Silvia Caprili

Evaluation of seismic vulnerability of a complex RC existing building by linear and nonlinear modeling approaches

Recent earthquakes, that stroked Italian regions in past decades (Umbria— Marche 1997; Molise 2002; L’Aquila 2009), pointed out the high vulnerability of reinforced concrete existing buildings causing severe damages in the structures and consequently life losses. This is mainly due to the fact that such structures were often built without reference to seismic actions or on the basis of old standard provisions. Nowadays in Italy, Public Authorities are requested to evaluate the seismic vulnerability of their building stock assessing the actual capacity of such structures, as a consequence of new hazard levels and seismic microzonation introduced by new standards. According to Eurocode 8 or Italian standard NTC 2008, the seismic analysis of existing reinforced concrete buildings can be performed by one of the established procedure (i.e. Linear Static Analysis LSA, Linear Dynamic Analysis LDA, Nonlinear Static Analysis NSA, Nonlinear Dynamic Analysis NDA), depending on the achieved knowledge level about the structural system and materials. In order to compare efficiency and differences of previously described approaches, a deep investigation was executed on a reinforced concrete existing building whose dynamic behaviour was evaluated by an experimental dynamic analysis. In such a way, updated models were obtained and adopted for seismic analysis performed by using linear and nonlinear approaches, taking into account the stiffness and strength contribution of masonry infill walls. It was so possible to get useful indications on the reliability and discrepancies of different modelling approaches as well as on the influence of masonry infills on the seismic response of existing r.c. buildings.

INFLUENCE OF STEEL MECHANICAL PROPERTIES ON EBF SEISMIC BEHAVIOUR

Among the resisting systems suitable for the design of ductile steel structures, Eurocode 8 proposes MRFs and EBFs. The formers are considered more efficient in terms of ductility, but they suffer a strong weakness in the lateral stiffness, with following cumbersome design procedures to avoid excessive lateral displacements maintaining a quite high ductile behaviour under seismic actions. Often, the design process leads to not optimized structural members, oversized with respect to the minimum seismic requirements due to lateral deformation limitations. EBFs combine high lateral stiffness, due to bracing elements, and high dissipative capacities, provided by the plastic hinges developed in links. Eurocode 8 proposes a design procedure for EBF structures in which iterative checks are required to design links with a defined level resistance dependent on all the other links’ strength. The present paper investigates the seismic behaviour of EBFs using Incremental Dynamic Analyses (IDA) to explore their mechanical response under increasing seismic action. IDAs are executed considering the influence of variability of steel mechanical properties on the behaviour of EBFs, using seven artificial accelerograms according to Eurocode 8. The aims of IDAs are the probabilistic assessment of the response of the system with respect to the variability of the material properties, the analysis of structural safety and the ability of the structures to internally redistribute plastic phenomena during the earthquake. Structural safety conditions will be defined according to a multi-level performance approach. The paper presents also some final suggestions for possible improvements and design simplifications.

Influence of variability of material mechanical properties on seismic performance of steel and steel–concrete composite structures

Modern standards for constructions in seismic zones allow the construction of buildings able to dissipate the energy of the seismic input through an appropriate location of cyclic plastic deformations involving the largest possible number of structural elements, forming thus a global collapse mechanisms without failure and instability phenomena both at local and global level. The key instrument for this purpose is the capacity design approach, which requires an appropriate selection of the design forces and an accurate definition of structural details within the plastic hinges zones, prescribing at the same time the oversizing of non-dissipative elements that shall remain in the elastic field during the earthquake. However, the localization of plastic hinges and the development of the global collapse mechanism is strongly influenced by the mechanical properties of materials, which are characterized by an inherent randomness. This variability can alter the final structural behaviour not matching the expected performance. In the present paper, the influence of the variability of material mechanical properties on the structural behaviour of steel and steel/concrete composite buildings is analyzed, evaluating the efficiency of the capacity design approach as proposed by Eurocode 8 and the possibility of introducing an upper limitation to the nominal yielding strength adopted in the design.

Seismic demand on steel reinforcing bars in reinforced concrete frame structures

Modern design standards for reinforced concrete (r.c.) buildings allow the achievement of ductile structures, able to globally dissipate seismic energy through the development of plastic deformations located in the dissipative regions (i.e. plastic hinges). The hysteretic capacity of r.c. structures is related to the ability of reinforcing steel bars to sustain many cycles of high plastic deformations without the exhibition significant decrease of strength and stiffness; this condition, typically due to cyclic/seismic action, shall be widely investigated in order to obtain a full and detailed knowledge of the structural behaviour of modern r.c. buildings. In the present paper, elaborated inside the European research project “Rusteel”, the evaluation of the seismic ductile demand on steel reinforcing bars due to real earthquake events was carried out. Representative r.c. case study buildings were designed following the actual European and Italian prescriptions and analyzed using the Incremental Dynamic Analysis technique for the assessment of the behaviour under real seismic events. The elaboration of a simplified mechanical model for the steel reinforcing bars, calibrated on the basis of experimental monotonic and cyclic tests, allowed the evaluation of the effective level of deformation and energy dissipation required by earthquakes and the assessment of the ability of the actual European production to satisfy the effective seismic ductile requirements.

