PORTAL FRAME BUILDINGS (Commercial or industrial)
• A Portal Frame can be defined as a continuous or rigid frame which has the characteristic of a rigid or restrained joint between the supporting member (or column) and the spanning member (or beam). • The object of this continuity of the portal frame is to reduce the bending moment in the spanning member by allowing the frame to act as one structural unit, thus distributing the loads and stresses throughout the frame.
Portal Frames 2 • If a conventional, simply supported beam, (steel or concrete) was used over a large span, an excessive bending moment would occur at midspan which would necessitate an uneconomical, probably impractical deep, heavy beam or an elaborate system of lattice struts. • The portal frame eliminates the need for a lattice of struts and ties within the roof space giving greater usable volume and a more pleasing internal appearance.
Points of Contraflexure
Rigid Joints
BENDING MOMENT DIAGRAM
Conventionally Supported Beam
Portal frames • Portal frames are constructed of steel, concrete and timber. • Usually for single storey, or low-rise industrial/commercial buildings and for either single span or multi-span units. • The frames are connected together over the spanning members with purlins designed to carry and accept the fixings of lightweight roof coverings. • The walls can be of similar lightweight materials fixed to sheeting materials or clad with brick/block infill panels. • Most portal frames are made under factory controlled conditions off-site which gives good quality control and dimensional control but can create transport problems.
PURLINS
Portal Frames Spaced as required
Portal Frames
Portal Frames
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• Hinges: • Portal frames of moderate height and span are connected directly to their foundation bases forming a rigid joint. • When the frames exceed 5 metres in height and 15/20 metres in span a hinged or pin-joint must be introduced at the base connection. • A hinge is a device which will allow an amount of free rotation to take place (caused by wind pressures and thermal movement), but at the same time transmitting load from one member to another. • These are sometimes called pin-joints, unrestrained joints & non-rigid joints.
Portal Frames • In practice this is not a true ‘pivot’ but a device to allow just enough movement to absorb rotational movement and stresses. • Hinges are introduced into a portal frame at the base connection and at the centre or apex of the spanning member giving three basic forms of portal frame: • 1. Fixed or Rigid Portal Frames • All connections are rigid, used for small to medium sized frames where stresses and therefore, bending moments, are low. • 2. Two pin portal frame • Hinges are used at the base connections to allow movement. Used when there are high base stresses and/or weak ground.
Positions for Hinged Joints
BENDING MOMENT DIAGRAM
Portal Frames • 3. Three-pinned portal frame • These have hinged joints at the base connections and at the centre of the spanning member. The effect of the third hinge is to reduce the bending moment in the spanning member but it will increase its deflection. • To overcome this disadvantage the spanning member must be given a moderate pitch to raise its apex well above eaves level. • The advantages of this type are that it is easier to construct, transport and erect on site, especially with pre-formed sections.
Concrete Portal Frames • Manufactured from high quality, pre-cast, reinforced concrete. • Usually low-pitched, single or multi-span frames, readily available, wide range of designs, just a few componants. • Designed to carry lightweight roof sheeting fixed to purlins. • Wall infills can be of various types, non-load bearing. • Foundations: • POCKET BASE-PLATE CONNECTION
Advantages of Concrete Portals • Factory controlled production, ensures quality, unlike in- situ casting. • A wide range of interchangeable standard components give easy and flexible design options. • Mass produced, short delivery times. • Little or no maintenance. • Own built-in fire resistance, no treatment required. • Wind resistance good. • Clean lines, can be aesthetically pleasing. • All work, fds., setting out, erection, carried out by the one firm.
TIMBER PORTAL FRAMES • Timber portal frames can be manufactured by several methods which produce a light, strong frame of pleasing, attractive appearance. • Particularly suitable for buildings where internal aesthetic appearance is important, as well as clear spanning space, swimming pools, gyms, churches, etc. • The three common methods are: • GLUED LAMINATE • PLYWOOD FACED • SOLID TIMBER
Glued Laminate • GLULAM • The main object of forming a laminated member, consisting of glued layers of thin section timber members, is to obtain an overall increase in the strength of the complete component, over which could be obtained from a similar sized solid section of a particular species of wood, even supposing such a section of timber should be obtained. • This type of portal frame is manufactured by specialist firms as the jigs and forming/cutting equipment would be too costly for small outputs. • The selection of suitable quality softwoods of the right moisture content is important for a successful design.
GLULAM • •
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As with other portal frames, these can be either fully rigid, 2-pin or 3pin structures. Site work is simple, consisting of connecting the foot of the supporting member to the metal shoe fixing or to a pivot joint housing bolted to the concrete foundation and connecting the joint at the apex or crown with a bolt fixing or a hinge device. Most glued laminated portal frames are fabricated in two halves which eases transportation problems and gives maximum usage of assembly jigs. The frames can be linked together at roof level with timber purlins and clad with lightweight sheeting. Or, they can be finished with traditional roof coverings, and any form of walling can be used in conjunction with these frames.
STEEL PORTAL FRAMES • Fabricated from standard Universal Beams, Columns, Box sections, or lattice of angles and tubulars. • Usually systems of welded fabrication of components, bolted together on site. • Most forms of lightweight roofing coverings can be used, and usual wall infilling. • Foundations: • POCKET CONNECTION • BASE PLATE CONNECTION • PIN or HINGED CONNECTION
Advantages Factory Controlled production Standard range of systems A frame of good wind resistance Easy, quick, all-weather site assembly, semiskilled labour • Overall dead load of steel portal frame is lighter than concrete or timber. • Disadvantage – steel is corrosive which will require life-long protective coating or regular repainting • Steel has a lower fire resistance than concrete but higher than timber. • • • •
