PORTAL FRAME CONSTRUCTION
The usual single storey buildings include terrace factories or workshops, detached
factories, warehouse, factory canteens, markets, hawker centers, sports halls,
industrial training centers, etc. Such buildings are characterized by large clear
open spaces under light roof structures. Steel structures, solely, or in combination
with reinforced concrete structures are usually adopted. If site conditions permit,
the ground floor slab should be unsuspended and designed to rest on well
compacted sub-grade and sub-base.
TYPES OF STEEL STRUCTURES
The usual forms of roof structures are triangulated truss frames, lattice
girders and solid web portal frames and their choice depends
considerably on the extent of clear floor space, headroom, type of roof
covering, roof pitch, natural lighting, drainage, ventilation, insulation and
the overall architectural requirement.
Truss frames or lattice girders are usually more economical in terms of
Portal frames are cheaper to fabricate faster in construction and have
advantages in aesthetic, ease of maintenance and added headroom,
unobstructed by ties or bracing members.
STEEL PORTAL FRAMES
Portal frame can be defined as two dimensional rigid frames which have the basic
characteristic of a rigid joint between the column and the beam. The main
objective of this form of design is to reduce the bending moment in the beam thus
allowing the frame to act as one structural unit.
To be effective a pitched roof portal frame should have as low a pitch as practical
to minimize spread at the knee of the portal frame (spread increases with the pitch
of the rafters of a portal frame). The knee of a portal frame is the rigid connection
of the rafter to the post of the portal.
The early use of the rigid portal frame coincided with the introduction of a wide
range of cold formed, profiled steel sheets for
roofing, which could be fixed at a low pitch and be
weather tight. The combination of low pitch steel
portal frames and profiled steel roof sheeting and
decking has led to the adoption of this form of
structure, particularly for single-bay single storey
Short span portal frames may be fabricated off site as one frame. Medium span
portal frames are generally fabricated in two halves for ease of transport and are
assembled on site with bolted connections of the rafters at the ridge, with high
friction grip bolts.
Many medium and long span steel portal frames have the connection of the rafters
to the posts at the knee, haunched to make the connection deeper than the main
rafter section for additional stiffness. In long span steel portal frames the posts
and lowest length of the rafters, towards the knee,
may often be fabricated from cut and welded I-
sections so that the post section and part of the
rafter is wider at the knee than at the base and
ridge of the rafter.
Steel portal frames may be fixed to or pinned to
bases to foundations. For short span portal frames,
where there is comparatively little spread at the
knee or haunch, a fixed base is often used. It will
be seen from that the steel base plate, which is
welded through gusset plates to the post of the
portal frame, is set level on a bed of cement grout
on the concrete pad foundation and is secured by four holding-down bolts set or
cast into the concrete foundation.
A pinned base is made by sitting the portal base
plate on a small steel packing on to a separate base
plate bearing on the concrete foundation. Two
anchor bolts, either cast or set into the concrete pad foundation, act as holding
down bolts to the foot of the portal frame. This type of base is the small packing
between the two plates allows some flexure of the portal post independent of the
foundation which may be less substantial than a comparable fixed base.
Portal frames with a span of up to 15m are defined as short span, frames with a
span of 16m to 35m as medium span and frames with a span of 36m to 60m as
Short span portal frames are usually spaced at from 3m to 5m apart and medium
span portal frames at from 4m to 8m apart to suit the use of angle or cold formed
purlins and sheeting rails. Long span portal frames are usually spaced at from 8m
to 12m apart to economize in the number of comparatively expensive frames, with
channel, I-sections or lattice purlins and sheeting rails to support roof sheeting or
decking and walling.
SYMMETRICAL PITCH REINFORCED CONCRETE PORTAL FRAME
This is the most structurally efficient and most commonly used type of concrete
It has been used for factories,
warehouses, barns, sheds and
single storey places for
assembly. The slope of the
rafters and spacing of purlins
and sheeting rails is usually
arranged to suit fiber cement
or profiled steel sheeting.
The rafter which is cast as one
unit is bolted to the posts at
the point of contra flexure. A
single post supports the rafters
of the frames below the valley
in the roof and these posts are
shaped to receive a pre-cast
reinforced concrete valley
gutter, bolted to the rafters,
which is laid without fall to
rainwater pipes and lined with
felt. The spacing of the internal
columns below valleys may be
increased by the use of a pre-
cast concrete valley beam to
support every other internal
roof frame. The bulky valley beam will obstruct clear head room and add
considerably to the cost of the structure.
The disadvantage of this multi-bay form of concrete
portal is the number of comparatively bulky internal
columns obstructing a free working area.
The pre-cast reinforced concrete purlins are usually
of angle section with stiffening ribs and cast in
lengths to span between portal frames. The purlins
are fixed by loops protruding from their ends which
fit over and are bolted to studs casts in the rafters,
with the joint being
completed with in situ
cast cement and sand
Corrugated fiber cement sheeting is hook bolted to the
concrete purlins over an insulating lining laid over the
purlins. As an alternative profiled steel sheeting with
an insulating lining may be fixed to Zed purlins bolted
to the portal frames. Walls may be of solid brick or
concrete blocks fixed between or across the posts of
the portal frames.
