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  • Writer's pictureRaj Singh

Constructability in Bridge Design #8: Final Crane Selection & Using Crane Charts

The final selection of the crane is a collaborative process involving both the erector and contractor, as there are a plethora of models and brands to choose from. The ultimate decision typically comes down to a commercial choice. When formulating erection plans, it's advisable to specify the smallest crane model that still meets the required capacity, while also allowing for the use of an equivalent. This strategy offers the flexibility to select an equivalent or larger crane, provided it satisfies the capacity and any other dimensional criteria. Once the type of crane has been determined, you can then delve into the detailed process of using technical specifications for the specific crane model.

For instance, if we choose the LTM 1250/1 as our 250-tonne crane, we would undertake the following study to confirm that the crane fulfills the requirements:

Detailed Load Calculation

At this point, it is essential to refine the load that will be lifted. Manufacturers can provide valuable information to this end. An exact weight assessment can be sourced from the fabricator or pre-caster for components such as girders, slabs, and secondary members. It's crucial to cross-verify these assessments with manual calculations to ensure there are no unintentional discrepancies in the data. For girder pairs, the weight should be estimated for the complete assembly, which includes cross members, splice plates, and bolts, among other elements.

The total load calculation should factor in the weight of the rigging, the hook block, and any lifting devices such as spreader bars, tongs, or lifting pad-eyes, in addition to the weight of the component being lifted. This estimate should be reassessed once the rigging design has been finalized.

For instance, let's say our detailed load calculation results in a total of 63 tonnes, which is approximately 139,000 pounds.

Refine Crane Placement and Lift Radius

An essential aspect of technical specifications is the crane's dimensions. This information is vital for understanding the crane's placement on the site and identifying any space constraints and special site preparation considerations. If the crane is to be operated in a highly confined space that doesn't allow the use of outriggers, it significantly affects the crane's stability and reduces its maximum capacity. In the initial planning stage, an assumed footprint of 14m x 8m was used. However, the actual footprint of the chosen crane is 50'2" x 27'11" (approximately 15.3m x 8.5m). This comparison reveals a slight difference in the crane's width and length, which should be considered in the final site preparation and execution.

Plan & Elevation Dimensions of LTM1250/1
Plan & Elevation Dimensions of LTM1250/1

From the above footprint, let's assume that the radius of the lift increases from 10m in the planning stage to 10.97m, which is approximately 36 feet. This information should be obtained from creating a CAD drawing to scale of the plan layout of the site, bridge girders and substructures, and crane.

Selection of Boom Length and Interference Assessment

The next crucial factor is choosing the boom length. It should allow for the lifting and relocation of the load. This process may require raising the boom, known as "boom-up," and/or rotating the boom, known as "swivel," to position the load at its final destination. During this process, conduct an interference check to ensure the boom doesn't have any obstructions. For example, in the working range chart below, a 85' boom obstructs with the pier cap when placing the far girder. However, a 102' boom avoids this obstruction. Therefore, we will use a boom length of 102'. Also, ensure that the chosen boom does not interfere with any overhead electrical lines or other possible obstructions.

Boom Lengths and Interference Check
Boom Lengths and Interference Check

Load Chart Visuals

The first step in this process is to select the appropriate load chart, which depends on the crane's configuration, including factors such as boom extensions or attachments (like jibs or luffers), the size of the counterweight, and the extension of outriggers. In our case, we won't use any attachments to the telescopic boom and will use full outrigger extension.

Next, we need to determine the smallest counterweight that allows the crane to provide the necessary lift capacity at the required radius and boom length. For us, we need to lift 139,000 lbs at a radius of 36ft with a boom length of 102ft.

Each axis on the chart (which can vary by manufacturer) corresponds to either the lift radius or the boom length. For example, on the LTM 1250 chart shown below, the horizontal axis represents boom length and the vertical axis represents the lift radius. Find the cell where these two meet; this value is your crane’s load capacity for the required lift.

The LTM1250/1 offers three counterweight sizes: 214,900 lbs, 187,400 lbs, and 37,500 lbs. The smallest size that provides the necessary capacity for these lift parameters is 187,400 lbs. This counterweight offers a capacity of 154,000 lbs, which exceeds the lift weight of 139,000 lbs at a utilization of 90%.

LTM1250/1 Chart for Telescopic Boom & 187,400 lbs Counterweight
LTM1250/1 Chart for Telescopic Boom & 187,400 lbs Counterweight

Note that the listed lifting capacities do not exceed 85% of the tipping load and include wind forces corresponding to a wind speed of 20 mph (~32 km/hr). If one parameter needs to change, such as needing to perform a heavier lift, you'll need to adjust other factors, like boom length or angle, until a safe lift is possible.

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