Roberto Nascimbene

Seismic Performance of Precast Industrial Facilities Following Major Earthquakes in the Italian Territory

Recent major earthquakes in the Italian territory have reaffirmed the seismic vulnerability of precast industrial buildings typical of past Italian building practices, highlighting structural deficiencies observed during previous events and primarily related to the transfer of horizontal forces between structural and nonstructural elements. An intrinsic lack of shear and ductility capacity has been observed in simply supported beam-to-joist and beam-to-column connections, primarily constituted by vertical steel dowels or solely relying on shear friction, with or without neoprene pads. These connections were designed neglecting seismic loads and their premature failure was observed during recent seismic events to cause a loss of support of beam elements, owing to the relative movements of elements, and the collapse of part of the buildings, primarily the roof. The seismic displacement demand of the industrial buildings under consideration is larger than traditional RC frame structures owing to their higher flexibility, according to both higher interstory height and to a cantilevered static scheme. Furthermore, this high flexibility may also result in displacement incompatibility between structural and nonstructural elements, such as precast cladding panels, causing their connection failure. On the basis of detailed field observations on a relevant number of buildings, collected just after the earthquakes, seven representative industrial facilities are examined to outline the primary vulnerabilities of one-story precast concrete structures not designed and detailed for seismic loads.

Review of Design Parameters of Concentrically Braced Frames with RHS Shape Braces

Past experimental data are collected to review the essential parameters of the design of ductile concentrically braced frames subjected to earthquake loading. The initial buckling load, the effective length of a brace, the degradation of compression resistance under cyclic loading, and the out-of-plane brace deformation will be examined in this study. The proposed equations for these parameters in the current seismic provisions are compared to the experimental data. In addition, new design equations are proposed for each parameter and verified with the experimental data. The current research is limited to rectangular hollow sections (RHS) because of their ease of connection to existing frames and their high radius of gyration compared to the other sections, such as wide-flange and double-tee sections.

Strain Life Analysis at Low-Cycle Fatigue on Concentrically Braced Steel Structures with RHS Shape Braces

The seismic performance of concentrically braced frame systems, with braces designed in order to buckle inelastically under a severe earthquake loading, mainly depends on the hysteretic response of the steel braces. As observed in past earthquakes and in experimental programs available in the literature, rectangular hollow section shape braces are more susceptible to early fatigue failures compared to other section shapes. This research, in particular, is focused on the evaluation of the hysteretic response of rectangular hollow section shape braces through appropriate low-cycle fatigue model implemented in a fiber-based element computer program. Firstly, the main influence regarding the number of integration points, number of subdivisions for single brace members, and initial camber on the local response of fiber-based inelastic beam-column element models based on force formulation is investigated. Secondly, the peak strain distribution is used to calibrate the input parameters of the low-cycle fatigue material model. Several data obtained from past experimental programs developed at the Universities of Montreal, Washington, and Calgary are also considered. Finally, an eight-story steel building is designed in order to evaluate the performance levels through incremental time history analyses.

Seismic Vulnerability Assessment of an Infilled Reinforced Concrete Frame Structure Designed for Gravity Loads

In the last decades, particular attention has been paid to the seismic vulnerability of existing reinforced concrete buildings designed for gravity loads only. Such buildings, designed before the introduction of capacity design in modern seismic codes, are very common, particularly in seismic prone countries of the Mediterranean area. Due to poor detailing and lacking of capacity design principles, high vulnerability has been highlighted in several past studies. In this article, inadequate seismic response and peculiar damage pattern are investigated by means of shake table tests performed on a 1:2 scaled 3-story infilled prototype. Particular attention is given to the role of beam-column joints and frame-panel interaction. The effectiveness of the EC8-based assessment approach is then evaluated; both linear and nonlinear numerical models, with different levels of sophistication, have been implemented in order to explore their behavioral aspects.

Seismic Performance of Brace-Beam-Column Connections in Concentrically Braced Frames

In typical brace-beam-column connections of concentrically braced frames (CBFs) with tubular braces, a gusset plate is used to connect the brace to the beam and column. The slotted tubular brace is welded to the gusset plate and subsequently the gusset plate is also welded to the beam and column. The beam-to-column connection at the gusset plate is either welded or bolted at the face of the column flange. Even though a bolted connection is provided at the face of the column flange, stiff gusset plates connected to beam and column could still provide a fully restrained beam-to-column connection as in the case of welded connection. Such a fully restrained beam-to-column connections in CBFs are such that plastic hinges will form in the column upon continued lateral deformation under severe shaking: hence, higher drift concentrations can be expected in the bottom storey when the partially restrained or pin-ended column-to-base connections are provided. For these reasons, this study investigates the local and global seismic performances of fully restrained brace-to-beam/column connections through numerical analyses. The global performance is examined using a 4, 8 and 12 storey concentrically braced prototype frames modelled in OpenSees, while the local performances are examined through the detailed finite element model of a single storey single bay frame located at the ground floor of the four storey brace frame using the finite element program MIDAS. Furthermore, this study introduces a partially restrained bolted connection at the corner of the gusset plate rather than providing it at the face of column flange in order to facilitate beam rotation at the bolted connection upon continued lateral deformation; this is expected to prevent the formation of plastic hinges in the columns and to distribute the

Seismic performance of non-structural elements during the 2016 Central Italy earthquake

Non-structural elements represent most of the total construction cost of typical buildings. A significant portion of the total losses in recent earthquakes worldwide, has been attributed to damage to non-structural elements. Damage to non-structural elements occurs at low levels of ground shaking, and can significantly affect the post-earthquake functionality of buildings. However, in Europe, limited prescriptions are provided in the codes for seismic design of non-structural elements and this may partially explain why it is so common for these elements to perform poorly during earthquakes. This paper describes the observed damage to non-structural elements following the 2016 Central Italy earthquake. The most commonly damaged elements were partition walls, ceiling systems, non-structural vaults, chimneys, and storage racks. As a result, it was highlighted the need to introduce seismic regulations devoted to improving the seismic performance of non-structural elements and to reduce the associated economic losses, loss of functionality, and potential threats to life safety.

