A Presentation of fib Commission 6 Prefabrication

Transcription

1 1 Encontro Nacional de Pesquisa-Projeto-Produção em Concreto Pré-Moldado USP EESC, São Carlos, SP 3-4 Novembro 2005 A Presentation of fib Commission 6 Prefabrication Marco Menegotto Chairman of fib Commission Prefabrication Professor of Structural Engineering Università La Sapienza, Roma, Italy marco.menegotto@uniroma1.it GENERAL The FIP Commission on Prefabrication was founded at the FIP Congress held in Amsterdam in It became the fib Commission 6 Prefabrication in 1998, after the merger of CEB (Euro- International Committee for Concrete) and FIP (International Federation of Prestressing) into fib (International Federation for Structural Concrete). While it celebrates its jubilee, one can say that the Commission, led by the past chairmen D.K. New, Karl Gustav Bernander, Arnold Van Acker and Gunnar Rise, has accompanied the birth and the growth of prefabrication of structural concrete in the second halve of 20 th century and has contributed quite substantially to its development. It must now keep coping with the problems rising continuously with the evolution of market demands and construction techniques. The Commission has enjoyed the participation of members from about 30 countries, of five continents. Today, 35 active members work in it and many observers and invited guests participate occasionally to the meetings. Membership is balanced, coming from industry, academia and profession. An active participation of Latin-American Countries would be more than welcome! Meetings regularly take place twice a year, always in different countries, often in connection with scientific events. The progress of the works and novelties in precast concrete are discussed. Each time, the hosting country organizes a technical visit to production plants or working sites or realizations, which gives opportunity of very interesting exchanges of views and shares of experiences among the participants. Interaction with other fib Commissions and with international associations and code-making committees is sought after and active, too. AREAS OF INTEREST Precast concrete construction is evolving continuously, to keep the pace with current society s habits and needs. The mission of the Commission is to enhance the progress, by stimulating guiding and coordinating research and developments on precast concrete internationally and to disseminate the knowledge, through seminars and short courses, educational material, state-of-the-art reports, guides to good practice, recommendations and prenormative studies, contributions to redaction, follow up and maintenance of new standards. Its scope covers not only issues directly related to precast concrete - i.e., elements, connections, systems, production, handling and assemblage - but also indirect matters such as material technology, structural analysis, building physics, equipment, etc. 1

2 The work in the Commission is related to the S.o.A. within the profession, which, in its turn, is based on the current marked demands. It might be interesting to look at these demands and at the most important factors they are based on. Structural efficiency Buildings should offer maximum efficiency to the user. Structures occupying lesser volume inside the building are increasingly requested. The capability of easily hosting equipments for building physics, as well as of withstanding direct exposure to weathering or fire is evaluated. Competition between different construction materials and systems is more and more judged in terms of performance and cost. The same applies to structures other than buildings, e.g., standard bridges, flyovers, etc., where precast concrete elements may perform quite efficiently. Structural efficiency helps also to save weight, which results in subsequent savings, namely in the foundations, and, particularly for seismic constructions, in the whole resisting system. Flexibility in use Certain types of buildings are frequently required to adapt to variable user s needs. This is especially the case with offices, but housing might also need to be more adaptable with the changes in family life. Not to mention shopping malls, storage houses, industrial halls and car parkings. The most suitable solution to this end is to create large free internal spaces, without restrictions to possible subdivisions. Also, elements typology must be open to variations according to specific requests. Best use of materials Each construction material possesses specific properties and optimal applications. Until recently, the structure of a building was mostly built in the same material. Now, the trend is to use a combination of different materials suited for the particular function among the structural and the architectural components. Speed of construction Today, the demand for a speedy return on investment is of paramount importance. The decision to start the work is postponed until the last moment, but the initially agreed construction term has to be met. Construction sites spoil the surrounding space, make noise, disturb the traffic; thus, they must last as short as possible. In addition, projects are getting more complex, which does not favour short construction times. One of the solutions taken by general contractors is to assign more responsibilities to precasters. Quality consciousness Quality has a broad meaning. Not only the quality of materials and execution has to respond to higher standards than before, but also the qualities regarding user-friendliness, comfort, aesthetics, duration are felt more and more important. Adaptability In future, there will be much less demolition of entire buildings and more demands to adapt older buildings to new requirements. The main reasons will be the high cost of demolition due to regulations regarding noise, dust, traffic problems and many other inconveniences. On the other hand, after many years, a building becomes less attractive for renting, and the owner will look for renovation, for example by providing it an up-to-date façade. The design concept should facilitate such renovations, without demolition of the rest of the structure. Preservation of the environment Preserving the environment is becoming increasingly important on global scale. Several governments are already imposing regulations as regards the disposal and recycling of waste, ground pollution by chemicals, etc. The energy consumption referred to the whole process is assessed. In addition to encouraging the reuse of structures as a further reason for the just mentioned adaptability, it is expected that increasingly severe restraints will be imposed on construction, e.g., with respect to emissivity of materials, shortage of raw materials, waste dumping, noise and dust, etc. 2

