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Investigating lateral buckling of joists as a cause of falls from elevation.

Hindman-DP; Nussbaum-MA
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R21-OH-008902, 2010 Nov; :1-78
Falls from elevation represent one of the most common construction safety incidents in residential construction. The focus of most previous fall-related research has been somewhat vague in the definition of the causes of falls as well as specific remediation. This research project investigated a particular fall mechanism - the lateral buckling of unbraced joists, particularly wood composite I-joists. The goal of this research was to explore the relationship between construction worker loadings and the mechanical behavior that causes lateral buckling of wood composite I-joists. This research project was novel in the study of a particular fall related mechanism, namely lateral buckling, which has not received much research interest. Intensive study of lateral buckling situations allowed researchers to formulate prevention strategies to enable workers to complete their task while reducing the occurrence of lateral buckling. Lateral buckling occurs in unbraced joists when a vertical load is applied. If the load is greater than the critical buckling load, the joist begins to deflect laterally (perpendicular to the length) and twist. When a dynamic load such as a person walking along the joist exceeds the critical buckling load, the joist can also begin to sway and wobble. As the worker moves towards the center of the joist, the amplitude of this sway and wobble become larger, possibly serving as the impetus for a loss of balance or fall. Many accident reports are nonspecific and give little evidence to help determine the root cause of falls, especially when lateral buckling is involved. Lateral buckling could easily be attributed to worker error or a slippery surface. Since lateral buckling is largely a stability concern with the joist, if the load is removed (i.e., worker steps off or falls), the joist returns to its initial position with no evidence that anything has happened. Lateral buckling occurs with all wood and wood composite beams as well as cold formed steel sections used in construction. Wood composite I-joists were chosen for this study as an example material due to the PI's familiarity and the tendency of the I-joists to buckle. This research used a P2R2P (practice to research to practice) approach to accomplish the specific aims. The first practice was the observation of construction workers walking on unbraced joists during their tasks. Special attention was provided to the position, and the amount of tools and materials carried. Observation of worker loadings demonstrated the need for workers to walk on unbraced I-joists. Typically, workers were observed with tool belts and off-center loads of materials and tools. Measurements of both the static and dynamic loading of the I-joists were conducted. Static loading used a point load applied at the mid-span of the I-joist using a specially designed loading fixture to eliminate any bracing effects. This testing examined the variability of load readings from the I-joists. Test subjects walking on a safety platform experienced lateral buckling behavior of the I-joists. Loading results indicated that the lateral load force applied to the I-joist by a worker walking varied from 9.3% to 72% of the test subject's static weight. This lateral load was greater than previous studies which had documented the lateral load associated with workers walking on a flat surface. Increased lateral load poses a particular problem in lateral buckling in that this tends to decrease the vertical load component at which buckling occurs. A finite element model was constructed using a pseudo-dynamic loading method. For two of the three I-joist types, the I-joist movement was predicted to within a cumulative error of 10% until the test subject reached the mid-span of the beam. The loading data for the test subject walking after mid-span was dominated by secondary vibrations and could not be predicted. A deterministic model was also presented which included the end support conditions and stiffness of bracing elements, but was not validated. To remediate lateral buckling, this research created a temporary joist stabilizer. This stabilizer could be used by workers to latch onto the joists in front of them creating a safe walking area. As work progressed along the length of the I-joists, the temporary brace could be moved to a new position. There is little to no research available on the movement of joists in partial bracing situations. To examine the amount of bracing stiffness and spacing needed, a second walking study was conducted using the safety platform. Braces of five different stiffness and two different spacing along the I-joist were tested. Bracing stiffness was found to be a more important factor than the bracing spacing to reduce the joist movement. Lateral displacement and twist of the joist increased as the test subject's static weight increased, while lateral acceleration did not change. This project provides the knowledge and background study needed to pursue the temporary joist stabilizer as a new invention to help workers avoid situations where joists may become unstable. Future plans include the production and development of this stabilizer as well as further testing and validation of the final product.
Construction; Construction-industry; Construction-workers; Fall-protection; Wood; Risk-factors; Workers; Accident-potential; Accidents; Injuries; Traumatic-injuries; Humans; Men; Women; Models; Weight-factors
Daniel P. Hindman, Associate Professor, Department of Wood Science and Forest Products, 1650 Ramble Road, Blacksburg, VA 24061
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National Institute for Occupational Safety and Health
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Virginia Polytechnic Institute and State University