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    1) Description of Multi-storey building system:
    The primary structural elements of a multi-storey steel building, namely the columns and floor beams, should be laid out with a view to minimising both the cost of the steelwork and the time required for its erection. For any given structure a layout can be determined that optimises the combined beam and column content of the structure, but in most cases functional and architectural considerations have to be taken into account, which mitigates against the design of optimal bay sizes. Consultation between the architect and the engineer in the early stages of planning could help to prevent an uneconomical layout having to be adopted.

    The third structural element, after the columns and beams, is the stabilising system necessary to provide lateral support to the building, i.e.to supply stability under gravity loading and to resist the overturning effects of wind. Obviously, the taller the building the more important the bracing system becomes and in very tall structures the provision of adequate lateral support does in fact become the dominant consideration.

    Lateral stability may be provided within the steel structure itself by means of bracing, or moment-resisting beam-column connections or steel shearwalls, or it may be imparted by other building elements, e.g. reinforced concrete service towers, or concrete or brick in-fill panels in the walls. In all cases the stabilising elements may be located within the plan dimensions of the building or in the perimeter walls, or they may even be external to the building.

    If a building is to incorporate the stabilising system within the steel structure, the framework can be of the two-way braced, one-way braced and one-way-rigid, or two-way rigid type.Triangulated bracing is usually cheaper than a stiff moment-resisting frame and should be used wherever access problems do not arise, i.e. where door, window or service openings are not required.

    In addition to vertical bracing systems it is necessary to provide stiffness within the plane of each floor, both to maintain the squareness of the floor in plan and to transmit the wind loading on the exterior of the building to the vertical bracing.

    Stabilising systems - examples
    Figs 7.1 to 7.5 give examples of various bracing systems that can be employed to provide stability to multi-storey buildings. The examples are of general application and illustrate the basic principles involved in such systems. The vertical steel bracing is shown as the X-type for simplicity, but could equally well be chevron bracing, knee bracing or someother type (the features of the various bracing types are discussed more fully in Chapter 11). The floors are shown as steel-braced, but in practice the bracing function could be furnished by the concrete floor slab, in which case only nominal squaring-up
    steel bracing would be required. The systems are applicable to buildings of almost any number of storeys.

    Two-way steel bracing
    The two-way steel braced system shown in Fig 7.1 is one of the most efficient in terms of stiffness, speed of erection and economy. All beam-to-column connections are of the simple (i.e. hinged) type, so labour input in both columns and beams is minimised and erection can proceed quickly. Being fully steel-framed, the structure is self-supporting and can be completely erected without having to be integrated with other trades. The only drawback is the presence of the braced panels in the exterior walls, which might interfere with the window pattern, but in the light of the current trend towards exposed steelwork
    the windows could be set back and the bracing system be expressed boldly as an architectural feature. In very long buildings it would be necessary to provide one or more interior sets of bracing, as shown dotted.

    One-way steel bracing
    The stiff frame shown in the alternative end elevation of Fig 7.1 is another method of providing transverse stiffness to the structure. All of the transverse frames not only the end ones, would be stiff, but the building would still rely for longitudinal stiffness on the two sets of one-way bracing in the sides. This would be a more costly arrangement than the two-way braced solution, but would remove the drawbacks of the triangulated bracing.

    It is more suited to long buildings and has the further advantage that the main (i.e.transverse) floor beams could be shallower, because of their continuity, with consequent reduction in storey height.It must be emphasised, however, that on purely economic grounds triangulated bracing is very much more cost-effective than a moment frame, both in shop fabrication and in erection.

    Central service core
    Where a building is fairly compact in plan and does not have a great length-to-width ratio,a central service core is a very efficient means of providing stability, as shown in Fig 7.2. e Elevation  

    Floor framing
    In steel-framed buildings the floor framing system almost invariably consists of a series of main and secondary beams at right angles to each other in plan, with the secondary beams framing into or passing over the tops of the main beams. The floor slab or deck is then carried on top of the secondary beams.

