Residential Pedestrian Bridge Design

[AZPE]

I have a client who wants to span a pedestrian bridge across a wash on his property to get to a guest house he wants to build. I rarely do anything other than wood framed custom home engineering and am a little out of my league. He wants to run a 7' wide x 50' long bridge with brick pavers. I initially just thought about two wide flange beams spaning the entire distance with a steel plate over them and angle iron at the edges for paver containment but when I ran the loads I got two W21x101 beams! With all that load I figured I would need a 3.5' dia. pier at each end of each beam and then tie them together with a grade beam. I am sure that there is a better way to do this, but, like I said, I am a little out of my league, any help would be appreciated.

 

[davidbeach]

Buy a used railroad flatcar.

 

[Qshake]

Used railroad cars have been used on occasion for simple structures such as culverts but this application (50') doesn't seem like the ideal condition even if you cut the car in half and used the two pieces side by side it's not 50'. And what doesn't go through the opening is backed-up on the upstream side in heavy rain. This could potentially flooded area where the owner might plant his cactus!

Anyway I would recommend a long boardwalk. You can use 2 -4x4s (cut to height desired) spaced at every 5' with two 2x8s nailed or screwed to each 4x4 post so that makes a frame little less than 7' wide. several 2x6s (longitudinal direction) can be placed atop the 2x8s or even framed into the 2x8s. The actual walkway boards (decking) will be placed transverse to the longitudinal 2x6s with the decking nailed or screwed into the the 2x6s.

Put some handrail on and your done. This should be cheaper than steel and more readily advanced by your neighborhood carpenter or weekend warrior.

The post should be founded in the ground so that water doesn't undermind them.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[AZPE]

Problem is that he has to clear span the wash area, it's a drainage easement and there isn't any building allowed in it, just over it.

 

[kelowna]

You could weld or bolt the two longitudinal beams to the top steel plate, that would make it structural and increase the inertia of the section hence reducing the size of the beams. You could also go for 3 beams instead of 2.

If possible get rid of the brick pavers, my guess is that they are not light. Is a wooden deck an option?

Another option is going for a truss system, the upper cord of the truss could form your handrail. Using square sections for the top and bottom chord, wooden deck in between with bracing underneath could be a nice lightweight solution.
Regardless, 50' is a considerable distance, and you should expect to use some beefy sections.

 

[jorton]

There are many companies that sell timber and steel manufactured bridges. They often have used ones for around 1/2 price, but you have to ask. We've helped a few customers out by designing the project around an available used bridge rather than the much costlier custom bridge. However, this is probably still more than the client is willing to spend.

 

[AZPE]

Actually, it's a pretty high end custom home. My guess is that they would rather pay more and have it all figured out for them. If you have any contact info for the company(ies) that you mentioned, I would appreciate it. Thanks in advance.

 

[cvg]

You might be able to install a steel multiplate arch culvert or even Con-arch bridge. You will probably need a floodplain use permit to do it, but might be cheaper or better

Con-Arch

http://www.con-arch.com/

Corrugated Steel Structural Plate

http://www.ncspa.org/product_detail.asp?id=9&catname=Bridge+Solutions&appid=7

 

[graybeach]

AZPE,
I designed a timber pedestrian bridge with a 49 foot span, a 6' clear width and 2x6 decking. The controlling design case was keeping the natural frequency above 3 hz to avoid uncomfortable vibrations. This required 3 - 11" by 30" laminated beams.
If you do not want a bouncy bridge - you might want to stay above 3 hz. Heavy pavers will make this difficult.

 

[DRC1]

There are a couple of firms that specilize in pedestrian bridges. Continetial Bridge is one. I have seen ads in Civil Engineering for them. They have quoted us for work. They have standard designs or will customize. Can com partially or totally prefabricated.

 

[AZPE]

Thank you very much for all of your input. I am a new poster and have found this forum very helpful.

 

[bridgebuster]

Here are links for two manufacturers:

http://www.contech-cpi.com/bridges/products/truss/273

http://www.usbridge.com/

 

[jorton]

AZPE,
I usually use Steadfast Bridges out of Fort Payne, AL. I work in Atlanta, and they are the closest supplier I know of. The delivery to the job site is usually included in the price, but you have to unload and place it. As I mentioned before, they usually have a few used bridges that were previously leased out for temporary detours, etc. If a concrete deck is required rather than timber, they are usually delivered with metal deck pans in place. Make sure the deck is reinforced rather than relying on composite deck pans, as these will eventually corrode. It may seem obvious, but I have worked on a few rehabilitations where this was the case.

