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

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

About: I'm a writer and illustrator of books for children and Marvin is a craftsman, carpenter, and retired building contractor. We build various things for our Funny Farm and I write about them.

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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    20 Discussions

    0
    JeffH352
    JeffH352

    Question 14 days ago on Step 4

    What keeps the stringers from sliding downward?

    0
    chip321
    chip321

    Question 4 months ago on Step 1

    would you be interested in designing a suspension bridge that spans 200' and can carry a 4 x 4 utility vehicle across.
    sincerely
    Chip

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Answer 4 months ago

    Thanks for asking, and sorry, no. I could design a bridge like this for us but not for someone else, since I am not a licensed engineer. Good luck! Maybe check with Bridges to Prosperity and see if they'll share a design with you?

    0
    svcamp4321
    svcamp4321

    5 months ago

    Here’s a picture of the bridge we built, will give u some idea and incorporate some of your own

    64044868-7F38-4138-BE8A-3521360B9908.jpegFDC6F4C8-E6B8-46B0-8372-12FDB7AE44F8.jpeg
    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 5 months ago

    Thanks for posting! Interesting structural components.

    0
    DanielB915
    DanielB915

    11 months ago

    I've read the book and loved it. Thank you for putting this info together. It's really motivating for people who'd like to start a similar undertaking. My question: is the suspended bridge style suitable for bridges with a grade, with two ends at different altitudes? If so, what kind of adjustments should be made to the project to account for the grade?

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 11 months ago

    Thanks for the compliments on our book, and for your question. I don’t know what kind of altitude changes you’re talking about, but if they are minor, then it shouldn’t make any real difference. You can adjust where the bridge deck levels to the posts and then build a ramp or stairs to get up to it from the low side.

    If you’re talking about a slope on the bridge deck (because cutting the high grade or ramping up to the low side isn’t an option) then the functionality of the bridge using the parts I employed won’t really be compromised because it’s a small bridge not meant for heavy loads. That said, I would however be concerned about safety on a bridge that isn’t level - not only the precarious feeling it could illicit in the person walking on it, it could be slippery.

    0
    ameliawildstar
    ameliawildstar

    2 years ago

    This is so cool!!! I posted a suspension bridge made out out of popsicle sticks but making a real one...Wow

    0
    DavidC1238
    DavidC1238

    2 years ago

    This is truly lovely. I have a creek that needs a bridge just like this. I have a couple questions though.

    1. How were the stringers attached at their tops?

    2. How did you anticipate the exact parabola of the suspended cable so as to precut the stringers to their proper lengths?

    3. Did you nail the collars to the 4x4s to prevent side slippage?

    4. How does one buy the collars?

    Thanks.

    0
    Tinker_001
    Tinker_001

    2 years ago

    Great bridge looks fantastic. On your cable locking system, how are the cables terminated (what tools were used) and what is the breaking strength of that termination. The keyhole idea is ingenious but is there any chance of the end wearing out and slipping through? On your specific bridge and out of curiosity, do you think it would support a riding lawnmower or a bicycle?

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 2 years ago

    Thanks! And thanks for your questions.
    I terminated the 3/16 cable (aircraft cable rated at about 4200# breaking strength) with aluminimum swagged end sleeves. According to some suppliers, a swagged end sleeve will achieve 95% of the breaking strength of the cable it is placed on (95% of 4200# = 3,900#). But not trusting everything I read, I decided to place 2 of them together, one right next to the other; besides they are cheap. The tool is a hand operated crimping tool - I made 3 craimps per sleeve.

    I would suppose there is a chance of the end crimp wearing down. My bridge has been in place for ove ten years and there are no signs of visible wear (as mentioned above, there are two stops to wear down). This is a private bridge with not a great deal of traffic. If this were a public bridge I’d probably use a machine swagged ends on the cables.

    As I have noted above, the breaking strength of the single (one side) cable is about 2 tons. That would be 4 tons for each support beam. The deck is the only limiting factor for a riding mower. I didn’t build a deck that would support it - the 2x wood deck is not strong enough nor is it wide enough. I had considered building the bridge to do just what you asked, but didn’t find a satisfactory or affordable deck material without moving the beam to beam span closer together. I wanted a flexible steel or aluminum deck but fairly light weight for handling and installation. I’m still working on that for a future bridge. But you could take a bicycle across, if you have good balance on a moving bridge! — Marvin

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 2 years ago

    Here's a photo of a swagged end.

    MVC-005F.JPG
    0
    GidonA
    GidonA

    2 years ago

    I have a dream of building a Simple suspension bridge between two neighborhoods in my city in Israel, Maale Adumim, built on two hill slopes separated by a ravine https://s18.postimg.org/t9brtg761/footbridge1.png I see that the longest such bridge in the world was recently completed in Switzerland, not much longer - about 500 meters longer (1620 feet). Is this WAY above your pay grade?

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 2 years ago

    I'm pretty much retired these days, but hopefully you can find someone to help with your project!

    0
    mrwonton
    mrwonton

    3 years ago

    this is spectacular!

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 3 years ago

    Thank you, it was fun to build and even more fun to cross!

    0
    PogueMahone1775
    PogueMahone1775

    3 years ago

    This is awesome! I was hearing Tangerine Dream as I read this...

    0
    Wildcat Man and Robin
    Wildcat Man and Robin

    Reply 3 years ago

    Thanks! I had to look them up, even though I'm a child of the 60s. :-)