VIBRATION
Minimum vibration at no extra charge.
An independent study from Arup underlines why reinforced concrete continues to be the most cost efffective solution for meeting stringent vibration criteria required for specialised buildings such as hospitals.
Recently, steel/composite floors have been marketed as suitable for hospital use on the basis that they can be designed to meet the stringent vibration criteria of hospital night wards and operating theatres. The Arup report was the first has considered the additional construction costs required to meet the vibration criteria.
It has been hypothesised that concrete floors would meet stringent vibration criteria at no or little extra cost, and that steel/composite solutions would require significant additional material to provide mass and stiffness. This hypothesis is based on the observation that, for a given span, concrete floors are heavier than composite floors, and that mass is a key factor in the dynamic response of floors to footfall forces.
Therefore, an independent study was commissioned by The Concrete Centre to quantify the additional steel, concrete and reinforcement required to upgrade 'normal' structural floor designs to meet vibration criteria for night-time wards and operating theatres.
Arup was commissioned to undertake the study - for two reasons. Firstly, Arup has developed a method of vibration prediction which has been extensively validated against measurements on both concrete and composite floors. Secondly, the Arup method is claimed to be the basis for the method in the Steel Construction Institute's recent hospital floor guide.
A number of simplified methods have been developed to predict footfall-induced vibration and have been incorporated into codes and guidance documents. Typically, the simpler the method, the less precise is the prediction - because simple methods necessarily do not take account of all the parameters that affect the vibration performance. To ensure that safe predictions are obtained, the simpler methods should be conservative. However, this is not always the case in vibration prediction.
The Arup method (published in full in the revision of the Concrete Society TR43) is based on first principles. The analysis itself uses accurate dynamic representations of floor structures. The footfall loading effects have been developed from the analysis of over 800 footfall force measurements. The overall process has been calibrated against measurements in scores of buildings in the UK and the USA.
Arup Study
The Arup study consists of four stages:
- Survey of recent hospital structural solutions, examplar designs recommended by healthcare clients and published literature by industry bodies
- Choice of structural solution type (flat slab, post tensioned slab, conventional steel and concrete composite floor and slimdek construction)
- Choice of design criteria (grid, loadings, durability, fire resistance, deflection criteria and vibration criteria from NHS Estates guidance)
- Design of structures for:
- Strength and deflection criteria only
- Night-time ward vibration criteria
- Operating theatre vibration criteria.
Dynamic analysis of a finite element model of a floor structure is used to determine the modal properties (natural frequencies, mode shapes and modal masses) required to make a prediction of the vibration caused by applied forces. Then, using footfall loading functions derived from hundreds of measurements, the footfall-induced vibration response is calculated.
The results of the study, tabulated below, are expressed as the % increase in mass and construction depth required to meet hospital vibration criteria relative to values for a normal 'strength and deflection' design - such as would generally be suitable for offices.
Structure Type |
Location |
Total Mass |
Construction depth |
|---|---|---|---|
Composite |
Office |
0 |
0 |
Night Ward |
131 |
37 |
|
Operating Theatre |
188 |
46 |
|
Flat Slab |
Office |
0 |
0 |
Night Ward |
9 |
10 |
|
Operating Theatre |
15 |
17 |
|
Post Tensioned |
Office |
0 |
0 |
Night Ward |
12 |
14 |
|
Operating Theatre |
27 |
32 |
|
Slimdek |
Office |
0 |
0 |
Night Ward |
59 |
34 |
|
Operating Theatre |
82 |
49 |
Whilst each of these structural forms can be designed to meet stringent vibration criteria, concrete solutions can do this with small increases in depth and material - and minimal additional cost. This is not the case for steel where material quantities and structural depth must be significantly increased in order to meet vibration criteria.
