External Wall Construction, Lateral Stiffness, Accidental Loading Factors - Multi-Storey Building Form.

External wall construction
The external skin of a multi-storey building is supported off the structural frame.

In most high quality commercial buildings the cost of external  cladding systems greatly exceeds the cost of the structure. This influences the design and construction of the structural system in a number of ways:

• The perimeter structure must provide a satisfactory platform to support the cladding system and be sufficiently rigid to limit deflections of the external wall.
• A reduction to the floor zone may significantly reduce the area and hence cost of cladding.
• Fixings to the structure should facilitate rapid erection of cladding panels.
• A reduction in the weight of cladding at the expense of cladding cost will not necessarily lead to a lower overall construction cost.

Lateral stiffness
Steel buildings must have sufficient lateral stiffness and strength to resist wind and other lateral loads. In tall buildings the means of providing sufficient lateral stiffness forms the dominant design consideration. This is not the case for low- to medium-rise buildings.

Most multi-storey buildings are designed on the basis that wind and/or notional horizontal forces acting on the external cladding are transmitted to the floors,which form horizontal diaphragms transferring the lateral load to rigid elements and then to the ground. These rigid elements are usually either braced-bay frames, rigid-jointed frames, reinforced concrete or steel–concrete–steel sandwich shear walls.

Low-rise unbraced frames up to about six storeys may be designed using the simplified wind-moment method. In this design procedure, the frame is made statically determinate by treating the connections as pinned under vertical load and rigid under horizontal loads. This approach can be used on both composite and non- composite frames, albeit with strict limitations on frame geometry, loading patterns and member classification.

British Standard BS 5950 sets a limit on lateral deflection of columns as height/300 but height/600 may be a more reasonable figure for buildings where the external envelope consists of sensitive or brittle materials such as stone facings.

Accidental loading
A series of incidents in the 1960s culminating in the partial collapse of a system-built tower block at Ronan Point in 1968 led to a fundamental reappraisal of the approach to structural stability in building.

Traditional load-bearing masonry buildings have many in-built elements providing inherent stability which are lacking in modern steel-framed buildings. Modern structures can be refined to a degree where they can resist the  horizontal and vertical design loadings with the required factor of safety but may lack the ability to cope with the unexpected.

It is this concern with the safety of the occupants and the need to limit the extent of any damage in the event of unforeseen or accidental loadings that has led to the concept of robustness in building design.Any element in the structure that supports a major part of the building either must be designed for blast  loading or must be capable of being supported by an alternative load path. In addition, suitable ties should be incorporated in the horizontal direction in the floors and in the vertical direction through the columns. The designer should be aware of the consequences of the sudden removal of key elements of the structure and ensure that such an event does not lead to the progressive collapse of the building or a substantial part of it. In practice,most modern steel structures can be shown to be adequate without any modification.

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