RECENT DEVELOPMENTS IN THE CONCEPTUAL DESIGN · PDF fileRECENT DEVELOPMENTS IN THE CONCEPTUAL...

RECENT DEVELOPMENTS IN THE CONCEPTUAL DESIGN OF R.C. AND P.C. STRUCTURES

F. Mola*, Politecnico di Milano, Italy

E. Mola, ECSD, Italy L.M. Pellegrini, ECSD, Italy

36th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 14 - 16 August 2011, Singapore

Article Online Id: 100036058

The online version of this article can be found at:

http://cipremier.com/100036058

This article is brought to you with the support of

Singapore Concrete Institute

www.scinst.org.sg

All Rights reserved for CI‐Premier PTE LTD

You are not Allowed to re‐distribute or re‐sale the article in any format without written approval of

CI‐Premier PTE LTD

Visit Our Website for more information

www.cipremier.com

36th Conference on Our World in Concrete & Structures

Singapore, August 14-16, 2011

__________________________

†ECSD, Srl, Milano, Italy

RECENT DEVELOPMENTS IN THE CONCEPTUAL DESIGN OF R.C. AND P.C. STRUCTURES

F. Mola*, E. Mola† and L.M. Pellegrini†

*Professor, Department of Structural Engineering

Politecnico di Milano Piazza Leonardo da Vinci, 32 – 20133 Milano, Italy

Email: <[email protected]>; Website: www.polimi.it, www.ecsd.it

Keywords: Conceptual design, concrete technology, construction techniques, bridges, tall buildings

Abstract. Some basic aspects of modern civil engineering are discussed, with a focus on the conceptual approach to structural design. After presenting the peculiar features of conceptual design, the development of normative documents is discussed, particularly for reinforced concrete and prestressed concrete structures. The development of concrete technologies and construction techniques for this kind of structures are then discussed. Finally, some outstanding and representative buildings and infrastructures are presented. The fundamental aspects of the evolution of the construction practice are thus clarified and the role of the modern structural engineering is highlighted. 1. INTRODUCTION Nowadays, the design of reinforced and prestressed concrete structures challenges engineers with

complex problems, whose solution is not always straightforward, which calls for the engineers to critically re-consider their role and cultural roots, in order to carry out their tasks and assume their responsibilities in modern society. These days, in fact, the choices made by structural designers are not only confined to the theoretical and practical fields, as the decisions have now taken a larger meaning and stronger impact on the community and make a very strong statement in modern architecture and on the environment. This calls for the engineers to deeply question their profession, their ethical reasons, their cultural background in order that the basic principles of the theoretical and scientific body of knowledge derived from the history of engineering can be preserved as the foundation for future achievements.

Before presenting some of the most outstanding structures of civil engineering built in the last twenty years, it is necessary to show the general pre-requisites of the conceptual design, both considered as an innovative creative act and as a critical review of the past. The balance between innovation and preservation of the past is a fundamental feature of conceptual design, which, in turn, is a basic pre-requisite for a design approach able to fully comply with all the performance objectives required of a structure.

Some general considerations on these broad and very sensitive subjects are thus expressed in the following, with the goal of offering the Author’s personal perspective, based on many years of experience in structural engineering, both in the academic and professional fields.

2. REMARKS REGARDING THE THEORETICAL ANALYSIS AND DESIGN OF REINFORCED CONCRETE AND PRESTRESSED CONCRETE STRUCTURES

Critical review and innovative creativity, representing the analytical and the design aspects of structural engineering, can appear as irreconcilable features. On the contrary, they have to be pursued in a strictly balanced approach, because, together, they become the tools for the creation of new,

F. Mola, E. Mola and L.M. Pellegrini

outstanding objects. In modern days, the economic resources involved in the financing of important structural and

infrastructural projects have become more and more relevant. The optimization of the use of money and time is now one of the basic pre-requisite that designers must comply with. If this effort is not carried out in a very careful and thorough way, even important projects might never see the light of day, because of unsolved financing issues.

Also, it is mandatory that a common definition of all the different levels of performance of the object is preliminary set forth and agreed upon, so that the design can be carried out to meet all the relevant criteria. Accurate quality control procedures and acceptance thresholds must be defined, in order to comply to all the design specifications.

It is thus apparent that it is not possible to clearly mark a boundary between theoretical and practical aspects of the design approach to a new structure because they are strictly intertwined and deeply affecting one another.

In classical mechanics mathematical speculation was not directly aimed at a practical design goal: the only motivation for research was speculation itself, and the refinement of previous solutions of general problems. This refinement process created the engineering cultural background enabling the designers of today to find solutions to more and more complex problems.

The huge body of theoretical knowledge inherited by modern engineers allowed our ancestors to build such wonders of structural engineering as the Pyramids, the ancient Greek temples, the Coliseum, the medieval cathedrals, Fig.1.

Fig.1 – The Pyramids, the Parthenon, Coliseum, Gothic Cathedral Such structures all comply with the law of equilibrium in a perfectly beautiful way, mostly due to an

empathic and intuitive act by the designer, rather than to a fully rational and mathematical approach, because at the time when they were created, a more holistic, undifferentiated approach to the design was used.

Later on, a stronger and stronger separation between theory and practice, analysis and design, came to life, but nowadays, as mentioned above, this process needs to be reverted, because there is no possibility, both for the architect and the structural designer, to ignore practical constraints while pursuing their creative act.

It is impossible to design any structure without a deep knowledge of the theory of structural mechanics, governing the statics of structures and the behavior of materials. This knowledge and understanding need to be deeper and deeper the more structures become complex and performance levels become many-folded and more difficult to comply with.

