监控建筑结构.pdf

Monitoring Building Structures Monitoring Building Structures Edited by J.F.A.MOORE Head of Structural Engineering Branch Building Regulations Division Department of the Environment London and erly Head of Structural Integrity Division Building Research Establishment Watford Blackie Glasgow and London Van Nostrand Reinhold New York Blackie and Son Ltd Bishopbriggs, Glasgow G64 2NZ and 7 Leicester Place, London WC2H 7BP Published in the United States of America by Van Nostrand Reinhold 115 Fifth Avenue New York, New York 10003 Distributed in Canada by Nelson Canada 1120 Birchmount Road Scarborough, Ontario M1K 5G4, Canada 1992 Blackie and Son Ltd First published 1992 This edition published in the Taylor 1986; 1987 and in long-span school and leisure halls Bate, 1974; 1984; DoE, 1974 amongst others. In the more temperate climates, failure of the infrastructure usually causes relatively short-term, if expensive, problems. For example, the storms in 1987 and January, 1990, in the UK Figure 1.1 caused disruption of road, rail, air and sea transport, the loss of electricity supply to around a million homes and overloading of the telephone system. For a short time following the latter storm, all the London railway termini were closed. These types of failure are most commonly associated with what may be called severe natural environmental conditions. It is unusual to experience widespread structural failure in these situations although, of course, there are often isolated cases which do occur. In many areas of the world, such as southern Europe, parts of Australasia and the Americas, the environment can be much harsher with the potential for earthquakes, hurricanes or typhoons to cause substantial destruction with consequent high costs, long repair or replacement times, and great costs in terms of human distress Figure 1.2. The design of structural systems to resist the forces generated in each of these extreme load conditions differs between the two cases. However, in both MONITORING BUILDING STRUCTURES2 instances the designer is expected to create a built environment which achieves a level of perance acceptable to the public. That is, acceptable in terms of the perceived severity of the hazard. In the event of a major earthquake or typhoon, when the failure of domestic and commercial buildings would not be surprising, then essential construction such as hospitals, power stations and emergency services accommodation would be expected to survive Figure 1.3. 1.2 Structural design philosophy The design options available to the structural engineer to meet his clients’ and the public’s requirements for buildings which per satisfactorily within various hazard scenarios may be categorised Armer, 1988 quite simply thus a Provide a strong basic structure. b Provide redundancy within the basic structure. c Provide sacrificial defence for the structure. Figure 1.1 Wind damage to an industrial steel-framed building. INTRODUCTION3 Figure 1.2 Storm damage in Australia. Figure 1.3 Structural damage due to an earthquake. MONITORING BUILDING STRUCTURES4 d Provide monitoring systems which warn of inadequate structural conditions. The first three of these are relatively familiar ground for the designer. The principle of making load-bearing structures strong enough to withstand all the loads which are likely to impinge during their lifetime is central to the universally applied structural design philosophy which has developed since the eighteenth century. It provides a straightforward basis on which to establish the level of structural safety by admitting the use of factors attached to either the design loads or to the response, i.e. the strength descriptor. By so doing, the designer is given a direct indication of the margins of safety against failure under the loads he has explicitly identified. The success of this approach depends heavily upon the engineer’s skills in predicting both the possible ranges and the combinations of loads which the structure will have to carry and the material and structural perance of the building itself. The principle of explicitly incorporating redundancy into a structure has more recent antecedents. In the early days of aircraft design, there was a major problem with the provision of sufficient power to get the machines off the ground. The two obvious strategies to surmount this difficulty were to build more powerful engines and/or to reduce the weight of the aeroplanes. The latter approach led to some very elegant mathematical solutions for the problem of ‘minimum weight design’, which in turn led to brittle behaviour of the resultant airframe structures To overcome this particular difficulty, the concept of ‘fail-safe’ design was developed, and the idea of beneficial redundancy was introduced into the designer’s vocabulary. By the intelligent use of redundant elements, it is possible to ensure that the failure of a single element does not precipitate the failure of the complete structure. As yet, there are no satisfactory theories to guide the designer in this field, but experience and empiricism must suffice. The provision of sacrificial defence for a building structure is even less scientifically founded. The use of bollards and crash barriers to prevent vehicle impacts is commonplace and their efficacy proven in practice. The use of venting elements such as windows and weak structural elements to reduce the level of explosive pressure on major structural elements is less widely adopted. Weighted trap-doors are sometimes used in factory buildings where severe cloud explosions are a high risk, but are arguably not sacrificial. Mainstone 1971 has given considerable data for the design of explosion venting windows, but great care must obviously be taken when using this design strategy. The provision of in-service structural monitoring systems in building construction has so far been limited to a few isolated cases such as the commercial fair complex in the USA IABSE, 1987. Some systems have found application in civil engineering construction, for example in special bridges and nuclear power stations. It is probably reasonable to assume that the use of monitoring in these circumstances reflects a ‘belt and braces’ INTRODUCTION5 approach to safety rather than a planned use of the technique in an integrated design philosophy. 