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Good vibrations

Technical

Specifiers need to consider the vibration response of floors in detail. Here we summarise the key issues

Any structure will vibrate if subjected to cyclic or sudden loading. In most cases, the vibrations are imperceptible and can be neglected in building design. But in some circumstances the response of the structure to a common cyclic load (eg walking) is sufficient to produce a response that is perceptible to the occupants of the building.

The most common example of small, but perceptible, vibration occurs in floor structures. This is not a new phenomenon, but it is more noticeable in the working environment of modern offices and residential dwellings.

For structures that are subjected to static loading, it is normal for the designer to calculate the vertical deflection of the floor to avoid undesirable deflections and limit the possibility of damage to brittle finishes. For floors that are subject to cyclic or sudden loading, though, human perception of motion is usually related to acceleration levels rather than displacement. In most practical building structures, if the magnitude of these acceleration levels is not limited, the typical reaction of the building occupants varies between irritation and a feeling of insecurity; this is based on the instinctive human perception that motion in a 'solid' building structure indicates structural inadequacy or failure.

Acceptability criteria

The evaluation of the exposure of humans to vibrations within buildings is covered by BS 6472: 1992 1(which is strongly linked to ISO2631-2: 1989 2), and is written to cover many vibration environments. This publication presents root-mean-square (rms) acceleration limits (for a sinusoidal response, this is defined as the peak acceleration divided by the square root of two) for vibrations as a function of the exposure time and the frequency, for both longitudinal and transverse directions of persons in standing, sitting and lying positions (see top illustration). These acceleration limits are given in terms of the 'base curves' (shown opposite).

The working environment also affects the perception of motion. For busy floors, where the occupant is surrounded by the activity that is producing the vibrations, the perception of motion is reduced. By contrast, in quiet environments (such as laboratories and residential dwellings), where the source of the vibration is unseen, the perception of motion is heightened significantly.

Limits for different environments are given in terms of the rms acceleration as multiples of the appropriate base curve.

Multipliers currently recommended for the UK are given in Table 1.

The multipliers in Table 1 are based on the assumption that the structure is subject to continuous vibrations (ie a floor that is heavily trafficked by pedestrians). However, in reality, the floor may be subject to sporadic vibrations over short periods of time.

In these circumstances, intermittent vibrations may be accounted for by adopting a vibration dose value (VDV) calculation based on BS6472: 1992. For example, by taking this approach, three occurrences each of eight seconds duration with vibration levels equivalent to a base curve multiplier of 25 would be tolerated in an office each day, provided vibrations at other times were low. But in the case of operating theatres and precision laboratories, HTM 2045 does not permit making allowance for the intermittency of events. In these circumstances, the multiplier of 1.0 given in Table 1 should be used (based on continuous vibration).

Assessment of vibrations on floors

Traditionally, the design of floors for occupant-induced vibrations has been based on providing a minimum natural frequency (which depends on the ratio of the floor stiffness to its mass), to 'tune' the structure out of the range of the activity frequency.

However, resonant accelerations can still arise from components of walking. Consequently, as natural frequency limits do not give any indication of the acceleration response (which depends on the effective mass of the floor and damping), it is now more common to predict the rms accelerations expected in service.

Guidance on designing floors for vibrations is given in the Steel Construction Institute (SCI) publication 076, 4 which has now been supplemented by three SCI Advisory Desk Notes.

5This guidance allows designers to make a conservative estimate of the base curve multiplier (denoted as a 'response factor') using hand calculations. Alternatively, as numerical modelling of floors becomes commonplace, more efficient designs may be produced by using finite-element-analysis techniques.

Case studies In December 2000, the SCI completed a three-year Department of the Environment Transport and the Regions Partners in Innovation project, where vibration tests on real floors were conducted to evaluate their in situ performance, so improvements to existing guidance could be made.

5In all cases, the actual floor performance was much better than required by current guidelines. A summary of the results appears in Table 2.

Current work The SCI is currently halfway through a three-year European Coal and Steel Community (ECSC) research project on the vibration of steel and composite floors with partners from Germany, Luxembourg and the Netherlands. The objective of this project is to improve on existing guidance and produce a general design publication that will receive the status of an acknowledged European guideline on floor vibrations. It is hoped that this guidance will be a key influence on CEN limits for floor vibrations and future revisions to the Eurocodes.

For further information please contact Dr Stephen Hicks at the SCI on tel 01344 623345, or email s. hicks@steel-sci. com

FOOTNOTES

1BS 6472 Evaluation of human exposure to vibration in buildings (1 Hz to 80 Hz), London, British Standards Institution, 1992 2ISO 2631-2: Evaluation of human exposure to wholebody vibration: Part 2: Continuous and shock-induced vibration in buildings (1 to 80 Hz), Switzerland, International Organisation for Standardization, 1989 3Health Technical Memorandum 2045: Acoustics: Design considerations, London, HMSO, 1996 4Wyatt, TA: Design Guide on the Vibration of Floors, SCI Publication 076, Ascot, Steel Construction Institute, 1989 5New Steel Construction AD253 (in Vol 9 No 6), AD254 (in Vol 10 No 1) and AD256 (in Vol 10 No 2) Design con -

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