Twine – 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)

Anchoring Pioneering Projects

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

Building pioneering projects often requires some type of strong point for attaching a guyline for a tower or derrick. An anchor point might also be needed to anchor one or both ends of a monkey bridge.

Sometimes nature will provide a tree or rock in just the right location or you might be able to shift the location of the project to take advantage of a natural anchor.

STAKES

Link to: Larger View — Pioneering Stakes

When nature does not provide a solution, anchors can be constructed using stout pioneering stakes.

Note: Under no conditions should tent pegs be used for pioneering stakes. They’re not long enough or strong enough to make a safe anchor.

Pioneering stakes should be made of hardwood, such as oak or hickory. The most common size of stake (for the projects shown in this pamphlet) is 2-1/2 inches in diameter and about 24 to 30 inches long (see figure 84). After cutting the stake to this size, cut a point on one end. Then bevel the top edge to prevent it from mushrooming or splitting when the stake is driven into the ground.

MALLET

When driving stakes into the ground, it’s best to use a wooden mallet. Using a metal sledge hammer or an ax head will damage the stake.

To make a wooden mallet, cut a 4-inch diameter piece of hardwood, such as hickory, elm, or sycamore, about 11 inches long (see figure 85). It should weigh about four pounds. Drill a 1-1/8-inch diameter hole to mount the handle. The handle can be made from a 24-inch length of hardwood (similar to making a stake). Use a knife or ax to round the end of the handle to fit the hole in the mallet head. Secure the handle in place with a wedge placed crosswise to the length of the head.

SOIL CONDITIONS

When driving the stake into the ground, drive it at about a 20° angle. Soil conditions can vary and will dictate how large and long a stake you need. If there will be a heavy strain on the anchor, you might need additional stakes as in the 3-2-1 configuration (shown in figure 89). After the stake is driven into the ground, keep your eye on it as strain is applied to see how it’s holding.

If ground conditions are unsuitable for even the largest stake you have, use a 4-inch diameter spar that’s buried 35 inches in the ground at a 30° angle and anchored in place with a stake (see figure 86).

GUYLINE

Always attach the guyline around the stake as close to the ground as you can get it. If the guyline is placed or slips higher on the stake, there will probably be enough leverage to pull the stake loose (See figure 87).

ANGLES FOR GUYLINES

Both the 3-2-1 anchor and the log-and-stake anchor should be positioned so that the guyline is at a 15° angle, or a maximum of 25°. To determine this, measure the height at the point where the guyline is attached. Double this distance to determine the minimum distance required between the base and the anchor. For example, if the guyline is attached 10′ up the pole, the anchor should be a minimum of 20′ from the base (see figure 88). If your line is long enough, it won’t hurt to place the anchor a few feet further out.

3-2-1 ANCHOR

As the name implies, the 3-2-1 anchor is made by driving stakes in a series: three stakes, then two stakes, and then one stake to form the anchor (see figure 89). All six stakes are 30 inches long and are driven 18 inches into the ground at a 20° angle.

First drive in the set of three stakes. Next drive in the set of two stakes about 24 inches away from the first set. Then tie a rope from the top of the three-stake set to the bottom of the two stake set using at least two loops of 1/4-inch manila rope, or six to eight loops of binder twine. Then use a small stick to twist the rope tight in a tourniquet. After the rope is twisted tight, push the end of the stick in the ground to keep it from unwinding.

Finally, drive a single stake in the ground about 12 inches from the two-stake set. Once again, use a twisted rope or binder twine as a tourniquet to hold the two-stake set tightly in place.

Depending on the strain, you can use other configurations, such as 2-1-1, or even 1-1-1 for a light strain. When using any stake anchor, be sure that it is in direct alignment with the strain being applied.

LOG-AND-STAKE ANCHOR

The log-and-stake anchor is easy to make and can hold a considerable amount of pull. You can tie the line directly to the log, or you can use a ring-and-rope grommet as shown in figure 90.

To make the log-and-stake anchor, place a log 4 to 6 inches in diameter perpendicular to the pull of the line. Then drive in four large stakes in front of the log. Next, slip the rope grommet through the ring and then slip the ends of the grommet around the log (see figure 90).

Drive a second row of stakes 24 inches behind the front stakes. Then anchor the front stakes to the rear stakes with a tourniquet made of binder twine or rope.

STROPS

It is good practice to use a device called a strop to avoid damage to your long lines. It also makes it easier to tie off your long lines and to make adjustments.

A strop can be made by using a 10-foot length of 1/2-inch diameter manila or polypropylene rope. To make a strop, splice a thimble and ring into one end of the rope (see figure 91), or use a screw pin shackle with a thimble.

The strop can then be wrapped around a rock or tree to attach the line (see figure 92). It can also be used around a spar that is anchored between two trees (see figure 93).

Note: Be sure to use a piece of canvas or burlap to protect your rope from sharp edges of a rock or to protect the bark of the tree from rope burns.

GROMMETS

A grommet is often used in conjunction with an anchor. A large grommet can be made by splicing together the ends of a 10-foot length of 1/2-inch manila or polypropylene rope. If you don’t have a spliced grommet in your pioneering kit, tie the ends of the rope with a square knot or a carrick bend. Be sure to secure the ends of the rope.

The completed grommet is useful when attaching a long line to an anchor of stakes. It provides a strong and more convenient way to attach a guyline or other long line.

The grommet you use must be made of a larger-diameter rope than the guyline to avoid creating a weak link in the chain between the structure and the anchor.

Single Pull Rope Tackle Monkey Bridge Configuration

PDF FILE for: Anchoring Pioneering Projects

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:

Using 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.

Link to: Double A-Frame Monkey Bridge PHOTOS — 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.

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

PDF FILE for: Double A-Frame Monkey Bridge

Scouting Magazine Article