The design philosophy of the new fib Model Code for Concrete Structures 2010 represents the state of the art with regard to performance-based approach to the design and assessment of concrete structures. Given the random nature of quantities determining structural behaviour, the assessment of structural performance cannot be well established by deterministic methods, instead requires a probabilistic approach. The performance-based approach is introduced in Part I of fib Model Code 2010 by applying the concept of performance requirements and reliability management during service life. Correct understanding of the reliability concept of fib Model Code 2010 is a basic prerequisite for applying its design philosophy in an appropriate manner. Therefore, the main objective of this paper is to explain some decidedly non-trivial issues related to safety and reliability management aspects. In this context, this paper indicates how this general philosophy in fib Model Code 2010 is further developed into a set of operational rules for the design and assessment of concrete structures.

Section 5.1 “Concrete” of the fib Model Code for Concrete Structures 2010 contains basic definitions and well-established constitutive relations for structural concrete. However, it also presents various new approaches and updated models compared with the earlier CEB-FIP Model Code 1990. This is particularly true for the strength, stress and strain characteristics of structural concrete, for creep and shrinkage and for sophisticated durability-related processes. The validity of the models has been extended to several types of concrete such as high strength concrete, self-compacting concrete and lightweight aggregate concrete. The durability-related models are either suitable for facilitating a full probabilistic service life design or for applying simpler code-type approaches.
This article provides a concise and selective overview of some of those models. Background information is summarized and there is a focus on improvements achieved by the updated models. In addition, some simple design aids are given to allow pre-design, for example

Most applications of fibre-reinforced polymers (FRP) deal with externally bonded reinforcement as a means of repairing and strengthening reinforced concrete (RC) structures or retrofitting RC structures in seismic regions. As internal reinforcement, FRP rebars or (more rarely) prestressing elements are used in special projects, combining material strength and durability characteristics. Over the last years, several national and international design guidelines have become available specifically for the design and application of FRP-strengthened or FRP-reinforced concrete structures. These efforts clearly demonstrate the interest in FRP as a novel reinforcing material for concrete construction. Hence, the time had come to introduce FRP reinforcement into the new fib Model Code for Concrete Structures 2010 as well. The main contributions to the fib Model Code 2010 relate to sections 5.5 “Non-metallic reinforcement” and 6.2 “Bond of non-metallic reinforcement”. The material presented in those two sections is further elaborated in this article.

In the fib Model Code for Concrete Structures 2010, fibre-reinforced concrete (FRC) is recognized as a new material for structures. This introduction will favour forthcoming structural applications because the need of adopting new design concepts and the lack of international building codes have significantly limited its use up to now. In the code, considerable effort has been devoted to introducing a material classification to standardize performance-based production and stimulate an open market for every kind of fibre, favouring the rise of a new technological player: the composite producer.
Starting from standard classification, the simple constitutive models introduced allow the designer to identify effective constitutive laws for design, trying to take into account the major contribution in terms of performance and providing good orientation for structural uses. Basic new concepts such as structural characteristic length and new factors related to fibre distribution and structural redistribution benefits are taken into account. A few examples of structural design starting from the constitutive laws identified are briefly shown.
FRC can be regarded as a special concrete characterized by a certain toughness after cracking. For this reason, the most important constitutive law introduced is the stress-crack opening response in uniaxial tension. A wide discussion of the constitutive models introduced to describe this behaviour, which controls all the main contributions of fibres for a prevailing mode I crack propagation, is proposed. The validity of the models is discussed with reference to ordinary cross-sections as well as thin-walled elements by adopting plane section or finite element models.

Conservation of concrete structures forms an essential part of the fib Model Code for Concrete Structures 2010 (fib Model Code 2010). In particular, Chapter 9 of fib Model Code 2010 addresses issues concerning conservation strategies and tactics, conservation management, condition surveys, condition assessment, condition evaluation and decision-making, making interventions and the recording of information for through-life management.
Chapter 9 incorporates the overall philosophy adopted in the development of fib Model Code 2010, which introduces a new integrated life cycle perspective into the design of concrete structures. Accordingly, Chapter 9 provides a response to concepts introduced earlier within fib Model Code 2010 relating to the service life design process, which requires the structure and its component parts to be allocated to a condition control category at the time of design. Different condition control categories are defined depending on factors such as the importance of the structure, its function, design service life, impact on third parties, environmental conditions, ease of maintenance and cost. The condition control levels and inspection regimes are defined in conjunction with these requirements. A through-life management process, outlined in Chapter 9, provides feedback for service life design and allows the associated theoretical model employed to be updated, in turn facilitating the assessment of compliance with the original design objectives.
An example of concrete structure conservation according to the fib Model Code 2010 concept is also presented.

