Between 1997 and 2010, there was a small, yet unexpected, number of flat-slab car park buildings that collapsed under service loading. In all the cases, the collapse was preceded by loss of bond (LoB) between the concrete and flexural reinforcement. All investigations of the causes of these collapses have since been inconclusive, particularly as regards the effect of LoB. Yet, LoB due to yielding of the flexural reinforcement in tension had already been proposed as the main cause of punching, and failure criteria derived on the basis of this proposal were found to produce predictions that correlated closely with experimentally established values of the punching failure load. The aim of the present work was to extend the range of application of these criteria to the case of LoB due to steel corrosion and test their validity against the available information on the car park building collapses.

Deviation of the flexural reinforcement spacing from code specifications appears to underly the cause of the loss of bond (LoB) between concrete and steel that preceded the unexpected collapse of the top-floor balcony of a building under service load conditions on 15 October 2021 in Athens. Although rare, LoB has also been identified as the cause of collapse of buildings under similar conditions in the UK and Canada. However, the codes of practice for reinforced concrete (RC) design neither make reference to the effect of LoB on structural behaviour, nor include LoB in the parameters that the formulae currently used for assessing load-carrying capacity are dependent on. In view of this, a description of the effect of LoB on the function of RC beams is provided in this paper and this description is used as the basis for the derivation of a formula linking LoB with load-carrying capacity. The validity of the proposed formula is verified using published experimental information on the subject. The proposed formula is used for the structural assessment of the collapsed balcony, taking into account the design details, physical state and loading conditions of structural members similar to the one that collapsed.

Reinforced concrete (RC) design is in a state of perpetual revision of ever-increasing complexity. However, preventing the recurrence of unexpected types of brittle failure is yet to be achieved. It is argued that the causes of such types of failure reflect conflicts between assumed and actual behaviour of concrete at both material and structural levels. This paper presents available information on the fundamental properties of concrete, which current design assumptions are incompatible with, and points towards the work required for really improving RC design.

Sustainability calls for reduction in the use of natural resources and man-made materials. In light of this, the present study demonstrates the potentials of the reduction of transverse reinforcement in structural walls. A structural wall 1.7 m long was designed following the Greek Code for Reinforced Concrete (GCRC). This wall was then constructed and tested under cyclic loading. The theoretical value of the uncracked stiffness was four times greater than the value calculated after the experiment. The wall was also designed according to the Compressive Force Path method (CFP), which allowed for a significant reduction in the transverse reinforcement for the same target values.

It has recently been shown that the ever-increasing complexity and inability of methods used for the design of reinforced concrete (RC) structures to consistently safeguard against brittle types of failure is the outcome of the conflict between the assumptions of the underlying theory and the fundamental concrete material characteristics. This paper demonstrates that these drawbacks can be eliminated through a coherent alternative theory developed on the basis of simplified beam theory modified to account for true concrete behaviour. Through the use of experimental information obtained from the literature, the proposed theory shows that, in contrast with widely held views, a structural member's load-carrying capacity depends solely on the strength of the compressive zone, which was found to be significantly larger than what it is widely considered to be. Moreover, load transfer is accomplished by the bending action of the concrete cantilevers formed between consecutive inclined or flexural cracks in the tensile zone. The above findings essentially underline the development of an alternative design method – the compressive force path method – and this is an indication of the suitability of the proposed theory to open up new possibilities for improving RC design.

In spite of the considerable number of reinforced-concrete structures invariably suffering earthquake damage in the regions of points of contraflexure of their beam or column elements, research on the subject to date has been sparse. This work discusses the causes of such damage and proposes a simple design method that does not require calibration through the use of experimental data. A comparison of the proposed method's predictions with experimental information and the predictions of typical current code methods not only demonstrates the validity of the proposed method, but also confirms the shortcomings of current design practice.

