1.5 – Overview – Tie Down Method
In order to meet the loading performance standards, loads need to be restrained. One of the two common methods for restraining loads is known as the tie-down method. This approach involves using lashings (typically webbing or chains) to “tie the load down onto the vehicle”. The principle that this method works by is to apply downward force on the load effectively clamping it onto the vehicle.
This additional force increases the friction between the bottom of the load and the vehicle providing additional resistance to movement. The lashings or chains also need to be sufficiently tensioned to keep the load in contact with the vehicle when it is travelling on uneven surfaces or cresting a hill to meet the vertical performance standards (20% of the mass of the load
Friction
Because this method relies on friction it is important that the surfaces on both the load and the vehicle are suitable and there are no other materials like oil, water or particles between the surfaces.
The amount of frictional force available to restrain the load depends on the total downward force and the nature of the two surfaces.
Click on the following animation to see how frictional forces work.
Changing the Friction Coefficient
Where the surfaces of the load and the vehicle result in low friction or very low levels of friction, consideration should be given to introducing rubber matting, dunnage (see Lesson 3.6), or other strategies to increase friction (see or changing to a direct restraint strategy.
Providing the Downward Force.
It is vital that operators understand that when the load is not blocked (in every direction) regardless of how heavy the load it the friction levels between the load and the vehicle surface will not be sufficient to restrain the load if the only force pushing down on the load down is gravity. Even high friction surfaces are not able to provide sufficient frictional force and in addition uneven road surfaces can cause a load to “Walk” as the vehicle moves up and down Lashing or chains are normally used to provide the additional force between the two surfaces but the force pushing the surfaces together is not the same as the tension in the chain or lashing because the chain or lashing is often not pulling at a right angle to the vehicle surface. This is referred to as the angle effect. Page 25 of the Load Restraint guide provides a table that illustrates the effectiveness of lashings at different angles.
Lesson 4.1 will discuss how many lashings or chains should be used and how these should be tensioned. It will also discuss how to calculate the normal force from the tension in the lashings and the angles between the lashing (or chain) and the vehicle surface.
When is the Tie-Down Method Not Appropriate?
The Tie Down method may not be appropriate where:
- The load is crushable or fragile and could be damaged by the lashings
- The load is not positioned along the centreline of the vehicle and could loosen if the load moves sideways
- Loads where the coefficient of friction between the load and the vehicle is low
- Bulk loads that cannot be effectively restrained by straps.