Numerical Modeling and Seismic Analysis of Tall Steel Buildings with Braced Frame Systems

The following paper presents the design and verification steps of tall steel frame structures with braced core, belt and outrigger trusses. The two case study structures refer to sky scrapers of 180m and 300m respectively, located in Istanbul, Turkey. The main objectives of this study are two: firstly designing the buildings through multilevel structural analysis, secondly to compare the results, in terms of seismic response, between response spectrum analysis (RSA) and nonlinear time history analysis (NLTHA). Such comparison has been made with the intention of investigating the relationship between the structure height and the accuracy of RSA predictions, considering that the latter approach tends to underestimate the influence of higher mode effects. In conclusion the capacity curves of the two structures, developed using incremental nonlinear dynamic analysis, are presented as an ulterior way to assess the seismic capacity of such type of high-rise structural systems

Progressive collapse fragility of European reinforced concrete buildings

Structural safety is generally assessed without consideration of abnormal load conditions that may give rise to global system collapse after local failure in one or a few components. Particularly in the case of high-risk structures, Eurocode 1 recommends a systematic risk assessment of the structure, considering either identified threats or unspecified damaging events. Nonetheless, a comprehensive probabilistic assessment of European structures is strongly needed. In such a context, this paper presents the outcomes of fragility analyses performed on reinforced concrete framed buildings, proposing a set of fragility models that can be used for probabilistic assessment and management of the risk of progressive collapse. Gravity-load designed and earthquake-resistant building structures were considered and respectively designed in accordance with Eurocodes 2 and 8. Fiber-based finite element models were developed and analyzed under sudden removal of one or more columns, allowing structural performance and damage propagation to be evaluated. Based upon statistics and probability distribution functions for material properties, geometry, and design loads of the building class under study, a Monte Carlo simulation was performed to generate both 2D and 3D models. Structural performance was assessed by incremental-mass nonlinear dynamic analysis, capturing the attainment of limit states either at sectional or global levels. Probability distribution functions were then fitted to fragility points in order to provide fragility functions at multiple damage states for their use in progressive collapse risk assessment. The analysis results show the significant impact of seismic design rules and secondary beams on progressive collapse fragility

Seismic Acceleration and Displacement Demand Profiles of Non-Structural Elements in Hospital Buildings

The importance of non-structural elements in performance-based seismic design of buildings is presently widely recognized. These elements may significantly affect the functionality of buildings even for low seismic intensities, in particular for the case of critical facilities, such as hospital buildings. One of the most important issues to deal with in the seismic performance assessment of non-structural elements is the definition of the seismic demand. This paper investigates the seismic demand to which the non-structural elements of a case-study hospital building located in a medium–high seismicity region in Italy, are prone. The seismic demand is evaluated for two seismic intensities that correspond to the definition of serviceability limit states, according to Italian and European design and assessment guidelines. Peak floor accelerations, interstorey drifts, absolute acceleration, and relative displacement floor response spectra are estimated through nonlinear time–history analyses. The absolute acceleration floor response spectra are then compared with those obtained from simplified code formulations, highlighting the main shortcomings surrounding the practical application of performance-based seismic design of non-structural elements. The absolute acceleration floor response spectra are then compared with those obtained from simplified code formulations. The results, both in terms of absolute acceleration and relative displacement floor response spectra, highlighted the influence of the higher modes of the structure and the inaccuracy of the code provisions, pointing out the need for more accurate simplified methodologies for the practical application of performance-based seismic design of non-structural elements

Towards Seismic Design of Nonstructural Elements: Italian Code-Compliant Acceleration Floor Response Spectra

Seismic risk reduction of a building system, meant as primary building structure and nonstructural elements (NSEs) as a whole, must rely upon an adequate design of each of these two items. As far as NSEs are concerned, adequate seismic design means understanding of some basic principles and concepts that involve different actors, such as designers, manufacturers, installers, and directors of works. The current Italian Building Code, referred to as NTC18 hereinafter, defines each set of tasks and responsibilities in a sufficiently detailed manner, rendering now evident that achieving the desired performance level stems from a jointed contribution of all actors involved. Bearing in mind that seismic design is nothing else than proportioning properly seismic demand, in terms of acceleration and/or displacement, and the corresponding capacity, this paper gives a synthetic and informative overview on how to evaluate these two parameters. To shed some light on this, the concept of acceleration floor response spectrum (AFRS) is firstly brought in, along with basics of building structure-NSEs interaction, and is then deepened by means of calculation methods. Both the most rigorous method based on nonlinear dynamic simulations and the simplified analytical formulations provided by the NTC18 are briefly discussed and reviewed, trying to make them clearer even to readers with no structural/earthquake engineering background because, as a matter of fact, NSEs are often selected by architects and/or mechanical or electrical engineers. Lastly, a simple case study, representative of a European code-compliant five-storey masonry-infilled reinforced concrete frame building, is presented to examine differences between numerical and analytical AFRS and to quantify accuracy of different NTC18 procedures

Progressive collapse fragility models of European reinforced concrete framed buildings based on pushdown analysis

