A truss is essentially a triangulated system of straight
interconnected structural elements. The most
common use of trusses is in buildings, where support
to roofs, the floors and internal loading such as
services and suspended ceilings, are readily
provided. The main reasons for using trusses are:
Reduced deflection (compared to plain members)
Opportunity to support considerable loads
Trusses are used in a broad range of buildings, mainly where there is a
requirement for very long spans, such as in airport terminals, aircraft hangers,
sports stadia roofs, auditoriums and other leisure. Trusses are also used to carry
heavy loads and are sometimes used as transfer structures. This article focuses
on typical single storey industrial building, where trusses are widely used to serve
two main functions:
• To carry the roof load
• To provide horizontal stability.
• The members of trusses are made of either rolled steel
sections or built-up sections depending upon the span
length, intensity of loading, etc. Rolled steel angles, tee
sections, hollow circular and rectangular structural tubes
are used in the case of roof trusses in industrial buildings
• In long span roof trusses and short span bridges heavier
rolled steel sections, such as channels, I sections are used
• Members built-up using I sections, channels, angles and
plates are used in the case of long span bridge trusses
In the first case the lateral stability of
the structure is provided by a series
of portal trusses; the connections
between the truss and the columns
provide resistance to a global
bending moment. Loads are applied
to the portal structure by purlins and
In the second case, (right) each truss
and the two columns between which it
spans, constitute a simple structure;
the connection between the truss and
a column does not resist the global
bending moment, and the two column
bases are pinned. Longitudinal
stability is also provided by a wind
girder in the roof and vertical bracing
in the elevations.
Trusses comprise assemblies of tension and compression elements. Under gravity
loads, the top and bottom chords of the truss provide the compression and tension
resistance to overall bending, and the bracing resists the shear forces. A wide range
of truss forms can be created. Each can vary in overall geometry and in the choice
of the individual elements. Some of the commonly used types are shown below
Pratt truss ('N' truss)
• Pratt trusses are commonly used in long span buildings ranging from 20 to 100 m
in span. In a conventional Pratt truss, diagonal members are in tension for gravity
loads. This type of truss is used where gravity loads are predominant .
• An alternative Pratt truss is shown where the diagonal members are in tension for
uplift loads. This type of truss is used where uplift loads are predominant, which
may be the case in open buildings such as aircraft hangers.
It is possible to add secondary members in Pratt truss to:
• Create intermediate support points for applied loads
• Limit the buckling length of members in compression (although in a 2D truss,
the buckling length is only modified in one axis).
When the floor span exceeds 15m, it is generally more
economical to change from a simple truss arrangement to
one employing wide span lattice girders which support
trusses at right angles.
In order to light up the space satisfactorily, roof lighting has
to replace or supplement, side lighting provision must also
be made for ventilation form the roof.
One of the oldest and economical methods of covering
large areas is the North Light and Lattice girder.
This roof consists of a series of trusses fixed to girders. The
short vertical side of the truss is glazed so that when the
roof is used in the Northern Hemisphere, the glazed portion
faces North for the best light.
It can be used for spans from 20-30m.
Used for industrial buildings, drawing rooms etc.
A lattice girder is a type of girder
with a criss-crossed web design,
such as in gardening lattices,
between the two edges of the
The diagonal lines of steel give
support in all directions, helping to
prevent the girder, which is one of
the main support elements in a
bridge design, from bending.
Often seen on older bridges or
buildings, lattice girders are also
widely used in mining tunnels for
roof support during excavations and
can be erected quickly.
Lattice girders are not used so
much in building or bridges
anymore as they have been
replaced by solid steel plate girders
Trusses consume a lot less material compared to beams to span the same length
and transfer moderate to heavy loads. However, the labour requirement for
fabrication and erection of trusses is higher and hence the relative economy is
dictated by different factors. In India these considerations are likely to favor the
trusses even more because of the lower labour cost. In order to fully utilize the
economy of the trusses the designers should ascertain the following:
Method of fabrication and erection to be followed, facility for shop fabrication
available, transportation restrictions, field assembly facilities.
Preferred practices and past experience.
Availability of materials and sections to be used in fabrication.
Erection technique to be followed and erection stresses.
Method of connection preferred by the contractor and client (bolting, welding or
Choice of as rolled or fabricated sections.
Simple design with maximum repetition and minimum inventory of material.
Portal frames are a type of
structural frame, that, in their
simplest form, are characterised
by a beam (or rafter) supported
at either end by columns,
however, the joints between the
beam and columns are 'rigid' so
that the bending moment in the
beam is transferred to the
Portal Frames are generally used for single storey construction which
require a large unobstructed floor space ie
Portal frames are made in a variety of shapes and sizes. They are
usually made from steel, but can also be made from concrete or
timber. The portal structure is designed in such a way that it has no
intermediate columns, as a result large open areas can easily be
created within the structure.
Speed and ease of erection
Building can be quickly closed in and made water tight.
Framework prefabricated in a workshop and not affected by weather.
Site works such as drainage, roads etc can be carried out until framework is ready for
No weather hold up during erecting the framework.
Connected together in factories by welding and site connections should be bolted.
Although steel is incombustible it has a poor resistance to fire as it bends easily when
Subject to corrosion
A high percentage of roofs are covered with
composite profile metal sheets with a coloured
These composite sheets have approximately 50mm
of insulation sandwiched between two thin metal
sheets or aluminium sheets. Coated steel is lowest
in cost but limited in life due to the durability of the
finish. Aluminium develops its own protective film.
Profile sheets are quick to erect, dismantle and
Galvanised steel purlins span between the steel
Shown here is a ridge joint or apex joint.
It is Important that this joint is strong
hence the use of wedge shaped pieces
called gusset pieces to strengthen and
increase the bolt area.
KNEE JOINT FOR PORTAL
Again the knee joint must be
strong to support the roof loads
and prevent bending.
Gusset pieces will be used to
increase strength, give greater
bolt area and prevent
deflection under load
With all types of frameworks we must
think on stability ie movement. To help
strengthen the framework and prevent
movement diagonal bracing will be used
This slide shows the
cladding rails for
attaching the external
metal cladding panels
These rails can be
fixed horizontal or
vertical depending on
the way the cladding
panels are fixed
These wire and
tubular ties are used
to prevent sagging of
the cladding rails
which can add
unto the joints of the
Here we see the finish of the external
cladding panels with the lower level
The blockwork behind creates a
protective wall or firewall.
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