Dora Foti

Response of middle-rise steel frames with and without passive dampers to near-field ground motions

This study analyses the response of middle-rise (6-stories) steel moment-resisting frame (MRF) with rigid joints subjected to near-field ground motions and also to artificial motions based on equivalent pulses. The near-field ground motions, recorded during the earthquakes in Northridge (1994), in Kobe (1995), and in Chi-Chi Taiwan (1999), show velocity and displacement peaks higher than the far-field ones. Indeed, in near-field areas other parameters are the key to defining the severity of the seismic input instead of the peak acceleration value. The ground motion velocity of fault-normal component is more severe than the velocity corresponded to fault-parallel component and presents large-amplitude pulses. The main objective of the present study is to acquire quantitative knowledge on near-field ground motion effects, to emphasize how these effects influence the structural response, and to point out the necessity to take into account the near-field effects in seismic design. Moreover, the responses of the aforementioned middle-rise frames equipped with shear link (SL) devices and subjected to near-field ground motions are discussed.

Dynamic Identification Techniques to Numerically Detect the Structural Damage

Damage detection in civil engineering structures using changes in measured modal parameters is an area of re- search that has received notable attention in literature in recent years. In this paper two different experimental techniques for predicting damage location and severity have been considered: the Change in Mode Shapes Method and the Mode Shapes Curvature Method. The techniques have been applied to a simply supported finite element bridge model in which damage is simulated by reducing opportunely the flexural stiffness EI. The results show that a change in modal curvature is a significant damage indicator, while indexes like MAC and COMAC - extensively and correctly used for finite ele- ment model updating - lose their usefulness in order to damage detection.

Identification of the Modal Properties of a Building of the Greek Heritage

In this paper, the experimental modal identification analysis of the public building “San Giacomo” in Corfu (Greece) is illustrated. It represents the unique example of a structure built utilising carves stones inside the city of Corfu. The building has a rectangular plan shape with dimensions 24.75 x 14 m, and height 9 m; all the floors are made by wood. The monitoring system consists of several elements properly connected: the units of acquisitions or piezoelectric accelerometers (in total 18 installed on the different walls) with a sensitivity of 1000 mV/g; the data acquisition system or DAQs positioned at each monitored level; the laptop with an acquisition software; the cables that connect all elements to each other. The paper describes the phases of the investigations, the technical details of the performed in-situ tests, the first identified frequencies of the building by means of the classical methods of Operational Modal Analysis (OMA) and the comments about the acquired data.

On the evaluation of the horizontal forces produced by grain-like material inside silos during earthquakes

This paper presents analytical developments devoted to the evaluation of the effective behaviour of grain in flat-bottom silos during an earthquake. This research work starts from all the same basic assumptions of Eurocode 8 except for the one regarding the horizontal shear forces among consecutive grains. Only this difference leads to a new physically-based evaluation of the effective mass of the grain which horizontally pushes on the silo walls. The analyses are developed by simulating the earthquake ground motion with time constant vertical and horizontal accelerations and are carried out by means of simple dynamic equilibrium equations that take into consideration the specific mutual actions developing in the ensiled grain. The findings indicate that, in case of squat silos (characterized by low, but usual, height/diameter slenderness ratios), the portion of the grain mass that interacts with the silo walls turns out to be noticeably smaller than the total mass of the grain in the silo and the effective mass adopted by Eurocode 8.

Response of frames seismically protected with passive systems in near-field areas

The present paper analyses the behaviour of moment resisting frames in reinforced concrete subject to earthquakes recorded in near-field areas. In these areas, the signals show impulsive-type accelerograms, with velocity and displacement peaks higher than in far-field zones especially for the fault-normal component of the ground motion velocity in the direction of propagation of the wave, which shows large-amplitude pulses. In the following, seven near-field signals scaled in agreement with the design spectrum of the Italian code (NTC, 2008) have been adopted to perform a nonlinear analysis on a six-story frame. The response has also been determined for the same frame protected once with hysteretic-type energy dissipaters and once with base isolators. The aim of the present study is to acquire quantitative knowledge on the near-field ground motion effects on frame buildings and on their damage also in the presence of passive seismic protection systems.

