Comparison of biomechanical loading during use of conventional stud welding equipment and an alternate system

Abstract

We investigated the effect of an alternative welding system designed to reduce exposure to extreme trunk flexion on measures of trunk inclination and muscle activity. Among 10 participants, data were collected while using conventional stud welding equipment and while using the alternate system. Paired t-tests were used to compare results between the two welding systems. Mean trunk inclination angle was reduced with the alternate system (34.4° versus 9.7°, p < 0.01). Percent time with trunk inclination angles greater than 60° was also reduced (40.0% versus 4.7%, p < 0.01). In general, the alternate system resulted in less desirable upper trapezius muscle activity levels. The alternate system appears to be effective in reducing exposure to extreme trunk flexion among stud welders. Continued development of the system should explore features designed to reduce shoulder forces and improve productivity.

Introduction

Musculoskeletal disorders are common among workers in the construction trades (Schneider, 2001). Low back pain, in particular, is a major source of morbidity and disability among workers in several trades, including ironworkers (Elders and Burdorf, 2004, Forde et al., 2005, Goldsheyder et al., 2002, Goldsheyder et al., 2004, Latza et al., 2000, Merlino et al., 2003). Physical risk factors in the working environment, including repetitive motion, awkward postures, forceful exertions, and whole-body vibration have been associated repeatedly with low back pain (Bernard, 1997, Burdorf and Sorock, 1997, National Research Council – Institute of Medicine, 2001). Evidence of these associations from ecologic, case-control, and cross-sectional studies is supported by the results of more recent prospective cohort studies (Hoogendoorn et al., 2000, Van Nieuwenhuyse et al., 2006).

Previous studies of physical risk factors associated with ironwork have focused on concrete reinforcement tasks (Albers and Hudock, 2007, Buchholz et al., 2003, Dababneh and Waters, 2000, Forde et al., 2005, Vi, 2006). Exposures to physical risk factors among ironworkers specializing in welding activities have not been reported.

Ironworkers weld shear stud connectors (Fig. 1) to structural steel to strengthen steel and concrete composite materials. On road bridges, for example, shear stud connectors are used to 1) anchor the concrete slab to the structural steel, 2) increase longitudinal shear load bearing capacity, and 3) reduce the amount of steel needed in the structural members, minimizing overall steel cost. A typical shear stud connector consists of a steel rod between approximately 8.0 cm and 26.0 cm length and up to 1.9 cm in diameter, with a flange at one end. A ceramic ferrule is used to keep heat and molten material within the appropriate zone when welding a stud to the steel beam (American Society of Civil Engineers, 2002).

Ironworkers who install shear stud connectors are exposed to prolonged, extreme forward flexion of the trunk during the welding activity and related subtasks (Fig. 2). Therefore, the primary exposure may be considered a stooped work posture, which Holmström et al. (1992) found to be associated with an elevated risk of low back pain when the duration of exposure exceeded 4 h per day.

Stud welding is a four-step arc welding operation (American Welding Society, 2004). First, a stud is loaded into a semi-automatic arc welding gun (stud gun) which is then positioned through a ferrule and in direct contact with the steel. Second, the operator triggers the stud gun, sending current through stud. The stud gun will then slightly elevate the stud, creating an arc of electricity between the stud and steel, forming a path for the weld current. Third, after the weld current has melted both the stud tip and base steel, the stud gun will plunge the stud downward into the molten material. The lift and plunge operations are performed automatically by the stud gun, but the operator must exert downward pressure to maintain the vertical position of the welding gun relative to the steel. Finally, once the completed weld has cooled, the ceramic ferrule is discarded. In the case of bridge construction, each weld location is prepared with an angle grinder (Fig. 2b).

Typically, stud welding is a two-person job. The overall job using conventional equipment consists of several subtasks, including setting up the welding equipment (generator, cabling, welding controller, and stud gun), grinding, laying out the ceramic ferrules and studs, welding studs, and occasionally repositioning the cabling and welding controller. A standard welding cable may weigh in excess of one pound per linear foot, depending on the amperage rating; therefore, high shoulder forces may be required to move cable. The ironworker designated as the stud welder will be solely responsible for setting up the welding equipment and welding studs. The support worker will lay out the majority of the ferrules and studs for the stud welder, assist with repositioning of the cabling and welding controller as necessary, and perform the majority of the grinding.

An alternate system with the potential to reduce exposure to extreme trunk flexion during stud welding is shown in Fig. 3. The system includes a wheeled cart with an articulating arm to which standard stud gun components are attached. The articulating arm allows the operator to maintain a more upright working posture, potentially reducing the mechanical load on the low back. The cart weighs 175 lbs. The system uses standard shear stud connectors, ferrules, welding cables, and welding controllers, and functions essentially as a pass-through for the weld current. The support worker is responsible for moving the cart, feeding studs into the cart, and will share in the responsibility of laying out ferrules using a semi-automatic dispenser (Fig. 3b). On a bridge site, the cart rests atop wheels, allowing the support worker to move it easily. A footbrake mechanism is used to lock the cart in place during active welding, which prevents the support worker from exerting forces to maintain the position of the cart. A brief multi-person lift is required to move the cart across the girders. During our preliminary observations, such lifts took less than 5 s, did not require the full weight of the cart to lifted at one time (i.e., the workers could “shuffle” the cart across the beams), and occurred infrequently (<5 per day).

The objective of this study was to compare estimates of trunk inclination, muscle activity, and spinal compression during use of conventional equipment and during use of the alternate system.