Tag Archives: Lashing (ropework)

Bridge Walkways

Author: Adolph PeschkeThe 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.

Lashing on the Walkway Cross Spars
Lashing on the Walkway Cross Spars

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.

Walkway Section Overview
Drawing 1: Walkway Section Overview

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).

Lashing on the Walkway Plank
Lashing on the Walkway Plank

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.

Driving in a Stake to Anchor Walkway
Driving in a Stake to Anchor Walkway

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.

Making a Trestle

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

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:

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.

Figure 125
Figure 125

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.

 

Lashing INFORMATION

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

We could imagine the first lashing made by man was wrapping a few strips of bark around a stone to hold it to a tree branch to make an ax to hunt and build with. Even today with all our modern ways to hold things together, it is still fascinating to lash sticks or spars together to make a camp gadget or useful structure.

There are still areas in the world where lashing spars (or bamboo poles) is the basic means of building structures. In Scouting, we use the same methods but have replaced strips of bark and vines with natural and synthetic fiber ropes.

The best choice of rope to use for lashing the type of pioneering projects (shown in this pamphlet) is pure manila rope. Therefore, all references to rope used for lashing in this section refer to manila rope.

Yet, sometimes we are faced with a problem—we have to use what is available and economical to get the job done. When making camp gadgets for temporary use, you could use lesser quality, less expensive rope or even binder twine for small projects.

SQUARE LASHINGS ***

The basic type of lashing for most projects is some form of a square lashing. This lashing is used to join two spars together, usually at a right angle, but not always. For example, square lashings are used when building a trestle to join the ledger and header to the legs at right angles. But it is also used to hold the ends of the X bracing to the legs at an angle. (Refer to “Making a Trestle.”)

In this section, three different square lashings are shown: (1) the Traditional Square Lashing, (2) the Modified Square Lashing, and (3) the Japanese Mark II. All three types of square lashings accomplish the same thing by making three wraps and two frapping turns around the spars being held together.

The only difference between these three different square lashings is the type of knot that is used to start and complete the lashing. You may learn that one of these knots is easier to tie, if so, you can stick with the one you are most comfortable with.

In addition to square lashings, you will most likely need to know how to make a diagonal and shear lashings. Some methods of making these types are also shown later in this section.

LASHING TERMS

No matter what kind of lashing you’re making, there are two basic terms you should be familiar with: wraps and fraps. The basic difference between the two terms is that a wrap is made around the spars, while a frap is made around the rope itself.

Wrap. A wrap is a turn made around the two spars to hold the spars tightly together. Usually, three wraps are made to form a Square Lashing. Some other lashings require more wraps.

Frap. A frap is a turn made between the spars. It goes around the wraps to pull the wraps tighter. Usually two frapping turns are made on a lashing.

Good lashings are not made in a hurry. Each wrap must be made with a strain on the rope. Frapping turns should be pulled up as tightly as possible before the final knot is tied.

ROPE LENGTH

When you set out to make a lashing, the size and length of the rope you need are among the first questions you have to answer. To determine the length of rope needed for a lashing, add the diameters (in inches) of the two spars at the point the lashing is being made. If one spar is 2-1/2 inches in diameter and the other is 3-1/2 inches in diameter, the total equals 6 inches. Multiply by 3 feet to get the length of the rope needed for lashing.

If you use a rope that is too short to make three wraps and two fraps for a lashing, you should add (Splice or join with a Square Knot) a length of rope to complete the lashing with three full wraps and two fraps. For safety, don’t leave the lashing short.

If you find you have extra rope, make more wraps or fraps to use up the rope to avoid cutting the rope or leaving long loose ends hanging out.

ROPE DIAMETER

In most cases, 1/4-inch diameter manila rope should take care of lashing two spars together as long as the combined diameter of both spars is 6″ or less. When the combined diameter is over 6 inches, use 3/8-inch diameter rope.

PIONEERING KIT

If your troop or camp puts together a pioneering kit, it should contain lashing ropes that are cut to standard lengths: 10, 15, 20, 30 and 50 feet.

