A Tower Crane Acquisition: Economic Feasibility, Model Selection, Lateral Support and Foundation Design

A Tower crane plan is divided into a tower crane selection phase, a lateral support and foundation design phase. The respective phases need to satisfy lifting, stability and economic requirements. After a prior examination of stability it is reasonable to determine the economic feasibility by analysing the input cost of the candidate cases. However, engineers do not analyse economic feasibility after establishing lifting plans that are suitable for the construction conditions once they have implemented the construction projects; instead, support design and economic analysis are executed based on data provided by equipment suppliers. This results in only certain parts of various plans being examined that may be drawn based on lateral support designs or the foundation designs of a single tower crane, which makes it difficult to minimize cost.

Flow by Economic Analysis Phase
For tower crane selection, the first step is to review drawings, analyse lifting loads, and examine site conditions to establish a basic lifting plan. Based on the established lifting plan, the location, capacity, and the number of tower cranes are determined. Engineers then select tower cranes that are suitable for the given site conditions. These variables are reviewed in accordance with the cause-effect sequence. After the tower cranes are selected, an appropriate jib length should be selected, and the tower crane type may be altered depending on the jib type. Moreover, tower crane and jib types affect stability, which should all be taken into consideration. After stability reviews of the selected tower crane and jib types have been conducted, their economic feasibility is analysed.

Lateral support design is performed mainly in 5 steps. In the first step, a tower crane is selected and a wall brace structure is designed. When the structure is determined, suitable supporting beams and brackets are selected to carry out a stability review of the axial force of members and support points. If no abnormality is detected during the stability review, the design is completed. Multiple design solutions that are capable of ensuring tower crane stability may be generated, and economic analyses are conducted for those solutions.

There are two types of tower crane foundation – a single foundation or a foundation with piles. These two types have different design and stability review features. In the case of a single foundation, its overturn, shear, bearing capacity, and pile resistance force should be examined, whereas a foundation with piles requires a stability review of the pile resistance force and pile pull-out. Economic analyses of the candidates are then conducted.

The process of making the tower crane plan is composed of steps 1-3. In step tower crane candidates are chosen based on jib types, installation methods, and operation conditions during the first step of tower crane selection. Here it is important to check whether the tower cranes can be rented and information regarding the selected tower crane candidates is utilized for the designs at steps 2 and 3. Step 2 of lateral support design and foundation design is conducted with detailed reviews, so the study proposes a process for each phase. For a simplified process, economic analysis is carried out for each phase of the tower crane selection, lateral support design, and foundation design. The features to be reviewed for the economic analyses (such as equipment types and members) are individually extracted in accordance with the selection process to generate a case.

Tower Crane Selection
As previously specified, tower crane types are selected based on plan requirements such as the site lifting load and working radius, as well as elevation requirements such as the lifting height. The lifting capacity determined by the slewing radius of a tower crane may vary by tower crane type, so plan analysis should be conducted based on the on-site lifting plan as described in the detailed process of tower crane selection (step 1). Here the aim is to minimize the tower crane rental cost and the working radius; and lifting capacity should meet or exceed the requirements. The highest lifting height is then examined to analyse the appropriateness of the given tower crane type. The information on the selected tower crane and jib candidates is saved for the calculation of the rental cost, which is the basis for the processes of step 2 and step 3.

When analysing the plane for tower crane selection, the lifting capacity, adjusted for the working radius, should meet the required lifting load. The working radius defined here differs from the slewing radius (which indicates the available working scope based on the lifting capacity), and it should be reviewed based on the maximum lifting load. That is, tower crane types and jib lengths should both be considered, implying that a single tower crane type may have various jibs that meet the required lifting load. In addition, lateral support and foundation designs may be altered depending on the jib length, even changing the lifting capacity.

Lateral Support Design
The lateral support process of the tower crane requires a consecutive review of stability. There are two types of lateral support methods: a wall bracing method and a rope guying method. The wall bracing method involves firmly fixing the tower crane mast to the building wall or slab depending on the site conditions and location of the tower crane installation. After selecting a tower crane and taking into account the site conditions, project characteristics, and work conditions, both stability and economic feasibility are reviewed for lateral support design, and the tower crane is then firmly braced and fixed. The wall bracing of the tower crane is largely composed of a master frame, a spacing support, and a bracket.

