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May. 06, 2024
I've encountered numerous individuals in the utility sector over the years. One notable person is a manager at a respected aerial device manufacturer. Back when OSHA introduced the 29 CFR 1926 Subpart CC, "Cranes and Derricks in Construction," we discussed the optimal ways for utility operations to adhere to the standards. The OSHA guidelines assumed typical construction site settings. Specifically, there was an expectation that the site’s general manager would inform the crane operator about potential underground hazards that might cause instability. This provision doesn't quite fit the realities of crane operations in right-of-way setups. Our conversation inevitably shifted to the topic of auxiliary outrigger pads. My friend had remarked, "We've had incidents where cranes tipped over, but never when auxiliary pads were used."
I can’t confirm if that remains true for the company, but it sparked my curiosity, leading me to investigate the significance of auxiliary pads in ensuring stable setups for aerial devices. Essentially, a modest increase in pad dimensions can tremendously boost ground support, which is crucial given the weight loads involved.
OSHA's emphasis on crane stability is apparent in the language used in Subpart CC. For instance, 1926.1402, "Ground conditions," highlights that understanding ground conditions can prevent many crane overturns, the primary cause of crane-related fatalities. OSHA also notes the utility industry's commendable record of low-incident operations.
For bucket trucks, boom trucks, digger derricks, and cranes, manufacturers provide outrigger feet as a bearing surface for user-placed auxiliary pads. These factory outrigger feet are designed for optimal support on solid ground. However, they aren’t meant for all ground conditions and should always be used in conjunction with outrigger pads.
OSHA doesn’t explicitly define the expectation for the stability of bucket trucks and digger derricks. Stability is generally covered under broader OSHA standards related to qualification and work-related safety skills, certified by employers. Let’s delve into some practical methods to enhance the stability of your equipment setup.
It's important to note that this guidance is based on my experience as a former safety director in a major line construction company, widely accepted practices in the lifting industry, and available public information. It’s intended to aid you in developing your own training or policy. Employers are ultimately responsible for establishing safe workplace practices, including proper setup corresponding to manufacturers’ instructions and regulatory standards.
Firstly, always use outrigger pads under all outriggers, regardless of the surface. Modern aerial devices usually come with synthetic outrigger pads, which should not be limited to sandy terrains. Setup rules often specify using manufacturer-provided pads. Additional support, known as cribbing, can be used under auxiliary pads, particularly in muddy or sloped conditions. Cribbing offers flexibility as you can create a larger support surface without hauling bulky auxiliary pads. Green wood dunnage pieces from local sawmills are an affordable, environmentally friendly option that can be left or shared once the job concludes.
The table below provides minimum cribbing lengths for various equipment when supplementing factory outriggers with built-up pads. These guidelines aren't foolproof and depend on proper ground conditions, but they’ve proven effective over the years. Check your operator manuals for similar recommendations. The table is based on outrigger foot widths with a safety factor of 2.5.
Cribbing Under Pads
Bucket trucks, digger derricks, and other equipment typically come with manufacturer-supplied outrigger pads which have demonstrated reliable performance. However, responsibility remains with the employer to ensure these pads and any added cribbing safely support the vehicle based on current conditions. Operators should inspect pads for signs of sinking or deformation under load, adding cribbing as necessary to maintain stability. A competent person should assess the load-bearing capacity of the soil to make informed decisions.
During my training as a crane operator, we used the NCCCO CraneTech method (April 2006) for determining crane outrigger pad sizes when soil compaction data is available. The following process helps estimate pad dimensions needed to support crane weight:
Divide the crane's loaded weight by the number of outriggers in contact with the ground to determine the weight per outrigger. Ensure this weight is within the ground's bearing capacity to avoid compression and instability. Soil compressive strength is usually given in tons per square foot (tsf), converted to pounds per square inch (psi) for the calculation.
Crane manufacturers design outriggers to handle the crane's total weight, providing a safety margin for pad sizing.
For more details, please visit XINXINGHUAGONG,Outrigger Pad.
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