trestle – SCOUT PIONEERING

Bridge Walkways

The following text is by Adolph E. Peschke as presented in the 1998 printing of the 1993 edition of the Pioneering Merit Badge Pamphlet:

Bridges are very popular pioneering projects. Essentially, a bridge consists of one or more trestles that support some sort of walkway. In the case of a monkey bridge, the walkway is just a rope that you walk on. But for many other bridges, you can build a walkway from spars that’s easier to walk on than is a monkey bridge.

In the Single Trestle Bridge, the Single Lock Bridge, and the A-Frame Bridge, the same type of walkway can be used. Each walkway can be 10 feet long and consists of two lateral spars and several cross spars. A 10-foot length of 2-inch x 10-inch construction lumber can be added as the plank to walk on.

MAKING A WALKWAY

To make a 10 foot section of walkway, select two spars with a butt diameter of 3-1/2 inches. These spars should be matched in the amount of sag they have when you stand on them with the ends supported above the ground. If one spar sags more than the other, it will make the walkway slant from side to side, making it hard to walk on.

Cross spars. The cross spars for the walkway should be approximately 2 to 2-1/2 inches in diameter and 3 feet long. You will need two additional cross spars that are 3-1/2 feet long for each walkway section. (The longer spars go at each end of the walkway.)

All of the cross spars are lashed to the lateral spars with 1/4-inch manila. Since the lashing is made only to hold the cross spars in position and not support weight, you can use a double strand of binder twine.

If you use binder twine, double it over and twist it a few times before you start the lashing. Make sure you have enough to complete the full lashing with the doubled-over binder twine. Don’t finish the lashing with only one strand if you run short. Instead, tie on more binder twine to complete the lashing.

Each of the cross spars is lashed to the lateral spars with a square lashing, making three wraps and two fraps. The Japanese Mark II is the easiest and quickest to tie.

There are two ways to approach lashing on the cross spars. If you are going to add a plank over the top of the cross spars, you will need a total of eight cross spars for each walkway. That is, six 3-foot cross spars, and two 3-1/2-foot cross spars (see Drawing 1).

Lashing on the Walkway's Underspar — Lashing on the Walkway’s Underspar

Start by lashing one of the 3-1/2-foot cross spars about 6 inches from the butt end of the lateral spars. Place this spar on top of the lateral spars so that the ends of the cross spar extend 3 to 4 inches out over both sides of the lateral spars. This additional length hanging out is used to lash the cross spar to the stakes, which anchors the ends of the walkway in place.

After the first cross spar is lashed in place, add six more 3-foot cross spars every 16 to 18 inches down the length of the lateral spars. The last cross spar should be lashed about 12 inches from the ends of the lateral spars to allow room for the “underspar.”

Underspar. An important feature of this type of walkway is to lash one 3-1/2-foot cross spar to the underside of the lateral spars 6 inches from the end. When the two walkway sections are placed on the trestle(s) to form the bridge, these underspars should contact the transom of the trestle(s). Then the three spars [two underspars on the two walkways and the transom spar of the trestle(s)] are lashed together at three points using a strop lashing (see Drawing 3).

Walkway plank. Before lashing the walkway to the trestle, the walkway plank should be lashed in at least three places using a strop lashing.

To make a strop lashing, use a length of doubled-over binder twine. Reach down and wrap the middle of this length of binder twine under one of the cross spars (see Drawing 2). Then wrap the binder twine over the walkway plank and down under the cross spar at the other side of the plank. Do this two or three times and finish with a square Knot.

If you are going to walk directly on the cross spars (with no plank on top), you will need enough cross spars to make a safe walkway, one that your foot cannot slip through. Start making the walkway as described before by lashing a 3-1/2-foot cross spar at the butt end of the lateral spars. Then lash the 3-foot cross spars about 3-inch apart, using as many cross spars as necessary to go the entire length of the walkway, ending about 1′ from the other end. Finally, add the 3-1/2-foot long underspar.

Anchoring the walkway. After the walkway is assembled, the butt ends are placed on the bank of the creek or ravine. This end is anchored in place by driving stakes in the outside corners formed by the lateral spars and the first (3-1/2′) cross spar. Lash this cross spar of the walkway to the stakes with a strop lashing.

The small ends of the walkway are attached to the trestle to form the bridge. On most bridges, walkways come from both directions to meet at the trestle(s). The ends of the walkways rest on a transom spar of the trestle(s). Then the two underspars of the walkways are lashed to the transom spar at three points with a strop lashing (see Drawing 2).

Drawing 3: Walkways Joined — Drawing 2: Walkways Joined

When the walkways are lashed to the stakes and to the trestle(s), all the walkway sections become joined to form a single unit that is very strong.

If you put together a pioneering kit, take some time to save the matched lateral spars to be used for walkways only.

While the above text describes how to make 10-foot walkways, you can make 8 or 12-foot sections the same way. If you use the longer walkways, be sure to test the strength of the spars before lashing them into a walkway that could be unsafe.