3 RESEARCH Research related with structural prefabrication is being done on various themes. Having often innovative features, precast elements themselves often require specific quality assurance, experimental and analytical research, reliability considerations, in order to assess their performance and to justify their safety. To be mentioned in particular is the Holcotors project under way, granted by the EC, on torsion in hollow-core floors. Then, the connections, comprising the adjacent zones of elements, are submitted to tests and modelled analytically, before they come to use in actual structures. Apart elements and connections, which are related to specific design solutions, research is active in fields of general interest, like structural behaviour, materials, fatigue, durability, production. As for materials, it deals with applications of concretes, such as high performance, selfcompacting, fiber-reinforced, lightweight aggregate, recycled of aggregates, as well as of nonmetallic reinforcements. Production is concerned with optimization of processes but also with respect of the environment, i.e., reduction of noise, of energy consumption and of waste. As far as structural behavior altogether is regarded, that of assemblies using only dry joints is considered. Special attention is paid to floors, actually to shear and torsional performance of one-way prestressed slabs, to floors supported on slender beams, to distribution of effects concentrated loads, to diaphragm behavior. Also to mention are researches on fatigue effects, on behaviour under fire conditions and on ductility of frames and cantilevering columns systems, related to seismic conditions. Some of the research projects imply costly experimental testing on full scale samples. Investigations on damaged structures and research on repair and retrofit have lead to valuable results, too. PUBLICATIONS A great deal of publications were authored by the Commission on Prefabrication in the last decades, many of whom have been translated in several languages and are still of actual interest: Guides to Good Practice (GGP), Technical Reports (TR), State of the Art Reports (SoA), Recommendations (Rec), Handbooks (HB), as listed below. GGP: Recommendations for Segmental Construction in Prestressed Concrete, FIP/9/1, February 1978 TR: Proposal for a Standard for Acceptance and Verification of Epoxy Bonding Agents for Segmental Construction, FIP/9/2, March 1978 TR: Bridge Decks with Pretensioned Precast Beams, FIP/9/3, August 1978 TR: Shear at the Interface of Precast and In-situ Concrete, FIP/9/4, August 1978 TR: Losses of Prestress in Tendons due to Steam Curing of Concrete, FIP/5/5, September 1978 GGP: Shear at the Interface of Precast and In-situ Concrete, FIP/9/6, January 1982 GGP: Design, Manufacture and Erection of Architectural Concrete Elements, FIP/9/5, February 1982 GGP: Acceleration of Concrete Hardening by Thermal Curing, FIP/9/7, March 1982 TR: Design Philosophy for Precast Buildings of Two or More Storeys, FIP/9/8, June 1982 TR: Ductility of Tie Connections for Concrete Components in Precast Structures, FIP/9/9, October 1982 TR: Design Principles for Hollow-Core Slabs regarding Shear, Transverse Load-bearing Capacity, Splitting and Quality Control, FIP/9/10, October 1982 FIP SoA: Prefabricated Thin-Walled Concrete Units, Th. Telford, London, 1984 FIP Rec: Design of Multi-Storey Precast Concrete Structures, Th. Telford, London, 1986 FIP TR: Precast Concrete Piles, Th. Telford, London, 1986 FIP SoA: Concrete Railway Sleepers, Th. Telford, London, 1987 FIP Rec: Precast Prestressed Hollow-Core Floors, Th. Telford, London,

