Pedestrain Wood Bridge Help 4

[TPWGM]

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

 

[woodman88]

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.

 

[MiketheEngineer]

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.

 

[msquared48]

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

 

[rowingengineer]

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

 

[TPWGM]

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.

 

[rowingengineer]

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

 

[Ron]

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!

 

[rowingengineer]

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

 

[Teguci]

Since you will neeed handrails anyhow...

The wood should be pressure treated and you will need a manufacturer to detail your connections.

 

[JAE]

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.

 

[woodman88]

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.

 

[port125]

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.

 

[TPWGM]

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.

 

[JAE]

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".

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.

 

[ztengguy]

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.

 

[woodman88]

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.

 

[Teguci]

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.

 

[ztengguy]

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?

 

[woodman88]

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.