Sky-N-Sun - Bridge Design

Suspension Bridge Design Considerations and Math
Deck

The deck provides the surface to walk on. The fire safety rules in most states require walkways to be at least 3 feet wide, and I wanted to have room to carry groceries with comfort. The old bridge used 2x4 boards as joists and I planned to do about the same, so the obvious length of the joists was half of 8 feet, or 48 inches. The joists would need to extend beyond the width of the deck by three inches to leave room to attach the suspenders, which left 42 inches (48-3-3=42) for the deck.

Looking through the sale flyer for the local lumber yard I discovered they carried pressure treated lumber for decking projects. I could use 12 foot 1.25x6 boards for the decking. Seven 6 inch wide boards would fit in the 42 inches available for decking. Actually, lumber is sold by the rough cut dimensions, so the boards are only 5.75 inches wide. Even with 0.125 inch spacing between boards for drainage of rain or melting snow, the deck width is only 41 inches. On the other hand, boards are usually an inch longer than the advertised length, so I ended up with a 41 inch deck and almost 4 inches of joist on each side for attaching the suspenders.

You may note in the graphic above that I alternated 2x4 and 2x6 joists. I wanted the 2x6 size where decking boards would join so that I would have enough surface area to stagger the placement of the screws fastening the decking to the joist. So 2x6 joists are every 4 feet, starting at the center of the bridge.

I did not want the decking boards to noticably bow under the weight of people crossing the bridge, so I chose the 1.25x6 decking boards instead of the 1x6 boards also available. Even with the 1.25x6 boards, the 4 foot span between joists will allow the boards to bow, so lighter weight 2x4 joists are placed between the main 2x6 joists resulting in a joist to joist span of 2 feet.

To calculate the weight of the lumber in the deck, multiply the width times the thickness times the length of each piece to get cubic inches. Then divide by 1728 to convert to cubic feet. Finally multiply by the weight per cubic foot.

Knowing that the lumber I used was white pine, and that the pressure treatment process nearly doubles the weight, I used 50 pounds per cubic foot in the formula.

Wood Type	Lbs/Cubic Foot
white pine	26
northern yellow pine	34
red oak	44
southern yellow pine	45
live oak	59

Wood Weight = ( width * thickness * length ) / 1728.0 * 50.0