65 research outputs found
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
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