Parametric study of the load-bearing mechanisms in RC beam-grids to resist progressive collapse

Recently, several structural failures demonstrated the disastrous consequences of progressive collapse and raised the awareness of the engineering community. However the low probability of progressive collapse makes it uneconomical to design every building against progressive collapse using conventional design methods. Furthermore in most cases the initiating events of progressive collapses are unknown during the design. As such, consideration of secondary load-carrying mechanisms can be an effective alternative. These mechanisms include compressive arch action (CAA) and tensile catenary action (TCA) in reinforced concrete (RC) beams. Several researchers have investigated the effects of CAA and TCA experimentally and numerically in individual RC beams. However to date limited studies have been carried out to study these mechanisms in RC beam-grids. Hence in this contribution a validated numerical model is developed to study and quantify the individual contributions and development of the different mechanisms in RC beam-grids. Parametric studies are performed in relation to the influence of the aspect ratio of the grid, reinforcement ratio and ultimate reinforcement strain

Pre-posterior analysis of inspections incorporating degradation of concrete structures

The framework of pre-posterior decision analysis has a large potential as a decision support tool in structural engineering. It seems ideally suited to tackle problems related to determining the value of Structural Health Monitoring and is commonly applied in inspection and maintenance planning. However, the application of this methodology for integrated life-cycle cost decision making related to monitoring of time-dependent and spatial degradation phenomena in concrete structures, needs further investigation. In this work, the timedependent and spatial degradation phenomena will be coupled to the pre-posterior decision making approach and applied on concrete beams under bending, subjected to corrosion of the reinforcement. A framework is set up to determine the value of information of inspections enabling adequate decision-making. The methodology incorporates Bayesian updating based on the uncertain inspection outcomes. The framework will be illustrated by application on a simply supported reinforced concrete beam

Post-cooling properties of concrete exposed to fire

Concrete structures are able to resist high temperatures due to fire relatively well and they can be repaired afterwards. In order to select appropriate repair strategies, assessment of the condition of a concrete structure after fire is of crucial importance. Previous research has mostly been focusing on the strength of concrete during fire and considering slow cooling of elements to room temperature. Guidelines and models related to these conditions have been incorporated into structural design codes. However, in reality, fast cooling of concrete by means of water occurs frequently and the effect of this cooling method has been much less the subject of research investigations. Nevertheless, the effect of water cooling can be significant. In this article the effect of water cooling on the residual compressive strength, stress-strain diagram and bond strength between concrete and reinforcement is investigated. Two cooling methods are considered,, i.e. quenching and spraying of specimens. It is found that the investigated properties are extremely sensitive to heating with subsequent water cooling

Evaluation of conformity criteria for reinforcing steel properties

Quality inspection of strength properties has to ensure that the used materials comply with their specified requirements. Based on the average outgoing quality limit concept, conformity criteria can be designed and evaluated in an objective way. In this contribution, the conformity criteria for the yield strength of steel reinforcement bars given in the European Standard EN 10080 and the German Standard DIN 488-6 are evaluated based on this concept. It was shown that the current conformity parameters suggested in these standards yield OC-lines that cross the limiting boundary. Hence, subsequently, alternative values for the conformity control parameters are proposed. Finally, it was shown that conformity control of reinforcing steel has a beneficial influence on the reliability level of a reinforced concrete beam

Calibration of partial factors for temporary structures

The Eurocodes currently do not provide a coherent reliability-based justification for the semi-probabilistic design format of temporary structures. Besides the need for suitable target reliability levels, a coherent definition of partial factors is needed, adjusted according to the chosen target reliability level and the intended reference period considered for the design of the temporary structure. When developing such a partial factor approach, attention should be given to the coherency with current Eurocodes to avoid conceptual discrepancies between the design of long-term and temporary structures. In this contribution a full-probabilistic framework for the structural reliability quantification of temporary structures is developed, based on Latin hypercube sampling. A sensitivity study is performed to detect the most important variables to be considered for the reliability analysis. The framework is subsequently used to determine the inherent reliability levels of scaffolds associated the design guidelines and partial factors according to current standards. Furthermore, recommendations for the target reliability levels for temporary structures are proposed, considering an economic optimization procedure. Finally, adjusted partial factors for temporary structures are derived, enabling a rather simple and straightforward, but objective and coherent safety evaluation of temporary structures by practitioners. Such adjusted partial factors are obtained using two methods: (1) an optimization procedure and (2) the Adjusted Partial Factor Method, which was originally developed for adjusting partial factors for existing structures