At the same time, it is impossible to focus only on theoretical solutions and very refined analytical calculations, based on the huge computational capacity of modern day computers, without carefully considering and incorporating into the design also a series of practical, constructive, budget and time restraints and without fully exploiting all the state-of-the-art innovations in construction techniques and construction site management.

When a synergic interaction between analysis and design is thus pursued and carried out, the most exciting and important goal of the modern engineer is obtained, because it enables the engineer to be at the same time the creator of a new object and the critical reviewer of his own creative process.

3. THE ROLE OF THE DESIGNER AND THE DEVELOPMENT OF DESIGN CODES

The two-faced role of modern engineers, both creators and analysts, should be well clarified and

supported by the latest generation of design codes. Such documents must be drafted so that they become the tool for the engineer to boost his skills and to improve his performance, both as a conceptual designer and as a design reviewer.

The body of European Codes (Eurocodes) has developed and evolved through a long path in the last decades, culminating, at present, in a complete set of documents, each devoted to a specific


structural material, in addition to the one defining the actions on structures and the one devoted to the specific prescriptions for seismic design.

At the beginning of the Fifties the normative situation in Europe was quite dispersed and no clear specifications for a good structural design were provided to practitioners, so that the structures of that time, even the more complex and representative ones, found the guarantee of a correct and efficient behavior only in the deep culture of masters of design, such as Torroja, Nervi, Freyssinet, Dischinger and others. The books of Nervi and Torroja on conceptual design, /1/,/2/, the Freyssinet memoire on his professional life, /3/, and the Dischinger work on the evaluation of creep effects in prestressed beams, /4/, are classic masterpieces which will inspire forever the work of structural engineers.

The situation improved in 1953, when CEB (Comité Européen du Béton) was founded. The activity of the Committee found its first expression in the drafting of the ‘Recommendations Unifiées’ in 1964, later followed by an updated version in 1970. Many theoretical and experimental results were included in the documents, so that many models for material behaviour were later derived from them, but, most importantly, the definition of a unique methodology for the measure of safety of structures was given, based on the semi-probabilistic limit state approach. After that, in cooperation with FIP (Fédération International de la Précontrainte), the Model Code was first published in 1978, /5/. This document was the first of the series of prototype Codes, i.e. reference documents, whose later versions came out in 1990, /6/, and in 2010, /7/. The latter was published by the newly created fib (Fédération International du Béton), born in 2000 from the unification of CEB and FIP.

The development of the Eurocodes evolved in the same decades, with the drafting of the different Eurocodes (i.e. from EC0-Basis to EC9-Aluminium). Eurocode 2, devoted to reinforced concrete and prestressed concrete structures, found its final draft in the document called EC2-EN-1992-1-1 for buildings and EC2-EN-1992-2 for bridges,/8/.

Even if the present body of Eurocodes has finally found a satisfactory balance between being detailed and comprehensive and, at the same time, user-friendly, still, a degree of damping persists between rules and practice, so that an inevitable gap is still open between the engineering practice, with the development of new techniques and the use of new materials, and their inclusion into normative documents.

Filling this gap is once again a task, and not a very easy one in many cases, that must be directly tackled by the engineer and designer, often making decisions based on their own judgment and culture, rather than on prescriptions, in order to reach more effectively the design goals. The theoretical knowledge and the analytical approach are the tools the engineers need to master in order to fully understand the meaning and the hypotheses informing the codes, so that they can use such normative documents as an enhancement of their own design capacity, rather than only a restraint to their creative abilities.

It’s undeniable that genius inventions, such as the first application of the technique of prestressing in the Pont de Veurdre by Freyssinet, Fig.2, or the early removal of the formworks used by Hennebique in the Risorgimento Bridge in Rome, Fig.3, were conceived and successfully transformed into innovative objects in an era where specific codes did not exist, but at the same time, given the innovative character of those inventions, they were also highly hazardous, implying levels of safety that modern society could never accept at the present day.

Fig. 2 – Pont De Veurdre Fig. 3 – Risorgimento Bridge Creativity, critique, theoretical knowledge and awareness of progress in the experimental field on which

the recommendations of codes are based are all fundamental ingredients of a common background for modern engineers, establishing the boundaries and the guidelines for a safe and sound approach to modern structural design. In the last three decades, this common background gave way to the creation of beautiful and complex structures, embodying the excellence of contemporary architecture


and engineering, both from the point of view of pure design, and from those of construction techniques and of the improvement of the performance levels of concrete, not only as for strength, but also durability, workability, sustainability.