The stability review of the lateral support is performed consecutively. Above all, the information concerning the tower crane selected during the stability review is used to input the lifting load and lateral force, which are the basis for the lateral support design. The wall bracing method is implemented in accordance with the site conditions based on the wall bracing design data and the information of the selected tower crane. Furthermore, after establishing the installation height and intervals of the wall bracing, the components and brackets for the support are set. A wide range of component and bracket specifications should be applied to confirm stability. When applicable components are chosen, compressive stress, reaction and the compressive stress of the bracing part are reviewed. If it lacks stability, such components and brackets should be replaced to ensure stability. These stability reviews and the selection of components and brackets are consecutively implemented to generate various lateral support plans.

The lateral support design process is composed of two steps – the supporting beam selection and bracket selection. Firstly, the information concerning the tower crane selection (tower crane type, section size, and jib length) determined in the previous stage is used while taking into account the lateral support method of the tower crane. The next step is to establish the installation height and spacing; first, the supporting beam installation height is inserted, and then the supporting beam separation is set. The designed lateral force (which is the lateral force applied to the spacing supporting beam at the height where the wall bracing is installed) should be reviewed, and the spacing supporting beam to be installed on a plane and the supporting point coordinates should be set. In addition, the bracing member type should be determined. Initially, structural shape steels such as H shape steel, L-2 shape steel, or L-shape steel should be determined, and then the final member type that can secure structural stability should be selected. Finally, the bracing cost is calculated based on the information regarding the installation method, component length and component types.

Foundation Design
The first part is to determine the foundation specifications by examining the stability of the soil bearing capacity of the foundation and the pile foundation. In particular, the geological conditions of the given site should be analysed when selecting the pile foundation, and if it lacks in soil bearing capacity or if the foundation cannot be enlarged to a certain size, a pile foundation is designed in the second part. Concrete strength is examined as basic data for the stability review of the soil bearing capacity that meets the site conditions. In addition, an allowable soil bearing capacity is the bearing capacity of the loaded ground, where the load per unit area is applied. The foundation size and thickness are analysed to determine the foundation specifications. The first priority is to review the stability of the soil bearing capacity; if it exceeds the allowable soil bearing capacity, the foundation may settle, overturning the tower crane. Thus, its risk should be reviewed. Several reviews should be carried out by changing the pile design or foundation specifications. Furthermore, the shear strength related to the foundation specifications needs to be reviewed. For a stability review of the pile, the maximum loading force and allowable loading force on the pile are examined. Foundation and pile reviews are intended to minimize the required cost.

Since tower cranes are selected in consideration of lifting load and working radius, the lifting load and free standing height are imported from the tower crane selected during foundation design, which is utilized in the design standards. When tower crane selection is completed, the tower crane information and foundation design standards are applied to design the foundation, and rebar specifications and reinforcement of rebar spacing are established. Here, the rebar specification and reinforcement of rebar spacing are generated based on the RC design information, and the minimum quantity of rebar is applied to prevent over-design of rebar. The next step is to review the stability of overturn and shear, and if it is proven to be unstable, the foundation size and reinforcement of rebar should be changed. After this process is repeated to secure the stability of overturn and shear, the bearing capacity is reviewed to determine whether to install the pile. Information pertaining to the economic review of the foundation includes the form installation cost, the quantity of reinforced rebar and concrete.

If it is impossible to secure the bearing capacity because the foundation size is limited due to the site conditions, or if the foundation size is too large to secure the bearing capacity, its efficiency and economic feasibility decrease. In such a case, the foundation size is restricted and the pile is reinforced to generate an efficient foundation design. Pile specifications and quantity should be established to ensure there is sufficient bearing capacity. The foundation size is considered for the designed pile to review spacing, pile bearing capacity, and pile pulling, and the bearing capacity of the completed design is reviewed once again. Moreover, information concerning the pile specifications and quantity are required for an economic review, which should be saved for each case.

