Options For Limited-Access Pedestrian Bridge Construction 2

[SlideRuleEra]

Seasons Greetings - In my new volunteer "job" as in-house engineer for our homeowner's association, we have two (2) pedestrian bridges that need replacement ASAP. My general arrangement sketches of the existing six (6) feet wide wood bridges are shown below:

In both cases replacement bridges need to be single span.

I would prefer to use prefabricated steel truss bridges such as those by Contech, or equal... but there is a problem. The pond is totally surrounded by individually owned single family homes on small lots. The only common area access is a relatively narrow tree-lined sidewalk. Doubt trucking in the bridges and equipment to set them is practical. Helicopter, maybe, but, of course cost is a big issue.

My question: Can you recommend another way to construct these limited-access bridges besides

1) Prefabricated

2) Site-built, piece-by-piece from manageable sized construction materials?

 

[SWComposites]

Look into prefabricated fiberglass bridges. Much lighter and thus easier to install. See articles at compositesworld.com.

 

[SWComposites]

https://www.creativecompositesgroup.com/products/pedestrian-bridges-boardwalks/pedestrian-bridges

https://aretestructures.com/

https://www.fs.usda.gov/t-d/pubs/htmlpubs/htm06232824/page03.htm

https://linksbridges.com/why-fiberglass-bridges/?amp=1

https://www.strongwell.com/products/bridge-components/

 

[BridgeSmith]

You might want to check with Acrow. They have modular bridges that can be assembled on site using individual steel truss and decking panels.

 

[LittleInch]

Why single span? That must be increasing your main span beam by a lot.

Also 6ft wide? How wide are the people walking across this?

Reduction in both would help a lot.

Suspension bridge?

Modular. Might be a bit " basic", but I'm sure you can paint it nice colours...

https://maadigroup.com/products/modular-bridges/

Or FRP. This sounds similar

https://www.strongwell.com/case-study-frp-goes-cross-country-with-mountain-bike-bridges/

Can it be curved? Glulam arch sounds OK.

Also look into golf course bridge or design. A lot of them are similarly in the middle of nowhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BridgeSmith]

If it's built on-site using steel beams, with bolted bracing, etc. and wooden decking and rails, everything should be light enough to put on a cart and roll in. My quick back-of-the-napkin numbers say a W14x22 would probably work for a 2 girder system. If I'm way off, you might need a W16x26, which is still only about 820 lbs per girder. If you do 3 or 4 girders, they'd be somewhat lighter.

 

[SlideRuleEra]

Many thanks for the suggestions.

SWComposites - Fiberglass appears to be exactly what we need, thanks for the useful links. I've started, and will continue reading them. Cost is going to be the biggest hurdle. We have several other projects that are competing with these bridges for available money (that's why the Board of Directors (BOD) asked me to join them... the "wheels are beginning to fall off" the 36 year old community's basic infrastructure).

BridgeSmith - Totally agree with your concept. I've sketched similar details for the 25' overflow bridge, which may be able to use much lighter M beams instead of W shapes. The 32' inlet stream bridge is going to be much more expensive:
Longer
Heavier
Too low - needs to be raised about 2'
Bridge location blocks routine maintenance clean out of silt accumulated in the stormwater pond inlet stream.
Note: Our stormwater ponds remain at full-pool all year because of high ground water table.

I'm going to try to convince the BOD and community to delete the longer bridge, for now. We can get by without one of the two bridges.

LittleInch - Clear span on the overflow bridge is needed to eliminate the two interior bents. Currently the 6x6 wood posts are not anchored to the spillway (just friction). The attached photo speaks for itself; that is less than 5" deep water tending to knock the "feet" out from under the interior bents. I'm not about to "open the can of worms" trying to anchor posts on a replacement bridge. This past spring we had to make major repairs to the spillway... that's when the BOD "woke up" and realized they needed in-house engineering input on significant projects.

Width of the bridges is, hopefully, negotiable, we'll see. I've not only dealing with technical issues, but also convincing intelligent, but non-technical BOD members to reduce scope of some project because of cost.

 

[SWComposites]

SRE - yes fiberglass cost tends to be a bit higher, but there is savings in the install cost and in the long term maintenance. Definitely go with a clear span bridge replacement; those posts held by "friction" look like a collapse waiting to happen.

 

[hokie66]

I suppose a Bailey bridge is not pretty enough?

 

[bugbus]

What is the reason for replacement out of curiosity? That bridge looks to be in OK condition

Edit: Never mind - I neglected to read the other posts.

 

[SlideRuleEra]

SWC - In addition to material cost, it will be a challenge for us to find a contractor who is willing to submit a reasonable bid on erecting a fiberglass bridge that he has probably never heard of.