Both ends of these lashing ropes should be properly whipped. It also helps to color-code the ends of all ropes with a bit of paint to denote each length. When storing ropes, make sure they are dry and properly coiled. Never “hank” ropes for storage. That is, don’t wrap them around your hand and elbow to form a coil. Tie each coil with a short piece of cord and store the coiled rope on pegs or in a ventilated storage box.*

TRADITIONAL SQUARE LASHING

Drawing Design from Rope Works by Gerald L. Findley http://www.ropeworks.biz/reader/squarlas.pdf
Drawing Design from Rope Works by Gerald L. Findley http://www.ropeworks.biz/reader/squarlas.pdf

In Scout Pioneering in the United States we most often see the square lashing started with a clove hitch. The clove hitch is tied on the vertical spar, just below where you want to join the crossing horizontal spar (see 1).

Using a clove hitch to start this lashing allows for two things. First, you can rest the crossing spar on the clove hitch to help support it as you begin the lashing while building your structure. Second, the clove hitch helps keep the structure from racking (twisting out of shape), causing the lashing to loosen as it is moved or hoisted into position.

After the clove hitch is tied, wrap the excess short end of the rope around the standing part of the rope (see 2). Hold the crossing spar up to the vertical spar and make three wraps around the spars using the long end (see 3, 4, and 5). Pull each wrap tight to hold the spars together. Make two frapping turns around the wraps (between the spars) to pull the wraps tight (see 6 and 7) and finish with another clove hitch on the horizontal spar (see 8 and 9).

One other point to make about the square lashing is that you shouldn’t be fooled by or limited by its name. Although two spars can be lashed together at 90° using a square lashing, it can also be used to lash two spars together at any angle. For example, a square lashing is used to lash the ends of two light spars to the uprights of a trestle to form the X bracing. A diagonal lashing is used at the center of the X to hold the crossed spars together.

MODIFIED SQUARE LASHING

The Modified Square Lashing was developed because of the difficulty usually experienced when tying a clove hitch to complete the traditional square lashing. The clove hitch that starts the lashing is easy enough to make, but tying a clove hitch at the end of the lashing is a different matter.

As shown in figure 104, the modified square lashing starts with a clove hitch. When tying the clove hitch, let the running end of the clove hitch extend about 12″. Also do not twist the short end around the standing part of the rope as in the traditional square lashing.

After tying the starting clove hitch, proceed as usual using the long end of the rope to make three wraps (see figure 105). Then make two frapping turns (see figure 106).

To complete the lashing, bring up the short end of the rope that extends from the clove hitch and tie a square knot (see figure 107). Bring the short end up in the opposite direction of the frapping turns.

As in the Traditional Square Lashing, there is some disadvantage in having to make the complete lashing using the one end of the rope.

Modified Square Lashing
Modified Square Lashing

JAPANESE MARK II SQUARE LASHING

This lashing is a straightforward approach to the task of lashing two spars together. (CLICK HERE FOR INFORMATION AND PROCEDURE.)

Square Lashing
Japanese Mark II Square Lashing Photographed Procedure

DIAGONAL LASHING ***

When putting crossed braces on a structure to keep it from racking (as used when making a trestle), the most important lashing is the diagonal lashing where the spars cross. (Refer to the “Making a Trestle” section.)

When the cross spars are properly assembled on the trestle, they will be standing apart where they cross. That is, there will be a few inches of space between the spars where they cross at the center of the X. To pull them tightly together, a timber hitch is used to start the lashing (figure 1). As the timber hitch is pulled tight, the spars are sprung together. Next, three wraps are made in each direction across the X (figures 2 thru 5). After the wraps, make two frapping turns between the spars, pulling the wrapping turns tightly together and taking up any slack (figures 6 and 7). Finally, tie a clove hitch on one spar to complete the lashing (figures 8 thru 10). When this lashing is added to the cross braces, it helps keep the trestle from racking. Filipino Diagonal Lashing.

Diagonal Lashing Diagram
Diagonal Lashing Diagram

WEST COUNTRY ROUND LASHING 

The pattern you make with the rope for this lashing is the same as the one to make the whipping. The only difference is that this lashing is tied around two spars to hold them together.

