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Advanced Cyclic Assessment of an Existing Steel-Concrete Composite Road Bridge
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The catastrophic effects of recent earthquakes around the world have highlighted the vulnerability of civil buildings; in Europe, most of the existing buildings were built after World War II and since they have largely exceeded their useful life, are characterized by a high vulnerability linked both to the durability of the materials and to the lack of seismic provisions in design. The current trend in constructional field is to design interventions aimed at the seismic and energy improvement of these structures jointly, in order to achieve a sustainable and integrated retrofit intervention. To this aim, the use of 2D orthogonal steel exoskeletons for the seismic strengthening represents a suitable solution, providing at the same time an enhancement in thermal insulation, through a 'double-skin' solution. Due to the growing interest in the design of steel exoskeleton interventions, a detailed dynamic characterization of this external strengthening solution is needed. In this framework, the present work, aims to investigate the effectiveness of the investigated strengthening solution by means of bi-directional non-linear dynamic analyses. Extensive numerical analysis was carried out. The selected case-study is an Italian existing pre-1980 s RC frame building located in Mugnano di Napoli (NA, Italy), which is a medium-high seismic hazard site in Italy (peak ground acceleration equal to 0.156 g). Two alternative strengthening solutions were numerically investigated by means of non-linear time-history and incremental dynamic analyses performed applying fifteen bidirectional ground motions on three-dimensional numerical models. The results of the dynamic non-linear analyses allow to quantify more appropriate performance indicators , both in terms of expected demand and overall collapse capacity, useful for the development of optimization strategy and design of exoskeleton system. The results highlighted that both the investigated strengthening scenarios allow to increase the stiffness of the existing building and allow it to meet the current code safety requirements reducing also the residual inter-storey drift. The obtained results are not strictly related to the investigated case study, but can be extended to all the structures retrofitted that follows the presented design procedure.
Steel moment resisting frames (MRFs) are drift‐sensitive structures and their design is largely influenced by code requirements to limit the lateral displacements and to control the second order effects. Current and next generation of Eurocode 8 recommend different rules to verify the lateral rigidity and resistance of steel MRFs that lead to structures that are substantially different in terms of size of members and overall performance. In order to investigate the influence of the design rules on the seismic behaviour of ductile steel MRFs a parametric numerical study was carried out on 48 structures designed according to Eurocodes as well as US codes, the latter analysed as benchmark, varying the number of storeys, moment resisting spans and seismic intensity. The results of non‐linear static and dynamic analyses show that the structures complaint with current Eurocode 8 are the most expensive and characterised by the greater lateral rigidity and resistance, while MRFs designed in accordance with ASCE7 rules have the smaller profiles and the lower overstrength. The frames designed with rules of the latest version of next generation of Eurocode 8 have an intermediate behaviour even if closer to ASCE7‐compliant structures. These results are also confirmed by the probability of collapse that is smaller than the minimum value specified by EN1990 for the considered consequence class of the examined buildings.
Fatigue performance is often a key aspect when dealing with existing steel structures such as steel bridges or offshore constructions. This issue proves to be more critical as these structures are usually located in aggressive environments and are thus exposed to progressive degradation. Indeed, disruptive phenomena such as corrosion can severely worsen the fatigue performance of the steel components. Currently, the normative standards do not provide a codified procedure for the fatigue checks of steel structures subjected to ongoing corrosion. Within this framework, in this paper a simplified approach for the life-cycle assessment of corroded steel structures is proposed. For this purpose, the concept of “critical corrosion degree” is introduced, allowing the expression of corrosion fatigue checks in a more direct “demand vs. capacity” form with respect to the currently available methods. A first validation of such methodology is reported for the corrosion fatigue tests drawn from the literature. The predicted levels of critical corrosion are in good agreement with the values of artificially induced corrosion (i.e., 4, 8, and 12% of mass loss, respectively), with a maximum relative error of ≈9.3% for the most corroded specimen. Finally, parametrical analyses are performed, highlighting the influence of the model parameters on the corrosion fatigue performance of the steel elements.
