As discussed in lesson 1.5 the tie down method relies on friction to provide the forces necessary to meet the loading performance standards.
The first considerations when considering using a tie down approach is whether the method is suitable for the load and vehicle you have chosen.
As discussed in lesson 1.5 there are three main situations where the tie down method may not be applicable.
These are where the:
It is worth noting that that the lower the coefficient of friction the higher the loads on the lashings will be which may increase the chance of crushing the load.
The addition of rubber matting or some other suitable material between the load and the vehicle surface may improve the friction coefficient reducing the load on the lashings (which may reduce the number of lashings required) and also reduce the risk of crushing the load.
If tie down method is being used in conjunction with a rated headboard or some other form of direct restraint which this may also reduce the number of lashings required.
Pages 243 to 246 of the load restraint guide provide technical advice on designing for the Tie-Down method and are compulsory reading for anyone wishing to do calculations. The following is an overview of the material in the load restraint guide.
The tables contained on pages 267 to 280 of the load restraint guide have two versions for each type of lashing one which assumes that the load is used with a headboard rated to provide a restraint force which is at least 30% of the weight. This means that the remaining 50% of the weight required to restrain the load will be met by the tie down method. This is also the amount required to meet the loading performance standards in the other directions (50% of the weight). There is also another version of each table which shows the lashings required to restrain the load when there is no headboard (unblocked) and tie down needs to provide 80% of the weight to resist forward movement.
If you decide to use the tie down method, there are five things you need to know to determine how many lashings you will need to adequately restrain the load. You need to know: