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

 

[Teguci]

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.

 

[ztengguy]

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.

 

[woodman88]

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.

 

[MiketheEngineer]

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!!!!!

 

[dhengr]

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.

 

[pmover]

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

 

[MJB315]

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

 

[graybeach]

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.

 

[MJB315]

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

 

[msquared48]

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

 

[ztengguy]

Yea, we ran him off good didnt we!