4. THE DEVELOPMENTS OF CONCRETE TECHNOLOGY

4.1 General aspects In the long history of concrete as a construction material, two distinct periods can be recognized, each of them characterized by a different approach of the engineers and architects to the design and construction of structures. The first period ranges from the birth of reinforced concrete structures to the Fifties: it can be defined as ‘heroic’ or ‘epic’, since during those decades the basic rules for the dimensioning of structures were established. In the mid-fifties, the birth of CEB, as mentioned above, boosted innovation in the evaluation of the safety of structures. In particular, with the new concept of limit state, the approach to the design of structures became more aware and less empiric, including the analysis of problems other than that of load bearing capacity. The notion that a much broader range of performance levels of structures must be investigated led the way for the introduction of specific methods and thresholds for the safety measures of a wider range of different limit states, among which durability took a very important role starting from the early Seventies. From that moment on, concrete technology has dramatically improved, so that today structures can be designed to prescribed levels of durability and load bearing capacity. It must be noted, though, that the behavior of reinforced concrete structures does not depend only on the durability of the material itself, but rather on the effective interaction between concrete and steel rebars. The development of concrete technology thus means the improvement of construction details and any measure taken in order to allow concrete and steel to work together in the most effective possible way. The different performances required of modern concrete are essentially related to: strength, workability, compactability, dimensional stability and resilience. These properties, defining in a broader sense high performance concrete, even though they are not independent on one another, anyway, for each of them, a purposely aimed mix design, with some peculiar features, is generally requested. For this reason the main character of concrete performance designed concrete will be separately discussed. 4.2 High strength concrete A marked increase in the strength of concrete is one of the first prerequisites that the development of concrete technologies could achieve. For this reason, for a while, strength was a privileged property of concrete, the one that both the designer and the technologists wanted to develop almost exclusively. Until 2009, the Italian Building Code, introduced in 1996, defined the highest concrete class as a characteristic cubic compressive strength of 55MPa, while the Italian Building Code approved in 2009, defines the highest concrete class as having a characteristic cubic strength of 105 MPa. This means that the allowed strength threshold has almost doubled in the last thirteen years. Today, high strength concrete can be relatively easily produced, and its performance can be guaranteed by strict quality control procedures, so that it can now be used as a construction material even for structures for which steel had always been the only possible material until a decade ago. In particular, for the vertical elements of tall buildings, at present, concrete is employed more and more extensively. This has reduced the global cost of such buildings, since the static efficiency ho=fc/pc, ratio between strength and unit weight of concrete, as illustrated in Fig.4, has increased by 4.7 times, while the cost of the material has increased about by 3 times.

Fig. 4 – Static efficiency of concrete The increase in strength also aggregate dimensions are reduced and the porosity limited. Nonetheless, specific mixes must be designed in order to fully reach the goals of high durability and high workability.

4.3 High workability concrete

The workability of concrete fundamental prerequisite in order to guarantee the development of the required properties once the concrete is cured and to allow the fullTechnological progress aimed at high workability found its inception in the early Eighties, when the so-called rheoplastic concretes were devisedfluidity and plasticity. Later on, these properties were further enhanced, leading to the design of concretes able to flow and selfThe industrial production of this kind of concrete started in the ‘self-compacting’ concrete (SCC). SCC is characterized by a very high workability and the ability to flow easily even through very congested rebar arrangements, Fig.5.Such concrete requires a careful mix design and a the formworks must be water proof, moreover, during castingconsiderable heights must be avoided, The quality control procedurestests with specified thresholds for acceptance. If the quality of fresh SCC is not strictly guaranteed, the properties of the resulting cured concrete cannot be Among the numerous researches carried out on these matters, in interesting features of SCC are shown: the first strength with respect to an ordinary concrete of the same class; the second onebonding between concrete and rebar in SCC with respect to ordinary concrete.These results show that structural durability can be enhanced if SCC is employed, even if they also suggest that a careful and thorough experimental and analytica 4.4 Reduced shrinkage concrete

One of the most undesirable featuresparticular architectural value, is the development of remarkably visible cracking patterns due to restrained shrinkage. Imposed deformation are mostly due to thermal effects, both endogenous ones, and those caused by the shrinkage of concrete. Since orstructures are strongly statically undetermined,the volume of the material during curingcauses cracking. Its negative impact on the aesthetics of the structure is the most evident shortcoming of cracking, but its most dangerous effect is the dramatic reduction of the durability of the structure, which becomes more exposed to the aggression of the environment.The goal of reducing or, better, eliminating shrinkagefrom the seventies, thus leading to the development of special concrete mixtures that increase their volume while curing. Since expansion is an imposed deformation opposed in siinduces compressive stresses the curing phase, an almost complete elimination of cracking. Still, the problem of the evaluation of cracking due to imposed deformatiosuccessfully mastered if the long term behavior of concrete is not taken into account


Static efficiency of concrete Fig. 5 – Casting of SCC concrete

The increase in strength also positively affects durability and workability, because normally the aggregate dimensions are reduced and the porosity limited. Nonetheless, specific mixes must be designed in order to fully reach the goals of high durability and high workability.

workability concrete

The workability of concrete is a crucial factor for an efficient casting procedure, which in order to guarantee the development of the required properties once the

concrete is cured and to allow the full interaction between concrete and rebar to take place. Technological progress aimed at high workability found its inception in the early Eighties, when the

called rheoplastic concretes were devised. The main properties of such concretes were high ty and plasticity. Later on, these properties were further enhanced, leading to the design of

concretes able to flow and self-compact under their own weight only, with no need of vibrationThe industrial production of this kind of concrete started in the mid-Nineties, and it was labelled as

compacting’ concrete (SCC). SCC is characterized by a very high workability and the ability to ry congested rebar arrangements, Fig.5.

te requires a careful mix design and a refined casting and curing procedure. Fformworks must be water proof, moreover, during casting free falling of

considerable heights must be avoided, to prevent bleeding and segregation. procedures for self-compacting concrete are quite strict and include a range of

tests with specified thresholds for acceptance. If the quality of fresh SCC is not strictly guaranteed, the properties of the resulting cured concrete cannot be exploited as well.

ous researches carried out on these matters, in /9/, two very important andare shown: the first one is that SCC exhibits an increas

strength with respect to an ordinary concrete of the same class; the second onebonding between concrete and rebar in SCC with respect to ordinary concrete.These results show that structural durability can be enhanced if SCC is employed, even if they also suggest that a careful and thorough experimental and analytical investigation mu

Reduced shrinkage concrete

features of concrete, traditionally limiting its use for surfaces that have architectural value, is the development of remarkably visible cracking patterns due to

sed deformation are mostly due to thermal effects, both ones, and those caused by the shrinkage of concrete. Since or

ctures are strongly statically undetermined, the presence of imposed deformations that reduce during curing, causes significant tensile stresses in concrete, which in turn

on the aesthetics of the structure is the most evident shortcoming of cracking, but its most dangerous effect is the dramatic reduction of the durability of the structure, which becomes more exposed to the aggression of the environment.

