Loading a semi truck1 with a forklift is very different from loading a small delivery truck by hand. Not only are the packages much larger and heavier, but they are generally all the same shape (and indeed often several dozen of the same thing). Furthermore, the truck is higher up off the ground, requiring a loading dock, and the forklift requires training to use safely.
A loading dock is usually either a raised platform inside the building, or a sunken ramp outside the building intended to bring the deck of the semi-trailer to the same height as ground level for the forklift. Occasionally it takes the form of a hydraulic lift used to raise the forklift up and down to match the trailer height, but this is much slower and generally found in buildings not originally designed for loading and unloading trucks.
First the truck, with its loading doors latched open, backs into the loading dock, being careful to set the trailer into the dock seal if there is one. A dock seal is a series of pads or flaps used to form a seal around the trailer's loading door to keep outside weather and cold air out of the building while loading is going on. The driver should then set the handbrake2.
Once the trailer is in position, it must be secured to prevent trailer creep (gradual movement away from the loading dock caused by action-reaction forces of the forklift moving back and forth). The wheels must be chocked, and if there is a dock lock, its hook should be secured to the underride guard as a second level of protection. Never use a dock lock instead of wheel chocks, only in conjunction with them. Most dock locks have a light that turns from red to green when the hook is secured, signaling that the truck is in position for loading.
Sometimes the tractor will remain attached to the trailer, and sometimes it will be removed. If the tractor is removed, the semi-trailer's landing gear will be set down. Many loading docks have a reinforced area under the landing gear to hold the extra pressure (it has much less surface area than the tractor's wheels do). It is important to note that the landing gear is set back several feet from the front of the trailer, meaning that if a heavy weight is set in the front without a counterweight in the rear, the trailer could tip forward.
To prevent this, a jack stand should be placed under the front of the trailer for additional support. A jack stand is an adjustable support post that should be placed under the front center of the trailer. However, if the jack stand is set firmly up against the trailer deck when it is not loaded, when the trailer is loaded the extra pressure will make it difficult to remove. For this reason, a jack stand is typically adjusted to about three inches below the deck, which is high enough to prevent accidents but not so high that the loaded trailer will be resting on it.
There is one last piece of safety equipment that needs to be put in place. There will be a small gap between the loading dock and the trailer that the forklift must drive over to get into the trailer. Additionally, the trailer will not be perfectly level with the height of the loading dock, and as it gets loaded and the suspension compresses, the height will continue to change. A metal bridge with two legs that fit into this gap needs to be placed across the gap to ensure safe loading and provide a ramp for the height mismatch.
Now that all the safety equipment is in place (wheel chocks, optional dock lock, jack stand if the tractor was removed, bridge), loading can begin. Typically, a forklift will pick up large rectangular cubes sitting on pallets and drive into the trailer with them. Usually all the pallets will be the same, so there is no need to decide what order to load them in. Sometimes foam or inflatable pads are used to take up extra space between the loads and the sides of the trailer to secure the load. Unloading is the same procedure in reverse.
There are two specific dangers when driving a forklift on to and off of the trailer. First is that the trailer is not the same as solid ground. If the forklift accelerates or brakes quickly, action-reaction forces of the forklift wheels on the trailer deck can rock the trailer back and forth. This is the primary cause of trailer creep and the reason wheel chocks and dock locks are used. Second is that the forklift is heavy, especially when carrying a load. The suspension of the truck may bounce when the forklift drives on and off. In extreme cases, the bouncing can contribute to trailer creep by putting extra pressure on the wheel chocks.
Loading a flatbed trailer can be done the same way as loading an enclosed trailer, but it doesn't need to be. Since a flatbed trailer has no sides, the forklift can raise and set the load onto the deck from the side without ever needing to actually drive onto the deck of the trailer. Two standard four foot by four foot pallets fit side by side down the length of the trailer, sometimes stacked two high, with only a single pallet centered in the very back for stability. In good weather, this can be done outside the building with no loading dock required. Many loading areas for flatbeds are enclosed or at least covered, however, to protect the loaders and the loads from bad weather. Regardless, the wheels must still be chocked for safety.
Since a flatbed trailer is not enclosed, most loads are covered with a tarp and secured with adjustable straps to protect them while the truck is driving to its destination. To place the tarp and set the straps, it is sometimes necessary to climb on top of the loads. If this is the case, OSHA requires that a safety harness and fall protection line is used.
1. Many people wonder why such a huge vehicle is called a "semi". The entire name of the vehicle is "semi-trailer tractor truck". Semi refers to the trailer only, and is so called because the trailer only has wheels in the back, not the front. The tractor part is the cab, or rig, or prime mover.
2. Thanks, archiewood, for pointing this out.