Developing cyclic loading protocols for regions of low to moderate seismicity

This paper deals with the development of loading protocols appropriate for cyclic testing in regions of low to moderate seismicity in Europe. To serve this goal, cumulative damage demands imposed by a set of 60 ground motion records, representative of a European moderate seismicity region for the 2% probability of exceedance in 50 years seismic hazard level, on a wide variety of SDOF systems are evaluated. To meet the calculated cumulative damage demands, several loading protocols are developed for different structural types, vibration periods and level of reliability (medians or 84th percentiles). Protocol comparisons reveal that the proposed loading protocols for the median estimates of cycle amplitudes are significantly less demanding than existing protocols. Consequently, they can result in more cost-effective structural configurations in European regions of low to moderate seismicity

Sensitivity of drift capacities of URM walls to cumulative damage demands and implications on loading protocols for quasi-static cyclic tests

Displacement-based seismic design and assessment procedures require as key input parameter estimates of displacement capacities of the critical elements. The displacement capacities of unreinforced masonry (URM) walls are typically determined by means of quasi-static cyclic tests. This paper shows that for URM walls failing in shear the number of applied cycles in quasi-static cyclic tests influences the drift capacities obtained from these tests. It is therefore important that loading protocols are used that reflect the expected cumulative damage demand. The latter will depend on the structural properties and the seismicity of the region. Existing loading protocols for quasi-static cyclic testing were derived to reflect the cumulative cyclic demands in regions of high seismicity and where derived for structural systems other than masonry buildings. For regions of moderate seismicity, these protocols are likely to underestimate the actual drift capacities. Based on statistical analysis of the displacement response of SDOF systems representative for URM buildings, the paper proposes new loading protocols for quasi-static cyclic tests on URM walls

Loading protocols for structures designed for different behaviour factors

All the existing loading protocols have been developed independently of the value of the behaviour factor q that the structure has been designed for. Conservatively, a single loading protocol is adopted, for all q values, based on the structural system with the most onerous cumulative seismic demands. However, analyses conducted by the authors, show that imposed cumulative seismic demands decrease significantly for structures designed for high behaviour factors. This drives to the conclusion that adopting a single loading protocol for the most demanding q value may lead to the derivation of highly conservative loading protocols for the rest of the structural systems. In this paper, new loading protocols will be developed for structures designed for different behaviour factors

Loading protocols for European regions of low to moderate seismicity

Existing loading protocols for quasi-static cyclic testing of structures are based on recordings from regions of high seismicity. For regions of low to moderate seismicity they overestimate imposed cumulative damage demands. Since structural capacities are a function of demand, existing loading protocols applied to specimens representative of structures in low to moderate seismicity regions might underestimate structural strength and deformation capacity. To overcome this problem, this paper deals with the development of cyclic loading protocols for European regions of low to moderate seismicity. Cumulative damage demands imposed by a set of 60 ground motion records are evaluated for a wide variety of SDOF systems that reflect the fundamental properties of a large portion of the existing building stock. The ground motions are representative of the seismic hazard level corresponding to a 2% probability of exceedance in 50years in a European moderate seismicity region. To meet the calculated cumulative damage demands, loading protocols for different structural types and vibration periods are developed. For comparison, cumulative seismic demands are also calculated for existing protocols and a set of records that was used in a previous study on loading protocols for regions of high seismicity. The median cumulative demands for regions of low to moderate seismicity are significantly less than those of existing protocols and records of high seismicity regions. For regions of low to moderate seismicity the new protocols might therefore result in larger strength and deformation capacities and hence in more cost-effective structural configurations or less expensive retrofit measures

Selection of earthquake ground motions for multiple objectives using genetic algorithms

Existing earthquake ground motion (GM) selection methods for the seismic assessment of structural systems focus on spectral compatibility in terms of either only central values or both central values and variability. In this way, important selection criteria related to the seismology of the region, local soil conditions, strong GM intensity and duration as well as the magnitude of scale factors are considered only indirectly by setting them as constraints in the pre-processing phase in the form of permissible ranges. In this study, a novel framework for the optimum selection of earthquake GMs is presented, where the aforementioned criteria are treated explicitly as selection objectives. The framework is based on the principles of multi-objective optimization that is addressed with the aid of the Weighted Sum Method, which supports decision making both in the pre-processing and post-processing phase of the GM selection procedure. The solution of the derived equivalent single-objective optimization problem is performed by the application of a mixed-integer Genetic Algorithm and the effects of its parameters on the efficiency of the selection procedure are investigated. Application of the proposed framework shows that it is able to track GM sets that not only provide excellent spectral matching but they are also able to simultaneously consider more explicitly a set of additional criteria

Optimum design of reinforced concrete retaining walls with the flower pollination algorithm

The flower pollination algorithm (FPA) is anefficient metaheuristicoptimizationalgorithm mimickingthe pollinationprocessof flowering species. In this study, FPA is applied, for first time, to the optimum design of reinforced concrete (RC) cantilever retaining walls. It is foundthat FPA offers important savings with respect to conventional design approachesand that it outperformsgenetic algorithm (GA)andthe particle swarm optimization (PSO) algorithm in this designproblem.Furthermore, parameter tuning reveals that the best FPA performance is achieved for switch probability values ranging between 0.4 and 0.7, a population size of 20 individualsand aLévy flightstep sizescale factor of 0.5. Finally, parametric optimum designs show that theoptimumcost of RC retaining walls increases rapidly with the wallheight and smoothly with the magnitude of surcharge loadin

