Bob Beckley, Project Leader

Trail crews and others working on backcountry maintenance and construction projects have to move heavy objects. Often, the work has to be done in areas without large trees that could be rigged with cables to support the operation. Portable steel tripod towers can help in such situations. When these towers are used with steel cable and a Griphoist winch (figure 1), heavy loads can be moved (figure 2).

Figure 1—The portable tripod was used to move rocks downhill
in this skyline operation with the load suspended from a cable.
The operator used a Griphoist winch to control the cable tension.
The tarp protects the cable from being damaged by the rocks.

Figure 2—It is easy to move and place a heavy load when the
portable tripod is used for rigging. Tether lines are used to
control the load's descent and to place the load.

Development of the Portable Tripod

During 1999, the Bitterroot National Forest's Steve Bull watched trail crews in Rocky Mountain National Park use a portable tripod. The tripod was designed to move relatively small rocks above timberline. That tripod was designed by Lester Kenway of Trail Services, Inc., Bangor, ME. The design used legs similar to those of the portable tripod, but did not have a top plate assembly. Instead, a bent length of "allthread" rod and nuts were used to bind the legs together and hang the snatch block.

In 2003, the Bitterroot National Forest trails program needed a portable tripod to install bridge stringers weighing more than 1,000 pounds that were being packed by stock to a remote location. Steve Bull created a conceptual design for a tripod head with the assistance of Charlie Mabbott and the leadership of Nick Hazelbaker. Sam Allsop of California State Parks had a top plate assembly design of his own. Bull created a modified design based on Allsop's design.

Later, engineer Dick Karsky at MTDC was asked to provide material strength data to assure that the tripod system could handle heavy loads safely. Kent Niles, a local welder and fabricator, built a set of tripods. Niles altered the top plate assembly design slightly (figure 3), made the foot and plate assembly one piece, and used trailer hitch pins with locking hairpin cotters (figure 4) to adjust the legs. Niles delivered the tripods during the summer of 2003.

Figure 3—The tripod's headplate with a snatch block and cable
hanging below it. Operators must follow all Occupational Safety
and Health Administration regulations for rigging operations.

Figure 4—The tripod's adjustable legs are locked in
place using hitch pins with locking hairpin cotters.

Bitterroot National Forest employees have used these tripods many times, transporting loads weighing up to 1,200 pounds. Any organization that needs to move heavy loads over short distances using equipment that can be packed to a remote location (figures 5a and 5b) should consider trying this versatile piece of equipment.

Figures 5a and 5b—The portable tripod can be transported
into the backcountry by mules. One mule carries the tripod
while another mule carries the block, Griphoist, and cables.

Safety and Training

Rigging operations can be dangerous. Equipment failure can lead to serious injury or death. Equipment must be inspected frequently to ensure it is in good, safe working condition. Pay special attention to frayed, bent, or kinked cables. They need to be repaired or replaced.

Employees working on or near rigging operations must be informed of the hazards and receive proper training. A Job Hazard Analysis must be completed and signed before starting a rigging operation and a safety briefing should be held before the start of each day's work.

Currently, the Forest Service does not have a certified Rigging Training Program. On-the-job training should be done by only those experienced in the type of rigging operations required and using equipment with which they are familiar. All rigging operations will follow applicable OSHA guidelines, such as 1910 Subpart N—Materials Handling and Storage. Private industry, State, or local technical schools may offer training or classes in rigging operations. Consult with your forest's safety officer before conducting a rigging operation.

Building and Using the Portable Tripod

The tripod is made from steel tubing and plates that are welded. It breaks down into sections that can be transported easily and that can be reassembled quickly using hitch pins with locking hairpin cotters. The leg sections are just 6 feet long so they can be packed on a horse or mule.

MTDC Drawing No. 1035 was prepared based on the Bitterroot National Forest's tripods. Fabricators must adhere to accepted welding practices and comply with the callouts in the drawing's materials list to assure that the tripod can support heavy loads safely.

