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To ensure the quality of produced concrete batches, it is often verified whether a specified concrete compressive strength is reached by the application of conformity control. The conformity criteria used for this purpose are related to the probability of accepting a batch, in relation to a specified quality. Batches fulfilling the conformity criteria are accepted, whereas others are rejected. In this way, the outgoing quality of the accepted concrete is higher than without the application of conformity control. This is the result of the filtering effect of conformity control and affects the probability of failure of concrete structures. Instead of only applying conformity control to the concrete compressive strength, a similar procedure could be applied to durability related variables. One example hereof is the diffusion coefficient of the concrete. When conformity criteria are defined and applied in relation to the diffusion coefficient, the result will be a filtering effect on the distribution of the diffusion coefficient. The outgoing distribution will have a lower mean value and standard deviation compared to the incoming distribution. This will influence the time-dependent reliability of reinforced concrete structures. This paper investigates the effect of conformity criteria for diffusion coefficients on the service life and failure probabilities of reinforced concrete elements. The filtering effect is investigated by a numerical study applying analytical methods and numerical integration.

In this work, a reinforced concrete slab bridge (instrumented and tested in 2018) is investigated. Based on field data, a finite element model of the bridge is calibrated. Model selection is performed both based on log evidence and posterior predictive capabilities. It is investigated if the models selected based on the log evidence also induce the most accurate posterior predictions. The influence of different assumptions on modelling the spatial distribution of the stiffness and different possible suggestions on how to include prediction errors and model bias are investigated. Comparing the conclusions based on log evidence and posterior predictions, only using the log evidence for model selection could be debated. Models performing best when considering the log evidence led to the least accurate posterior predictions, and models rejected based on the log evidence could still have good predictive capabilities. Considering the different model classes, introducing spatial variation of the stiffness leads to a posterior prediction closer to the measurements. Introducing a global model bias leads to a better match between predictions and measurements compared to not including this model bias. Even better posterior predictions are achieved if this model bias is quantified locally for the different considered datapoints.

Reinforced concrete structures are often subjected to corrosion, inducing a decreasing reliability over time. This corrosion often exhibits spatial variation, which can be modelled by random fields...

In many countries concrete bridges are reaching the end of their service-life, showing signs of deterioration, e.g. due to corrosion. Hence, the question arises whether their safety level is still acceptable. To improve the estimate of the safety level, information extracted from different tests, e.g. proof-loading, operational modal analysis, etc., where deflections, strains or accelerations are measured, can be used. Unfortunately, in practice inspection results are often not used directly to improve our knowledge of the degree of deterioration and it is difficult to combine information from different types of tests when making inferences. In this contribution, a methodology is developed and demonstrated in order to update estimations of parameters of service life models for concrete girders subjected to chloride-induced corrosion based on heterogeneous measurement data, with focus on strains measured under proof-loading and modal parameters extracted from ambient vibration tests. A Bayesian framework is adopted, where posterior distributions of the parameters describing the corrosion process are generated based on Markov Chain Monte Carlo sampling. These updated distributions reflect the information on the actual deterioration state of the bridge, which can be extracted from limited data. The updating procedure based on strain and modal data is illustrated by application on a simply supported beam and a case study.

Realistic structural models incorporating the time-dependent effects of concrete are essential in order to make accurate predictions of the time-dependent deflections at any time of the service life. Experimental databases are used to calibrate and validate existing models for creep and shrinkage available in international standards. However, extensive research campaigns on large-scale prestressed beams are scarce. In 1967-1985, a research program comprising a unique set of long-term experimental data on concrete beams was conducted in joint collaboration with four Belgian research institutes to determine the influence of creep and shrinkage on the long-term behavior of reinforced and prestressed concrete members. The main aim of the final part of the research campaign was the determination of the long-term behavior of prestressed and partially prestressed beams subjected to permanent loads, considering the influence of the magnitude of the loads, the degree of prestressing, the shape of the cross-section, the type of prestressing and the stress conditions. This paper reports on the obtained unique set of long-term tests. Additionally, also information related to the creep and shrinkage data of prisms and the results of the static tests in a four-point bending configuration until failure at the age of 28 days and 5 years are presented in this paper. The measurements of the prestressed members are compared with a simplified calculation method based on the direct stiffness method, which accounts for aging, creep, and shrinkage. The proposed simplified calculation model allows fast and accurate predictions of strains, stresses, and deflections as a function of time. The results show that the direct stiffness method in combination with the current models of EN1992-1-1 and fib Model Code 2010 can predict the long-term behavior of concrete beams in good agreement with the available experimental data. The research allows to develop more accurate calculation guidelines with respect to the evolution of deflections, concrete deformations and stresses of prestressed beams as function of the prestress-degree, shape of the cross-section, and the type of prestressing.

