Numerical Simulation of the Behavior of Cracked Reinforced Concrete Members

Refined non-linear static or dynamic analyses of reinforced concrete structures require the knowledge of the actual force-displacement or bending moment-rotation curves of each structural member, which depend on the crack widths and on the crack pattern, and after all on the slip between concrete and reinforcing steel. For this reason the definition of improved local models taking into account all these local aspects is a fundamental prerequisite for advanced assessment of r.c. structures. A numerical procedure which allows to predict the relative displacement between steel reinforcement and the surrounding concrete in a reinforced concrete element, once assigned the stress in the naked steel bar and the bond-slip law is discussed. The method provides as final outcomes the sequence of crack openings and the individual crack widths, regardless of the particular bond-slip correlation adopted. The proposed procedure is implemented referring to two relevant experimental case studies, demonstrating that it is able to predict satisfactorily actual strain fields and slips along the investigated reinforced concrete elements

Reliability of roof structures subjected to snow loads

A proper evaluation of snow loads on roofs is crucial for structural design especially to guarantee an adequate reliability level of lightweight roof structures. The definition of roof snow load in structural codes is based on both the evaluation of ground snow loads and conversion factors from ground to roof load, which are function of the roof’s geometry, its exposure to wind and its thermal properties. However, reference values of roof snow loads are based only on an extreme value analysis carried out to derive characteristic values of ground snow load, while conversion factors are considered as deterministic quantities due to the lack of the data. In this paper, first a methodology to evaluate the reference value of roof snow load is presented based on the definition of probability density functions for ground snow loads and conversion factors accounting for roof’s geometry and its exposure to wind. The results lead to the definition of a design conversion factor which depend on the coefficient of variation of ground snow loads and are compared with the constant values provided by the Eurocode models, in EN1991-1-3:2003. Then, structural reliability is assessed for reference steel and timber structures located in different sites. Considering different proportions between variable and permanent loads, the reliability of flat roofs designed according to Eurocode provisions, provided by the current version and the new draft, is finally compared with the required target reliability levels

Probabilistic methodology for the assessment of the impact of climate change on structural safety

Structural design is often governed by climatic actions, such as snow, wind, thermal and atmospheric icing loads, that will occur during the design service life. Since in structural standards climatic actions are usually derived from historical data series assuming stationary climate, alterations induced by climate change should be specifically evaluated, also to assess their influence on structural reliability. In the paper, a probabilistic methodology for the assessment of climate change impact on long-term structural reliability is presented, based on the analysis of observed data series and climate projections, provided by high resolution climate models. Factor of change uncertainty maps for climate extremes are derived starting from the analysis of weather series generated by an ad hoc weather generator, which considers homogenous populations of data suitably derived from climate model output. The long-term structural reliability is then assessed for reference structures at a given site considering the non-stationary nature of climatic actions by means of the pdfs of changes in extreme value parameters. Specifically, variations of the failure probability with time due to climate change are evaluated by moving time windows of forty years considering changes in mean load intensity and standard deviation of yearly maxima of the investigated climatic action. The results show the capability of the method to assess the impact of climate change on structural safety, highlighting the necessity of adaptation measure to maintain the required target reliability of the structure during its life

Extreme ground snow loads in Europe from 1951 to 2100

Lightweight roofs are extremely sensitive to extreme snow loads, as confirmed by recently occurring failures all over Europe. Obviously, the problem is further emphasized in warmer climatic areas, where low design values are generally foreseen for snow loads. Like other climatic actions, representative values of snow loads provided in structural codes are usually derived by means of suitable elaborations of extreme statistics, assuming climate stationarity over time. As climate change impacts are becoming more and more evident over time, that hypothesis is becoming controversial, so that suitable adaptation strategies aiming to define climate resilient design loads need to be implemented. In the paper, past and future trends of ground snow load in Europe are assessed for the period 1950–2100, starting from high-resolution climate simulations, recently issued by the CORDEX program. Maps of representative values of snow loads adopted for structural design, associated with an annual probability of exceedance p = 2%, are elaborated for Europe. Referring to the historical period, the obtained maps are critically compared with the current European maps based on observations. Factors of change maps, referred to subsequent time windows are presented considering RCP4.5 and RCP8.5 emission trajectories, corresponding to medium and maximum greenhouse gas concentration scenarios. Factors of change are thus evaluated considering suitably selected weather stations in Switzerland and Germany, for which high quality point measurements, sufficiently extended over time are available. Focusing on the investigated weather stations, the study demonstrates that climate models can appropriately reproduce historical trends and that a decrease of characteristic values of the snow loads is expected over time. However, it must be remarked that, if on one hand the mean value of the annual maxima tends to reduce, on the other hand, its standard deviation tends to increase, locally leading to an increase of the extreme values, which should be duly considered in the evaluation of structural reliability over time

