Dr Juan Sagaseta

This paper outlines a refinement to current grillage and linear finite element analysis methods to better estimate the behaviour of reinforced concrete deck slabs on prestressed beam or steel girder bridges, suitable for modern codes of practice and computerised methods. The work is part of research aimed at unlocking the potential of compressive membrane action. The paper proposes a three-phase approach for the prediction of cracking, deflections, ductility and load capacity. The method increases the accuracy of current grillage and conventional linear finite element methods by taking into account flexural cracking extensions using an effective strain method. The method gives better estimates of deflections at serviceability and allows for better estimates of the ultimate load capacity of existing bridges. This results in more economic reinforcement designs with lower carbon footprint for new bridges.

This paper focuses on robustness considerations of tall buildings with reinforced concrete flat slabs supported by a central core and one row of perimeter columns. This layout is commonly used in office and residential buildings to reduce storey height and maximise natural daylighting. The core and lateral bracing of tall buildings is generally less sensitive to local failure than the secondary load transfer system provided by the floors and perimeter columns, considering the high vulnerability of perimeter columns to accidental actions and punching of the slab around the supports. This paper analyses sudden corner and edge column removal situations in slab-core-perimeter column systems using dimensions and loading representative of current tall building design and construction, looking at potential punching at the connections and flexural failure in the slab. The dynamic punching model previously developed by the authors, was validated in this work for further cases of punching around edge columns and slabs with low amount of flexural reinforcement. The dynamic punching model is based on the Critical Shear Crack Theory for quasi-static punching and the Ductility-Centred Robustness Assessment method. A simplified level of approximation is proposed for a preliminary dynamic punching check during the conceptual design stage to optimise the position columns and level of flexural and punching reinforcement needed. The approaches proposed are consistent with Model Code 2010 and can be used to arrest the horizontal propagation of failure in the slab. The analyses of the case studies in this work showed that removal of an edge column adjacent to a corner column can be critical and alternative robustness design considerations are needed in this case.

As building structures in different parts of the world become increasingly exposed to extreme events, there has been a notable research and professional effort to ensure the design of more robust buildings which are insensitive to local failures. At the same time, several works performed in the field of forensic structural engineering have contributed to advancing knowledge on causes and risk factors of structural failure. This includes the creation of several collapse databases, most of which focus mainly on the underlying hazards causing failure. While such databases have provided invaluable insights for preventing structural failures, they do not lend themselves well to the analysis of how failure propagates, which can be useful for improving the progressive collapse resistance of buildings. To this end, a novel database of building collapses is presented in this article which systematically collects information on the hazards, initial failures and their corresponding propagation mechanisms. In addition, key information related to the context and the consequences of collapse is also gathered. Based on the information compiled in the database, this article provides an in-depth analysis of the most commonly occurring initial failures and propagation mechanisms, with significant conclusions extracted from the study of past collapses. The application of different consequence models for estimating fatalities and reconstruction costs is also presented, leading to recommendations for improving such models and related data collection strategies. •Novel database of building collapses.•Systematic collection of information on initial failures and propagation mechanisms.•Analysis of most commonly occurring hazards, initial failures, and propagation phenomena.•Application and evaluation of consequence models for estimating casualties.•Estimation of reconstruction and replacement costs.

Many theories and empirical formulae have been proposed to estimate the shear strength of reinforced concrete members without transverse reinforcement. It can be noted that these approaches differ not only in the resulting design expressions, but also on the governing parameters and on the interpretation of the failure mechanisms and governing shear-transfer actions. Also, no general consensus is yet available on the role that size and strain effects exhibit on the shear strength and how should they be accounted. This paper reviews the various potential shear-transfer actions in reinforced concrete beams with rectangular cross-section and discusses on their role, governing parameters and the influences that the size and level of deformation may exhibit on them. This is performed by means of an analytical integration of the stresses developed at the critical shear crack and accounting for the member kinematics. The results according to this analysis are discussed, leading to a number of conclusions. Finally, the resulting shear strength criteria are compared and related to the Critical Shear Crack Theory. This comparison shows the latter to be physically consistent, accounting for the governing mechanical parameters and leading to a smooth transition between limit analysis and Linear Elastic Fracture Mechanics in agreement to the size-effect law provided by Bažant et al.