To make this lashing, tie a series of half-knots (overhand knots) around the two spars (see figure 115). Tie one half-knot in front and the next half-knot in back (see figure 116). Make sure each half-knot is pulled up as tight as possible. After tying six to ten half-knots, finish off the lashing with a square knot (see figure 117). By using six to ten half-knots in this lashing, it makes it very strong and effective, but can be a little difficult to untie.

The West Country Round Lashing is used to tie two spars together to extend the overall length of the spars. When this is done, you should make two sets of lashings, not just one lashing. Make one lashing at each end of the overlapping spars.

West Country Shear Lashing Diagram
West Country Shear Lashing Diagram

TWO-SPAR SHEAR LASHING (Shear Lashing)

The main application for the Two-Spar Shear Lashing is when spar legs are to be spread apart to form an A-frame.

The Two-Spar Shear Lashing starts with a clove hitch on one spar (see figure 118). After making the clove hitch, wrap the excess part of the short running end around the standing part of the rope (see figure 119).

Two-Spar Shear Lashing Diagram
Two-Spar Shear Lashing Diagram (this drawing has been modified for the purpose of clarity)
Completed Shear Lashing Before the Spars have been Spread
Completed Shear Lashing Before the Spars have been Spread
Shear Lashing After Spreading the Shear Legs
Shear Lashing After Spreading the Shear Legs

Unlike square lashings, the shear lashing requires eight or ten wraps around the spars before making the frapping turns between the spars to pull the wraps tight (see figure 120). This lashing then ends with a clove hitch on the other spar (see figure 121).

If you’re making an A-frame, start the spars side by side and tie a clove hitch on one spar, about 1 foot from the top end of the spars depending on the project. Then you can take ten wrapping turns around the spars, making the wraps somewhat loose. The legs can then be spread to the required distance. This should put strain on the wraps.

With the legs apart, you can make the frapping turns around the wraps to pull them tight. Finally, complete the lashing by tying a clove hitch on the opposite spar.

The Somewhat Ambiguous Shear Lashing

STROP LASHING

In some pioneering situations all that’s needed is a few wraps with a rope, a light cord, or binder twine to hold two small spars or sticks together. Wrap the rope or cord around the spars a few times and finish with a Square Knot. This is called a strop lashing.

A strop lashing can be drawn down tight, or it can be made as a loose wrap so that it allows movement or acts as a hinge.

The strop lashing can have several simple applications at camp. For example, if you don’t want to dig a hole for the staff of your patrol flag, drive a tall stake in the ground. Then use a light cord or binder twine to make two strop lashings about 1 foot apart to hold the staff to the stake (see figure 122).

If your patrol just completed a signal tower and you want to show who did it, lash your patrol flag to the top of one of the legs with a strop lashing.

When you’re adding walkways to a bridge, they need to be joined to the trestle to form a single unit. The way to do that is to lash the two walkways to the transom at the center of the bridge with two or three strop lashings (see figure 123).

The ends of the walkways also need to be held to stakes. Use a strop lashing to hold the ends of the walkway to the stakes (see figure 124).

Strop Lashing
Strop Lashing

The Boy-Sized projects provided and designed by Adolph Peschke and featured in the older Pioneering Merit Badge Pamphlet (Single Lock BridgeSingle Trestle Bridge, A-Frame Bridge, Double A-Frame Monkey Bridge14′ Double Ladder Signal Towerdo not require the frequently employed tripod lashing, and hence it’s not included herein.

* An approach that is seen as a practical way to preserve and organize your lashing ropes is to coil ropes of the same length into manageable groupings, i.e. a large coil containing more than a single rope, and then tying up the grouping at one end with a 3-foot light cord.

** Round lashings are more frequently used to very effectively extend Scout staves or smaller diameter spars.

*** In a square lashing, the wraps and fraps form a square. In a diagonal lashing the wrapping turns cross the poles diagonally. Both lashings can be used to join two poles together that cross each other from 45º to 90º.  It’s the square lashing that is used most because there is more contact between the rope and the poles, and hence it is stronger. A diagonal lashing is used when there is a need to close a gap between two spars or when they spring apart, in other words, when we want to bind poles together where they cross each other but do not touch. This most commonly occurs when the ends of the spars are already lashed in place in a structure, as in forming the X-brace of an H-Trestle.