Structural safety for extreme loads that may cause local damage to single primary components or even the progressive collapse of the structure has been probabilistically assessed in a few studies, hence neglecting uncertainties in loads and system capacity. As such, this paper moves from a deterministic to a probabilistic framework, proposing new progressive collapse fragility models based on pushdown analysis of low-rise, reinforced concrete framed bare structures. Two building classes representative of structures designed for either gravity loads or earthquake resistance in accordance with current European codes were investigated. Monte Carlo simulation was used to generate random realizations of 2D and 3D structural models. Fiber-based finite element models were developed within an open source platform. The primary output consisted of fragility functions for each damage state of interest, given the loss of corner column at the ground floor. The fragility models were compared to those derived through incremental dynamic analysis (IDA) to assess the inaccuracy of progressive collapse fragility functions derived through pushdown analysis. Load capacity predictions provided by those analysis methods were used to develop regression models for a quick estimation of dynamic amplification factor at a given displacement/drift level. The analysis results show a significant influence of both seismic design and secondary beams on robustness of the case-study building classes

Nonlinear material modelling for fibre-based progressive collapse analysis of RC framed buildings

The choice of the most suitable constitutive models for fibre-based progressive collapse analysis of reinforced concrete (RC) structures is still an open issue, mainly because nonlinear material modelling has never been treated as a variable involved in the assessment problem so far. To support analysts in selecting which models could be more representative of the actual inelastic material response for progressive collapse analysis of RC framed buildings, this paper presents a numerical investigation where fibre modelling was integrated with different combinations of stress–strain relationships for concrete and reinforcing steel. A series of pushdown simulations of a two-bay perimeter frame mock-up were then carried out to assess the gravity load capacity under downward displacement. Analysis results are compared with available experimental test data for performance quantification of numerical models, which is based upon an experimentalto- numerical load capacity ratio and overall statistical parameters. Sensitivity to the material modelling approach, load eccentricity and boundary conditions is evaluated, involving strain indicators that can be used in performance-based robustness assessment of RC framed buildings. Finally, a number of parametric analyses are presented to show how the load capacity is influenced by capacity model properties, such as material strengths, beam span length, and span length ratio of asymmetric frames

Macro-Modelling of IP-OoP Interaction in Unreinforced Solid Masonry Infills under Earthquake-Induced Actions: A Review

Unreinforced masonry-infilled reinforced concrete frames are a prevalent taxonomy class not only in the Mediterranean region but also in Europe and worldwide, where buildings of this type abound or are ubiquitous. Thus, somewhat expectedly, various earthquake events and sequences have repeatedly shown the poor seismic behaviour of masonry infill walls, which, in turn, have brought into question issues of the variability, uncertainty, and interaction of in-plane (IP) and out-of-plane (OoP) responses. The latter aspect is examined in this paper, which provides a systematic review concerning the conceptualisation and development of numerical macro-models for simulating the behaviour of solid infill wall panels taking their IP–OoP interaction into account. To this end, the most important parameters involved in the cyclic behaviour of unreinforced solid masonry infill walls are addressed first, and then the main models currently available in the literature are scrutinised and key features discussed, with emphasis posed on issues of accuracy/suitability and easiness or level of complexity/sophistication

Performance limit states for progressive collapse analysis of reinforced concrete framed buildings

The definition of performance limit states for progressive collapse design and assessment of civil structures is an open issue and deserves special attention in view of future building codes and standards. In this study, the main findings of a multilevel sensitivity analysis are presented to characterize the progressive collapse capacity of a selected class of modern European buildings having a reinforced concrete framed structure. After that a group of capacity model properties are assumed as random variables on the basis of earlier studies, the sensitivity of both ultimate load capacity and corresponding maximum and residual drifts to the ultimate steel strain and location of column-removal scenario is assessed. Then, the influence of the capacity model properties on maximum drift demand corresponding to a codecompliant design gravity load is evaluated. Finally, five performance limit states associated with increasing levels of damage are introduced and the corresponding load capacity is quantified under varying capacity model properties. Analysis results indicate a high sensitivity to the ultimate steel strain, column location in plan, beam span and yield steel strength

Seismic Acceleration and Displacement Demand Profiles of Non-Structural Elements in Hospital Buildings

The importance of non-structural elements in performance-based seismic design of buildings is presently widely recognized. These elements may significantly affect the functionality of buildings even for low seismic intensities, in particular for the case of critical facilities, such as hospital buildings. One of the most important issues to deal with in the seismic performance assessment of non-structural elements is the definition of the seismic demand. This paper investigates the seismic demand to which the non-structural elements of a case-study hospital building located in a medium–high seismicity region in Italy, are prone. The seismic demand is evaluated for two seismic intensities that correspond to the definition of serviceability limit states, according to Italian and European design and assessment guidelines. Peak floor accelerations, interstorey drifts, absolute acceleration, and relative displacement floor response spectra are estimated through nonlinear time–history analyses. The absolute acceleration floor response spectra are then compared with those obtained from simplified code formulations, highlighting the main shortcomings surrounding the practical application of performance-based seismic design of non-structural elements. The absolute acceleration floor response spectra are then compared with those obtained from simplified code formulations. The results, both in terms of absolute acceleration and relative displacement floor response spectra, highlighted the influence of the higher modes of the structure and the inaccuracy of the code provisions, pointing out the need for more accurate simplified methodologies for the practical application of performance-based seismic design of non-structural elements