The Tree Care Industry Magazine recently published an article by Emily Duane that covers the importance of proper outrigger pad, crane pad, and cribbing setup. It also addresses the magnitude of risk crane operators face every day.
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Highlights of that article are captured below. To read the article in its entirety, see the TCI website here.
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.
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, specializing in engineered outrigger pads.
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.
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.”
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.
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.”
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.
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“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.”
Resources
“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.”
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.
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Written by Jeff Steiner on September 16, 2019. Posted in Worksite Safety.
An outrigger pad is a safety tool that can be used with any equipment that has outriggers, down jacks, or stabilizers. It is a must for stability when a piece of equipment – such as a crane – lifts loads or personnel aloft. This article will provide an overview of outrigger pads, including how to use them safely and what kinds of pads are available on the market today.
The Basics
Outrigger pads are placed on the ground under the equipment’s outrigger, shoe, float, or foot. The size and thickness of the outrigger pads to be used should be selected based upon the type of equipment, soil conditions of the work site, and type of lift being performed.
When working with outrigger systems, it’s important to understand that the point of contact between an outrigger and the ground is quite small. Because of the pressure of the outrigger, the ground underneath may shift, be displaced, or collapse if an outrigger pad is not used. If any of those things happen, there is the potential for the equipment to shift or tip the load, which could lead to the equipment toppling over. In fact, approximately half of crane lifting accidents are caused by improper use of outriggers.
Outrigger pads were created to stabilize equipment so that the ground below doesn’t shift and the equipment doesn’t topple over. Stability is dependent upon the equipment’s footprint and center of mass. The footprint is the total area enclosed by the support structure of the equipment. The center of mass is the point at which the equipment would balance if it were set on top of a single point to support it. If the center of mass is on the edge of the footprint or outside of it, the equipment will topple. There are two ways to provide extra stability. The first is to use a counterweight to help shift the center of mass back over the footprint. The second is to make the footprint bigger with an extendable outrigger system.
All cranes have counterweights, but there is a limit to how heavy they can be due to transportation and setup considerations. As noted earlier, the ground can only sustain so much pressure before it gets displaced or collapses. Even if you can transport a crane to the work location, there is no guarantee the ground conditions will be stable enough to set up and hold the crane. Because of that, it is essential for the stability of the equipment to use retractable outriggers to extend the crane’s footprint. After deploying the outrigger system, 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.
Six Important Points
As with any safety tools, outrigger systems and outrigger pads must be used according to the instructions of their manufacturers. In addition, keep the following six points in mind when working with an outrigger system.
1. Proper setup and leveling are critical to the appropriate function of any style of outrigger system. The operator is required to follow the manufacturer’s chart regarding the allowable level and grade percentage when setting up the equipment. If the level is incorrect, the equipment’s lifting capacity will be reduced.
2. Prior to setting up, try to acquire 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. OSHA 29 CFR 1926.1402, “Ground conditions,” states the following in paragraph 1926.1402(b): “The equipment must not be assembled or used unless ground conditions are firm, drained, and graded to a sufficient extent so that, in conjunction (if necessary) with the use of supporting materials, the equipment manufacturer’s specifications for adequate support and degree of level of the equipment are met. The requirement for the ground to be drained does not apply to marshes/wetlands.” Per 1926.1402(a)(2), “supporting materials” refers to blocking, mats, cribbing, marsh buggies, or similar supporting materials or devices.
For those who do not work in construction, ASME B30.5-3.2.1.5(i) regarding mobile and locomotive cranes states that “[b]locking under outrigger floats, when required, shall meet the following requirements: (1) sufficient strength to prevent crushing, bending, or shear failure; (2) such thickness, width, and length, as to completely support the float, transmit the load to the supporting surface, and prevent shifting, toppling, or excessive settlement under load ...”
3. Check for current locates and know where utilities are overhead and underground. Maintain proper clearances around overhead power lines. It’s an absolute must to find out the known voltage of the overhead power lines and determine the minimum approach distance for qualified and unqualified personnel.
4. Outrigger pads must not be used to bridge any gaps or span any voids.
5. The maximum lift capacity of the equipment must be known, which includes understanding how to follow the load chart. Be sure to factor in not only the load being lifted but the rigging being used to lift the load as well. 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. A strong job briefing prior to work also is a must.
6. Understand that different pieces of equipment have different outrigger systems. Workers will need to understand how to set up those systems on each piece of equipment they work with. Following are short descriptions of some common outrigger systems found on today’s equipment.
- An out-and-down or vertical pressure outrigger system is very common on 40-ton cranes and above.
- A-frame or 45-degree outrigger systems are common with bucket trucks and digger derricks.
- You will find combination units with both A-frame and out-and-down outrigger systems on many boom cranes, from 30 tons on down.
- Down jacks typically are found on trailers and heavy-haul equipment.
A Rule of Thumb
A known ground-bearing capacity may not be available for every job site or equipment setup. And in a number of industries – including line work, tree trimming, and some crane work – determining the capacity may not be practical, particularly during storm response work. To address the issue, I suggest following a rule that I learned from my father and have been using for more than 20 years. An outrigger pad should be at least three times the square surface area of the outrigger shoe or float, provided you are working on Grade A soil conditions. As soil conditions worsen, continue to create a bigger footprint using outrigger pads and cribbing. After the equipment is set up and level, or within the manufacturer’s grade tolerance, and outrigger pads have been deployed, make a dry run of the boom. Use a spotter to ensure the outrigger pads are not shifting, sliding, or sinking into the ground. Then check to see that the equipment is still level or within grade. If any issues are identified, stop and stow the boom. Now is the time to add cribbing, blocking, or more outrigger pads. The pads should be built to support the maximum load of the pick or lift; to displace the weight of the equipment while keeping the equipment level; and to help the crew work safely.
A Variety of Pads
Outrigger pads are built in different lengths, widths, thicknesses, and shapes. When it comes to shapes, there are pros and cons depending on what you choose. For example, because large square outrigger pads have a large surface area, they are not as easy to move around on the job site as round outrigger pads, which usually can be rolled into position by one person but have less surface area.
In terms of the materials used to build outrigger pads, I have found only three – American custom composite, premium birch, and American steel – that I believe should be used when personnel are aloft or with any critical pick or lift with a load of 10,000 pounds or more.
American custom composite has 3,000 psi and maintains its value for a lifetime. This material is engineered to perform in the most extreme conditions and typically will outlast the equipment it is being used with.
Used in the field since 1991, premium birch has been battle-tested in the harshest conditions with some of the largest equipment. These outrigger pads have a typical lifespan of 10 years.
American steel crane mats are built with high-grade steel and will outlast the equipment they are being used with.
Conclusion
As with any tool of the trade for line
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