This work investigates the error in estimating the Strain Energy Density value through a free coarse mesh model in a finite element analysis. Several numerical simulations were carried out to acquire this value both according to the conventional methodology of the Strain Energy Density method and according to the procedure shown in the present work that can be applied directly in the post‐processing phase of the simulation without requirements for the model. The main advantage of this new procedure is the possibility to apply the method also to those numerical simulations already done for other purposes decreasing considerably the effort of the researcher and the calculation time. The numerical simulations carried out show that the Strain Energy Density value can be estimated with an error of 1% also with a free coarse mesh model having a mesh size near the notch tip of 1/8 of the control volume radius.
The seismic performance of cold formed steel (CFS) strap-braced stud walls was evaluated in previous tests by monotonic and cyclic tests on full-scale specimens designed according to the elastic approach. Even if an elastic seismic design approach was followed, based on the experimental results, these walls exhibited a low but not negligible energy dissipative behaviour. Therefore, the current paper is focused on the seismic performance evaluation of low energy dissipative CFS strap-braced stud walls. In particular, starting from the critical analysis of current seismic codes, the paper presents a procedure for the prediction of strength and stiffness according to design rules provided by EN 1993-1-3, which was used to design case studies. Then, a test-based evaluation of behaviour was performed and values of the behaviour factor from 1.52 to 3.61 were obtained for low energy dissipative CFS strap-braced structures.
本书介绍了当前材料与结构抗疲劳设计领域的最新研究成果及进展（包括裂纹闭合、变幅加载、鱼眼现象等），重点分章节阐述了低周、超高周疲劳裂纹萌生和扩展寿命的实验技术及估算方法，同时从断裂力学（短裂纹行为、裂尖塑性区等）和实验观测（如Paris唯象模型等）角度揭示了材料疲劳损伤演变的微观（如夹杂、裂纹等缺陷）和外部机制（如加载模式、腐蚀、环境介质等），尤其给出了多种材料（金属单晶、合金及非金属等）详实的实测数据。
本书内容丰富完整、涵盖学科范围广阔，可供从事材料与结构抗疲劳断裂设计的工程技术人员、相关专业的高校教师及高年级本科生或者研究生参考。
The last 15 years have seen fundamental advances in both the definition of the hazard and on the criteria in codified seismic design, creating a situation whereby even relatively modern constructions may represent an unchecked risk. This article deals with a case of particular relevance, i.e., the seismic risk of bridges and viaducts on the Italian highway network. After a brief outline of the present state of the network, the article concentrates on the solutions adopted in the upgrading, for increasing both the traffic and the seismic capacities, through the illustration of two case studies. The second, main part of the article deals in more detail with one of the case studies, examining alternative design options and the corresponding analytical aspects. Finally, the opportunity is taken to investigate the relevance of two aspects not commonly taken into account in design practice, i.e., differential support motion and soil-structure interaction.
In the present paper, the influence of the mean stress on the fatigue behavior of smooth and notched medium carbon steel components (C45 grade) is investigated through conventional and more advanced design methodologies. Original experimental data are provided for two notched specimen geometries and three different high load ratios (R = {0.7, 0.8, 0.9}). To assess the effect of the mean stress in a larger range, the bulk of fatigue data has been widened through more than 150 tests on C45 steel drawn from literature. The considered experimental data ranged between a load ratio of −2 and 0.9. The mean stress effect has been assessed in a first attempt through well‐established literature models resulting in a data correlation characterized by a large scatter. Finally, an attempt to uniquely consider both geometrical and load ratio effects has been performed through a numerical application of the strain energy density (SED) method. Elevated load ratios (R ≥ 0.7) prove to be highly detrimental for C45 steel with a fatigue life reduction of up to 3 orders of magnitude; however, this effect can be considered with a sufficient degree of accuracy through the SED method.
