What Are Struck-By Accidents? Understanding the Four Categories of Hazards
Struck-by accidents in construction occur when a worker is hit by an object, vehicle, or piece of equipment that was set in motion by an external force. Unlike "caught-in" or "caught-between" hazards where workers become trapped, struck-by accidents involve objects impacting workers with sufficient force to cause injury or death. OSHA categorizes struck-by hazards into four main types, each presenting distinct risks and requiring specific prevention measures.
The Physics of Impact: Why Struck-By Accidents Are So Devastating
The severity of struck-by injuries relates directly to physics: force equals mass times acceleration. Even relatively light objects become deadly weapons when falling from height or propelled at high velocity. A one-pound wrench dropped from 50 feet impacts with approximately 356 foot-pounds of energy. A five-pound hammer falling the same distance delivers nearly 1,800 foot-pounds. These forces easily fracture skulls, crush vertebrae, and cause fatal internal injuries.
Vehicle strikes present even greater force because of the mass involved. A 10,000-pound forklift traveling at just 5 mph impacts with over 25,000 foot-pounds of energy, equivalent to being struck by a small car. Even low-speed vehicle impacts frequently cause fatal crushing injuries.
Understanding these physics helps explain why struck-by accidents rank among construction's deadliest hazards and why prevention through engineering controls, protective equipment, and safe work practices is so critical.
Struck-By Flying Objects: The Projectile Hazard
Flying object hazards involve tools, debris, or materials that become high-velocity projectiles through mechanical force, compressed air, or explosive release. Unlike falling objects governed primarily by gravity, flying objects can travel horizontally or even upward, making them difficult to predict and protect against.
Common sources of flying object hazards include:
*Powder-Actuated Tools*: These tools use explosive charges to drive fasteners into concrete, steel, and other hard materials. When fasteners strike reinforcing steel, voids, or unsuitable substrate, they can ricochet with lethal velocity. Spall material (chips of concrete or steel) can also become projectiles. OSHA requires that only trained operators use powder-actuated tools and that all persons in the area wear eye and face protection.
*Compressed Air Tools*: Pneumatic nailers, staple guns, and other compressed air tools propel fasteners with tremendous force. Accidental discharge, double-fires, and fasteners that penetrate through materials create flying object hazards. Workers have been killed by nails penetrating through thin materials and striking workers on the opposite side.
*Grinding and Cutting Operations*: Abrasive wheels, circular saws, and cut-off tools create flying debris as they work. Wheel failures can launch fragments at extremely high velocities. Cutting through unknown materials can release unexpected projectiles when hidden components are struck.
*Demolition Activities*: Breaking concrete, cutting steel, and dismantling structures releases fragments that can travel significant distances. Chunks of concrete, pieces of reinforcing steel, and building components become projectiles during demolition.
*Chipping and Breaking Operations*: Jackhammers, chipping guns, and breaking operations create continuous streams of flying debris. Workers and bystanders face ongoing exposure to flying material throughout these operations.
Struck-By Falling Objects: Gravity's Deadly Force
Falling object hazards are perhaps the most common struck-by risk on construction sites, particularly on multi-story projects where work occurs at multiple elevations simultaneously. Objects fall from scaffolds, aerial lifts, structural steel, roofs, and through floor openings.
Key falling object hazard sources include:
*Tools Dropped from Height*: Unsecured hand tools are frequently dropped from scaffolds, ladders, and aerial work platforms. A falling hammer or wrench can easily kill a worker below. Tool tethering requirements exist specifically because this hazard is so common and so preventable.
*Materials Falling Through Openings*: Floor openings, shaft openings, and wall openings allow materials to fall from upper levels to lower levels. Debris, small materials, and even larger components can fall through openings that are not properly covered or guarded.
*Overhead Work Debris*: Workers above generate debris that falls on workers below. Cutting, grinding, welding, and other overhead operations create streams of falling material. Paint chips, rust, dust, and small debris may seem minor but can cause eye injuries and, in larger quantities, create substantial hazards.
