The Single A-Frame Bridge is made up of three subassemblies. Please refer to Bridge Walkways as a point of reference for two of the three of these subassemblies. The following photos will enliven the text and instructions featured in Adolph Peschke’s informative, older pioneering merit badge pamphlet. Click on the photos for larger views:
In the Pioneering Area of the 2013 national jamboree, we put together a couple of Single A-Frame Bridge kits, so Scouts and Venturers could build this simple crossing bridge during their visit to Garden Ground Mountain. Each kit included:
- two pre-constructed walkways
- two 12-foot leg spars (shear legs)
- one 5-foot transom spar
- one 6-foot ledger spar
- two pre-positioned anchors
- four pioneering stakes
- two guylines
- five lashing ropes
Whenever a crew wanted to build a bridge, we provided an overview of the design and gave them a quick introduction to tying a rope tackle and the Japanese Mark II Square Lashing. What follows are some photo montages of the Single A-Frame bridges built from the kits during the jamboree. For larger and largest views, click on the photos once, and then once again:
On occasion, a pair of Scouts wanted to build a bridge, and with persistence, and the help of staff or friendly Scouter, they were able to get it done.
- Single A-Frame Bridge (scoutpioneering.com)
The simplest of all crossing bridges is the Single A-Frame Bridge. The design was featured at the 2013 National Jamboree which afforded Scouts an opportunity to construct the bridge right on the spot.
Building this bridge is quite simple because there are very few lashings needed for the center A-frame. The A-frame is a triangular shape that resists racking and provides strength for the structure.
A-frame. Start this project by determining the depth of the creek or ravine to be spanned. You have to add 8 feet to that measurement to get the total height of the legs for the A-frame. For example, to span a creek 4 feet deep, the legs of the A-frame should be about 12 feet or longer.
This total length allows for the distance from the butt ends of the A-frame legs up to the transom that supports the walkways. The transom should be about 1 foot higher than the banks of the creek. It also allows for the height from the walkways up to the tops of the legs to permit free passage for a person along the walkways.
Lay the A-frame subassembly out on the ground to check if the spars are long enough when lashed together for the two requirements mentioned above.
A-frame Legs. When you’ve determined the length of the spars for the legs of the A-frame, lash them together at the top with a shear lashing, not a diagonal lashing. This lashing should be made somewhat loose so that you can spread the spar legs apart to form the A-frame. As you spread the spar legs, the Shear Lashing will tighten. A little practice will show you how loose to make the shear lashing initially in order for it to be tight when the A-frame is formed.
Ledger and transom. To complete the A-frame, use square lashings to lash the bottom ledger across the legs about 1 foot from the bottom of the legs. Then lash a transom spar to support the walkways at the proper height in relation to the banks of the creek.
Walkways. The two 10-foot walkway sections are made as separate subassemblies. (Refer to “Bridge Walkways.”)
Assembly. After the walkways are made, take them to the assembly site along with the A-frame. Place the A-frame in the center of the creek and heel in the legs about 4 to 6 inches deep. As the legs are being heeled in, level the transom to accept the walkways in a level position.
When the A-frame is upright and the transom is level, lash both underspars on the walkways to the transom with strop lashings at three points. Finally, lash the cross spars at the ends of the walkways to stakes on the banks of the creek with Strop Lashings.
For safety, it’s best to add a light 1/4-inch guyline from the top of the A-frame to both sides of the creek to prevent it from tipping over.
List of Materials for a A-Frame Bridge
- two 3-inch x 12-foot A-frame legs
- one 2-inch x 6-foot bottom ledger
- one 3-inch x 6-foot transom
- four 3-inch x 10-foot walkway lateral spars
- twelve 2-inch x 3-foot walkway cross spars
- four 2-1/2-foot walkway cross spars
- two 2-inch x 10-foot walkway planks
- four stakes
Pioneering is the knowledge and skill of using simple materials to build structures that are used in a wide range of Scouting activities. These skills are sometimes referred to as “backwoods engineering.”
Down through the ages, people have used ropes, spars, and simple hardware to build bridges, towers, and even their own shelters. In the early development of our country, pioneering methods were used in mining and transportation, to clear the wilderness, and to build roads and bridges. So it is understandable that the term “backwoods engineering” was applied.
Whatever the project, the same applied principles of physics, geometry, and math are used to build pioneering projects and structures. But, keep in mind that all the information (in this pamphlet*) is eventually used for a practical, hands-on application—that is, to build something.
Pioneering is a good foundation for many Scouting activities. You must learn, and then use, such disciplines as planning ahead and teamwork. You can also put to use the basic skills learned in rank advancement, such as knot tying.
But most of all, pioneering provides a practical way to experience the joy of accomplishment when you’ve built something that is needed for yourself or others; it can be something that makes living in camp easier and more comfortable. Pioneering can be both fun and challenging when you use your skill and knowledge to choose the right materials (ropes and spars) and build a usable structure.
The basics of pioneering, such as tying knots, making lashings, using rope tackle, constructing anchors, and basic rope knowledge can be done at home. The projects and structures (shown in this pamphlet**) can usually be constructed with materials available at summer camp or at council camping events.