The steel exoskeleton systems are widespread structures applied on the external perimeter of an existing building designed to absorb the horizontal actions providing at the same time useful support for energy efficiency upgrading and architectural restyling. The growing interest in the use of exoskeletons is due to the possibility of creating an integrated retrofit (structural, energetic, formal and functional), combining “structural safety” with the concepts of “deep renovation”. Among many solutions present in the literature, in the present work, the steel exoskeletons placed orthogonally to the facade of the existing building have been analysed; this structural typology has the advantage to allow rapid retrofit execution without interfering with the activities carried out within the existing structure. In this framework, the aim of the present work is the introduction of a detailed step-by-step design procedure for steel exoskeletons systems adopted for seismic retrofit of existing buildings looking also at the accessibility and operability aspects. Each step of the procedure was individually explained and it was applied for the design of a strengthening intervention on a real single-storey steel industrial building. The existing building is located in Nusco (Av, Italy) which is classified as in medium–high seismic intensity area which corresponds a peak ground acceleration (ag) equal to 0.238 g. Non-linear analyses were conducted to assess the existing structure’s performance as well as to verify the effectiveness of the strengthening solutions designed. The use of the exoskeleton systems allows to strongly increase both the elastic stiffness and the resistance of the investigated industrial building without interrupting its product activity.
High strength steel (HSS) wire ropes are widely used in civil engineering to act as hangers or bridge cables. These kinds of structural members are particularly prone to both corrosion and fatigue damage owing to typical exposition and load conditions. Within this framework, in the present paper a comprehensive state of the art review dealing with literature and normative models for fatigue checks of corroded wire ropes is presented. Most peculiar aspects related to correct fatigue demand and strength evaluation in wire ropes are discussed on the basis of over 40 research works.
The mechanical features of beam-to-column joints may highly affect the resistance of steel structures at elevated temperature. Therefore, the behaviour of steel joints under elevated temperature has been widely studied. However, few studies have been specifically devoted at investigating the response of bolted end-plate joints under elevated temperature that were formerly damaged by seismic actions. Hence, the study summarized in this article was addressed to: (i) to analyse the response of seismically prequalified extended stiffened end-plate joints under high temperature; (ii) study the influence of the seismic damage on performance of this type of joints at elevated temperature. Parametric finite element simulations were carried out to achieve these purposes varying the dimensions of members, the design criteria of the joints, the vertical loads, the presence of lateral-torsional
restraints and the level of imposed damage. The obtained results in terms of moment-rotation-temperature characteristics and pattern of plastic deformations showed in which terms the type of cyclic damage may affect the performance of the joints under high temperature as well as the influence of the efficiency of bracings
to restrain the beams against lateral-torsional buckling.
Cold formed steel (CFS) wood sheathed shear walls are one of the most often used type of shear walls applied as the main lateral force resisting system (LFRS) in a lightweight steel (LWS) building. This paper evaluates the behavior factor used in the seismic design according to Eurocodes for buildings equipped with CFS wood sheathed shear walls as the main LFRS. FEMA P695 methodology is used to evaluate the behavior factor. A set of archetypes are selected and designed, which reflected the applicable building heights, seismic hazard and occupancies for LWS buildings in Europe. Nonlinear three-dimensional finite element (FE) models of building archetypes are developed in OpenSees software using the individual shear wall models, which are calibrated on results from tests on individual shear wall specimens. The resilience of building models against collapse is checked by analyzing the models using lateral static (pushover) and incremental dynamic analysis (IDA). Uncertainties arising from test and design information, numerical modelling and archetype response variability to different ground motions is also taken into account. Based on the collapse performance evaluation results, a value of 2.5 for behavior factor, used in the design, is found to provide appropriate level of safety against the collapse in LWS buildings equipped with CFS wood sheathed shear walls.