Current practice is mostly focused on prescriptive design approaches where the performance of the structure in case of fire is assessed based on its performance in standardized fire tests. Those tests indicate whether the structural member can withstand standardized ISO 834 fire exposure for a certain code specified time. This method however does not provide an explicit safety level. This issue is enlarged even more by the fact that the ISO 834 fire exposure does not represent a natural fire exposure, but a pseudo-worst-case exposure, making the correlation between the standardized fire test results and real-life behaviour of structural members exposed to fire questionable. However, there has been a century-old tradition of standardized fire tests with a lot of experience and infrastructure based on it. For that reason, here, a methodology is presented to obtain more information on the behaviour of structural members exposed to a natural fire from the standardized fire test results by using a Bayesian framework. As an example structure, a simply supported concrete slab is considered. Its failure during the standardized fire test is modelled and the parameters affecting the time when it fails (i.e., parameters affecting the nominal fire resistance time) are determined. The model is then used in a Markov chain Monte Carlo procedure to update parameter distributions based on the measured fire resistance time. Using these updated distributions, a full probabilistic calculation of the performance of the slab considering a natural fire exposure is then conducted to assess the failure probability.

When a concrete slab is excessively loaded due to an accidental event, compressive membrane action can be activated in order to generate an alternative load transfer to the remaining supports, which can considerably enhance the load-carrying capacity. This can increase the structural reliability of the components, delay or even prevent a progressive collapse and consequently increase the robustness of concrete structures. Although the beneficial effects of compressive membrane action in reinforced concrete slabs have been recognized for decades, only limited research has been focusing on this effect in prestressed concrete members, like hollow core slabs. Therefore, a novel realscale test set-up has been developed in order to assess this phenomenon in real-scale hollow core slabs. The results indicate that, despite the presence of the large hollow cores, compressive membrane action can significantly improve the load-carrying capacity of hollow core slabs under excessive loading. In parallel, a numerical finite element model has been established in order to model the structural behavior (i.e., compressive membrane action) as per the investigated test program. The model was able to correlate the results from the experiments very well, and emphasized the large influence of the concrete tensile strength on the ultimate failure load of concrete hollow core slabs in compressive membrane action.

The fib Model Code offers pre-normative guidance based on the synthesis of international research, industry and engineering expertise. Its new edition (draft MC 2020) will bring together coherent knowledge and experience for both the design of new concrete structures and the assessment of existing concrete structures. This contribution presents an overview of the main developments related to the partial factors for materials. In the draft MC2020, the partial factors are presented in tables for clusters of cases depending on consequence classes and variability of basic variables. Furthermore, formulas and background information are provided to facilitate updating of the partial factors. This contribution discusses the different assumptions adopted in MC 2020 for design and assessment. Main changes with respect to the previous version are related to description of the difference between in-situ concrete strength and the material strength measured on control specimens, and to modelling of geometrical variables. The presented comparison of the requirements imposed by Eurocodes and MC 2020 for design reveals insignificant differences. The assessment requirements may be decreased by about 25% when the conditions specified in MC 2020 are satisfied. Hence, the revised MC 2020 will provide designers and code makers with wider possibilities to utilise actual data and long-term experience in assessments of existing structures.

Corrosion is one of the most frequently occurring problems in reinforced concrete (RC) structures. The formation of rust products results in deterioration processes that decrease the durability and structural capacity. Therefore, frequent inspection and monitoring of corrosion damage play a vital role in establishing reliable asset management of civil structures. Vibration-based monitoring (VBM) is particularly useful in studying the global response of a deteriorated structure, since a change in modal characteristics may indicate damage. Although VBM has already been studied thoroughly, a systematic understanding of how corrosion contributes to the changes in modal characteristics is still lacking. In this study, an overview and assessment of the existing literature for dynamic tests on corroded RC beams is provided, aiming to give a critical review of the current knowledge, possibilities and challenges related to this topic. Moreover, this study presents additional data from two test programmes, which enable to point out the challenges in VBM of corroding RC beams. The experimental test programmes include two locally corroded beams and three uniformly corroded beams, as well as non-corroding reference beams. For high corrosion levels, the natural frequencies of beams reported in the literature, as well as the beams of the test programmes, decreased. However, comparison with a non-corroding reference beam proved to be essential to eliminate the influence of other non-negligible factors such as creep and shrinkage. Other modal characteristics, such as the damping ratio and mode shapes, are found to be less sensitive to monitor uniform or local corrosion in RC beams.

ispartof: STRUCTURAL CONCRETE vol:23 issue:2 pages:669-681 status: published

ispartof: STRUCTURAL CONCRETE vol:22 issue:5 pages:2556-2580 status: published

Summarization: Guest editorial - Special issue on the Sixth International Symposium on Life-Cycle Civil Engineering (IALCCE 2018): held in Ghent, Belgium, 28-31 October, 2018 (http://www.ialcce2018.org). Life-cycle, risk, resilience and sustainability of civil infrastructure Presented on

In this work, partial factors for the design of temporary scaffold structures are calibrated based on reliability calculations. Scaffolds are commonly designed in accordance with the rules in stand...

Despite the recent amount of theoretical and technological developements, structural robustness is still an issue of controversy being underlined by serveral progressive collapses in te past. Current design codes point out different strategies, among which strategies to limit progressive structual damage by applying prescriptive design and detailing rules. For example, for consequence class 2 structures, EN1991-1-7 defines a risk class CC2b for which also vertical ties are required. However, te background of the design values of vertical ties in current code is not clear and their adequacy should be validated. Moreover, effects such as membrane action and Vierendeel action are important to consider when assessing structural robustness and are difficult to incorporate when applying only traditional design methodologies. To this extent, a set of numerical simulations have been executed in this contribution in order to verify and investigate the progressive collapse behavior of RC frames including the redistribution of internal forces and the response of the vertical ties in columns.

