DICA was recently featured in the rigging review section of the February 2016 issue of American Cranes & Transport. Part 1 of a 2-part series can be read below or using the above link. This information was originally presented at the 2015 SC&RA Crane & Rigging Workshop in Denver, Colo. where it received high marks for an “excellent, informing and engaging session”.
Setting Up For Success
Part 1 of a 2-part series is excerpted from a presentation made by Kris Koberg, CEO of DICA Outrigger Pads, at the 2015 SC&RA Crane & Rigging Workshop, in Denver, Colo. It reviews pertinent standards and regulations governing crane setup. In addition, it discusses the importance for crane owners and operators to know and understand the outrigger reaction forces of their equipment, ground bearing capacity and ground bearing pressures, rather than relying on rules of thumb to estimate this critical information. This article is specific to outrigger enabled cranes and equipment, however the same principals apply to crawler cranes and other track driven equipment.
OSHA states that cranes must be assembled on ground that is firm, drained and graded sufficiently, in conjunction with supporting materials, such as blocking, cribbing, pads, or mats, to provide adequate support and levelness. (OSHA 1926.1402). It can be difficult to know if the ground beneath your crane will support the weight and pressure under your outriggers, yet knowing this is critical to safe crane setup.
In response to this need, U.S. regulatory bodies and site owners continue to place greater emphasis on understanding ground conditions and using outrigger pads or crane mats that are sufficient to provide the necessary foundation support. In fact, Item No. 1 on OSHA’s site inspection checklist for OSHA compliance officers (Compliance Directive for Cranes and Derricks in Construction Standard issued 10/17/14) is to determine the adequacy of ground conditions beneath the equipment set-up area such as the support/foundation, matting, cribbing, blocking, etc.
OSHA has established that the responsibility for providing adequate ground conditions, including identifying underground hazards, belongs to the controlling entity. If there is not a controlling entity, then this responsibility shifts to the employer. While this is a welcome addition to the newest Cranes & Derricks in Construction rule, OSHA’s multi-employer policy remains in force.
Multiple employers can be cited for the same violation of hazardous conditions. Crane rental companies and their operators must remain vigilant in the evaluation of ground conditions and use of proper outrigger pads and crane pads. They must also notify the appropriate party if the ground conditions are inadequate for safe crane setup before beginning or continuing operations.
Although OSHA does not have a rule regulating the construction, design, or materials used as foundation support, ASME B30.5 provides some insight. ASME states that outrigger blocking or cribbing must have sufficient strength to prevent crushing, bending, or shear failure. And it needs to be of 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 the load. (ASME B30.5-2011).
Essential Elements to Know Before Sizing Outrigger Pads
Knowing the outrigger reaction forces and the ground strength (ground bearing capacity), or the allowable ground bearing pressure, are the essential pieces of information to establish a safe crane set-up (see chart 2). These data points will enable you to know what size outrigger pads or crane pads are needed for a specific crane or lift. Unfortunately, industry rules of thumb are often used to estimate outrigger reaction forces and outrigger pad sizes. Beware, there are inherent flaws in these estimating methods. Rather than using rules of thumb, crane owners and operators need to clearly understand the forces that are being emitted through the outriggers of their equipment, and they need to have an understanding of what pressures the ground they are setting up on can support. If the pressures being emitted to the ground are greater than the ground can support a clear problem will arise.
Understanding Maximum Outrigger Reaction Force (MORF) – The maximum outrigger reaction force that a crane can exert is critical for understanding the strength needs of properly sized pads or mats for individual pieces of equipment. Every manufacturer provides MORF information, with some doing a better job than others. Some manufactures clearly provide this information in their technical specifications, while others provide online calculators that allow users to calculate lift specific outrigger reaction forces. Third-party software, such as 3-D Lift plan, is also a source for similar information. It is not likely that a crane will be used in a scenario that generates the maximum outrigger reaction force. Generally, this force is generated when the crane is at maximum counterweight, maximum outrigger spread, shortest boom length and radius, and executing the maximum pick possible for that crane with the boom positioned directly over one of the front outriggers. For a 100 Ton crane this would mean that it would be making a 100 ton lift. All other lifts would generate an outrigger force that is less than the maximum outrigger reaction force for that crane.
As noted above, rules of thumb have historically been a “go to” estimating tool, despite their inherent flaws. Two examples of common rules of thumb and their significantly overestimated results are highlighted below.