Hot dip galvanized steel bolted joints has been tested under fatigue loading to evaluate the effect of galvanizing coating on the fatigue strength of S355 structural steel. The experimental results showed that the decrease of the fatigue life of coated specimens in comparison with that of uncoated joints is very limited and the results are in good agreement with Eurocode detail category, without substantial reductions. The procedure for coating and preparation of the bolted joints is described in detail in this paper providing a useful tool for engineers involved in similar practical applications. The experimental results are compared with the previously published data on central hole notched galvanized and not treated specimens characterized by the same geometry.
In five chapters, this volume presents recent developments in fatigue assessment. In the first chapter, a generalized Neuber concept of fictitious notch rounding is presented where the microstructural support factors depend on the notch opening angle besides the loading mode. The second chapter specifies the notch stress factor including the strain energy density and J-integral concept while the SED approach is applied to common fillet welded joints and to thin-sheet lap welded joints in the third chapter. The forth chapter analyses elastic-plastic deformations in the near crack tip zone and discusses driving force parameters. The last chapter discusses thermomechanical fatigue, stress, and strain ranges. © Springer-Verlag Berlin Heidelberg 2013. All rights are reserved.
This study focuses on the evaluation of the seismic vulnerability of the Italian roadway bridge stock, within the framework of a Civil Protection sponsored project. A comprehensive database of existing bridges (17,000 bridges with different level of knowledge) was implemented. At the core of the study stands a procedure for automatically carrying out state-of-the-art analytical evaluation of fragility curves for two performance levels damage and collapse on an individual bridge basis. A WebGIS was developed to handle data and results. The main outputs are maps of bridge seismic risk (from the fragilities and the hazard maps) at the national level and real-time scenario damage-probability maps (from the fragilities and the scenario shake maps). In the latter case, the WebGIS also performs network analysis to identify routes to be followed by rescue teams. Consistency of the fragility derivation over the entire bridge stock is regarded as a major advantage of the adopted approach.
Brittle failure of components weakened by cracks or sharp and blunt V-notches is a topic of active and continuous research. It is attractive for all researchers who face the problem of fracture of materials under different loading conditions and deals with a large number of applications in different engineering fields, not only with the mechanical one. This topic is significant in all the cases where intrinsic defects of the material or geometrical discontinuities give rise to localized stress concentration which, in brittle materials, may generate a crack leading to catastrophic failure or to a shortening of the assessed structural life. Whereas cracks are viewed as unpleasant entities in most engineering materials, U- and V-notches of different acuities are sometimes deliberately introduced in design and manufacturing of structural components.
Dealing with brittle failure of notched components and summarizing some recent experimental results reported in the literature, the main aim of the present contribution is to present a review of some local approaches applicable near stress raisers both sharp and blunt. The reviewed criteria allowed the present authors to develop a new approach based on the volume strain energy density (SED), which has been recently applied to assess the brittle failure of a large number of materials. The main features of the SED approach are outlined in the paper and its peculiarities and advantages accurately underlined. Some examples of applications are reported, as well. The present review is based on the authors’ experience over more than 15 years and the contents of their personal library. It is not a dispassionate literature survey.
In the presence of sharp V-notches the stress distributions are singular and the intensity of the stress fields is given in terms of the notch stress intensity factors which are largely used in the literature for fatigue strength assessments of welded joints and other notched components. Very refined meshes are necessary to determine directly the NSIFs from the local stress distributions. Refined meshes are not necessary when the aim of the finite element analysis is to determine the mean value of the local strain energy density on a control volume surrounding the points of stress singularity. The SED in fact can be derived directly from nodal displacements, so that also coarse meshes are able to give sufficiently accurate values for it.The link between local SED and NSIFs is discussed with reference to some typical welded joints and to plates weakened by sharp V-notches. The SED-based procedure is found to be useful to determine theoretical stress concentration factors for holes and blunt U- and V-shaped notches.
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