We’ve seen how block and tackles and even commercial-grade come-alongs have been employed to tighten hand and foot ropes during the construction of various monkey bridge projects, especially those spanning longer distances and using larger diameter ropes. On the other side of the coin, in the presentation of his Double A-Frame Monkey Bridge, Adolph Peschke says, “Whatever strain three or four Scouts can put on the foot rope by pulling it by hand will be enough.”
The simple rope tackle provides a 3:1 mechanical advantage and is frequently used to apply the desired tension to both hand ropes and foot rope—often on both sides of the bridge. Adjusting the strain on individual ropes during use of the bridge can result in a slight complication—the spanner ropes can lose their symmetry.
A method that will alleviate this issue, and also provide a greatly increased mechanical advantage, is to utilize a Single Pull 2-Ring Configuration. After attaching the hand ropes to their respective spars with Clove Hitches, (adjusting the strain on the sections of the hand ropes between the double A-frames to match the sag of the foot rope) this approach is executed as follows:
Use a roundturn with two half hitches to attach the hand and foot ropes to a rope grommet at one anchor point. At that side of the bridge, they will remain fixed.
Pulling the three ropes so each receives the same degree of strain, attach each to one large ring at the other side of the bridge, again using Roundturns with Two Half Hitches.
With a length of 1/4 or 3/8-inch manila (preferred), using a roundturn with two half hitches, connect one end to the same large ring.
Reeve the running end of this rope through the ring in a rope grommet which is situated about four feet away and already fixed at the anchor point on this side of the bridge.
Carry the running end back and reeve it through the large ring. (If you now pull on the running end, there’s a 2:1 mechanical advantage.)
To increase the strength of this connection enabling it to withstand all the strain exerted on the bridge during heavy operation, reeve the running end back through the ring in the rope grommet and then through the large ring two or more times.
Now when you adjust the tension of the hand and foot ropes with this pull rope, the mechanical advantage is greatly increased. Secure the entire configuration with two Half Hitches cinched up against one of the rings.
Some Notes: This arrangement can be configured any way you like, e.g. when using 3/8-inch manila, attach the pull rope to the large ring, reeve it through both rings twice, and then finally secure the configuration at the large ring. Or, instead of first tying the pull rope to the large ring (the ring that’s functioning as the moving block) initially tie it to the ring in the rope grommet (fixed block). Then proceed to reeve it through the other ring, back again, etc.
With each turn on the rings, make sure the rope doesn’t cross on top of itself as this would interfere with adjusting the strain.
This kind of rope tackle can exert too much force on the bridge components, so carefully monitor how tight everything is getting and don’t just give the rope a willy-nilly pull.
The National Camp Accreditation Program (NCAP) Program Specific Standard PS-212 states: “Scout camp structures such as monkey bridges, obstacle courses, and pioneering towers are expected to meet safety standards in equipment and supervision comparable to COPE but are not subject to COPE standards, do not require COPE inspection, and do not require an on-site COPE Level II instructor. “VERIFICATION: If a project has participants elevated more than 6 feet above the ground, evidence of council enterprise risk management approval. This approval may be part of the general program design review in Standard PD-112.”
The following text is by Adolph E. Peschke as presented in the 1998 printing of the 1993 edition of the Pioneering Merit Badge Pamphlet:
This project solves the old problem of wanting to build a signal tower when there aren’t enough big spars to do the job. The double ladder tower requires four 14-foot spars and several smaller spars, but not nearly the amount needed for a four-leg signal tower. It also cuts down the number of lashings required.
This tower is not free standing. It requires the use of guylines to hold it steady. Review the sections on anchors and rope tackle if this is your first encounter with guylines.
Assemble the ladders. This project begins with building two ladders: a climbing ladder and a supporting ladder. Lay out two pairs of spars on the ground for the legs of the ladders. Be sure the butt ends are even at the bottom so that the tower will stand up straight. Before you begin any lashing, mark the positions where the spars that will hold the top platform are to be lashed onto the legs. This is about 4 feet from the top ends of the legs.
To make the climbing ladder, lash ten rungs on one pair of legs at about 1-foot intervals. The top rung should be lashed on where you marked the position of the platform, 4 feet from the top. Also the top handrail is lashed on to complete the climbing ladder.
To make the supporting ladder, lash three spars on the other set of legs to serve as the bottom, center, and top spreaders. The top spreader should be lashed at the point you marked for the platform, 4 feet from the top. Then lash on the top handrail, as on the climbing ladder.
Lash the ladders together. Now you have to join the two ladders to form the tower. Turn the two ladders up on their sides so they’re parallel to each other and approximately 6 feet apart. Check to see that the bottoms are even. Now lash on the base spreader to join the bottoms of the two ladders.
Lash on the platform supporting spar just above the top rung and top spreader on the ladders. Before proceeding, check the measurements from the bottoms of the legs to the platform supporting spar to make sure they’re equal on both legs so that the platform will be level.
