Marston Mat Mudway

Marston Mats, aka perforated steel planking (PSP), were developed by the U.S. before World War II. The idea was to quickly build temporary runways and landing strips. They were first used at Camp Mackall airfield near Marston, North Carolina, in the U.S.

marston

A Curtiss P-40 Warhawk on a Marston Mat runway at Milne Bay, Papua New Guinea in September, 1942. – public domain

marstonmat

 

I honestly don’t remember where or when I acquired four of these heavy metal planks, probably saw them advertised somewhere. I know I didn’t pay much for them. The idea at the time was to see about using them for my bridge deck. I wasn’t able to acquire any more at the time, so opted for the wood deck you see in the photos. One of these days I’ll replace that deck with something metal. I like that the perforations would let air flow through so the bridge would not be as vulnerable to high wind gusts.

Meanwhile during our rainy season, areas where we like to walk get more than a little muddy. The small creeks flood and send water everywhere. I’m always digging ditches to try to control the flow somewhat, but this particular area is low, so it’s always a mucky mess.

mats
So I installed some 2×10 planks I had as well as the four Marston Mats. Nothing much to it, other than first setting out short cross members for everything to rest on. While you could just toss the planks into the mud, raising them up a bit makes them more effective, and you can get them level to some extent. I screwed down the wooden planks, but the mats were heavy enough to be stable without reinforcement.

jeepmud
When finished with the project, we noticed that Jeep the dog appreciated the wooden planks, but not so much the mats. He proved the point that the Marston Mats would not be the best material for a bridge, assuming you want to allow your four legged friends to come across with you. We’ll be watching for other options.

Thanks for stopping by! Be sure to check out our suspension bridge book. Here is the link:

Building a Small Cable Suspension Bridge with the Cable Locking System

Images, diagrams, and text copyright 2013-2016 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

A Simple Swamp Bridge

We continue to expand the trail system on our property in the Great Northwet, and that’s not a spelling error. Here in western Oregon many areas that are passable in summer become higher-than-boots water in winter. One spot we have is a small wetland that is fed by a stream that runs year-round, just not as torrential during the dry months. I decided it would be a good place to put a small stump-and-plank bridge so that we could connect two trails we like to use.
swampbridge4
swampbridge3
I had some 20 inch diameter cedar posts that would work well for the bases. Planted in water and being cedar, they would not rot anytime soon. I cut them with a chain saw and then the fun part began – digging holes in deep mud, in water. Even in summer, it is a very boggy area. My goal was to dig until I hit more solid ground, but of course the sludge would just pour back into the hole as I shoveled. I dammed up in front of each hole as I dug to divert some of the water, and that helped. Mostly it just took persistence and a bit of mind over matter.
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swampbridge2
I set the posts 10 feet apart for my 2×10 planks. If you use something smaller for planks, it is probably best to space the posts closer together. I cut the planks so that they centered on the posts and screwed them down. Then I used metal strapping to tie them together. I could add another deck using smaller dimension lumber to really beef it up, which I’ve done for other bridges on the property (more on them later!), but after walking across this bridge every day for several months now, it seems to be all we need for such a short span – about 40 feet. I might punch some metal posts on each end and the center and add a rope railing for balance. In any case, it’s not a bridge for the non-sober!
finishedbridge
Thanks for stopping by! Be sure to check out our suspension bridge book. 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.

Evolution of the Cable Locking System and an Alternative Plan

When the idea of the cable locking system first came to me, it was to assemble and disassemble the means of holding the beams for the deck of a suspension bridge. How it was to be produced and what that might cost were of secondary consideration then. After working out just how I would have it made, I found a manufacturer that would do the job, and it was a reasonable cost.

It was a long process getting the patent for the cable locking system, but once that was accomplished we set about writing a book – Building a Small Cable Suspension Bridge with the Cable Locking System. The book was meant to promote the cable locking system and talk about all the steps we took to build the bridge.

I never thought there was much of a market for small suspension bridges, after all, how many people have a creek or ravine that they need to cross on their property? But after we published the book, there was a fair amount of interest in the cable locking system, mostly in the United States, but also from around the globe. And we had put in the book, and on our website, “contact us if you are interested in the CLS.”

About a year later, some people were asking about purchasing CLS components. We had three companies that could do the job, but we were only considering about 50 “units.” After some waiting they gave quotes on 100 or 300 units. The prices seemed expensive, and after informing the interested buyers, we heard no more from them.

The cost of production seemed to be a problem and we weren’t willing to buy and store parts in case someone ordered them. So I wondered: Can I simplify the CLS so that it could be produced more cheaply, and by just about anyone, with a minimum of tools? I came up with a possible answer.

