What then is the starting point for calculating the required mat size for a crane, after all four outriggers have been fully extended and pinned, so that they cannot retract under any circumstances. The author proposes a step-by-step methodology (see box).
This formula could be applied to the crane that tipped to the rear in New Zealand on Thursday 24 September 2020. The crane was fully telescoped out and at a low boom angle.
The crane owner said that the lifted load was only 150kg. So how can a 90-tonne capacity crane be overturned by a 150kg load? The answer is physics, as described by Isaac Newton in his 1687 work, Mathematical Principles of Natural Philosophy (Philosophiae Naturalis Principia Mathematica).
So let’s apply physics to this overturned crane. First, the stated 150kg load held at the end of its 50m-long boom applied 7.5t of load moment to the crane depicted above. According to the manufacturer’s duty chart for this particular crane, lifting 1.3t at 48m radius requires a full ballast requirement of 28t. However, no rated capacity is given for the lower boom angle used in the incident, and the crane was only fitted with two ballast blocks of 1.5 and 3.6 tonnes each (5.2t total) for counterbalance.
The crane has a gross weight of 48t, plus 5.2t of counterbalance ballast fitted, plus the load. The total gross mass was 53.35t sitting on four outrigger mats of approximately one square metre each; they imposed a load of approximately 13.34t/m2 on the earth below.
What led to this crane overturning was a combination of factors, as identified in WorkSafe’s report: such as a fully-extended boom at a low angle and a long radius which applied a high tonnage per square metre to the soft earth under the crane’s four hydraulic jacks on fully-extended outriggers. This was more than the ground could withstand, and gravity took over, tipping this crane to the rear and dropping the boom on the steelwork in the background, in a self-evident breach of the employer’s duty of care to all others in the vicinity of its work. Fortunately no-one was killed on this occasion.
This unsafe system of work caused by poor outrigger set-up is not unusual, and has occurred in nearly half of the 1,291 crane and lifting incidents worldwide personally recorded by the author since May 2007.
It is now highly recommended to train, instruct and supervise all lifting operations, as force, mass and gravity are ever-present factors that can never be completely excluded from lifting operations.
Calculation
The site engineer advises the appointed person (AP) of the ground bearing pressure of the actual lift site using the normal formula of tonnes per metre squared, or t/m2, which the AP can then use to produce a lift plan.
2. The AP then calculates the gross weight of the crane, including all standard and extra ballast blocks actually fitted.
3. Also to be included is the total gross weight of the load to be lifted, including the weight of all slings, shackles and lifting beams.
When the totals of steps 2 and 3 are known, the outrigger mat size calculation continues.
4. Take the crane gross vehicle weight, including ballast, and add the load and rigging. This determines the gross weight to go through all four outriggers.
5. Divide the gross weight by the ground bearing pressure. This yields the total area required for all outriggers, in t/m2.
6. Divide the quotient by three, not four (to give a 25% safety margin) to find the area required for each of the four outriggers, in m2.
7. Take the square root of that quotient to determine the length, in metres, of one side of a square outrigger pad. This should always be rounded up (never down) to achieve a 25% safety margin per mat.