An important characteristic of self-healing concrete is its ability to regain liquid tightness after it has been damaged. This ability can be tested using water permeability setups. However, a lack of standardized test methods makes it difficult to compare results between different studies. Additionally, the large variation on crack widths between specimens results in a large spread of the permeability results. This, together with all other factors (e.g. internal crack geometry) contributing to the variability of permeability results, should be clearly assessed in order to develop a standardized permeability test. In this study a new active crack width control technique has been developed to significantly reduce the variation on the crack width within a series of specimens, resulting in more consistent permeability results. By analysing permeability results of specimens from test series with different nominal crack widths the factors contributing to variability could be assessed. The two main contributors are the variability on the mean crack width at the crack mouth and the variability on the internal crack geometry. The variability of the mean crack width can induce a 3 times higher variability of the permeability results. In contrast to the crack width, the internal geometry of a crack cannot be determined directly, yet it can cause a difference in permeability of more than 25% for specimens with an identical nominal surface crack width. It can be concluded that when crack widths are actively controlled, the main source of variability on permeability results is the internal geometry which cannot be controlled, regardless of the chosen cracking technique. In order to reduce the variance on the mean permeability, it is proposed to use at least six specimens per series.

Before implementing a bridge monitoring strategy, a bridge manager would like to know the return on investment. Moreover, in order to spend the available budget as efficiently as possible, the monitoring strategy should be optimized, i.e., the type of measurements but also the time and locations at which these are performed. For this purpose, the Value of Information (VoI) can be used. The VoI represents an estimate of the benefit that can be gained from a monitoring strategy before it is actually implemented. By comparing the VoI of different alternative strategies, the one with the highest VoI can be selected. As such, the VoI is a tool for objective decision-making. The calculation of the VoI is based on pre-posterior analyses, including Bayesian updating of model parameters based on yet unknown monitoring outcomes. When calculating the VoI for an actual case, some challenges arise. First, the calculation of the VoI requires a number of assumptions on different input parameters. Second, the VoI is computed by evaluating life-cycle costs for different possible outcomes of the monitoring strategy, leading to a high computational cost. However, for practical implementations, results are preferably available within an acceptable time span and are robust with respect to the chosen input parameters. In this work, the implementation of the VoI approach for optimization of monitoring strategies is investigated by a problem statement in a case study where a reinforced concrete girder bridge is considered. To perform this optimization, the VoI for different monitoring strategies is compared. The calculation time required for the Bayesian updating of the model parameters based on the available data is limited by using Maximum A Posteriori (MAP) estimates to approximate the posterior distribution. The VoI can be used both to optimize a monitoring strategy or for comparison of different strategies. To limit the number of required (computationally expensive) evaluations of the VoI, optimization of the monitoring strategy itself can be simplified by determining the optimal sensor locations beforehand, based on a different metric than the VoI. For this purpose, the information entropy is used, which expresses the difference between the prior and posterior uncertainty of the model parameters. Finally, the sensitivity of the VoI to different input parameters is investigated.

A simple framework for setting risk tolerability limits is proposed following a literature review of risk acceptance in fire safety engineering and a feedback round with international fire safety professionals. The framework provides practical guidance for the application of international fire safety guidance, such as the recently published UK guidance document on probabilistic risk assessment (PRA) for the fire design of buildings, PD 7974-7:2019, which provides a state-of-the-art framework to assess adequate safety for complex fire engineering designs. However, the application of this guidance document is hindered by two main constraints: (i) PD7974-7:2019 requires risk tolerability limits to be set, but lacks guidance on defining them for a specific building project, and (ii) no reference case studies are given that demonstrate the application of PRA methods to fire engineering design. In order to help fill this gap, a risk tolerability framework is developed and proposed herein. Finally, it is applied to a reference case study of a UK office building. Considering the developed framework, further research needs are identified. It is expected that the proposed risk tolerability framework and the presented reference case study can support the application of PRA to fire engineering design and ease stakeholder communication on setting risk tolerability limits.

Reinforced concrete structures degrade over time, among others due to corrosion. This degradation process is not uniform along the structure. Monitoring and inspections can be used to detect damage and update a deterioration model of the structure. In this work, a model-based method is used to identify damage through the calibration of a FEM model based on modal characteristics. The model calibration is performed by means of a Bayesian method, using MCMC to sample the posterior distributions of model parameters for the time-dependent degradation of material properties. The calibrated model can then be used for a more accurate prediction of the remaining service life. Furthermore, it enables to be implemented in a pre-posterior analysis framework to determine theVoI of the monitoring and inspection program for comparison of different monitoring and inspection strategies.