The tripod was designed to be used with a materials handling winch, such as the Griphoist-Tirfor model TU-28 (pulling capacity of 4,000 pounds). Operators must ensure that the wire rope, blocks and fittings, and anchor points are in good working condition and are compatible with this load. They must observe all Occupational Safety and Health Administration guidelines for the use of cable clamps, safety latches, chains, and slings.

When two tripods are needed, snatch blocks should be used to suspend the cable from the top of the tripods. The blocks reduce wear on the cable and help prevent the tripods from being pulled over as the winch pulls in cable. The wire rope cable between the tripods must always have some deflection. Post signs to warn people of the dangerous work (figure 6a) and tie flagging on rigging cables (figure 6b).

Figures 6a and 6b—Signs should warn people about work
occurring in the area. Rigging support cables must be flagged
so workers do not trip over them or walk into them.

Tripods can lift heavy objects, but are dangerous when loads pull to the side. Crews should not attempt to pick up or drag heavy loads that are not directly under the overhead cable (figure 7). If the load is off to the side of the cable, the tripods should be moved so that the load is directly beneath the cable. Otherwise, one or both of the tripods may tip over.

Figure 7—One portable tripod can be used by itself when lifting heavy
objects, such as one end of a log bridge. The safety chain around the
bottom prevents the legs from spreading too far apart. Hooks welded
near the bottom of each leg hold the chain in place.

Assembling and Setting Up the Tripods

The part numbers are shown on MTDC Drawing No. 1035. Before assembling a tripod, gather all the parts. Run the cable through the snatch block and choose a location where the cable can be anchored near each of the tripods.

Assembly Steps

1. Attach the eyebolt (No. 11) to the top plate assembly (No. 7).
   
   
2. Attach the three legs (No. 1) to the top plate assembly (No. 7) using the hitch pins and hairpin cotters (No. 14).
   
   
3. Attach a sleeve (No. 2) to the bottom of each leg (No. 1) using a hitch pin and hairpin cotter (No. 14). The sleeve allows the length of the tripod's legs to be adjusted.
   
   
4. Depending on the type of ground under the tripod, attach the foot and plate assembly (No. 4) to each leg (No. 1) using a hitch pin and locking hairpin cotter (No. 14).
   
   
5. Attach the snatch block (with the cable running through it) to the eyebolt.
   
   
6. With the assembled tripod lying on the ground and with plenty of slack in the cable, have one person pull one leg of the tripod forward while two persons push the other two legs up until the tripod is standing.
   
   
7. Adjust the lengths of the legs until the tripod head is relatively level, keeping the legs evenly spaced.
   
   
8. Using a level, make small adjustments to the legs so the tripod head is level, keeping the legs evenly spaced.
   
   
9. Attach the chain to the hook on each of the foot and plate assemblies. The chain prevents the legs from spreading.
   
   
10. After completing the rigging setup (connecting the cable to the anchors, attaching the Griphoist to the cable, attaching the load to the snatchblock, and attaching a tether line to the load) load the cable slowly and gently. If the tripods shift, repeat steps 7 and 8 until the tripods are loaded and level.

Drawing

MTDC Drawing No. 1035 shows how to construct the portable tripods.

About the Author

Bob Beckley received a bachelor's degree in political science from the University of Montana in 1982. He began his Forest Service career as a timber technician on the Nez Perce National Forest. Bob was a smokejumper when he joined MTDC in 1990. He works as a project leader, public affairs specialist, and blaster.

Additional Information

For additional information about the Portable Backcountry Rigging Tripod, contact MTDC:
USDA Forest Service
Missoula Technology and Development Center
5785 Hwy. 10 West
Missoula, MT 59808-9361
Phone: 406–329–3900
Fax: 406–329–3719

Electronic copies of MTDC’s publications are available on the Internet at:
http://www.fs.fed.us/eng/pubs

Forest Service and Bureau of Land Management employees can search MTDC’s documents, CDs, DVDs, and videos on their internal computer networks at:
http://fsweb.mtdc.wo.fs.fed.us/search/