What Must Be Placed On Timbers Or Cribbed

You're standing on a job site at 6:47 AM. The crane operator is waiting. Which means the lift plan says "crib the outriggers. " The foreman hands you a stack of 4x4s and says, "Make it solid.

Do you know exactly what goes on those timbers — and what never should?

Most people don't. This leads to they guess. And that's how accidents happen.

What Is Cribbing, Really

Cribbing isn't just stacking wood. It's an engineered temporary foundation. You're building a load-bearing structure that might hold 50,000 pounds — or more — on soil that could be anything from compacted gravel to last week's mud.

The timbers (usually 4x4, 4x6, or 6x6 hardwood) get stacked in alternating layers. But each layer crosses the one below at 90 degrees. It spreads the load. It resists tipping. That's the "crib" part. It gives you a flat, stable platform on ground that isn't.

But here's what gets missed: the timbers aren't the foundation. They're the load distribution system. What you place on top of them — and how you place it — determines whether the whole thing holds or folds.

The difference between cribbing and dunnage

Dunnage is loose lumber used to keep materials off the ground, separate loads, or create airflow. It's inspected. Cribbing is structural. It's built to a pattern. It's rated.

If you're using 2x4s scrap-piled under a crane outrigger, that's not cribbing. That's a lawsuit waiting to happen.

Why This Matters More Than You Think

A 90-ton crane exerts 100+ psi on its outrigger pads. On bare soil? But that pad sinks. The crane goes out of level. Because of that, the load chart goes out the window. The boom comes down.

Cribbing done right drops that pressure to 15–25 psi. Soil can handle that.

Done wrong — wrong timber size, wrong stacking, wrong material on top — and you've built a house of cards that looks solid until the moment it isn't.

OSHA 1926.5 says outriggers must be "properly supported.So " Neither tells you exactly what goes on the timbers. ASME B30.That's why 1402 requires that ground conditions support the equipment. That's on you.

What Must Be Placed on Timbers or Cribbing

This is the core question. The answer depends on the application, but the principles don't change.

Crane and equipment outriggers

This is the most common use. The outrigger float (pad) sits directly on the top layer of cribbing. Nothing else. In practice, no rubber mats. In real terms, no steel plates between. No plywood "to protect the wood.

The outrigger float bears on the timber. Period.

Why? So because you need full, even contact across the entire float surface. Day to day, plywood compresses unevenly. In practice, a steel plate creates a hard spot. Both defeat the load-spreading geometry of the crib.

If the float is damaged, warped, or smaller than spec — fix the float. Don't shim it with random material.

Jacking and shoring loads

When you're jacking a bridge segment, a pre-cast panel, or a structural beam, the jack head (or load cell, or bearing plate) goes on the crib.

Key rule: the bearing surface must be flat, rigid, and larger than the jack base.

A 12-inch jack base needs at least a 14x14 timber cap. But they sit on a timber cap layer, not directly on the crossed timbers. Steel bearing plates are standard here — they are the interface. The cap distributes the point load across multiple timbers below.

Heavy equipment tracks and wheels

Sometimes you crib under a dozer, excavator, or haul truck to level it for maintenance or to get it off soft ground.

The track shoe or tire goes on the crib. But — and this matters — you need a continuous timber mat (a "crib mat") across the full track width. Not individual blocks under each shoe. Point loading on cribbing crushes the top timbers and destabilizes the stack.

Rigging hardware under load

Snatch blocks, equalizer beams, spreader bars — if they're landing on cribbing during a pick, the load-bearing fitting goes on the timber.

Not the hook. Not the shackle body. The bearing surface designed for compression.

A shackle pin bearing on a 4x4 will crush the wood fibers. A spreader bar's end fitting, designed for flat bearing, won't. Know the difference.

Structural components during erection

Columns, beams, trusses — when they're stood up and temporarily landed on cribbing before final connection, the base plate or bearing stiffener goes on the timber.

Not the flange. Worth adding: not the web. The engineered bearing surface.

And the crib must be tall enough to get the piece to connection height without shimming the top. Every shim is a failure point.

What Must NOT Go on Cribbing

This list is longer than most people realize.