Optimization Model
The issue of crane acquisition and management does not only require a study of economic feasibility which considers all the cost influence factors of each acquisition method, but also the policy of managing the equipment. The economic feasibility study is not complicated, but requires a series of cost analyses using cash flow charts and net present value analyses. The policy of managing equipment is a fairly complicated issue to deal with. This study, thus, aims to develop an optimization model for generating cases that require a minimized cost by examining multiple cases, where the stability is already ensured, and calculating the costs required, limited to the rental and installation costs.

When analysing economic feasibility by case, the cost per type differs (including tower crane type, lateral support, and foundation), so all the costs should be converted into the net present value (NPV) for an accurate analysis. The installation cost of the foundation is calculated first, and there is no additional cost until its removal. However, tower crane and lateral support components have rental costs to be paid depending on the rental period after their installation, which is highly influenced by the installation and removal schedule.

The lateral support cost is composed of the installation cost, member rental cost (x2), and removal cost (x3) depending on the installation schedule. The total cost is the cumulative sum of all costs. Firstly, when the lateral support is installed, the installation cost (x1) is the input and the rental cost (x2) generated every month after its installation, increasing the rental cost as the lateral support increases. The removal cost (x3) is the cost of removing all lateral supports, which is generated as soon as the tower crane is removed.

The cost for lateral supports should be calculated for each location, and the cost for lateral supports used for removal cost can be calculated by adding together the current value of the installation cost and the monthly rental cost with the present value of the removal cost.

Multiple cases should be generated in connection with the stability reviews as specified in the economic optimization. This should be conducted, and the costs required can be calculated simultaneously using consecutive selection. The lifting load and lifting height affect the tower crane rental cost as well as the installation/removal costs and telescoping cost. However, the additional mast delivery cost is unrelated to the lifting, and is instead affected by the lifting height, which is related to the quantity of additional masts.

All the costs related to the tower crane are calculated to determine the total cost, which is the basis for the selection of the minimum cost. The stability of the optimum tower crane selection comes before price. This sequential flow in tower crane plans is clear and only cases where stability has been ensured will receive a cost calculation

Conclusion
The optimum tower crane and supporting design management that provides stability and economic feasibility suitable for construction conditions should integrate and consider all the different factors, including engineering, economy, simulation and optimization methods. However, it is difficult for site engineers to have all the related knowledge and such work is time consuming. If the related methods are built into a system, engineers will be able to perform the related works easily and promptly. This study establishes TC selection and support design processes and a related database to achieve this goal, and proposes a method to optimize economic feasibility (cost) assuming that lifting conditions and stability are satisfied. The results of this study are summarized as follows.

First, the database built from the available tower cranes in the market for optimum tower crane selection generated candidate cases in accordance with the lifting conditions, and we confirmed that it is capable of easily and quickly generating cases using a simulation afterwards.

Second, the design components required for lateral support and foundation designs with which to build a database were collected, which made it possible to easily and quickly perform stability reviews. Furthermore, the algorithm proposed for the optimum supporting design was confirmed as being able to generate the optimum design plan after calculating the costs of design plans that meet stability.

Third, we confirmed that the system run time can be reduced by examining the necessity of piles during the support design process related to the selected TC prior to foundation design, and then determining the bracing frame and the bracket details for lateral support.

Fourth, it was proven that even though the tower crane rental cost during cost reviews is minimized, increases in support system costs such as lateral support and foundation may not always minimize the total cost. This is because the TC selected with the minimum cost may require supplementary support systems to secure structural stability, which may increase the total cost.

Fifth, the algorithm proposed in this study simulates all the possible candidate cases, saves them in the database, reviews practical alternatives, and generates a case with minimized cost as the final solution. It should be checked whether the design components can be procured on-site.

Finally, all the costs used in the economic analysis should be converted into the net present value (NPV) for an accurate comparison, since all the costs are generated at different periods. In particular, tower crane and lateral support members (which are rental items) have rental costs to be paid according to the rental period, which is greatly influenced by the installation and removal schedule.