RESPONSE OF UNREINFORCED MASONRY BUILDINGS

Abstract

A summary of dynamic measurements are presented that illustrate relations between linear seismic demand and true nonlinear response of unreinforced masonry buildings with flexible diaphragms and rocking piers subjected to a series of simulated earthquake motions.     

SEISMIC DESIGN AND RESPONSE OF BARE AND MASONRY-INFILLED REINFORCED CONCRETE BUILDINGS. PART I: BARE STRUCTURES

Abstract

Eurocode 8 is applied for the complete design of 26 multi-storey reinforced concrete buildings to study its operationally and compare the implications of trading strength for ductility through designing the same structure for a different Ductility Class. The difference between the conventional full Capacity Design of columns in bending and the relaxed one allowed by Eurocode 8 is quantified, and the implications on the column capacities are examined. About half of the designed buildings, representative of the class of regular frames, are subjected to nonlinear dynamic response analyses to spectrum-compatible motions with intensities up to twice that of the design motion. Nonlinear modeling is very simple, but gives satisfactory agreement with available quasistatic or pseudodynamic test results on full scale structures. Results show that the three Ductility Classes of Eurocode 8 are essentially equivalent in terms of material quantities and seismic performance. Within the limitations of the nonlinear modelling, the response results suggest very satisfactory performance of structures designed to Eurocode 8, even under twice the design motion intensity. Softening of the structure due to concrete cracking and steel yielding significantly reduces the seismic force demands and contributes to the satisfactory performance, despite the increased P — 6 effects. Another important contributor to the good performance is the significant overstrength of the members considered in the analyses with their average as-built properties. Beam overstrength due to the contribution of the slab to flexural capacity is large enough to overcome the effects of the application of the relaxed Capacity Design rule to columns in bending. However, the resulting column plastic hinging does not lead to drift concentrations suggesting formation of storey-sway mechanisms.     

SEISMIC DESIGN AND RESPONSE OF BARE AND MASONRY-INFILLED REINFORCED CONCRETE BUILDINGS. PART II: INFILLED STRUCTURES

The effects of masonry infills on the global seismic response of reinforced concrete structures is studied through numerical analyses. Response spectra of elastic SDOF frames with nonlinear infills show that, despite their apparent stiffening effect on the system, infills reduce spectral displacements and forces mainly through their high damping in the first large post-cracking excursion. Parametric analyses on a large variety of multi-storey infilled reinforced concrete structures show that, due to the hysteretic energy dissipation in the infills, if the infilling is uniform in all storeys, drifts and structural damage are dramatically reduced, without an increase in the seismic force demands. Soft-storey effects due to the absence of infills in the bottom storey are not so important for seismic motions at the design intensity, but may be very large at higher motion intensities, if the ultimate strength of the infills amounts to a large percentage of the building weight. The Eurocode 8 provisions for designing the weak storey elements against the effects of infill irregularity are found to be quite effective, in general, for the columns, but unnecessary and often counterproductive for the beams.     

SEISMIC HAZARD ASSESSMENT IN FLORENCE CITY ITALY

A seismic hazard analysis of Florence city was performed in the frame of a project concerning the dynamic behaviour of cable-stayed bridges. Both a probabilistic approach and a methodology based on the use of a local macroseismic catalogue were applied. A local catalogue was expressly compiled for this purpose, to collect the macroseismic intensities actually observed at the site as a result of past earthquakes. This sort of catalogue is an independent tool to verify the assumptions of the probabilistic approach (seismic zoning, earthquake recurrence relation, attenuation model), though it can supply results in terms of macroseismic intensity only and reflects the effective seismic history at the site, without taking into account any variability. The Cornell' methodology was used to assess probabilistic hazard in terms of macroseismic intensity, peak ground acceleration, peak ground velocity, and pseudovelocity uniform response spectra. The local catalogue points out level VII of the Mercalli-Cancani-Sieberg scale (MCS) as the maximum intensity historically observed in Florence. The probabilistic approach leads to the consideration of intensity VIII MCS as the maximum credible for the city. The probabilistic analysis in terms of ground motion was performed using attenuation relations estimated for alluvium sites, since the geology of Florence area is represented by fluvial and lacustrine deposits of various thickness. Peak ground acceleration values with 90% non exceedence probability in 50 and 500 years are respectively 145 and 219 cm/s's for a shallow alluvium site, and 95 and 157 cm/s's for a deep alluvium site; the corresponding peak ground velocity values for sites located on alluvium are 6.41 and 11.76 cm/s. Uniform response spectra are provided for shallow and deep alluvium sites, according to frequency-dependent attenuation relations estimated from strong Italian earthquakes.     

Seismic vulnerability assessment of historic centers: description of a predictive method and application to the case study of scanno (Abruzzi,Italy)

ABSTRACT

In this article, a predictive model for the seismic vulnerability assessment of old Italian historic centers is presented through its direct application to a meaningful case study, the historic center of Scanno, in Abruzzi, Italy.

The proposed method is calibrated on the basis of the observations carried out on similar historic centers hit by the 2009 L’Aquila earthquake and is applied in order to provide likely damage scenarios by means of fragility curves. The method is based on the evaluation of a limited number of structural and typological parameters that can be obtained by simple and rapid inspections on buildings. In addition, it is conceived in order to provide useful information on the most effective anti-seismic strategies to be implemented on urban scale for pursuing a global mitigation of the seismic risk and for the application of suitable risk reduction policies.

