Jet-Pacs Project: Dynamic Experimental Tests and Numerical Results Obtained for a Steel Frame Equipped with Hysteretic Damped Chevron Braces

The experimental and numerical results obtained by Research Units of the University of Basilicata and University of Calabria for a steel frame, bare or equipped with metallic yielding hysteretic dampers (HYDs), are compared. The shaking table tests were performed at the Structural Laboratory of the University of Basilicata within a wide research program, named JETPACS (“Joint Experimental Testing on Passive and semiActive Control Systems”), which involved many Research Units working for the Research Line 7 of the ReLUIS (Italian Network of University Laboratories of Earthquake Engineering) 2005–2008 project. The project was entirely founded by the Italian Department of Civil Protection. The test structure is a 1/1.5 scaled two-story, single-bay, three-dimensional steel frame. Four HYDs, two for each story, are inserted at the top of chevron braces installed within the bays of two parallel plane frames along the test direction. The HYDs, constituted of a low-carbon U-shaped steel plate, were designed with the performance objective of limiting the inter-story drifts so that the frame yielding is prevented. Two design solutions are considered, assuming the same stiffness of the chevron braces with HYDs, but different values of both ductility demand and yield strength of the HYDs. Seven recorded accelerograms matching on average the response spectrum of Eurocode 8 for a high-risk seismic region and a medium subsoil class are considered as seismic input. The experimental results are compared with the numerical ones obtained considering an elastic-linear law for the chevron braces (in tension and compression), providing that the buckling be prevented, and the Bouc-Wen model to simulate the response of HYDs.     

Wavelet-Based Damage Detection in Reinforced Concrete Structures Subjected to Seismic Excitations

The feasibility of using output-only model-free wavelet-based techniques for damage detection in reinforced concrete structures subjected to seismic loads is explored through the analysis of the results of a full scale shake table test of a reinforced concrete bridge column recently performed at the NEES Large High Performance Outdoor Shake Table. The evaluated approaches are based solely in the analysis of the acceleration time histories recorded in the structure. The viability of using numerical models to validate this type of damage detection methodologies is also evaluated. Wavelet analyses were capable of identifying the rebar fracture episodes and partially identified the frequency shifts in the structure as the inelastic demand increased. It was also found that, depending on the methodology employed, the use of numerical models to validate damage detection techniques can oversimplify the actual problem and/or induce spurious irregularities.     

Selection of the Most Unfavorable Real Ground Motions for Low- and Mid-rise RC Frame Structures

The goal of this article is to select those real (or recorded) ground motions capable of exposing the low- and mid-rise reinforced concrete frame structures to an extreme limit state. By performing correlation analyses, two optimal intensity measures are first selected to represent the ground motion damage potential. Then based on each record's damage potential, four subsets of strong ground motions, referred to as the most unfavorable ground motions, are identified and preliminarily confirmed to be applicable to the low- and mid-rise RC frame structures.     

Transverse Seismic Behavior Studies of a Medium Span Cable-Stayed Bridge Model with Two Concrete Towers

Due to lack of investigation on nonlinear seismic behavior of cable-stayed bridges under strong earthquake excitation, the concrete towers, as the main gravity-carrying component, are usually required to remain nearly elastic. However, in order to achieve this high seismic performance objective, the reinforcement ratio of the tower legs and the tower struts need to be greatly increased in addition to its static loading requirement. To study the potential plastic region and possible failure mode of the cable-stayed bridge, a 1/20-scale full bridge model from a typical medium span concrete cable-stayed bridge was designed, constructed and tested on 4 linear shake tables using a site specific artificial wave in the transverse direction. Test results showed that the damage characteristics of the bridge model were as follows: (1) the severe damage was observed at the upper strut, with several steel bars fractured at both ends; (2) the repairable damage was observed at tower legs at the bottom and the middle part, with concrete cover spalling and exposure of steel bars; (3) the minimal damage was observed at the lower strut and the both sides of the side bents, with only slightly concrete spalling; and (4) no damage was observed at the auxiliary bents, the superstructure and the cables. Numerical results and test results were further compared and showed good agreement in low amplitudes of excitations. The test also proved that the bridge system was stable in flexural failure of upper struts, and had the negligible residual displacement subjected to high amplitudes of excitations.     