The paper presents typical examples of brittle failure that reinforced concrete structures continue to suffer to date in spite of the advances in shear design claimed to have been made in recent years. The key concepts underlying the development of shear design methods are concisely presented and it is shown that the cause of the apparent inability of the methods to consistently achieve their aim is the incompatibility between the assumed mechanisms of load transfer underlying the methods and fundamental concrete behavior. Simpler and more efficient design methods are presented, which have been developed on the basis of alternative mechanisms of load transfer compatible with fundamental concrete behavior at the structure's ultimate limit state.

It was long ago stated by Priestley, and now applies more than ever, that the design of reinforced concrete is so full of myths, fallacies and contradictions that it is hard to know where to begin in an examination of current design. Such an examination forms the subject of the present paper, with a focus on current methods for designing stirrup arrangements for confining the transverse expansion of concrete in the compressive zone of the critical lengths of structural concrete members. These methods have often been found inapplicable in practice and the reason for this is attributed to the misconception that the structural performance code requirements can only be satisfied by confining the transverse expansion of concrete with an appropriate stirrup arrangement. It is argued that stirrups are required for securing a mechanism of load transfer, rather than for producing confinement, within the critical length when bond between concrete and steel is lost after yielding of the flexural reinforcement in tension. A method for assessing the stirrups required for this purpose is proposed and shown to produce design solutions that not only satisfy the structural performance code requirements but also are free of the shortcomings of current methods.

Current codes of practice for the design of structures have been developed within the context of the limit-state philosophy [1] (American Concrete Institute 2011, Eurocode 2 2004): A structure or member is first designed so as to exhibit specified performance after attaining its load-carrying capacity, i.e., when its ultimate limit state is reached; the design is complemented or even revised during a process of checking whether the structure or member exhibits the desired behavioural characteristics under service conditions, i.e., at the serviceability limit state.

The work presented concerns an investigation of the effect of bond between concrete and longitudinal reinforcement on the behaviour of reinforced concrete beams, without transverse reinforcement, subjected to transverse loading combined with an axial force in selected cases. The results showed that the development of bond anywhere within the shear span inevitably leads to inclined cracking, which is the cause of 'shear' failure. Similarly, eliminating bond throughout the beam's span was also found not to safeguard against premature failure. On the other hand, allowing the development of bond only within the shear-free region of the beam was found not only to prevent failure within the shear span, but also to allow the calculation of flexural capacity as for the case of beams with steel bonded to concrete throughout their span.

The application of the compressive force path method for the design of earthquake-resistant reinforced concrete structural walls with a shear span-to-depth ratio larger than 2.5 has been shown by experiment to lead to a significant reduction of the code specified transverse reinforcement within the critical lengths without compromising the code requirements for structural performance. The present work complements these findings with experimental results obtained from tests on structural walls with a shear span-to-depth ratio smaller than 2.5. The results show that the compressive force path method is capable of safeguarding the code performance requirements without the need of transverse reinforcement confining concrete within the critical lengths. Moreover, it is shown that ductility can be considerably increased by improving the strength of the two bottom edges of the walls through the use of structural steel elements extending to a small distance of the order of 100 mm from the wall base.

In the present study a shear wall of 1.7 m length, 1.7 m height and 0.15 m width was designed, in compliance with the Greek Code for Reinforced Concrete (GCRC) and the Compressive Force Path method (CFP). The 1.7 m long wall, designed according to the current GCRC was constructed and tested under cyclic loading, applied in two phases. Under the first one, the specimen reached a displacement of 38.5 mm and a load of 710 kN and under the second one, the maximum displacement was 72 mm and the load 675 kN. It was concluded that the load carrying capacity of the wall was 25% greater than the design value estimated by the GCRC. The experimental value of uncracked stiffness was ¼ of the value delivered according to the GCRC. The ductility of the specimen was 3.3 in the first phase of the testing procedure (uncracked state) while in the second (first crack had occurred) was 6.2. The widest and longest crack was formed at the base of the wall, where predicted. Moreover, the steel structure used for the experiment remained flexible, notwithstanding alterations made. The comparison of the wall reinforcement designed according to the GCRC and the CFP showed that the latter method demands less amount of transverse reinforcement to achieve the same objectives as the former.

