Αποτίμηση Φέρουσας Ικανότητας Πενταώροφου Κτηρίου από Οπλισμένο Σκυρόδεμα

Abstract

Η παρούσα μεταπτυχιακή διπλωματική εργασία πραγματεύεται την αποτίμηση και ανάλυση της συμπεριφοράς και επάρκειας ενός πολυώροφου κτηρίου από οπλισμένο σκυρόδεμα, τόσο με ελαστικές όσο και με ανελαστικές μεθόδους. Η υφιστάμενη κατασκευή είναι ένα πενταώροφο κτήριο με υπόγειο, πιλοτή και δώμα στην περιοχή της Άνω Γλυφάδας η οποία μελετήθηκε και σχεδιάστηκε το έτος 1999. Οι ελαστικές μέθοδοι ανάλυσης που εφαρμόστηκαν είναι αυτές της δυναμικής φασματικής και της γραμμικής ανάλυσης χρονοϊστορίας, ενώ οι ανελαστικές μέθοδοι που εφαρμόστηκαν περιλαμβάνουν την ανελαστική στατική μέθοδο (Pushover) και την ανελαστική δυναμική μέθοδο (ανάλυση χρονοϊστοριών). Η έκβαση της αποτίμησης υποδεικνύει την επάρκεια της κατασκευής έναντι των διάφορων φορτιστικών σεναρίων, αν και κατά πόσο αυτή χρήζει επέμβασης/επισκευής ή ενίσχυσης. Πιο συγκεκριμένα: Στο πρώτο κεφάλαιο γίνεται μία σύντομη αναφορά στη σπουδαιότητα αποτίμησης των κατασκευών, υφιστάμενων ή νέων, καθώς και μία σύντομη αναφορά στο αντικείμενο της παρούσας εργασίας και των λογισμικών που χρησιμοποιήθηκαν. Στο δεύτερο κεφάλαιο παρουσιάζεται η υπό μελέτη κατασκευή και περιγράφεται η διαδικασία που ακολουθήθηκε για την προσομοίωσή της στο λογισμικό SAP2000 ώστε να γίνει η περαιτέρω ανάλυση. Καθορίζονται οι ιδιότητες των υλικών, τα χαρακτηριστικά των κατόψεων και των διατομών αλλά και οι φορτίσεις που επιβλήθηκαν. Στο τρίτο κεφάλαιο πραγματοποιείται η ιδιομορφική ανάλυση του φορέα και παρουσιάζονται οι ελαστικές μέθοδοι ανάλυσης που εφαρμόστηκαν στο προσομοίωμα. Αρχικά παρουσιάζεται η μεθοδολογία εφαρμογής και τα αποτελέσματα της δυναμικής φασματικής μεθόδου με χρήση του φάσματος του Ευρωκώδικα 8. Εν συνεχεία εκτελείται η γραμμική ανάλυση χρονοϊστορίας κατά την οποία γίνεται χρήση τριών διαφορετικών σεισμικών γεγονότων, του Kobe στην Ιαπωνία (1995, Takatori station), του σεισμού του Northridge του Los Angeles της California (1994, Rinaldi station) και του σεισμού του Düzce στην Τουρκία (1999, Bolu station). Παρουσιάζονται τα μέγιστα παραμορφωσιακά και εντατικά μεγέθη που προκύπτουν από την εκάστοτε ανάλυση και γίνεται μία σύντομη σύγκριση μεταξύ τους. Στο τέταρτο κεφάλαιο παρατίθεται η θεωρία του σύγχρονου τρόπου σχεδιασμού των κατασκευών που βασίζεται στον αντισεισμικό σχεδιασμό με στάθμες επιτελεστικότητας. Το πέμπτο κεφάλαιο αφορά στις ανελαστικές μεθόδους ανάλυσης. Αρχικά αναπτύσσεται το θεωρητικό υπόβαθρο και οι προϋποθέσεις εφαρμογής της ανελαστικής στατικής μεθόδου (Pushover). Παρουσιάζονται οι μέθοδοι υπολογισμού του σημείου επιτελεστικότητας της κατασκευής και η εφαρμογή της ανάλυσης στο προσομοίωμα. Τέλος, παρουσιάζονται τα αποτελέσματα της ανάλυσης για τις δύο κατανομές της οριζόντιας φόρτισης, ομοιόμορφης και τριγωνικής, που εφαρμόζονται στην κατασκευή. Στη συνέχεια πραγματοποιείται η ανελαστική δυναμική ανάλυση κάνοντας χρήση των τριών σεισμικών καταγραφών που χρησιμοποιήθηκαν και κατά την ελαστική ανάλυση. Παρατίθενται τα αποτελέσματα και αξιοποιούνται στην αποτίμηση της επάρκειας της κατασκευής. Στο έκτο κεφάλαιο δίνονται τα κυριότερα συμπεράσματα που εξάγονται για την υπό μελέτη υφιστάμενη κατασκευή μετά το πέρας όλων των αναλύσεων, ελαστικών και ανελαστικών.