Flat slab concrete buildings are widely found in infrastructure such as office and residential buildings or industrial facilities. The susceptibility of progressive collapse of such structures due to accidental loads is highly dependent on the structural performance of the slab-column connections. This paper presents a framework for a simplified reliability analysis and derivation of safety factors for computing the probability of punching of flat slab concrete buildings subjected to accidental loads such as column removal, slab falling from above or blast load. The main advantage of the proposed approach is that it considers in a simple manner, the uncertainty in the gravity load applied in the slab before the accidental event, which affects the inertial effects and demand/capacity ratio in the slab-column connections. Eurocode 2 and the Critical Shear Crack Theory for punching are used and extended to dynamic cases for the assessment of the demand/capacity ratio using computer-based time history finite element simulations. The proposed reliability method is applied to a case study of an existing building showing that the column removal situation is not always critical whereas the slab falling from above is much more detrimental.

The topic of robustness and progressive collapse of structures has attracted significant attention within the field of structural engineering recently. This is reflected by the rise in the number of scientific papers published in recent years as well as efforts in reviewing and developing codes for design. Although important numerical and experimental studies have been carried out to date simulating the sudden removal of columns to reproduce the possible consequences of an extreme event, most of these studies focus on subassembly systems and internal columns. Edge and corner columns are most vulnerable to accidental events. This paper gives the results of a test carried out on a purpose-built full-scale reinforced concrete building with a specially designed corner steel column used for the sudden column removal. The test was highly instrumented, involving 38 strain gauges, 38 displacement transducers and 2 accelerometers to monitor the vertical and lateral response. The results were used to analyse the dynamic performance of the structure after the sudden column removal as well as the alternative load paths (ALPs) mobilised during the test (i.e. flexural and Vierendeel action). The test showed a clear dynamic amplification of the strains and displacements (with high peaks); dynamic amplification factors (DAFs) were obtained accordingly. The load initially carried by the removed column was redistributed through the entire building system (not just the neighbouring columns). Tests on full-scale buildings, including the one described here, can be used to compile a database 26 to validate codes and future numerical studies.

Progressive collapse of concrete flat slab structures is mainly governed by the structural response of the slab-column connections before and after local failure of the connections. A novel model is proposed to predict the structural response of column-slab connections after punching shear failure (post-punching shear). The proposed model considers the gradual activation and deactivation of flexural bars within and outside the punching shear cone as well as the contribution of integrity reinforcement. The model is formulated in terms of slab vertical displacements so that it can be applied to progressive collapse analyses including asymmetric cases such as in column removal scenarios commonly adopted in structural robustness design. The proposed model is validated using existing experimental data covering symmetric post-punching tests without shear reinforcement, with and without integrity reinforcement. Numerical modelling was also applied to verify the analytical model by means of 3D explicit finite element modelling. Analytical and numerical models showed that asymmetric post-punching lead to a reduction of the peak post-punching strength due to an early fracture of the reinforcement whereas the residual shear strength in the connection immediately after punching can be higher in asymmetric cases.

