Tag: H-Trestle

Pioneering Curriculum IX: Using a Diagonal Lashing and Making a Trestle

This is the ninth post in a series that will eventually comprise an activity-based, unit pioneering program curriculum.

SUPPORTING VIDEO: How to Tie a Diagonal Lashing

Link to: Larger Image
Trestles are often used to support bridge walkways.

IX A. The diagonal lashing gets its name because the wraps form a diagonal where they cross the spars. It’s primary use is to spring together two spars that are not touching when the ends are lashed in place on a structure (as in a trestle).

IX B. The trestle is a basic component of many pioneering projects. When we speak of a trestle, we’re referring to what has been dubbed an H-trestle. The way it’s designed yields a very strong supporting structure that is often used as a subassembly for something larger, and frequently serves to support the walkways of a bridge.

OBJECTIVES

  • Each Scout will tie a diagonal lashing to effectively spring together the X-brace of a trestle.
  • Working individually or in small groups, Scouts will properly construct a trestle.

MATERIALS

  • two 6-foot x 2 to 3-inch spars and 15-foot lashing rope for demonstration
  • six Scout Staves for every four Scouts (six will be needed for each trestle)
  • two 6-foot x 1/4-inch manila lashing ropes for every Scout (eight will be needed for the square lashings on each trestle)
  • one 10-foot x 1/4-inch manila lashing rope for every Scout (one will be needed for the diagonal lashing on each trestle)

PROCEDURE A

Basic Four Steps to Tying a Traditional Diagonal Lashing

Using the 6-foot spars and the 15-foot lashing rope, Instructor demonstrates a diagonal lashing, then using a 10-foot lashing rope and two Scout Staves, with the aid of an instructor and following the basic four steps, Scouts will tie a diagonal lashing.

  1. Cinch the poles together by tying a timber hitch around them where they cross.
  2. Make three wrapping turns on the opposite diagonal to the timber hitch.
  3. Keep the wraps parallel to one another and pull them tight. Make three more tight wraps across the first three, again keeping them parallel.
  4. Take two frapping turns between the poles, tightly around both sets of wraps and complete the lashing with a clove hitch around one of the poles.

PROCEDURE B

With eight 6-foot lashing ropes, one 10-foot lashing rope, and working individually or in a group of up to four individuals, Scouts will follow the following procedure and build a trestle:

Trestle Formed with Scout Staves
Trestle Formed with Scout Staves

 

  1. Lay out two poles parallel to one another as the trestle legs.
  2. Place a pole over the legs at the top and bottom to serve as the ledgers, and then lash them to the legs with four tight Square Lashings using the 6-foot lashing ropes.
  3. Turn the legs and ledgers over and place one pole diagonally over two of the legs as one of the X-braces.
  4. Where it intersects, join this pole to the legs with two tight square lashings.
  5. Place the last pole diagonally OVER one leg and UNDER the other leg, as the other X-brace.
  6. Where it intersects, join this pole to the legs with two tight square lashings.
  7. Stand the structure up. There should be a gap between the the two X-braces where they cross in the middle.
  8. Using the 10-foot lashing rope, spring the X-braces together with a tight diagonal lashing.
  9. The completed trestle should be sturdy enough to be lifted and shaken without losing its shape or becoming loose. After building their trestles, Scouts are ready to have a Roman Chariot Race.

INTERPATROL COMPETITON: Roman Chariot Race

PIONEERING PROJECT: Single Trestle Bridge

PIONEERING PROJECT: Single Lock Bridge

PIONEERING CURRICULUM: MAIN PAGE

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.