NORTH LIGHT PRECAST REINFORCED CONCRETE PORTAL FRAME
The most economical span for this profile of frame is up to about 9m to minimize
the volume of roof space inside the frames and to avoid the large sections of frame
that would be necessary with greater spans.
For convenience in casting and transport the rafter is cast in two sections which
are bolted together at the point of contra flexure and in turn bolted to the posts.
Because of the limited spans and the obstruction of many internal columns this
type of frame is much less used.
TIMBER PORTAL FRAMES
Combination of slender timber sections
glued or glued and nailed together are
used in portal frames for medium and
long span roofs for such buildings as
churches, assembly halls, sports halls
and other single storey structures where
the timber portal frames are exposed for
The advantages of timber as a structural
material in this form are its low self
weight and are comparatively low maintenance required to preserve and maintain
its strength and appearance, particularly where there levels of high humidity as in
Symmetrical pitch glued laminated timber portal frames are usually fabricated
in two sections for ease of transport and are bolted together at the ridge. These
comparatively expensive portal frames are spaced fairly widely apart to support
timber or steel purlins which can be covered with any of the sheet material, slates
The laminations of timber from which the portal is made are arranged to taper in
both the rafter and the post so that the depth is greatest at the knee, where the
frame tends to spread under load, and the slender at the apex or ridge and the foot
of the post, where least section is required for strength and rigidity.
Because f their graceful arch like appearance, glued laminated portal frames are
used as much for appearance as practicality.
Flat glued and nailed timber portal is a one-off
design for an aircraft hanger. The flat portal frame
is designed for the most economic use of timber
and consists of a web of small section timbers
glued together with the top and bottom booms of
glued laminate with web stiffeners. The portal
frames are widely spaced to support metal decking
on the roof and profiled sheeting on the walls.
This structural form was chosen both for
appearance and the long span structure which is
lightweight, free from maintenance and has
adequate fire resistance.
TYPICAL SINGLE STOREY PORTAL
Fixed or rigid portal frame- all joints or connections are rigid giving lower
bending moments than other formats. Used for small to medium span
frames where moments at foundations are not excessive.
Two pin portal frame- pin joints or hinges used at foundation connections
to eliminate tendency of base to rotate. Used where high moments and weak
ground are encountered.
Three pin portal frame- pin joints or hinges used at foundation connection
to eliminate tendency of base to rotate. Used where high base moments and
weak ground are encountered.
SOLID WEB PORTAL FRAMES
They are widely used in the construction of large sheds for industrial
buildings. They are also used in stiffening large span girders.
Portal frames can be used for single bay and multi-bay building and can be
economical for span up to 30 m.
It consists of vertical members (called columns) and top member who may
be horizontal, curved or pitched. The vertical and top members are rigidly
Footings have to be designed to resist horizontal reaction and when the span
exceed 25 m or when the
column height to span
ratio are small, the
horizontal reactions may
be too large to be resisted
by the footings.
Either the footing or the
column bases have to be
tied together or supported
by some other rigid type
The portal frames are
spaced at suitable
distance and support the
roof which may consist of
either a continuous slab
or beam-slab system.
It is subjected to vertical
loads from its roof etc. as
well as horizontal loads
due to wind.
They are such structural
frames which are cast
monolithically due to
which continuity effect is
developed in the frames and amount of bending moments are considerably
They are mostly use as building frames in the construction of high-
L B Structures has approval to supply the
Metspan ‘cold rolled’ portal frame system
which offers economical ‘fast track’ construction at competitive rates. The
Metspan approach to construction enables us to offer a quick response to
structural frame projects.
This new method of construction is ideally suited to the industrial, retail and
service sectors, particularly for the small to medium sized buildings of
traditional portal design.
Metspan is designed to cater for clear
spans from 12 to 18 meters and has a
standard roof pitch of 10 degrees. There
is a choice of standard eaves heights
ranging from 3 to 6.5 meters.
Bay centers will be automatically
determined to suit most economical
arrangements, but bespoke layouts can
be accommodated to facilitate specific
requirements such as door positioning.
Name- Hallstrom Park Leisure Centre
Architect- Devine, Erby Mazlin
Structural engineer- Tierney and
Function- Two Basketball Courts
Lapping pool (25m)
Leisure Pool (35 sq. m)
Location- Small Street, Willoughby,
Cost- $10 million
Type- integrated sport & leisure centre
Plan shape- three rectilinear spaces - 36m x 34m ( both pool hall and gym hall)
Height to roof base- 10 m
Basic roof module- shallow cylindrical
Number of modules-3
Floor area-3,345 sq m
Relationship to ground- entry at ground level pool and gym lower, following site
Material- steel (encased in concrete for fire
roof structure type- rolled portal frames
support structures- portal frame, bracing
footings- deep bored piers to depth of 15m
(BUILDING CONSTRUCTION AND MATERIALS)