Application of Optimisation Techniques to Planning and Estimating Decisions in the Building Process
An integrated computer model for time and cost optimisation has been developed for multi-storey reinforced concrete office buildings. The development of the model has been based on interviews completed with Planners, Estimators and Researchers within 2 of the top 20 (in terms of turnover) UK main contractors, and on published literature, bar charts and bills of quantities of concrete framed commercial buildings. The duration and cost of construction of a typical multistorey reinforced concrete office building is calculated through the first part of the integrated model, i.e. the simulation model. The model provides a set of choices for the selection of materials and plant and possible methods of work. It also requires the user to input the quantities of work, gang sizes and the quantity of plant required, lag values between activities, output rates, unit costs of plant, labour costs and indirect costs. A linked bar chart is drawn automatically by using the data available from the simulation model. The second part of the model, (optimisation) uses the data provided by the simulation part and provides sets of solutions of time vs. cost from which the minimum project cost corresponding to the optimum project duration is calculated under the given schedule restrictions. Linear programming is used for the optimisation problem. The objective function is set to be the minimisation of the project cost which is the total of the direct costs of all the activities creating the project and the indirect costs of the project. The constraints are formulated from the precedence relationship, lag values, and normal and crash values of time and cost for the activities supplied by the simulation model. The simulation part has been validated by comparing and contrasting the results with those methods and practices adopted by commercial planners and estimators. The validation of the optimisation part has been undertaken by plotting time-direct cost curves from the results and checking the convexity of the curves. Additionally, the validation procedures included taking account of the opinions of practitioners in the industry on the practical and commercial viability of the model.
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