Continue by lashing on the top long handrail. The lash on the two side X-braces diagonally between the legs using square lashings to lash the ends to the legs, and a diagonal lashing where they cross.
Lash the other side. To make the lashings on the other side, you have to get the whole crew together to roll the tower over 180° so that it’s laying on the X braces and the other sides of the ladders are up where they will be easier to get to.
Then proceed as before. Lash on the base spreader spar and the platform supporting spar. Again, measure to make sure there’s equal distance from both ends of the platform support spar to the bottoms of both legs. Continue to lash on the top long handrail and finish with the X-braces.
Lash on two more platform X-braces under the platform. These braces go diagonally across the legs just under the platform to help the tower resist racking (see figure 137). Use square lashings to lash them to the legs and a diagonal lashing where they cross.
Before standing the tower upright, lash on the spars to form the platform floor.
Anchors and Guylines. When all the lashings are done, move the tower to where it will be hoisted. Before actually hoisting the tower, lay out the position of the four legs on the ground. Then determine where the four anchors for the guylines will be placed to steady the legs of the tower. (Refer to the Anchors section to determine the position of the anchors.)
If the tower is positioned to make use of a natural anchor (such as a tree), prepare anchor strops to attach the guylines. For any guylines that won’t be using natural anchors, build anchors using pioneering stakes. At a minimum, you’ll need to build well constructed 1-1 anchors at all four corners.
Attach the four guylines to the legs just above the platform. The guylines should be 3/8-inch diameter manila or polypropylene rope. They’re attached to the legs of the tower using a roundturn with two half hitches and securing the running end of the rope.
Note: For safety reasons, never use a taut-line hitch on guylines, or for that matter, in any pioneering work. This hitch is used when adjustments in the tension are called for. It can slip.
Hoisting the tower. Hoisting the tower up into a vertical position is done with separate ropes. Do not use the guylines. Tie two lines on the side of the tower being lifted and one line on the opposite side to prevent over pulling and toppling the tower.
You’ll need a whole crew to do the hoisting. First there should be a safety officer who observes for all safety considerations and signs of trouble during the hoisting. There should also be a signal caller who tells the crew members when and how fast to pull on the hoisting ropes and when to stop pulling. Two or more Scouts should be on each of the two ropes. And one or two Scouts should be on the rope on the other side to prevent over pulling the tower.
When everyone is in position, the signal caller should direct the Scouts on the hoisting ropes to hoist the tower into position. As soon as it’s up, temporarily tie the guylines to the anchors using a roundturn with two half hitches.
Heeling in the legs. When the tower is upright, heel in the butt ends of the tower legs in holes about 4 to 6 inches deep. This is done to steady the tower and can also help in leveling the tower to make sure that the platform is level and the tower itself is vertical.
Tighten the guylines. To hold the tower steady, gradually apply strain to each of the four guylines at the same time. One of the easiest ways to adjust the strain is to tie a rope tackle on the anchor ends of the guylines.
As soon as the tower is in position and the legs are heeled in, go to each of the anchors and untie the roundturns with two half hitches and replace it with a rope tackle.
Do this by tying a butterfly knot in each guyline about 6 to 8 feet from the anchor. Then wrap the running end of the guyline around the forward stake of the anchor and back through the loop in the butterfly knot. When rope tackles are tied to all four anchors, gradually tighten the lines. Apply enough strain to each of the guylines to hold the tower firm and in a vertical position. Then tie off the rope tackles and secure the running ends with half hitches.
Test the structure. Before the tower can be put into general use, make a test climb while the safety officer and the whole crew observe all the lashings and anchors to ensure they are all secure.
Note: Some people are not comfortable climbing up to a high place. They should not be encouraged to climb if they are not sure of themselves. Do not pressure anyone to climb the tower if they don’t want to.
four 4-inch x 14-foot tower legs
ten 2-inch x 3-foot climbing ladder rungs
three 2-inch x 3-foot support ladder spreaders
two 2-1/2-inch x 6-foot base spreaders
two 2-1/2-inch x 6-foot platform supporting spars
two 2-inch x 3-foot platform handrails
two 2-inch x 6-foot platform long handrails
four 2-1/2-inch x 10-foot X braces
two 2-1/2-inch x 8-foot X braces
eighteen 2-inch x 3-1/2-foot platform support slats
eight pioneering stakes
four 3/8-inch x 50-foot manila guylines
thirty-one 1/4-inch x 15-foot manila lashing ropes (for 28 square Lashings and 3 diagonal lashings)
In accordance with current regulations, a fine adaptation consists of replacing the ladder rungs with support side spreaders, and dispensing with the platform floor slats. Lashing one or more long flag poles to the top of the legs and flying banners or flags never fails to elicit a rousing array of cheers, as the Scouts hoist their tower into an upright position! Click here for project description and materials.