Allocation of time buffer to construction project task durations

A variety of buffers are used in design and construction to absorb variation caused by the inherent complexity and uncertainty present in construction projects. This research focuses on time buffers added to construction project task durations. In this research, time buffer is defined as an amount of extra time added during planning to individual task durations to compensate for uncertainty and protect against workflow variation. Although previous research has acknowledged this addition of time buffers, their reasons for use and allocation in practice have not been studied. Excessive time buffers mask the sources of uncertainty that make them necessary. Additionally, overly large buffers are wasteful and can reduce project performance by extending duration, reducing work discipline and coordination. A nationwide survey was administered to project managers, superintendents, and foremen to identify the most frequent and severe causes of the adding of time buffers to construction task durations. Forty-seven individual causes of time buffer were grouped into nine categories: project characteristics, prerequisite work, detailed design/working method, labor force, tools and equipment, material and components, work/jobsite conditions, management/supervision/information flow, and weather. The similarities and differences in perceptions between project managers, superintendents, and foremen in regards to what causes them to add and size time buffers were examined. The research also quantitatively developed risk profiles of the causes of buffer through an integrated risk assessment approach. Further, the causes of time buffer were hypothesized to be correlated and not independent of one another. As a result, the underlying structure of the causes of buffer was examined using factor analysis. This underlying structure served as a basis for the development of structural equation models (SEM) to identify the causal structure used by construction personnel at different levels of management when buffering and planning for uncertainty in their task durations. A model survey was distributed in order to provide a level of model validation and to obtain feedback of recommended adjustments to the proposed models. Also, case studies were conducted to examine the effects of using the collaborative planning philosophy of Lean Construction on time buffer and project performance. The case studies also investigated the buffer allocation associated with various trades and activity types. This research is significant and valuable as previous research has not been conducted in regards to the causes and allocation of time buffer. Previous research also did not investigate the relationship among variation, buffer, and productivity, at least not using empirical data. The topic is of significant importance in its relation to planning and the inherent complexity, uncertainty, and resulting variation in construction. Construction personnel at every level of management are constantly planning and trying to figure out how best to deal with the uncertainty and variability of construction projects. Understanding why time buffer is added and quantifying the influence of different causes on the way construction personnel at various level of management buffer and plan for uncertainty allows construction companies to take effective steps towards addressing those causes, managing uncertainty, and reducing the associated time buffer in construction projects.