Single Lock Bridge (scoutpioneering.com)
Single Trestle Bridge (scoutpioneering.com)
Single A-Frame Bridge (scoutpioneering.com)

Making a Trestle

Lashing together a Trestle from Bamboo Spars (note the friction tape)

The following text is by Adolph E. Peschke as presented in the 1998 printing of the 1993 edition of the Pioneering Merit Badge Pamphlet:

A trestle is the basic component for building a bridge in a pioneering project. It is used to support the walkways.

The most basic form of a trestle is an H-frame. It consists of two legs, two ledgers, and two cross braces (see figure 125). When building a bridge, the top ledger is also called a transom. This is the part that supports the walkways.

To make an H-frame trestle, the two ledgers are lashed near the top and bottom of the legs and the cross braces are added, lashing them to the legs.

All of the lashing on the H-frame trestle is done with two types of lashings: a square lashing and a diagonal lashing. The ledgers are lashed to the legs with square lashings. Although it might not look like it, the cross braces are also lashed to the legs with square lashings, not a diagonal lashing. A diagonal lashing is used to lash the two cross braces together where they cross in the center.

When setting out to build an H-frame trestle, choose the two spars for legs first. These spars can be most any length, depending on the type and height of the structure you’re building.

To build a basic H-frame, lay the two legs on the ground with the two butt ends of the spars at the same end and even with each other. Then add the ledgers.

Ledgers. The ledgers are spars that are typically 2 to 2-1/2 inches in diameter. They are lashed to the legs with square lashings. Any of the three square lashings (shown in this pamphlet) can be used. The position of the ledgers on the legs will depend on the structure you’re building. There are a couple of general rules to keep in mind.

Assembling the X-Brace Side — This Trestle is One of Several Subassemblies in a Larger Project

First, always keep the legs parallel and the butt ends of the legs even with each other as you’re lashing on the ledgers.* If you don’t, the trestle will stand crooked when you stand it up. As you add the ledgers, they should not stick out too far beyond the legs. You must leave enough room at the ends to tie the lashing, Any more will get in the way.

When using a Traditional Square Lashing or a Modified Square Lashing to tie the ledgers to the legs, be sure the starting clove hitch is placed on the leg so it’s beneath the ledger. When the clove hitch is below the ledger it will support it when the trestle is stood upright. As you tie the lashings, make sure they ar all very tight.

If you use a Japanese Mark II Square Lashing, you can start this lashing with a clove hitch in the middle of the rope to help support the ledger.

Cross braces. Next, the cross braces are added. The cross braces are spars that are usually 2 inches in diameter. They are lashed to the legs in a particular sequence.

First, flip the trestle over and work on the opposite side from the ledgers (see figure 125). Lash one cross brace to the back side of both legs. As mentioned before, use a square lashing (not a diagonal lashing) to attach the ends of the cross braces to the legs.

The second cross brace is added so that the bottom end is on the same side as both ends of the first cross brace. The other end is placed on the front side, the side with the ledgers (see figure 125). This is done so that the cross braces are standing slightly apart. There will be a gap where they cross at the center.

Trestle built with 5' Scout Staves — Trestle built with 5′ Scout Staves

Diagonal Lashing. After the ends of the ledgers and the cross braces are lashed to the legs, stand the trestle up on end. Adjust the trestle so that the legs are parallel. Also check to see that the top ledger is parallel to the ground. If it is not, lower the trestle, untie the lashing, and adjust it.

When the legs are parallel and the top ledger is parallel to the ground, you’re ready to tie the diagonal lashing to the cross braces while the trestle is standing upright. This lashing is very important to the strength of the trestle.

The diagonal lashing creates triangles that are important to stiffen the arrangement of the spars and to keep the trestle from racking. Look around at steel towers, bridges, or buildings being erected and you will see the triangle used in many places for the same reasons as we use it to build a trestle.

When the cross braces are lashed to the legs, there is a slight gap between them where they crossed at the center. A diagonal lashing is used here because it starts out with a timber hitch. The timber hitch pulls the cross braces tightly together. This adds strength to the whole trestle. You have to keep a strain on the lashing rope as you complete the diagonal lashing with three wraps in each direction around the X. Then make two frapping turns between the cross braces to pull the wraps tight. Finally finish by tying another clove hitch on one cross brace.

Once the possibility of racking has been taken care of with the diagonal lashing, the trestle’s vertical legs provide support for a large downward load. Since this is a downward force, also known as a shearing force, the legs don’t have to be very big. In fact, the overall shape of the trestle is an engineered structure that is able to support quite a bit of weight with rather small-diameter spars for legs.

* An exception is building a Single Lock Bridge when the top of one trestle has to fit between the legs of the other.

Single Lock Bridge

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

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.

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.

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).

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

Tag: trestle

Bridge Walkways

Making a Trestle

Single Lock Bridge

Single Trestle Bridge

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