5 minute read
CICA Tech Corner
In the CICA Tech Corner article in the September/October 2020 issue, the CICA technical committee discussed the impact of changing ground conditions have on the operation of articulated pick and carry cranes. Since then, CICA, together with Multiplex, has developed an easy-to-use side slope deration calculation tool according to the manufacturer’s instructions to assist crane crews with their lift planning on site.
WHY IS IT IMPORTANT TO CALCULATE
the derated capacity of the articulated pick and carry crane when the crane is operating on the side slope? Crane load chart ratings are based on the machine being levelled in all directions. Mobile cranes operating on outriggers achieve levelling through adjusting the configuration of the crane outriggers, their load charts are designed to operate within the one per cent gradient (or 0.57°) ground slope. Articulated pick and carry cranes are not designed to operate on outriggers – any deviation to firm, level conditions could require the rated capacity to be reduced accordingly to the manufacturer’s recommendations. This is because when the crane is out-of-level, side loads develop in the boom. This side loading causes mobile cranes to lose capacity as the degree of out-of-level increases.
Sometimes people are not aware that they are operating their pick and carry cranes on a side slope. For example, when operating the crane to lift and carry a load on site, if the crane drove on uneven road surface (potholes, etc.), the ground effectively creates a side slope condition. To prevent overloading the crane from these ground conditions, it is recommended that, before the lifting job starts, walk or drive around the site to confirm the lift path/route.
How is the side slope deration calculated? As mentioned in the previous article, deration load charts are provided by manufacturers to guide the operator how to calculate the rated capacity reduction when a side slope is encountered. Crane operators follow the deration percentage specified on the chart to calculate the appropriate crane rated capacity for their lifting configuration and environment.
SIDE SLOPE DERATION CALCULATION EXAMPLE
A Franna AT-20 crane is selected to perform a lifting task with the following configurations: • Lifting on winch • Boom length: 11m, load radius: 7m • Articulation angle: 0°, straight ahead • Ground slope 2°
To calculate the rated capacity for this crane, the first step is to select the correct load chart and interpret the chart with the relevant crane configurations. This crane is intended to lift using the winch with 11m boom length, 7m radius at greater than 10° articulation. According to the chart, the rated capacity of the crane at this configuration is 3,750kg.
The next step is to determine the deration percentage. The rated capacity obtained from the first step is for when the crane is operated on level ground (0.57°) – this crane
operates on ground with a 2° slope. Deration should be made following the manufacturer’s instructions.
From the deration chart, with 11m boom length, 7m lifting radius and 34° boom angle, the deration percentage for this crane at this configuration is 40 per cent.
Rated capacity of the crane = 3,750kg – 40 per cent x 3,750 kg = 2,250kg. In other words, the crane can pick up 60 per cent of its level ground rated capacity.
The CICA articulated pick and carry crane side slope deration calculation tool follows the same calculation process. Follow the three steps below to plan your lift with articulated pick and carry cranes. • Select your crane model, boom length and radius • Enter the articulation degree and side slope degrees • Choose the load weight required to lift, to determine whether the load is within the crane’s rated capacity
The calculation tool is available to access from the CICA website at: https://cica.com.au/resources-safety. CICA Members can save, store, and send the calculations through the CICA Member Portal.
First, select the correct crane model and the lifting arrangement (lifting point, boom length, radius) from the drop-down list. This determines which rated capacity chart will be used. The tool currently has two crane models in the system, AT 20-3 and MAC 25-3.
With the selected crane model and the lift configuration, the tool will display with results for when the crane is lifting at 0° articulation and on level ground.
The next step is to select the articulation degrees and the side slope degrees. The calculation result, which is the rated capacity of the crane under the configuration selected for the lifting operation. If the mass of the load to be lifted is selected, the result can also show the percentage of the rated capacity used.
The calculation tool can also assist with selecting the appropriate crane configuration for the lift. The bar chart at the bottom righthand corner shows the rated capacity of the crane under different configurations.
The red line is the load to be lifted, the dotted rectangle is the actual crane configuration.
Before setting the crane to lift the load, put the numbers in the tool to see which configuration has the rated capacity to lift the load.
For a set boom length and radius, the crane rated capacity is enough to lift a load with a mass of 2.3t when the crane is articulated and operating on a side slope. If the load mass increased to 4t, then the crane can only lift it with 0° articulation (straight ahead) on level ground. This feature could assist with the lift planning, before setting up the crane, run the calculation to see which configuration is suitable for the lift.
For more details on the calculation tool, please contact CICA technical project engineer Alice Edwards by email: projeng@cica.com.au.
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