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criteria when designing a
steel structure and check afterwards
whether the deflection is
within the requirement.
When designing an aluminium
structure, it will often be the
deflection criterion that will be
governing. For that reason, the
design procedure will start with
the deflection criterion and it will
be checked afterwards if the
stress or the resistance of the
structure is within the limits.
The deflection of members under
bending load depends on the
modulus of elasticity (E) and on
the moment of inertia (I) together
with the load and the span.
With the same span and load, it
will be the product E x I that will
determine the deflection.
To get the same deflection of
steel and aluminium beams in
bending, the moment of inertia
of the aluminium beam must be
three times that of steel. If the
increase in the moment of inertia
is to be done only by increasing
the thickness of the web and
flanges, the aluminium beam will
have the same weight as the
steel beam.
4. Aluminium versus Steel
59
EUROPEAN ALUMINIUM ASSOCIATION
To save weight, the aluminium
beams in bending have to be
higher. An example will illustrate
this:
An aluminium beam shall have
the same deflection as an IPE
240 steel beam. The moment of
inertia and the mass of the IPE
240-beam are
I = 38.9 · 106 mm4.
mass = 30.7 kg/m.
The aluminium beam must have
a moment of inertia of
I =116.7 · 106 mm4
to get the same deflection.
If the height of the aluminium
alloy beam shall be 240 mm, this
will be satisfied by an I-beam of
I 240 x 240 x 12 x 18.3
which has a moment of inertia
and the mass of
I = 116.6 · 106 mm4
mass = 30.3 kg/m
If the height of the aluminium
alloy beam can be 300 mm, the
deflection criteria will be satisfied
by an I 300 x 200 x 6 x 12.9
which has a moment of inertia of
I = 116.7 · 106 mm4
and a mass =18.4 kg/m
which is a weight saving of 40%.
An I 330 x 200 x 6 x 10 will have
a moment of inertia of
I = 117.3 · 106 mm4
and a mass = 15.8 kg/m
which give a weight saving of 49%.
These three different aluminium
beams will give the same deflection
as an IPE 240 steel beam. It will be
the shape and stability of the beam
that will determine the weight of
the beam. Table VI.1 shows the
beams and the weight savings.
t h
w
b
Steel Aluminium Aluminium Aluminium
Moment in inertia in mm4 38.9 106 116.6 106 116.7 106 117.3 106
E x I (N/mm2) 8.17 1012 8.16 1012 8.17 1012 8.21 1012
h (mm) 240 240 300 330
b(mm) 120 240 200 200
w (mm) 6.2 12 6 6
t (mm) 9.8 18.3 12.9 10
Unit weight (kg/m) 30.7 30.3 18.4 15.8
Weight in %
of the steel beam 100 % 99 % 60 % 51 %
TABLE VI.1
60
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VI DESIGN AND CALCULATION 60 | 61
The stress in an aluminium structure
designed according to
deflection criteria is very often
low. In the following example a
steel beam, IPE 240 is compared
with an aluminium beam I 330 x
200 x 6 x 10 (both beams are
shown in the table VI.1). The
deflection criterion is 1/250 of
span (24 mm), the span is 6000
mm and the load is 11.6 kN/m. In
the Figure VI.1. the stress-strain
curves for steel S355 and aluminium
EN AW-6082 T6 is
shown. The stress and strain for
both the steel and aluminium
beam is also shown. With the
same deflection, the same load
and the same span, the steel
beam has a bending stress of
161 MPa while the aluminium
beam has a bending stress of 73
MPa. This is the maximum stress
when the deflection is 24 mm for
both beams.
EN AW-6082 T6
S355
0,1 0,2 0,3 0,4 0,5 0,6 0,7
300
200
100
STRESS COMPARISON BETWEEN ALUMINIUM AND STEEL BEAMS
FIGURE VI.1
0,105
0,077
161
73
σ (Mpa)
ε (%)
Additional comparison of weight-optimized beams are also given in
Chapter III, section 2.1
61
EUROPEAN ALUMINIUM ASSOCIATION
5.1. Philosophy
Limit state design and partial
safety factor method are the
methods that the new design
standards are based on. In
Europe the EN 19xx standards
are the basis for this method for
all structural materials in civil
engineering. For aluminium the
actual standards are:
EN 1990 Eurocode – Basis for structural design
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