The following text is by Adolph E. Peschke as presented in the 1998 printing of the 1993 edition of the Pioneering Merit Badge Pamphlet:
The single lock bridge shown here is a well-established and basic design. The list of spars shown for this project should build a bridge to span a creek or ravine approximately 4 feet deep and 18 feet from bank to bank.
Trestles. The bridge consists of two trestles and two walkways. Begin by building the two trestles as subassemblies. Adjust the length of the spars for the trestle so that when they are placed in the creek, as shown in Drawing 2, the tops of the ledgers will be about 1 foot above the level of the banks of the creek. This will give a comfortable slant to the walkways.
When constructing the two trestles, build only one trestle first. Then as the second trestle is being built, make sure that the legs are narrower at the top and fit between the legs of the first trestle (see Drawing 1).
Walkways. Next, the two walkways are constructed as subassemblies. Each walkway consists of two lateral spars. six cross spars, and two longer cross spars. One of these two longer cross spars is used as an underspar at the end of the walkway that is attached to the transom. The other longer cross spar is used to attach to the stakes. (Refer to Bridge Walkways.)
Assembly. After building the trestles and walkways, take them to the assembly site (the creek or ravine). Place the trestles in the center of the creek so that the tops of the trestles are interlocked (see Drawing 2). Then lift a 3-inch diameter transom spar to fit on top of the interlocked trestle legs. Now, heel in the bases of the legs in holes 4 to 6 inches deep. As you’re heeling in the legs, level the transom spar so that the walkways don’t slant when they’re added.
Next, the two walkways are put into position (see Drawing 3). Lash the underspars on the walkways to the transom spar with Strop Lashings at three points. Finally, the cross spars at the ends of the walkways are lashed to the stakes.
By lashing the walkways to the transom spar and lashing the ends of the walkways to the stakes, you make a complete walkway unit that will prevent movement and provide a sturdy bridge deck.
List of Materials for a Single Lock Bridge
- four 3-inch x 6-foot trestle legs
- four 2-1/2-inch x 4-foot trestle ledgers
- one 3-inch x 4-foot trestle transom
- four 2-inch x 6-foot cross braces
- four 3-inch x 10-foot walkway lateral spars
- twelve 2-inch x 3-foot walkway cross spars
- four 2-inch x 3-1/2-foot walkway cross spars
- two 2-inch x 10-inch x 10-foot walkway planks
- four stakes
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