The idea for this issue was born in late September 2004 during a visit by Professor Howard Wright, then Chairman of the Editorial Advisory Panel of this journal and external examiner to the MSc course at the Department of Civil and Structural Engineering at the University of Sheffield. We discussed student comments of my module called ‘Vibration Engineering’ and their perception of the content as interesting but unfamiliar, and therefore hard to master. This module is based on the last 15 years of research and consultancy which I and my colleagues at the University of Sheffield have conducted into vibration serviceability (http://vibration.shef.ac.uk). The module is one of very few attempts in UK universities to educate formally civil and structural engineers about vibration serviceability. The module was launched five years ago as a direct response to the following recommendation given in the 13th annual report of the influential ICE/IStructE Standing Committee on Structural Safety (SCOSS)1
The design of dynamically responsive structures for safety and to meet performance requirements for acceleration and frequency is a relatively complex subject. It is perhaps not sufficiently well covered as a matter of course in the education and formation of civil and structural engineers. Today, these engineers should, it is suggested, learn the principles of the subject as undergraduates. There may also be a need for more postgraduate courses specialising in structural dynamics. In addition, practising engineers should perhaps have more opportunity to develop their skills in identifying and designing dynamically responsive structures as part of their continuing professional development.
The SCOSS report was published in 2001 and it came hot on the heels of the London Millennium Bridge lateral sway problem and a number of more or less publicly known problems with vibration performance of grandstands under crowd dynamic loading. Therefore, Howard's idea was that I could guest edit a special issue of Structures and Buildings devoted to vibration serviceability. Wisely or not, I accepted the challenge, and its outcome is in your hands, after two years of hard work. In the call for papers for this special issue, I made a non-exclusive ‘shopping list’ of seven topics related to vibration serviceability which I would like to see covered. These were
statistical and risk assessment approaches to vibration serviceability
human-induced vibrations and dynamic human – structure interaction
ground-borne vibrations and related vibration serviceability issues
acceptability of vibrations for humans and high-quality processes
analytical and/or experimental assessment of vibration serviceability in design and in completed structures
best design and management practice when dealing with vibration serviceability
education and training of engineers to enable them to deal with vibration serviceability.
The response to the call was good. Many papers which did not make it into this special issue are good submissions which will, after review, find their place in future issues of the journal.
Personally, I am satisfied that the papers selected cover more or less the first five topics from my list. I really tried hard to solicit a paper on the best design and management practice when dealing with vibration serviceability in real-life projects. Logically, I approached a number of practising engineers in the UK and elsewhere (those who were not approached, please do not be offended). I knew that these had dealings with vibration serviceability in the past. I failed miserably and a paper on this topic was not produced. This may indicate three things. First, that people who know a thing or two about handling vibration serviceability in real-life projects are quite busy due to a general lack of available expertise, so they do not have the time to write. Second, that the area is such a dangerous professional and legal minefield that nobody wants to stick their neck out and propose something to guide engineering practice through the minefield. Third, and most probably, both of the previous two reasons are correct: vibration serviceability is indeed a professional and legal minefield for an average practising engineer, but there is a handful of good operators around who can navigate safely through it. These operators are, however, so busy with the rapidly increasing level of vibration serviceability-related work in the UK and elsewhere that they do not have the time to write. Similarly, no paper was produced explicitly on education and training of engineers to enable them to deal with vibration serviceability. As I could not solicit papers from others on the last two items from my shopping list, the remainder of this editorial is my personal view on these matters.
Although often neglected or overlooked, vibration serviceability is becoming a governing design criterion. This is particularly so for light and slender civil engineering structures occupied and dynamically excited by humans. Four typical examples of these structures are long-span floors, staircases, grandstands and footbridges. As the trend toward lighter and more slender structures continues, it is becoming apparent that a significant number of important aspects when dealing with vibration serviceability have been inadequately addressed in the past. One of these, as mentioned above, is inadequate training of civil engineers in the UK (and elsewhere) to deal with dynamically responsive structures. Another is that the dynamic behaviour of civil structures under human-induced dynamic loading has not been adequately researched and reported in the past. Despite these rather deep-rooted problems, which are neither easy nor quick to solve, design decisions have to be made in contemporary design practice based on the current skills and knowledge.
Vibration is an omnipresent form of dynamic motion in which the structure oscillates about an equilibrium. In principle, everything vibrates all the time. The problems with vibration occur when it becomes excessive, causing annoyance, malfunction of sensitive equipment, damage or structural failure. The first step towards the assessment of vibration serviceability of a civil engineering structure of whatever kind is to identify and characterise the following three factors
the vibration source
the transmission path, i.e. mass, stiffness and damping properties of the structure
the receiver.
Although this rationalisation of the vibration serviceability problem may seem simple, it is actually a very difficult task requiring a thorough understanding of the phenomenon. Quite often this rationalisation is not performed when doing a scheme design and preparing tendering documentation. As a result, consulting engineers often accept a very broad responsibility that the structure will have satisfactory vibration performance without actually knowing what the dynamic excitation is going to be, or what the vibration performance criteria are that the structure needs to satisfy. This approach may lead to nasty surprises. Also, a vibration serviceability check is often left until very late in the project when the scheme is well on its way. If the problem is noticed then, it is very hard to deal with it. The message that vibration serviceability is rapidly becoming a governing design criterion, which can make or break a scheme, is seemingly not getting through.
This is not helped by the quite limited usefulness of design guidelines, not only in the UK, but also throughout the world. It is interesting to note here that the UK was always at the forefront of vibration serviceability research worldwide. At the moment, key design guidelines dealing with vibration serviceability in the UK are in a state of flux. The 30-year old footbridge guidelines in BS54002 are in need of urgent updating, the building vibration guidelines in BS64723 have been being updated for some time now, the stadia guidelines are being written as I write these lines, and floor design guidelines are likely to be in need of attention too, as demonstrated by the heated exchange of correspondence in the Structural Engineer, published earlier this year.4
Bearing in mind this state of affairs with vibration serviceability guidelines, to achieve the required understanding, which also means awareness of methods and procedures published in the past but proven to be inadequate (such as the infamous heel-drop test in the case of long-span floors), it is necessary to have a good command of the information published in more-or-less obscure technical literature. When this in-depth understanding is lacking, it is my experience that problems with vibration serviceability typically occur in practice due to poor design specifications (when it is not clear what the design should satisfy), lack of awareness and planning (in particular during the tendering process), over-simplification and inadequate modelling in the design stage, or change of usage of already existing structures which performed well in the past under different conditions. Here ‘problems’ do not only mean excessive vibrations after the structure has been built or retrofitted, but also a production of over-conservative and expensive designs which are clearly not competitive and/or do not provide the client with the best value for money.
