Construction sites are facing increases in the size of component modules, aimed at reducing field labour costs and schedules. This prompts the criticality of cranes in completing successfully jobs. As such, the process of selecting a heavy lift crane in completion of a specific project involves an extensive consideration of certain factors that influence the capabilities of handling load on the job site.
When assessing crane requirements in any given project, the loads that ought to be moved form the basis of reference. As such, it is important to determine the heaviest required lift as it determines the size of crane that is to be applied in the project. Other factors that have to be determined under the load include:
- The largest module’s dimensions – This provides an insight on the attachment of a jib at the end of the project in the case of very large modules.
- Crane reach – This is important in setting the crane’s jib and boom configuration. As such, more boom and jib is required with an increase in the crane reach.
- The module’s weight at the furthest point of reach – Considering the furthest point of reach, the weight of the module directly relates to the reach of the crane. This applies in the sense that the furthest reach requires the minimum capacity, an aspect that may subsequently lead to the overall reduction of the crane’s capacity due to the additional boom and jib lengths.
- The rigging type required to lift the module – In the making of proper rigging choices, one has to consider the rigging’s height and the rigging concept. In the case where the module is significantly wide, or where there is use of spreader bars, there is need to apply a fairly tall rigging, an aspect that leads to an increase in the overall required headroom for lifting and setting of the load.
In the process of crane selection, it is important to carry out a review of the site prior to selection. The conditions that exist within a site may determine the choice of crane regardless of the load that is to be moved. Among other factors, the most important factors that should be considered when assessing the site include the sites location, access to the crane and around, underground utilities, soil conditions, overhead lines, prevailing wind conditions, and existing buildings.
The site’s location determines the type and size of the crane that is to be applied. As such, heavy lift cranes require added time to mobilize and demobilize, and add the costs of the project. As such, there is need to select equipment providers that are in proximity. Crane access also presents an important issue of consideration. In this case, it is important not only to consider the module’s weight and the distance to the final location, but also the distance to the pick location. In addition, there is need for enough space in the assembly of the crane. It is also important to consider the underground utilities and the type of soil that exists within a site. For instance, in the case of increased ground utilities, or poor soil conditions, there is need to select a crane with a lower imposed ground bearing pressure. Other existing conditions within the site such as existing buildings or overhead power lines determine the type of jib and boom attachment configuration. Wind conditions have an effect on the stability of the load and the boom during lifting.
Every crane is accompanied with crane load charts, which provide for the crane’s capacity at any given radius. In most charts involving heavy lift cranes, this is referred to as the gross load that is to be lifted below the tip of the boom. As such, one has to look beyond the weight of the module at a given radius as compared to the provided crane’s capacity at a matching radius. There are various deductions that ought to be considered along with the module’s weight, which include: the block’s weight, weight of the rigging, the line’s weight, weight of any ball or jib attached to the point of the lifted load, wind restrictions, and various parts of the line.
The ground-bearing pressure and the existing soil conditions within a site are important factors to consider in the application of cranes within a given project. It is important to consider the ground-bearing pressure at different radii, loads, and swing angles in order to determine the worst-case scenario. Case in point, the worst ground-bearing pressure may not be caused by the heaviest load selected, and thus constructors are discouraged from making such assumptions. If given, the ground-bearing pressure that is allowable determines the selection of the appropriate crane for the job. Generally, the amount of ground-bearing pressure increased with an increase in the capacity of the crawler crane. In some cases, the track arrangements and application of wider trucks may be utilized in the reduction of the ground-bearing pressure.
Being part of the total project cost, operation cost is an important factor in the selection of the appropriate crane for a given project. In general terms, the rental rate of a crane increases with an increase in the crane’s capacity and added accessories such as a wheeled counterweight or a luffing jib. Such an increase in the rental rate also leads to an increase in the cost of mobilization and demobilisation. Other cost considerations include the assembly and disassembly time, time taken to supply the site with the crane, and the cost of improving the ground to accommodate the ground-bearing pressure.
On a normal scale, it is important to choose a crane that applies 85% of its rated capacity during the project. This optimizes the rate of renting the crane. For instance, in the case where a selected heavy lift crane is applied in setting of structural steel, yet it only uses 40% of its capacity for all the lifts, it would mean that the selected crane is excessively large for the job and that the resources