Building a Small Cable Suspension Bridge - the Basics Plus a Video Demo

We built this bridge and wrote a “how we did it” book about the process a few years ago. I thought it would be fun to share the basics of this design as an Instructable for people who have enough skill to be able to take the information and work with it. And as we do in our book, we recommend having your specific design approved by an engineer just to be on the safe side.

So here you go! We also have a blog with a fairly recent materials price list, hints and tips, and other projects. Link is at the end of this Instructable.

PS Thank you all so much for the votes in the Outdoors Contest! We are a finalist as of October 6, 2017, and are delighted!

Step 1: The Design

Here is our design for an 80’ long walking bridge that spans our creek and is set back far enough for a serious flood. The challenge was to do as much as possible without the need of machinery or swimming. Even if your creek isn’t that wide, consider flood stage and go from there.

The drawing shows everything used to build the bridge other than the cable locking system, which is pictured below and in the video.

Step 2: Install Four Posts

After clearing a site, we installed four posts.

Step 3: The Dead Men...

Dead men are a vital part of this design (the foundation in the ground). Engineered based on the load, they were were buried and secured before the suspension cables were brought in.

Step 4: Build the Cable System

The suspension cables and stringers were cut and assembled off-site. Because of the Golden Gate style, the stringers each had to be a specific height to keep the deck level, actually, slightly crowned.

Step 5: Consider Resonance

The stringers were also spaced to deal with harmonic resonance. Though keep in mind, it’s a suspension bridge and it will move when you walk on it. Whee!

Step 6: Span the Cables and Stringers

Use of a truck on one side of the creek transported the cables across the creek with the stringers rolled up so they didn't tangle. The cables were secured into place.

Step 7: Add the Support Beams

We used our patented Cable Locking System to attach the beams. This system allows for assembly “on the fly” and easy repair/replacement of the beams (joists) if needed. Temporary planks were set out as we worked our way across the span.

The movie shows exactly how the cable locking system works. The magic of the system is the reversed keyhole arrangements between the two components. To note, the email and website given at the end are no more. YouTube doesn't let us edit videos.

Step 8: Add the Decking

Once the beams were in place, we added decking.

Step 9: Now We Can Get to the Other Side and Stay Dry!

This bridge has been our access across the creek to get to our spring for many years, and is holding up great through several major floods. A couple of trees have hit it, but it wasn’t phased. We recently adjusted the turnbuckles on the dead men cables to pull up the slack in the deck from tree hits.

Step 10: For More Information...

Note that all photos and illustrations are from our book, Building a Small Cable Suspension Bridge with the Cable Locking System. The book is easily found on Amazon.com if you look it up, and our blog with more info is wildcatman.wordpress.com. Hopefully this is enough information to get you inspired!

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33 Comments

rond85

Question 7 months ago on Step 2

What is cost for your bridge? Our park needs a 100 ft pedestrian bridge but get quotes in millions $!!
This is for City of Goleta. Ca. USA.
Thanks.

Gsobeck

Question 16 days ago on Step 9

Hello,
I purchased and read your book, I enjoyed it very much! I’m interested in building a similar bridge but was hoping you might share some info on the calculations and assumptions you used as mine will have different parameters for length and load capacity.
Thanks
Gregg

Wildcat Man and Robin

Answer 13 days ago

Hello Gregg,

I wish I could be more helpful. As I say in the book and will repeat, I am not an engineer. I will say calculating stresses in a cable is not as straight forward as those for a rigid wood structure.

Having said that, I know enough to do things/projects that I do, taking into account the risks I am willing to take for myself and my family. But, I don’t know enough to advise anyone else on their project. Not being an engineer I would be in too deep, and I wouldn’t do that to someone.

What I can say (and what I stated in the book) is what assumptions of weight (force) were going to be put on the bridge. And you will want to do this anyway, for yourself and for what I will suggest below.

My bridge is a fairly modest proposal and for me would only be used occasionally by few people. So I assumed maybe five people, giving them a weight of 200 pounds each. That gives me 1,000 pounds of dynamic load. If you plan for equipment to cross then you have to account for that. That is, you have to account for everything that will be on the bridge, either separately or all at one time — keep that in mind.

Next you need to total all of the weight of the bridge itself. This is principally the deck portion. What’s on the opposite side of the posts, with the posts, don’t go into the calculations (but they are included in the total for weight bearing support). So you are totaling every item/material that makes up the “deck” bridge. I don’t recall the number but I’m guessing around a thousand pounds. That is the static load. I just analyzed each component to see if it could carry it’s share.

This is my suggestion: draw out your bridge with accurate dimensions with all of the parts and components, sized (labeled/called out). Do a plan view looking down on the bridge, again showing sizes. Then do an elevation view (side view) making sure all items are identified: make a list and number or name the item and point with arrow. Submit these plans to a structural engineer. Have them analyze your proposal and make a list of modifications. Then redraw and resubmit.

Build bridges,
Marvin

AzadT2

Question 7 months ago on Introduction

Hello
Do u sell the cable lock system? Or if not where can I buy those?
Want to build a suspension bridge on our property here in WA
Thx

jonnyknz2021

Question 1 year ago

What size/width steel cable did you use?
(I apoloize if I missed somewhere in your instructions)

chapelparkdrive

Question 2 years ago

im trying to make a suspension mini bridge for school to have a competition to see which bridge can holod the most weight how do i make a stable good suspension bridge?