Solid as a rock

The construction of lighter and longer floor structures in recent years has resulted in an increasing incidence of floor vibration. For buildings where users or equipment are sensitive to movements, tight vibration criteria are appropriate. Buildings designed for more general purposes will not need to meet such strict criteria, but the need to correctly analyse and design for vibration is always more pressing when longer and lighter floor constructions are used.

The NHS set about tackling this with the introduction of Health Technical Memorandum 2045, which sets out vibration requirements for hospital design. So how much more does it cost to design to NHS vibration criteria than to the specifications used for a “normal” office? This was the question that The Concrete Centre commissioned Arup to independently find the answer to.

In order to improve the vibration performance of such floors, their mass and stiffness needs to be increased. Because concrete floors are heavier than composite ones, Arup’s working hypothesis was that concrete floors would meet the vibration criteria at no or little extra cost, whereas steel and composite solutions would require significant additional material to provide mass and stiffness.

Arup was comissioned to undertake the study because it has developed a method of vibration prediction that has been extensively validated against measurements on both concrete and composite floors and has been independently peer reviewed.

The Arup study consisted of four stages:

1 Survey of recent hospital structural solutions, exemplar designs recommended by healthcare clients and published literature by industry bodies

2 Choice of structural solution (flat-slab, post-tensioned slab, conventional steel and concrete composite floor and Slimdek construction)

3 Choice of design criteria (grid, loadings, durability, fire resistance, deflection criteria and vibration criteria from NHS Estates guidance)

4 Design of structures for:

• strength and deflection criteria only (in office)
• night-time ward vibration criteria
• operating theatre vibration criteria.

The study considered the mass and construction depth that would be required for a normal design, such as would generally be suitable for offices. It then assessed the percentage increase in strength and deflection that would be needed to meet hospital vibration criteria (see table, below).

While each of these structural forms can be designed to meet stringent vibration criteria, the table shows that 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.

This finding is supported by Peter Young and Michael Willford of Arup, who concluded in the 18 April edition of Structural engineer: “Steel-framed floors designed for the commercial sector have perceptible footfall induced vibration and are not suitable for all uses in the healthcare sector without significant modification.”

Analysis methods

A large number of methods for the assessment of vibration performance are available to the designer, but in an area that is unfamiliar to most structural engineers it is difficult to assess them critically, and tempting to use simplified methods. Such simplified methods have been incorporated into a number of codes and guidance documents, but it is important to remember that, typically, the simpler the method, the less precise the prediction. Therefore, to ensure that safe predictions are obtained, the simpler methods should be conservative.

The Arup method (published in full in the revision of the Concrete Society TR43 Edition 2) is based on first principles. The analysis itself uses accurate dynamic representations of floor structures and the footfall loading functions 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.