Struck-By and Caught-In Hazards

1. Why This Topic Matters

Struck-by and caught-in/between hazards are two of OSHA's Focus Four (also called the Fatal Four) in construction, alongside falls and electrocution. Together these four categories account for the majority of construction fatalities every year. Struck-by events are the leading cause of nonfatal injuries in construction and the second leading cause of construction deaths after falls. Roughly three-quarters of struck-by fatalities involve heavy equipment such as trucks, cranes, or earthmoving machines. Caught-in/between incidents, especially trench collapses and machinery entanglement, are notorious for producing catastrophic, often unsurvivable, crushing injuries.

For the BCSP Certified Safety Professional (CSP) exam, this topic sits inside the Advanced Application of Safety Principles domain. The exam does not simply ask you to recite definitions. It expects you to recognize these hazards in a scenario, distinguish between them, apply the hierarchy of controls, reference the correct standards and numerical thresholds, and choose the most effective intervention. Mastering this area pays off because the same reasoning pattern (recognize, classify, control, verify) is tested across many other exam domains.

2. What Struck-By and Caught-In Hazards Are

Struck-by hazards occur when a worker is hit by an object, piece of equipment, or vehicle and the impact alone causes the injury. OSHA groups struck-by events into four sub-categories:
• Flying objects: material ejected from power tools, nail guns, grinders, powder-actuated tools, compressed air, or failed abrasive wheels; also whipping hoses and rebar under tension.
• Falling objects: tools dropped from elevation, loads falling from cranes or forklifts, unsecured material on scaffolds, falling rock or trench spoil.
• Swinging objects: crane loads, suspended materials, excavator counterweights, gates and doors, tag line failures.
• Rolling objects: vehicles, mobile equipment, unchocked trailers, rolling pipe or logs, runaway carts on slopes.

Caught-in or caught-between hazards occur when a worker is squeezed, pinched, crushed, or compressed between two or more objects, or between parts of an object. OSHA's distinguishing rule: if the injury results primarily from crushing between objects, it is caught-in/between; if the impact alone caused the harm, it is struck-by. Typical caught-in/between examples include:
• Cave-ins during trenching and excavation (the most lethal sub-category; one cubic yard of soil can weigh roughly 3,000 pounds, about the weight of a small car).
• Machinery pinch points and nip points: unguarded belts, gears, rollers, conveyors, and rotating shafts that pull in clothing, hair, or limbs.
• Between equipment and a fixed object: a worker pinned between a backing truck and a wall, or between a swinging excavator counterweight and a structure.
• Rollovers of mobile equipment without rollover protective structures (ROPS) or without seat belt use.
• Collapse of unbraced structures, formwork, scaffolds, or stacked materials.

A single incident may involve both. Example: a worker is struck by a swinging load (struck-by) and pushed against a steel column where the load pins him (caught-between). The exam may ask you to classify the primary mechanism of injury, so read the stem carefully for words like impact, hit, thrown versus pinned, crushed, trapped, engulfed.

3. How These Hazards Work: Mechanisms and Energy

Both hazard categories are fundamentally about uncontrolled release or transfer of kinetic or potential energy into the human body. Understanding energy helps you reason through unfamiliar scenarios:
• Potential energy (height, tension, stored pressure) becomes kinetic energy when an object falls, a spring releases, or soil loses cohesion.
• Kinetic energy scales with mass and the square of velocity. A doubling of speed quadruples impact energy, which is why vehicle speed in work zones and tool ejection velocity matter so much.
• Gravity and soil mechanics drive cave-ins. Soil fails when the shear stress from its own weight (plus surcharge loads such as spoil piles or equipment near the edge) exceeds its shear strength. Water, vibration, previously disturbed soil, and time all reduce stability.
• Mechanical advantage drives caught-in machinery injuries. Rotating equipment and in-running nip points generate forces far beyond what a person can resist.

The practical consequence: controls that eliminate or isolate the energy source (lockout, protective trench systems, guarding, exclusion zones) are far more reliable than controls that depend on human behavior or PPE.

4. Key Standards and Numbers to Know

Excavation and Trenching (29 CFR 1926 Subpart P):
• A protective system (sloping, benching, shoring, or shielding/trench box) is required at 5 feet or deeper unless the excavation is entirely in stable rock. A competent person may require protection in shallower trenches if conditions warrant.
• Excavations 20 feet or deeper require protective systems designed by a registered professional engineer (or based on tabulated data approved by a PE).
• A safe means of egress (ladder, ramp, stairway) is required in trenches 4 feet or deeper, located within 25 feet of lateral travel for every worker.
• Spoil piles, materials, and equipment must be kept at least 2 feet from the edge of the excavation.
• Soil classification and maximum allowable slopes: Type A (most cohesive, e.g., clay) 3/4:1 (53 degrees); Type B 1:1 (45 degrees); Type C (least stable, e.g., sand, gravel, submerged soil) 1-1/2:1 (34 degrees). Stable rock may be vertical (90 degrees). Soil that has been previously disturbed can never be classified as Type A.
• Daily inspections by a competent person before each shift, after rainstorms, and after any hazard-increasing occurrence.
• Atmospheric testing is required when oxygen deficiency or hazardous atmosphere is reasonably expected at depths greater than 4 feet.

