Portal frame 1

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PORTAL FRAME
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PORTAL FRAME
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3. With single storey buildings natural lighting is
gained by placing clear sheets in roof layout.

4. Steel portal frame buildings are a very
common form of industrial building in
Malaysia.
They are formed by a series of parallel steel
portal frames as the major framing elements
which support the roof structure.
Large clear spans of up to approximately 40
metres can be achieved economically using
steel Universal Beams (UB).
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5. Large unobstructed floor space

6. Single-storey warehouse and industrial
buildings are commonly constructed from
structural steel portal frames with steel,
masonry or concrete cladding.
The steel portal frames are formed by a steel
rafter spanning between two steel columns.
It is common to encase all or part of the steel
portal frame column leg with concrete, or to
use a reinforced concrete column for the
lower part of the portal frame leg.
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7. The portal frames are usually spaced at 5 to
10 metres apart and the steel columns are
fixed or partially fixed at the base.
The roofs of such buildings are always
constructed using corrugated steel sheeting
with 5% to 15% translucent plastic skylights,
supported on steel purlins which are in turn
supported by the rafters.
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Concrete walls attached to partly protected columns after the fire

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Concrete spalling on the surface exposed to high temperatures

12. Concrete tilt-up wall panels are suitable to be
used as boundary walls due to their fast
erection method and on-site fabrication of the
panels.
From a fire resistance perspective, the concrete
walls must act as effective compartmentation to
prevent fire spread to adjacent property.
The inwards collapse of the walls can increase
the fire separation distance to the relevant
boundary and reduce the likelihood of
horizontal fire spread by radiation.
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13. In addition, they must not collapse outwards
which may endanger the lives of the fire-
fighters undertaking rescue and fire fighting
operations in close vicinity to the building.
The concrete panels are commonly pinned at
the base and to the steel portal frames.
The side walls are usually attached to the top
and mid-height of the steel columns and the
end walls are connected to the purlins near
the top.
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Typical end walls attached to steel columns or cast in-
situ concrete columns

15. Under fire conditions, the collapse
mechanisms of these walls are in turn
dependent on the performance of the
supporting frames under elevated
temperatures, providing the connections
between the walls and the frame do not fail.
Passive protection is sometimes applied
to two-thirds height of the column legs.
This can be economically achieved by
encasing the columns in cast in-situ concrete.
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Collapse of steel rafter

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Typical rigid connection

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1.If a conventional simply supported
beam was used over a large span,
an excessive bending moment
would occur at mid-span which
would require a deep heavy beam or
a beam shaped to give a large cross
section at mid-span.
2.No bending moment is induced into
the supporting member or column in
the simply supported beam frame

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Large
Depth
Large cross-
section
Beam for large span

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CG
A SIMPLY SUPPORTED BEAM
Load of the beam acts through the
centre of gravity
SPAN
Supporting members or columns

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PORTAL FRAME THEORY
1.A portal frame is a continuous or rigid
frame which has the basic characteristic
of a rigid or restrained joint between the
supporting member or column and the
spanning member or beam.
2.The object of the portal frame is to
reduce the bending moment in the
spanning member by allowing the frame
to act as one structural entity, thus
distributing the stresses throughout the
frame.

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3.The transfer of stresses from the
beam to the column in rigid frames
will require special care in the
design of the joint between the
members and also at the foundation
connections

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A Pin Joint

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3.They are sometimes called pin joints,
unrestrained joints and non-rigid joints.
4.No bending moment is transmitted
through a hinged joint
5.Hinges can be introduced into a portal
frame design at the base connections
and at the centre or apex of the spanning
member :-

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HINGE
HINGE
apex
FOUNDATION
COLUMN or
SUPPORTING
MEMBER
RAFTER or SPANNING
MEMBER

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Fixed or rigid portal frame
1.All connections between frame members
are rigid. This will give bending moments
lower magnitude more evenly distributed
than other forms.
2.Used for small to medium size frames
where the moments transferred to the
foundations will not be excessive.

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1.A deeper beam must be used
2.The spanning member must be given a
moderate pitch to raise the apex well above
the eave level
3.Two other advantages of the three pin portal
frame are:
a) The design is simplified since the frame
is statically determined
b) On site they easier to erect, particularly
when preformed in sections

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4.Most portal frames are made under
factory controlled conditions off site
which gives good dimensional and
quality control but can create
transportation problems.
5.To lessen this problem and that of site
erection splices may be used
6.These splices can be positioned at the
points of contraflexure, junction between
spanning and supporting members and
at the crown or apex of the beam

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7.Portal frames constructed of steel, concrete or
timber can take the form of the usual roof
profiles used for single or multi-span buildings
such as flats pitched, northlight, monitor and
arch.
8.The frames are generally connected over the
spanning members with purlins designed to
carry and accept the fixing of lightweight roof
coverings or deckings
9.The walls can be of similar material fixed to
sheeting rails attached to the supporting
members or alternatively clad with brick or infill
panels.

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Comparison of volume of roof space and area of
truss of one single and four trusses

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STEEL PORTAL FRAMES
1.To be effective a pitched roof portal
frame should have as low a pitch as
practical to minimise spread at the knee
of the portal frame (spread increases
with the pitch of the rafters of a portal
frame)

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APEX
KNEE

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2.The knee of a portal frame is the rigid
connection of the rafter (or spanning beam) to
the post of the portal (column)
3.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 buildings.
4.For short and medium span frames the apex or
ridge, where the rafters connect, is generally
made as an on-site, rigid bolted connection for
convenience in transporting half portal frames.

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5.Long-span portal frames may have a pin joint
connection at the ridge to allow some flexure
between the rafters of the frame which are pin
jointed to foundation bases to allow flexure of
posts due to spread at the knees under load.
6.For economy in the use of a standard section,
short- and medium-span steel portal frames
are often fabricated from one mild steel 1-
section for both rafters and posts, with the
rafters welded to the posts without any
increase in depth at the knee

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7.Short-span portal frames may be fabricated off
site as one frame.
8.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 strength friction grip bolts.
9.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.

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10.In long-span steel portal frames the posts and
lowest length of the rafter, 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
11.The haunched connection of the rafters to the
posts can be fabricated either by welding a cut
I-section to the underside of the rafter, or by
cutting and bending the bottom flange of the
rafter and welding in a steel gusset plate.

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WASHERS
NUT
BOLT

55. Knee joint for Portal Frame
•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.

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HAUNCH

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12.The junction of the rafter at the ridge is
often stiffened by welding cut I-sections
to the underside of the rafters at the
bolted site connection.
13.Steel portal frames may be fixed to or
pinned to bases to foundations.
14.For short-span portal frames, where
there is comparatively little spread at the
knee or haunch, a fixed base is often
used.

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62. Ridge joint for Portal FrameShown 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.

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67. Steel portal frame buildings are usually
designed by assuming pinned support
conditions at the column bases of the steel
portal frames.
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Typical connection details for fixed and pinned conditions at column
base

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15.It will be seen 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.
16.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.

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17.Two anchor bolts, either cast or set into
the concrete pad foundation, act as
holding-down bolts to the foot of the
portal frame.
18.This type of base is described as a
pinned base as the small packing
between the two plates allow some
flexure of the portal post independent of
the foundation which in consequence
may be less substantial than a
comparable fixed base.