The introduction of new vertical steel bolts is an easy, practical and common solution for retrofitting and strengthening slabs for punching. Although a common option where punching strengthening is concerned, few studies exist regarding how the bolt's anchorage dimensions and its embedment in the concrete slab affect the strengthening efficiency. This work presents an analytical approach that is able to predict the punching capacity of slabs strengthened with post-installed vertical steel bolts, taking into account the anchorage dimensions and positioning plus the material properties. This approach results from the combination of two physical models: one provided in the fib Model Code for Concrete Structures 2010 regarding the punching capacity estimation, and another that allows the deformation (crushing) of the concrete beneath the head of the anchorage to be taken into account. The predicted values are compared with experimental results, showing that the analytical approach is able to simulate correctly the anchorage behaviour and its influence regarding a slab's loadbearing capacity. A parametrical analysis is carried out in order to study the importance of different factors such as concrete compressive strength, longitudinal reinforcement ratio and steel bolt length, always accompanied by the effect of anchorage head size and embedment.

High-performance concretes such as high-strength concrete (HSC) or lightweight aggregate concrete (LWAC) are generally used to reduce member sizes and self-weight, and to optimize the construction of reinforced concrete structures. The bond between the aggregate particles and the cement paste can be strong enough in HSC and LWAC to cause the aggregate to fracture at cracks, which in turn reduces the shear stress that can be transferred across cracks by means of aggregate interlock. Relatively smooth cracks can also develop in self-compacting concrete due to the low coarse aggregate content. The contribution of aggregate interlock to the shear strength of RC beams is uncertain and depends on parameters such as the amount of shear reinforcement or the contribution of arching action for loads applied close to the support. Existing tests on slender RC beams without shear reinforcement have shown that shear strength is reduced by aggregate fracture. However, there is a lack of similar test data for members with stirrups and for members with varying shear span/effective depth ratios. This paper reviews the findings and contributions in this area from the experimental and analytical research of the author's PhD thesis, which was awarded the fib Achievement Award for Young Engineers in 2011.

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This paper examines the evidence for the one-way shear model developed for the fib Model Code for Concrete Structures 2010 and provides examples of its application. For the design and analysis for shear, for members with and without shear reinforcement, the fib Model Code 2010 procedures have been developed from physical-mechanical models that are based on observed behaviour at the meso-scale level; they represent a significant advance over previous standardized empirical methods. In addition, an approach referred to as “level of approximation” (LoA) is incorporated where advanced models are simplified in a consistent and conservative way such that the designer can select the effort needed to justify their design. To illustrate the practical use of the models and the LoA approach, two examples are presented. The first is a deck slab of a cut-and-cover tunnel where design and possible refinements are discussed for a given configuration. The second is a prestressed concrete bridge girder, which is considered for the cases of design and for the analysis of an existing structure.

This paper outlines the theoretical background to the punching shear provisions implemented in the fib Model Code for Concrete Structures 2010 and presents a practical example of their application. The aim is to explain the mechanical model that forms the basis for the punching design equations, to justify the relevance of the provisions and to show their suitability for the design and assessment of structures.

The scope of fib Model Code for Concrete Structures 2010 includes the fully fledged performance- and displacement-based seismic design of new structures and assessment of existing ones. This part of fib Model Code 2010 covers buildings, bridges or similar concrete structures and aims to provide well-defined performance levels for specific seismic hazard levels. Detailing of members for ductility is not based on opaque prescriptions, as in current codes, but on transparent, explicit verification of inelastic deformation demands against capacity limits. The reference analysis method is non-linear dynamic, but under certain conditions inelastic deformation demands may be estimated from linear analysis and the 5%-damped elastic response spectrum; in that case force demands on force-controlled, brittle failure modes are estimated from the plastic mechanism through equilibrium. In order to predict the seismic deformation demands with some confidence, the analysis should use realistic values for the member secant stiffness up to the yield point. The paper explains the background to the expressions given for this property in fib Model Code 2010 as well as of those for the deformation limits used when verifying seismic deformation demands. The modifications to the shear resistance approach of fib Model Code 2010, which takes cyclic loading into account, are also explained and justified.

