer you should care about your friend's liability exposure when working on something like this. You did say you had done some structural design, earlier in your career. Many of us would ask, is this guy really an engineer, has he forgotten everything he learned in school, can I even begin to trust that he would apply what I told him in a correct engineering manor, and yet Woodman gave you a perfectly practical answer, and correctly implied that your friend needed an engineer involved in this project. My first reaction to your OP was not to participate in this thread, because the original idea was so ill thought out. Someone needed a 34' beam, but beyond that had little grasp of his problem, and no foundation for it. Stick around, this isn't intentionally an unfriendly place, but do give your questions some better engineering thought before you post the next time. And, reread some of the threads that stuck in your craw, and see if some of what I'm ranting about above wasn't the reason for the rough treatment.
As for anonymity on this sight, I'm with Woodman again, and I've expressed that several times before. While we assume some anonymity here, what you say here could easily come back to haunt you, so you better be able to defend it. I've had stuff read back to me in court, which I said in a depo. or at trial ten years ago, and then had to explain why that didn't apply here, in this case. While it may be pretty shallow lawyering to assume that any advice given here could be relied upon for a final design. If push came to shove I suspect that lawyer would have Teguci's name, address, e-mail, etc. in short order and then you could spend your time and money defending your advice and proving that that lawyer's reasoning was shallower than the pond in question here.
RE: Pedestrain Wood Bridge Help
TPWGM,
a floating pedestrian bridge is another recommendation.
using "internet design tables" for engineering design activities . . . i would think industrial strength & proven design tables for the material you plan to use are the preferred method. i'm not ruling out information obtained from the internet, but then again, we are not familiar with your practices/knowledge.
regardless, good luck.
-pmover
RE: Pedestrain Wood Bridge Help
I do understand your sentiment and your desire to help your friend. At the heart of it, its a simple structural engineering problem and, being a PE (in some field of Civil Engineering) for twenty years, you are an obvious choice for your friend to ask for assistance.
But whereas the problem is simple in concept, the design it can be quite involved. As a PE for twenty years, you should know this upfront. I don't think the subsequent lines of questionings and comments from your peers is out of line, and I thought it was counterproductive for you to get defensive early. Engineers are here to help people; Firefighters are here to help people. But each knows that assistance must be given in a safe and professional way to prevent people from getting hurt. This we all know.
So, we can assist you as peers, but we require the proper information. And, even then, its just helpful, non-liable, assistance. You're the PE, and if you are not comfortable with all aspects of the design, it would be prudent for you to sit across a table with someone who is.
And my recommenddation whould be to follow the codes to the letter. Because that point made about the Owner's daughter's wedding pictures is a good one.
"We shape our buildings, thereafter they shape us." -WSC
RE: Pedestrain Wood Bridge Help
Based on this, I think 2x10s will not be sufficient. Plus, the moment splice in the middle will not work.
Maybe you could replicate a "stress laminated" deck by bolting a solid width of 2x lumber together and staggering the splices.
RE: Pedestrain Wood Bridge Help
There is a product called Lite Steel Beam (LSB). It is a cold formed steel beam product (formed in the shape of a channel) which has some decent properties. It is apparently easier to handle, cut, and connect to in the field. It may be possible to use two or more of these sections to form your span. You could fasten your decking down directly to it using self-drilling screws (or attached to wood plates that are fastened to the LSBs). But, of course, there would be a lot of details to consider (bracing, hand rails, connections at supports, etc.).
Full disclaimer: I have never used these products myself, and I could be unaware of any "gotchas" associated with them. I am not sure of their durability in an exterior environment (thin steel sections and corrosion typically don't mix well), their availability, and cost. I wonder if anyone who is reading this post has worked with LSBs in the past and could share their experience.
I offer this just as a thought. I have provided a link to the manufacturer's website below for reference.
http://www.litesteelbeam.com/index.html
"We shape our buildings, thereafter they shape us." -WSC
RE: Pedestrain Wood Bridge Help
Mike McCann
MMC Engineering
RE: Pedestrain Wood Bridge Help