The “inverse” CLS employs the cable locking system but places it somewhat in reverse of the
original application. It eliminates some of the materials and simplifies the manufacturing process while still using the patented cable locking system. Here’s a drawing:

CableLockingNew

The new system has some drawbacks, in that it has to be placed beneath the beam during assembly as opposed to slipping the beam into the original CLS. Also, the beam has to be pre-drilled and a PVC tubing should be inserted to protect the cable from corrosion. But overall, assembly is not all that different from the original cable locking system design.

Picture a swing: hung by two ropes (chains, cables, etc) one on each side. To assemble the “inverse” system, while cradling the beam in your lap, grasp the first of the two suspender cables, push it through the pre-drilled hole ( with tube protector) such that the cable can be inserted through the “inverse” CLS. Pull tight. Proceed to other side and repeat. A screw through the side of the inverse will lock it to the beam so that it can’t move about. I actually use a screw in my original design, if you noticed in the book.

Hopefully this design will be something more affordable and something that people can have made locally without a lot of hassle. I prefer the original set-up for ease of assembling “on the fly” and less intrusion to the beam, but this is an option for you to consider.

Thanks for stopping by! Be sure to check out the book. It includes some cool ideas that apply to other projects, like how to put a really tall post into a deep hole when you aren’t that tall. 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 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

Zip Line!

When I created this blog I promised something would be posted about zip lines aka ziplines. So, here is the first post about all that.

I visited Costa Rica a few years ago and rode on my first wild zip line. That is, it wasn’t a carnival ride or a rope in someone’s yard. It was down a mountain, in the rainforest, and attached to trees, and pretty freeking amazing. When I got home, my spousal unit said, “You’re going to build one of these, aren’t you?” And I said “Of course!”

I set up a a 420 foot zipline on our property. So far I’ve built the tower and ladder, which is a great treehouse getaway:
ZipBuilder

and I built the lower platform:
ZipLanding

and have installed and tested a temporary (smaller diameter) cable. Here is a fun movie of a sandbag wearing my spousal unit’s blouse slamming into the platform:
Zip Line Test with Fake Spousal Unit Getting Seriously Hurt.

While I was pleased at the 32 MPH speed, this test proved that I need to raise the final cable a bit on the tower tree and on the base post as well when I finalize everything.

There are more photos of the building process on my website: Zipline Photos.

The full size cable is up on the hill ready to install but at this point the path down the mountain needs to be cleared (again) before we continue with the construction. For now, I’m working to finish building our house, so this project is on hold for the moment, but will be completed at some point.

What’s funny about this particular project is that when my spousal unit’s elderly relative heard about it, she wrote us out of her will, telling another relative in private (which of course didn’t remain private) that “Building a zip line is pure Tom Foolery! These people have time to burn!” Well, building a zip line is what I do on the weekends rather than watch football. Just don’t call me Tom.

Thanks for stopping by! Be sure to check out my book about building a bridge. It includes some cool ideas that apply to other projects, like how to put a really tall post into a deep hole when you aren’t that tall. 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 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

Can You Dig It?

There were a total of eight holes to be dug for my small cable suspension bridge. They would house four posts and four dead-men (more on dead-men later). The posts were going to be three feet into the ground, but the plan also called for concrete punch pads, so that meant another ten or so inches of hole to dig.

Why a punch pad? A two-foot wide pad of concrete would help to distribute the weight. The posts were bearing much of the weight of the loads. If I just set the post to the ground, the point load might exceed the bearing capacity of the native soil. If there were a big load on the deck, such as a tree that fell on it, the posts could sink into the ground and cause the bridge deck to be lower over the creek. The point load would be far less than if I distributed the weight in a concrete pad.

I figured 1,500 pounds of bearing load per square foot, so three square feet of concrete pad totaling twelve square feet would allow for a pretty hefty weight capacity. I also planned to make the pads about ten inches thick to give them more tensile strength in order to prevent cracking.

Meanwhile, the plan also called for backfill with 3/4 minus gravel, which would be tamped at regular intervals. The gravel would help to protect the posts from insects and water damage, plus it would help hold the posts more firmly in the ground.

OvalHole

To allow for the backfill, the holes needed to be about two feet in diameter. I set to work and soon discovered that digging a hole that was only two feet wide was awkward to work in. After digging the first 18 inches or so, I connected the two post holes, creating an oval around both of them. It was easier to work in, plus, there would be that much more gravel backfill to add more security to the posts.

I always use ready-mix concrete for small projects like these punch pads. The bags have the correct ratio of sand, gravel, and cement in them, so all one has to add is water and an energetic mixer. Some people don’t even bother to mix, but if you want a strong concrete base, you will want to mix it well.