Machine Guarding (29 CFR 1910.212, 1910.219, 1926.300):
• Guards must prevent any part of the body from reaching the danger zone through, over, under, or around the guard.
• Guard types: fixed (preferred), interlocked, adjustable, self-adjusting. Devices include presence-sensing (light curtains), pullbacks, restraints, two-hand controls, and gates.
• Point of operation, power transmission apparatus, and other moving parts all require protection. Fan blades must be guarded when within 7 feet of the floor (openings no larger than 1/2 inch).

Control of Hazardous Energy (29 CFR 1910.147): Lockout/tagout prevents caught-in injuries during servicing and maintenance; lockout is preferred over tagout; the affected and authorized employee distinction is frequently tested.

Hand and Power Tools (1910.242, 1926.302): Compressed air for cleaning must be reduced to less than 30 psi with effective chip guarding and PPE. Powder-actuated tool operators must be trained and certified; tools must not be loaded until immediately before use. Abrasive wheels must be ring-tested and guarded, with work rests within 1/8 inch and tongue guards within 1/4 inch.

Mobile Equipment and Vehicles (1926.600 through 1926.602, 1926 Subpart W): Backup alarms audible above surrounding noise or a spotter when the view to the rear is obstructed; ROPS and seat belts on earthmoving equipment; equipment not in use must have blades, buckets, and forks lowered; parked vehicles chocked or braked; no riders unless a seat is provided.

Cranes and Rigging (1926 Subpart CC, 1926.1400 series): Swing radius of the rotating superstructure must be barricaded; no one may work under a suspended load; tag lines for load control; rigging inspected before each shift; qualified rigger and signal person; power line clearances (minimum 10 feet for lines up to 50 kV, increasing with voltage).

Falling Object Protection (1926.451, 1926.502, 1926.100): Toeboards at least 3-1/2 inches high, debris nets, screens, canopies, catch platforms, and barricaded drop zones. Hard hats meeting ANSI/ISEA Z89.1: Type I protects the crown; Type II also protects against lateral impact. Class G (general, 2,200 V), Class E (electrical, 20,000 V), Class C (conductive, no electrical rating). Tool tethering programs (ANSI/ISEA 121) address dropped-object hazards.

Work Zone and Traffic Safety: MUTCD Part 6 temporary traffic control; ANSI/ISEA 107 high-visibility apparel (Class 2 minimum for most roadway work, Class 3 for high-speed or low-visibility conditions); internal traffic control plans to separate workers on foot from equipment; blind-spot awareness and spotter communication.

Rollover Protection (1926.1000 through 1926.1003, ISO 3471): ROPS plus seat belt as a system; ROPS alone does not protect an unbelted operator who can be thrown and crushed.

5. Applying the Hierarchy of Controls

The exam heavily rewards choosing controls higher in the hierarchy. Apply the hierarchy to each hazard type:

Elimination/Substitution: Use trenchless technology (horizontal directional drilling) instead of open trenching; prefabricate components at grade instead of working beneath hoisted loads; use remote-controlled demolition equipment; eliminate the need to enter a machine by designing in automatic lubrication and clean-out.

Engineering Controls: Trench boxes, hydraulic shoring, sloping and benching; fixed guards and interlocks; presence-sensing devices; ROPS and FOPS (falling object protective structures); backup cameras and proximity detection; barricaded swing radius; physical separation of pedestrians and equipment; debris nets and canopies; toeboards; anti-whip restraints on compressed-air hoses; positive wheel chocks; load-moment indicators on cranes.

Administrative Controls: Competent-person daily inspections; internal traffic control plans; spotter protocols and hand signals; exclusion zones under loads; permit systems for excavation entry; LOTO procedures; training and certification (rigger, signal person, powder-actuated tool operator); job hazard analyses; pre-shift equipment inspections; limiting speed in work zones.

PPE: Hard hats, safety glasses and face shields, steel- or composite-toe footwear with metatarsal guards where appropriate, high-visibility apparel, and gloves. Remember that PPE mitigates consequence, not likelihood, and is the exam's typical distractor when a higher-order control is available.

6. Leading Indicators, Program Elements, and the CSP Perspective

At the CSP level you are expected to think like a program manager, not just a field inspector. Strong struck-by/caught-in programs include:
• Hazard identification through JHAs and pre-task planning that specifically flag drop zones, pinch points, swing radii, and excavation depth.
• Risk assessment using severity and likelihood matrices; struck-by and caught-in events almost always score high on severity, which pushes them toward engineering solutions.
• Competent-person designation and documented authority to stop work.
• Equipment selection and procurement standards (ROPS, cameras, guards, alarms) so that hazards are controlled at the design and purchase stage.
• Leading indicators: percentage of excavations inspected before entry, number of near-miss reports involving mobile equipment, guard audit completion, LOTO audit scores.
• Incident investigation that correctly classifies the event type, because misclassification distorts trending and prevention priorities.

