LATIN AMERICAN SEMINAR ON CONCEPTUAL DESIGN AND APPLICATIONS OF PRECAST CONCRETE STRUCTURES DEVELOPMENT OF STRUCURAL PREFABRICATION IN EUROPE

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1 LATIN AMERICAN SEMINAR ON CONCEPTUAL DESIGN AND APPLICATIONS OF PRECAST CONCRETE STRUCTURES THE fib COMMISSION PREFABRICATION DEVELOPMENT OF STRUCURAL PREFABRICATION IN EUROPE by Marco Menegotto Chairman, fib Commission on Prefabrication Keywords Prefabrication, Structural concrete, Structural design, Seismic construction, Applications, Research, Recommendations. Summary The development of the industrial prefabrication of structural concrete, based on the experience in Italy and Europe in the second halve of 20 th century, with reference also to seismic constructions, is outlined in this paper, that will be thoroughly illustrated in the oral presentation at the Seminar. The activity of Commission 6 Prefabrication of the International Federation for Structural Concrete fib (and formerly within FIP) which grew in parallel and accompanied such development, is presented, too. 1. Historical Overlook Both precast and monolithic structures have existed for long time in history. In Roman times, for instance, concrete was widely used for building monolithic structures, while precast structures were made in fact of large pieces of stone. Some have survived till now, proving their soundness, in spite of attacks of various types, including earthquakes. In modern times, after concrete produced with Portland cements was developed together with steel reinforcement and with prestressing means, it naturally suited the so called monolithic construction, with continuous beams, frames, slabs, walls. Indeed, also the pre-fabrication of concrete elements, to be subsequently assembled into a structure, revealed effective and took its place in construction industry. Although some outstanding applications of the 1930s exist in Italy, e.g. those of P.L. Nervi, extended prefabrication of structural concrete in industrial plants took place only in the second halve of the 20 th century. Up to that time, due to availability of cheap manpower, construction in general was not much industrialized. Steel structures were never common in the Country, due to the cost of that imported material. Thus, mainly masonry and in-situ concrete structures were being built. In the years 1950s, two main factors, one related to technical and one to economic development, modified the terms of convenience: the introduction of prestressing and the growth of manpower costs. Moreover, Italy was changing from an agricultural into an industrial country and a demand of halls for new buildings for industrial plants of all kinds really exploded. This type of market demand, much sensitive to costs and delivery times, turned rapidly toward precast solutions. Another innovation that favored prefabrication were lightweight aggregates, better dealt with in plants, than on construction sites. Precast concrete industrial halls became a kind of typical item in the Italian built landscape while a variety of solutions developed, often personalized, well fitted in the environment and with aesthetic appeal. Numerous precasters rose, who, having reached the manufacturing experience with those items, laid out their production to other fields, like commercial buildings (storehouses, markets, malls), complex

2 industrial buildings (multi-storey), social buildings (schools, hospitals, gymnasia), parking garages, office and hotel buildings and, finally, dwellings. The above evolution, based on skeleton structures, had implied the transition from simple, modular schemes to more flexible ones and the adaptation to particular needs, up to tailor-made solutions. In parallel, also the loadbearing precast wall-panels appeared, this time following foreign systems, for dwelling and social buildings. They had a certain development within large projects but, due to their functional rigidity, they are no longer widely used. However, successful tentatives of making These systems have been very popular in Eastern Europe. Beside the so-called fully prefabricated structures, there have been always so-called mixed construction or partially prefabricated, too, i.e., with only some parts made with precast concrete elements, combined with cast-in-situ concrete or steel or masonry. Floor slabs play an important part in these kinds of structures. Practically the most of floors, within any kind of structure, are made of precast concrete (planks, beam-and-blocks, hollow-core, double tees, etc.). In some cases, the whole deck (slabs and beams) is precast, while laying on cast-in-situ columns. This is because horizontal elements, if poured on site, require expensive propping for a certain time and, if they are prestressed, also skilled workmanship is needed, which is available better in plants than on construction sites. In a different field, the development of highway network and the improvement of the railway network gave lieu to a large market of precast elements for bridges. This oversight of the Italian example as a country story may be taken as a typical profile for other European countries. However, there have been some differences, due to the various market and social situations, namely the actual competition of precast concrete with structural steel in economies more advanced at that time and ore rich. In fact, the European Community of Coal and Steel (CECA), the anticipator of the Common Market, was created in the 1950s, too. At present, several advantages play in favor of structural prefabrication. First of all, those linked to structural concrete per sé, vs. other materials, like easy manufacture, lower cost, good fire resistance, no need of maintenance and, in general, good durability. Concrete as a material has had recently a considerable evolution, allowing for more efficient structures, and its higher performances may be better obtained in dedicated plants rather than on construction sites. In addition, compared with cast-in-situ, construction using precast elements is more environmentfriendly, namely within inhabited areas, in terms of dust, noise, occupancy time, etc. When it is the case, the recycling of materials or of entire units is easier, too. Surface finishing obtains much better results in prefabrication, whose products are installed only after acceptance checks. All this explains the market share kept by prefabrication in Italy, in spite of some prejudices spread among traditional technicians against non monolithic structures. 2. Schemes of Precast Concrete Structures The selection of the structural scheme may follow two main criteria. One tends to emulate the (questionable) monolithic behavior of cast-in-situ concrete. Connections are normally wet cast, with through re-bars rendered continuous by welding, threading, overlapping, or coupling. The other one, more proper to fabrication in individual units, accepts flexural discontinuities due to dry joints, possibly integrated with devices. Loadbearing panels Typical example of the second criterion are the loadbearing panels systems. They are composed by concrete plates, laying in three mutually perpendicular planes, which stabilize each other only being connected by linear hinges. Some systems (so-called 3D) prefabricate assembled sets of plates, vertical and horizontal, thus avoiding even temporary stabilizers during erection. The overall resistance, as well as the prevention of progressive collapse, are provided by steel ties, that can be incorporated within the panels ore laid aside them.