The UCO textile factory was built in Ghent (Belgium) in the period 1947-1948. The roof structure covered an area of about 35,000 m(2) and consisted of 100 primary beams and 600 secondary beams. These post-tensioned beams were designed by prof. Gustave Magnel, who was a world-leading expert in the field of prestressed concrete. This project was one of the first important large-scale applications of prestressed concrete in industrial buildings in the world and also the first large scale application of on site prefabrication. Recently, part of the factory building was demolished and a primary beam and secondary beam of the roof structure were transferred to the Magnel-Vandepitte Laboratory for Building Materials and Structures for testing at an age of approximately 70 years. The primary beams have a span of 20.5 m and a maximum depth of 1.7 m. The secondary beams have a span of 13.7 m and a depth of 1 m. Only prestressing tendons (Blaton-Magnel system with o 5 mm wires) were present in the beams but no passive reinforcement nor stirrups are present except for the reinforcement in the corbels and the end blocks. This article describes the results of the loading tests up to failure on the beams. The experimental results are compared with results from analytical calculations. Furthermore, a nonlinear finite element model was developed and validated. Subsequently, this numerical model was used for a Bayesian-based assessment of the prestressing losses in these beams. As such, the article presents novel information related to time-dependent prestress losses after 70 years in service as well as the structural performance of such existing post-tensioned concrete structures dating from that pioneering period.

Traditional design and assessment approaches usually assume that e.g. material properties and environmental influences are uniform in space. However, it is well-known that such parameters can show considerable spatial variability. Furthermore, it has been shown that such spatial variability can significantly influence structural reliability. One way to account for spatial variability is by means of random fields. However, the use of such advanced calculations has not found its way to everyday engineering practice. Therefore, a methodology is developed in order to include spatial variability in the partial factor method in a way which is consistent with the current Eurocode format for design. This is done by introducing a separate partial factor which depends on the correlation length and the variability of the parameter under consideration. As such, an easy-to-use graph is generated, which can be applied in practice for the adjustment of partial factors to take into account spatial correlation. Finally, the proposed approach is validated by means of full-probabilistic calculations.

Existing concrete bridges can show signs of deterioration, e.g. due to corrosion, in which case the question arises whether these bridges still have an acceptable safety level. To obtain additional information about the state of the structure, different types of information can be gathered. However, the measurements performed are often not used directly to update the estimate of the corrosion degree of the bridge and the resulting information from different measurement types is not combined. In this contribution, it is investigated how parameters of the service life models for concrete girders subjected to chloride-induced corrosion can be updated based on heterogeneous measurement data, with the focus on modal measurements and strain measurements. The posterior distributions of these parameters are generated based on Bayesian updating. These updated distributions can be used to get a more accurate estimate of the real deterioration state of the bridge. Based on this, the failure probability of the bridge and its remaining service life can be estimated more accurately.

Aging bridges in combination with an ever-growing traffic volume are a matter of concern all over the world. Consequently, the reassessment of existing bridges is gaining importance rapidly. This paper presents two bridge case studies considered within the IABSE Task Group 1.3 “Calibration of Partial Safety Factors for the Assessment of Existing Bridges”. The so-called design value method (DVM) and adjusted partial factor method (APFM), introduced in fib Bulletin 80 and both relying on a partial factor format, are considered in this paper. The objectives are (i) to illustrate how DVM and APFM can be used when specifying partial safety factors for assessment of existing bridges, and (ii) to discuss some of the assumptions that are implied by these methods. Two case studies are considered for illustration in this paper: a single span reinforced concrete slab and a 3-span continuous reinforced concrete slab.

Carbonation of concrete results in a drop of the pH which can induce steel corrosion. This is costly to repair and therefore an accurate assessment of the carbonation resistance is required. Carbonation of uncracked concrete has been well-studied. However, in reality many of the concrete structures exhibit cracks, but only limited data is available on the influence of cracks on the carbonation process. Two types of cracks in cylindrical High-Volume Fly Ash mortar specimens were investigated: (1) an artificial crack created by pulling out a cast-in metal plate with a thickness of 100, 200 or 300 µm, and (2) a realistic crack created by splitting cylinders and reattaching them with silicon spacers (with a thickness of 100, 200 or 300 µm). After crack creation the specimens were stored in a carbonation chamber for different durations and the carbonation front was visualised using phenolphthalein. This paper reports on the influence of the crack width and the crack geometry on the CO2 ingress. Artificial cracks with different crack widths did not show a difference in carbonation front. On the other hand, a significant difference could be observed for realistic cracks with different crack widths; realistic cracks with a nominal crack width of 100 µm induced a substantially lower carbonation depth. This demonstrates the influence of crack tortuosity on carbonation; the smaller the crack width, the larger the influence of the tortuosity. Comparing artificial cracks to realistic cracks, it could be concluded that the carbonation depth is lower for realistic cracks. Due to the wall effect near artificial cracks, more pores can be found in the crack area which makes them more susceptible to CO2 ingress.

Many existing bridges are reaching the end of their technical service life. To estimate the remaining lifetime of reinforced concrete structures, it is important to assess the parameters of the corrosion process (i.e. parameters governing the corrosion propagation, but also parameters of the chloride or carbonation ingress). The level of corrosion and its spatial distribution can be inferred from strain data under static loading or modal data. In addition, information from visual inspections is often available as well, although not often used to quantify the level of corrosion. In this work, it is investigated how visual inspections can supplement the strain and/or modal data and provide information on corrosion parameters. The developed framework is applied to a simply supported beam subjected to corrosion.

