Spectrum-to-spectrum methods for the generation of elastic floor acceleration spectra

In seismic codes, the acceleration demand of nonstructural components is commonly expressed in terms of floor response spectra and estimated by means of simple predictive equations. By using the latter, response-history analysis of the structure is not required, being floor spectra calculated directly from the peak ground acceleration expected at the site. The price for this simplicity in the method used for the estimation of floor spectra is the generally poor approximation of the obtained predictions. Codes’ equations, in fact, do not explicitly account for important factors influencing floor spectra, such as the contribution of the higher modes of vibration of the structure and the actual value of the nonstructural components’ damping ratio. Alternative spectrum-to-spectrum methods for direct generation of floor spectra have been proposed, which include these factors and improve the accuracy of the predictions. Different approaches have been used and several methods developed. Despite large research effort, however, a comparative evaluation of the currently available proposals is still lacking. The objective of this paper is to fill this gap, by reviewing selected proposals representative of practice-oriented spectrum-to-spectrum methods. A case study consisting in a six-story frame is analyzed and predictions obtained with the investigated methods are compared with exact floor spectra derived from time-history analyses of the structure, as well as spectra calculated using the Eurocode 8 equation

Comparative assessment of strut models for the modelling of in-plane seismic response of infill walls

The influence of infills on the seismic response of frame structures has long been recognised. Typically, stiffness and strength of the infill and connections between infill and frame are such that the infill affects the global seismic behaviour of the structure. Hence, the presence of infills should be considered in the analysis and design of new buildings and in the seismic assessment of existing ones. To this aim, simple models for infill walls, such as the equivalent diagonal no-tension strut model, have been developed in the last decades. The objective of the present study is to assess the validity of different strut models. To this aim, 162 experimental tests available in the literature are considered. The data set includes both reinforced concrete and steel frames, as well as confined masonry structures. The mechanical characteristics of masonry and the boundary conditions between frames and infills of the test specimens take into account a large set of situations, reflecting the great variability in the materials and in the construction techniques adopted in different countries. Moreover, the type of tests and the related results are not uniform; in some cases monotonic experiments are performed, whereas in other cases cyclic tests are carried out. As expected, the presence of different types of infill-frame systems results in a large scatter of the data. However, the comparison between experimental results and predictions show that, on the average, the infill strength can be adequately estimated by resorting to the strut model whereas major uncertainties are found for the stiffness prediction

3D printed geopolymeric lattices: Effect of different filler materials on mechanical properties

Our group developed mixtures based on geopolymer for additive manufacturing of porous components via direct ink writing (DIW). We optimized the rheological properties in order to obtain suitable inks for the production of highly porous lattices. It should be noted that, as geopolymer mixtures are subjected to ongoing poly-condensation reactions, their viscosity changes with time in what can be seen as a 4D printing process. Different materials were added to the mixture, such as glass and plastic fibers, as well as fillers like sand, to produce innovative 3D printed geopolymeric composites. The influence of these materials on the mechanical properties was evaluated

Reliability‐based partial factors for seismic design and assessment consistent with second‐generation Eurocode 8

Second-generation Eurocode 8 introduces displacement-based design alongside the traditional force-based one with reduced equivalent forces; it provides resistance models needed to apply the approach to reinforced concrete, steel and composite frame or wall-frame structures: chord rotation for members, and shear strength for members and joints; finally, it declares a target reliability level for near collapse. This paper discusses the formulation of partial factors to achieve a target reliability when using the displacement-based approach. The factors are derived following a traditional, time-invariant way of performing code-calibration for other non-seismic design situations, starting from the probability distributions for the seismic action effect and the resistance. It is shown that the problem can be simplified to one with two Lognormal variables, a uniform reliability can be achieved over the calibration space when both a load-side and a resistance-side factor are used, and constant sensitivity coefficients can be used, with values different from the original König and Hosser ones and reflecting the larger weight of the action-related uncertainty in the seismic case. The problem with this otherwise good result is that a partial factor on the load side must be used and, further, that its value is site dependent. This approach is not in line with current European practice, thus a corrected site-independent resistance-side partial factor is proposed, which is compatible with the framework of Eurocode 8. As a further advantage, the proposed format coincides with that proposed in the second-generation EN1990/EN1992 for the assessment of existing structures under non-seismic design situations

Seismic Vulnerability of the Italian Roadway Bridge Stock

This study focuses on the seismic vulnerability evaluation 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

Risk‐based optimization of concentrically braced tall timber buildings: Derivative free optimization algorithm

Mass timber materials are attractive alternatives for tall-timber buildings (TBs), where the need for sustainability is apparent. Innovative structural systems and design methodologies are needed to fulfil performance requirements according to modern performance based approaches. This paper deals with the design and optimization of buckling restrained braces as earthquake protection system for tall-TBs through risk-based design procedure. This procedure controls the mean annual frequency of exceedance of several limit states evaluated through a SAC-FEMA approach and using response spectrum linear analyses on linearized models for demand assessment. The features of the optimization procedure and the linearized models are shown through an application on a 20-story mass-TB located in a high seismic zone. The optimization is executed through a derivative-free algorithm, the generalized pattern Search, adopting several solution strategies whose efficiency and effectiveness for this kind of applications are shown and discussed. Finally, the results are compared and validated through the execution of non-linear analyses within a multiple stripe framework