Interface shear transfer between differently aged concretes is a topic that crops up frequently and in different situations in structural design. In the fib Model Code for Concrete Structures 2010 the fundamental basics of concrete-to-concrete load transfer are given in section 6.3 and the corresponding design rules in 7.3.3.6. The different potential mechanisms contributing to the shear resistance along the interface, i.e. adhesive bond, aggregate interlock, friction and dowel action, are thus combined and their relationship taken into account by interaction factors. This article summarizes the most important results from past and ongoing studies and presents the background to the theory forming the design basis of fib Model Code 2010, the “extended shear friction theory” (ESF).

The design shear resistance of an existing structure can be evaluated with analytical design procedures and numerical procedures provided by non-linear finite element analyses. The new fib Model Code 2010 proposes different calculation methods that fall into four different levels of approximation. As the level of approximation rises, so the complexity and the accuracy of the calculated shear resistance increases. Non-linear finite element analyses belong to the highest level of approximation, but although they are more and more becoming a customary tool in the daily design process, building codes do not provide guidance on how to perform these analyses.
This paper describes non-linear finite element analyses performed on prestressed beams, which underwent shear failure during experimental loading, in order to assess and criticize the finite element approaches. The aim of this work is to propose guidelines for numerical simulations in order to reduce model and user factors. The results obtained from the non-linear finite element analyses have been compared with the analytical results using different levels of approximation. The design shear resistance obtained with the highest level of approximation, level IV, derived from non-linear finite element analyses, turned out to be higher than the design shear resistance obtained with analytical procedures (levels I/II/III).

Conceptual design is the approach that creates an idea in order to find a solution to a new proposal for a structure or solve a detail in a specific structure. It is a personal approach that is learned over time and with experience. It is not normally dealt with at university, but is vitally important for producing sound structures.
The fib Model Code for Concrete Structures 2010 introduced this concept in the first section of chapter 7 “Design”. The content of that section explains the general approach to developing conceptual design.
This paper will show different examples of conceptual design following the general guidelines stated in the Model Code.

This paper provides an overview of serviceability specifications given by the fib Model Code for Concrete Structures 2010 (fib MC2010 [1]). First, the reasons behind crack control and deflection control are discussed, then specific design rules are provided. Simple rules as well as detailed models are also presented. Numerical examples are provided in order to assist in the application of the design recommendations for crack control and deflection control (reinforced and prestressed concrete elements).
Simple rules mean indirect control of cracking or deflections without calculations. Indirect crack control may include limitation of stresses and selection of maximum bar diameter or maximum bar spacing. Indirect deflection control normally means limiting the span-to-depth ratio.
Detailed models are based on physical and mathematical approaches to cracking and deflections. The design crack width is expressed as the maximum bond transfer length multiplied by the mean strain between cracks. Deflection analysis can be provided by integrating curvatures or by using a simplified or refined method. Vibrations and numerical modelling of cracking are also briefly discussed.

Prestressing in flat slabs helps to control deformations and cracking under service loads and allows the required slab thickness to be reduced, thus leading to more slender structures and an economic solution for long spans. However, as a consequence of the limited thickness of these members, punching typically governs at the ultimate limit state. Studies of the punching shear strength have shown that the presence of prestressing in flat slabs has a number of potential beneficial effects, namely the vertical component (force) carried by inclined tendons, the in-plane compression stresses and the bending moments developed near the supported region. However, the approach provided by codes of practice for punching design in the presence of prestressing may differ significantly. Some codes neglect the influence of the bending moments introduced as a result of prestressing and the sections at which the deviation forces of the tendons are considered may be located at different distances from the edge of the supported region. The influence of prestressing on the punching shear strength of members without shear reinforcement is investigated in this paper by using the fundamentals of the critical shear crack theory. Using that as a basis, and also taking into account 65 tests available in the scientific literature, the suitability and accuracy of a number of design codes, e.g. Model Code 2010, Eurocode 2 and ACI 318-11, are investigated and compared.

No short description available.

The Model Code for Concrete Structures 2010 is a recommendation for the design of structural concrete, written with the intention of giving guidance for future codes. As such, the results of the newest research and development work are used to generate recommendations for structural concrete at the level of the latest state of the art. While carrying out this exercise, areas are inevitably found where information is insufficient, thus inviting further study. This paper begins with a brief introduction to the new expertise and ideas implemented in fib Model Code 2010, followed by a treatment of areas where knowledge appeared to be insufficient or even lacking and where further research might be useful.

