4.2.1 Cargo at rest is prevented from sliding
by static friction. When movement has been initiated the resistance
of the material contact is reduced and sliding is counteracted by
dynamic friction, see 4.2.6, instead.
4.2.2 The static friction may be determined by
an inclination test. The angle ρ is measured when the timber cargo
starts to slide. The static friction is calculated as:
μ = tan(r)
4.2.3 Five inclination tests should be performed
with the same combination of materials. The highest and the lowest
values should be disregarded and the friction factor is taken as the
average of the three middle values. This average figure should be
rounded down to the nearest fraction of 0.05.
4.2.4 If the values are intended to be used for
non-winter conditions, the coefficient of friction for both dry and
wet contact surfaces should be measured in separate series of tests
and the lower of the two values are to be the used when designing
cargo securing arrangements.
4.2.5 If the values are intended to be used for
winter conditions when exposed surfaces are covered by snow and ice,
the lowest coefficient of friction found for either dry, wet or snowy
and icy contact surfaces should be used when designing cargo securing
arrangements.
4.2.6 If not specially measured the dynamic friction
factor may be taken as 70% of the static values.
4.2.7 The following values of static friction
for the mentioned conditions may be used when designing securing arrangements
for timber deck cargoes unless the actual coefficient of friction
is measured and documented as described above.
Table 4.2 Typical values of static
friction for different material combinations
| Contact surface
|
Non-winter
conditions
|
Winter
conditions
|
| Dry or wet
|
|
Sawn wooden package
against painted steel
|
0.45
|
0.05
|
| against sawn wood
|
0.50
|
0.30
|
| against plastic cover or webbing slings
|
0.30
|
0.25
|
Round wood
coniferous round wood (bark on) against painted steel
|
0.35
|
|
| coniferous round wood (bark on)
between layers
|
0.75
|
|
4.2.8 Static friction may be used for tight block
stowage arrangements as well as for the design of frictional lashing
systems such as top-over lashing systems.
4.2.9 Dynamic friction should be used for non-rigid
lashing systems, which due to elasticity of securing equipment allow
for minor dislocation of the cargo before full capacity of the securing
arrangement is reached.