g or, better, eliminating shrinkage-induced cracking has been pursued starting from the seventies, thus leading to the development of special concrete mixtures that increase their volume while curing. Since expansion is an imposed deformation opposed in si

compressive stresses that oppose those induced by shrinkage, thus allowing, at the end of the curing phase, an almost complete elimination of cracking. Still, the problem of the evaluation of cracking due to imposed deformationssuccessfully mastered if the long term behavior of concrete is not taken into account

Casting of SCC concrete

positively affects durability and workability, because normally the aggregate dimensions are reduced and the porosity limited. Nonetheless, specific mixes must be designed in order to fully reach the goals of high durability and high workability.

is a crucial factor for an efficient casting procedure, which is a in order to guarantee the development of the required properties once the

interaction between concrete and rebar to take place. Technological progress aimed at high workability found its inception in the early Eighties, when the

he main properties of such concretes were high ty and plasticity. Later on, these properties were further enhanced, leading to the design of

, with no need of vibration. Nineties, and it was labelled as

compacting’ concrete (SCC). SCC is characterized by a very high workability and the ability to

refined casting and curing procedure. First of all, free falling of concrete from

compacting concrete are quite strict and include a range of tests with specified thresholds for acceptance. If the quality of fresh SCC is not strictly guaranteed,

two very important and is that SCC exhibits an increase in compressive

strength with respect to an ordinary concrete of the same class; the second one is the improved

These results show that structural durability can be enhanced if SCC is employed, even if they also l investigation must be carried out.

se for surfaces that have architectural value, is the development of remarkably visible cracking patterns due to

sed deformation are mostly due to thermal effects, both exogenous and ones, and those caused by the shrinkage of concrete. Since ordinary concrete

the presence of imposed deformations that reduce concrete, which in turn

on the aesthetics of the structure is the most evident shortcoming of cracking, but its most dangerous effect is the dramatic reduction of the durability of the structure, which becomes

induced cracking has been pursued starting from the seventies, thus leading to the development of special concrete mixtures that increase their volume while curing. Since expansion is an imposed deformation opposed in sign to shrinkage, it

those induced by shrinkage, thus allowing, at the end of

ns cannot be fully and successfully mastered if the long term behavior of concrete is not taken into account. A purely elastic


analysis of the problem leads to the deceitful conclusion that an expansion-induced deformation pattern having the same asymptotic value as the one of the shrinkage induced stresses would give way to the complete elimination of the latter. This is not true, because the final stress pattern induced in the material is affected not only by the asymptotic values, but also by the rate of development of the phenomena. For this reason, as illustrated in Fig.6, long terms effects due to shrinkage are generally larger than those due to expansion.

Fig. 6 – Shrinkage and expansion plots Fig. 7 – MAXXI Museum, Rome Since state-of-the-art concrete technology is not able to produce delayed expansion concrete, alternative attempts were made, such as the reduction of shrinkage itself, with the use of special admixtures in the mix design. The new generation of expansive concrete with shrinkage reducing admixtures proved to be very effective, since the reduction of the asymptotic value of shrinkage deformations also reduces the associated tensile stresses, which can be fully kept under the limit of tensile strength of concrete. Interesting applications involving the use of these materials have recently been carried out, in the construction of the XXI Century Modern Art Museum (MAXXI) in Rome, Fig.7. 4.5 Fiber reinforced concrete

This kind of concrete was developed in order to increase ductility and resilience under tensile stresses. The presence of micro-cracks in the concrete mass is the main reason for its very low tensile strength and its fragile failure. The addiction of fibers of different kinds, either metallic or glass or plastic ones, introduces into the concrete mass elements able to prevent the formation of micro-cracks and limits the opening of existing ones. Once the problems of decreased workability and increased density due to fibers are solved, the use of fiber reinforced concrete is highly beneficial, since durability, ductility and resilience are significantly improved. Even if the tensile strength of concrete is increased by the presence of fibers, still the use of ordinary steel rebar is mandatory, in order to provide tensile strength to the structural elements after the concrete cracking takes place, at the ultimate limit state. In any case fibers reduce the cracking pattern, increasing the global structural durability.

5. DEVELOPMENT OF CONSTRUCTION TECHNIQUES 5.1 New construction techniques for bridges and infrastructures One of the first applications of innovative construction techniques for p.c. bridge structures was the segmental construction, i.e. on-site casting with sliding formworks. This method brilliantly solved the problem related to the height of the piers. It also allowed the bridges to be built starting from the piers and casting the decks as cantilevers, without any need for propping or provisional supports, with a final casting at the end of the construction, which creates continuity in the completed deck. In Fig.8 it can be seen that the construction process must start from two opposite piers, moving towards the mid-span in a symmetrical way, in order to prevent excessive flexural actions in the piers. It becomes important to correctly predict and monitor the vertical and lateral displacements of the cantilevered parts of the deck, in order to achieve the correct final continuous configuration.

Viscoelastic Analysis

Elastic Analysis


Fig. 8 – Segmental construction process for bridges In order to do this, the development of long term deformations in concrete must be accurately investigated, since creep deformations are remarkable in concrete when it is loaded at a young age. Another construction technique for bridges and flyovers, which is the natural evolution of the former, is the use of precast segments instead of cast-in-situ ones, with the use of prestressing to allow the cantilevered configuration in the construction phase, Fig.9.

