Fall Protection
Engineering controls and personal systems designed to prevent workers from falling or to safely arrest a fall if it occurs. Falls remain the leading cause of death in construction worldwide.
Overview
Fall protection engineering is dictated by the physics of gravity and human physiology. The human body can only withstand a certain amount of deceleration force before internal organs rupture, the spine breaks, or severe trauma occurs. OSHA limits the maximum arresting force on an employee to 1,800 pounds (8 kN) when using a full-body harness, and strictly prohibits the use of body belts for fall arrest.
The Physics of Falling
The primary goal of fall protection is to either prevent the fall entirely or limit the free-fall distance and deceleration forces to survivable levels.
Fall Protection Systems
Checklist
- Guardrail Systems: Passive engineering controls that prevent the worker from reaching the edge. Top rails must be 42 inches ( inches) high and withstand 200 lbs of lateral force without deflecting below 39 inches.
- Safety Net Systems: Passive catch systems installed below the working surface to safely arrest a falling worker or debris.
- Personal Fall Arrest Systems (PFAS): Active systems consisting of an anchorage, a full-body harness, and a connecting device (lanyard/deceleration device or self-retracting lifeline).
Key Takeaways
- Fall protection engineering requires balancing the free-fall distance with maximum arresting forces to prevent severe physiological damage to the worker.
- Passive systems (guardrails) are always preferred over active systems (PFAS) because they prevent the fall entirely.
Clearance Distance Calculations
When engineering a PFAS, the most critical calculation is the Total Required Clearance. If a worker falls, the system must arrest them before they strike the lower level, equipment, or any obstruction.
The calculation must account for the length of the lanyard, the deployment (elongation) of the shock absorber, the height of the worker, and a safety factor to ensure clearance.
Where:
- = Total Required Clearance from the anchorage point
- = Lanyard Length (typically 6 ft)
- = Deceleration Distance (max 3.5 ft for standard shock absorbers; this is the distance it takes to slow the fall)
- = Height of Worker (often calculated from the D-ring to the feet, approx 5 to 6 ft)
- = Safety Factor / Harness Stretch / D-ring slide (typically 2 to 3 ft)
Personal Fall Arrest System Clearance Calculator
Calculate the required clearance distance to prevent a worker from striking the lower level during a fall.
6
3.5
6
2
15
Required Clearance: 17.5 ft
6 + 3.5 + 6 + 2 = 17.5
Unsafe! The required clearance exceeds the anchorage height. The worker will strike the ground.
Anchorage (15 ft)
Required Drop (17.5 ft)
Ground (0 ft)
Note
In cases where clearance is insufficient (e.g., working 10 feet above ground), self-retracting lifelines (SRLs) that lock up within 2 feet of a fall, or a higher anchor point directly overhead must be engineered instead of using a 6-foot lanyard.
Key Takeaways
- Total required clearance () must always be calculated to ensure the fall arrest system engages fully before the worker impacts the ground or lower level.
- A standard 6-foot shock-absorbing lanyard requires nearly 18 feet of total clearance below the anchorage point.
Implementing Fall Protection
Procedure
Identify Fall Hazards:
Any unprotected edge 6 feet (1.8m) or more above a lower level requires fall protection. Perform a site survey to identify edges, floor holes, elevator shafts, and leading edges.
Design Anchorages:
Anchorage points used for personal fall arrest systems must be capable of supporting at least 5,000 pounds (22.2 kN) per employee attached, or be designed, installed, and used as part of a complete PFAS under the supervision of a qualified engineer with a safety factor of at least 2.0. This is the most critical structural component of the system.
Calculate Clearance:
Perform the total required clearance distance calculation () based on the specific equipment and the worker's position relative to the anchor to ensure they will not strike the lower level.
Rescue Plan:
Develop and implement a prompt, written rescue plan. Prolonged suspension in a harness after a fall is arrested can lead to suspension trauma (orthostatic intolerance), which can be fatal in minutes due to blood pooling in the legs and depriving the brain of oxygen.
Suspension Trauma
A potentially fatal condition where blood pools in the legs of a worker suspended motionless in a harness, depriving the brain of oxygen and leading to syncope and death. Immediate rescue (typically within 15 minutes) is critical to survival.
Key Takeaways
- Anchorage strength (5,000 lbs minimum per worker) is the foundational requirement for any Personal Fall Arrest System.
- A PFAS is incomplete without a site-specific rescue plan to prevent suspension trauma following an arrested fall.