Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS the specification of a performance level and an acceptable low probability that poorer performance could occur within a specific period of time. To evaluate a performance objective, selection of a performance level of interest is required. The performance levels can be, for example, the ultimate state limits or the service state limits. A desired probability that damage in a period of time will be worse than this performance level has to correspond for each selected level. Moreover an average estimate of the ground shaking intensity at the probability of exceedance identified in the performance objective definition has to be determined for each probabilistic performance objective. The performance of a building during an earthquake depends on many factors including: the structure’s configuration and proportions, its dynamic characteristics, the hysteretic behaviour of the elements and joints, the type of non-structural components employed, the quality of materials and workmanship, the adequacy of maintenance, the site conditions, and the intensity and dynamic characteristics of the earthquake ground motion experienced. Consequently, seismic performance prediction for buildings, either as part of a design or evaluation, should consider, either explicitly or implicitly, all of these factors. Therefore, a reliability-based performance-oriented approach, adopted by the SAC project for design and evaluation, was taken in order to explicitly account for uncertainties and randomness in seismic demand and capacities in a consistent manner and to satisfy with defined reliability identifiable performance objectives corresponding to various occupancies, damage states and seismic hazards. Consistently with the Eurocode 8 (2002) and on the basis of the FEMA 302 NEHRP Recommended Provisions for Seismic Regulation for New Buildings and Other Structures (BSSC, 1998a) and the FEMA 273 NEHRP Guidelines for Seismic Rehabilitation of Buildings (BSSC, 1997), two performance levels are considered. These are termed Serviceability Limit State (S.L.S.) and Ultimate Limit State (U.L.S.): • The Serviceability Limit State (named Immediate Occupancy (IO) level in the FEMA Provisions) is defined as the post-earthquake damage state where only minor structural damage has occurred with no substantial reduction in building gravity or lateral resistance. Damage in this state could include some localized yielding and limited fracturing of connections. Damage is anticipated to be so slight that if it is not found during inspection there is no cause for concern. For pre-Northridge buildings, fewer than 15% of the connections on any floor may experience connection fractures without exceeding the IO level. • The Ultimate Limit State (named Collapse Prevention (CP) level in the FEMA Provisions) is defined as the post-earthquake damage state, in 123
4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS 124 which the structure is on the verge of experiencing either local or total collapse. Significant damage to the building has occurred, including significant degradation in strength and stiffness of the lateral force resisting system, large permanent deformation of the structure and possibly some degradation of the gravity load carrying system. However, all significant components of the gravity load carrying system must continue to function. The probability that a building may experience greater damage than foreseen depends on the vulnerability of the building and the seismic hazard to which it is exposed. Vulnerability is related to the capacity of the building, which may be a function of the global or interstory drift, plastic rotations or member forces. Ground accelerations associated with an earthquake cause building response resulting in global and interstorey drifts and member forces, all of which can be classified as demands. If both the demand produced by ground motion and the capacity of the structure to resist this demand could be predicted with certainty, the engineer could design a building and have 100% confidence that the building would achieve the desired performance objectives. Unfortunately, neither capacity nor demand can be precisely determined because of uncertainties and randomness inherent in our prediction of the ground motion, the structure’s response to this motion and its capacity to resist damage, given these demands. On the basis of the important advancements in performance evaluation, developed under the SAC project, a procedure for associating a level of confidence with the conclusion that the designed structure is capable of meeting the aforementioned performance levels (S.L.S. or S.L.U.) has been performed. The procedure includes the following steps: i. Determining the structure performance objective to be evaluated. This requires the selection of one or more performance levels, that is, either S.L.S. or U.L.S., and the appropriate hazard level, that is exceedance probability desired for this performance. The NEHRP guidelines (BSSC, 1998) recommend that design solutions provide a 90% level of confidence that the building satisfy desired performance from a global perspective and a 50% level of confidence that it satisfy the performance at a local level. ii. Determining the ground motion characteristics for the chosen performance objective. The ground motion intensity for each performance level should be chosen in order to have the same probability of exceedance as the hazard level of the design objective. It is assumed that a peak ground acceleration, p.g.a. equal to = 0.40 g should have the 90 % probability of not being exceeded in 50 years for the U.L.S. According to a Poisson model, this corresponds to a reference return period of 476.7 years; conversely a p.g.a. = 0.1 g should have the 90 % probability of not being exceeded in 10 years for
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Dipartimento di Ingegneria Meccanic
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UNIVERSITÀ DEGLI STUDI DI TRENTO D
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Ai miei genitori
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INDEX 1 INTRODUCTION...............
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INDEX iii 4.10.3 Results of the PsD
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1.1 Introduction 1 1 INTRODUCTION I
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1. INTRODUCTION imposes precise con
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1. INTRODUCTION joints whilst 2D mo
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7 2 DUCTILITY AND SEISMIC RESPONSE
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2. DUCTILITY AND SEISMIC RESPONSE O
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3. SEISMIC BEHAVIOUR OF BOLTED END
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4. SEISMIC RESPONSE OF PARTIAL-STRE
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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