    Except where stiff-frame action is required, as discussed under Stabilising Systems above, the main beams are usually simply-supported spans between the columns. If the secondary beams have their top flanges flush with the tops of the main beams they would be framed into the webs of the main beams and would thus also be simply-supported; this would produce a floor grid of minimum depth and would result in a reduction in storey height. However, underfloor services running at right angles to the main beams would then have to pass through holes formed in the webs of these beams, or else be routed
    below the main beams, which would increase the floor depth.

    If the secondary beams pass over the tops of the main beams, however, they would no longer be simply-supported but be continuous, significantly reducing the mass and especially deflection. The routing of services in both rectangular directions in plan would be facilitated by reason of the space available above the main beams.

    The two beam framing systems referred to above represent conventional practice as used on the great majority of small to medium-sized buildings. The beams are ofstraightforward construction and employ standard end connections and are thus easy and cheap to fabricate. A number of non-standard options are available and are worth considering for larger buildings where a high level of repetition of components would justify their use. These are discussed below.

    Twin beams
    Main beams span between columns and can therefore not normally be made continuous.Continuity can, however, be achieved by replacing the beam by a pair of closely-spaced twin beams passing on each side of the column, as shown in Fig 7.6.

    Because of their continuity the main beams can now be designed plastically, for acombined moment on the two beams of 70 per cent or less of that for the simply-supported single beam, and at a combined mass m about equal to that of the single
    beam. As regards deflection, the twin-beam system would tend to be more stiff than a single simply-supported beam of the same load capacity because of the continuity. The labour input for the twin beams would be more, but this alternative is useful when it is desired to reduce the depth of the floor (and thus the storey height), or on long spans where the twin rolled I-sections replace a more expensive single welded plate girder.

    Elevation and floor plan of a typcial multi-story steel building:

     

    1) Description of Multi-storey building system:
    The primary structural elements of a multi-storey steel building, namely the columns and floor beams, should be laid out with a view to minimising both the cost of the steelwork and the time required for its erection. For any given structure a layout can be determined that optimises the combined beam and column content of the structure, but in most cases functional and architectural considerations have to be taken into account, which mitigates against the design of optimal bay sizes. Consultation between the architect and the engineer in the early stages of planning could help to prevent an uneconomical layout having to be adopted.

    The third structural element, after the columns and beams, is the stabilising system necessary to provide lateral support to the building, i.e.to supply stability under gravity loading and to resist the overturning effects of wind. Obviously, the taller the building the more important the bracing system becomes and in very tall structures the provision of adequate lateral support does in fact become the dominant consideration.

    Lateral stability may be provided within the steel structure itself by means of bracing, or moment-resisting beam-column connections or steel shearwalls, or it may be imparted by other building elements, e.g. reinforced concrete service towers, or concrete or brick in-fill panels in the walls. In all cases the stabilising elements may be located within the plan dimensions of the building or in the perimeter walls, or they may even be external to the building.

    If a building is to incorporate the stabilising system within the steel structure, the framework can be of the two-way braced, one-way braced and one-way-rigid, or two-way rigid type.Triangulated bracing is usually cheaper than a stiff moment-resisting frame and should be used wherever access problems do not arise, i.e. where door, window or service openings are not required.

    In addition to vertical bracing systems it is necessary to provide stiffness within the plane of each floor, both to maintain the squareness of the floor in plan and to transmit the wind loading on the exterior of the building to the vertical bracing.

    Stabilising systems - examples
    Figs 7.1 to 7.5 give examples of various bracing systems that can be employed to provide stability to multi-storey buildings. The examples are of general application and illustrate the basic principles involved in such systems. The vertical steel bracing is shown as the X-type for simplicity, but could equally well be chevron bracing, knee bracing or someother type (the features of the various bracing types are discussed more fully in Chapter 11). The floors are shown as steel-braced, but in practice the bracing function could be furnished by the concrete floor slab, in which case only nominal squaring-up
    steel bracing would be required. The systems are applicable to buildings of almost any number of storeys.

