A Practical Guide to Using Outrigger Pads
I’ve met a lot of people over the years while working in the utility industry. One of those people is in management with a respected manufacturer of aerial devices. Back when OSHA published 29 CFR 1926 Subpart CC, “Cranes and Derricks in Construction,” he and I and a few others were discussing how a utility operation could best comply with some of the standard’s requirements. The OSHA rules were formed with the perspective of typical construction sites in mind. In particular, we discussed the rule’s expectation that the site’s general manager will tell the crane operator about underground obstructions that might collapse and cause a crane to become unstable. It’s obvious that a crane operator setting structures on a right-of-way doesn’t have that luxury, so we were thinking about things we could do. The discussion landed on auxiliary outrigger pads. At the time, my friend from the aerial device company had this to say: “We have occasionally been sued by folks who turned over one of our cranes or aerial devices, but we have never been sued by anyone who had set up on auxiliary pads.”
I don’t know if that’s still the case with that company, but at the time I began to research why auxiliary pads appeared to be an important part of stable setup for aerial devices. Basically, it’s because sometimes even a few square inches of additional pad dimension can increase ground support by tons per square foot. When it comes to the four-point support of an aerial device that weighs in at tons, tons-per-square-foot increases are a good thing.
The expectation for the stability of cranes is clearly demonstrated by the language OSHA uses in the Subpart CC standard. Take 1926.1402, “Ground conditions,” for example. In the preamble, OSHA explains that due diligence in determining ground conditions will prevent numerous overturns, which are the most frequent cause of crane-related fatalities. The preamble also mentions OSHA’s recognition of the utility industry and our good record of low-incident operation compared to the rest of industry.
On bucket trucks, boom trucks, digger derricks and cranes, the manufacturer-supplied outrigger foot is designed to be used as a bearing surface against an auxiliary pad placed by the user. The fixed factory outrigger foot is optimally sized to provide support for all boom configurations on solid foundations. The fixed outrigger foot size also takes into account space and weight, and the qualified operator is expected to be able to determine what additional support is needed to assure stability. The fixed foot on the outrigger is not designed to accommodate all ground conditions and should always be used with an outrigger pad.
Practical Considerations for Stability
In our industry, OSHA’s expectation for stable setup of bucket trucks and digger derricks is not called out literally. Setup stability is expected to be covered as a collective part of the OSHA standards regarding qualification and work-related safety skills that the employer must certify after observing an employee’s demonstrated skill. So, let’s take a look at some practical things that can improve your bucket truck and digger derrick setup stability.
I need to clarify here that there is little – if any – consensus guidance that a policy writer can turn to. The information that follows are workable methods I used for years when I served as the safety director for a big line construction company. So, keep that in mind. The guidance in this document is based on practices common to the lifting industry, information available from public sources and industry experience. I am providing this guidance as a tool to help the reader in developing their own training or policy because I haven’t found any detailed guide on device setup. Also keep in mind that it is the employer’s responsibility to devise policies and practices to establish workplace safety, including performing due diligence in setting up cranes and aerial equipment in accordance with the equipment manufacturers as well as state and federal requirements.
First, outrigger pads should be used under all outriggers in all surface conditions. If you purchase an aerial device today, it is likely to come with synthetic outrigger pads. They should not be relegated to use in sandy areas only. Bucket truck and digger derrick operating rules often call out setting up on manufacturer-provided outrigger pads. Cribbing (dunnage) is additional support used under an auxiliary outrigger pad. It is added in muddy conditions and stacked to achieve leveling on sloped ground. Cribbing is convenient to add additional size because you can build a 4-foot-by-4-foot pad over a soft spot or mud without having to cart around a 4-foot-square, 200-pound auxiliary outrigger pad. Cribbing also comes out of mud easier (tie a rope to one end) than a 3- or 4-foot-square outrigger pad, and it can be used to raise a pad to level a truck. During my time as a safety director for construction, I would survey the site before mobilization. If we weren’t using crane mats, I would frequently identify a local sawmill that could run a truckload of 3-foot 4x4s to keep on-site for our cranes and buckets used to perform transmission construction. These green wood dunnage pieces are inexpensive, environmentally friendly and can be left behind or given away when no longer needed. They also become a handy goodwill tool for the people you have been inconveniencing for the last few months, although there are rarely any left because lineworkers tend to burn them during winter for heat in the laydown area.
Below is a table that offers guidance on minimum cribbing lengths for digger derricks, bucket trucks and light boom trucks when supplementing factory outriggers with built-up pads or when providing additional support for factory-provided pads. There is no guarantee this table is foolproof since it relies on the proper performance of certain ground conditions. However, after years of following these guidelines, they seem to work well since no bucket or derrick I was overseeing failed to remain upright. Check your operator manuals and you likely will find similar guidance. This table is based on the widths of outrigger feet and a pad dimension increased safety factor of 2.5.
Note: Minimum cribbing lengths shall be 2.5 times the width of the digger derrick/truck crane outrigger foot. Use this table to select minimum lengths of cribbing planks.
