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PORTAL FRAME

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PORTAL FRAME 6

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

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

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Large unobstructed floor space

Single-storey warehouse and industrial buildings are commonly constructed fromstructural 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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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 alwaysconstructed 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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10Concrete walls attached to partly protected columns after the fire

11Concrete spalling on the surface exposed to high temperatures

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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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

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 appliedto two-thirds height of the column legs. This can be economically achieved byencasing the columns in cast in-situ concrete.

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18Collapse 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 used2. 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 determinedb) 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 FRAMES1. 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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WASHERSNUT

BOLT

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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Ridge joint for Portal Frame 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.

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Steel portal frame buildings are usually designed by assuming pinned support conditions at the column bases of the steel portal frames.

67Typical 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.