A Spiral Staircase Project

Here’s a staircase that I built for our house back in 1981 or thereabouts. It was about 9 feet 6 inches from floor to floor and I wanted about an 8 inch rise. That meant 12 steps or treads with the second floor representing step 13.


I figured out the location of the landings and designed my stairway so it would wrap about 3/4 of the way around the column. I built the column using 16 2x4s that I cut at 22-1/2 degree angles with an outward face equal to 1-1/2 inches or the width of a 2×4.

The wedges were glued together and strapped. I used a block of wood and a hammer to get the pieces flush with each other (see inset). Next were the supports and stairs.

I used a skill saw, drill, chisel and rasp to create holes through the column. I added blocks to keep them from slipping back. The stairs and rail posts were attached to the supports. I built the rail out of thin strips of redwood called bender board. I glued them together and sanded. I put one on the bottom as well, to stabilize the staircase and also look cool.

I never quite finished the project. In fact, about five years later, I changed my mind about the location of the landings and reversed everything. It wasn’t as easy as I thought it would be, and I wound up having to cut the column and then add a new piece in the middle, which you can see in the top photo.

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.

Patent for Sale!

We’ve been, off and on, trying to market the patent for my cable locking system. If you don’t know what that is, please visit http://www.wildcatman.com for information about the integral part of my suspension bridge. We had minor interest from Bridges to Prosperity (http://www.bridgestoprosperity.org/), but that’s about it so far. So, if you know anyone who wants to manufacture a cool cable locking system and send me a percentage of the profits, let me know!

Here’s a video Robin created about the system – http://www.youtube.com/watch?v=cLXrzC9K5wQ

Here’s the history of how I wound up with a patent for sale. It all started when I decided to apply for a utility patent. With no knowledge or experience in patents, Robin and I hired a patent attorney to do the initial search. They determined that it had never been done and, it also passed the test for being “unobvious” and therefore was patentable, in their expert opinion.

We knew that hiring an attorney to actually write the patent would cost more money than we had, but when we talked to these people on the phone they assured us that we could write it ourselves and do the drawings and they would charge a much smaller fee to help from that point on.

However, once the search was done, they wanted another $3-5,000 to pursue the patent for us. They denied ever telling us they would simply assist in some capacity. So we told them no thanks and bought David Pressman’s book, Patent it Yourself, (http://www.amazon.com/Patent-It-Yourself-Step-Step/dp/1413317197) and got to work. It took us a full month to get it all done to the best of our knowledge and understanding of the process. We wrote a check to the USPTO and sent them our specs, abstract, drawings, and claims – the main components of a patent application. Then we waited about 18 months (which they promise is typical). We did file online so could check in to see if any action had been done.

The eighteen months passed and our patent was rejected, with a “non- final” action. Our $500.00 patent search had failed to turn up another patent that the USPTO found and decided was too much like my invention. I looked at it and said no way, so the games began. We answered with revised claims, and it was rejected again. We filed a continued patent examination (get David’s book if you want more information) and tried again.

We got some help from the USPTO help-line who assured us that patent examiners are told to help people who weren’t using a patent attorney. That is, if we were lucky enough to get someone who was sympathetic. While our examiner did appear to be on our side, her letters always suggested that we needed to hire an attorney and she didn’t seem all that sympathetic. Considering that the attorney we did hire blew it on the patent search, we weren’t willing to spend the money even if we had it. I called the examiner and we had a few long conversations. It finally all came down to including the cable in the claims. Without it, my system did not lock.

Finally the examiner agreed to show our new claims to her supervisor. We waited some more, then one morning there was a message on our answering machine from the examiner. Her supervisor had told her she should not have rejected our most recent claim. She offered to rewrite it to include some things she felt needed to be there. We still have that recording.


On November 15, 2010, the cable locking system was allowed for a U.S. patent. It took almost three years and about $2,000. An attorney would have cost far more. For instance, had we hired the one who did the search, they would have charged $3-5,000 for the initial patent application. Then when it was rejected for the same reasons ours was rejected, they would have dinged us another several thousand to rewrite it and argue with the examiner. They all have the disclaimer that they can’t guarantee the initial patent search is thorough. All we can say about the entire patent process for the do-it-yourselfer is, “Good luck, and don’t give up!”

You can read more about all this 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.

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.


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.


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


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.


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.


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.

Welcome to Wildcat Man’s Blog!


I’ve been a designer and carpenter for 40+ years and have constructed or remodeled just about everything from chicken houses to restaurants. For me, the challenge of building is figuring out how to create something in an efficient, practical way, while making it esthetically pleasing. This blog will highlight a lot of techniques and/or designs that I have come up with or observed over the years.

The photo above is me working on my 80 foot long suspension bridge. I created a “cable locking system” for this design (U.S. Patent No. 7,866,909), and I published a book:

Building a Small Cable Suspension Bridge With the Cable Locking System: http://www.wildcatman.com.

Thanks for stopping by! I’ll be posting more soon.