Tag Archives: Cross

Single Lock Bridge

CLICK HERE FOR COLOR PHOTOS, COMMENTARY, AND SOME FURTHER GUIDELINES ON BUILDING A SINGLE LOCK BRIDGE.

Single Lock Bridge Photo Scanned from 1967 Field Book
Single Lock Bridge Photo Scanned from 1967 Field Book

Link to: Older Pamphlet InfoThe 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.

Drawing 1: Trestle Schematic
Drawing 1: Trestle Schematic

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.)

Interlocking Trestles
Drawing 2: Interlocking Trestles

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.

Drawing 3: Fully Assembled Single Lock Bridge
Drawing 3: Fully Assembled Single Lock Bridge

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

CLICK HERE FOR COLOR PHOTOS, COMMENTARY, AND SOME FURTHER GUIDELINES ON BUILDING A SINGLE LOCK BRIDGE.

Single Trestle Bridge

SingleLockVertical
Summer Camp: Pioneering Merit Badge Class

CLICK HERE FOR COLOR PHOTOS AND COMMENTARY.

This simple crossing bridge uses only a single trestle and two walkways. The legs of the trestle are extended up above the walkway to provide a way to attach a handrail. The length of the spars listed for the walkways and trestle will be enough to build a bridge that will span a creek or ravine that’s up to 4 feet deep and 18 inches wide.

This project can be broken into three subassemblies: the trestle, the two walkways, and the four light spars for handrails.

Trestle. Begin by building the trestle. The legs for the trestle should be spars that are about 3 inches in  diameter and 8 to 10 feet long. When choosing these spars, take into account the depth of the creek you’re crossing.

The distance from the base of the legs to the top ledger (transom) on the trestle should be about 1 foot higher than the level of the banks of the creek. This will allow the walkways to slant up. Then allow an additional 4 feet in height on the legs from the top ledger up to the top of the legs for attaching the handrail.

The top ledger of the trestle should be about 3″ in diameter since it also acts as the transom and carries all the weight of the walkways and the person using it. The bottom ledger can be smaller: a 2 inch diameter spar will work here.

Drawing 1: Trestle Heeled in with one walkway positioned.
Drawing 1: Trestle Heeled in with one walkway positioned.

The trestle is assembled with Square Lashings to hold the ledgers and the ends of the cross braces to the legs. The center of the cross braces is lashed together with a Diagonal Lashing.

Walkways. The two walkways are assembled as separate sub assemblies. (Refer to Bridge Walkways.) Be sure to make the cross spar at the end of the walkway long enough to attach to both the stakes and the handrails without getting in the passageway.

Assembly. To assemble the bridge, set the trestle in the center of the creek. Heel in the bottoms of the trestle legs by setting them in holes approximately 4 to 6 inches deep (see Drawing 1). This will prevent the trestle from shifting, and is also a way to level the transom spar as the trestle is set in place so that the walkways are level.

Next, put the walkways in position from both sides and lash the walkways’ underspars to the transom (top ledger) of the trestle. Then drive stakes at the other end of the walkways. Lash the ends of the cross spars on the walkways to the stakes.

Handrails. Finally, handrails are provided to help those crossing the bridge and also add strength to the structure of the bridge. When the handrails are added, they form triangles with the walkway and the trestle leg. These triangles produce a strong structure that prevents the bridge from racking. Lash the handrails to the top of the trestle legs and to the stakes with simple Strop Lashings (see Drawing 2).

single_trestle_bridge
Drawing 2: view of assembled bridge with handrails.

List of Materials for a Single Trestle Bridge

  • 2    3-inch x 8 or 10-foot trestle legs
  • 1    3-inch x 4 -foot trestle top ledger (transom)
  • 1    2-inch x 4-foot trestle bottom ledger
  • 2    2-inch x 6-foot cross braces
  • 4    3-inch x 10-foot walkway lateral spars
  • 12  2-inch x 3-foot walkway cross spars
  • 4    2-inch x 3-1/2-foot walkway cross spars
  • 2    2-inch x 10″ x 10-foot walkway planks
  • 4    2-1/2-inch x 12-foot handrails
  • 4    stakes