The final aim of the article is to give an applicative vision of the method, by providing instructions on how to judge the features of the buildings that are influential on their seismic behavior, as well as by showing the potentiality of the method itself in providing likely damage scenarios, also with the support of GIS-based representations.     

The Use of a Large-Scale Seismic Vulnerability Assessment Approach for Masonry Façade Walls as an Effective Tool for Evaluating,Managing and Mitigating Seismic Risk in Historical Centers

ABSTRACT

The vulnerability assessment of the building stock in a given territorial area, such as a city or an entire country, is a key prerequisite for evaluating risk, not only because of the potential physical consequences resulting from the occurrence of an event, but also because it is one of the few aspects in which engineering research can intervene. In fact, the rigorous vulnerability assessment of existing buildings followed by the implementation of appropriate retrofitting solutions can help to substantially reduce the levels of physical damage and economic impact of future events. Particularly regarding the seismic vulnerability assessment of historical centers, the amount of knowledge that has been accumulated over the past decades, together with the broad damage data obtained from post-earthquake damage surveys, provides a singular opportunity to develop and calibrate innovative large-scale seismic vulnerability assessment approaches, which can be used to outline and support risk mitigation and management strategies. This article addresses this issue by discussing the use of a large-scale seismic vulnerability assessment methodology for masonry façade walls as a tool for evaluating the potential benefit resulting from the application of different seismic retrofitting strategies, both considering their contribution to reduce post-event urban losses and accessibility.     

Regeneration and Structural Strengthening of the Fortress of Arnara,Latium, Italy

The fortress of Arnara is a small castle in a high seismic hazard zone in Latium, Italy, which represents a landmark for the local population. It was chosen as a case study, being a perfect example of Middle Age fortress, well preserved during the centuries despite some strong earthquakes damaged it in the past. The project presented here is a significant example of how the interest of a community to regenerate an ancient structure for its re-use can pave the way to a complete structural strengthening, which includes anti-seismic concepts aiming at retrofitting the ancient fortress walls. The overall project joins architectural and structural solutions. The architectural solution aims at a functional reconfiguration of the spaces, so to attract the locals to reuse it, while at the same time highlighting some valuable architectural features of the fortress. The structural solution aims at improving the seismic resistance of the masonry walls with a structural intervention that is respectful and integrates with the existing forms. This was achieved by means of a tenso-structure that has the two-fold role of reconnecting the different portions of the structures and of endowing it with a distinctive architectural sign. This project demonstrates that rediscovery of architectural heritage can also be pursued by proposing a new use for these structures, following an appropriate regeneration strategy that must take into account the cultural features of the territory, being at the same time respectful of the past history of the buildings.     

Seismic Assessment of the Lima Cathedral Bell Towers via Kinematic and Nonlinear Static Pushover Analyses

ABSTRACT

The protection of cultural heritage against earthquake induced actions is one of the main challenges the earthquake engineering science and practice are facing. This article presents a seismic assessment study on one of the most ancient colonial buildings present in Peru, the Cathedral of Lima, focusing on its towers. A historical review highlighted how these structures, together with the whole Cathedral, suffered intense damage and partial collapse during previous earthquakes. In order to identify the structure main deficiencies, both linear kinematic analyses and nonlinear static analyses have been performed. Different nonlinear finite element models have been created to evaluate the influence of the adjacent walls. Different load distributions have been compared to evaluate how simplified patterns could provide results close to load distributions taken from a modal analysis of the complex. A simple retrofit strategy, consisting on the introduction of steel ties, has also been studied as a reference. Results show good correlation between kinematic and pushover analyses. The construction, when compared to the requirements of the national code for new buildings, results significantly vulnerable, pointing out the need to accept some structural damage even after seismic retrofit.     

Lithology and fluid prediction from amplitude versus offset (AVO) seismic data

Seismic reflection data as used in the oil industry is acquired and processed as multitrace data with source‐receiver offsets from a few hundred metres (short offset) to several kilometres (long offset). This set of data is referred to as ‘pre‐stack’. The traces are processed by velocity analysis, migration and stacking to yield a data volume of traces with ‘zero‐offset’. The signal‐to‐noise enhancement resulting from this approach is very significant. However, reflection amplitude changes in the pre‐stack domain may also be analysed to yield enhanced rock physics parameter estimates. Pre‐stack seismic data is widely used to predict lithology, reservoir quality and fluid distribution in exploration and production studies. Amplitude versus offset (AVO) data, especially anomalous signals, have been used for decades as indicators of hydrocarbon saturation and favourable reservoir development. Recently, enhanced quantification of these types of measurement, using seismic inversion techniques in the pre‐stack domain, have significantly enhanced the utility of such measurements. Using these techniques, for example, probability of the occurrence of hydrocarbons throughout the seismic data can be estimated, and as a consequence the many pre‐stack volumes acquired in a three‐dimensional (3D) can be survey, reduced to a single, more interpretable volume. The possibilities of 4D time lapse observation extend the measurements to changes in fluid content (and pressure) with time, and with obvious benefits in establishing the accuracy of dynamic reservoir models and improvements in field development planning. As an illustration, recent results from the Nelson Field (UK North Sea), are presented where we show the method by which probability volumes for oil sands may be calculated. The oil–sand probability volumes for three 3D seismic datasets acquired in 1990, 1997 and 2000 are compared and production effects in these data are demonstrated.