When stabilizing a load after rigging and moving it to a specified location, the rigger should ensure it is well blocked before the rigging is removed.
- Select wood blocking that is straight and uniform. Preferably the rigger should use a hardwood like hickory or oak.
- When creating a “Lincoln Log Stack” the height of any blocking stack should not exceed 2 times its width. With large loads, three stacks (cross laid) or four should be used to provide stability. The load’s center-of-gravity should be well contained within the stacks’ boundary.
- A load edge like a dozer blade should rest cross-grain on the wood immediately underneath it. If it rests in-line with the grain of the wood, it can split the wood causing a collapse.
- To create additional stability between stacks, long timbers can be used to “inter-tie” the stacks.
- Wedges should be used to fill gaps under curved load surfaces in order to distribute the weight and avoid point loading on a stack’s corner, causing instability.
- A flat table of blocking can be laid on the top layer of a cribbing stack. The blocking should be cross-grain to the load’s bearing surface.
- The ground surface that supports the blocking should be firm and level before building a cribbing stack.
From ASME B30.1 Jacks
(a) While jacking, blocking should follow the load up or down to serve as an arresting device if the load falls.
(b) Wedges or shims should be used as necessary to ensure the load is completely supported at its blocking points.
(c) Blocking should never be taller than twice the width of the blocking base.
(d) Blocking shall be of sufficient thickness, width, and length to prevent shifting, toppling, or excessive settlement of the load.
(e) Blocking shall be of sufficient strength to prevent crushing, bending failure or shear failure, and to adequately transmit the load’s weight to the supporting surface.
Happy trails my rigging friends,
Mike Parnell, ITI – Field Services