Optimum solution of discrete time-cost trade-off problem

Completion of construction activities within their normal duration provides the optimum direct cost. However, in this case summation of the necessary expenditures for the operation of the construction site which are defined as overhead cost may reach significant amounts. Therefore in the initial schedule, the sum of the direct and the overhead costs which are considered as total project cost can be higher than the minumum obtainable cost. The project duration can be shortened by crashing some of the activities with an additional increase in the direct cost. This increases the direct cost while decreases the overhead cost. Total project cost decreases when the savings of the overhead cost is higher than the increase of the direct cost. Consequently, obtaining construction schedule which provides the minimum total project cost is defined as time-cost trade-off problem. In this study, discrete time-cost trade-off problem is solved. Determination of construction alternatives for the time-cost trade-off problem requires considerable amount of data collection and editing. Type of equipment to be used, sizes of the crews, and the implemented construction technique directly effects the duration and cost of an activity. While preparing the schedule, the planning department is usually under pressure of time and it is not feasible to determine construction alternatives for every activity which significantly increases the workload of the planning department. In this study, a spreadsheet application is developed which systematically prepares the construction schedule and computes the duration and cost of each activity without human intervention. A macro which implements genetic algorithm is written to minimize the summation of the labor and the overhead costs. The developed macro application does not require any pre-knowledge on the Genetic Algorithm or optimization procedure. Construction schedule which prorites the laeast total construction cost is prepared by the developed tool, this approach decreases the workload of the planning department. The spreadsheet application developed in this study provides the opportunity of maximizing the profits of the contractors by minimizing the total project cost.