7
•
A truss is defined as a structural unit made
of aluminium round tubes welded together
to form a regular module. Trusses are usually
made according to standard measures, but
they can also be customized. Aluminium has
been chosen on purpose in order to make the
most of some of its special features:
•
low self weight, aprox. 1/3 of the weight of
steel
•
maintenance is almost nil, since it is highly
resistant to the corrosion caused by external
agents (e.g. Atmospheric phenomena)
•
high tensile strength
•
attractive finish (it looks good being bright
and shiny)
•
fully recyclable material
The basic elements making up an aluminium
truss are the following: the main tubes or
chords, the diagonal braces and the coupling
elements.
There are several different types of trussing on
the market, depending on the cross section:
usually the most used are the flat section,
the triangular section and the square section
trusses. Each one of them presents different
features in terms of performance, versatility
of use,
appearance and cost.
TYPES OF FORCES A TRUSS IS EXPOSED TO
bending
:
•
Considering a span, the action of a force
upon it (e.g. gravity) causes its deflection: the
top chords are exposed to compression and
the bottom chords to tension.
The bending force can be quantified through
application of a bending moment (resulting
from the principle of a force multiplied by
a distance) that leads to the bending of a
truss around the neutral axis. The bending
force has specific formulas for typical loading
situations such as UDL (Uniformly Distributed
Load) and CPL (Central Point Load).
The parameter expressing the entity of
bending is defined with the term “deflection”.
It expresses the maximum distance between
the position of the truss under load and its
position at rest.
Torsion
:
•
Considering a truss bound at one end and
free at the other one, the torsion force, applied
to the free end, tends to make the truss twist
around its own axis.
In a system of trusses, the torsion force is
basically active in the vertical elements
(towers). In order to oppose this phenomenon,
it is recommendable, especially for towers,
the use of an anti-torsion truss, where the
diagonals contrast the unwanted effect.
Shear
:
•
The shear force is the load acting across
a truss at its supports or at the position of a
point load, in such a way as to cut the truss
vertically at the supports.
The maximum shear force is usually active
at the changes of direction of the truss (from
vertical to horizontal).
•
The tables show the entity of the bending
effect for each kind of truss, depending on the
load applied (distributed or concentrated).
In case a parameter is not indicated, the
load needs to be reduced or, if not possible, a
higher capacity truss needs to be used.
E.g. The table shows the load on the QUADRO
truss with a 250 mm side section (connection
with Spigot and Pins). Supposing we want
to check what is the bending effect of an 8
mt span with a distributed load of 50 kg/mt,
crossing the raw “8m” with the column “UDL”
we obtain on the column “Def” a deflection of
66 mm.
•
The charts, finally, stress the performance
of the total loads with different spans, both
in the distributed and concentrated load
configurations. For example, a QUADRO
truss with a 250 mm side section and an 8
mt span (connection with Spigots and Pins),
can support a load of about 50Kg/m with the
load evenly distributed and about 200Kg with
a concentrated load applied to the middle.
WHAT IS A TRUSS?
HOW TO READ THE DIAgRAmS AND TAblES
QUADRO 25 (COUPLING WITH SPIGOTS & PINS)
DIMENSION
CPL
UDL
DPL
TPL
QPL
Def.
m
kg
kg/m
kg
kg
kg
mm
4
432
161
320
215
160
13
6
279
93
210
140
116
37
8
201
50
151
100
84
66
10
152
30
114
76
63
105
12
117
20
88
59
49
150
14
91
13
68
46
38
201
Trussing
inFo
TecHnical inTroducTion