Fig. 9 – Segmental construction by using precast Following this method, the perfect compatibility between the surfaces of two adjacent segments must always be guaranteed, in order to achieve the correct longitudinal and transverse final geometric configuration of the structure, the correct distribution of the interlocking stresses at the interface and the correct safety coefficient against the decompression limit state. In this case, it’s even more important to carry out a refined analysis of the time evolution of the deformations and of the stresses in the segments. Another possible construction technique is the so-called ‘incremental launching’ method, which implies the pre-assemblage of large parts of the decks in areas where workers have easy access. After that, the pre-assembled parts are moved into place sliding on tracks with translational or rototranslational movements. The procedure of incremental translational launching for prestressed concrete bridges, Fig.10, was developed in the Eighties, when jacking operations and the constructions of sliding supports able to minimize friction forces became possible. The incremental launching technique can be profitably used for bridges with straight axis and nearly constant spans, ranging between 40m and 70m, nevertheless, bridges with curved axis in the vertical or horizontal plane have recently been constructed.

Fig. 10 – Construction phases for a launched bridge


The technique based on the rotational movement of the bridge after its construction, as shown in Fig.11, is particularly useful when a river or railroad must be crossed, because the additional costs due to the track system are much lower than those due to the discontinuation of service or the use of very high propping system or the aggravation given by the presence of water.

Fig. 11 – Construction technique for cable stayed bridges involving rotation of the deck after construction Finally, an interesting construction method is the one based on the introduction or provisionary restraints that are released in later phases. This method is typically used for arch bridges, Fig.12. The two halves of the bridge are built separately, in an almost vertical configuration, then they are rotated into the final horizontal configuration and the provisionary hinges at their bases are transformed into a continuous joint with an additional casting or with imposed deformations. Alternatively, the two halves of the bridge are built segmentally in their final configuration, but are provisionally restrained by stays, until they are made continuous at the end of the construction phase with the last casting at mid-span.

Fig. 12 – Construction techniques for arch bridges 5.2 New construction techniques for tall buildings In recent years the use of concrete has become more and more widespread in tall buildings, due to the increase in the strength of concrete, its workability and its durability, coupled with an easier construction procedure with respect to that required for steel. At the end of the Sixties the tallest concrete building in the world was the Pirelli Building, 127m high, whereas at the end of the Nineties the record had increased to 310m with the Telekom Malaysian Headquarters, to get to the 800m of Burj Khalifa in Dubai, currently the tallest building in the world, having the main structural elements made of concrete, Fig.13.


Fig. 13 – Pirelli Tower, Malaysian Telekom Tower, Burj Khalifa The main structural elements in tall buildings are the foundations, the shear-resistant cores, the columns and the floor slabs. Foundations are usually concrete plates, 3m thick and up, normally made of self compacting concrete, with reduced hydration heat and reduced shrinkage. In this way, it’s possible to cast them continuously, covering extensive surfaces with volumes of concrete in the order of 10

4 m

3 or more

without the development of cracking patterns due to shrinkage and hydration heat. The construction technique of stairways cores involves the use of dedicated formworks, with highly specialized features consisting in self advancing formworks, windshields, concrete pumps, coupled with a strongly industrialized production and arrangement of rebar, that are usually made of pre-assembled parts. A scheme of such devices is represented in Fig.14, whereas in Fig. 15 the construction method used for Burj Khalifa is shown.

Fig. 14 – Concrete casting system Fig. 15 – Burj Khalifa construction Fig. 16 – Steel encasing of columns

Another state-of-the-art technique, used for Palazzo Lombardia in Milan, currently the tallest building in Italy, is the use of steel encasing for the reinforced concrete vertical elements. The steel formworks, not having a static role, can be erected for a height of 3 to four storeys, leaving room at the floor level to insert the floor rebar, before the slabs are cast. Inside the steel encasing, the longitudinal and confinement rebars are arranged for the whole height, thus eliminating the need to move formworks and install the rebar floor by floor, Fig.16.

Fig. 17 –Polyethilene spheres in the slabs Fig. 18 – Prestressed slab with unbonded tendons Once the slab formworks are ready and the floor rebar is also arranged, concrete can be cast. The steel encasing thus acts as a perishable formwork, providing a very consistent reduction of construction times, also allowing the slab-to-column joints to be cast much more effectively and


guaranteeing a better monolithic quality of the frame, which in turn is crucial for an improved global structural behavior. Finally, as for slabs, the modern trend is to have cast in situ reinforced concrete slabs or reduced-weight partially precast slabs, sometimes coupled with unbonded prestressing cables. Among the reduced-weight slabs, those with polyethylene spheres are very interesting: they were very successfully employed in Palazzo Lombardia, as shown in Fig. 17. Prestressed slabs allow a significant reduction of flexural deformations, the reduction of the thickness of the cross section, the almost complete elimination of cracking phenomena, thus increasing global structural efficiency and durability, Fig.18.

6. SOME REMARKS REGARDING STRUCTURAL ANALYSIS In large-scale structures, such as tall buildings or important bridges and flyovers, usually built segmentally, the correct prediction of the long term deformations due to creep becomes very important. Concrete creep can affect the structural service life causing excessive deformations, cracking or other negative phenomena which can sometimes be impossible to repair or require high maintenance costs. In the Thirties, the first theories regarding visco-elasticity, aimed at developing formulations for the structural analysis that would take into account concrete ageing were developed. From then on, the study of the problem of the long term variation of stresses and deformations in concrete and its analytical solutions boomed, so that new technologies and construction techniques could be attempted and mastered, leading to the achievements of the present days. Structural solutions that were unthinkable a few decades ago are today a reality and new, even more impressive challenges are already being undertaken. The more challenging the applications become, the more refined and theoretically complex the solutions of the problem must become as well. For this reason, the theoretical and analytical approaches are strictly intertwined with the practical needs, and both must be beneficial for each other, which requires designers and researchers synergic efforts. 6.1 Examples related to long term structural analysis In Fig.19 the construction procedure for a segmental prestressed concrete bridge is shown. Construction starts from the central bay, then moving to the lateral ones, which are connected to the central bay by means of hinges and then closed by means of prestressing, creating a continuity.