I needed to mix about three cubic feet of concrete for each punch pad, which amounted to six bags of 60 lb. ready mix per hole. Rebar and drift pins were already set into place. The pins stuck up about two inches from the center once the pads were poured. When the punch pads were cured, it was time to set the posts.

CoveredHole-Funny
I kept the holes covered until the posts and gravel were done. We didn’t need a critter or human falling in there.

Be sure to check out my new book, Building a Small Cable Suspension Bridge. There is a link to purchase it here: http://www.wildcatman.com. There is also a link there to contact me.

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

To Gallop or Not to Gallop?

Another consideration when building my suspension bridge was harmonic resonance. While harmonic resonance was not completely to blame for the failure of Galloping Gertie (http://www.wsdot.wa.gov/tnbhistory/Connections/connections3.htm), I decided to address the problem because it could be an issue. After all, the bridge was bound to be a little bouncy, so why make it worse?

All objects have a frequency or a set of frequencies with which they will naturally resonate when disturbed in some way – be it plucking a guitar or stepping on a bridge deck. Each of these natural frequencies is associated with a wave pattern. When the object resonates at one of its natural frequencies, it vibrates in a manner so that a standing wave is formed within the confines of the object.

HarmonicSeriesDrawing

A set of standing waves, in a “container” of a specific length. This set of waves is called a harmonic series, the grey dots are the nodes. (illustration by Marvin Denmark)

Harmonic resonance refers to the multiples of the strongest resonance of, in this case, a mechanical system. Resonance is the tendency of the system to absorb more energy when its oscillations match the system’s natural frequency of vibration. Resonance can cause swaying motions, but there are ways to reduce those motions. Since this was just an 80-foot long bridge, resonance was not a serious concern, but I decided to use one trick to prevent it from waving “on its nodes.”

In the case of resonance for this bridge, the waves will bounce back and forth between two boundaries: the posts. Nodes are always at equally spaced intervals where the wave amplitude (motion) is zero. The points where the cable connects to the post are two nodes. There is a possibility for one node in the middle, at third points, at quarter points, and so on, as seen in the previous drawing showing standing waves. The more excitation, the more nodes will potentially form.

I decided to space the suspenders so that they were not positioned on the nodes of the bridge span. I determined the exact points on my string model, then just moved the suspenders a couple of inches over to avoid the nodes.

NodesStringers

Drawing of the suspenders with measurements. (illustration by Marvin Denmark)

I assembled everything on dry land according to my plan so that I could tweek things fairly easily. The final product doesn’t bounce much, unless two people and a dog are all walking on it at the same time. Or maybe a bobcat. But at least my bridge doesn’t gallop!

StringersPasture2 Sunplus

You can read more in my book, Building a Small Cable Suspension Bridge. There is a link to purchase it on my website: http://www.wildcatman.com.

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

 

The Catenary Curve on a Homemade Suspension Bridge

arch

The St. Louis Gateway Arch is built in the shape of an upside-down catenary curve.
Photo by HHsu/Shutterstock.com

An integral part of my suspension bridge was the catenary curve of the two main cables. The catenary curve is the curve assumed by a heavy uniform flexible cord hanging freely from two points. The curve of the main cables would evenly distribute the weight of the deck among all of the suspenders, which are the cables that run from the main cables to the deck assembly. The curve also determines the relative lengths of all the suspenders.

M-StyleIllustration

The catenary is the curve formed by a perfectly flexible inextensible chain of uniform density that is hanging from two supports

I had to figure out how the cables were going to curve in the most efficient and eye-pleasing manner to support the structure while still looking cool. The factors to consider for the curve can get complicated. There truly is a mathematical formula to get the “ideal” weight load distribution in a curve. Engineers call that ideal an “equal resistance catenary.” This is when the cable’s resistance to breaking is equal along its entire length. To accomplish this bit of perfection, we consulted my wife’s brother the engineer. He was a great help in getting a better understanding of the physics behind catenary curves.

But, being a hands-on guy, I created a scale model for determining the catenary curve for my bridge design. This way, I could draw up the posts and deck at the pre-determined height and length, respectively, then set the catenary curve to my liking by using pins and heavy nylon fishing line. I could double-check using the math, but it was nice to have something physical that could be adjusted.

curve2

This approach at determining the catenary, though being less precise than the mathematical one, can be used to solve undetermined configurations.

You can read more about the catenary curve and another aspect I had to consider, harmonic resonance, in my book, Building a Small Cable Suspension Bridge. There is a link to purchase it on my website: http://www.wildcatman.com.

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