4 FIP HB: Planning and Design of Precast Building Structures, SETO Ltd, London, 1994 FIP Rec: Design of Thin-Walled Units, fib, May 1998 FIP GGP: Composite Floor Structures, fib, May 1998 fib GGP: Special Considerations for Precast Prestressed Hollow-Core Floors, fib Bulletin 6, January 2000 fib SoA: Precast Concrete in Mixed Construction, fib Bulletin 19, June 2002 fib SoA: Environmental Issues in Prefabrication, fib Bulletin 21, January 2003 fib SoA: Seismic Design of Precast Building Structures, fib Bulletin 27, October 2003 (by C7, with contribution of C6) fib SoA: Precast Concrete Bridges, fib Bulletin 29, November 2004 fib GGP: Structural Connections for Precast Concrete Buildings, fib Bulletin under publication. These documents witness the continuous and productive work of the Commission in its life. TASKS AND PROGRAMS The number of Task Groups (TG) pertaining to Commission 6 is varying, according to needs and accomplishments, and rose in the past up to sixteen. At present, they are the following. Cooperations with other Commissions in dealing with intersecting topics are also active. TG 6.1 Prestressed hollow-core floors Hollow-core slabs have been the object of the greater consideration, since they have represented a revolutionary innovation in structural concrete, being cast without forms and reinforced only by one-way pre-tensioned strands. After a TR in 1982, the Group has edited in 1988 the FIP Recommendations for the design of prestressed hollow core floors. They were intended to complement the CEB-FIP Model Code 1990 and have been used as a basis for several national and international standards, e.g., part of the Eurocode 2 (EN ) and the CEN Product Standard EN They offered models for the specific mechanisms of transfer of prestress in the end zones, shear capacity, transverse load distribution, fire safety and connections. Since then, the knowledge on performances of prestressed hollow-core floors in various applications has progressed noticeably. A number of new topics have already been dealt with in a complementary fib GGP (see above) and are being studied yet, e.g., restrained ends, shear and torsion interaction; design rules for large openings; fire resistance; diaphragm action in seismic conditions, floors supported by flexible beams, dynamic loading. A paper on shear resistance of prestressed hollow-core floors exposed to fire has been published on fib journal in 2003, too. The TG is now preparing a revised Report to the state-of-art today, for updating the design Recommendations. TG 6.2 Connections Structural connections are among the most essential parts in prefabrication. Effective connections are the key to successful systems. They must meet a variety of design and performance requirements, related to manufacture, assemblage, serviceability and resistance. A thorough understanding of the basic mechanisms and the role of the connections within the overall structural system form the basis of a conceptual design. Designers should understand how the connections are part of the system and be well aware of the flow of forces, as well as of the deformations, throughout the whole structure and the connections. In order to explain and to encourage the correct design of structural connections in precast concrete structures, the TG has prepared a Guide to Good Practice, containing the general philosophy and several examples, which is going to be published as a fib Bulletin. 4

