2.2 Crane Setup & Outriggers
Key Takeaways
- Most mobile crane on-outrigger load charts assume the crane is fully extended on outriggers, level, with floats firmly supported and tires off the ground unless the chart specifically allows partial extension
- Cribbing and blocking must be sound, large enough to keep ground bearing pressure within soil capacity, and stacked so the outrigger float bears fully and squarely on the support
- On-rubber (on-tires) lifting uses a separate, much lower rated chart and typically restricts travel speed, boom position, and load, and is only permitted when the manufacturer provides an on-rubber chart
- OSHA 29 CFR 1926.1404 requires an assembly/disassembly (A/D) director who understands the procedures and has authority to stop work during crane assembly and disassembly
- The crane must be re-leveled and outrigger support re-verified whenever the setup, ground, or configuration changes, not just once at the start of the shift
Setup Decisions Drive the Load Chart
The setup is where the operator commits the crane to a specific load chart. Choose the wrong configuration — partial outrigger extension when the chart assumes full, on-rubber when only an on-outrigger chart exists, or an unlevel machine — and the posted capacities no longer apply. The exam expects you to connect each setup choice back to which chart governs and why.
Outrigger and Stabilizer Deployment
Outriggers are extendable beams with floats (pads) that lift the crane off its tires and transfer crane and load forces into the ground over a wider, more stable base. Most mobile crane on-outrigger charts assume:
- Outriggers fully extended (and fully set) unless a separate intermediate/partial chart is provided
- The crane leveled (typically within about 1 percent of level)
- Floats firmly supported on adequate ground or matting
- Tires raised clear of the ground so the crane's weight is on the outriggers, not the suspension
If the manufacturer provides intermediate or partially extended outrigger charts, the operator must use the chart that matches the actual extension — partial extension always reduces capacity, especially over the side and rear quadrants where the support base is narrowest.
Float Bearing
Each float must bear fully and squarely on its support. A float that is partly off a mat, bridging a crack, or perched on the edge of cribbing concentrates load on a small area and can punch through or kick out. After setting outriggers and lifting the tires, the operator should walk the crane and confirm every float is solidly seated and the machine is level.
Cribbing and Blocking
Cribbing (also called blocking) is the structural material placed between the outrigger float and the ground to spread load and create a sound bearing surface. Good cribbing practice:
| Requirement | Reason |
|---|---|
| Sound, undamaged material rated for the load | Cracked or rotten timber fails without warning |
| Large enough footprint | Keeps ground bearing pressure below soil capacity |
| Stacked stable and crib-style when tall | Prevents tipping or shifting of the stack |
| Float centered and fully bearing on the crib | Avoids edge loading and point loads |
| Sized from calculated reaction loads | Guessing leads to undersized support |
The size of cribbing or mats is driven by the maximum outrigger reaction during the lift compared with the allowable soil bearing capacity from Section 2.1. Cribbing does not increase the crane's rated capacity — it only protects the supporting surface so the rated chart remains valid.
On-Rubber vs. On-Outriggers
Lifting on-rubber (on tires) and lifting on-outriggers are governed by entirely different rated charts.
| Aspect | On-Outriggers | On-Rubber (On-Tires) |
|---|---|---|
| Stability base | Wide — outrigger footprint | Narrow — tire track/wheelbase |
| Rated capacity | Higher | Substantially lower |
| Chart required | Standard on-outrigger chart | Separate on-rubber chart, if provided |
| Typical restrictions | Level, fully set, tires clear | Tire pressure, boom over front/within limits, low/zero travel speed, reduced radius |
Key concept: a crane may only lift on-rubber if the manufacturer publishes an on-rubber chart, and the operator must follow every condition that chart imposes (tire pressure, boom position relative to direction of travel, allowable travel with load, and reduced capacities). When in doubt, set up on outriggers.
Leveling
Level the crane on the outriggers and confirm with the machine's level indicator. An out-of-level crane increases the loaded radius on the low side, reduces stability, and can defeat the rated capacity limiter. Re-level any time the ground settles, cribbing shifts, or the crane is repositioned.
The Assembly/Disassembly Director (OSHA 1926.1404)
When a mobile crane is assembled or disassembled — for example, erecting lattice boom sections, adding a jib, or rigging counterweights — OSHA 29 CFR 1926.1404 requires that the work be directed by an assembly/disassembly (A/D) director.
The A/D director:
- Must understand the applicable assembly/disassembly procedures (manufacturer's or an approved equivalent)
- Must meet the criteria for both a competent person and a qualified person, or be a competent person assisted by a qualified person
- Has the authority to stop the operation at any time during assembly or disassembly when a hazard arises
- Must address known hazards such as assembly/disassembly area placement, boom and jib pick points, center of gravity of components, lifting capacity of supporting cranes, outrigger/stabilizer setup, wind and weather, and dunnage/blocking under the crane
The A/D director ensures crew members understand their tasks, the hazards, and the warning signals before assembly begins, and confirms that pins, bolts, and locking devices are properly installed before the crane is put into service.
Bottom line: Setup defines the chart. Fully deploy and properly support outriggers, level the crane, never lift on-rubber without an on-rubber chart, and have a qualified A/D director with stop-work authority during assembly and disassembly.
A mobile crane is set up with outriggers only partially extended, but the operator uses the fully extended on-outrigger load chart. Why is this unsafe?
Which statement about lifting on-rubber (on tires) versus on-outriggers is correct?
Under OSHA 29 CFR 1926.1404, which is a required authority or duty of the assembly/disassembly (A/D) director?