The alternative load path (ALP) method is widely used to investigate the performance of reinforced concrete (RC) buildings in case of progressive collapse (or disproportional collapse) scenarios. This kind of analysis can be carried out either in a static way or in a dynamic way. In this contribution, these methods are compared by means of nonlinear static and nonlinear dynamic analysis methods. A 5-storey RC frame subjected to two different sudden column removal scenarios is adopted as a case study. The probabilistic analysis considers 12 random variables. On the basis of the stochastic results, a dynamic amplification factor (DAF) is calculated. The mean values of the DAF are 1.114 and 1.102 for the external column removal scenario (Case A) and the internal column removal scenario (Case B), respectively. Compared to the failure probabilities obtained through a static analysis, the failure probabilities for the incremental dynamic analysis results are 184.0% and 180.7% higher for Case A and Case B, respectively.

Membrane structures are widely used because of their lightweight properties and expressive architectural shapes. However, there does not yet exist a unified design approach for membrane structures as is available for conventional buildings. Because of their flexibility, membrane structures exhibit a different structural behaviour than conventional buildings. Membrane structures are doubly curved, prestressed and the prestress contributes to the stiffness. The structural response of these structures is non-linear, and the hanging and arching directions interact. A probabilistic framework for tensioned membrane structures is developed herein. Using this tool, partial factors are calibrated for a representative tensioned hypar membrane structure made of PVC-coated polyester fabric. Reliability analyses are performed using a First Order Reliability Method in combination with Latin Hypercube Sampling. The approach is illustrated for two load cases, i.e. considering snow load and wind uplift load, respectively. Three hypars are investigated, each with a different rise to span ratio. The partial factors are determined through a minimisation process. As a result of the performed process, the recommended partial factor for prestress is 1.0, for snow load is 2.0 and for wind uplift load is 2.0 for the investigated membrane structure. In this study, the hanging direction under snow load proves to be decisive for the calibration of the partial factors. The obtained partial factors are not in line with the partial factors found in the Eurocode (1.5 for variable loads). Alternatively, when considering a partial factor for snow load and wind uplift load of 1.5, partial factor for prestress should be increased to 1.2 for the structural type which have been investigated. The application of Latin Hypercube Sampling in combination with the structural analysis of membrane structures enables to perform the reliability analysis calculations in a more computationally efficient way. Moreover, the used optimisation procedure for the calibration of the partial factors makes it possible to interpret the obtained results with respect to the complex non-linear behaviour of the investigated tensile membrane structures.

The design of tensioned membrane structures is not yet addressed by standards like the Eurocodes. Currently, in cooperation with the TensiNet Association, Working Group 5 of CEN TC250, is working on a general standardised design approach for membrane structures, consistent with the partial factor framework used in the Eurocodes (as specified in EN1990). To achieve such a design approach, research focusing on developing a methodology to verify the reliability of a membrane structure is needed. The structural behaviour of a tensioned membrane can be represented by a tensioned cable-net. This article starts from the investigation of a cable-net structure, as such structures are addressed in EN1993-1-11, followed by applying the established method to a membrane structure. This article evaluates the influence of the partial factor for pretension. If the cable-net and membrane structure are dimensioned according to a partial factor 1.0, the obtained reliability index is lower than the considered target reliability index. If the structures are dimensioned according to a partial factor 1.35 the obtained reliability index is higher than the considered target reliability index and therefore meet the safety requirements. The study shows that the influence of the structurally non-dominant direction plays an important role in the reliability analysis.

Due to the negative impact of construction processes on the environment and a decrease in investments, there is a need for concrete structures to operate longer while maintaining their high performance. Self-healing concrete has the ability to heal itself when it is cracked, thereby protecting the interior matrix as well as the reinforcement steel, resulting in an increased service life. Most research has focused on mortar specimens at lab-scale. Yet, to demonstrate the feasibility of applying self-healing concrete in practice, demonstrators of large-scale applications are necessary. A roof slab of an inspection pit was cast with bacterial self-healing concrete and is now in normal operation. As a bacterial additive to the concrete, a mixture called MUC+, made out of a Mixed Ureolytic Culture together with anaerobic granular bacteria, was added to the concrete during mixing. This article reports on the tests carried out on laboratory control specimens made from the same concrete batch, as well as the findings of an inspection of the roof slab under operating conditions. Lab tests showed that cracks at the bottom of specimens and subjected to wet/dry cycles had the best visual crack closure. Additionally, the sealing efficiency of cracked specimens submersed for 27 weeks in water, measured by means of a water permeability setup, was at least equal to 90%, with an efficiency of at least 98.5% for the largest part of the specimens. An inspection of the roof slab showed no signs of cracking, yet favorable conditions for healing were observed. So, despite the high healing potential that was recorded during lab experiments, an assessment under real-life conditions was not yet possible. ispartof: MATERIALS vol:13 issue:4 ispartof: location:Switzerland status: published

Although traditionally the fire resistance rating of concrete elements is determined through standardized tests or tabulated data, there is a growing trend towards the use of performance-based approaches to evaluate structural behaviour during or after a fire. The safety format to be applied with these advanced numerical evaluations is however unclear. In this paper, the applicability of the concept of a global resistance factor (GRF) safety format is explored for simply supported concrete slabs exposed to the Eurocode parametric fire curve for a wide range of parameters. The safety of the slab is evaluated in relation to its ability to withstand a complete burnout scenario, i.e. its ability to resist the applied loads throughout the entire duration of a fire including the cooling phase. Using a full-probabilistic model, the required GRF is numerically derived for a specified target safety level in case of fire. Additionally, a calculation method is provided which allows to determine the GRF of fire exposed slabs for any given compartment through the use of a reference compartment and equivalency equations.