PDF FILE for: Lashing INFORMATION

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

Double A-Frame Monkey Bridge

—> View Video of Bridge Construction

Assembly
Adjusting knots and lashings after the first test crossing.

The well-known, time-tested, traditional Monkey Bridge is perhaps the most familiar of all Scout pioneering projects. It’s frequently featured at Scout Expos, Camporees, Scout Camps, and is often a central attraction at public gatherings where Scouting is represented.

The following instructions and guidelines are provided by Adolph Peschke, taken from the 1993 edition of the Pioneering Merit Badge Pamphlet:

Link to: Older Pamphlet InfoUsing a double A-frame to build a monkey bridge is a departure from the usual X-frame that supports the foot rope and hand ropes. This new method has two distinct advantages over the X- frame version. First, the double A-frame provides a wider base making it less likely to tip over. The second advantage is that the positions of the A-frames can be adjusted so the span between the hand ropes can be narrowed for better balance as you make the crossing.

Building the A-frames. The first step in building the monkey bridge is to build four A-frames using the 8-foot spars for the two legs, and 6-foot spars for the ledger. Lay out the first set of three spars (two legs and one ledger) on the ground in position for lashing. Before lashing, drive three stakes, as follows, to help you make all four A-frames the same size: Drive a stake at the top to mark where the leg spars cross. Then drive stakes to mark the positions of where the bottom ledger crosses the legs. This will also indicate how far the legs are spread apart. Now you can lash the four A-frames together, laying them out one at a time using the stakes. Remember that all three lashings on the A-frames are square lashings, even though the spars cross at less than 90˚ angle.

Schematic of A-frame alignment

Double A-frame. When you have four A-frames, you can lash two of them together to form a double A-frame. (see figure 140). Lay one A-frame on the ground and then put another on top of it so that the bottom ledgers overlap one-half their length (approximately 3 feet). The first step in lashing the A-frames together is to go up where the two legs cross (the X formed by one leg from each A-frame). Then with a good tight square lashing, lash the two legs together.

Note: The point where these two legs are lashed together is where the foot rope will rest. You can adjust the overlap of the two A-frames to adjust how high the foot rope will be off the ground. Also note where the tops of the A-frames are, because this is where the hand ropes will be. To complete the double A-frame, stand it up so the butt ends of all four legs rest solidly on level ground. Lash the two bottom ledgers together where they overlap with three strop lashings. Now repeat the entire process to build the second double A-frame.

Laying out the distance between A-frames and anchors

Site preparation. Before you can erect the double A-frames, you need to prepare the site. Begin by stretching a length of binder twine along the center line of where the monkey bridge is to be built.  Working from the center, measure 10 feet toward each end to mark where the A-frames are to be placed. They should be 20 feet apart. Then mark out another 10′ from each A-frame to where the anchors are to be built.

Note: These dimensions are for building a bridge with a 20-foot span. This is the maximum span for a bridge using a 50-foot rope. The extra 30 feet of rope is needed to have 15 feet of rope at each end for the proper distance from the A-frames to the anchors (10 feet) and for the knots at the anchors (5 feet).

Build the anchors. The foot rope will be attached to anchors at both ends. Before erecting the double A-frames, build a 3-2-1 anchor, or a log and stake anchor, 10 feet from where the A-frames will be erected (see figure 141).

Rope grommet. After the anchors are built, attach a rope grommet with a ring or shackle in it. (You can make the rope grommet with a 10-foot length of 1/2-inch diameter polypropylene rope. Tie the ends together using a carrick bend, and permanently secure the ends with some strong twine).

Position the A-frames. Prepare to erect the monkey bridge by moving the A-frames into position no more than 20 feet apart. Lay them down on the binder twine that marks the center line of the bridge.

Hand and foot ropes. Now you can prepare the foot and hand ropes for the monkey bridge. Lay the foot rope in a straight line off to the side of where the A-frames are laying. Then lay the two hand ropes on the ground next to each other so they’re parallel to the foot rope and 42 inches away.