Fig. 19 – Segmental prestressed bridge: construction process The bending moment vs time plots in the two hinges, starting from the time when each of them becomes moment resisting, are affected by prestressing and differential shrinkage. A remarkable difference can be observed between the final diagram of flexural actions and the elastic initial one. The final diagram is non-symmetric because of the differences in the ages of concrete in the different segments. When different rheological models are assumed for concrete, a quite scattered range of values of actions and deformation at final time can be obtained. Consequently the correct choice of


constitutive models is a key factor to obtain reliable solutions. The use of the different formulations suggested by Codes must always be supported by a wise and cultured engineering approach. Another example is the cable stayed bridge, represented in Fig. 20. If the second order effect of the self-weight induced vertical deformations of the stays is taken into account, then a careful determination of the tension in the stays, interacting with the concrete deck, is needed, because the two affect each other. If the stays are prestressed to tension values equal to the values they would have if considered undeformable, then such values of tension will not vary in time, as represented in Fig. 21. This is the consequence of a fundamental theorem of viscoelasticity, which is still valid in the case of geometric non-linearity. Still, this situation is impossible to pursue in real bridges, because of their rheological non-homogeneity, /10/. As a consequence, the stress patterns in the stays and in the deck will vary in time, as shown in Fig.22, so that the designer must always be able to predict and limit values of this variation, to prevent negative effects.

Fig. 20 – Cable stayed bridge

Fig. 21 – Axial forces in the stays Fig.22 Bending moments in the deck Even more difficult problems arise in launched bridges. Referring to the four span bridge of Fig.23, the bending moment X1 vs time plot is reported in Fig. 24 a. The abrupt changes are related to the time when the bridge is launched, while the smooth lines describe the structural relaxation. Furthermore, as illustrated in Fig.24 b, the values of the bending moment are higher in comparison with the elastic ones, as during launching the vertical displacement of the metallic nose increases in time owing to concrete creep.

Fig. 23 – Lauched bridge: construction phases


Finally, it can be observed that displacements larger than those in the final configuration are present during the construction process, as shown in Fig.24 c), in which the envelope of the transverse displacements during the construction process has been reported, /11/. The envelope of the alternate deflections maintains its basic properties in every span except for the last, in which the presence of the previous spans reduces the related displacements.

Fig. 24 – a) Bending moment X1 vs time plot; b) Evolution of elastic and viscoelastic bending moments; c)Transverse displacement envelope in launching Another very important problem is that of column shortening in tall buildings. It causes deformations in the slabs and remarkable differential displacements in the supports of the non-structural elements, such as facades. This phenomenon, discussed in /12/, must be analyzed by taking into account the compensation of displacements that is carried out during the construction. The analysis shows that during the service life of the structure, remarkable increases in the vertical displacements of the columns take place. Referring to the building in Fig. 25, studied in /13/,the displacements of the columns are originated by two effects: a long term effect due to the loads applied below the level where the displacement is being measured and an elastic and long term effect due to the loads applied above the level where the displacement is being measured. In Fig. 26 the global vertical displacements, sum of the two effects, are reported. As for beams, as shown in Fig. 27, the reductions in the flexural stresses are small, because the displacements in the columns are induced by static loads.

Fig. 25 – Reference building Fig. 26 – Sum of vertical displacements Fig. 27 – Time evolution of flexural stresses The Codes often provide simplified design aids, such as tables or plots, to predict long term effects. These simplified solutions can sometimes be too simplistic, leading to mistakes in the predictions of complex phenomena. An example of this concept can be found in Fig.28 a),b), where, for a continuous 3-bay beam, the time evolution of the bending moments at the supports is shown, following the introduction of continuity in the supports N.1 and N.2, after the loading is applied. In Fig.28 a), representing a homogeneous structure, the plots Hij (i=j=1,2), governing the solution of the problem, are reported. In Fig.28 b), the same plots, but for a structure where the lateral bays are assumed to be much older than the central one are represented. In both cases, the two sets of Hij

(i=j=1,2) plots are not very different from one another, whereas the Hij (i,≠j) plots are equal to zero in the first case, while they are non-zero, even if their values are small, in the second case. Given the similarity of the the two sets of Hij (i=j=1,2) plots and the small relative values of the Hij

(i,≠j) plots in the second case, one could be tempted to assume the solution of the first case, i.e. homogeneous structure, as a good approximation for the solution of the second case, related to a


non-homogeneous structure. However, this is not a wise choice, because it does not allow the evaluation of the peculiarity of the behavior in the latter case. Unfortunately, though, while determining the Hij plots in the first case is quite easy, it is not as easy in the second case, /14/, but this challenge should be consciously faced by the designer by perceiving it as an advancement of his own knowledge.

a) b) Fig. 28 – Time evolution of the bending moments at the supports of a three-bay beam

7. SOME REPRESENTATIVE CASE STUDIES In order to highlight the above discussed concepts, some representative structures are presented. The chosen examples are some of the most important reinforced concrete and prestressed concrete structures built in the last decades. Three bridge structures and some buildings will be presented, all of them representing, at the time of their construction, innovative challenges, both for the designers and the constructors, some of which even breaking world records. 7.1 Bridges The first is the Gatweay Bridge, built in Brisbane, Australia, in 1986, Fig.29. It is a multi-span prestressed concrete viaduct, whose central bay spans 260m. This span set the length record for a prestressed concrete bridge and held it for 15 years, till the beginning of the New Millennium, when the new and current world record was set by the Stolma bridge in Norway at 303m.

