Research from North Carolina State University identified factors that cause construction site managers to schedule more time than necessary for specific tasks. Understanding these factors and whether they can be reduced or eliminated could help the industry complete construction projects more quickly.
At issue is a construction planning concept called a time buffer. A time buffer is the difference between how long it should take to accomplish a task based on optimum productivity, and how long you think it will take in the real world. On any job, things can go wrong; bad weather or broken equipment can delay completion of a task. To account for these unforeseen events, construction foremen add time buffers into their schedules.
For example, if the optimum time for a task is three days, and a foreman adds one day of buffer time, the foreman tells his supervisor and project manager that the task will take four days.
“This is important, because construction projects – like building a school or hospital – can consist of thousands of tasks,” says Dr. Min Liu, an assistant professor of civil, construction and environmental engineering at NC State and senior author of a paper on the research. “If every site manager builds a small buffer into every task, it can come to thousands of hours.
“Time buffers are contingencies that are built in, in case something goes wrong – but there is something called student syndrome,” Liu says. “Student syndrome says a student won’t do his homework until the night before it is due. Similarly, if a foreman thinks a task will take three days, but allots four days to do the work, the work is more likely to take the full four days. It’s similar to Parkinson’s Law, which says that a task will fill the amount of time allotted to complete it.
“We did this study to better understand how people determine when to add time buffers, and the length of those time buffers,” Liu says. “This helps us determine how much of a time buffer is actually necessary, and will help us find ways to minimize wasted time in construction projects.”
The researchers analyzed survey results of 180 construction industry professionals from across the United States. They found a number of factors that contribute to time buffers.
Some factors are frequent contributors to time buffers, but do not increase the time buffer by very much. An example of this is a desire to protect the reputation of the construction company. Some factors occur infrequently, but can significantly lengthen a time buffer. An example of this is a delay in getting a necessary permit. And some factors are both frequent and significant. For example, if the task is part of a complex project – like a laboratory facility – that complexity often leads to lengthy time buffers.
“Project managers can use the factors we’ve identified to prioritize their review of construction tasks and target issues related to time buffers,” Liu says. “For example, managers can pay particular attention to factors that are most likely to result in lengthy time buffers in order to determine if those time buffers are necessary or can be reduced.”
The paper, “Application of Time Buffers to Construction Project Task Durations,” is published online in the Journal of Construction Engineering and Management. Lead author of the study is former NC State Ph.D. student Major Marion Russell. Co-authors include Gregory Howell of the Lean Construction Institute and Dr. Simon Hsiang of Texas Tech University.
Note to Editors: The study abstract follows.
“Application of Time Buffers to Construction Project Task Durations”
Authors: Marion M. Russell and Min Liu, North Carolina State University; Gregory Howell, Lean Construction Institute; Simon M. Hsiang, Texas Tech University
Published: online, Journal of Construction Engineering and Management
Abstract: For this research, a time buffer is defined as the extra time added during planning to individual task durations to compensate for uncertainty and protect against workflow variation to assure a predictable hand-off to subsequent crews. Although previous research has acknowledged this addition of time buffers, their use in practice has not been studied. This paper reports on what causes people to add and size time buffers. A nationwide survey was administered to project managers, superintendents, and foremen to identify the most frequent and severe reasons for adding time buffers to construction task durations. Forty-seven buffer factors 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. Contributions to the body of knowledge include (1) identifying the 12 most frequent and severe causes of time buffer; (2) analyzing (understanding) how buffers are viewed differently by foremen, superintendents, and project managers, between trades and between general contractors and subcontractors, and the perception among different levels of experience; and (3) investigating how companies that do not regularly use the Last Planner System and those that do view those factors differently. Additionally, the research quantitatively developed risk profiles of the buffer factors through an integrated risk assessment approach. Understanding the application of time buffers and their associated frequency and severity will help construction managers address potential problem areas and inefficiencies in a prioritized manner.