A quick Google search of “failed crane tipped over” produces nearly 4.5 million results in less than a second. A search of OSHA’s accident results for “crane outrigger” lists 55 incidents across three pages, including nine relating to cranes tipping over on the first page alone. Each month, the Tree Care Industry Association (TCIA) compiles an accident report from published reports or incidents reported directly to TCIA staff that, unfortunately, often includes crane incidents. Accidents involving cranes result in costly damage, can be lethal and are entirely avoidable with the correct knowledge and preparation.
Cranes are only as safe as their setup and operation. This begins, quite literally, with setting a proper foundation for the equipment using outrigger pads.
Why use outrigger pads?
“An outrigger pad is a safety tool that can be used with any equipment that has outriggers, down jacks or stabilizers,” says Jeff Steiner, co-owner of Bigfoot Construction Equipment, Inc., a 15-year TCIA Corporate Member company based in Woodstock, Illinois. “It is a must for stability when a piece of equipment – such as a crane – lifts loads or personnel aloft.”
While proper selection and setup of outrigger pads is relevant to other equipment, due to the much larger forces involved with cranes, this article will focus there.
Proper outrigger-pad selection and placement helps prevent the ground below the outrigger from shifting and toppling the equipment; they can be used on hard and soft surfaces. “Installing outrigger pads underneath the outriggers will expand their point of contact with the ground and displace the pressure from the equipment through the outrigger pad to the ground,” says Steiner.
He also notes that outrigger pads must not be used to bridge gaps or span visible voids.
Selecting the correct pad size and material
Pads come in a variety of shapes, sizes and materials – it’s important to pick the correct outrigger pad for your needs. Equipment manufacturers are doing a good job of providing outrigger reaction forces for the specific model of equipment you are using.
“Outrigger pads must be large enough to reduce ground-bearing pressures to levels the ground can support and provide stability to keep the crane level during operations,” states Kris Koberg, CEO of DICA, a 22-year TCIA Corporate Member company based in Guthrie Center, Iowa, and specializing in engineered outrigger pads.
Steiner adds, “Pad size depends on the size of the outrigger, or float, on the equipment, soil conditions and the maximum-outrigger force.”
Shape brings an added dimension to the selection process. Pads can be quite heavy, so round pads can be tipped on their edge and rolled around for easy positioning; square pads do not offer this same level of simplicity, especially when the pad is larger. Additionally, your rig’s storage capabilities will determine the size – and, to a degree, shape – that your pads can be. It’s best to familiarize yourself with the size and storage capacity of your equipment and discuss your needs with a trusted manufacturer.
As for material composition of the outrigger pads, common options on the market include engineered composite, wood, steel, FRP (fiberglass reinforced plastic) and fiberglass.
“Pads must be strong enough not to fail under maximum loads and pressures, and they must be rigid enough to effectively spread the load,” Koberg says. “Performance, weight, durability and longevity all are factors to consider. The larger the load-distribution area needed, the more rigid the pad must be.”
Does material stiffness relate to strength?
Stronger does not necessarily mean that a material is stiffer. “Strength, specifically mechanical strength, can be defined as the measure of stress that can be applied to a material before it permanently deforms or breaks,” says Koberg.
Stiffness is a material’s ability to resist bending or deforming. The ability of an outrigger pad to spread load is based on the stiffness of the pad relative to the stiffness of the ground.
“If the pad is not rigid enough to overcome the stiffness of the ground, the load concentrates in a smaller area on the pad,” explains Koberg. “This results in increased ground-bearing pressure, which will cause more pad deflection and shear stress.” These scenarios could result in functional or, worst case, physical failures.
Which forces need to be calculated for the pad/ground?
Prior to setting up, Steiner recommends acquiring recent soil conditions for the work area.
“Knowing the ground-bearing capacity will help workers determine what type of outrigger pad is the best fit for the equipment that will be used.” (See OSHA sidebar)
It’s important to calculate the following:
• Force exerted on the ground based on the crane weight, rigging, load weight, etc.;
• Ground-bearing capacity (GBC) – or pressure – for the surface you’re setting up on;
• Area, defined as force divided by pressure; and
• The square root of the area so that blocking dimensions can be determined.
Ground conditions can be a problem. “According to ProSight Specialty Insurance, ground conditions are the third-leading non-auto cause of operational failures in the crane and rigging industry,” reports Koberg. “Avoid making assumptions about outrigger loads or pressures because that data is readily available. Knowing this information and understanding how pads are designed to mitigate pressure and spread load will improve your safety IQ and reduce your risk of experiencing a ground-conditions failure.”
Is there a cycles-to-failure with outrigger pad use?
The team at DICA defines failure in two ways: functional failure and physical failure. “Physical failure is the physical breaking or yield of a pad. This should not happen if the pad is designed, used and inspected correctly. Functional failure occurs when excessive pad deflection occurs, when the ground-bearing pressure is too great for the application or the specific requirements,” explains Koberg. “Excessive deflection is a sign that the pad is not rigid or stiff enough for the specific scenario.”