Numerical Investigation of Variable Length of Seismic Damage Region for Reinforced Concrete Columns

ABSTRACT

The seldom investigation of variable length of damage region prevents the estimation of probabilistic drift limits of reinforced concrete columns at different performance levels for the performance-based seismic design. However, if using the numerical approach to predict the variability of damage region within the framework of force-based beam-column element, the current force-based beam-column element is unable to model the spreading of damage region. Therefore, a new numerical simulation method is proposed to compute the emergence, propagation and termination of damage region of reinforced concrete columns. Then, based on the developed numerical simulation method, the measured response of experimental testing is calibrated. From the calibration, it can be observed that there is a rapid increase on the variable length of damage region with the increasing of lateral displacement and then followed by a stable stage. The propagation of the longitudinal reinforcement yielding and concrete tensile cracking mainly occurs in the ascending branch of the load–displacement response. Then, based on the growth characteristic of the damage region from the numerical simulation, an empirical equation is proposed to describe the variable length of damage region by using the least-square regression analysis to fit the computed responses for its simplicity to use in engineering practices. Finally, the stable length of damage region is reinvestigated by carrying out a parametric study with the developed numerical simulation method, indicating that two critical design parameters, specifically the axial load ratio and the shear span ratio, have considerable influences on this quantity of interest.     

Cyclic Response of Non-ductile RC Frame with Steel Fibers at Beam-Column Joints and Plastic Hinge Regions

An experimental study has been conducted on a reduced-scale gravity-load designed test frame to investigate its overall performance due to the addition of steel fiber-reinforced concrete (SFRC) at the critical regions. Two geometrically similar specimens, namely, reinforced concrete (RC) and SFRC, are tested under slow-cyclic lateral loading. End-hooked steel fibers (aspect ratio = 80) of 1.0% volume fraction were used in the SFRC mix for a distance of one-and-half times the member size near the joint regions. The addition of steel fibers improved the damage tolerance, lateral load resisting capacity, lateral stiffness, ductility, and energy dissipation of the frame.     

Probabilistic Performance Assessment of Retrofitted Skewed Multi Span Continuous Concrete I-girder Bridges

The unique dynamic response of skewed bridges causes them to experience more noticeable damage compared to straight bridges during seismic events. The effectiveness of different retrofit strategies on the fragility of skewed bridges can change with the skew angle. This article assesses the impact of skew angle and various retrofit strategies on the fragility of multi span continuous concrete I-girder bridges. The results indicate that the level of effectiveness of a retrofit strategy is highly dependent on the skew angle and damage state of interest and an appropriate retrofit strategy should be chosen based on the vulnerability of the components.     

Mapping Mesolithic and Neolithic cultures behaviours and interactions with nature and properties of rocks in Western France

Multivariate statistical methods are used to study recent Prehistory lithics from the Armorican Massif, allowing us to classify them in more or less distinct groups according to the structural and mineralogical qualities of rock types involved. Analyses are carried out on metamorphic, sedimentary and magmatic rocks making up lithics collected from 31 localities. The petrographic properties are determined by polarizing microscope (POM), while the mechanical properties are obtained by Vickers's indentation method (VI), compression/traction (CT) and ultrasound echography (UE). The results provide us with important information about the management of lithic raw materials used by various human and cultural groups during the Mesolithic and Neolithic of the Armorican Massif. The more commonly used materials show a homogeneous and stable distribution of regional technology styles. The rock types exhibiting mature and homogeneous structures possess the better qualities for knapping (fracture energy G_Ic) and use (edge toughness M₁), and are more involved in exchange. The application of Upward Hierarchical Classification (UHC) and Principal Component Analysis (PCA) allows us to identify the classes of materials as well as the causes of variations of rock type abundance within the lithic assemblages of the Mesolithic and the Neolithic. In this way, we can then also predict the discovery of archaeological artefacts in areas lacking any known indications of the exploitation of outcropping materials.     

Flexural and Shear Damage Models for MRF Structures Excited by Far-Field Horizontal Strong Ground Motions

The unexpected damages in structures during severe earthquakes have been reported frequently so far. In this study, the damage-based inelastic behavior of special moment resisting frame (SMRF) structures designed according to the new versions of general earthquake loading codes (International building code [IBC] 2012 & American Society of Civil Engineers [ASCE] 7-2010) and seismic design references (National Earthquake Hazards Reduction Program [NEHRP] 2009 & Federal Emergency Management Agency [FEMA] P-750) has been investigated. The final results presented based on distinctive shear and flexural failure modes show that a non-uniform distribution of severe damage in structural height occurs during design level seismic excitations. Also it is observed that the shear and flexural damages are more critical in short and tall MRF structures, respectively.