*Crane Load Drops*: Failure of rigging, improper load attachment, or mechanical failure can cause crane loads to drop. Given the mass of materials handled by cranes, load drops are frequently fatal to anyone struck.
*Structural Collapse*: During demolition, renovation, or construction, structural components can fall unexpectedly. Walls, columns, beams, and roof sections have collapsed onto workers, causing mass casualty events.
*Material Storage Failures*: Improperly stacked or secured materials can fall when disturbed or when storage structures fail. Lumber stacks, pipe bundles, masonry units, and other stacked materials create substantial falling object hazards.
Struck-By Swinging Objects: The Pendulum Effect
Swinging object hazards occur when loads, equipment, or materials move in an arc and strike workers who are in the swing path. The pendulum effect of swinging loads can generate tremendous force, and the arc of swing may be difficult to predict.
Primary swinging object hazards include:
*Crane Load Swing*: When crane loads are lifted off the ground, they naturally seek the center point beneath the boom tip. This movement can cause significant side-to-side swing. Improper rigging, sudden boom movements, and wind can all cause loads to swing unpredictably. Workers in the swing radius can be struck with tremendous force.
*Excavator and Backhoe Operations*: The rotating superstructure of excavators creates a continuous swing hazard. Workers within the swing radius can be struck by the counterweight, bucket, or boom. Backhoe operations create similar hazards with the swinging bucket and boom.
*Structural Steel Erection*: During steel erection, beams and columns suspended from cranes can swing when the wind catches them or when they're released from temporary supports. Ironworkers face particular exposure to swinging steel hazards.
*Suspended Equipment*: Welding leads, air hoses, suspended platforms, and other equipment hanging from overhead structures can swing when disturbed or when support fails. The weight and momentum of suspended equipment can cause serious injuries when it strikes workers.
*Improperly Secured Loads on Vehicles*: When loads shift on trucks, flatbeds, or trailers, they can swing outward and strike workers during loading, unloading, or transport. Materials that break loose during transport can swing with their remaining attachments.
Struck-By Rolling Objects: Mobile Hazards
Rolling object hazards involve vehicles, equipment, and materials that move along the ground and strike workers. This category includes both powered vehicles and unpowered objects that roll due to gravity or other forces.
*Construction Vehicles*: Dump trucks, concrete trucks, delivery vehicles, and other construction vehicles create significant struck-by hazards, particularly when backing. Limited visibility, lack of warning systems, and congested site conditions contribute to vehicle strikes.
*Material Handling Equipment*: Forklifts, telehandlers, and other material handling equipment operate in close proximity to pedestrian workers. The combination of limited operator visibility, material obstructing sight lines, and workers focused on their tasks creates frequent near-misses and strikes.
*Cylindrical Materials*: Pipes, tubes, conduit, and other round materials can roll when disturbed or when blocking is removed. Workers have been killed when pipe bundles broke loose and rolled over them.
*Equipment Roll-Away*: Equipment that is not properly parked, blocked, or secured can roll due to grade, vibration, or accidental contact. Equipment rolling into excavations, off edges, or into workers creates both struck-by and caught-between hazards.
New York City's Amplified Risks
Construction in New York City presents amplified struck-by risks due to the unique characteristics of urban high-rise construction:
*Vertical Construction*: Multi-story buildings under construction have workers at many different elevations simultaneously. Objects dropped from upper floors have more distance to accelerate and more workers below who could be struck.
*Congested Sites*: NYC construction sites are typically tight, with minimal space between the work area and adjacent buildings, streets, and pedestrian areas. This congestion places more workers in hazard zones and limits escape routes.
*Multiple Contractors*: Large NYC projects involve numerous subcontractors working simultaneously. Coordination challenges mean that workers from one contractor may not know what hazards workers from another contractor are creating overhead or nearby.
*Street-Level Risks*: Construction adjacent to public sidewalks and streets creates struck-by risks for pedestrians and passersby. Falling object protection (sidewalk sheds, debris nets) is required but not always adequate.
*Limited Staging Areas*: Deliveries often occur directly into work areas due to lack of staging space. This brings vehicle and material handling hazards into areas where workers are performing other tasks.