7. Worked Scenario Reasoning

Scenario A: A 6-foot-deep trench in previously disturbed clay with a spoil pile 1 foot from the edge and no ladder. Workers want to enter for 15 minutes. What is the greatest deficiency? Reasoning: depth exceeds 5 feet so a protective system is mandatory; disturbed soil cannot be Type A so steeper slopes are not allowed; spoil pile violates the 2-foot setback and adds surcharge; egress is required at 4 feet or deeper. All are violations, but the greatest hazard and the first correction is the absence of a protective system. Short duration is never a justification for entry.

Scenario B: A worker is pinned between a reversing dump truck and a concrete barrier. Classification: caught-between (crushing between two objects), even though the truck struck him. Most effective preventive control: an internal traffic control plan that physically separates workers on foot from equipment paths (engineering/administrative), supported by spotters, backup alarms, cameras, and high-visibility apparel.

Scenario C: Maintenance technician reaches into a conveyor to clear a jam while it is energized and is pulled in. Primary failure: lack of hazardous energy control (lockout) and possibly inadequate guarding or interlock. The best answer will involve LOTO plus engineered guarding, not a sign or retraining alone.

8. Exam Tips: Answering Questions on Struck-By and Caught-In Hazards

Tip 1: Classify the mechanism first. Scan the stem for action verbs. Hit, struck, ejected, fell on, swung into indicate struck-by. Pinned, crushed, trapped, engulfed, buried, pulled in indicate caught-in/between. If both appear, identify which caused the actual injury.

Tip 2: Apply the hierarchy of controls ruthlessly. When a question asks for the most effective, best, or preferred control, choose elimination or engineering over administrative or PPE. Hard hats, training, and signage are classic distractors.

Tip 3: Memorize the trench numbers. 5 feet (protective system), 4 feet (egress), 25 feet (lateral travel to egress), 2 feet (spoil setback), 20 feet (PE design), soil slopes 3/4:1, 1:1, and 1-1/2:1 for Types A, B, and C. These appear in both recall and calculation items.

Tip 4: Be ready for simple calculations. You may need to compute the required top width of a sloped trench. Example: a 10-foot-deep trench in Type C soil with a 4-foot bottom width requires 1.5 x 10 = 15 feet of horizontal slope on each side, so top width = 4 + 15 + 15 = 34 feet. Also know how to estimate soil weight (about 100 to 125 pounds per cubic foot) and kinetic energy relationships.

Tip 5: Know who does what. Distinguish competent person (authorized to identify hazards and take corrective action) from qualified person (has the degree, certificate, or demonstrated expertise) and from a registered professional engineer. Trench inspections require a competent person; deep excavation designs require a PE.

Tip 6: Watch for absolute language and shortcuts. Answers claiming that short-duration entry, dry weather, or experienced crews justify skipping protection are wrong. Answers that rely solely on a single barrier such as a backup alarm are usually incomplete compared with layered controls.

Tip 7: ROPS and seat belts are a system. Any answer implying ROPS alone protects an operator is incorrect; without a seat belt the operator can be ejected and crushed by the ROPS itself.

Tip 8: Link hard hat classes and types correctly. Type I vs Type II refers to impact direction; Class G, E, and C refer to electrical rating. Do not confuse these.

Tip 9: Remember the compressed-air rule. Cleaning with compressed air requires pressure reduced to below 30 psi plus chip guarding and PPE. Exam items frequently test the exact figure.

Tip 10: Think like a program manager. CSP items often frame the question at the system level: which procurement specification, program element, or leading indicator would most reduce struck-by risk across a company? Choose answers that address root causes, design, and management systems rather than individual behavior.

Tip 11: Eliminate before you select. Cross out any option that is a lower hierarchy level, that misstates a number, or that assigns responsibility to the wrong role. Usually two options fall away quickly, improving your odds on close calls.

Tip 12: Manage time and avoid overthinking. Most struck-by/caught-in questions have one clearly defensible best answer grounded in OSHA definitions and the hierarchy of controls. If two options both seem correct, pick the one that controls the hazard closest to its source and earliest in the process.

9. Quick Review Summary

• Struck-by = impact alone; caught-in/between = crushing or compression between objects.
• Four struck-by sub-types: flying, falling, swinging, rolling.
• Leading caught-in killers: trench cave-ins, unguarded machinery, equipment pinning, rollovers.
• Trench numbers: 5 / 4 / 25 / 2 / 20 feet; slopes 3/4:1, 1:1, 1-1/2:1.
• Prefer engineering controls: protective systems, guarding, interlocks, ROPS, barricaded swing radius, physical separation.
• PPE and training are last-line defenses, not primary answers.
• Competent person inspects daily; PE designs 20-foot-plus excavations.
• Hard hats ANSI Z89.1; hi-vis ANSI 107; tool tethering ANSI 121; LOTO 1910.147; compressed air under 30 psi.

Master the definitions, the numbers, and the hierarchy, then practice classifying realistic scenarios. That combination is exactly what the CSP exam measures in this domain.