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Pedestrain Wood Bridge Help
4

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
 

RE: Pedestrain Wood Bridge Help

I doubt you can get sawn lumber that span and any attempt to splice shorter pieces together, correctly, would be more expensive that buying GLB's. Considering the fact that your friend does not want to hire an engineer, he probably will not want to pay the cost of a 34' free spanning bridge. For starters  he/you might want to find a GLB supplier and talk to them about it.

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

Seems a bit of over kill.  EXACTLY what are your loads??  I am an old wood/timber engineer and can/may 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

I helped a friend years ago span 25 feet over a pond with PT 2X12's in a modified collar tie truss arrangement that worked well.  Still there today and simple to erect.  Must be properly designed though.  Should be able to do 35 feet if you have enough rise in the span to develop the necessary moment.

Mike McCann
MMC Engineering

 

RE: Pedestrain Wood Bridge Help

I bet we wants to be able to drive his BMW over it on weekends. This is how rich get richer and I get more helpful.

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

(OP)
I am a little surprised and two of the responses.  Woodman88 I never said he was not willing to pay for an engineer and he is very capable of buying the lumber needed.  He is a very capable handyman like myself and likes to build things himself.  The question was for clarification of what size lumber he needs.  He asked me because we are friends of over 20 years and he knows I am an engineer and we help each other.  rowingengineer your comment is totally irrelevant.  If you have nothing helpful to offer then why are you on the forum to begin with.

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

Call me unprofessional, I can't possibly attain the high standards of saintly hood that you are suggesting. However I would point out that in a friendship there is usually some give and take, while I may help a mate with a small engineering problem that is within my discipline, I would expect a beer or two for the effort. But would I go onto a free forum for the first time and ask for free help on a bridge that isn't described in anywhere enough detail to provide any meaning full answers, NO.

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

Without regard to it being located in someone's back yard, it still should be designed to meet code requirements; which will likely be on the order of 100 psf.  

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

That sounds like a good idea, I might just have a few extra.  

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 would second Ron's comment - this should be built to code despite being on someone's property.

"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

TPWGM,
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

There has been talk of getting engineered wood products to span the 34'. I was taught that it was usually not a good idea to use engineered wood in exterior applications. Do they have products that will hold up to the moisture?

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

(OP)
rowingengineer I never came on here with a saint hood attitude; I came with a simple question.  Structures are not my discipline, therefore I went to a place that had professionals and asked for advice.  Asking for free advice on a free forum, I think that's the point of the forum.  As far as give and take, you don't know either of us so don't assume you know the give and take that has occurred or will.  I do have many engineering friends in the area and I have also asked one to help me as well.  Thought I would come here for some input as well.  Again, if you're not on here to help others then why are you on here at all.
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

TPWGM - I've been on this site for a while as well and participated in many posts, answered many and asked questions as well.  The value of the advice here is unparalleled.  Yes there are many folks that come on here and are do-it-yourselfers looking, in many cases, for free engineering services.

You yourself posted here looking for a more experienced engineer to give you some assurance that a beam you designed was "OK".

Quote:

Could someone be so kind to confirm if the proposed beams will or will not work and if not, what would you recommend

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

You are on the right track with the wood, etc. Deflection might be an issue, so along the lines of another reply, a couple wide flange beams (W12x26 Seems to work)  cut to length would work, painted or galvanized. bolt a top plate, nail in your decking, rails, good to go. The beams might be lighter than the wood glams you might end up with.

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

ztengguy,
"(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

This is a more flushed out concept for the truss bridge.  For architectural reasons, I'd want to try to induce some kind of camber into the span.  You could try building it upside down or cut the tension web members (negative) short and the compression web members (positive) long and bend the chords to fit.  

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

woodman88 I assumed braced at 1/4 points, some x bracing could be used, and decking would brace the compression flange. What size are you coming up with?

RE: Pedestrain Wood Bridge Help

ztengguy,
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

Woodman - good point regarding assumptions.  

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

I didn't design a bridge either, I was just giving some order of magnitude and advice for this fellow engineer to get some comparison to the wood. I would have hoped he would take this disinformation and check it himself, as I would if he said a 24" drainage pipe would work if I asked a civil question.
Sorry for confusing you and others.
  

RE: Pedestrain Wood Bridge Help

Teguci,
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

Man we are getting touchy here - bad week??

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

3
This whole thread kinda makes my a$$ tired.  I generally agree with the comments made by Woodman, Rowing, Ron, and JAE.  We should not be doing someone's design for them on this forum, nor should people be coming here expecting that.  Those who do should be run out of the forum on a rail, and quickly, because these forums do not offer free design or engineering services.  We should be, or might be, giving them ideas to help solve their problem, as Teguci did, without giving them the exact design or answer; and telling them where to dig a little deeper when we think they are going astray, in our opinion.  And, good sound opinions, in that vein, generally illicit seconds and thirds, with a little more detail.  I'm not a union man, and I don't think the P.E. behind my name is there primarily to protect my turf.  But, I do object strongly to the misapplication and misuse of the terms Professional and Engineer.  When you ain't one, de-puff your chest a little, and tell us who you really are, and you'll probably get more and better help, assuming you come here with an intelligent, well thought out question.  If your question is put together in such a way as to show that you know very little about the subject, but then you get all uppity when called on that, you will probably be given the bums-rush in short order.  If on the other hand, the hair dresser down the street wants to know how a beam (that thingie carrying her garage roof over the OH door) works, most of the regulars here will give a very good explanation for that level of understanding.  There are way too many people coming here pretending to be something they are not, and I've always wondered what that (Struct., Mech. or whatever) behind our handles really means to people.  Are they really an engineer of that stripe, or do they just think their question fits that category, but they're not quite sure?  I always thought these were forums for engineers and/or well qualified technical people, doing this work for a living, not as a hobby or a handyman.  If you are a designer, drafter, or tech. person, etc. say so; and if you come here with a meaningful question, well thought out, and with enough of the pertinent info. needed for a meaningful discussion, to show that you have some basic understanding of your own problem...  you more than likely will generate some good sound discussion and advice for your initial effort.

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

Amen dhengr!

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

TPWGM,

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

A few years back I designed a 30' span wood pedestrian bridge in accordance with the AASHTO pedestrian bridge code.  Vibration controlled the design. The bridge is 6'-5" wide and consists of three equally spaced 11"x19" pressure treated parallam beams with 2x6 decking.

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

I was thinking a bit more on the issue and, in addition to sizes and advice given by others in this thread, let me offer this.

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

I don't think that the OP's going to be back any time soon, so, to me, further posting is really moot at this point.

Mike McCann
MMC Engineering

 

RE: Pedestrain Wood Bridge Help

Yea, we ran him off good didnt we!
 

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