Optimal sub-contractor selection and allocation in a multiple construction project: Project portfolio planning in practice

Subcontracting is known as a global concern in construction industry. Appropriate selection of sub-contractors (SCs) significantly affects scheduling. However, review of the recent literature suggests that the issues of selecting SC and scheduling construction projects are investigated separately. Moreover, general contractors (GCs) usually conduct a number of projects simultaneously, while the literature demonstrates that subcontracting is often studied when the GC is conducting only one project. In order to fill these gaps, the current research study is conducted to investigate the subjects of subcontracting and scheduling construction projects when more than one project must be done at the same time. To this aim, a mixed-integer linear programming model is developed to minimize the cost of the project in two different conditions. In the first condition, the deadline can be postponed by paying a penalty. In contrast, in the second condition, the deadline cannot be postponed under no circumstances. Then, the proposed model is solved by applying the general algebraic modelling system. The findings demonstrate that the proposed model is reliable to provide an optimized schedule when several projects must be conducted at the same time.