How do I Build a Bridge in the Wild?

A rope and plank suspension bridge allows foot traffic across a river.

Building a bridge in the wild requires the use of simple materials that can be easily transported to a remote area. One type of bridge that works well is a suspension bridge. It is constructed between anchors on either side of the area spanned. Solid anchors are key to the stability of a suspension bridge. When available, sturdy trees can serve as anchors. For a permanent bridge, anchors are constructed of posts placed deep in the ground. Steel cable is preferable to manila rope for a bridge that will receive a lot of traffic.

Things You'll Need

• 4 12-foot long 4-by-6 posts
• 4 4-foot long 4-by-4 posts
• Post hole digger
• Steel cable
• 20 mm manila rope
• 2-by-6 lumber
• 2-by-4 lumber
• Manual drill
• Nails
• Hammer
• 4 ground anchor bolts
• 4 eye bolts
• 8 turnbuckles

Instructions

Preparation

1. Cut the 2-by-6 lumber into 3-foot lengths. Cut enough boards of this length to create a platform to span the area to be bridged.

2. Drill 2 holes in each narrow end of the board. Holes should be 3 inches from the outside edge of the narrow end of the board and 1 1/2 inches from each long side of the board.

3. Drill holes in the center of each 4-by-6 post 6 inches from top of post, or large enough to allow free passage of steel cable. Also drill holes 2 feet 4 inches from the bottom of the post and insert eye bolts. Drill additional holes 3 1/2 feet from the bottom hole large enough to allow free passage of 20 mm manila rope.

4. Cut two cross members 3-feet long out of the 2-by-6 lumber. Lay two 4-by-6 posts on the ground, spacing the posts 3 feet apart from outside edge to outside edge. Lay one cross member across the posts, covering the holes. Drill matching holes in the cross member.

5. Cut 4 lengths of 2-by-4 lumber 4 feet long. Cut 2 lengths of 2-by-4 lumber 3 feet long. This lumber is used to construct the base and arms of the anchor platform.

6. Cut 5 lengths of 2-by-6 lumber each 4 feet long. These boards are used for the anchor platform base.

Construct Towers

1. Dig two 2-foot deep holes for anchor posts on each side of the area to be spanned. Place the posts with the pre-drilled holes facing the posts on the opposite side of the expanse.

2. Nail a cross member in place at the top of one set of 4-by-6 posts, matching the holes in the posts.

3. Build an anchor platform. Nail a 3-foot long 2-by-4 board to the 4-by-6 posts, 2 inches below the lowest drilled hole in the posts, forming a cross member.

4. Dig 2 parallel holes approximately 12-inches deep and 3 feet apart. Place a 4-by-4 post in each hole, leveling the top of the post and making certain the outside edge of the post is 4 feet from the anchor posts.

5. Nail a 3-foot long 2-by-4 near the bottom of the 4-by-4 posts, creating a cross member. Make certain the cross member is level with the bottom cross member attached to the anchor posts. Nail 2-by-4 arms from 4-by-4 posts to anchor the posts. Nail five 2-by-6 platform boards to the bottom cross members, creating an entrance platform. Space the boards evenly apart.

String Bridge

1. 

   Pull the cable tight with a turnbuckle.

   Attach a steel cable to a ground anchor bolt placed approximately 10 feet directly behind the anchor post. Run cable through the top hole in one of the anchor posts. Extend the cable to the top of the matching anchor post on the other side of the expanse. Tie it down with a ground anchor bolt. Follow the instructions of the bolt manufacturer. Repeat for the other top cable.

2. Attach steel wire to an eye-bolt at the bottom of one of the posts. Attach a turnbuckle and run the cable to the opposite post. Fasten it in the same manner. Repeat for the other two posts.

3. Lash 3-foot long 2-by-4 boards to the two bottom steel cables, creating a walkway. Tie a double overhand stopper knot in one end of the 20 mm manila rope. Do this by making a loop and passing the end of the rope through it, then passing it through again and tightening the knot. Run the rope through the remaining hole in one of the posts. Pull across to the opposite post and tie another stopper knot. This rope creates a handrail.

4. Tie manila rope to the bottom steel cable at one end of one side, then run it up and over the top steel cable lacing rope in this fashion every 3 to 4 feet. Fasten at the opposite side. Lace rope from the bottom steel cable to the handrail for added safety.

Tips & Warnings

• For a wider bridge, you will need more than two base logs. If you want a bridge that's 3 to 4 feet wide, you will need 3 base logs, and one additional log for every additional 2 feet of width. Then lay the sticks in a serrated fashion across the logs. In other words, lay one on one "space between logs" and the one on an adjacent "space between logs", and then back to the first space, and so on. Once all the sticks are laid, you can continue building the bridge as usual. However, if your bridge is a wide version, you can stack up more dirt, such as 2 feet thick.

• During the paving process, add large, flat, heavy rocks. This will reduce the chance of your bridge being washed away. But, because of the increased weight, make it weaker. (Your bridge will easily be strong enough if it's a pedestrian only bridge.)

• Do NOT attempt to drive a car over one of these bridges! Although a VERY VERY well built bridge might hold a small car, it probably won't and it's not worth the risk. Although, if you REALLY make sure that the bridge you built is very strong, you can probably drive a lawn mower or similar small vehicle over it, but do it at your own risk! (Note: If you are trying to get your bridge to be as strong as possible, then only put a thin layer of dirt on, as dirt is very heavy!)

• Never "depend" on your bridge! Unless you use treated wood as the base timbers and cross-sticks, your bridge will start to rot, and get weaker over time.

References

• The Treehouse Guide: Bridges
• Design Technology: Suspension Bridges: Design Technology
• Education Center Online: How to Build a Suspension Bridge
• Machovec: Rope Bridges
• Pioneering Projects: Scout Engineering

Resources

• Ohio Department of Transportation: Bridge Terms Definitions
• U.S. Forest Service; "Packable Trail Bridges"; John Kattell
• Waagmeester; "Special Project: The Postman"; Steve Waagmeester