Floor Loadings Factor - Multi-Storey Building Form.

Because steel-framed buildings are relatively light in weight, excessive imposed loadings will have a greater effect on the sizing of structural components, particularly floor beams, than with reinforced concrete structures.

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 imposed loading, which is the load that the floor is likely to sustain during its life and which will depend on the building use. Imposed floor loads for various types of building are governed by BS 6399 but the standard loading for office buildings is usually 4kN/m2 with an additional allowance of 1kN/m2 for movable  partitioning.

For normal office loadings, dead and imposed loadings are roughly equal in proportion but higher imposed load allowances will be necessary in plantrooms or to accommodate special requirements such as storage or heavy equipment.

Floor beams will be designed to limit deflection under the imposed loadings.

British Standard BS 5950 governing the design of structural steelwork sets a limit for deflection under imposed loading of (span/200) generally and (span/360) where there are brittle finishes. 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 clearance for service routes.The designer should therefore check the cumulative effect of deflections in the individual members of a floor system although the actual maximum dis- placement is in practice almost always less than that predicted. In some instances, vibrations of floor components may cause discomfort or affect sensitive equipment, and the designer should check the fundamental response of the floor system. The threshold of perceptible vibrations in building 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.

However, in most situations a simple check on the natural frequency of the floor system is all that is required.

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