Choose Cribbing Blocks for the Whole Support System

Cribbing blocks are individual pieces assembled into a temporary support, often called a crib. On a construction site, they may support raised machinery during maintenance or form part of an approved outrigger-support system. Choose them by the load at each support, required height, access beneath the equipment and ground conditions—not simply by block size or advertised tonnage.

A lifting device raises the load; cribbing provides support against falling or shifting. For U.S. construction work, OSHA requires heavy equipment held aloft by slings, hoists or jacks to be substantially blocked or cribbed before employees work under or between it. OSHA’s jack rule also requires a raised load to be cribbed, blocked or otherwise secured at once. OSHA equipment requirements and jack requirements establish those duties; they do not assign a universal capacity to cribbing blocks.

Start with the support load and available space

Before ordering blocks, establish:

• Load at each support: Use equipment documentation or an engineered support plan. Dividing total machine weight by the number of cribs is not enough to establish actual reactions; support locations and center of gravity affect load sharing.
• Approved contact points: Identify where the equipment can bear safely, along with any required top plate or load spreader.
• Height and footprint: Measure both the final support height and the space available to place blocks without entering beneath an unsecured load.
• Foundation: Check the soil, paving or slab beneath the crib—not just whether its surface looks level.
• Application and exposure: Confirm that the blocks are approved for the task, temperatures and expected duration.

Turtle Plastics’ Dura Crib manual specifically requires users to understand the equipment’s load and center of gravity, use trained personnel and place the crib on a flat, level surface. Dura Crib safety manual

Wood versus manufactured composite blocks

Material	Selection advantage	What must be verified
Timber cribbing	Can be cut to suit a documented crib configuration	Species, actual dimensions, condition, bearing intersections and the applicable design
Manufactured plastic or composite cribbing	Available with identified models, load ratings and interlocking surfaces	Permitted orientation, complete-stack configuration, plates, temperature limits and intended use

Timber cribbing carries load through the areas where members cross. Crushing at those bearings can lower the stack; uneven crushing can also destabilize it. The DHS Field Guide for Building Stabilization and Shoring Techniques bases its timber-crib design loads on bearing area, number of bearing points and allowable cross-grain bearing pressure. Those rescue-shoring designs are not a blanket rating for any jobsite 4×4 or 6×6 offcut. DHS field guide, cribbing section

Manufactured blocks reduce some identification uncertainty, but “plastic cribbing” is not one interchangeable product class. ESCO’s composite system, for example, requires its bottom and top plates, prohibits mixing other cribbing materials into the system and requires a complete box crib with no voids. ESCO instruction and safety manual

Read the rating as a system limit

A working load limit is the permitted working load, not the ultimate load that causes complete failure. Use the published working limit, not a failure-test figure. Dura Crib manual, definitions

Additional vertical layers do not multiply capacity: each layer still transfers the supported load downward, and the foundation also carries the crib’s own weight.

Check whether the published limit applies to an individual block or an assembled crib, and under what contact conditions. A sharp bolt, narrow edge or small equipment foot can concentrate force instead of distributing it across the rated bearing surface. ESCO prohibits point loading on edges, points and bolts and requires flat mating surfaces on its top plate. ESCO manual

Do not borrow another product’s height limit or a generic height-to-width rule. Obtain the permitted stack height, footprint and arrangement for the actual blocks. A stack may have enough compressive capacity yet remain vulnerable to shifting or tipping.

Crane approval is a separate check. DICA documents ProStack Pyramid Lock blocks for crane-outrigger support with an interlocking base pad. By contrast, Turtle Plastics’ December 2025 Dura Crib manual excludes stabilization applications with crane equipment. Similar appearance or a high load rating does not make those systems substitutes. DICA setup sheet and Dura Crib manual

Check the foundation beneath the blocks

The crib and its foundation have separate capacity checks. As an illustration, a total downward force of 20,000 lb spread uniformly over an effective 2-by-2-foot bearing area produces an average pressure of 5,000 lb/ft². That arithmetic does not establish soil capacity or prove that a pad distributes load uniformly.

A larger pad helps only if it has sufficient stiffness and strength to spread the load. A concrete slab also needs a capacity check; do not treat a slab over a basement or void as ordinary ground support. The DHS guide explicitly calls for verifying the supporting concrete’s stiffness and capacity and checking that there is no basement below. For covered crane operations, OSHA requires ground conditions, together with supporting materials where necessary, to meet the equipment manufacturer’s support and levelness requirements. DHS field guide and OSHA ground-condition requirements

Before transferring the load—and while it remains supported

Have trained personnel follow the equipment manual and the cribbing system’s instructions. Inspect blocks and plates, confirm ratings and identification, and remove damaged components from service. Keep mating surfaces clean and fully engage the intended interlocks.

Inspection does not end once the load is supported. Follow the system’s loaded-inspection and duration requirements: ESCO calls for inspecting loaded cribs at least daily, while Dura Crib’s manual specifies temporary use and prohibits leaving supported equipment unattended or for prolonged periods. ESCO manual and Dura Crib manual

During transfer, keep personnel out from beneath unsecured equipment. If blocks deform, the foundation settles or the stack shifts, stop the operation and keep people clear; do not pull out a loaded block to correct it. Transfer the load safely before replacement or rebuilding. ESCO’s manual expressly requires safe load transfer before replacing damaged blocks. ESCO manual