Stringer ropes. Now you can add the stringer ropes that will go from the foot rope to the hand ropes. Start by tying the center of an 8-foot long stringer rope (use 1/4-inch manila rope) at the center of the foot rope, using a clove hitch. The stringer rope is tied around the foot rope so that both ends are 4 feet long. Add two more stringer ropes on both sides of the center stringer rope (so there are five stringer ropes in all), tying them about 4 feet apart. Tie one end of each stringer rope to one of the hand ropes, again using a clove hitch. Then do the same with the other ends of the stringer ropes, attaching them to the other hand rope.

Assemble the bridge. You’re just about ready to assemble the bridge. First place a piece of heavy canvas (called a “saddle”) in the V formed by both double A-frames. This will protect the foot rope and allow it to slide a little in the V without interfering with the lashing rope.

Fig142
Schematic of foot, hand ropes and anchors

Now get the crew together to erect the bridge. You will need a safety officer to watch for any problems that might occur, and a signal caller to tell the crew members what to do. You will need two Scouts to lift and hold each double A-frame in place, two more Scouts to lift the foot rope into the V of the double A-frames, and two more Scouts to lift the two hand ropes into place at the tops of the A-frames. Lift everything into place. Then, holding the A-frames steady, temporarily tie the hand and foot ropes into the rings of the grommets using a roundturn and two half hitches (see figure 142).

Tighten the foot rope. Now you can put a strain on the foot rope. It’s not necessary to use block and tackle since this will put too much strain on the lashings, anchors, and the foot rope itself when there is a load on the bridge.* Whatever strain three or four Scouts can put on the foot rope by pulling it by hand will be enough. As soon as the bridge is used a few times, there will be a sag in the rope. This is fine because it means that you are working with reduced strain on the foot rope as a safety measure.

Tighten the hand ropes. Next, tie the hand ropes to the top ends of the A-frames. First, loosen one end at a time from the anchors. Then, use a clove hitch to tie the hand rope to the top end of the leg of the double A-frame. As you’re tying these clove hitches, adjust the strain on the sections of the hand ropes between the double A-frames to match the sag of the foot rope. Also, adjust the length of the stringer ropes so there is even strain between the foot rope and both hand ropes. After the hand ropes are tied to the tops of the A-frames, move down and retie the ends of the hand ropes to the rings in the grommets using a roundturn and two half hitches.

Final testing. With caution, one crew member can get on the bridge as all lashings, anchors, and knots are observed by the safety officer and all other crew members. Make adjustments as required. Then secure the running ends of the hand ropes and foot rope with a piece of cord. Safe operation calls for only one Scout to be on the foot rope of the monkey bridge at a time.

LIST OF MATERIALS FOR DOUBLE A-FRAME MONKEY BRIDGE

  • eight 4-inch x 8-foot A-frame legs
  • four 3-inch x 6-foot ledgers
  • fourteen 1/4-inch x 15-foot lashing ropes for Square Lashings
  • one 1/2-inch or 3/4-inch x 50-foot rope
  • two 1/2-inch x 50-foot hand ropes
  • five 1/4-inch x 8-foot stringer ropes
  • six 1/4-inch x 10-foot lashing ropes for Strop Lashings
  • six pioneering stakes for each 3-2-1 anchor
  • eight pioneering stakes for each log-and-stake anchor
  • one 5-inch x 4-foot spar for log-and-stake anchor
  • two 1/2-inch x 10-foot polypropylene ropes for rope grommets
  • two pieces of scrap canvas for foot rope saddle
  • binder twine for anchor tieback straps

* It has been found that a rope tackle in the foot rope at each end (not a block and tackle) tightened by one Scout is an excellent procedure to maintain the optimum foot rope tension, and an easy-to-use remedy for too much sagging due to repeated, heavy use and over stretching. There are other configurations used to initially tighten and keep the hand and foot ropes at the optimum tension during use, depending on the weight the bridge must withstand and the amount of traffic it will bear.

More Photos

2013 Jamboree Monkey Bridge Modified A-Frame Construction Photo Illustration

Single Pull Rope Tackle Monkey Bridge Configuration

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