The most frequently used technique to construct reinforced concrete (RC) building structures is the shoring or propping of successive floors, in which the slabs are supported by the shores until the concrete acquires sufficient strength. A significant number of structural failures have been reported during construction in recent years leading in some cases to the progressive collapse of the whole structure. The collapse often starts with the local failure of a single element which could be due to errors in design or construction and/or due to accidental events. Although this is a well-recognized problem, studies on the effects of local failure in the shoring elements on the integrity of the shoring-structure system have not been carried out in the past. In this work advanced numerical finite element models were carried out of a three20 storey RC building and its shoring system. Four scenarios of local failure were considered: sudden removal of a (1) shore, (2) joist and (3) complete shore line; and (4) incorrect selection of shores. The results indicated that the structure-shoring system was able to develop alternative load paths without dynamic amplification effects due to the large stiffness and redundancy of the system without compromising the integrity of the structure but leading to significant damage in the concrete slabs. Design recommendations are also given based on the results from this study, which pretend to be the first study to focus on the structural response and damage of a building structure under construction after the sudden failure 27 of one or more shores.

The paper presents a series of fifteen RC slab tests subjected to low-velocity impacts with different impact energies, diameter and shape of impactor. The discussion presented here only refers to flat impactors (series F). Tests are very well reported and failure was described as punching. FE models were validated against the tests and a parametric analysis of 54 cases was carried out for two slab thicknesses, three impactor sizes, concrete strengths and reinforcement ratios. The results from the parametric analysis were used to derive empirically close-form expressions for the minimum impact energy required to cause punching. The experimental work presented has enabled the discussers to validate further the analytical model for punching developed at University of Surrey (in collaboration with EPFL) and establishing a fruitful comparison with the numerical predictions and empirical formulation presented in the paper. This discussion should complement the work presented in the paper.

Reinforced concrete (RC) slabs and panels are commonly encountered in critical infrastructure and industrial facilities with a high risk of close-range explosions due to accidents or terrorist attacks. Close-in detonations lead to high intensity concentrated loads which can cause a premature brittle punching failure of the member. The assessment of such type of failure mode is challenging since the loading source varies its magnitude in space and time. This paper proposes an analytical method by which the occurrence of punching (or otherwise) is assessed by comparing the dynamic shear demand and capacity (supply). An exponentially decaying distribution of reflected overpressures on the RC surface is presented for this analysis. The punching shear demand is estimated from the pressure and inertial forces acting in the free body diagram. The dynamic punching shear capacity is obtained using the Critical Shear Crack Theory with small slab deformations which are predicted from an equivalent single-degree-of freedom model. The proposed approach takes into account the impulsive behaviour of the member leading to a higher punching capacity and provides better predictions than using existing formulae for punching which are based on tests with quasi-static loading and deformations. The proposed analytical equations are further supported by numerical explicit finite element models providing useful information of crack development, dynamic reactions and deflections. The application of the proposed method has been illustrated and validated by comparison with various tests with scale distances from 0.2 to 1.5m/kg1/3. A practical example is presented to illustrate the applicability of the proposed method.

A dynamic punching shear model is presented for general sudden column removal cases which was validated against data from a purpose-built full-scale two-storey reinforced concrete building subjected to a sudden corner column removal. Such analyses are generally performed in structural robustness or integrity design against progressive collapse and several simplifications are generally adopted to avoid complex dynamic nonlinear analyses. These simplifications are generally on the conservative side and punching can be predicted incorrectly. The test results presented showed that Vierendeel action at small deformations was predominant after column removal. The dynamic amplification of the deformations and shear was significant although punching did not occur as predicted by the model. It was found that in general cases punching around edge columns after sudden corner column removal was not critical using design accidental load combinations, although a dynamic punching check is still needed especially for higher live loads and low flexural and punching reinforcement ratios.