Porous Geopolymer Components

Geopolymers are based on an inorganic 3D network of alumino-silicate units usually synthesized through reaction of alumino-silicate powders in presence of a silicate alkaline solution. The rheological characteristics of the reactive mixtures and the fact that these systems can consolidate at low or even room temperature, together with their intrinsic micro- and meso-porosity and mechanical properties, are the reason why they are considered for a wide range of applications, such as construction materials, thermal insulation, filters, adsorbers and so on. Open cell alkali or acid-based geopolymer foams were produced by direct foaming using different fabrication approaches. Potassium-based foams with a porosity up to 85 vol% were obtained from metakaolin, potassium silicate and potassium hydroxide, while metakaolin and phosphoric acid were used to fabricate foams containing an aluminum phosphate crystal phase already after synthesis at room temperature, and a total porosity of ~80 vol%. The strength of the foams depended on the porosity of the components as well as the heat treatment temperature. Components with designed, non-stochastic porosity were also produced by additive manufacturing, specifically Direct Ink Writing. Paste with suitable pseudo-plastic rheology were developed and we fabricated components with overhangs and spanning features, including highly porous 3D lattices

Approximate Bayesian Network Formulation for the Rapid Loss Assessment of Real-World Infrastructure Systems

This paper proposes to learn an approximate Bayesian Network (BN) model from Monte-Carlo simulations of an infrastructure system exposed to seismic hazard. Exploiting preliminary physical simulations has the twofold benefit of building a drastically simplified BN and of predicting complex system performance metrics. While the approximate BN cannot yield exact probabilities for predictive analyses, its use in backward analyses based on evidenced variables yields promising results as a decision support tool for post-earthquake rapid response. Only a reduced set of infrastructure components, whose importance is ranked through a random forest algorithm, is selected to predict the performance of the system. Further, owing to the higher importance of evidenced nodes, the ranking method is enhanced with a recursive evidence-driven BN-building algorithm, which iteratively inserts evidenced components into the subset identified by the random forest algorithm. This approach is applied to a French road network, where only 5 to 10 components out of 58 are kept to estimate the distribution of system performance metrics that are based on traffic flow. Sensitivity studies on the number of selected components, the number of off-line simulation runs and the discretization of variables reveal that the reduced BN applied to this specific example generates trustworthy estimates

On the seismic performance of straight integral abutment bridges: From advanced numerical modelling to a practice‐oriented analysis method

The seismic performance of integral abutment bridges (IABs) is affected by the interaction with the surrounding soil, and specifically by the development of interaction forces in the embankment-abutment and soil-piles systems. In principle, these effects could be evaluated by means of highly demanding numerical computations that, however, can be carried out only for detailed studies of specific cases. By contrast, a low-demanding analysis method is needed for a design-oriented assessment of the longitudinal seismic performance of IABs. To this purpose, the present paper describes a design technique in which the frequency- and amplitude-dependency of the soil-structure interaction is modelled in a simplified manner. Specifically, the method consists of a time-domain analysis of a simplified soil-bridge model, in which soil-structure interaction is simulated by means of distributed nonlinear springs connecting a free-field ground response analysis model to the structural system. The results of this simplified method are validated against the results of advanced numerical analyses, considering different seismic scenarios. In its present state of development, the proposed simplified nonlinear model can be used for an efficient evaluation of the longitudinal response of straight IABs and can constitute a starting point for a prospective generalisation to three-dimensional response

Eurocode 8: Seismic Design of Buildings - Worked examples

This document is a Technical Report with worked examples for seismic design of buildings following the Eurocodes. It summarizes important points of the Eurocode 8 for the seismic design of concrete and steel buildings including foundations utilizing a common generic building as a basis. An overview of EN 1998 with focus on the performance requirements and compliance criteria for structures, ground conditions and seismic actions is presented at the first section. An introduction to the example reinforced concrete building with its geometrical and material properties as well as the main assumptions for analysis and the detailed structural analysis calculations are presented in the second chapter of the report. Specific rules for design of the building for ductility and the design of concrete foundation elements are presented in the following chapters. For the sake of completeness, the details of design and detailing of the same example as a steel building with three different configurations, namely; with (i) steel moment resisting frames, (ii) composite steel concrete moment resisting frames, and (iii) composite steel concrete frames with eccentric and concentric bracings is also presented afterwards. Key concepts of base isolation is summarized by utilizing the example building. Seismic performance assessment and retrofitting according to EN 1998-Part 3 is explained as the last past of the report. The reinforced concrete/steel building as worked example analyzed in this report was prepared and presented at the workshop “Eurocode 8: Seismic Design of Buildings” that was held on 10-11 February 2011 in Lisbon, Portugal. The workshop was organized by JRC with the support of DG ENTR and CEN and in collaboration with CEN/TC250/Sub-Committee 8 and the National Laboratory for Structural Design (Laboratorio Nacional de Engenharia Civil - LNEC, Lisbon). The document is part of the Report Series ‘Support to the implementation, harmonization and further development of the Eurocodes’ prepared by JRC in collaboration with DG ENTR and CEN/TC250 “Structural Eurocodes”.JRC.G.5-European laboratory for structural assessmen