RINTC PROJECT: NONLINEAR ANALYSES OF ITALIAN CODE-CONFORMING PRECAST R/C INDUSTRIAL BUILDINGS FOR RISK OF COLLAPSE ASSESSMENT

Most of modern seismic design codes adopt the performance-based approach by defining a pre-defined level of safety for different limit states. These safety margins are implicitly defined into the definition of detailing provisions as well as in partial safety factors for both actions and materials strength. Moreover, such implicit provisions are related to the specific structural typology; hence, the structural safety can change for different typologies designed for the same site. This study investigates the seismic performance of typical Italian RC precast buildings by means of an extensive parametric study. Both the structural typology and the geometrical configurations are defined in order to represent the most common practice in the country. The investigated buildings are located in five sites with different levels of seismic hazard and two soil types are also considered. The collapse assessment is investigated by means of multi-stripe analyses, performed by non-linear dynamic analyses at 10 intensity levels. 1636 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 1636-1644

PROGRESSIVE COLLAPSE FRAGILITY MODELS OF RC FRAMED BUILDINGS BASED ON PUSHDOWN ANALYSIS

Partial or total progressive collapse under abnormal loading conditions (e.g. deliberate terrorist attacks, uncontrolled gas releases, and vehicle or aircraft impacts) is one of the most vivid examples of low probability-high consequence (LPHC) event that may occur in the lifetime of a structure. Despite this, structural safety for extreme loads that may lead to disproportionate (or progressive) collapse has been probabilistically assessed and controlled in a few cases, thus neglecting uncertainties in loads and system capacity. As such, this paper moves from a deterministic to a probabilistic framework, proposing fragility models at multiple damage states for low-rise reinforced concrete (RC) framed bare structures which may be applied for progressive collapse risk assessment and management. Two building classes representative of structures designed for either gravity loads or earthquake resistance in accordance with current European rules were investigated. Monte Carlo (MC) simulation was used to generate random realizations of two-dimensional (2D) and three-dimensional (3D) structural models. Their fiber-based finite element (FE) representations were developed within an open source platform for nonlinear static pushdown analysis. The output consisted of fragility functions for each damage state of interest. Such fragility models were then compared to those derived through incremental dynamic analysis (IDA) in a previous study. IDAbased and pushdown-based capacities were additionally used to propose regression models for quick estimation of dynamic amplification factor (DAF) at a given displacement/drift target. The analysis results show a significant influence of both seismic design/detailing and secondary beams on robustness of the case-study building classes.

SEISMIC PERFORMANCE OF HIGH-RISE STEEL MRFS WITH OUTRIGGER AND BELT TRUSSES THROUGH NONLINEAR DYNAMIC FE SIMULATIONS

The work reported herein summarizes the results of a series of nonlinear dynamic FE analyses devoted to assess the main criticalities in the seismic response of high-rise steel MRFs with outrigger and belt trusses. Thirtyand sixty-storey planar frames, extracted from reference three-dimensional structures composed of an internal one-way braced core, are designed in accordance with European rules. The core consists of a CBF system, while outriggers are placed every fifteen stories to limit inter-storey drifts and second order effects. FE models able to account for material and geometric nonlinearities have been developed within an open source FE code, using inelastic force-based fiber elements to model structural members and equivalent nonlinear links to reproduce the behaviour of bolted beam-column joints and welded gusset-plate connections. Out-of-plane imperfections are explicitly included in the braces to allow for potential buckling mechanisms in both braces and gusset plates. NLTHAs have been performed, in comparison with response spectrum analysis, aiming to quantify the potential of such systems, when included in the lateral-force resisting system of modern highrise steel MRFs. Global and local performance have been investigated in terms of inter-storey drift and acceleration peak profiles and axial force-displacement curves and static-to-seismic load ratios in critical braces at different floor levels. Sensitivity to the structure height has been explored by comparing the response of the two prototype MRFs. Trends are discussed to show that, if accurately designed and detailed, these structural systems provide an optimum combination of stiffness and strength.

Seismic Fragility Analysis of MRFs with PR Bolted Connections Using IDA Approach

Partially restrained (PR) bolted beam-to-column connections are a promising typology of connection in modern steel moment resisting frames (MRFs). Both high-fidelity three-dimensional solid models and mechanics-based idealisations have been extensively explored in order to investigate the behaviour of this attractive solution, applicable both to new construction and to retrofitting of existing structures. Despite this, structural safety has been probabilistically assessed and controlled in a relatively few cases, thus neglecting characterisation, modelling and propagation of uncertainties. As such, this paper moves from a deterministic to a probabilistic framework, proposing fragility models at multiple damage states for low-and medium-rise MRF structures with T-stub and top-and-seat angle connections which may be applied for seismic risk assessment and management. After validation against past experimental data, use was made of component-based modelling to reproduce the seismic response of these PR bolted connection systems within planar MRFs designed for earthquake resistance in accordance with current European rules. A set of 44 records scaled at increasing seismic intensity was considered to perform a series of incremental dynamic analyses (IDAs). Fragility functions for each damage state of interest were then derived and compared. The analysis results show the influence of connection typology and structure height.