Reinforced concrete flat slab structures are used widely in construction projects due to their economic and functional advantages. Punching shear failure in such structures can have catastrophic effects in the case of, for example, multi-storey framed structures and the designer aims to ensure that ductile flexural deformation occurs before the brittle shear failure. Shear mechanisms generally govern the behaviour of reinforced concrete structures subjected to localised impact loads. Existing experimental results investigating punching shear in flat slabs subjected to impact loading shows that when increasing the loading rate, the punching shear strength also increases whereas the deformation capacity reduces. This behaviour is due to a combination of inertial effects and material strain-rate effects which leads to a stiffer behaviour of the slab for higher loading rates. This can also lead to a change of mode of failure from flexural to pure punching shear with increasing loading rates. Current empirical formulae for punching shear are unable to predict this behaviour since the slab deformations are not considered for calculating the punching shear strength. This paper presents an analytical model based on the Critical Shear Crack Theory which can be applied to flat slabs subjected to impact loading. This model is particularly useful for cases such as progressive collapse analysis and flat slab-column connections subjected to an impulsive axial load in the column. The novelty of the approach is that it considers (a) the dynamic punching shear capacity and (b) the dynamic shear demand, both in terms of the slab deformation (slab rotation). The model considers inertial effects and material strain-rate effects although it is shown that the former has a more significant effect. Moreover, the model allows a further physical understanding of the phenomena and it can be applied to different cases (slabs with and without transverse reinforcement) showing a good correlation with experimental data. © 2014 The Authors. Published by Elsevier Ltd.

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. Copyright © 2013 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

This paper presents the results of a series of tests on short span reinforced concrete beams which were strengthened in shear with various arrangements of externally bonded carbon fibre reinforced polymer (CFRP) sheets. The objective of the tests was to determine the effect of changing the area and location of the CFRP sheet within the shear span. A total of fifteen 150 mm × 300 mm × 1,675 mm concrete beams were tested of which four were un-strengthened control specimens. The remaining 11 beams were strengthened with varying configurations of CFRP sheets. Parameters varied in the tests included the area of CFRP sheet, its anchorage length and the distance of the CFRP sheet from the support. The experimental results revealed that the CFRP is more effective when it is placed close to the supports and even small areas of CFRP can give significant increases in shear strength. The experimental results were compared with the three different existing shear prediction models for estimating shear contribution of CFRP sheets. A simple strut-and-tie model is presented which gives reasonable predictions of shear strength for the beam specimens, which were strengthened with CFRP over the full depth of the beam. The superposition method of design is replaced in EC2 by the variable angle truss model in which all the shear is assumed to be resisted by the truss mechanism. A simple regression equation is proposed for the calculation of effective stress in FRP to be used in EC2. © 2012 King Fahd University of Petroleum and Minerals.

This study is primarily focused on the approximate analysis of reinforced concrete outriggers which are commonly used in the design and construction of Supertall Buildings subject to distributed horizontal loads. Existing global analysis formulae that provide preliminary results for lateral deflections and moments are reviewed for two lateral load resisting systems, namely Core-Supported-with-Outrigger system (CSOR) and less frequent Tube-in-Tube-with-Outrigger system (TTOR). These formulae are only applicable for CSOR and neglect the reverse rotation of the outrigger actually suffered due to the propping action from the outer columns; and give rather high predictions of the deflections compared to advanced numerical FE models. An improved model is proposed which overcomes this issue, and provides more consistent results to FE predictions. The same can also be extended to TTOR. Several case studies are investigated to verify the accuracy of the proposed methodologies. The global analysis is followed by the local analysis of reinforced concrete outrigger beams using strut-and-tie modelling and nonlinear finite element analysis to obtain optimised reinforcement layouts (reduction of quantities of reinforcement). The results highlight the different challenges in detailing such structural members which are heavily loaded (high congestion of reinforcement) and the behaviour at failure can be brittle.

