aluminium in commercial vehicles - European Aluminium Association

64 EUROPEAN ALUMINIUM ASSOCIATION ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER VI
6. Serviceability limit state 7. Ultimate
limit state
All calculations in serviceability
limit state are elastic calculations.
Elastic deformations are calculated
and compared with the limits
for deflections. The sizes of vibrations
have to be calculated in the
same manner. If the vibration has
a high number of cycles, the
members and the connection
details have to be checked for
fatigue.
Normally the calculations of elastic
deflections are based on the
moment of inertia for the gross
cross-section of the member. For
members in cross-section class 4
(see section 7.2.4 in EN 1999-1-1)
it is necessary to reduce the
moment of inertia, if the stresses
of the compression part of the
cross section are higher than the
stresses when local buckling
occurs.
Moment of inertia for calculation
of deflections for cross section
class 4 members:
I = I - ser gr σgr fo (I gr - I eff )
Where:
σ is maximum compressive
gr
stress in serviceability limit state
in the cross section, based on the
gross cross-section properties
(positive in the formula)
I is the moment of inertia for
gr
the gross cross-section
I is the moment of inertia of the
eff
effective cross-section in ultimate
limit state, with allowance for
local buckling
7.1. Cross section classes
Cross-sections are classified in 4
classes. In Table VI.2 the different
classes identify how the
cross-section behaves during
compression and bending. This
is directly linked to the resistance
(load bearing capacity) of the
cross-section.
Thin parts of a cross-section may
buckle at low stresses, and this
will reduce the resistance of the
cross-section. This is taken care
of with the rules for cross-section
classification.