Non-destructive characterization and identification of the modal parameters of an old masonry tower

The aim of the paper is to present a structural characterization of the bell tower of the Cathedral of Trani (Bari, Italy) collecting the data of two different non-destructive techniques. The tower, built in 1200, is about 60 meters high and has a square plan with a side of about 7.50 meters; moreover, it is connected to the church through a step supported by a pointed arch. Firstly, an Operational Modal Analysis was performed on the tower by using the data taken by an experimental campaign of measurements of specific accelerometers. The tower vibrations due to ambient actions have been recorded and analyzed by means of two different algorithms in such a way as to estimate the modal parameters of the tower. Moreover, a recent procedure based on the data acquired by means of a ground penetrating radar (GPR) tests for estimating the thicknesses of the internal and external layers at each level of the tower was used. The results of the identification procedure have characterized the dynamic behavior of the masonry tower and were utilized also to evaluate the dynamic interaction between the tower and the Cathedral walls. GPR technology has permitted of foreseeing the internal composition of the building walls. Finally, the combination of the results of the two techniques, has allowed realizing a complete 3D finite element model.

Non-Destructive Techniques and Monitoring for the Evolutive Damage Detection of an Ancient Masonry Structure

In the present article non-destructive testing evaluation of the existing damage evolution has been applied to some buildings of the medieval town of Craco (Matera, Italy) and, in particular, to the Normand tower. Actually the little town of Craco is totally abandoned as a consequence of the activation of the landslide motions of its soil depth. Nevertheless, the Normand tower still stands because it is located on a stable and stiffer foundation ground. The tower was built in the XII century. It is 20 m tall and has a symmetric square plan, with dimensions 11 m x 11 m; it was built for defense against enemy attacks. Inside the tower a cistern in reinforced concrete was placed in 1949. It is not connected to the structural walls of the tower; however it represents an obstacle for installing the damage detection equipment and sensors. In this article a preliminary study on the masonry structure of the Normand tower is carried on. A finite element numerical model has been developed and a modal analysis has been performed. The final aim of the research is to find out the evolutionary stage of the cracks and to propose a possible retrofit of the tower.

On the Seismic Response of Protected and Unprotected Middle-Rise Steel Frames in Far-Field and Near-Field Areas

Several steel moment-resisting framed buildings were seriously damaged during Northridge (1994); Kobe (1995); Kocaeli, Turkey (1999), earthquakes. Indeed, for all these cases, the earthquake source was located under the urban area and most victims were in near-field areas. In fact near-field ground motions show velocity and displacement peaks higher than far-field ones. Therefore, the importance of considering near-field ground motion effects in the seismic design of structures is clear. This study analyzes the seismic response of five-story steel moment-resisting frames subjected to Loma Prieta (1989) earthquake—Gilroy (far-field) register and Santa Cruz (near-field) register. The design of the frames verifies all the resistance and stability Eurocodes’ requirements and the first mode has been determined from previous shaking-table tests. In the frames two diagonal braces are installed in different positions. Therefore, ten cases with different periods are considered. Also, friction dampers are installed in substitution of the braces. The behaviour of the braced models under the far-field and the near-field records is analysed. The responses of the aforementioned frames equipped with friction dampers and subjected to the same ground motions are discussed. The maximum response of the examined model structures with and without passive dampers is analysed in terms of damage indices, acceleration amplification, base shear, and interstory drifts.

Post‐tensioned masonry: state of the art

A state of the art of post-tensioned prestressed masonry is presented. The paper especially deals with research done on the effects of creep, shrinkage and deformations due to humidity variations, apart from the loads applied to the masonry wall.

On Modeling an Innovative Monitoring Network for Protecting and Managing Cultural Heritage from Risk Events

In this paper the model of an Innovative Monitoring Network involving properly connected nodes to develop an Information and Communication Technology (ICT) solution for preventive maintenance of historical centres from early warnings is proposed. It is well known that the protection of historical centres generally goes from a large-scale monitoring to a local one and it could be supported by a unique ICT solution. More in detail, the models of a virtually organized monitoring system could enable the implementation of automated analyses by presenting various alert levels. An adequate ICT solution tool would allow to define a monitoring network for a shared processing of data and results. Thus, a possible retrofit solution could be planned for pilot cases shared among the nodes of the network on the basis of a suitable procedure utilizing a retrofit catalogue. The final objective would consist in providing a model of an innovative tool to identify hazards, damages and possible retrofit solutions for historical centres, assuring an easy early warning support for stakeholders. The action could proactively target the needs and requirements of users, such as decision makers responsible for damage mitigation and safeguarding of cultural heritage assets.