Extreme events (i.e. terrorist attacks, vehicle impacts, explosions, etc.) often cause local damage to building structures and pose a serious threat when one or more vertical load-bearing components fail, leading to the progressive collapse of the entire structure or a large part of it. Since the beginning of the 21st century there has been growing interest in the risks associated with extreme events, especially after the attacks on the Alfred P. Murrah Federal Building in Oklahoma in 1995 and on the World Trade Center in New York in 2001. The accent is now on achieving resilient buildings that can remain operational after such an event, especially when they form part of critical infrastructures, are occupied by a large number of people, or are open to the public. This paper presents an ambitious review that describes all the main advances that have taken place since the beginning of the 21st century in the field of progressive collapse and robustness of buildings. Widely diverse aspects are dealt with, including: (1) a collection of conceptual definitions, (2) bibliometric details, (3) the present situation and evolution of codes and design recommendations, (4) quantification of robustness, (5) assessing the risk of progressive collapse, (6) experimental tests, (7) numerical modelling, and (8) research needs. Considering the comprehensive range of these aspects, this paper could be of great use to professionals and researchers who intend to enter the field of the progressive collapse of building structures and also to other experts who require an extensive and up-to-date view of this topic.

The risk of structural failure of buildings can be significant during construction. Temporary adjustable telescopic steel shores or props are commonly used in building construction. The failure of shores is sudden and therefore structural fuses as load limiters (LL) can be introduced to provide ductility in the temporary member for a specified limit failure load. Previous work by the authors showed that the design of shoring systems can be improved using LL for standard cases of imposed loads applied during construction. This paper extends this work to cases of accidental loading where the shoring system-permanent structure interaction is less known. The main principles of LLs are discussed and implemented in advanced numerical simulations of a real case RC building during construction by means of explicit nonlinear dynamic finite element analyses. Different local failure scenarios were investigated corresponding to cases observed in practice. The comparison of the numerical results obtained with and without LLs demonstrated for the first time the benefits of using LLs in terms of: (a) mitigating the risk of failure of the temporary structure; and (b) reducing permanent damage (cracking and short-term deflections in the slab) affecting the durability and functionality of the building.

It is over a century ago that testing of reinforced concrete slabs by the pioneers of the material such as Lord, Turner and Maillart showed that restrained slabs could carry significant loads. Since that time the interest in and knowledge of the internal arching, or compressive membrane action, that enhances the strength of reinforced concrete has waxed and waned. In this paper, definitions of key terms such as arching action, compressive membrane action and geometric arching are given. A review of key twentieth century research and testing is also given, with particular emphasis on aspects related to bridge decks. The more recent advances in compressive membrane action and punching shear are then outlined. A graphical summary of key tests is presented, together with an initial analysis of these data. The current American, British and European bridge codes incorporating compressive membrane action are reviewed and the major differences outlined in the light of this recent research. Finally the key issues are summarised and a few thoughts on future research and the codification of arching action and compressive membrane action are given.

Pile caps used in foundations are commonly designed for simple cases of loading and geometry using the strut-and-tie method. This approach is known to provide safe designs and rather conservative predictions of the ultimate failure load of tests. This level of conservatism is due mainly to the large simplifications made in the geometry assumed which in many cases ignore relevant parameters such as the size of the column. A three-dimensional strut-and-tie model is presented for four-pile caps in which the geometry adopted is optimized. The inclination of the direct strut from the column to the pile is obtained analytically through the maximization process of the resisting load carried by the truss assuming different modes of failure (flexural and shear). This approach is shown to provide more accurate predictions of strength of existing deep pile cap tests with lower scatter compared to design approaches in the literature and ACI 318 Code.

Quadripartite vaults are found in many historic buildings and are often below ground level. When new infrastructure is developed above these structures it is necessary to assess the strength of the existing vaults. A new formula for assessing the failure load of quadripartite masonry vaults under uniform loads is presented. The approach is based on the upper bound solution from limit analysis of the elliptical arch defined at the intersection between the cylindrical surfaces. The predictions from the analytical solution are consistent with numerical results from a symmetrical 3D finite element analysis developed using a damaged plasticity model with homogeneous material properties for the masonry. A case study is also presented corresponding to the quadripartite vaults in the undercroft of London Bridge station. This case study is used to examine the effect of the presence of ribs and horizontal movement restraint introduced by surrounding structures. The presence of ribs was found to have the largest effect on the strength of the vault. The non-linear FE analysis showed that modifying the boundary conditions to restrain the edges of the vault from horizontal movement increased the failure load by approximately 30% for a vault without ribs. When ribs were included the introduction of horizontal restraint had a much larger impact and in such cases a more refined model including explicitly the surrounding structure might be needed.

