Book-inspired Suspension Bridge in Virginia

We were delighted to finally receive photos of a suspension bridge that was inspired by our book, Building a Small Cable Suspension Bridge With the Cable Locking System.

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According to the builder, Mo Goldman, the bridge is just under 40′ feet in length and 4′ wide (basically half the length of our bridge) and is located in Virginia just outside of Charlottesville.

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The posts are aluminum, 13′ in length, 6″ round with 1/4″ thick side walls, easy for two guys to carry. The post holes are 3′ deep and about 2′ around; the posts are placed on a concrete footer prior to pouring around them. Everything was hand-dug and poured because they were limited to access with a Polaris on one side.

It was fun to see that Mo set up a temporary cable to move materials across. That’s how we moved our gravel for the opposite side, one bucket at a time. But Mo took it further and carried the posts, other materials, plus wheelbarrow and even himself across their “zip line.”

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Mo also followed the idea of setting up the catenary curve between two trees/structures located away from the creek to plan and build the cables and stringers on dry ground.

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Mo didn’t use our cable-locking system, but instead used a system often used for this type bridge – an appropriate length “eye” bolt placed in a drilled hole in the beam. The suspenders were then connected with a chain connecting link, which uses a threaded portion mating to a free spinning nut to open or close it.

He wrote to us about the bounce in his bridge which was more than he expected, though not a big deal. I noticed that he paid attention to harmonic resonance in the arrangement of the stringers so they were assumed off the “nodes,” so wondered if the decking material he used could be partly responsible (a suspension bridge is going to bounce, that’s a given). He used a material called Trex, which is a deck material made from recycled plastic and wood fiber. Trex tends to flex more than standard lumber does. We concluded that he needed to stiffen the deck, so now he is working on some ideas.

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Mo even put up a sign on his bridge similar to ours and inspired us to remake our sign so that it names the creek, too. We hope others who build a bridge based on what we did will also send us photos and notes about their building experience.

Meanwhile our book is available in paperback and as an ebook via Amazon.com.

 

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A Conflict between treated wood and metal parts

Pressure treated wood is a popular product for outdoor construction, i.e. bridges and decks. The chemicals (including chromated copper arsenate – CCA) that were used to treat wood in the U.S. and perhaps other countries were revised in recent years to remove at least some of the potentially harmful properties. The new formulations did help with the environmental risks, but they caused new problems for construction. When wood treated with the new chemicals was exposed to metal parts, the metal tended to corrode, even moreso in water contact environments, in only a few years. The pressure treatments I’m talking about are alkaline copper quaternary (ACQ) and copper azole (CA), which are both active corrosion materials.

So, the experts started recommending using galvanized steel or stainless steel for any metal parts, i.e. connector plates, joist hangars, and fasteners, that came into contact with pressure treated wood. Galvanized steel or stainless steel would withstand the chemicals for a while. So at least, the corrosion rate would be slowed down.

In my experience in remodeling, and specifically rebuilding decks, I found that some structures that had used ACQ or CA showed exactly how corrosive the treatments were. Galvanized steel joist hangers were nearly destroyed in only a few years.

I had this concern in mind when I built my suspension bridge. The cable locking system parts are hot-dip galvanized steel. But I also added a padding-wrap made of a bituminous material to add more protection between them and the painted (another layer of protection) pressure-treated beams that supported the bridge. Maybe I over-did it a little, but I like the things that I build to outlive me.

In building our new house, I ran into the problem again. In Oregon (and I’m guessing other states in the U.S.) it is required to install foundation anchor bolts in the stem wall foundation. These are to tie the house to the foundation. The bolts go through the pressure treated wooden plates and are secured with nuts. I had the issue again of metal making contact with pressure treated wood. I used ungalvanized (standard, but oversized to 5/8″ vs 1/2″ bolts) because I’m not convinced simple galvanization is the answer anyway. I opted to protect the bolts with a sleeve that I made from polyethylene tubing (5/8″ interior) material.

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The bolt came through the hole in the pressure treated plate…

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and then I used a water impervious roofing material tab…

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before adding the galvanized steel plate.

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At the completion all metal was completely isolated from the treated wood.

Thanks for stopping by! Be sure to check out our bridge book. Amazon has the book on sale for about $12.00 right now. Here is the link:

Building a Small Cable Suspension Bridge with the Cable Locking System

Images, diagrams, and text copyright 2013-2014 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.