The following simplified equation may be used to calculate, with satisfactory accuracy, the forces involved in securing rigid, non-rolling items such as cases, parcels, timber, etc. For a load consisting of a large number of separate items (bulk goods, rolling goods, etc) specially selected methods of calculation must be applied.
P = Qa – (10Q + S) μ
Where
Q = the mass of the load (kg).
P = force which acts on the restraint equipment such as headboards, side post, etc (Newtons, N).
a = design acceleration level (m/s2). In the forward direction a = 10m/s2 approximately, in the rearward and sideways direction a = 5m/s2 approximately.
S = the sum of the tensile forces exerted by the vertical parts of the lashings (Newtons, N).
μ = friction coefficient for the contact surface between the load and its support. It may be assumed for these calculations that the coefficient of friction is 0.2. If a higher value is used, it must be justified by evidence, for example, a result obtained in a practical experiment.
One method is to let the load unit, while resting on its support, be subjected to a horizontally acting force. The magnitude of the force F is measured at the moment when the load begins to slide on the support. The friction coefficient may then be calculated from the following relationship:
μ = F/10Q
Another method is to incline the load and its support on a progressively increasing angle until the load begins to slide on the support. The angle of inclination, α, is observed at the moment the load begins to slide. The friction coefficient can then be calculated from the relationship:
μ = tan α
When experiments are carried out using this method, special measures must be taken to prevent loads with a high centre of gravity from tipping over during the trial.
While the load is resting on its support and fastened in a manner which is the subject of the test, the plane of the support is inclined on a progressively increasing angle, thereby stressing the load fastening devices. For the load fastening method to be considered to meet the requirements it must be capable of holding the load in position at the angle of inclination shown in the diagram at the design acceleration in question. The magnitude of the angle of inclination depends not only on the acceleration, but also on the friction coefficient, μ, the value of which must therefore be determined first.