Many of modern life activities involve the risk of fire, explosions and impacts. In addition, natural extreme events are becoming more and more common. Thus, robustness, the ability to avoid disproportionate collapse due to an initial damage, and resilience, the ability to adapt to and recover from the effects of changing external conditions, represent two important characteristics of current structures and infrastructures. Their definitions are reviewed in this paper with the aim of sorting and describing the different approaches proposed in the literature and in the international standards. A simple example is also analysed in order to compare different methods

After the collapse of Ronan Point tower building in 1968 there was an unprecedented discussion about the issue of progressive collapse in structural design. In particular, recommendations were published that precast panel structures should include tying elements to hold a structure together after an element loss. The initial investigation into the causes of the collapse and the majority of the subsequent discussion was focused on ensuring precast structures had the same monolithic behaviour as conventional forms. However, the prescriptive recommendations were then applied to all structural forms without amendment to account for the different mechanical behaviour. This paper presents the findings from a novel bibliographic study of historical documents published soon after Ronan Point collapse which influenced the development of relevant design guidelines. The technical information was analysed chronologically to determine the intended purpose of such requirements and the assumptions they were based on. It then traces the development of progressive collapse design requirements to the current Eurocodes to consider if they are being applied as intended. This critical review is timely since robustness considerations in Eurocodes and other international codes are currently being reviewed and general misconceptions regarding existing prescriptive rules have been identified among practitioners in the UK and internationally.

Structural robustness is a significant property towards improving resilience of buildings, i.e. enhance their ability to withstand and recover from extreme events which often can cause local damage and progressive collapse. It is widely accepted that robustness depends on the capacity of the structure to activate alternative load paths (ALPs) after the failure of load-bearing elements, e.g. columns. Early evidence during World War II showed that progressive collapse of some buildings was avoided by the presence of masonry infill walls. Subsequent studies focused on this effect for cases of sudden column removal although most of these studies were analytical, numerical and only looked at internal columns which are generally less vulnerable to accidental events compared to corner and edge columns. The aim of this study was to analyse how infill walls can improve the robustness of reinforced concrete (RC) buildings in corner columns failure scenarios. A purpose-built 3D two-storey full-scale RC building structure with infill masonry walls was tested. The contribution of masonry infill walls was analysed in terms of: (i) load redistribution, (ii) ALPs, and (iii) Dynamic Amplification Factors (DAFs) to be applied in linear-static analyses. The test was highly monitored by 38 strain gauges, 38 LVDTs and 2 accelerometers to register the vertical and lateral response. The results showed that masonry infill walls had a significant influence on the structural response and activated the predominant ALPs at very small deflections.

Fifteen 1200 x 1200 x 150 mm (47.2 x 47.2 x 5.9 in.) reinforced concrete (RC) slabs were tested under low-velocity impact loadings. These slabs were fixed on their four sides and vertically impacted by a drop-weight system. The influences of impact energy, diameter of impacted area, and nose shape of impactor on the damage of RC specimens are studied. The damage of slabs under low-velocity impact increases with increasing impact energy. Moreover, punching shear failure mode was observed for all the specimens that failed during the test. Besides experimental work, three-dimensional (3-D) finite element (FE) analysis was conducted using the LS-DYNA software to help determine the impact energy that would cause punching shear failure of RC slabs. In the FE model, 3-D elements with strain-rate-sensitive material models were used to model concrete and steel reinforcement. The support and impactor were idealized as rigid body. Based on the results from FE analysis, two dimensionless empirical equations are proposed in term of various parameters to assess the energy capacity of slab under low-velocity impact.