Glass is increasingly used to produce structural elements for buildings. For the case of glass beams, a lot of hybrid glass concepts, in which glass is combined with another material (e.g. steel, reinforced concrete, fibre reinforced polymers) were developed and successfully tested to overcome the material’s brittle nature and hence provide post-fracture capacity. Being one of the most widely tested concepts, the reinforced laminated glass beam demonstrates satisfactory load-carrying behaviour, both for statically determinate and indeterminate support conditions, for a variety of environmental conditions (temperature, humidity, etc.) and for several reinforcement percentages and beam sizes. Contemporary building codes do not only require sufficient structural safety and robustness for individual components, but also for parts -or the entire structural system- of a building. For the reinforced glass beam concept, component safety is demonstrated by post-fracture strength and ductile ultimate collapse behaviour. System action can be achieved through plastic hinge formation and stress redistribution. Additional system safety and robustness can be provided through the activation of membrane action in case of horizontal restraints at the beam ends. When a support is (accidentally) removed, membrane action can provide an additional safety mechanism which prevents collapse of the beam system. This paper presents an experimental program to assess the potential of activating membrane action for reinforced laminated glass beams. Statically indeterminate five-point bending tests with horizontal restraints, with or without intermediate support, are performed on 4.3 m long beam specimens. In addition, the effect of reinforcement percentage is investigated.

Transparency has become a trend in architecture. In the near future, the scale of application of structural glass will grow from elements (beams, columns, etc.) to entire systems (floors, roofs, facades, frames, etc.). Especially beams, being one of the most important and common building components, will be combined into load-carrying systems (e.g. a supporting grid for floors and roofs). However, this scale-up cannot be realized without incorporating structural safety and robustness on an element and system level, as required by modern building codes. For glass beams, research resulted in the concept of hybrid glass beams, in which a glass web is combined with flanges or reinforced with another material that provides post-fracture strength and ductility, hence providing structural element safety and robustness. Especially the stainless steel reinforced glass beam, based on the principle of reinforced concrete proved satisfactory. To investigate the feasibility of applying this concept in innovative beam systems, experimental five-point bending tests on 3.0 m beams and membrane action tests on 4.3 m beams were performed, resulting in satisfactory load-carrying behaviour with significant stress redistribution due to plastic hinge formation and membrane action, both compressive and tensile. Both features/mechanisms resulted in significant increases in the post-fracture capacity of the beam systems. For the latter's design, focusing only on glass fracture is insufficient, as the element and system safety requirements should also be satisfied. Therefore, post-fracture capacity and ductility should be incorparated in design. the latter requires an amount of reinforcing material, wihich decreases the beam's overall transparency. Therefore it is important that design aims at achieving maximum capacity and ductility with as little reinforcing material as possible. this paper investigates the effect of stress redistribution and membrane action on the achievable capacity of a reinforced glass beam system. It is concluded that both features/mechanisms, and especially membrane action can make a significant contribution to the ultimate capacity and ductility of the beam system. This paper also considers the incorporation of these mechanisms in design, resulting in optimal (i.e. minimal) usage of materials. Compared to statically determinate beams, a decrease in reinforcing material is possible, resulting in ever more transparent structures.

Many theoretical methods for evaluating robustness have been developed in the last decades. Especially probability‐based robustness quantification promises to provide a rational basis for decision‐making. To allow for this type of robustness evaluation, the probability of failure (or the corresponding reliability index) of the structural system in the damaged and the undamaged state has to be determined. In this paper, the Probability Density Evolution Method (PDEM) is used to evaluate the dynamic reliability, taking into account the randomness which comes both from the excitation and from the stochastic system properties. The efficiency of the PDEM for robustness quantification is evaluated through a simple cargo crane example case. In addition, the instant equivalent extreme‐value event, which is quite different from the equivalent extreme‐value event for one variable, is applied to obtain the extreme value of reaction forces for each sample at instant time.

© 2018, Springer Science+Business Media, LLC, part of Springer Nature. An integrated probabilistic risk assessment methodology is developed for the purpose of quantifying the life safety level of people present in buildings in the context of fire safety design. Multiple risk based concepts and tools have been developed in previous research to objectify performance based design methods for simple building types and layouts. However, these available models lack an integrated approach for challenging building designs and moreover they are not adequately coupled, most often resulting in a significant computational effort. Hence, there is a need for a practical and efficient framework for dealing with complicated building layouts and different occupancy types. Therefore, a computationally efficient quantitative risk assessment method is developed that provides a framework by combining deterministic sub-models and probabilistic techniques to quantify the fire safety level by means of failure probabilities, individual and societal risk. The deterministic framework is supported by analytical and numerical models. The probabilistic framework is supported by response surface modelling, sampling techniques and limit state design. Following the theoretical description of the model, a case study of a five storey commercial shopping mall of 25,000 m2 is elaborated and discussed as proof of concept. Multiple fire, building and occupant variables are implemented in the model. Three different fire safety designs are compared, resulting in quantified risks between 10−6 and 10−8. The case study proves the validity of the newly developed integrated methodology for this type of buildings and its benefits in fire safety engineering. ispartof: FIRE TECHNOLOGY vol:55 issue:4 pages:1211-1242 status: published

Robustness, resilience and anticipation are essential aspects of a system’s first line of defence against exceptional hazards and high-impact unexpected events. This paper questions both the princi...