3 Skeletons - Industrial halls Schemes of typical industrial halls belong to the second criterion, too, being represented by simply supported slabs on Δ beams, in their turn simply supported on columns cantilevering from the foundation. A large variety of roofing elements, which may replace beam and slabs, are available, normally 2.5 m wide, up to more than 20 m long, with manifold thin cross-sections, from simple T, TT, Y, to elaborated curved shapes (wingers), laid in alternation with lightings. The structural scheme is statically determined (so called inverted pendulum). - Buildings Design of buildings with skeleton structures often follows the first criterion (emulative of monolithic), mainly because it must adapt to an initial design thought (yet already approved) for a castin-situ solution. Not of common use are frames cut for connections at a distance from the beam-column joints (cross shaped precast units), where action effects are lower and their location is theoretically most rational and effective in seismic structures, for observing the capacity design criterion. This occurs for practical production and transportation reasons, unless great amounts of pieces are to be made, for special applications. When buildings are conceived for prefabrication since their lay out, the structure follows mostly the dual scheme, where gravity loads are assigned to columns and horizontal actions to shear walls. This scheme is particularly suited for seismic constructions, where it allows for long spans, without having heavily stressed beam-column joints and consequent large cross sections in the frames. Instead, the joints can be, at limit, hinged. - Other structures Commercial buildings, like shopping malls, are now quite similar, with respect to structural layout and possible use of precast components, to modern multi-span halls for light industry plants, which have in fact similar requirements of free horizontal space, height and lighting. Multi-storey car parks are among the most common buildings adopting fully prefabricated structures. Up to two storeys, they can rely on a simply cantilevered columns scheme. Above, they move to the dual schemes, where the bracing structures are normally provided by staircase and elevator shafts. Another very important field of application are highway and railway bridges, whose great majority have the deck made of precast concrete beams. However, the latter do not play any role in seismic performance, which affects the piers, mostly cast-in-situ in practice. Particular structures, such as stadiums, churches, domes, reservoirs, etc., may contain, to greater or lesser extent, very special prefabricated parts. These are often produced on the construction site itself by the contractor and are not very relevant for the prefabrication industry, although the may be of outstanding interest, from the technical point of view. All above is exemplified and illustrated in the oral presentation. 3. Activities of fib Commission 6 Prefabrication Industrial prefabrication developed internationally in the same years. In 1955, the FIP Commission on Prefabrication was founded. In 1998, with the merger of CEB (Euro-International Committee for Concrete) and FIP (International Federation of Prestressing) into fib (International Federation for Structural Concrete), it became the present fib Commission. One can say that the Commission 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

4 occasionally to the meetings. Membership is balanced, coming from industry, academia and profession. 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. The work in the Commission is related to the SoA within the profession, which, in its turn, is based on the current marked demands. These demands and at the most important factors they are based on may be summarized by the following items: Structural efficiency Flexibility in use Best use of materials Speed of construction Quality consciousness Adaptability Preservation of the environment Research related to structural prefabrication is being run on various themes. Having often innovative features, precast elements themselves require specific quality assurance, experimental and analytical research, reliability considerations, in order to assess their performance and to justify their safety. 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, self-compacting, fiber-reinforced, lightweight aggregate, recycled of aggregates, as well as of non-metallic 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; last but not least, with aesthetics. 4. Commission s Publications A great number of publications have been 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: they are 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

5 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, 1988 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 SoA: Precast Concrete Railway Track Systems, fib Bulletin 37, September 2006 fib GGP: Structural Connections for Precast Concrete Buildings, fib Bulletin 43, Feb 2008 fib SoA: Prefabrication for Affordable Housing, fib Bulletin 60, Aug 2011 fib GGP: Design of Precast Structures against Accidental Actions, fib Bulletin 63, (in print) Several more are being prepared at present. All these documents witness the continuous and productive work of the Commission. 5. Commission s Tasks and Programs The number of Task Groups (TGs) pertaining to Commission 6 is varying, according to needs and accomplishments, and rose in the past up to sixteen. They are the following: TG 6.1 Prestressed hollow-core floors TG 6.2 Connections TG 6.3 P/C in Mixed Construction (dis) TG 6.4 Precast bridges (dis) TG 6.5 P/C Railway Track Systems (dis) TG 6.6 New Model Code - Precast Concrete TG 6.7 Affordable housing TG 6.8 Treatment of imperfections in precast concrete members TG 6.9 Precast concrete building structures for accidental loading TG 6.10 Design provisions for moderate seismic areas TG 6.11 Sandwich Panels TG 6.12 New Design Handbook TG 6.13 Quality Control TG 6.14 Wind Towers Cooperations with other fib Commissions, on intersecting topics, are also active.