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Big Truck Extrication - Part 1: Anatamy 101

This is the first of a three part article on big truck extrication. We are going to first take a look at basic truck anatomy. In article two we will discuss scene approach, hazards, and stabilization. In part three we will talk about extrication techniques. Trucks have been around since shortly after the coming of the motorized automobile. They have become much bigger, more advanced, and travel much faster than ever before. In order to be able to execute a good extrication, a good basic knowledge of trucks and there anatomy would be essential. Trucks present us with quite a different challenge and we need to stay abreast to meet this challenge. So, “OK good buddy, let’s put the hammer down, and get this eighteen wheeler rolling.”

We are going to approach this in a similar way I got my medic training some years ago. Before one can learn to treat a patient, we must first learn the basic human anatomy, which will give us a better understanding of how to stabilize and treat our patient. I use the same philosophy when I teach extrication classes. By first learning about some basic truck anatomy, styles, and types, we will have a better understanding when it comes to stabilizing a light duty truck weighing 12,000 pounds verses a heavy truck weighing 140,000 pounds. You will also see that by knowing the construction of the cabs, you can better choose the tools needed to gain access to your victim.

Trucks play a vital role in the economy of our nation and the lives of each and every one of us. Because of this, there are a large number of trucks traveling our highways all the time. Trucks make up over 3% of all vehicles on our roads. The one problem they present for us as extrication technicians, is they weigh up to thirty times more than the average automobile. They also carry all types of cargo from the groceries we purchase from the store, to hazardous chemicals used in a variety of manufacturing processes. Trucks are involved in approximately one out of every eight motor vehicle crashes. When looking at truck crashes we will find that 60% of them occur on major interstates, 25% occur on major highways, and 10% occur on secondary roads. Most fire departments and rescue squads have one, if not all, of these types of roads running through their response district. So, hence the need for training in large truck extrication.

Trucks are put into two basic categories, medium and heavy. Medium trucks are put into a class 3, 4, or 5, and have a gross vehicle weight rating (GVWR) of between 10.000 pounds and 19,499 pounds. Heavy trucks are put into classes 6, 7 and 8. Class 6 is trucks that have a GVWR of 19,500 pounds to 29,000 pounds. Class 7 trucks have a GVWR of 29,100 pounds to 33,000 pounds, and class 8 are those 33,001 and greater. Trucks come in several types of designs. The first and most common is the straight truck. These are built on a solid frame and not designed to pull a trailer. Most of these truck have two to three axles and have a GVWR of 10,000 pounds to 40,000 pounds. Next we have the specialty trucks which are designed for a specific purpose. Some examples of these types of trucks would be concrete trucks, dump trucks, wreckers, and fire apparatus.

The next type truck we have is the truck / semi-trailer combination. They are compiled of a truck, also called a tractor, and one or more trailers pulled by the truck. The tractors are either two or three axle and may weigh up to 18,000 pounds. The entire tractor trailer rig may weight up to 140,000 pounds. The trailers also come in a variety of types including a flatbed for hauling building materials, a closed box trailer for general cargo, tankers for hauling fuels, chemicals, and grain and vehicle transports. Remember that some trucks will display placards for hazardous materials and some may not. Trucks can haul 440lbs of hazardous materials without a placard.

Basic Styles and Construction

We will now look at some basic truck construction and configuration. The conventional style construction is the trucks that have the engine in front of the passenger compartment and are longer. The engine compartment is either hinged down the center or may pull towards the front. These cabs are usually made of a combination of steel, aluminum and fiberglass. The other style of truck is the cab over with the engine being located under the midline of the driver and passenger compartment. These units tilt forward to access the engine. These are also constructed of a combination of steel, aluminum, and fiberglass. Many tractor trailers have sleepers. These allow for the drivers to stop and sleep or a pair of drivers to rotate off driving so they do not exceed their federal driving limit. Sleepers are an extension of the cab and not a separate unit. There is access through the cab and usually through a side door. Rescuers should always consider there may be a driver trapped or pinned in the sleeper compartment when assessing the wreck scene.

