Calculating Shock Loads

Shock load is the term used to describe the sudden force exerted when an object suddenly accelerates or decelerates, such as when a falling object hits the ground, a fastball strikes a catcher’s glove or a diver begins to leap off a diving board. This force is exerted on both the moving object and the object being acted on. Determining shock load can be very important in a variety of safety-related situations, for example, determining the effectiveness of a safety harness or the wire lanyard attached to it. Most harness lanyards are made to withstand a certain amount of force, and you can calculate the shock load for a falling object attached to a somewhat elastic wire rope.

Determining Shock Load

Step 1

Write down the equation to determine shock load in pounds: shock load = load x [1 + (1 + (2 x FD x A x E)/(load x L))^1/2].

Step 2

Plug in the values in the following example: load = 200 pounds, falling distance = 12 inches, area factor = 0.472, diameter of rope = 0.25 inches, metallic area = 0.0295 inches^2, modulus of elasticity = 15,000,000 pounds per square inch, and length of cord = 10 feet (120 inches). Therefore, in this example, shock load = 200 x [1 + (1 + (2 x 12 x 0.0295 x 15,000,000)/(200 x 120))^1/2].

Step 3

Calculate the numerator then the denominator separately, as per the order of operations. So in this example, the equation simplifies to shock load = 200 x [1 + (1 + (10,620,000)/(24,000))^1/2].

Step 4

Divide the numerator by the denominator, as per the order of operations. So now you have shock load = 200 x [1 + (1 + 442.5)^1/2]. Add 442.5 to 1 within the parentheses to get shock load = 200 x [1 + (443.5)^1/2].

Step 5

Take the square root of 443.5 and then add 1 to perform the calculations within the brackets and get shock load = 200 x 22.059.

Step 6

Multiply for the final result: shock load = 4,411.88 pounds.

Things Needed

• Calculating shock load for an elastic wire requires knowing several factors:
• Weight of the object in pounds (load)
• Falling distance in inches (FD)
• Length of the cord in feet (L)
• Modulus of elasticity (E) = 11,500,00 pounds per square inch (new rope) or 15,000,000 pounds per square inch (stretched rope)
• Area factor of rope (area factor)—each particular kind of rope has an associated area factor
• Metallic area of the rope (A) = diameter of the rope in inches x diameter of the rope in inches x area factor

TL;DR (Too Long; Didn’t Read)

Area factors for wire ropes typically range from around 0.35 to over 0.55.

Warning

Safety harnesses should be checked regularly for damage and replaced as necessary.