Cribbing Under Pads
As I noted earlier, digger derricks and bucket trucks often come from the manufacturer with outrigger pads. Manufacturer pads have historically performed well in support of the bucket trucks and derricks they accompany. However, manufacturer-supplied pads do not relieve the employer of the responsibility to assure pads and cribbing under an outrigger will safely support the vehicle in the conditions present. The operator still must carefully observe the manufacturer’s pads for sinking or deformation during loading. Adding cribbing as described above will limit sinking and bending of the auxiliary pads in soft conditions. If you see one of your pads sink or bend, add dunnage supplement pads as needed. A competent person should attempt to quantify the load-bearing capacity of the soil when conditions are suitable for making those calculations.
Calculations for Outrigger Pads
I was qualified as a crane operator many years ago and recently found some training materials from that class, which provide the following recommendations.
When compaction information is available or a pocket penetrometer is used to measure soil compaction, lift planners may use the following calculations to compute support limits by outrigger pad area for constructed pads. This method was published by NCCCO CraneTech in April 2006.
Method for Determining Crane Outrigger Pad Dimensions When Soil Compaction is Known
The total loaded weight of a crane is divided by the total number of outriggers in touch with the earth to determine the maximum weight that will be placed on each outrigger. The total weight on the outrigger must be less than the weight that can be supported by the earth without further compressing. If the earth beneath an outrigger should further compress during a lift, the rig will become unstable. The weight-load capability of compacted soil, known as the soil’s compressive strength, usually is rated in tons per square foot (tsf). The following process requires that soil compaction be stated in pounds per square inch (psi) in order to estimate the pad dimensions in square inches needed to support the weight to be applied.
The weight of a crane and load cannot be evenly divided among the outriggers because swinging over a single outrigger loads that point more than all of the others. Crane manufacturers design each outrigger to handle the total weight of the crane and load. Using the total weight of the crane plus the load weight computed against the ground resistance to calculate pad size matches the manufacturer’s capacity for the outrigger and ultimately provides a good safety factor for pad applications.
Step 1: Convert tsf to pounds per square foot (psf).
Formula: tsf * 2,000 = psf
Example: 1.5 tsf * 2,000 = 3,000 psf
Step 2: Convert psf (when known) to psi.
Formula: psf ÷ 144 = psi the soil will support
Example: 3,000 ÷ 144 = 20.83 psi
To compute pad area for a lift for calculated crane-plus-load weight:
Formula: square root of total crane weight ÷ soil psi = pad dimension
For example, let’s say the crane and load weight are 78,000 pounds and soil compaction is 20.83 psi. Given that Ö(78,000 ÷ 20.83) = 61, the pad size is 61 inches by 61 inches (approximately 5 feet square).
Cribbing Best Practices
If you are using outrigger pads for cranes that are constructed of wood cribbing (beams or blocks), the following best practices should be followed:
- Cribbing planks should not be less than 3-inch-thick hardwood or built-up plywood.
- Cribbing assembled on-site shall be a minimum of three layers for 3-inch-thick planks or two layers for 4-inch-thick planks.
- Cribbing shall not bend or deform in any manner under loading.
- Each successive layer of cribbing shall be laid at a right angle to the layer below.
- The top cribbing layer in contact with the crane outrigger foot shall be at least as wide as the outrigger foot.
- Cribbing should not be used if it is split, warped or excessively worn.
Guidance on Cribbing Constructed Pads
Cribbing is laid at a bias (right angle) to evenly spread out the load from the crane outrigger foot. Using three layers for 2-inch planking distributes the stresses across all of the boards in the lower two layers, creating a single, larger unit of resistance.
Pads constructed of cribbing (dunnage) planks that are 4 inches thick may be constructed in two layers, provided that the top layer of cribbing is wider than the crane outrigger foot and that the bottom layer is at least as wide as the length of the top layer. If a built-up pad bends under load, additional layers must be laid.
When to Crib, When to Excavate
Cranes, derricks, boom trucks and bucket trucks must be set up levelly in accordance with manufacturer standards. Cribbing alone will not always solve leveling problem, and in some cases, cribbing will make the setup less stable than an incline.
There are no national standards or limits regarding how to build cribbing or how high cribbing can be built. The operator must understand the physics at work to determine how and when cribbing and excavating must be used.
Soil finds its natural slope in accordance with its granular weight, granular shape, moisture content and organic content. This slope is called the angle of repose. The angle of repose can be disturbed by pressure and cause the soil to slide or otherwise be displaced. The greater the angle of repose, the greater the chance the soil can be disturbed.
When setting up a crane on a slight angle, the use of cribbing stacks can level the crane with careful placement of interlocked cribbing of two or three layers. The more cribbing layers used, the less stable the cribbing may be if not properly interlocked and constructed. In addition, cribbing can be destabilized by the outriggers opposite the cribbed outrigger, especially where high angles are concerned. If the angle is too great, an articulating outrigger may engage the soil at a fairly high angle. The higher the angle, the more push is created toward the opposite-side outrigger set up on cribbing.
Sometimes a better choice is to excavate a pad into a slope on which to set up a crane. There is no standard for the size of a pad excavation, but experience teaches that a pad three times the longest length of the equipment to be set up is reliable.
About the Author: After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 22 years to safety and training. A noted author, trainer and lecturer, he is a senior consultant for the Institute for Safety in Powerline Construction. He can be reached at [email protected].