Research on the Construction Schedule and Cost Optimization of Grid Structure based on BIM and Genetic Algorithm

With the rapid development of power engineering in China, grid structure engineering has become one of the most important infrastructure projects in the electric power industry. At the same time, the grid structure has a variety of difficulties, such as complex modeling, many members and so on, which makes the grid engineering in the construction management process particularly complex. However, there are many problems in the traditional grid construction management, such as low production efficiency, more surplus materials, more collision of installation components, which not only increases the construction management cost, but also increases the cost. BIM is the most important construction project management tool, which can directly manage the construction by building BIM model. Through BIM software, the construction unit can realize the three-dimensional visualization and process controllability of on-site construction. Through intelligent member construction, the construction unit can solve the construction management problems of complex grid structure engineering. In the construction process, the project will encounter rush events, which will encounter construction schedule and cost optimization problems. Through the genetic algorithm, this paper constructs the construction schedule and cost optimization model, which will better realize the cost optimization.


Introduction
In recent years, with the development of grid technology in China, the design institute has designed many excellent grid projects. Grid structure is one of the forms of long-span space structure, which has good mechanical performance, stiffness and integrity. Grid technology can effectively bear various loads, which is convenient for structural fabrication and installation [1] . Power transmission and transformation project management involves all aspects, among which the most important is schedule control and cost control, which will affect the smooth implementation of the whole power transmission and transformation project [2] . In the actual grid engineering project management, the project economy mostly adopts the schedule cost optimization method to manage the objectives. For example, reducing the progress of project activities by increasing the cost will meet the requirement of shortening the project schedule with the least cost increase. With the progress of society, the traditional schedule optimization method has been unable to meet the needs of the greatest interests of all parties in the project construction, which leads to the construction schedule cost optimization has become an important research topic of project management [3] . In practical engineering application, schedule control is to compare the deviation between the actual progress and the planned progress. By taking corresponding measures, we can minimize the cost of compression. At present, BIM has become the most widely used schedule control tool in the progress management and control of grid engineering projects. Through BIM, we can broaden our thinking, which can make the project manager pay more attention to the progress control and monitoring. Through the control of the key chain technology, the control of the grid project progress can not only be integrated into the whole task, but also can truly determine the time required for the project. Through a systematic and holistic perspective, we can better carry out schedule management and cost optimization [4] .

Software sharing in BIM Era
BIM system can form a working platform through a server, which can work together with various disciplines. By making the workflow orderly and concise, all kinds of information will be shared, which can achieve the consistency of design results and actual products. In recent years, BIM Technology has been developed rapidly, which has produced a lot of BIM software. Through BIM integrated database, we can carry out correlation and parameterization, which can realize the working platform of different professional cooperation [5] . Through BIM, we can achieve accurate transmission and timely sharing of information between different disciplines, as shown in Figure 1.

Genetic algorithm
Genetic algorithm is a search optimization method based on the theory of biological evolution and molecular genetics, which is an adaptive global optimization probability search algorithm. Through the group search strategy and information exchange between individuals in the group, GA search does not rely on gradient information, which will have a strong macro search ability [6] . Genetic algorithm simulates reproduction, mating and mutation in natural selection and genetics. Through random selection, crossover and mutation, we can generate a new group of individuals that are more adaptable to the environment, which can make the population evolve to better and better areas in the search space. Genetic algorithm is a new global optimization algorithm, which has been widely used in function optimization, combinatorial optimization, production scheduling and other fields [7] .

Basic steps of genetic algorithm
Genetic algorithm can be divided into two kinds of operation process: genetic operation and evolutionary operation. Genetic operation is used to simulate the population replacement mechanism in natural evolution. By generating new species, the algorithm can search the global optimal solution. Among them, evolutionary operation is a process of constantly updating population through competition mechanism. The basic steps of genetic algorithm are as follows. First, the initial population pop (k) is generated randomly. Secondly, based on fitness function, we can evaluate chromosomes. Thirdly, we can select chromosomes to form new pop (k) based on the fitness value. Fourth, through crossover and mutation, we can produce a new chromosome, namely offspring. Fifthly, by iterating over and over again, we can get to a predetermined evolutionary algebra. The iterative flow chart of genetic algorithm is shown in Figure 2.

Establish optimization model
This paper takes into account the owner's payment for a certain work in the project, which is carried out periodically during its implementation. Assuming that the one-time payment in the middle of work is simplified, the relationship between jobs is compiled according to the single code network. At the same time, this paper considers the objective existence of the time value of funds. In this paper, the optimization model can be established, as shown in Formula 1.

Process design of construction schedule cost optimization
Based on genetic algorithm, this paper establishes the resource balance optimization process of construction system, as shown in Figure 3.

Conclusion
Based on BIM, we can establish the overall model framework of grid engineering, which can make the original geometric information more intelligent. Through BIM, we can import all the information such as grid member, welding ball node connection and splicing weld into the whole model through 3D solid modeling. Combined with bar code technology, we can directly present the information as bar code, which will facilitate the site construction personnel to accurately and quickly find the bar. Through the network planning schedule and cost optimization method, this paper calculates the schedule and cost optimization method based on genetic algorithm, which can more accurately consider the direct cost and schedule of grid engineering.