Structural principles

The structure must be designed safely to carry the applied loadings.

The structure must have adequate strength and stiffness to resist the applied loads due to gravity and wind. The function of the structure in resisting vertical loads due to gravity and horizontal loads due to wind is generally considered separately.

The principal floor loadings are due to the self weight of the building and its occupancy. These are referred to as 'dead' and 'superimposed' (or 'imposed') loads respectively.

The floor loadings to be supported by the structure have two components:

  • The permanent or dead loading comprising the self-weight of the flooring and the supporting structure together with the weight of finishes, raised flooring, ceiling, air-conditioning ducts and equipment.
  • The superimposed loading which is the load that the floor is likely to sustain during its life and will depend on the use. Superimposed floor loading for various types of building are governed by BS 6399, but the standard loading for office buildings required by developers and funding agencies is usually 4kN/m2 where movable partitioning is used.

Dead and superimposed loads in commercial buildings are often approximately equal.

For normal office loadings, dead and superimposed loadings are roughly equal in proportion but higher superimposed load allowances will be necessary in areas of plant or to accommodate special requirements such as storage or heavy equipment. The optimum structural solution is to locate any heavier loadings close to columns or where the floor spans are shorter.

The design of the floor structure is concerned mainly with vertical loads. The criteria determining member sizes depend on floor span.

The criteria determining the choice of a member size in a floor system varies with the span.

The minimum size is fixed by practical considerations such as fitting practical connections. As the span increases, the size will be determined by the bending strength of the member and, for longer spans, by the rigidity necessary to prevent excessive deflection under superimposed load or excessive sensitivity to induced vibrations.

Structural criteria governing choice of floor beam

Structural criteria governing choice of floor beam

Composite construction is widely used because of the economic advantages it offers.

Floors may either act compositely with the supporting beam, or independently of it. Composite action enables the floor slab to work with the beam, enhancing its strength and reducing deflection. It has therefore become very popular in steel framed construction for multi-storey buildings.

Composite and non-composite floor systems

Composite and non-composite floor systems

In some cases deflection limits may need to be stricter than those specified in design codes.

In practice, floors will be designed to limit sagging deflection under the superimposed loadings. The British Standard BS 5950 governing the design of structural steelwork sets a limit on deflection under superimposed loading of span/200 generally, and span/360 where there are brittle finishes. For very long spans, this limit is likely to be inadequate; for example, the sag allowed by the code on a 15m span girder would be 42mm and the designer may consider setting more stringent limits. Edge beams supporting cladding will be subject to restriction on deflection of 10-15mm. Deflections may be noticeable in the ceiling layout, and should be taken into account when determining the available cumulative effect of deflections in the individual members of a floor system, although the actual maximum displacement is in practice almost always less than those predicted by theoretical analysis.

Floor vibrations may need to be controlled.

In some instances, vibrations of floor components may cause discomfort or affect sensitive equipment, and the designer should always check the fundamental frequency of the floor system. The threshold of perceptible vibrations in buildings is difficult to define and present limits are rather arbitrary. There is some evidence that modern lightweight floors can be sensitive to dynamic loads which may have an effect on delicate equipment, but generally only for very long spans or light floors.

Building structures should have sufficient lateral rigidity to resist likely wind loads.

Steel buildings have to be rigid enough in the horizontal direction to resist wind and other lateral loads. In tall buildings, the means of providing sufficient lateral rigidity forms the dominant design consideration and developments in this field have led to the construction of taller and taller buildings such as the John Hancock Building or the Sears Tower in Chicago.

Most multi-storey buildings are designed on the basis that wind 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 lattice or rigid jointed frames or reinforced concrete shear walls.

Lateral load bearing systems

Lateral load bearing systems

For most multi-storey buildings, functional requirements will determine the column grid which will dictate spans where the limiting criteria will be rigidity rather than strength.

Floor framing systems may be either simply supported or rigid at the supports. Continuous construction is more efficient structurally, giving shallower floors, but heavier columns, increased complexity at junctions and connections with higher fabrication costs. In practice, the great majority of steel framed multi-storey buildings use simple construction.

Floor framing

Floor framing

In addition to strength and stiffness, building structures must be designed to avoid progressive collapse in the event of a catastrophic accident.

The partial collapse of a system-built multi-storey building at Ronan Point in 1968 following a gas explosion, led to a fundamental reappraisal of the approach to structural stability in buildings. This centred on the need to limit the extent of any damage in the event of catastrophic or accidental loadings. This concept of robustness in building design requires that any major structural element must either be designed for blast loading, or be capable of being removed without precipitating progressive collapse of other parts of the structure. This can be demonstrated by considering alternative load paths and structural actions in the damaged state.

In addition, there is a requirement for suitable ties to 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.

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