• People. Never stand on cribbing. It's not a scaffold. It's not rated for live loads.
• Materials storage. Pallets of brick, bundles of rebar, stacks of formwork — these are sustained loads. Cribbing is for temporary, monitored loads during active operations.
• Vehicles not part of the lift. Don't park the crew truck on the crane cribbing "just for a minute."
• Improvised shims. 2x4s, plywood scraps, rocks, hard hats — I've seen all of them. Each one creates a pivot point.
• Multiple pieces of equipment on one crib. One crib, one load. Sharing defeats the engineering.

How to Build Cribbing That Actually Works

Timber selection

Species matters. Douglas fir, southern yellow pine, oak — dense, straight-grained, knot-free. No spruce. No hem-fir. No pallet wood. No treated lumber (chemicals weaken fiber structure over time).

For more on this topic, read our article on safety data sheet has how many sections or check out height of a railing in stairwell.

Dimensions: Minimum 4x4 for light loads. 6x6 or 8x8 for heavy. Length: 24–48 inches. Longer timbers span soft spots better but are heavier to handle.

Condition: No splits longer than 1/4 the timber width. No rot. No insect damage. No oil or chemical saturation. If you wouldn't trust it under your own truck, don't use it.

Stacking pattern

Standard box crib: Two timbers parallel, two on top perpendicular. Repeat. Each layer centered on the one below.

Overlap: Minimum 4 inches past the crossing timbers. Less than that and the ends crush.

Height-to-width ratio: Never exceed 3:1. A 24-inch wide crib goes max 72 inches high. Taller? Widen the base.

Level every layer. Use a torpedo level. Not your eye. Not "looks good." A 1/4-inch lean at the bottom becomes 2 inches

at the top. Day to day, check each layer. Tap it true with a sledgehammer if needed.

No gaps between timbers. Solid bearing. Air gaps concentrate load on the edges and crush fibers.

Base preparation

Ground must be competent. Compacted subgrade. Gravel pad. Mud mat. Never on backfill, frozen ground, or saturated clay without verification.

First layer flat. If the bottom timbers rock, the whole stack walks. Level the pad. Use a plate compactor. Take the time.

Drainage. Water under cribbing = moving cribbing. Slope the pad. Trench if you have to.

Inspection Protocol

Before every shift. Walk every crib. Look for:

• Crushed fibers at contact points
• Splitting propagating from ends
• Timbers shifting out of alignment
• Base settlement or water accumulation
• Missing or damaged pieces

During the lift. Assign a spotter only watching the cribbing. Not the load. Not the boom. The cribbing. Radio communication to the operator. "Crib holding." "Crib shifting — stop."

After load-off. Inspect again. Cribbing that took a heavy pick may be compromised for the next one. Rotate stock. Mark used timbers. Retire anything questionable.

Common Failures — And the Incidents They Cause

The "tall and skinny" crib. 6x6 timbers stacked 10 feet high on a 24-inch base. Lateral load from wind on the piece — or just the crane's swing torque — kicks the top out. Column drops. Structure collapses. Seen it.

The "shim stack" disaster. Three 2x4s, a piece of plywood, a wedge driven in with a sledge. Load shifts. Shim shoots out like a projectile. Piece tilts. Rigging slips. Worker in the line of fire.

The "pallet wood" collapse. Free pallets broken up for cribbing. Mixed species. Hidden cracks. Nail holes. One timber crushes at 40% of rated capacity. Progressive failure takes the whole stack.

The "frozen ground" surprise. Cribbing placed on hard-frozen clay. Afternoon sun thaws the top inch. Mud slurry. Crib sinks unevenly. Load goes eccentric. Shackle side-loads. Pin bends. Hook opens.

The Discipline That Prevents All of It

Cribbing isn't carpentry. It's structural engineering executed by hand, in real time, under load.

Every timber placed with intent. Also, every layer checked. Every assumption verified. Every shortcut refused — especially when the schedule screams and the crew is tired and "it'll hold, we've done it a hundred times.

The hundred-and-first time is the one that kills someone.

Build it right. Inspect it relentlessly. Respect the physics. The timber doesn't know your deadline. It only knows the load.