Design guide for steel trusses (part 1).
Trusses are structures comprising one or more triangular units constructed with straight members whose
ends are connected at joints referred to as nodes.They are the most cost effective solution to support a large
amount of weight and span great distances. This is the first out of two articles that will present some basic
tips for the preliminary design of steel trusses.
Fink (triangular) trusses are more commonly used for the roof of houses and can span a
distance 8-20m.
Trusses with N shaped diagonals can span a distance 15-30m.
Trusses with N shaped diagonals and secondary members can span a distance 30-50m. We
use secondary members in order to create loading points and reduce the buckling length of the
diagonals under compression.
There are trusses where their top and bottom chords are parallel to each other. In this case,
the distance, H, between the chords should be L/15-L/10.
The purlins (and all point loads) should only be applied at nodes in order the members to be
subjected only to axial forces and not to bending moments. The distance between the top chord
nodes (and the purlins) should be 1.80-2.40m. Every half-span should be divided into an even
number of intervals. By doing this, x bracings of equal length will be placed every three purlins.
The minimum height, h, of an N shaped truss is h=0.40-0.50m for L=20m and h=1.00-1.20m
for L=45m.
The maximum height, H, of an N shaped truss is H=4.50m so that it can be easily transported
to the construction site.
The inclination of the top chord should be 3-25% in order not have water accumulation on the
roof. In general it is better the inclination to be more than 20% in order to avoid such problems.
The angle between the vertical members and the diagonals should be greater than 30o so
that the node will be better formed. If this can’t be done, secondary members should be used.
The diagonal members should be always in tension for the predominant loads that are
subjected to the truss otherwise the longest members should be subjected in tension and the
shorter in compression.
A truss can either be simply supported or can be arranged as a portal frame. In the first case
the truss doesn’t contribute to the lateral stability of the structure whereas in the second case it
does. For simply supported trusses the upper chord is in compression for gravity loading and the
bottom chord is in compression for uplift loading. Uplift loads are predominant in open buildings
(sheds). For portal trusses each chord is partly in compression and partly in tension.
Trusses can give economic solutions for spans over 20-25m.
The span to depth ratio should be chosen in the range of 15-10.
Long span trusses should be divided into two or more parts with maximum length 14-15m in
order to be easily transported to the construction site.
Design guide fo
f r steel
e tru
r sses (part
r 2).
This is the second and last article presenting some basic tips fo
f r the preliminary design of steel trusses. This
design guide is fo
f cused mainly on the trusses’ cross-sections (shape and orientation) and the in and out-off
plane buckling lengths.
Cross-Sections
The most cost eff
ffective
v sections fo
f r trusses are channels and angles. Specifically, tw
t o angles
are bolted on ve
v rtical gusset plates and intermediately batt
t ened.
When the members of a truss are heavily loaded (large span trusses) or subj
b ected to bending
moments we can use I shaped sections (IPE, HEA & HEB) fo
f r chords and batt
t ened angles or
channels as diagonal and ve
v rtical members. The web of the top chord should be ve
v rtical and the
web of the bott
t om chord horizontal.
Hollow sections (CHS, SHS, RHS) can also be used as chord and web members. Howeve
v r,
they are ve
v ry expensive
v compared to other solutions.
Buckl
k ing Lengths
When a truss is subj
b ected to gravity
t loads, the upper
pp chord is compressed
p
and we have
v to
design it against in and out-off plane buckling. The in-plane buckling length is the distance
betw
t een the nodes of the truss.. The out-off plane buckling length is the distance betw
t een the
purlins. It
I is ve
v ry important to mention that the purlins should be restrained by bracings or ve
v ry
stiff
f roof sheeting of class 1 or 2 (second picture). If
I the steel deck is placed on the roof truss
without being supported by purlins and it is class 1 or 2, the top chord is restrained against
buckling.
When the wind uplift
f is the critical load case fo
f r the design of a truss, the bott
t om chord is
subj
b ected to compression. Bo
B tt
t om chord has also to be checke
k d fo
f r in and out-off plane buckling.
As in the top flange, the in plane buckling length is the distance betw
t een the nodes of the truss.
The out-of-plane buckling length is either the total length of the bottom chord or the distance
between the longitudinal beams which are placed between every truss and connect their bottom
chord nodes to each other. These beams should have at their both ends horizontal bracings in
order to be restrained (third picture). The capacity of the compressed bottom chord can also be
checked by taking into account the stiffness of the connected parts but this procedure is very
complicated.
The trusses should be connected to each other with vertical bracings. These bracings are
placed between every truss along the structure (third picture, a-a cross-section). The main role
of these vertical bracings is robustness. Their presence can also reduce the out-of-plane
buckling length of the bottom chord and prevents vertical vibration modes.
The diagonals and the vertical members of a truss should also be checked against
g
in and outof-plane
buckling.
out-of-plane
p
g Their in plane buckling length is 0.9L and their out-off plane L. L is the length
of these members.
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Categories: Steel Design
Tags: buckling lengths, truss design
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