Although many analytical and experimental studies have been carried out to date on the 1 progressive collapse and robustness of cast-in-place RC and steel building structures, very few 2 experimental research works have been performed on precast concrete (PC) structures. The small 3 number of publications on these experiments have focused on analysing the behaviour of 4 subassemblies after the sudden loss of an internal column. This paper is the first to describe the 5 construction of a full-scale purpose-built experimental PC building to study the sudden removal 6 of corner columns and the structure's ability to find alternative load paths (ALPs). The 7 connections between the precast members were designed using existing simplified guidance for 8 robustness. The results obtained from the test using gravity loads corresponding to typical load 9 combinations defined in building codes for accidental design situations, showed a structural 10 response governed by Vierendeel action with a clear contribution from the floor slabs. Load 11 increase factors were obtained from the test results which can be applied by practitioners using 12 simplified linear finite element models to account for non-linear dynamic effects in a simplified 13 manner. This work is expected to form part of a large database of experimental results that are 14 useful for developing advanced numerical simulations and parametric analyses. 15

Building progressive collapse is currently one of the hottest topics in the structural engineering field. Most of the research carried out to date on this topic has been focused on the structural analysis of the failure of one or more columns in a building to determine the Alternative Load Paths (ALPs) the structure can activate. Past research was mainly focused on extreme situations with high loads and large structural deformations and, to a lesser extent, research looked at lower loads used in design accidental situations, which requires a different set of assumptions in the analysis. This paper describes a study aimed at analysing accidental design situations in corner-column removal scenarios in reinforced concrete (RC) building structures and evaluating the available real ALPs in order to establish practical recommendations for design situations that could be taken into account in future design codes. A wide parametric computational analysis was carried out with advanced Finite Element (FE) models which the authors validated by full-scale tests on a purpose-built building structure. The findings allowed us to: (i) establish design recommendations, (ii) demonstrate the importance of Vierendeel action and (iii) recommend Dynamic Amplification Factors (DAFs) for design situations.

The progressive collapse of the World Trade Centre showed the devastating consequences of pancake-type collapses, triggering significant research on failed floor impacts for different forms of construction. In current high-rise construction, concrete flat slabs supported on columns are widely used and, in this case, the fall of the slab could be prevented depending on the detailing and the horizontal propagation of the collapse from one support to adjacent supports. The activation and interaction of different phenomena during horizontal propagation governed by the slab-column response is investigated in this study for cases of flat slabs with and without integrity reinforcement. The paper focuses on slabs without punching reinforcement which are more critical. A slab system analytical model is presented based on a column removal scenario considering the dynamic response of the column-slab connections before and after punching including membrane effects. The model was verified using finite element models with solid elements at the connections. The results highlighted the key role of integrity reinforcement towards preventing slabs from falling by means of activating tensile membrane action concentrated around the columns.

Steel-concrete-steel (SCS) sandwich panels consist of two steel plates connected with tie bars filled with concrete; composite action is achieved using headed studs in the plates. This form of composite construction has recently regained interest in the construction industry as it allows modular construction and decongestion of reinforcement which is particularly useful in large infrastructure such as tunnels, wind turbines and nuclear energy facilities. This paper investigates the out-of-plane shear resistance of SCS panels without shear reinforcement. In practice, the spacing between tie bars acting as shear reinforcement can be significant and the shear resistance is governed in some cases by that of a member without shear reinforcement. A qualitative comparison of the shear transfer actions is presented between SCS and conventional reinforced concrete (RC) members without shear reinforcement. Existing design formulae for shear in RC are applied to existing experimental data of SCS panels. This study shows that the shear resistance models for RC give conservative predictions of strength of SCS slender panels with low or medium levels of shear connection at the interface between the concrete and the steel. This inbuilt conservatism is due to the bond-slip of the interface resulting into a concentration of the flexural cracks towards mid-span which allows the development of full arching action (shift of Kani’s valley). A strut-and-tie model is presented for SCS which provides more accurate predictions of strength in such cases and also in other cases such as short-span members (discontinuity region).

