Should You Rely on Rules of Thumb for Ground Bearing?

This past July, I penned an article for Crane Hot Line (“Growing Support”) in which I summarized current industry efforts in the development of best practices and standards for lift handling equipment (LHE) ground support. While that “Guide to the Guides” highlights some of the latest documents under development, along with other foundational (pun intended) technical works that we regularly rely upon, it was silent about another set of guides that our industry regularly uses — the widespread, seemingly eternal, and somewhat controversial, “Rules of Thumb.” 

Skyrocketing demand for increased load handling capability has in turn led to breathtaking increases in LHE size and capacity. These increases have culminated in higher ground force reactions to be dealt with. Thankfully, a quasi-epiphany in attention to ground conditions and load distribution methods is now underway. While increased recognition of these hazards is leading to the development of enhanced best practices and standards, compressed schedules and increasing workloads continue to put pressure on day-to-day LHE operations to find and use quick workarounds, driving pressure to rely on rules of thumb. Controversy arises in the clash between using codified guides versus relying on anecdotal “knowledge.”

Before evaluating the merits of using rules of thumb, let’s start by establishing a working definition of the term. Though the exact origin of its use is unknown, the idiom is generally believed to have been coined in the 17th century, evolving from various trades and crafts using the width of their thumbs as a means of measurement. Today, it’s defined in the Cambridge Academic Dictionary as “a method of judging a situation or condition that is not exact but is based on experience.” The Oxford Dictionary defines a rule of thumb as a “broadly accurate guide or principle, based on experience or practice, rather than theory.” 

In short, we can view rules of thumb as broad, experience-based guides used within a decision-making process. These are problem solving approaches that rely on practice and experience rather than scientific research or theoretical foundation. They provide approximate solutions which may not be optimal but may be adequate in resolving an immediate need. Developed from previous experience in similar situations, things may generally perform as expected, however, systematic, or unanticipated errors potentially can result.

The use of rules of thumb is prevalent among all industries and honestly, we use them daily in our own lives. As an example, in daily living applications, some personal financial planning “rules” that are widely touted include:

• Keep 3 to 6 months of living expenses saved.

• Maintain 5 to 10 times your income value in life insurance.

• 4% withdrawal rate during retirement.

While guidelines such as these may prove adequate for any number of people, they are anecdotal, highly generalized, and do not consider risk profiles, timing, or extended financial demands potentially affecting individual needs.

The heavy lift industry possesses its own rules of thumb. Examples, unique to the world of ground conditions and outrigger pads include:

• “Rule of 3”: using the area (in square feet) of a single LHE outrigger float, multiply the value by a factor of 3, and round up to the next whole number. This represents the minimum pad size to be placed beneath each outrigger. Using this rule of thumb, a 2' x 2' float would require a minimum 12-sq.-ft. pad beneath each outrigger float.

• “Rule of 5”: using the LHE’s maximum lifting capacity in tons, divide the value by 5, again rounding up to the next whole number. This represents the minimum pad size to be placed beneath each outrigger. Using this rule of thumb, a 60t US capacity crane would require a minimum 12-sq.-ft. pad beneath each outrigger float.

• “75% Rule”: Quoting from “Assessment and management of outrigger loading,” published by the UK’s Temporary Works forum (TWf2022:02):  “In the UK, the most frequently used rule of thumb for all-terrain type mobile cranes is the ‘75% rule’, which is taught on the CPCS “Appointed Persons: Lifting Operations” course, and asserts that the maximum outrigger load will not exceed 75% of gross weight of rigged crane (including all counterweight) plus 100% of the hoisted load (including all accessories and hook block etc.). It is recommended that this rule is not used for cranes with a capacity in excess of 160 tonne.”

Similar to the general applicability of the personal finance guidelines, these outrigger pad rules may yield adequate ground support in many situations, while failing horribly in others by not addressing specific soil or lift conditions. 

Because of the potential for failure, there are many that argue vehemently against the use of rules of thumb. Despite the generalized nature of these rules, there are those that do advocate for them. It may seem surprising, but the medical profession in particular sees value in their use and is studying their effectiveness in clinical applications. A 2015 article in the New England Journal of Medicine (NJEM Knowledge+) quoted the late Alvan Feinstein, a Yale professor known as the “father” of clinical epidemiology: “Every observant clinician has discovered that certain ‘short-cuts’ or other maneuvers, either of intellect or of action, can increase the efficiency of his work in clinical practice.”

Quoting further from the NJEM article: “These cognitive shortcuts, or rules of thumb are also known as heuristics. Because they assist in simplifying difficult decisions, they help clinicians (decision makers) avoid ‘paralysis by analysis’ when operating under conditions of uncertainty that demand speed and immediacy of action. In that way, they can improve decision-making effectiveness and help shape the decision-making process. That said, they can lead to mistakes by failing to include elements that are not readily apparent.”

My opinion on rule of thumb use in our industry has shifted from hardline opposition in applying any and all such rules, to something slightly more in keeping with that of the NJEM article. Heuristics can and do have value in helping shape the decision-making process and they do offer a bit of a “gut-check” or “smell test” in developing a solution.

Regarding outrigger pad sizing rules of thumb, I strongly advise against continued use of both the Rule of 3, and the Rule of 5. Both carry the appearance of having some science behind them as there is rudimentary math involved in “calculating” the pad size, but I’m unaware of anything in the way of documented research that factually validates relying on either numeric factor. 

Simply relying on these rules ignores the other highly critical half of the issue, the ground itself. Without having an established, allowable ground bearing capacity to compare the outrigger reaction force against, the adequacy of the sizes arrived at under Rules 3 or 5 have no basis in reality – they’re simply glorified rolls of the dice. 

The 75% Rule does possess a level of legitimacy insofar as generating a potential maximum reaction force imposed on the ground by any single outrigger on the LHE. However, in and of itself, the calculation does not yield a surface area value for the underlying outrigger pad — more information and math are required to size the pad. Again, without an established, allowable ground bearing capacity to compare the outrigger reaction force against, the size of the pad realistically can’t be determined. Inasmuch as virtually no lift will reach the LHE’s maximum lifting capacity, this method will most likely also result in oversizing the pads.

Unlike doctors in an Emergency Room or operating suite, we are rarely faced with truly emergent, life or death scenarios that demand immediate action. Difficult as it may seem, schedules actually can be controlled to allow time to calculate lift-specific ground bearing pressures (reaction forces), and virtually all LHE manufacturers now offer software and apps that greatly ease calculating those lift-specific forces. 

Given the tools and technology available to our industry today, my “rule of thumb” is to take the time and get the needed information and do the math – be specific, be accurate, and move on from the approximations. 

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Michael Walsh is president of the Dearborn Companies, leaders in construction and temporary-works consulting and specialists in heavy lift and rigging engineering.  He can be reached at mwalsh@dearborncos.com