The work presented is concerned with the application of the compressive force path (CFP) method for the design of earthquake resistant reinforced concrete structural walls. It is based on a comparative study of the results obtained from tests on structural walls under cyclic loading mimicking seismic action. Of the walls tested, half have been designed in accordance with the CFP method and the remainder in accordance with the provisions of euro-codes 2 and 8. The results obtained show that both methods of design adopted lead to solutions which satisfy the requirements of current codes for structural performance in all cases investigated. Moreover, the solutions obtained from the application of the CFP method result in a significant reduction of the amount of stirrup reinforcement placed at the critical lengths of the walls' vertical edges. In fact, such reinforcement is not specified by the latter method for the case of walls with a span-to-depth ratio smaller than 2.5; for the case of walls with a shear span-to-depth ratio larger than 2.5, not only is it placed over a length which is considerably smaller, but also its spacing is significantly larger, than the code specified values.

From both practical experience and published experimental evidence, it becomes clear that the methods adopted by current codes for the design of earthquake-resistant RC structures have two significant shortcomings: Not only do they lead to reinforcement congestion which may cause difficulties in concreting and often incomplete compaction, but, also, in spite of the large amount of reinforcement specified, they have been found unable to always prevent the brittle types of failure which they are widely considered to safeguard against.

This chapter presents a qualitative description of the behaviour and function of a structural concrete member at its ultimate limit state, together with a description of the mechanism which underlies the transfer of external load from its point of application to the supports of the structural member. This qualitative description, which is compatible with all available experimental information, is made by reference to the case of a simply-supported beam, without stirrups, at its ultimate limit state under transverse loading (the effect of axial loading is also considered). Such a structural member is chosen because, not only is there ample experimental information describing its behaviour but, also, the description of how the beam actually functions forms the theory underlying the design methodology proposed in the following chapters. This theory has been termed the ‘compressive-force path (CFP) concept’ since, as deduced from the description of how the beam functions, the main characteristic of the beam is that both its loading capacity and failure mechanism are related to the region of the member containing the path of the compressive stress resultant which develops within the beam due to bending, just before failure occurs. Experimental information on the validity of the concept is also presented, and it is shown that this provides a realistic description of the causes which dictate the various types of beam behaviour as established by the experimental information available to date. The generalisation of the concept, so as to extend its applicability to any structural configuration and, in particular, to the case of frame-type structures, forms the subject of the Chap. 6 .

This study makes use of the findings of previously published experimental information to demonstrate that the underlying concepts of the methods adopted by current codes of practice (for example, ACI 318 and EC 2) for the shear and, to a certain extent, the flexural design of reinforced concrete structures are in conflict with -fundamental properties of structural concrete at both the material and the structure levels. It is shown that this conflict is the cause of various types of unexpected premature brittle failure of reinforced concrete structures. Experimental evidence is presented that indicates the aforementioned types of failure can be prevented by adopting alternative design methods that allow for a more realistic description of structural concrete behavior and, unlike the code methbds, have been found to consistently satisfy the performance requirements of current codes of practice.

Punching, which may be suffered by two-dimensional (2D) reinforced-concrete (RC) structural elements (such as flat slabs, plates, footings, etc.) in regions under the action of concentrated load, is widely considered to be a 'shear' (non-flexural) type of failure. As a result, the methods adopted by current codes of practice (such as, for example, those adopted by ACI318 [American Concrete Institute: Building code requirements for structural concrete (ACI 318-02) and commentary (ACI 318R-02), 2002] and EC2 [EN 1992-1: Eurocode 2: design of concrete structures—part 1–1: general rules and rules for buildings, 2004.]) for designing against punching are essentially those applied to the 'shear' design of RC beam-like elements with modifications that allow for characteristics of structural behaviour particular to punching and to the geometry of the relevant structural elements.