The design and construction of civil engineering structures take into great consideration the sensitivity of such structures in the event of local failures. Flat slab structural systems are very prone to progressive collapse after the failure of a connection or column. Hence, to improve their robustness the introduction of integrity reinforcement is recommended in Eurocode 2, ACI 318-11 and Model Code 2010. However, very little investigation has been carried out on the asymmetric post-punching response of these connections or the actual contribution of designed integrity reinforcement to robustness at a system level. Presented in this paper, is a numeric approach developed for modelling the response of isolated RC flat slab test specimens using the finite element (FE) software LS-DYNA. This is in view of their incorporation into system models for both quasi-static and dynamic assessments of robustness in flat slab structures. Quarter FE models of four symmetric isolated RC flat slab specimens with experimental responses available in literature were developed. These quarter FE models were analysed numerically using a quasi-static displacement controlled approach and their flexural, punching shear and post-punching shear responses observed. A sensitivity analysis was carried out to obtain the optimum element characteristics for punching shear strength as well as other response criteria. Half asymmetric F.E models of two slab specimens were also developed and analysed. These provided the asymmetric punching and post-punching shear response of the slab specimens, assuming the loss of an interior column. Results of quarter symmetric FE models gave accurate predictions of slab load-deformation responses, punching and post-punching shear strengths. Maximum percentage differences of 2% and 3% were obtained when comparing test and FE results of symmetric slab specimens for peak punching and post-punching shear strengths respectively. Asymmetric FE models gave post-punching shear strengths lower than values obtained from tests on symmetric specimens. Robustness of flat slab structures after the loss of an internal column could be significantly overestimated where models adopted do not take into consideration such reductions in post punching shear strength. The results presented validate the use of this FE approach on LS-DYNA to predict the response of concrete flat slab connections.

Reinforced concrete panels and slabs are commonly used in industrial, military and high- security facilities for protective purposes. These structural members are designed against accidental events such as fire, blast and impact loading. This paper focuses on localised hard impacts such as falling objects or debris in which the kinematic energy of the impactor is entirely absorbed by the deformation of the struck body. It is well known that the structural behaviour in such cases is highly non-linear and therefore the structural assessment is often carried out by means of complex numerical models. The accuracy of such numerical tools in predicting the type of failure can be questionable unless the models have been rigorously validated beforehand. This paper presents the numerical predictions (non-linear FE using solid elements) of existing slab tests subjected to drop objects and a comparison with analytical predictions using a dynamic punching model developed at University of Surrey. The results obtained numerically and analytically are consistent with the experimental data. In addition, a combined analytical/numerical approach using dynamic punching formulas and simplified FE (shell elements) is shown to provide consistent predictions of punching failures. This combined approach, which is suitable for design purposes, offers a good compromise in terms of ease of application and level of simplification needed in the dynamic models.

Current building regulations for design against progressive collapse normally use prescriptive rules and risk based qualitative scales which are insufficient to cover current needs in design. Structural robustness of concrete flat slab structures is examined using different theoretical models to capture the dynamic behavior under accidental events. In such extreme events, the large dynamic reactions at the connections could potentially lead to punching and progressive collapse. Punching formulae based on load-deformation response relationships such as the Critical Shear Crack Theory (CSCT) are particularly useful in dynamic situations. The Ductility-Centred Robustness Assessment developed at Imperial College London is also used in this paper to derive simple design formulae to assess punching of adjacent columns in the sudden column removal scenario which is commonly adopted in practice. The approach can be extended to assess flat slab systems upon considering membrane action in the slab and post-punching behavior in the connections. Analytical models for tensile membrane are used in combination with the CSCT to demonstrate that the tying forces required in codes of practice cannot be achieved without prior punching of the connections. It is also shown that numerical modelling of post-punching is a promising tool to review detailing provisions for integrity reinforcement.