We are seeing this on all of our infrastructure projects... for this community, the cost is "high", but for qualified contractors our projects are "small potatoes" not worth bothering with. The Charleston, SC area is in the midst of a long-term "building boom"... both consulting engineers and contractors have more profitable work begging for them to take it.

hokie66 - The Board would probably faint. I have seriously considered designing the bridge to be more like a platform or elevated walkway that I would have used at one of our electric generating stations. Then we may be able to get some interest from small contractors who perform industrial work.

bugbus - I see your edit, but will comment anyway because I asked myself the same question when this project first came up. The devil is in the details... when I reverse engineered the existing bridge "design" results were shocking:

These bridges were "built" by a contractor, not designed by an engineer... and when you look, it shows.

Live load rating tops out at about 30 psf (when it was new). Should be more like 85 psf for a pedestrian bridge.

Wet, subtropical climate in Charleston. Bridges are at least 10 to 15 years old. Even though the wood is "pressure treated" (to who knows what level of preservation) much of it is visibly rotting. I have down rated live load to 20 psf based on engineering judgement.

Then, of course, the overflow bridge is laterally unstable because of the unanchored interior bents.

 

[LittleInch]

SRE,

Thanks for the reply. You've got a piece of work on your hands here....

The real issue is that I assume the HOA have not really built up a big enough fund to pay for replacement of things and a soon as you get contractors in you're into a different scale of things that just boggle the brain of most homeowners who can barely play the mortgage.

So I would approach this like any concept design and look at all the options, write a report and give people a few options to argue about.

So
1) Do you really NEED these bridges? What would happen if they just weren't there? Edit Just read you could do without one of them, but both?
2) Can you reduce the width/ span or divide into two? This could be by e,g. dropping in a few gabion baskets as a mid point? Or a couple of concrete blocks?
3) You're definitely into small 2 to 5 man company size here where this is worth their while, So need to find one or two to see what their capabilites are and design around them rather the other way around.
4) Figure out what machine you can actually get in there. Even a small tracked digger will help hugely. Many will reduce track width when travelling to 3 feet wide or so. Again design around their lifting capacity rather than the other way around.
5) Longevity might be the best long term solution but I guess most owners won't want to fund a replacement that will last 50 years when they won't be there in 10 years, so I would concentrate on timber solutions or FRP main beams at best.

Good luck, Merry Christmas, and let us know how it goes...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Also off the wall a bit here but could you use two or more telegraph poles? They are about 30 ft long....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SWComposites]

SRE - the composite bridge companies might do the install, or know of contractors experienced with their products. Worth a few calls. (note, I don’t have any involvement in the civil engineering side of composite materials/structures; just have read a lot of articles about those applications over the years)

 

[SlideRuleEra]

Thanks, SWC. Have given this bridge project a good bit of thought since yesterday:

Soon after new year begins will see if I can convince the eight (8) other Board members to eliminate the longer bridge over the inlet stream.

The HOA is set up to bid projects through a property management company (working with them on technical issues is a story in itself... at first, the "manager" considered engineering requirements to be "suggestions". A little arm-twisting and that has been resolved.)

I'll prepare a "Scope of Work" (to keep things simple I just combine a real Scope of Work with Engineering Specifications... nobody here knows the difference.) This document will list all of the FRP companies including the links you provided, Acrow (per BridgeSmith), Contech, and perhaps a few other companies in the steel prefab bridge business.

Then sit back and see what we get.

If that approach does not work, then the Board will have justification to bypass the property management company and try again.

I appreciate all the input and will let you know what happens. Simultaneously will move on to road rebuilding, community-wide above/underground stormwater system restoration/improvements, repave/repurpose worn-out tennis court to pickleball, etc. Lucky that I consider all this to be a really interesting "hobby"... at least for now.

 

[hokie66]

Careful about the pickleball. Noisier than tennis, which could cause problems depending on proximity to dwellings.

 

[SlideRuleEra]

hokie66 - Yes, we have discussed noise quite a bit. The pickleball players taped temporary court lines onto the dilapidated court about a year ago. When asked about noise, nearby residents have no complaints, so far. Two advantages:
Most of the year is too hot for pickleball since the court is in direct sunlight.
No lights and no plans (or money) to install them, so no night pickleball.

The Board's finance officer and I have convinced enough of the other Board members to put this court project at the bottom of the priority list.

 

[SlideRuleEra]

Going out for bids this week for bids on a prefabricated or modular pedestrian bridge. I incorporated all of the manufacturers suggested in this thread (thanks) as acceptable suppliers. Will see what happens next.