    Two-way steel bracing
    The two-way steel braced system shown in Fig 7.1 is one of the most efficient in terms of stiffness, speed of erection and economy. All beam-to-column connections are of the simple (i.e. hinged) type, so labour input in both columns and beams is minimised and erection can proceed quickly. Being fully steel-framed, the structure is self-supporting and can be completely erected without having to be integrated with other trades. The only drawback is the presence of the braced panels in the exterior walls, which might interfere with the window pattern, but in the light of the current trend towards exposed steelwork
    the windows could be set back and the bracing system be expressed boldly as an architectural feature. In very long buildings it would be necessary to provide one or more interior sets of bracing, as shown dotted.

    One-way steel bracing
    The stiff frame shown in the alternative end elevation of Fig 7.1 is another method of providing transverse stiffness to the structure. All of the transverse frames not only the end ones, would be stiff, but the building would still rely for longitudinal stiffness on the two sets of one-way bracing in the sides. This would be a more costly arrangement than the two-way braced solution, but would remove the drawbacks of the triangulated bracing.

    It is more suited to long buildings and has the further advantage that the main (i.e.transverse) floor beams could be shallower, because of their continuity, with consequent reduction in storey height.It must be emphasised, however, that on purely economic grounds triangulated bracing is very much more cost-effective than a moment frame, both in shop fabrication and in erection.

    Central service core
    Where a building is fairly compact in plan and does not have a great length-to-width ratio,a central service core is a very efficient means of providing stability, as shown in Fig 7.2. e Elevation  

    Floor framing
    In steel-framed buildings the floor framing system almost invariably consists of a series of main and secondary beams at right angles to each other in plan, with the secondary beams framing into or passing over the tops of the main beams. The floor slab or deck is then carried on top of the secondary beams.

    Except where stiff-frame action is required, as discussed under Stabilising Systems above, the main beams are usually simply-supported spans between the columns. If the secondary beams have their top flanges flush with the tops of the main beams they would be framed into the webs of the main beams and would thus also be simply-supported; this would produce a floor grid of minimum depth and would result in a reduction in storey height. However, underfloor services running at right angles to the main beams would then have to pass through holes formed in the webs of these beams, or else be routed
    below the main beams, which would increase the floor depth.

    If the secondary beams pass over the tops of the main beams, however, they would no longer be simply-supported but be continuous, significantly reducing the mass and especially deflection. The routing of services in both rectangular directions in plan would be facilitated by reason of the space available above the main beams.

    The two beam framing systems referred to above represent conventional practice as used on the great majority of small to medium-sized buildings. The beams are ofstraightforward construction and employ standard end connections and are thus easy and cheap to fabricate. A number of non-standard options are available and are worth considering for larger buildings where a high level of repetition of components would justify their use. These are discussed below.

    Twin beams
    Main beams span between columns and can therefore not normally be made continuous.Continuity can, however, be achieved by replacing the beam by a pair of closely-spaced twin beams passing on each side of the column, as shown in Fig 7.6.

    Because of their continuity the main beams can now be designed plastically, for acombined moment on the two beams of 70 per cent or less of that for the simply-supported single beam, and at a combined mass m about equal to that of the single
    beam. As regards deflection, the twin-beam system would tend to be more stiff than a single simply-supported beam of the same load capacity because of the continuity. The labour input for the twin beams would be more, but this alternative is useful when it is desired to reduce the depth of the floor (and thus the storey height), or on long spans where the twin rolled I-sections replace a more expensive single welded plate girder.

    Elevation and floor plan of a typcial multi-story steel building:

     

    Our last concern would be the EIFS(EXTERNAL INSULATION FINISHING SYSTEM).Our suggestion would be the integrated insulated decoration panel because it has perfect thermal insulation performance with various pattern and color choices.

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