- Crane Lifting Capacity x 85%
- (Total Crane Mass + Load) x 65%
Identifying the Ground Bearing Capacity (GBC) — The ground bearing capacity can be simply defined as the strength of the ground, or its ability to support a pressure. Because identifying this critical factor is very difficult, geotechnical reports, ground penetrating radar, and a geotechnical engineer should be considered as resources whenever possible.
As noted earlier, OSHA has established that the responsibility for providing adequate ground conditions, including identifying underground hazards, belongs to the controlling entity. If there is not a controlling entity, then this responsibility shifts to the employer.
To assist in the assessment of the ground, many companies have implemented the use of roll tests or plate tests as methods for testing the surface, or crust of the ground. These tests are useful and are recommended in the assessment of the top layer of the ground. But be careful. The surface is supported by the subgrade, or the ground beneath the surface layers. In a recent accident in Queensland, Australia (December 2015) the subgrade gave way under the pressure of the outrigger causing the crane to topple to the ground resulting in two fatalities.
To see below the surface of the ground and evaluate the subgrade you will need to employ the use of soil borings, dynamic cone penetrometers, ground penetrating radar or other more advanced techniques to understand the strength of the subgrade and any unknown hazards.
Understanding the Ground Bearing Pressure (GBP) – The ground bearing pressure is defined as the pressure that is exerted onto the ground. The objective is to ensure that the ground bearing pressure (GBP) is always less than the ground bearing capacity (GBC), (see chart 2).
If the ground bearing pressure is greater than the ground bearing capacity the ground will move. As the ground moves it will compress and compact to a point where it will support the pressure being imposed on it, or it will completely fail under the load. Unfortunately, sufficient ground compaction may not occur prior to the occurrence of a catastrophic accident.
Outrigger Reaction Force = 150,000 Lbs Outrigger Float = 2’ x 2’, Area = 4 ft2
Assumption – Outrigger Float Only. No Outrigger or Crane pad is used.
- Pressure under the outrigger float = outrigger reaction force / outrigger float area
- 150,000 Lbs / 4 ft2 = 37,500 psf of Ground Bearing Pressure (GBP)
Assumption: A 6’ x 5’ properly designed and constructed crane pad is used. 6’ x 5’ = 30 ft2 of area.
- Pressure under the crane pad = outrigger reaction force / crane pad area
- 150,000 Lbs / 30 ft2 = 5,000 psf of Ground Bearing Pressure (GBP)
By using the 6’x5’ crane pad the ground bearing pressure can be effectively reduced from 37,500 psf to 5,000 psf, approximately an 86% reduction.
How to Decrease Ground Bearing Pressure and Increase Ground Bearing Capacity
The most common way to prevent the failure of the ground (shear failure) due to ground bearing pressures exceeding ground bearing capacities, is to use outrigger pads or crane pads. Accurately sized and engineered outrigger pads increase the area that the force is emitted through, thereby reducing the ground bearing pressure.
ASME states that “outrigger blocking or cribbing must have sufficient strength to prevent crushing, bending, or shear failure. And it needs to be of 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 the load.” (ASME B30.5-2011).
To ensure that you have selected the right solution based on the ASME standard you will need the assistance of an engineer or a knowledgeable outrigger pad manufacturer. Remember, outrigger pads and crane pads need to be sized for both the outrigger reaction force and the ground bearing capacity, or the desired ground bearing pressure. The larger the area the load needs to be distributed over, the more rigid (generally thicker) the pad must be. If using materials that are not engineered such as wood, be sure to account for degradation that is caused by water, UV, insects, rotting and previous stress.
An additional measure to decrease ground bearing pressure and prevent failure of the ground, is to improve the ground itself. The ground can be improved in many ways, such as compaction, adding rock or other dense inorganic materials, removing un-compacted surfaces, or allowing wet ground to dry.
According to Keith Anderson, author of Rigging Engineering Basics, 1st Edition, “It might make sense to do some of both…” (That is, improve the ground and use outrigger pads or crane pads.) See photo below example of this method).
Editor’s Note Setting up for Success Part II will guide crane owners in how to select the right size outrigger pad or crane mats. As mentioned previously, undersized pads can put you in an unstable condition, but oversized pads are inefficient in terms of purchase, transportation costs and set-up and tear down time. Just as rules of thumb can provide inaccurate results in determining outrigger reaction forces, many are unreliable for selecting outrigger pad sizes. Part 2 will show how sizing rules of thumb lead to widely varying conclusions. It will also show how DICA’s Three Step Fitting Process provides accurate outrigger pad and mat sizing.