Load intensities which exceed the UD load intensities, “concentrated loads”… — Page 63, Lesson 81

I want to walk you through how we handle loads that are heavier than what the floor is normally designed to take. We call these "concentrated loads" — they exceed the UD, or uniform, load intensities. The key point is that they can still be carried, but only if we use approved load spreaders to distribute the load over enough area so that we don't exceed the loading limits. Now, what are load spreaders? When you have a concentrated load, or an item with hard or sharp areas, the floor needs protection. The standard practice is to use load spreaders made of 2-inch thick timber. That's the professional standard — 2 inch thick timber spreaders. Let me walk you through a worked example so you see exactly how this calculation works. We have a cargo item with dimensions 3 ft × 4 ft × 12 ft, and it weighs 900 kg. The compartment has a maximum running load of 20 kg per inch, and a maximum load intensity of 70 kg per square foot. The question is: what are the running load values and floor intensity values, and are there any limitations on how we can carry it? First, let's handle the running load, which relates to underfloor protection. The running load is the load divided by a length. We calculate three values: the maximum running load uses the shortest length, the mid running load uses the mid length, and the minimum running load uses the longest length. For the maximum running load, we take 900 kg divided by the shortest length, which is 3 ft. Converting to inches, that's 3 ft × 12 inches per foot = 36 inches. So 900 divided by 36 gives us 25 kg per inch. The limit is 20 kg per inch, so this exceeds the limit. For the mid running load, we use the mid length of 4 ft, which is 48 inches. 900 divided by 48 gives us 18.75 kg per inch. That's below the 20 kg per inch limit, so it's okay. For the minimum running load, we use the longest length of 12 ft, which is 144 inches. 900 divided by 144 gives us 6.25 kg per inch. That's also below the limit, so it's okay. Now let's look at the distribution load intensity, which relates to floor protection. Here we divide the load by an area. The maximum distribution load uses the smallest area, the mid uses the medium area, and the minimum uses the largest area. For the maximum distribution load, we take 900 kg divided by the smallest area, which is 3 ft × 4 ft = 12 square feet. That gives us 75 kg per square foot. The limit is 70 kg per square foot, so this exceeds the limit. For the mid distribution load, we use the medium area of 3 ft × 12 ft = 36 square feet. 900 divided by 36 gives us 25 kg per square foot. That's below the limit, so it's okay. For the minimum distribution load, we use the largest area of 4 ft × 12 ft = 48 square feet. 900 divided by 48 gives us 18.75 kg per square foot. That's below the limit, so it's okay. So here's the conclusion. The cargo can be carried, but only under specific conditions. It must be placed on the cargo bay floor on either its medium area or its largest area to prevent floor damage. And either the 4-foot length or the 12-foot length must be placed parallel to the longitudinal axis of the aeroplane to prevent underfloor damage — this spreads the load across the underfloor frames. The key insight here is that orientation matters. By choosing which face of the cargo rests on the floor, and which dimension runs along the aircraft's length, we control both the floor intensity and the running load. The smallest area would give us 75 kg per square foot, which exceeds the 70 limit, so we can't use that face. And the shortest length would give us 25 kg per inch, which exceeds the 20 running load limit, so we can't orient the cargo that way either.