1.3 The role of monitoring A dictionary definition for ‘to monitor’ is ‘to watch or listen to something carefully over a certain period of time for a special purpose’. The end of this definition is perhaps the most important part since it identifies the need to establish purpose as an essential element of the activity of monitoring. There have been a number of examples where considerable quantities of data have been collected, particularly in the field of research, in the vain hope that some brilliant thought will arise as to what should be done with this ination. Many who have been associated with structural testing will be familiar with the thesis that putting on a few more gauges would be ‘useful’ since more ination must be helpful. Unfortunately, insight rarely arrives in such circumstances and, if it does, the inevitable conclusion is that something else should have been measured So before any consideration is given to s of observing the perance of construction, the question must be asked ‘why is this expensive process to be established’ The answer to this question necessarily involves clear understanding of the positive actions which could result from the data-gathering rcise. The role of construction monitoring has to be established against a background of requirements emanating from the public i.e. the state, the owner and the user. These requirements have to be expressed in a which is compatible with the data generated by any monitoring systems which are established. Putting this point more explicitly, whilst it is relatively easy to gather enormous amounts of data from most monitoring systems, it is usually very difficult to interpret such data and to develop useful consequential responses. Public concern with the perance of building structures is principally directed at those aspects which impinge on the individual, that is upon his own safety and welfare. The acceptable degree of safety is encapsulated in the various building regulations, standards and codes of practice. Unfortunately, there is not a simple correlation between the safety of people in and around construction and the security/safety of that construction, as shown by Armer 1988. Neither is it possible to express a level of safety for a construction as a number. In spite of the large number of academic papers which purport to discuss safety levels in explicit terms and to offer the designer the opportunity to choose his own value, the concept is entirely subjective and acceptability will vary with construction type and location, current political situation and so on. Any post-construction monitoring of building works carried out by government or state authorities acting as the public cutive rather than the owner is essentially limited to the collection of statistical data on failures. MONITORING BUILDING STRUCTURES6 This of monitoring matches the nature of the response which can be implemented practically by a national regulating body. For example, certain types of construction can be outlawed or special requirements can be included in regulations, such as the provision for buildings of over four storeys to be designed to resist disproportionate collapse following accidental damage. The effect of this of monitoring and response is the control of the perance of the population of constructions and not that of an individual building. This point is sometimes misunderstood in discussions on the function of codes of practice for good design. By giving rules for the design of construction, i.e. codes of practice, the regulating body can ensure that a particular building is part of the population of buildings modelled by a particular code, but it does not, however, guarantee further its quality. Since within any population of artefacts the perance of individual elements will vary from good through average to bad, such will be the lot of construction. Thus the objective of monitoring construction by public authorities is twofold firstly, to ensure that acceptable levels of safety are sustained for the people and, secondly, to ensure that a degree of consumer protection is provided to meet the same end. The owner of a building, if not also the user, will require to protect his investment both as capital and as a generator of funds. This can be achieved by regular monitoring and by consequential appropriate maintenance. There are, of course, many quirks in the financial world which may make this simplistic description somewhat inaccurate for particular owners and at particular times but nevertheless it represents a realistic generality. The user will be concerned that the building he rents or leases provides a safe working environment for both his staff and his business. It is therefore in the user’s interest to monitor the building he occupies to ensure that his business is not damaged by the loss of facility. It is therefore of concern at many levels to ensure the proper functioning of the building stock, and a necessary part of this process is to monitor current condition. The discussion so far is predicated on the thesis that monitoring construction should be a legitimate weapon in the armoury of those concerned with the in-service life of buildings. It must be admitted, however, that its use in the roles described is only in its infancy. Most practical experience in the instrumentation of building has been gained by researchers. Their objectives have usually been to validate, or sometimes to calibrate, theoretical models of structural behaviour. By so doing they hope to provide another design aid for the engineer. In the technical literature, there are many reported examples of this use of the technique. Ellis and Littler 1988 on the dynamics of tall buildings, Sanada et al. 1982 on chimneys and Jolly and Moy 1987 on a factory building illustrate the variety of work undertaken so far. Since there is quite a limited number of structural perance indicators which can be monitored, and likewise a limited number of suitable which is INTRODUCTION7 really a euphemism for stable instruments, there is considerable scope for the exchange of techniques and s between the various applications for which some experience exists. References Armer, G.S.T. 1988 Structural safety Some problems of Achievement and Control. The Second Century of the Skyscraper. Van Nostrand Reinhold, New York. Bate, S.C.C. 1974 Report on the failure of roof beams at Sir John Cass’s Foundation and Red Coat Church of England Secondary School. BRE Current Paper CP58/ 74, Building Research Establishment, Watford. Bate, S.C.C. 1984 High alumina cement concrete in existing building superstructures. BRE Report. HMSO, London. BRE 1985 The structure of Ronan Point and other Taylor Woodrow-Anglian buildings. BRE Report BR 63. Building Research Establishment, Watford. BRE 1986 Large panel system dwellings Preliminary ination on ownership and condition. BRE Report BR 74, Building Research Establishment, Watford. BRE 1987 The structural adequacy and durability of large panel system buildings. BRE Report BR 107. Building Research Establishment, Watford. DoE 1974 Collapse of roof beams. Circular Letter BRA/1086/2 May 1974. Department of the Environment, London. Ellis, B.R. and Littler, J.D. 1988 Dynamic response of nine similar tower blocks, Journal of Wind Engineering and Industrial Aerodynamics, 28, 339–349. IABSE 1987 Monitoring of large scale structures and assessment of their safety. Proceedings of Conference, Bergamo, Italy. Jolly, C.K. and Moy, S.S.J. 1987 Instrumentation and Monitoring of a Factory Building. Structural Assessment. Butterworths, London. Mainstone, R.J. 1971 The breakage of glass windows by gas explosions. BRE Current Paper CP 26/71. Building Research Establishment, Watford. Sanada, S., Nakamura, E.A., Yoshida, M. et al. 1982 Full-scale measurement of excitation and wind force on a 200 m concrete chimney. Proceedings of the 7th Symposium on Wind Engineering, Tokyo. 2Surveying J.F.A.MOORE and B.J.R.PING 2.1 Introduction 2.1.1 Scope Surveying is used here as a term to cover a wide range of techniques which rely primarily on visual observation with instrumentation to obtain a measurement of length or determination of position from which movement may be deduced. The s generally require the presence of an observer on site at the time of measurement, as opposed to the techniques described in chapter 5, which can be operated remotely once installed. This does