Structural robustness assessment of concrete frames considering membrane action effects

One way to increase the structural robustness is to take into account membrane action effects in beams and slabs. This membrane action generates an additional load transfer to neighbouring supports, which can considerably increase the load-carrying capacity of the member under consideration. However, the effect of membrane action on commonly used robustness indicators is still unknown. In previous contributions of the authors, a numerical model for reinforced concrete slabs and beams under large deformations was developed and validated. In this contribution, a framework is developed in order to incorporate this numerical model in the analysis of a simple concrete frame in case of column loss, in order to assess the influence of membrane action on commonly used robustness indicators

Application of a multi-level probabilistic framework for the risk-based robustness assessment of a RC frame structure

Despite the increased interest and research about structural robustness, one has to notice that no practical framework is available yet to quantify and assess the robustness of structures which takes into account both local structural behavior of the elements under large deformations and the uncertainties of the acting loads and materials. In this contribution advanced calculation methods and risk-based quantification approaches for robustness are combined by a multi-level calculation scheme which is applied for two alternative designs. The developed approach is able to quantify the reliability and structural robustness of planar reinforced concrete frames in an objective way while using a conditional risk-based robustness index and taking into account the developed membrane action. Additionally the assessment and influence of the direct and indirect costs on risk-based robustness quantification are studied

Model uncertainty quantification for column removal scenario calculations using the energy-based method

Progressive collapse resistance of a building structure is often investigated by the notional removal of one or more vertical load bearing elements from the structural system. Usually, a nonlinear dynamic analysis is needed to perform such an analysis. To avoid the complex nonlinear dynamic analysis, the energy-based method (EBM) is a promising method to predict the maximum dynamic responses of a structural system, where the dynamic load-bearing capacity curve is derived from the static load-displacement curve based on the principle of energy conservation. In this contribution, the performance of the EBM is evaluated based on a validated finite element model of a tested RC slab. Subsequently, 60 samples are generated by using Latin Hypercube Sampling (LHS), taking into account probability distributions for the most important variables. Both static analyses and direct dynamic analyses are executed for every sample set. Based on the results of the stochastic analyses, the EBM is observed to perform well. Furthermore, in the analyzed case study, the model uncertainty of the ultimate load bearing capacity obtained through the EBM compared to direct dynamic analysis is found to be represented well by a lognormal distribution with mean (i.e. bias) of 0.96 and a standard deviation of 0.13. Model uncertainties are also obtained in relation to ultimate displacements and displacements at different load levels

Das neue fib Bulletin 80 : Teilsicherheitsbeiwerte für die Nachrechnung bestehender Massivbauwerke

Die zuverlässigkeitsbasierte Nachrechnung bestehender Tragwerke kann auf Basis verschiedener wissenschaftlicher Verfahren erfolgen. Da jedoch bisher nur wenige mit den Eurocodes kompatible Verfahren für eine praxisgerechte Anwendung existieren, kommen für die Nachrechnung bestehender Tragwerke meist semiprobabilistische Verfahren und Teilsicherheitsbeiwerte zur Anwendung, wie sie auch zur Bemessung von Neubauwerken verwendet werden. Eine solche Beurteilung bzw. Nachrechnung bestehender Tragwerke führt oft zu sehr konservativen Ergebnissen, verbunden mit aufwendigen und kostenintensiven Verstärkungsmaßnahmen. Vor diesem Hintergrund wurde in der fib Task Group 3.1 „Reliability and safety evaluation: full-probabilistic and semi-probabilistic methods for existing structures“ das neue Bulletin 80 „Teilsicherheitsbeiwerte für die Nachrechnung bestehender Massivbauwerke“ [1] erarbeitet, welches sowohl Empfehlungen für die Teilsicherheitsbeiwerte der Einwirkungs- als auch der Widerstandsseite enthält. Im vorliegenden Beitrag werden neben zwei Verfahren zur Bestimmung modifizierter Teilsicherheitsbeiwerte für die Nachrechnung bestehender Massivbauwerke nach fib Bulletin 80 [1] mit schwerpunktmäßiger Betrachtung der Widerstandsseite auch Methoden zur Anpassung des Zielzuverlässigkeitsindex nach fib Bulletin 80 [1] vorgestellt. Darüber hinaus wird das Vorgehen an einem Anwendungsbeispiel erläutert