 

[Qshake]

Another option is to use steel beams but provide external post-tensioning in order to reduce the weight of the beams give a more slender profile to the bridge. The post-tensioning can be placed in-between the two beams thereby keeping away from rainfall.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BRGENG]

davidbeach – used railroad flatcar – a very reasonable and cheap alternative, good suggestion and your not structural, A+

Qshake – I think you are getting flat car and box cars mixed up. Correct a box car will behave like a culvert and if not sized properly could result in debris piles, however your boardwalk will defiantly not allow debris to pass and would probably be washed out and need replacing. While it could be built by about anyone with a saw, timber is not cheap and replacing it over and over is really not cheap. D

Kelowna – buying bigger beams is cheaper then fab. work. Wood deck over brick pavers is a good idea and will save weight, good idea. The truss system is another good idea however there we go with the fabrication again, beams are cheaper. Better idea is to use a concrete deck and make the beams composite. Will cut down weight and increase stiffness. 50 feet is short for a bridge and no you should not expect heavy sections W21x101 are major overkill, run your own numbers before you respond. D

Jorton – excellent honest answer and yes they cost more. We just bid out a bridge with two options 40 foot precast bridge vs. 90 foot steel girder bridge. The precast bridge package (bridge only) delivered to site cost more then the 90foot steel bridge and roadway work. A

Cvg – finally someone brings up the permitting. We have had hayfield that shed water when it rains require a Nationwide permit from the Corp of Engineers. Trust me, you don’t want to get caught without a nationwide permit if it is required. They are easy to obtain and can save you a giant headache later. Depending where you are at you could be looking at several permits that you would need to obtain. Again prefab bridges/culverts mean $$$$$$$. A, for the permit comment.

Graybeach – Vibration is a good point, we have all worked in an office where the floor moves when the mailman comes by, however I would not let vibrations be the controlling design load for a bridge between two homes. However making the owner aware and let them decide is always the best bet. A

DRC1 - $$$$$ Continetial Bridge makes a beautiful product and you pay greatly for that. However if the owner has the money and looks are important this is a good option. However, a steel beam bridge with a decorative guardrail fabricated locally can achieve the same look the prefab companies offer and is usually cheaper. They have to pay for there national advertising and sales force somehow. Also with a local fab bridge you have more options on looks, try altering a Continetial or Steadfast bridge from there typical design and watch the price soar. But again fab = $$$, B

Jorton – yes always reinforce the slab, typically the metal pans are designed for construction loads only and not service loads. A

Qshake – This sounds like a grad student doing research for his thesis. Yes it is an option but I would only use it if I came across some used beams that would not work any other way.

Everyone, including me, you can not design a bridge without seeing the site firsthand. Maybe I should say that you can not design the best fitting most economically bridge without seeing the site. No one mentioned a hydraulic study to determine if the proposed bridge would create backwater and flood upstream properties. Or if the proposed bridge would be flooded by a 2 yr storm event or a 25 yr storm event. What about precast concrete slab beams, they can easily span 50 feet for a pedestrian bridge without being prestressed and maintaining a shallower profile then originally proposed by the owner.

How come no one questioned a W21x101, when was the last time you ever used a W21x101, have you ever? Since deflection usually controls on steel bridges a W27x84 would not only save you weight but provide a stronger section. I doubt that the project was limited to a 21 inch beam. If you run the numbers you will see that a W27x84 is overkill.

You want ideas on what might work ask here and you will get a response, although are you able to tell what is valid and what is not? If you want the best solution for your problem, contact a local structural engineer who specializes in bridge design. If you don’t know one contact your local County Commissioners office and ask for referrals, they hire more consultants to design bridges then anyone else. You state DOT is another source for referrals but which do you see more often 50 ft county bridges or 50 ft DOT bridges?

Good luck,

 

[civilperson]

Price out a 8' wide double tee from a concrete prestressed precast manufacturer. If the abutments are cast in place and the double tee set on the abutments a single unit will easily carry pedestrian loading plus decorative brick, timber, handrails, etc.