Dr. Ken-Yu and Dr. Zhenyu Zhang, SHARE Lab College of Built Environments, University of Washington
In last month’s issue, we discussed overexertion in cart operations, cart condition and maintenance and tire selection. In this article we’ll address workspace setup and site pre-task planning.
Workspace Setup – Space Constraints
Space constraints or tight spaces, are common on a commercial roofing site. They come from physical restrictions of the site or are a by-product of bad workspace setups. Be cautious when pulling a cart in tight spaces as they require a higher degree of cart control precision and will increase the risk of overexertion. Here are some examples of space constraints.
Example 1: Poor workspace setup – A temporary ramp is placed in front of a penetration, which creates a limited landing area for moving a cart onto the ramp. The landing is uneven and poorly set up. The worker has to overcome the uneven surface from the limited space and then pull along the ramp. This significantly increases the risk of overexertion.
Example 2: Narrow passage – Workers have to apply a greater force to position a cart into the narrow passage. The extra force could be adding 12 percent more pressure on the worker’s lower back and shoulder. Additionally, when moving a cart across the narrow passage, extra force is needed to minimize lane deviation.
Example 3: Bad housekeeping – Construction debris sits along the path, forcing workers to pull the cart through a tight space. Overcoming and pulling over an obstacle in an open area is relatively easy. An open space allows operators to build enough cart
momentum and sustain relatively constant velocity before contacting the obstacle. In contrast, cart speed is relatively lower in a tight space like the example shown above. Operators cannot depend on cart momentum and thus need to apply a much larger force to not only get over the obstacle but also initiate the cart movement. In this case, even a small obstacle will make a big impact.
Example 4: Improperly stored materials – Bundled materials are improperly placed in the path of cart movement which creates a very narrow passage for cart operation.
■ Position the ramp at a better place to eliminate space constraints. Build a smooth landing for the ramp to ensure the path of travel is free of obstacles.
■ Team pulling or pushing is needed when the space constraint is unavoidable. Ensure good housekeeping so the path of travel is free of obstacles.
■ When an obstacle cannot be removed, keep an open and clean workstation so that workers can start by moving the cart at least two feet away from the obstacle. Always consider setting up a ramp to bridge an obstacle.
■ A two foot clearance is needed on both sides of hallways or passages to avoid space congestion.
It takes more than two feet to get a cart moving and significant pulling force is required during this time. This initial force is three times more than the forces required to sustain the cart movement. A tight space won’t allow a cart to move fully, so operators must continually apply the initial force, which increases the risk of overexertion.
Keeping an open space is extremely important when operators need to pull over obstacles. As shown in the diagram to the right, moving the cart at least two feet away can reduce injury rates by five to nine times because open space allows operators to move the cart fully and build momentum to overcome the obstacles.
Ramps provide an aid for raising or lowering material carts, but they will cause safety problems when not set up properly. Building a stable ramp with a gentle slope is the key to keep cart handling safe and productive.
Moving a cart over a four-degree ramp is unacceptable for 3 in 10 of the male population and presents little additional risk compared to a flat surface. However, a ramp soon becomes a hazard when it reaches eight degrees or higher. The time required to overcome the ramp also surges by 23 to 47 percent as the slope increases. This means that workers need to apply a greater hand force for a longer time when overcoming a steeper ramp, which will greatly increase the workers’ chance of getting hurt.
Ramps should be stable with a smooth running slope and landings. Poorly set up ramps often come with gaps, obstacles and ridges that are harmful. This is because workers cannot depend on the cart momentum to overcome these hazards while moving a cart over the ramp. In the following examples, a worker’s chance of injury increases from 10 to 90 percent because of these “small” problems.
Here are some solutions to consider when building a ramp:
■ Plan ahead before building a ramp.
■ Secure boards the day before to stabilize the ramp.
■ Use heavy sheets to bridge gaps and make a smooth surface.
■ Extend ramp lengths or lower ramp rises to make certain the slopes are less than four degrees.
■ Some construction sites might not allow for a gentle slope because there is no room for extended ramp lengths or ramp rises are a part of the building structure and nonremovable. In these cases, a lower cartload is recommended.
These small changes will yield big results.
Obstacles are common on a construction site. Moving a cart over an obstacle will increase the risk of overexertion. It is important to make certain that the path of cart movement is free of obstacles. Examples include tear-off debris, parapet walls, gas hoses or other conduits and gaps between different floor surfaces when installing roofing sheets. The larger an obstacle is, the riskier it becomes. Eliminate obstacles whenever possible:
■ Use tape or strips to secure boards and avoid movement.
