Dynamic Loading and Environmental Effects Flashcards

A crane is lifting a 20,000 lb structural panel at 50 fpm when the operator suddenly brakes to a full stop in 0.5 seconds. Applying a dynamic load factor (DLF) of 1.3 for abrupt deceleration, what is the approximate dynamic load imposed on the hoist line during the stop?

Answer: 26,000 lb

Dynamic load factor is applied by multiplying the static load by the DLF. 20,000 lb × 1.3 = 26,000 lb. Abrupt braking converts kinetic energy into an instantaneous load spike, and a DLF of 1.3 is commonly used for sudden stops — the resulting dynamic load is what the hoist line and crane structure must momentarily absorb.

A 10,000 lb load is suspended from a boom-tip in 25 mph steady wind. The load has a flat vertical surface area of 400 sq ft. Using a wind pressure of 1.3 psf at 25 mph, what is the approximate horizontal wind force on the load, and how does this primarily affect crane stability?

Answer: 520 lb force, increasing the overturning moment by shifting the effective load radius outward

Wind force = pressure × area = 1.3 psf × 400 sq ft = 520 lb. This horizontal force creates a swing of the suspended load outward from the crane centerline, effectively increasing the load radius. Since crane load charts are based on static radius, any outward displacement of the load increases the overturning moment — the primary stability risk in wind.

During a pick, a rigger reports the load is 'hung up' and an operator applies hoist power to break it free. The load releases suddenly, causing a jerk condition. Which of the following statements BEST describes the load condition at the moment of release?

Answer: The hoist line tension momentarily exceeds the static load weight because stored elastic energy in the rope and boom is released instantaneously

When a stuck load is pulled against with hoist power, energy is stored elastically in wire rope stretch and boom deflection. At the instant the load breaks free, this stored energy releases — the line tension can briefly spike well above the static load weight. This is the classic 'snatch load' or shock load condition, and is one of the most dangerous dynamic load scenarios because the spike is nearly instantaneous.

A lattice boom crawler crane is operating near the coast in a tropical environment (95°F, 90% relative humidity). The rated capacity chart was developed at standard conditions. Which combination of environmental effects requires the operator to be MOST cautious about structural integrity compared to a dry inland day at 70°F?

Answer: Elevated temperature reducing steel yield strength combined with accelerated wire rope corrosion from salt air humidity

Steel loses yield strength at elevated temperatures (significant above 300°F but measurable at sustained high ambient), and salt-laden humid air dramatically accelerates corrosion in wire rope — reducing both rope breaking strength and fatigue life. Together these two effects degrade structural and rigging integrity in ways not reflected on standard load charts, making them the most critical combination. The other options either describe effects that go in the wrong direction (air density actually decreases with humidity) or are minor concerns.

A 60-foot boom crane is lifting a load that begins to pendulum (swing). The load is suspended on 40 feet of hoist line. An engineer states the load's natural pendulum period is approximately 7 seconds. The operator's hoist and swing cycle happens to match this 7-second period. What phenomenon is occurring and what is the correct corrective action?

Answer: Resonance is occurring; the operator must immediately change the operating rhythm to a cycle time that does NOT match the pendulum period

When the crane's operational cycle matches the load's natural pendulum frequency, resonance occurs — each cycle adds energy to the swing, causing amplitude to grow dangerously. The correct response is to deliberately break the matching rhythm by changing the timing of hoist/swing operations. Locking the swing brake or increasing hoist speed would not address the resonant energy input and could worsen the dynamic condition.

A crane is operating at 90% of its chart capacity when wind speed increases from 20 mph to 35 mph. The operator notes the load has a sail area of 300 sq ft. Wind pressure at 35 mph is approximately 3.2 psf. Assuming the chart capacity was established at 0 mph wind (no wind derating applied), which OSHA-aligned action is MOST appropriate?

Answer: Stop hoisting, determine the equivalent added radius from horizontal wind force, verify the resulting load moment does not exceed the chart capacity at that radius, and lower the load if it does

At 35 mph with a 300 sq ft sail area, wind force = 3.2 × 300 = 960 lb horizontal. This force swings the load outward, increasing the effective radius. The correct action is to determine what radius the load has shifted to, compare the load moment to the chart capacity at that new radius, and lower the load if it exceeds the rated capacity. Simply subtracting the wind force from load weight or increasing boom angle does not correctly address the physics of increased overturning moment.