Modified forms of steel sections
Non standard steel sections can be fabricated where architectural or structural solutions dictate standard sections inappropriate.
Such shapes might be used for aesthetic reasons or where the largest suitable standard section is structurally inadequate.
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Whilst the vast majority of fabricated steel in construction is based on standard sized hot rolled sections such as straight beams and columns, modified sections including tapered beams, plate girders, castellated beams, and curved sections can also be used. Variations to the standard cross-sections can be made by cutting and welding. Different cross-sectional shapes can also be produced from plate. These other fabrication techniques which lead to the production of special forms of members are relevant to only a very small proportion of typical building work but they have been included because of the high level of architectural interest which they generate.
In addition to the modified forms described, a variety of methods can be used to shape steel components.
Pressing, folding and forming may involve several processes to create shapes to supplement the range of rolled sections.
Press brakes are used for bending sheet and plate, with a ram and bed.
Plate girders are made by welding plates together to form I sections. They are principally used where standard shapes have inadequate strength.
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The figure shows a plate girder made by welding together steel plates, which has then been stiffened further by welding additional plates transversely and sometimes longitudinally. Because the flange to web welds are long and straight they can be completed by machine relatively cheaply.
Plate girders are typically used where the structural requirements are particularly onerous, such as in bridge construction or in transfer structures supporting a number of storeys over an open plan ground floor area.
Castellated beams are formed by cutting the web of a rolled section along its length in a 'wave form' and welding two pieces together to form a deeper section.
A castellated beam is fabricated from a standard Universal Beam section. The beam is initially split along its length by a profiled flame cut as shown below. The two halves of the beam are then separated, displaced by one profile and reconnected by welding
Castellated beams have greater resistance to deflection, and can accommodate services.
Castellated beams have a deeper section than a comparable solid beam, which has a greater resistance to deflection. They are therefore most often used in long span applications with light or moderate loadings, particularly roofs. Since the weight of steel has not changed, the structural efficiency of the section in bending has been increased. A further advantage of castellated beams is the holes in the web which provide a route for services.
The web, however, is the main source of shear strength. Care must therefore be taken to ensure that the shear resistance is sufficient at points of high shear. For this reason the holes are often filled in using welded plate, at points of support and/or concentrated load.
The increased depth of a castellated beam will also increase its tendency to fail under lateral-torsional buckling. Where this form of failure is an important factor in the structural design, the improved efficiency of castellated beams may not be realised unless additional structural restraint is provided.
A variation of the castellated beam is the 'cellform beam'.
The cellform beam is fabricated in a similar way to the conventional castellated beam, but some additional cuts are made in the web to create circular holes when the cut sections are joined.
Tapered beams follow the pattern of bending moments - the greatest depth at mid-span reflecting the maximum moment. They are most often made of plate, but can be fabricated by cutting a standard section. The latter method, however, may produce more waste.
Floor beams in multi-storey frame are usually designed as simply supported. Hence bending moments diminish from a maximum at mid-span to zero at the supporting columns. It would therefore seem logical to make a corresponding reduction in the bending strength of the beam. This is the principle of the tapered beam which, for lengths less than 15m, is made from plate using automatic cutting and welding processes. As mentioned above, the relatively small savings in material are likely to be outweighed by the increased production costs. However tapered beams do offer a practical advantage over standard universal sections in that they greater provides greater vertical space near the column through which service trunking can pass.
The shape of the taper reduces the shear strength towards the supports, where the maximum shear force often occurs. This aspect of the beam's performance must be carefully checked.
Tapered beams are likely to be particularly suitable for wide span floors.
The optimum spans between 15 and 18 metres, where a considerable depth of universal section would otherwise be necessary throughout its whole length. This is about twice the span most commonly adopted in office construction.
Other cross-section variations can be fabricated by cutting and welding standard sections in a very wide variety of ways.