5 TG 6.3 Precast Concrete in mixed construction Mixed construction is being used at present in many structures. An illustration of how precast concrete elements may be mixed in combination with other structural materials, such as cast-in-situ concrete, steel, timber, masonry, glass, in order to optimize the overall building performance, has been the purpose of the TG. Having accomplished its task, by publishing the SoA Report in fib Bulletin 19, the TG is now disbanded. TG 6.4 Precast bridges Bridges and viaducts are an important segment within the construction activity. Most publications regarding realisations and guidance on design and execution are generally focusing on outstanding bridge projects with extremely long spans and challenging execution conditions. However, the bulk of the bridge market concerns standard bridges, over motorways, railways and watercourses, characterised by short spans, execution under limited traffic restrictions and, above all, cost effectiveness. This represents an eligible domain for prefabrication, with its high strength concrete, speed of construction, absence of scaffolding, industrialised production process, etc. The knowledge of modern precast bridges is often limited among authorities and designers and a lot of prejudices exist, with regard to appearance, technical possibilities and innovations. Just published is the SoA on fib Bulletin 29, with detailed information. The scope is bound to industrially precast bridges, with components manufactured in permanent precasting plants. It is largely based on European experience, since most members of the TG are from Europe and the collection of information from other continents was incomplete. The TG intends to complement the Report with guidelines on the design and execution. TG 6.5 Precast concrete railway track systems The Commission edited in 1987 a SoA on railway sleepers. Since then, many changes and developments have taken place in railways, which increased speeds and axle loads The precast concrete industry has responded with new designs, more efficient production methods and the adoption of highly effective quality assurance schemes. Precast concrete crossing bearers for switch and crossing are now being used currently. The slab track incorporating precast concrete is already used in special cases. New codes of practice, covering all aspects of precast railway track products, from design and manufacture to installation and maintenance, have been developed. It is considered therefore that an up-to-date SoA Report for precast concrete track products is timely and would show to the specifier and the end user that precast concrete track systems provide durable, long lasting and economical solutions. The TG is finalising its preparation. TG 6.6 Model Code Precast Concrete Following the tradition of CEB-FIP, who edited three editions of Model Code for Concrete Structures, advanced documents deemed to inspire actual operational codes, which were in fact the basis of the Structural Eurocodes, fib is now preparing a New Model Code, based on the life-time design of structures. The NMC should appear in the occasion of the 2 nd fib Congress, to be held in June The TG interfaces the works of the fib Special Activity Group for the NMC and contributes actively by setting proposals, discussing provisions and checking drafts on the issues concerning directly and indirectly total or partial prefabrication. TG 6.7 Affordable housing The TG has been recently appointed. Its aim is to prepare a State of the Art Report on prefabricated housing, taking into account particularly the cost and the local means of construction. Housing is intended to be cost effective, fast and easy to build, while maintaining all characteristics of good housing. The main performances taken into account are load-bearing capacity, earthquake and adverse weather resistance, as well as shelter performances like water-tightness and thermal insulation. The TG has started by organising two Workshops in India in 2003 and 2004, to share the experiences among local and fib experts and to find possible new research topics. 5

6 TG 6.8 Treatment of imperfections in precast concrete members Imperfections and damages are not avoidable and give lieu to uncertainty on their impact on structural performance and appearance. Informal documents exist, but a recognized basis for taking decisions would be very helpful. The target of the TG is to prepare a Report, dealing with precast units showing imperfections or other deviations, with respect to the design and specifications. It would relate the deviations to the performance requirements, in order to enable an evaluation of the effects and the necessary actions to be taken. The precast members considered are hollow-core slabs, beams, walls and columns, made of normal weight concrete; water retaining structures are out of this scope. The document will serve as a guide for the QC systems, in identifying and dealing with quality deviations, from the manufacturing stage to the installation on site. Activity in other TGs A TG on Environmental Matters in Prefabrication has been transferred to the Commission 3 Environmental aspects of design and construction. It had edited the fib SoA Environmental issues in Prefabrication in Bulletin 21. Contributions by Commission 6 were also given to TG 7.3, in preparing the fib SoA Seismic design of precast building structures in Bulletin 27. The formation of new TGs is planned for the meeting which is going to take place in connection with the next fib Congress, to be held in Naples in June CONCLUDING REMARKS The fib Commission on Prefabrication occupies a privileged position, thanks also to the strong support from industry. This position came not along as a matter of course, but because of substantial contributions by the Commission to the sake of precast concrete: technical promotion of precast concrete in general, scientific research, dissemination of information on novelties, elaboration of sound design methods, implementation and endorsement of knowledge in standardisation and coding work. Industry is participating with its best experts to the meetings. The profession, as well as the universities and technical institutions, are well represented in the Commission, which is an open forum with exchange of information, discussions on interesting items and technical visits to upto-date projects and plants; all this giving also opportunities to take part in common research projects. The climate is very friendly and the sound turnover of generations is continuous. Such motivated interaction among producers, designers and researchers constitutes the basis of the success of the group, which constantly gives the members more than what they are bringing in individually. The oral presentation is showing images of the Commission 6 activities, in parallel with the development of industrial prefabrication in Europe. 6