■ Cut parapet walls. If an obstacle is immovable or unavoidable:
■ Plan ahead and select the best possible path of movement to avoid obstacles.
■ Reduce cartloads.
■ Use materials (e.g., plywood) to bridge the gaps created by obstacles.
Roofers often need to handle heavy materials. Even with the aid of wheeled equipment, roofing materials can still be too heavy for
a single worker. We recommend 960 lbs. as the maximum load weight when one worker pulls a cart over a flat roof (maximum two hours per day). Given that most of the materials exceed a cart’s ergonomic load capacity, there is a high chance a worker can get hurt if teamwork is not adopted.
Load weight directly determines the amount of force that workers must exert when handling a cart. By applying team pulling and
pushing, workers can share cartloads and minimize the risk of overexertion. This ergonomic recommendation is particularly useful when physical hazards cannot be eliminated. In addition to preventing overexertion injuries, team pulling and pushing can help shape the safety culture in your workplace:
■ Encourages workers to watch out for each other and offer a helping hand when needed.
■ Empowers workers to build a habit of asking for help.
■ Develops a sense of unity as workers work toward the same goal.
When engaging in team pulling and pushing, set up the following guidelines:
■ Assign a team leader.
■ Determine a set of commands and make sure that everyone knows what to do when they hear a command.
■ The team leader should survey the field and pre-determine the path of cart movement (enough space for movement, obstacle
free).
If you feel it’s too hard for employees to remember all the information regarding team pulling and pushing, warning signage will help. Keep these in mind:
■ Signage to assemble teamwork – when loading materials, a crane unloads bundled materials onto a cart which is then moved
by workers to storage locations. By attaching signage to the bundled materials, workers will be reminded of an explicit prompt for team pulling.
■ Signage to articulate capacity – during installation, workers need to split bundled materials and transport them across a site.
Signage can remind them of the maximum load sizes a worker can handle by themselves.
■ Carts can be remodeled to allow teammates to push more easily by installing a rear handle. Similar to the warning signage,
this additional handle can offer a clear reminder to consider team pulling.
You may also consider engineering controls. Although the control might come at a cost, it can bring considerable long-term benefits. Mechanical devices, such as motorized push-pullers, can help workers pull carts that exceed the load weight limit. Note that motorized push-pullers are not suitable for all facilities as they require extra hallway space and steering room. When working on a large roof, a motorized cart is also an option which can be used to tow a cart or several carts together without the need for manual effort.
Job rotation requires workers to rotate between different tasks and can prevent workers from being exposed to some particular ergonomic stressors for extended periods of time. By being involved in different tasks, workers can also increase their skill base, enjoy the work opportunity and become more productive. Job rotation is recommended to prevent overexertion in cart handling.
A two-hour rotation schedule is recommended and each worker should be able to handle this task within an eight-hour work shift with no more than one pull every 30 minutes. No other heavy physical activities should be performed between pulls.
From a biomechanical perspective, cart handling involves forceful, sustained shoulder elevation and a forward bending posture. Workers should be rotated through some tasks that do not involve these ergonomic risk factors. Here are some generic principles
you can follow:
■ No forceful pushing or pulling of equipment or materials.
■ No forceful lifting or carrying of 50 lbs. or more.
■ No objects handled above the shoulders.
■ No awkward back angles involved (>30°).
What tasks should you consider when performing job rotation for cart operators? Consider these:
■ Assemble equipment or structures.
■ Inspect equipment or materials.
■ Cut or install materials.
■ Apply sealants or other protective coatings.
■ Smooth surfaces with abrasive materials or tools.
Before implementing job rotation, a qualified person should conduct a jobsite analysis to ensure that the same ergonomic risk factors are not involved. A “qualified person” is one who has thorough training and experience sufficient to identify ergonomic hazards in the workplace and can recommend an effective means of correction. Formal job rotation demands a consistent and systematic method. For more detailed instructions, visit www.ergo-plus.com/learn-job-rotation/ for a step-by-step guide.
Next month we’ll focus on developing an ergonomic intervention and provide you with a cart inspection checklist.
Dr. Ken-Yu Lin and Dr. Zhenyu Zhang, SHARE Lab College of Built Environments, University of Washington developed Ergonomic Guidelines, Using Four-Wheel Carts in the Roofing Trade with funding and support for this project provided by the State of
Washington, Department of Labor and Industries, Safety and Health Investment Projects (grant number 2018ZH00361).
Previous Article