Doors on tractor trailers are usually heavy and have one of two types of hinges. They will either have a conventional style hinge like that found on a standard automobile or a piano style hinge. Both are heavy and well attached. The door latches are found at the lower rear corner of the door due to their height above the ground. They usually have a single latch with a two step locking mechanism. The windshield and rear window are the standard laminated safety glass as found in automobiles. However, the glass will be set in a rubber gasket that will make it easier to remove and normally it will not need to be cut out. The side windows are tempered safety glass that is heavier than those in autos. The roofs are made of steel ribs that usually run from front to back and are covered with either aluminum or fiberglass. Some trucks have a fiberglass wind faring or deflector on the roof as well. Behind this you may also find the air conditioner unit or air horns. Most trucks have multiple batteries that are wired in series.

Most large trucks utilize diesel fuel to reduce the possibility of a flashover. The fuel tanks are usually saddle tanks that are attached to one or both sides of the truck. The size of the tanks can vary from fifty gallons to three hundred gallons. If the truck has duel tanks, most are interconnected by a fuel line that equalizes both tanks. There is a cut off at one or both ends of this tube, so fuel can be shutoff should a leak occur. Most large trucks are equipped with air braking systems. In 1995 NHTSA (National Highway Traffic Safety Administration) made it mandatory that anti-lock brakes be on all heavy and medium duty trucks. In 1997 they required that all new tractors manufactured have anti-lock brakes and in 1998 all trailers, single unit trucks, and buses were required to have them as well. Some slightly smaller trucks may have hydraulic brakes or a combination of hydraulic and air. With air brakes the system uses compressed air to apply the brakes under normal operations. The air tanks for this system can be located at different locations around the truck depending on the manufacturer. If there is a problem with the air system that engages the brakes and air pressure is lost, heavy springs will activate the braking system. The trailers being pulled also have an air brake system that connects into the tractors air system and work in conjunction with the tractor. These airlines run behind the cab and connect into the front of the trailer. These lines are equipped with a breakaway valve or a glad-hand connector. These airlines are colored coded for easy recognition. The blue line is for normal operations. The red line is the emergency airline that serves to lock the brakes on the trailer, should an emergency arise. Should the air to the trailer be interrupted, there are air chambers located under each axle that will activate a large spring that is used to mechanically apply the brakes. These are sometimes called piggyback chambers. Most large trucks also have air ride suspension. These are large air bellows that are fixed under each axle. These can hold up to 120psi of air and should this pressure be suddenly lost, the truck could settle several inches or more. Mounted on the back of the tractors is a flat piece of steel called the fifth wheel. This plate has a wedge shaped slot cut into it that allows for the “king pin” on the trailer to slide into it. This pin is locked into place by two spring loaded jaws. These locks can be released by using the pull handles located on the drivers side. This system allows for the trailer to rotate up to 90 degrees.

By understanding some of the trailers being pulled, a rescuer may have a better idea of what they may be up against. This could range from a tanker full of fuel to a livestock trailer full of angry and confused cattle. Trailers that are pulled behind tractors come in a variety of sizes and shapes. These are as unique as the cargo they carry. Trailers are freestanding units with wheels in the front and back. Semi-trailers have only wheels in the rear and are supported in the front by the tractor. Several of the more common types of trailers you might see are; flatbeds for hauling supplies and equipment, tankers for hauling grain, fuel, chemicals and other liquid and gaseous substances, box trailers for hauling a wide assortment of materials and cargo, and livestock trailers for moving animals. When semi-trailers are not attached to the tractors, they are supported in the front by a set of landing gear. This device has a handle for lowering and raising the front of the trailer. The rear of the trailers will have a bumper for collision control. The older trailers had bumpers that offered little protection on rear end collisions. In 1998 all trailers had to be equipped with a much stronger bumper that would help prevent vehicles from running under the trailer and resulting in serious injury or death.

In this first article we have looked at some real basic large truck construction and design. We have given you a basic knowledge of truck construction that will aid you in deciding how you should approach, stabilize and extricate your victim. In our next article, we will look at scene assessment and approach, as well as vehicle stabilization. Until next time, stay safe, train to your best, and remember that to have not achieved, is to not have tried.

Read the other parts of this column:
- Big Truck Extrication – Part 2: Assessment and Stabilization
- Big Truck Extrication – Part 3: The Extrication

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