Blog post written by Professor Aleksandar Pavic, Managing Director

Recently, the University of Cambridge invited me to give a seminar to their Structures Research Group on a topic of my own choosing. Of course, I accepted this kind invitation and duly delivered a 35-minute seminar entitled: “The need and state-of-the-art tools for rapid advances in global vibration serviceability research”. In this I highlighted key topics that are occupying my research and commercial activities these days, namely:

  1. the rise of vibration serviceability as a governing design criterion in civil structural engineering and the level of preparedness of the structural engineering profession to deal with this effectively and efficiently;
  2. the evidence for this;
  3. the consequences of this; and
  4. the priorities and available tools for the structural engineering profession to effectively and efficiently handle the rapidly growing issue of vibration serviceability.

This blog post summarises a first portion of this talk and looks at some of the evidence of this phenomenon. This evidence was actually developed not less than 12 years ago by other experts in this field, so I am here just a messenger that vibration serviceability is becoming a governing design criterion. Discussion(s) of other equally important items in my talk mentioned above will come to this blog in future posts.

Summary of the research

The following figure shows a ‘standard’ composite steel-structure floor plate featuring floor panels which was designed to have minimum dimensions and still satisfy all design requirements (strength, deflection, fire protection, etc.) apart from vibrations for human office occupancy.

Figure showing structural arrangement of a generic 'standard' floor plate
Generic ‘standard’ floor plate

That floor structural configuration is described in the first column of the table below titled ‘Joist System’ where it can be seen that the ‘Joist System’ floor is susceptible to vibration despite meeting all other design criteria. Needless to say, this is not surprising and happens very often in practice today using any reasonable design guidance in the UK or elsewhere.

Table showing composite floor configurations examined to satisfy vibration serviceability
Composite floor configurations examined to satisfy vibration serviceability

This table then presents four other structural configurations for the same floor which were explored so that they have optimum characteristics but are not susceptible to vibrations i.e. they do satisfy vibration serviceability.

The cost of meeting vibration requirements

It can be seen that in order to satisfy vibration serviceability:

  1. the ‘Non-Composite System’ needs to be more than twice heavier than the starting ‘Joist System’ which is also non-composite;
  2. the ‘Composite Beam System’ is thinner but a third heavier;
  3. the ‘Redesign no. 1’ (non-composite) is only slightly heavier, but leads to a 100mm greater story height which may be more expensive to cool and heat, will cost more to clad and may lose a whole floor or two to the client in a multi-storey building designed within a usually limited height envelope.
  4. The ‘Redesign no. 2’ (non-composite) is a whopping 77% heavier and has 150mm greater storey height with all of the above listed problems increasing cost and potential loss of space even more.

The reference paper from which this information was taken is the first attempt known to me where somebody tried to analyse the real cost of meeting floor ‘vibration serviceability’ requirements by so called and so omnipresent ‘structural modification’. In other words, by throwing considerably more construction materials at the problem: not to improve strength but purely to increase mass and stiffness to reduce floor vibrations caused by the relatively tiny dynamic forces from a single person walking. If designers of cars and aeroplanes were designing like this to solve excessive vibrations in their moving and flying structures, nothing ever would be moving or flying.

The consequences of such a design approach in structural engineering, which – as I write this – is the approach used around the world to design millions of m2 of floors, are clear from the previous table. This is very much against principles of ‘build light’ which is the key strategy in the just published 2020 Climate Emergency Design Guide to reduce embodied energy in buildings to meet net zero carbon emissions target by 2050 in the UK.

Using so much construction materials just to reduce calculated resonant vibrations is incredibly wasteful and is really not a good engineering considering that floors move only a few microns during such vibrations causing equally tiny stresses. This further reduces the level of utilisation of materials in buildings which is currently estimated to be not more than around 50%.

Is there a better way?

Damping is a much, much more effective way to reduce resonant vibrations compared with increasing mass and stiffness, as every quality textbook on structural dynamics will point out. And yet, the structural design profession is still not embracing this concept as much as it should and indeed, must. This is particularly important, considering that in just a few years the tax on construction materials such as steel and concrete may not be far away from the tax on fuel in order to protect the planet.

Adding otherwise unnecessary materials is not the way forward to control resonant floor vibrations due to human footfall, so what is the alternative? In FSD Ltd we utilise a two-pronged approach: 1) increased accuracy of calculations through highly sophisticated and realistic calculations, as described in my previous blog post; and 2) the use of state-of-the-art passive and active damping technologies to control floor vibrations without adding more materials to it. We have successfully deployed numerous such systems over recent years and are leading the way with our constant R&D. If you would like to hear more about how our expertise in this area could help you to save money on your project, get in touch using our contact form and we’d love to discuss your project with you.