Fig. 29 – Gateway Bridge The main beam of the Gateway Bridge, having a hollow core cross section, is the largest ever built. It is 15m high on the supports, 15m wide and supports a 22m wide deck. The bridge was built by means of a climbing formwork. In 2010 a new viaduct, parallel to the first, was open and the two of them coupled took the name of ‘Sir Leo Hirsher Bridge’. In Fig.30 the first bridge and the second one, under construction, are represented.

Fig. 30 –Second Gateway Bridge under construction


A second structure, which is very interesting for its architectural features and its very challenging configuration, is the Millau Bridge, built on the Paris-Montpeller highway in France, Fig.31. The structure is a cable-stayed bridge, having 342m spans, supported by seven piles. The maximum height of the piles, 343m, set a world record, while the maximum height of the deck above the ground, i.e. 270m, ranks twelfth in the world. The stays are not parallel and the piles are very slender, so the aesthetics of the bridge is very peculiar and a beautiful example of refined elegance. This result is the consequence of the complete achievement of the potential of conceptual design, when coupled to architectural innovation and aesthetics. For the construction of the deck the launching technique was adopted, with eight provisional supports. In Fig. 32 the construction phases of one pile is represented.

Fig. 31 – Millau Bridge, France Fig. 32 – Millau Bridge: construction A third, very interesting example, is the Krka River Bridge, built in 2005 in South Croatia, Fig.33. It is a reinforced concrete arch bridge, with a steel-concrete deck consisting in a grid of steel beams with two main beams spanning 7.6m and two transverse beams spanning 4m. The arch spans 204m, one of the longest ever built. The construction technique was segmental, with provisional stays and steel proppings on top of the piles, so that no scaffolding for the formworks was needed, Fig.34. The segments were 5.20m thick, 10m wide and 3m high. The state-of-the-art construction technique allowed the use of arch bridges to be re-considered and rated, since it allows very pleasant aesthetics results to be achieved in areas where the environment would be spoiled by more invasive structures.

Fig. 33 – Krka Bridge, Croatia Fig. 34 – Krka Bridge: construction phase 7.2 Tall buildings Two elegant buildings, related to the Italian experience and representative of the evolution of tall buildings in Italy in the last fifty years will be discussed at first. The older one is the Pirelli Tower, represented in Fig. 35, 127m tall, which, when it was built, was the tallest building in the world having a reinforced concrete structural system.

Fig. 35 – Pirelli Tower, Milano: view and plan

The structure is made of a central core and two a diamond shape. Between the cores two frames arespan 18m, including main and secondary beams.The construction techniques was traditional, with casting in construction techniques had not yet been employed, still the results were very good from an architectural point of view. In 2009, the Pirelli Building was outgrown by the new represented in Fig.36. For this building, stateaesthetically refined and beautiful object, which has now become a landmark in Milan’s skylight, could be successfully built in a very short time.Two other important tall buildings are the Trump International Hotel and Tower in Chicago, USA, 423m high and built in the first decade of the current century,in Canton, China, 321.9m high and completed in 1997concrete structural system and both held the time of their construction.

Fig. 37 – a) Trump Intl. Hotel and Tower, Chicago, b) CParis, France, d) Bayerische Hypotheken Bank, This fact shows that the record height increased by 30% in about ten years, as a consequence of the impressive development of concrete technology. To build the Trump Tostrength of 70MPa was employed, with enough workability to be pumped upwards for over 400m prior to casting. Even higher performance concrete was used for the building which is currently the tallest in the world, i.e. the Burj Khalifa in Dubai, 80advancements show that the development of the potential of concrete is fundamental for tall buildings, at present and even more in the near future. Finally, tcoupled to prestressing allows complex and elegant structures to be built. This is the case of the Grande Arche in Paris, FranceGermany, Fig.37 d), where the use of prestressitechnological and construction issues.


: view and plan Fig. 36 – Palazzo Lombardia Complex, Milano: view and plan

The structure is made of a central core and two lateral cores located at the sides of the plan, having a diamond shape. Between the cores two frames are located, supporting the slabs, with maximum

main and secondary beams. The construction techniques was traditional, with casting in ordinary formworks, but, even if the new

ction techniques had not yet been employed, still the results were very good from an

In 2009, the Pirelli Building was outgrown by the new Palazzo Lombardia Complex, 161.30m high,. For this building, state-of-the-art technologies were employed, so that a very

aesthetically refined and beautiful object, which has now become a landmark in Milan’s skylight, could be successfully built in a very short time.

her important tall buildings are the Trump International Hotel and Tower in Chicago, USA, 423m high and built in the first decade of the current century, Fig.37 a), and the CITIC Plaza Building in Canton, China, 321.9m high and completed in 1997, Fig.37 b). Both buildings have a reinforced concrete structural system and both held the world record of height for this kind of buildings, at the

a) Trump Intl. Hotel and Tower, Chicago, b) CITIC Plaza, Canton, China, c)

Bayerische Hypotheken Bank, Munich, Germany

This fact shows that the record height increased by 30% in about ten years, as a consequence of the impressive development of concrete technology. To build the Trump Tower, concrete having a cubic strength of 70MPa was employed, with enough workability to be pumped upwards for over 400m

Even higher performance concrete was used for the building which is currently the tallest in the Khalifa in Dubai, 800m high, having a reinforced concrete main structure. These

advancements show that the development of the potential of concrete is fundamental for tall even more in the near future. Finally, the use of high

stressing allows complex and elegant structures to be built. This is the case of the ris, France, Fig.37 c) and of the Bayerische Hypoteken Bank in Munichwhere the use of prestressing was the only means to solve some very delicate

ogical and construction issues.