The present master thesis deals with the assessment and analysis of the behavior and integrity of an existing multi – storey reinforced concrete building, with both elastic and inelastic analysis methods. This existing structure is a five – storey building with also a basement, a ground floor and a loft, and is set in the area of Ano Glyfada, in the city of Athens. It was studied and designed in the year 1999. The elastic analysis methods applied are those of spectral static analysis and linear time history analysis, while inelastic methods applied include the non – linear static analysis and the non – linear dynamic analysis (time history analyses). The outcome of the evaluation indicates the adequacy of the construction against various scenarios of load cases, and whether the existing building needs any additional repair or retrofitting measures. In the first chapter a brief reference to the importance of evaluation of buildings is presented, either existing or new ones, as well as a brief reference to the main subject of the present thesis and of the utilized software. In the second chapter a detailed presentation of the existing structure is given along with the description of its simulation in the software SAP2000v15 so as to proceed further with the analysis. The geometry, the material properties, the design of the concrete members and the loads that were used in the studied building are also defined in this chapter. More specifically the building has a rectangular plan (with dimensions 8,30m×14,20m). The vertical structural system of the building consists of columns and walls, while the basement is surrounded by perimeter walls of reinforced concrete. The height of the floors is determined at 3,00m while the height of the basement and the loft is 2,90m and 2,20m respectively. The influence of infill masonry walls was ignored in carrying horizontal loads. The simulation of the horizontal and vertical members, as well as strip footings was made with the use of frame elements with six degrees of freedom per node. The perimeter walls of the basement were simulated with x - braces of high stiffness. Finally the interaction between the soil and the structure was modeled considering appropriate springs. The material properties, in SI units, are for concrete C20/25 (fck=20MPa) and S500 (fyk=500MPa) for both longitudinal and shear steel reinforcement. The third chapter presents the modal analysis of the structure and the elastic analysis methods that were imposed to simulate the building’s response along with their results. The first step of the analyses is the definition of the design spectrum which was selected in accordance with the provisions of Eurocode 8 – Part 3 and the initial parameters which were used in the initial design of the existing structure. According to the results of modal analysis, the eigenperiods of fundamental modes in X and Y directions are Τx1=0,8602sec and Τy2=0,7717sec respectively. The third mode shape is a torsional one with a period of Τ3=0,6521sec. Subsequently, the spectral static analysis, as well as its results, are presented. Modal response spectrum analysis may be applied to all types of buildings without restrictions. All modes of vibration that contribute to the structure’s global response are taken into account. The vertical component of the seismic action is ignored in this analysis. Finally, the building is subjected to linear time history analysis considering three different seismic excitation records in order to estimate the seismic behavior of the building under realistic seismic actions and compare the results with the respective results of spectral analysis. The three seismic events that are used are those of Kobe in Japan (1995, Takatori station), the Northridge earthquake in Los Angeles of California (1994, Rinaldi station) and Düzce earthquake in Turkey (1999, Bolu station). The accelerograms were downloaded from the PEER (Pacific Earthquake Engineering Research Center) Ground Motion Database, University of Berkeley, California, after they were scaled according to the target, design spectrum. The PEER tool evaluates automatically the appropriate linear scale factor with which the selected accelerograms are multiplied. The scale factor does not alter the relative frequency content of the acceleration time series, and thus does not change the shape of time series. From the comparison between the spectral static analysis and the linear time history analysis it turned out that the results of the linear time history analysis are a lightly worse from those of the spectral analysis, both for the element forces and moments, and the joint displacements. The fourth chapter presents the theory guiding modern design of structures based on the so called Performance Based Design. The objective of Performance Based Design is the evaluation of the elastoplastic response of a structure subject to seismic actions. Seismic performance is described by designating the maximum allowable damage state (performance level) for a specific seismic hazard (earthquake ground motion). Performance level describes a limiting damage condition which may be considered satisfactory for a given building for a given ground motion. The limiting condition is described by the physical damage of the structure’s frame, the threat to life safety of the building’s occupants corresponding to the sustained damage and the post- earthquake serviceability of the building. The fifth chapter deals with the inelastic analysis methods. Initially, the theoretical background and the requirements for applying nonlinear static method (Pushover) is developed. Subsequently, the methods for the estimation of the performance point of the structure and the analysis results of the model are presented in detail. First of all, the nonlinear material properties were taken into consideration and were imported in the software. The pushover analysis uses a series of successive elastic analyses with modified stiffness, to approximate the force – displacement capacity diagram of the overall structure, commonly known as pushover curve. The mathematical model of the structure is modified to account for reduced resistance of yielding components. Lateral force distribution is applied until additional components yield. This process carries on until the structure becomes unstable or until a predetermined limit is reached. Uniform as well as triangular distributions of horizontal loads were applied accordingly. The analysis performed shows that for either distribution of lateral forces in both directions the structure reaches a performance point which is characterized as of «Life Safety» category, which means that the structure is expected to suffer serious damage in its structural members, but without collapse, as there will remain substantial lateral strength and stiffness reserves, and the vertical elements will be able to withstand the vertical loads. Thus the structure may withstand moderate and intense seismic events, experiencing a repairable damage. Finally, an inelastic dynamic analysis is performed using the three accelerograms used also in the linear time history analysis. The nonlinear material results were taken accordingly to those that were defined for the inelastic static analysis. The results concerning the adequacy of the structure result quite similar to those of the pushover analysis regarding the performance point reached. This is mainly due to scaling of the initial accelerograms to meet the target design spectrum. Alternatively a more accurate scaling would have been to use the seismic records scaled to the spectrum that corresponds to the performance point emerged during the pushover analysis. The sixth chapter provides the main conclusions drawn for the studied existing structure using both elastic and inelastic analyses.