How to Build a Bridge Tower

Bridge towers provide support to suspension bridges.

A bridge tower is an integral part of suspension and cable-stayed bridge construction. Smaller bridges may only have one tower on each side of the span, while longer bridges have towers at intervals along the span. In a suspension bridge, the supporting cables extend over the tops of the towers and down to an anchorage on either end. In a cable-stayed bridge design, the cables extend from the towers at varying angles to the bridge deck. For a small suspension or cable-stayed bridge, the towers can be a fairly simple arrangement of poles planted securely in the ground and braced.

Things You'll Need

• Shovel
• 10 bags of pre-mix concrete, 60 pounds each
• 8 pieces of metal reinforcement bar (rebar), about 24 inches long
• 2 pieces of metal rebar, about 10 inches long
• 2 wooden posts, 6-by-6 inch, length determined by bridge design
• Drill and 1/2-inch bit
• Hammer
• Nails, about 3 inches long
• 8 boards, 2-by-4 inch, lengths determined by bridge design
• 4 wooden stakes
• 4 bolts, nuts and washers, 1/2-inch, 8 inches long
• 2 bolts, nuts and washers, 1/2-inch, 10 inches long
• 3/4-inch minus gravel
• Tamper

Instructions

Excavation and Concrete

1. Dig two holes 4 feet deep and 3 feet wide. Space them according to the bridge design.

2. Mix the concrete according to the directions on the bag and pour approximately 4 inches of concrete in each hole.

3. Place two pieces of 24-inch rebar on the concrete, about 12 inches apart. Place two pieces of 24-inch rebar at a 90 degree angle on top of the first two pieces. You should have a tic-tac-toe pattern.

4. Pour 4 inches of concrete on top of the rebar.

5. Repeat the process for the second hole.

6. When the concrete starts to set up, place a rebar piece straight up in the center of each pad so that about 4 inches is exposed. Let the concrete set up for at least eight hours.

Tower Assembly

1. Cut the posts to the desired length for the tower. Allow 3 1/2 feet to be planted in the ground.

2. Drill a 4-inch deep hole in the middle of one end of each post.

3. Place a post over the rebar piece in the concrete.

4. Nail a 2-by-4 inch board about two-thirds of the way up the post. Pound a stake in the ground so that you can nail the other end of the board to it, making sure the post is standing perpendicular to the ground.

5. Nail a board a quarter of the way around the post in the same way. Repeat the process on the other post.

6. Bolt two 2-by-4 inch boards on either side of the posts using the 10-inch bolts, about 8 inches from the tops of the posts. Allow enough room for the cable assembly.

7. Set a 2-by-4 inch board under top board, slanting it about 30 degrees to the other post. Bolt it into place. Bolt a board on the opposite side of the posts in an opposing direction, creating an X pattern.

8. Fill the hole with gravel. Tamp the gravel about every six inches.

9. Remove the nailed boards and stakes.

Tips & Warnings

• Consult an expert for tips on bridge design and construction before starting.

• Use a level to stand the posts vertically before attaching the bracing assembly.

• Cover the holes with plywood or set up a warning fence until construction is complete.

References

• Marvin Denmark; Building Contractor; Oregon
• Subcommittee on Public Affairs: Bridge Inspection Definitions
• Drake Ready Mix Inc.: Concrete Calculator
• NOVA Online: Suspension Bridges

Resources

• "Suspension Bridges for Mountain Warfare"; War Department; 1944