Climate Change: impact on snow loads on structures

A general procedure to evaluate future trends in snow loads on structures is illustrated aiming to study influences of climate change at European scale, to assess its impact on the design of new structures as well as on the reliability levels of existing ones, also in view of the next revision of the Eurocodes. Analysing high quality registered meteorological data of daily temperatures, rain and snow precipitations in nine Italian weather stations, conditional probability functions of occurrence of snow precipitation, accumulation and melting have been preliminarily determined as functions of daily air temperatures. By means of Monte Carlo simulations and based upon daily output of climate models (daily max. and min. temperatures and water precipitation) yearly maxima of snow loads for various time intervals of 40 years in the period 1980-2100 have been simulated, deriving, via the extreme value theory, the characteristic ground snow loads at the sites. Then, the proposed procedure has been implemented in a more general methodology for snow map updating, in such a way that the influence of gridded data of precipitation, predicted by global climate models, on extreme values of snow loads is duly assessed. Preliminary results demonstrate that the outlined procedure is very promising and allows to estimate the evolution of characteristic ground snow loads and to define updated ground snow load maps for different climate models and scenarios

Towards new European snow load map: Support to policies and standards for sustainable construction

The Mandate M/515 of the European Commission to CEN requested the assessment of the climate change implications for the Eurocodes, the European standards for structural design. The European Commission Mandate M/526 requested the European Standards Organisations (ESOs) to contribute to building and maintaining a more climate resilient infrastructure throughout the EU in the three priority sectors: transport infrastructure, energy infrastructure, and buildings/construction. To proceed with the envisaged adaptation of the European standards to the implications of climate change, the expected changes in the climatic loading shall be assessed in terms of the Eurocodes concept for the characteristic values of the variable climatic actions. The present report justifies the need of a European research project to develop an advanced procedure for deriving snow load on structures, taking into account climate change projections, and to set up a new European snow load map based on this procedure

Statistical Parameters of Steel Rebars of Reinforced Concrete Existing Structures

Historical and cognitive investigations supported by in-situ and/or laboratory tests are needed for a robust reliability assessment of existing structures. Indeed, an adequate knowledge of material properties and their statistical description is the basis for carrying out accurate reliability analyses and verifications on the investigated structures. In this paper, a procedure for the definition of pdfs of mechanical parameters of steel rebars is proposed based on secondary experimental test data. This information is very helpful for the reliability assessment of existing r.c. buildings, where estimation of statistical parameters of mechanical properties of steel reinforcement is very difficult. In fact. It must be highlighted on the one hand that direct information about the examined structure are commonly not sufficient, on the other hand that the number of rebar samples extracted from the structure, if available, is so limited that it does not allow a complete statistical analysis. The first step has been the collection of experimental acceptance tests carried out by Department of Civil and Industrial Engineering of University of Pisa on steel rebars of reinforced concrete (r.c.) structures during the 1960s. The yield strength and the tensile strength are extrapolated for each sample defining a significant database of experimental test results for existing r.c. structures. Then, probability distribution models for the mechanical properties of steel reinforcement have been defined as already done by the authors for concrete strength. A cluster analysis has been carried out based on the Gaussian Mixture Model applying the Expectation-Maximization algorithm to identify homogeneous material classes and their associated pdfs of material mechanical parameters. The main advantage of proposed procedure consists in its “blindness”, In fact, not requiring subjective information like pre-classification of data, the methodology is not sensitive to alterations caused by engineering judgement or by inexact identification of declared strength class of the tested samples, due for example to downgraded materials

influence of mechanical parameters on non linear static analysis of masonry buildings a relevant case study