Similarly, Bigfoot’s team states that custom-composite outrigger pads should not fail pending correct inspection and use. However, Steiner notes that wood has a lifespan of approximately 10 years depending on use, weather and a few other factors. “It’s important to regularly inspect your outrigger pads for wear and damage regardless of the material,” urges Steiner.
Shear failure as a force factor
When outrigger pads can no longer withstand the amount of compression applied on top and tension on the bottom, the material can tear.
“Shear failure may occur because the loads exerted on the material cause it to bend or deform past its yield point,” Koberg says. “Most outrigger pads that fail do so from shear failure, while most accidents involving outriggers are a result of the ground giving way or the outriggers not being set up correctly. This is why it is important to assess every situation to ensure you have enough pad area to reduce the load, and pad rigidity to spread the load, sufficiently.”
How to inspect and store outrigger pads
Manufacturers provide inspection and usage guidelines for their products that must be adhered to.
“Outrigger pads are a safety tool and must be inspected before each use. Some materials are less likely to show cracks or stress lines, such as FRP and fiberglass. However, most materials will give indicators,” says Steiner. “Proper storage and care depend on the material of the outrigger pads. For example, Bigfoot’s custom-composite pads are not affected by weather when stored on trucks.”
Similar to any other equipment inspection, inspect outrigger pads and mats for material-integrity issues such as cracking, warping, rotting and permanent deformation before using. If any of these conditions are present, do not use the pad or mat. “If using materials that are not engineered, such as wood, be sure to account for degradation that is caused by moisture, UV light, insects, rotting and stress from previous use,” recommends Koberg.
Cribbing and pads
Cribbing serves a different purpose than that of outrigger pads.
“When uneven ground conditions exist, equipment must be set up within level tolerances required by the manufacturer,” explains Koberg. “Cribbing provides additional height where needed to attain the proper degree of level.”
Sometimes both cribbing and outrigger pads are needed. Steiner cautions, “The outrigger pad needs to be bigger than the cribbing, and it must be placed on the bottom. We recommend using the same material for the cribbing as the outrigger pad to address the shear and PSI (pounds-per-square-inch) values.” Proper setup and leveling are critical. “The operator is required to follow the manufacturer’s chart regarding the allowable level and grade percentage when setting up the equipment,” says Steiner. “If the level is incorrect, the equipment’s lifting capacity will be reduced.”
Contributions to safety culture
You might set a crane up on asphalt today and someone’s lawn tomorrow. Manufacturers can’t predict your environment, so they build the machine and a process that is capable of performing a task, and, when the environment changes, you are responsible for mitigating that change.
“When you present outrigger pads to your fleet, you’re introducing a process and, inherently, a safety culture,” says Tyler Elliff, vice president of Precision Crane Service in Windsor, California, and a DICA customer. “It is subconscious at first. You’re putting down your pads, setting up your crane or truck and you may or may not know you just avoided an accident because everything worked well. Any piece of equipment with outriggers is less susceptible to an accident while using outrigger pads. When you don’t use them, you are putting yourself at risk, and when – or if – something happens, it can happen quickly, not allowing enough time to react.”
When confronted with a situation where the subsurface geology doesn’t reflect the norm, by implementing a process using outrigger pads, you very well could have a near miss rather than an accident. “Anyone in their right mind will start to understand the value of that pad. It’s a significant change in your usable surface area and gives you more time to react and make a decision,” says Elliff.
Crane failure as a result of improper outrigger-pad selection and setup is preventable. As the saying goes, “Knowledge is power.” Familiarize yourself with the manufacturer’s specifications for the weight and limitations of the specific equipment you use.
“The maximum lift capacity of the equipment must be known, which includes understanding how to follow the load chart,” emphasizes Steiner. “Be sure to factor in not only the load being lifted, but the rigging being used to lift the load as well.”
“A good manufacturer will help you figure out how to take unacceptable ground pressure and make it acceptable,” says Elliff. “Leveraging your vendors is essential – they’re there to help. Good products and pads with good education helps bridge the gap if you don’t have an engineer on your team.”
DICA (https://dicausa.com) and Bigfoot (https://www.outriggerpads.com) offer a variety of resources to aid in the selection of outrigger pads, as well as safety information and charts detailing ground-bearing
capacities of various soil types, on their websites.
Steiner adds, “A key to safe lifting work is employing qualified personnel who know how to read load charts and have been trained to run the equipment.”
There are many educational resources available on the market, including TCIA’s Best Practices for Crane Use in Arboriculture and its Tree Care Academy Crane Operations Specialist credentialing program, which covers topics such as assessing and overcoming poor ground conditions, safe lifting procedures, rigging and hoisting and job-site safety.
Emily W. Duane, a former TCIA staff member, is a freelance writer specializing in business and marketing topics for the outdoor trades and recreation industries. Her background is in non-profit trade associations, corporate-retail and professional-services firms. She currently is based in Denver, Colorado.
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