|NEN 2018 & NEN 2019||
To come to a calculation for a construction, first the load combination is
chosen, based on situations occurring during its lifetime. With the help
of the several classes, like class of utilization (number of load cycles)
and the load spectrum, the group factor M and load factor ψ can be
determined. The combination of the load combination multiplied by the group
factor and load factor leads to the appropriate safety factors belonging to
the right loads. The result of this is that the design load is much higher
than the actual load.
In NEN-EN 13001 load combinations are used, but each type of load has got
its own safety factors, instead of the group factor and load factor, as in
NEN 2018. Here, each load is multiplied with its belonging safety factor.
Then each load is multiplied with another factor (partial safety factor).
At the end every part is multiplied with the same risk factor. The result
of this is that the design load is much higher than the actual load.
The construction has to be proven for yield, fatigue and stability. At first,
the yield calculation is done with the yield point divided by 1.5 for load
combination 1 and 1.33 for other load combinations. The fatigue calculation
is done by dividing the notches in groups and determining the stress ratio
(κ). This stress ratio is the minimum stress divided by the maximum
stress and can be anywhere between -1 for a completely alternating situation
and 1 for a non alternating situation with the minimum stress the same as the
maximum stress. In case of a very low stress ration, the fatigue condition is
the one which has to be focussed on, because the maximum stress can be very
low due to the alternating situation.
The construction has to be proven for yield, fatigue and stability. At
first, the yield calculation is done with the help of the yield point
divided by 1.1 for rolled and non-rolled materials and between 1.1 and
1.47 when tensile stresses perpendicular to the plane of rolling. The
fatigue calculation is done with the help of the stress history factor S.
This stress history factor is determined by using several classes and
parameters, like the load spectrum, the class of utilization and a
parameters F2, mass of moving crane
parts and mass of hoisting load. This class S is used in the calculation for
the permissible stress range (maximum - minimum stress) and the
characteristic value of the weld.