In contemporary architecture, glass is more and more applied not only to make up the cladding of the building, but also for structural members such as beams and facade fins. The trend to increase the scale of these structural entities makes it increasingly structurally efficient to apply statically indeterminate beam systems. For these systems to be approved, contemporary building codes require them to provide structural safety on an element and system level. Therefore, a lot of hybrid glass beam concepts, in which glass is combined with another material that provides post-fracture capacity, were developed and investigated. Among others, reinforced glass beams have experimentally proven their feasibility for statically determinate (element safety) and indeterminate (system safety) support conditions by providing significant post-fracture capacity, ductility, plastic hinge formation and load redistribution capacity. However, system safety can also benefit from the effects of membrane action, which can result in yet higher post-fracture performance. Moreover, the latter can make a significant contribution to the robustness of a beam system when an accidental event occurs such as collapse of one of the supports. This paper presents experimental test results of statically indeterminate five-point bending tests with clamping end-supports on twelve 4.3 m long stainless steel reinforced beam specimens in which the horizontal membrane forces are assessed. Two series of tests were performed, with and without intermediate support, for two types of beams with varying reinforcement percentage (i.e. solid and hollow profile reinforcement). During the tests, significant compressive as well as tensile membrane action was observed. The effects of the latter on the load-carrying behaviour is discussed by comparing the load-deflection diagrams with those of similar reinforced glass beam systems without clamped external supports. From this comparison, it is concluded that membrane action provides significant contribution to the load-carrying behaviour of such beam systems. Significantly higher post-fracture capacities can be achieved for both reinforcement sections and both types of tests. It is concluded that membrane action can be incorporated in design, which will lead to more economical, slender and more transparent reinforced glass beam systems.

Recent research has shown that the compressive membrane action (CMA) significantly enhances the load bearing capacity of FRP strengthened concrete beams. It is of great interest to investigate the effect of CMA on the structural reliability of such beams and how to incorporate the benefits of CMA into partial factor based design. Following a CMA model and the probabilistic models of its corresponding design variables, the effect of CMA on the reliability indices of FRP strengthened concrete beams was investigated. A parameter study as well as a sensitivity analysis were also conducted with parameters including properties of FRP and steel reinforcement, concrete properties and geometrical properties. The reliability indices with respect to load ratios i.e. ratios of the variable load to the total loads is selected to quantify the effect of CMA. The results show that the CMA effect significantly improves the structural reliability of FRP strengthened concrete beams. The parameter study indicates that an increase of the concrete strength and yield strain has positive effect on the structural reliability while an increase of FRP ratio, FRP modulus, steel ratio as well as the concrete ultimate strain has an adverse effect. Furthermore, it is found that the variations of the concrete strength, the FRP Young’s modulus as well as the concrete cover have a significant influence on the reliability index; the variations of the ultimate strain of FRP, the yield strain of steel reinforcement and the ultimate strain of concrete have a moderate influence on the reliability index. Finally, an adjusted partial factor for the FRP strength is derived for cases where CMA would already be considered in the design stage.

Structural robustness has become an important research topic in the engineering community since several large failures in the past decades led to the public awareness and indicated the importance to consider structural robustness during the structural design. So far most research has been focusing on structural measures to improve structural robustness and on theoretical methods to quantify structural robustness. However with respect to the analysis of concrete frames, there are only a limited number of examples which try to quantify the achieved robustness level for the available structural measures, such as the development of alternate load paths by membrane action. In this paper both advanced calculation methods and quantification approaches for robustness are combined by a computational efficient calculation scheme which considers different levels of structural idealisation. 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. As an illustration the developed calculation scheme is applied and discussed for two alternative designs of a regular office building. The results show the importance of the uncertainty on membrane action effects on the structure in case of an unforeseeable event leading to a notional column removal.

In order to assess the structural reliability and redundancy with respect to deterioration, appropriate models have to be selected which adequately describe the deterioration process. The parameters associated with these models have to be estimated based on statistical inference. In general, the uncertainties that arise from the estimation of parameters are not accounted for in reliability assessment and the obtained structural reliability indices are assumed to be constant values. When parameter uncertainties are considered, the structural reliability index can, however, be considered as a random variable which inevitably influences the full-probabilistic decision-making process. Furthermore, the structural reliability indices which are used in a reliability-based redundancy factor can be considered as random variables. Hence, this redundancy factor itself is a random variable as well. In this paper, a full-probabilistic framework is developed which allows for the service life assessment of reinforced concrete elements subjected to corrosion based on a probability-based redundancy factor and taking into account parameter uncertainties. It is proven that these parameter uncertainties have a significant influence on the lifetime estimates of concrete elements subjected to corrosion. Finally, a simplified method is proposed which allows to incorporate parameter uncertainties in the redundancy assessment.