��

��

��

�

��

��

��

� ��

��

��

��

��

�⌧� ��

� ��

��

��

��

��

��

��

��

� ��

� ��

�

��

��

��

��

��

Palazzo Lombardia Complex, Milano: view and plan

lateral cores located at the sides of the plan, having the slabs, with maximum

ordinary formworks, but, even if the new ction techniques had not yet been employed, still the results were very good from an

Palazzo Lombardia Complex, 161.30m high, art technologies were employed, so that a very

aesthetically refined and beautiful object, which has now become a landmark in Milan’s skylight,

her important tall buildings are the Trump International Hotel and Tower in Chicago, USA, and the CITIC Plaza Building

oth buildings have a reinforced kind of buildings, at the

China, c) Grande Arche,

This fact shows that the record height increased by 30% in about ten years, as a consequence of the wer, concrete having a cubic

strength of 70MPa was employed, with enough workability to be pumped upwards for over 400m

Even higher performance concrete was used for the building which is currently the tallest in the 0m high, having a reinforced concrete main structure. These

advancements show that the development of the potential of concrete is fundamental for tall he use of high performance concrete

stressing allows complex and elegant structures to be built. This is the case of the erische Hypoteken Bank in Munich,

ng was the only means to solve some very delicate

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

�

�

��

�

�

�

�

�

�

��

��

��

��

��

�

��

��

��

��

��

��

��

�

��

��

��

��

��

��

��

��

��

��⌧

� ��

�

��

��

��

� ��

�

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

� ��

��

��

��

�

��

��

��

��

��

��

� �

��

� �

��

��

��

��

��

��

��

��

��

��

��

��

� �

�

��

��

��

��⌧��

��

��

��

��

�� ⌧��

��

��

�

�� ⌧��

��

��

�

��

�⌧��

��

��

��

��

��

��

��

��

��


8. CONCLUSIONS Modern structural engineering challenges designers to extremely complex tasks, calling them to very though efforts to find reliable and efficient solutions. On one side, structural designers have to conceive efficient and reliable structures based on their knowledge of structural mechanics and their theoretical background. On the other side, state-of-the-art analytical and numerical techniques must be mastered by modern engineers and extensively used to investigate in detail even the most complicated issues of the structural response. The two main features of conceptual design of structures are thus expressed: the ability to conceive beautiful and efficient structures and the ability to thoroughly investigate their behavior. This approach to structural design has brought to very impressive developments in the last three decades, which can be summed up in three main aspects: scientific advancements, technological progress, construction technique development. The advancements in the scientific and theoretical knowledge of structures led to the statement of normative documents for the measure of structural safety, which is a fundamental prerequisite for designers, technological advancements allowed higher and higher concrete performance levels to be achieved, state-of-the-art construction techniques allowed even very complex structures to be successfully built. These three aspects, harmonically conceived, allowed a very fruitful expression of the full potential of the conceptual design approach to structures, as represented by the outstanding case studies hereby discussed.

9. REFERENCES

/1/ Nervi, P.L., “Scienza o Arte del Costruire? Caratteristiche e possibilità del cemento armato”, Rome, 1945 (in Italian) /2/ Torroja, E. “Razon y Ser de los tipos estructurales”, Madrid, 1956 (in Spanish) /3/ Freyssinet, E., « Naissance du beton precontraite et vues d’avenir », Travaux, Paris, June 1954 (in French) /4/ Dischinger, F., « Untersuchung uber die Kincksichereit, die elastische verformung und das kriechen des betons bei bogenbrucken”, Der Bauingenieur, H. 33/34, 1937 /5/ CEB Model Code 78, Système International de Réglementation Technique Unifiée des Structures, Vol.II, Paris, France, 1978 /6/ CEB/FIP Model Code 90, Thomas Thelford, London,UK, 1993 /7/ fib Model Code 2010,Vol.2, Design, Bulletin 56, Apr. 2010 /8/ Eurocode 2, EN 1992-1-1 - Design of Concrete Structures - Part 1.1 General Rules and Rules for Buildings, EN 1992-2 - Part 2 Concrete Bridges /9/ Mola, F. , “The chemical, physical, mechanical properties of SCC, a wide research programme in progress in Italy, Proc. of. 29th Intl. Conference on ‘Our World in concrete and structures’, Singapore, 2004 /10/ Mola,F.,Pisani, Creep Effects on Long Term Behaviour of R.C. and P.C. Cable-Stayed Bridges, Proceedings of the Int. Symposium for Innovation in Cable-Stayed Bridges, Fukuoka, Japan, 1991 /11/ Mola, F., Pisani, M.A., Mapelli, M., “Time dependent analysis of launched bridges”, Intl. Journ. Of Structural Eng. and Mechanics, Vol. 24, Dec. 2006 /12/ Mola, F., Pellegrini, L.M. “Effects of column shortening in R.C. tall buildings”, Proc. of. 35

th Intl.

Conf. on ‘Our World in Concrete and Structures’, Singapore, 2010 /13/ Mola,F., Pellegrini, L.M., Giussani, F. , “Gli effetti dell’accorciamento delle colonne negli edifici alti in c.a.”, Proc. Of. 26th ‘Convegno AICAP’, Padova, 2011 (in Italian) /14/ Mola,F., Pisani, M.A.,Creeep Analysis of Non-Homogeneous Concrete Structures, Proceedings of the Fifth Int. RILEM Symposium, Barcelona, Spain, 1993

RECENT DEVELOPMENTS IN THE CONCEPTUAL DESIGN · PDF fileRECENT DEVELOPMENTS IN THE CONCEPTUAL...

Documents

Transcript of RECENT DEVELOPMENTS IN THE CONCEPTUAL DESIGN · PDF fileRECENT DEVELOPMENTS IN THE CONCEPTUAL...