CEB/FIP Model Code 1990 (MC-1990) did represent the technology and focus some 20 years ago. However, it soon became evident that the document had some notable lacunas. In 1995 the general assemblies of the two organizations endorsed CEB/FIP bulletin No. 228, extensions to MC 1990 for high-strength concrete, and in 2000 a similar extension to MC 1990 for lightweight aggregate concrete as bulletin No. 4.
The fib approved bulletin No. 34 Model Code for Service Life Design (MC SLD) in 2006. All these three additions have since matured and are now incorporated in the new fib Model Code for Concrete Structures 2010 (MC-2010).
The main purpose of an fib Model Code is to act as a model for operational standards. The obvious counterpart for a body such as fib operating worldwide is ISO. The initiative taken by MC SLD has therefore further matured in ISO TC-71/SC-3/WG-4 and it was accepted as ISO 16204 “Durability - Service Life Design of Concrete Structures” during the summer of 2012.
According to the obligations given in the WTO Agreement on Technical Barriers to Trade, it is hoped that these principles will be further implemented in national and regional standards.
This article describes the need for a transparent methodology when dealing with service life design, and the process - originating from a group of enthusiasts one decade ago - through fib and finally reaching international consensus in ISO.

The fib Model Code 2010 for Concrete Structures introduces numerical simulation as a new tool for designing reinforced concrete structures. The model of resistance based on non-linear analysis requires adequate model validation and a global safety format for verifying designs. The numerical simulations combined with random sampling offer the chance of an advanced safety assessment. Approximate methods of global safety assessment are discussed and compared in a case study. An example of a bridge design supported by non-linear analysis is shown.

This article describes an experimental programme aimed at studying the effect of cover, ratio between diameter and effective reinforcement ratio (&phgr;/&rgr;s, ef) and the influence of stirrup spacing on the cracking behaviour of reinforced concrete elements. The experimental programme was conceived in order to contribute to the debate - fuelled by the publication in recent years of Eurocode 2 EN1992-1-1 and the revision of the Model Code under way when the tests were carried out (and now published as a finalized document) - regarding the influence of these parameters on cracking. Important theoretical aspects are discussed, including where the crack width is estimated by current code formulations and what relevance this may have on the correlation between crack opening and durability of RC structures, especially with regard to structures with large covers. The effect of stirrup spacing, a variable absent from current codes, is also discussed.

This paper discusses the possibility of using precast tunnel segments in fibre-reinforced concrete without traditional reinforcement. The case study of a hydraulic tunnel in Monte Lirio, Panama, excavated with a tunnel boring machine (TBM) by SELI S.p.A., has been analysed.
The segments were designed according to the draft of Model Code 2010. In order to achieve the required performance and to optimize the structural behaviour, three different types of steel fibre were considered in the research.
The design was backed up by full-scale tests on precast segments. In particular, 18 full-scale tests were performed, including point load tests simulating the thrust of the TBM and bending tests. The results show the good behaviour of the elements and indicate the fibre-reinforced concrete suitable for the precast elements.

Fatigue design according to CEB-FIP Model Code 90 is limited to concrete grades up to C80. In addition, the design rules include a strength-dependent reduction in the fatigue reference strength, which leads to uneconomical design of high-strength concrete. Considering comprehensive knowledge now available concerning the fatigue behaviour of normal-strength and high-strength concretes, the amount of this reduction can no longer be justified. A new design model for compressive fatigue loading and its derivation is presented in this article. A comparison between the new design model and the current standard ones reveals that the new design model ensures safe and economical design of normal-strength, high-strength and ultra-high-strength concrete. This new design model is included in the new fib Model Code 2010.

In order to address how new knowledge influences design expressions, design codes have in most cases become significantly more complex over the last decades. However, this tendency is leading to codes that are too complicated for preliminary design but still not sufficiently accurate for assessing existing structures (where even more realistic models of behaviour are sometimes required). An alternative code strategy is that proposed by codes based on a levels-of-approximation (LoA) approach. This approach is based on the use of theories based on physical parameters where the hypotheses for their application can be refined as the accuracy required increases. The approach proposes adopting safe hypotheses during the first stages of design, leading to relatively quick and simple analyses. In cases where such a degree of accuracy is not sufficient (e.g. design of complex structures, assessment of existing structures, significant potential economic savings), the hypotheses can be refined in a number of steps, leading to better estimates of the behaviour and strength of members. This approach, recently adopted in the first complete draft of Model Code 2010 for a number of design issues, is discussed within this paper with reference to punching shear provisions.

• fib General Assembly approves the Model Code 2010
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• Hans Wittfoht † 1924-2011
• Andrew Beeby † 1939-2011
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• Acknowledgement
• 2011 reviewers