Mechanical Performance versus Corrosion Damage Indicators for Corroded Steel Reinforcing Bars

The experimental results of a testing campaign including tensile and low-cycle fatigue tests on different reinforcing steel bar types in the as-delivered and corroded condition are presented. Experimental data were statistically analyzed adopting ANOVA technique; Performance Indicators (PIs), describing the mechanical performance characteristics of reinforcements, and Corrosion Damage Indicators (CDIs), describing the detrimental effects of corrosion phenomena, were determined and correlated in order to evaluate the influence of corrosion on the behaviour of reinforcing steels, providing useful information for designers in addition to what is presented in current standards.

A knowledge-based approach for the structural assessment of cultural heritage, a case study: La Sapienza Palace in Pisa

The full knowledge of the morphological evolution of an historical masonry building, defined more as ‘structural aggregate’ than as ‘single construction’, together with the analysis of the architectural, structural, geological and geotechnical aspects, allow the assessment of the static safety and seismic vulnerability of the complex and the design of retrofit interventions. In the present paper, a Knowledge-Based-Approach is applied to the historical building ‘Palazzo La Sapienza’ in Pisa, allowing to provide reliable results concerning the actual structural condition of the building avoiding the strong computational effort usually associated to the execution of refined numerical analyses. In case of complex buildings, characterized by a high heterogeneity of materials, structural typologies, geometries and so on, the adoption of a global model is not always useful to represent the effective structural behaviour. The proposed approach shows how a deep multidisciplinary knowledge of the construction can limit the use of cumbersome numerical modelling and analysis, however reaching reliable and accurate results usable also in the current practice.

PALAZZO LA SAPIENZA IN PISA: STRUCTURAL ASSESSMENT AND RETROFIT OF AN HISTORICAL MASONRY BUILDING IN ITALY

Abstract. Palazzo La Sapienza, historical seat of the University of Pisa since the XVI century, represents one of the most important examples of the Tuscany cultural heritage, nowadays not in use due to several problems related to maintenance and structural deficiencies. The building, in its current form, is the result of several modifications, enlargements, elevations, connection with adjacent parts, resulting finally more similar to a “structural aggregate” made up of single units connected together without specific scheme and organization than to a single unitary building. After the earthquake of May 2012, the Palace was temporarily closed in relation to the ordinance issued by the Major’s Office and then subjected to wide in situ investigations aiming at analyzing in a very detailed manner all the criticisms related to structural, nonstructural, geotechnical and maintenance problems; the deep in situ survey campaigns allowed to reach a very deep knowledge of the building, of its progressive morphological evolution and of its actual condition, including problems not directly related to the structural condition but, as an example, to the instability and heterogeneity of the ground soil and of the foundation system. All the information so obtained allowed the execution of static and seismic vulnerability assessment – according to the actual prescriptions of Italian Standard for Constructions (D.M. 14/01/2008) – through the elaboration of complex global and local models and the final elaboration of an executive retrofit design comprehensive of local interventions on significant structural elements or subportions of the building. In the present paper, after a short description of the knowledge analysis of the building and of its safety verification, the retrofit executed to obtain a satisfying level of safety is presented.

The Influence Of Soil–foundation–structure Interaction On The Overall Behaviour And Diseases Of A Medieval Building In Pisa

The University of Pisa was established in 1343, but it was only in the 16th century that a specific venue, Palazzo La Sapienza, was built. The building was subjected to various modifications in relation to the users’ requirements, with the following irregular structural growth due the absence of a specific and organized global and general scheme. The present paper describes the investigations carried out on the construction and on the foundation soil to clarify their mutual interactions and explain some of the damage today affecting the building. A deep in situ experimental test campaign was executed to define masonry typology, dimensions of structural elements, mechanical properties of materials, geotechnical parameters of the soil and foundations’ geometry and masonry characteristics. Dynamic properties of the subsoil were investigated in order to properly define the local seismic action and the local influence of subsoil profile, necessary for the execution of safety assessments on the global model of the building.

Dataset on the cyclic experimental behavior of Steel frames with Reinforced Concrete infill Walls

This paper presents the experimental data on the cyclic behavior of Steel frames with Reinforced Concrete infill Walls (SRCW). Two specimens, characterized by a different shear studs distribution, have been tested: the first one is provided with shear studs positioned only in the four corners of the steel frame; the second one presents shear studs all distributed along the perimeter of the steel frame except for the zone of the dissipative fuses. The overall setup, loading protocol, collapse mechanisms, force-displacement curves for both the whole system and the main single components are described for the two tested prototypes.

Mechanical performance of steel reinforcing bars in uncorroded and corroded conditions

The paper presents data coming from a wide experimental test campaign executed on different typologies of steel reinforcing bars representative of the actual European production scenario. Tensile and low-cycle fatigue tests have been executed to assess the mechanical performance of reinforcing bars under monotonic and cyclic/seismic conditions. The effects of exposure to aggressive environmental conditions have been reproduced through accelerated salt-spray chamber. Residual mechanical performance of corroded specimens has been analyzed as function of corrosion indicators such as mass loss and necking.