The alternative load path method is widely used to assess the progressive collapse performance of reinforced concrete structures. As an alternative to an accurate non–linear dynamic analysis, an energy–based method (EBM) can also be adopted to approximately calculate the dynamic load–bearing capacity curve or the dynamic resistance based on a static capacity curve. However, dynamic effects cannot be explicitly taken into account in the EBM. The model uncertainty associated with the use of the EBM for evaluating the dynamic ultimate capacity of structural frames has not yet been quantified. Knowledge of this model uncertainty is however necessary when applying EBM as part of reliability calculations, for example, in relation to structural robustness quantification. Hence, this article focuses on the evaluation of the performance of the EBM and the quantification of its model uncertainty in the context of reliability–based assessments of progressive or disproportionate collapse. The influences of damping effects and different column removal scenarios are investigated. As a result, it is found that damping effects have a limited influence on the performance of the EBM. In the case of an external column removal scenario, the performance of the EBM is lower as the response is not a single deformation mode according to the results in the frequency domain. However, a good performance is found in the case of an internal column removal scenario in which the assumption of a single deformation mode is found to be sufficiently adequate. Probabilistic models for the model uncertainties related to the use of the EBM compared to direct dynamic analyses are proposed in relation to both the resistances and the associated displacements. Overall, the EBM shows to be an adequate approximation, resulting in a small bias and small standard deviation for its associated model uncertainty.

Traditional design of reinforced concrete structures is usually based on small deformation theories whereas in order to assess the ultimate load bearing capacity of RC structures against progressive collapse significant deformations should be taken into account. The alternate load paths facilitated by membrane action which develop under large deformation can activate a significant strength reserve, redistribute the loads and hence limit the damage progression in the structure. Up to date, no uniform approach is available on how to incorporate these membrane effects in the structural design or verification procedures against progressive collapse. In this article, sensitivity analyses were performed to investigate the effect of the stochastic nature of membrane action on the load bearing capacity for three distinct RC elements, i.e. a beam, one-way slab and two-way slab. It was found that variables which are usually not explicitly taken into account by traditional design methods, such as the axial restraint stiffness and ultimate reinforcement strain have a significant influence on the resistance of the considered elements. Subsequently, the design resistance of the elements was assessed using a probabilistic method and the ECOV method. It was found that the ECOV method and the probabilistic method considering a lognormal distribution provide similar results in case one observes a single failure mode for which the variability of the resistance is limited. In case of multiple failure modes and a large variability of the resistance, a mixed lognormal distribution proved to enable a more accurate design resistance estimation.

If deterministic creep prediction models are compared with actual measurement data, often significant differences can be observed. These inconsistencies are associated with different causes, i.e. model uncertainty, uncertain input parameters, measurement errors and wrongfully applying creep prediction models outside their limitations. First, the physical mechanism causing creep of concrete is not yet fully understood. Therefore, it is very likely that certain influences on creep of concrete are not considered in these prediction models, resulting in systematic model errors. The model errors can be quantified by comparing prediction results with experimental data. Secondly, the stochastic character of the input parameters form an additional source of uncertainty which can be quantified by the variance of the model response. The coefficient of variation in function of time-duration, i.e. the time since the application of the load, is a useful measure to quantify the level of uncertainty. In the literature, statistical analysis by means of numerical simulations are often used for this matter. However, even for specialized sampling techniques, a large amount of samples is necessary to cover the relevant ranges of various input parameters. The aim of the present study is to provide an approximate uncertainty quantification of the creep prediction models given uncertain input parameters. This approximation is based on a Taylor series approach. This approach has the advantage that is does not require numerical simulations nor does it require the knowledge of the probability density function of the input parameters. This method is evaluated and compared with the statistical analysis for several creep prediction models available in literature and design codes.

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.

In 1967–1985, a research campaign comprising a unique set of long-term experimental data on concrete beams was conducted in joint collaboration with four Belgian research institutes to determine the influence of creep and shrinkage on the long-term behavior of reinforced concrete members. The main aim of the research campaign was the determination of the long-term behavior of cracking and deformations subjected to permanent loads considering the influence of the magnitude of the loads and various reinforcement ratios. The objective of this article is twofold: to provide an overview of the measured data of the reinforced beams of the research campaign, which has never been published before, and to propose a simplified calculation method based on available models in literature that can predict the available measurement data. A simplified calculation model is proposed, which accounts for nonlinear creep strains due to high stresses, shrinkage, aging, and cracking in reinforced concrete beams. This numerical method is based on a cross-sectional analysis formulated using the layered Euler-Bernoulli beam theory, allowing fast and accurate predictions of strains, stresses, and deflections as a function of time based on fib Model Code 2010 and EN1992-1-1. The measurements of the beams subjected to high permanent loads during a time period of 4 years are compared to the results evaluated with the proposed simplified calculation model. The results show that the proposed simplified calculation method based on the current models of EN1992-1-1 and fib Model Code 2010 can predict the long-term behavior of reinforced concrete beams subjected to high loads in good agreement with the measurements.