Abstract In seismic zones, suitable procedures to assess the seismic vulnerability of existing buildings are necessary also in view of optimal planning of interventions. Starting from the agreement between the Municipality of Florence and the Department of Civil and Industrial Engineering of the University of Pisa, a research program is ongoing, devoted to setup a simplified, fast but reliable procedure for the evaluation of seismic performance of masonry buildings. In this paper, a simplified non-linear pushover type method for the verification of unreinforced multi-story masonry buildings with both deformable and non-deformable slabs is presented, starting from some of the basic assumptions of the POR method. Various tests on the procedure show that the method is able to give results that are comparable with those obtained by the classical pushover analysis performed on equivalent frame models. The intuitiveness of the method and the low computational effort required by the new algorithm allow the evaluation of the sensitivity of non-linear static analysis regarding the definition of mechanical parameters. In particular, the relevant influence of the modulus of elasticity as well as the ultimate inter-story displacement assumed for masonry walls on the assessment of seismic performance are discussed in detail. The results are presented for a significant case study, the Primary School "G. Carducci" in Florence, a four-story masonry building, with a horseshoe layout where lateral appendixes detached from the central block

Fatigue behaviour of composite timber-concrete beams

Refurbishment of existing buildings often claims for strengthening and stiffening of timber floors. To avoid too heavy interventions, this need is particularly relevant in seismic zones and/or for historical buildings, not only to preserve historical value, but also to contain the masses. A solution commonly adopted is to substitute the screed with a thin reinforced concrete or lightweight reinforced concrete slab duly connected to the timber beams, in such a way that a composite timber-concrete floor is obtained, granting also a sufficient rigidity in the horizontal plane. Moreover, this solution has also the advantage to improve the acoustic performance. Of course, the behavior of the composite structure depends on the rigidity of the shear connections. Since several type of shear connectors are available, the experimental assessment of its static and fatigue behavior is a prerequisite for a suitable design of the intervention. Aiming to compare their performances, an ad hoc experimental study has been carried out on three different types of shear connectors. The fatigue tests have been performed on a composite wood-concrete beam. During each test, 15000 loading-unloading cycles have been applied, recording the deformations and the relative slip. After completion of the load cycles, static load has been applied till to collapse. In the paper, the experimental tests and results are widely discussed, also in comparison with commonly used theoretical models, and relevant conclusions are draw

Climate Change Impact on Corrosion of Reinforced Concrete Bridges and Their Seismic Performance

As a consequence of climate change impact, a significant variation in terms of temperature, atmospheric humidity, and carbon dioxide concentration levels is happening. This condition leads to several negative effects on the safety and the life cycle of existing concrete structures, such as the increase in the rate of material degradation, due to corrosion phenomena. In fact, the presence of carbonation and corrosion phenomena significantly influence the load-bearing capacity of existing reinforced concrete (RC) structures, under both static and dynamic loads. Among the wide range of existing RC constructions, bridges stand out for their importance. Furthermore, as structures directly exposed to the weather effects, they are more susceptible to these phenomena. In this paper, the influence of corrosion on existing RC motorway viaducts’ seismic behavior, considering the impact of climate change, is investigated, by means of an efficient procedure based on the implementation of 3D simplified finite element models and the use of analytical relations to obtain the amount of reduction in the steel reinforcement area as a function of the age of the bridge and of the different corrosion scenarios analyzed. Several scenarios for the expected variations in CO2 concentrations, temperature, and relative humidity are evaluated, considering that most of the viaducts present in the Italian motorway network were built between the 1960s and the 1970s. The results obtained using the projection of climate change impacts are compared with those calculated considering the corrosion scenarios resulting from the DuraCrete research project, to understand if the evolution of climate change leads to worse scenarios than those previously assessed