Rocking isolation of a typical bridge pier on spread foundation

It has been observed that after some earthquakes a number of structures resting on spread footings responded to seismic excitation by rocking on their foundation and in some cases this enabled them to avoid failure. Through application to a standard bridge supported by direct foundations, this paper discusses the major differences in response when foundation uplift is taken into consideration. Special focus is given on the modifications of rocking response under biaxial and tri-axial excitation with respect to uniaxial excitation. It is found that inelastic rocking has a significant isolation effect. It is also shown that this effect increases under biaxial excitation while it is less sensitive to the vertical component of the earthquake. Finally, parametric analyses show that the isolation effect of foundation rocking increases as the size of the footing and the yield strength of the underlying soil decreases

Loading protocols for European regions of low to moderate seismicity

Existing loading protocols for quasi-static cyclic testing of structures are based on recordings from regions of high seismicity. For regions of low to moderate seismicity they overestimate imposed cumulative damage demands. Since structural capacities are a function of demand, existing loading protocols applied to specimens representative of structures in low to moderate seismicity regions might underestimate structural strength and deformation capacity. To overcome this problem, this paper deals with the development of cyclic loading protocols for European regions of low to moderate seismicity. Cumulative damage demands imposed by a set of 60 ground motion records are evaluated for a wide variety of SDOF systems that reflect the fundamental properties of a large portion of the existing building stock. The ground motions are representative of the seismic hazard level corresponding to a 2% probability of exceedance in 50 years in a European moderate seismicity region. To meet the calculated cumulative damage demands, loading protocols for different structural types and vibration periods are developed. For comparison, cumulative seismic demands are also calculated for existing protocols and a set of records that was used in a previous study on loading protocols for regions of high seismicity. The median cumulative demands for regions of low to moderate seismicity are significantly less than those of existing protocols and records of high seismicity regions. For regions of low to moderate seismicity the new protocols might therefore result in larger strength and deformation capacities and hence in more cost-effective structural configurations or less expensive retrofit measures

Assessment of seismic behaviour of existing reinforced concrete structures

The goal of this research is to investigate seismic response of existing RC structures. To serve this goal, a new beam-column type finite element model is developed for inelastic analysis of RC structures. The finite element is formulated on the basis of flexibility method. It consists of three subelements which are connected in series and represent respectively hysteretic flexural, shear and anchorage slip response. The flexural subelement is used for modelling flexural response of the RC member before and after yielding of longitudinal reinforcement. It is able to model variation of section flexibility along the member and gradual spread of inelastic flexural deformations from the ends towards the midspan. The shear subelement represents RC member hysteretic shear behaviour. Following a dual coupling procedure with the flexural subelement, proposed for first time in this thesis, this subelement is capable of modelling accession of shear deformations in the plastic hinge regions of RC members as well as their propagation to the midspan. In addition, the proposed subelement accounts for shear flexibility variation along the member due to variation of the shear force caused by distributed loading. Finally, the bond-slip subelement accounts for the fixed-end rotations which arise at the interfaces of adjacent RC members due to bond deterioration and slippage of the reinforcement in the joint regions. In addition to the above, an innovative local seismic damage index is proposed herein, according to which flexural, shear and bond-slip damage are treated in an explicit manner. The proposed index is able to take into consideration the additive character of degradation arising from the three deformation mechanisms as well as the increase of damage caused by their interaction. The proposed finite element and damage index are calibrated against experimental data coming from six experimental column specimens and three experimental plane frame structures. The analytical results of the finite element are in accordance with the experimental values in terms of stiffness, strength and displacement demands. Moreover, the proposed damage index describes adequately the damage state of all specimens under examination.Αντικείμενο της διατριβής είναι η αποτίμηση της σεισμικής απόκρισης κατασκευών Ο/Σ που δεν έχουν σχεδιαστεί σύμφωνα με τις σύγχρονες αντισεισμικές προδιαγραφές. Για τον σκοπό αυτόν αναπτύσσεται ένα πεπερασμένο στοιχείο προσομοίωσης της υστερητικής συμπεριφοράς γραμμικών μελών Ο/Σ. Το στοιχείο βασίζεται στη μέθοδο της ευκαμψίας και συντίθεται από τρία επιμέρους υποστοιχεία που συνδέονται μεταξύ τους σε σειρά και προσδιορίζουν αντίστοιχα την απόκριση του μέλους σε κάμψη, διάτμηση και ολίσθηση της αγκύρωσης. Το καμπτικό υποστοιχείο προσομοιώνει την καμπτική συμπεριφορά του δομικού μέλους πριν και μετά τη διαρροή των άκρων του και είναι σε θέση να προσομοιώσει την εξάπλωση της καμπτικής διαρροής από τα άκρα προς το εσωτερικό του. Το διατμητικό υποστοιχείο προσομοιώνει τη διατμητική υστερητική συμπεριφορά. Με κατάλληλη διαδικασία διπλής αλληλεπίδρασής του με το καμπτικό υποστοιχείο, που προτείνεται για πρώτη φορά στη διατριβή, επιτυγχάνεται η ορθή προσομοίωση αφενός της αύξησης των διατμητικών παραμορφώσεων στις περιοχές των πλαστικών αρθρώσεων και αφετέρου της εξάπλωσης τους από τα άκρα προς το εσωτερικό του δομικού μέλους. Μέριμνα λαμβάνεται επίσης για τη μεταβολή της δυστμησίας στην περίπτωση που η δρώσα τέμνουσα μεταβάλλεται κατά μήκος του στοιχείου από την ύπαρξη κατανεμημένων φορτίων. Τέλος, το ολισθητικό υποστοιχείο προσομοιώνει την ανάπτυξη των τοπικών στροφών που προκαλούνται στα άκρα των δομικών μελών από τη διείσδυση της διαρροής και την ολίσθηση της αγκύρωσης του οπλισμού στους γειτονικούς κόμβους. Στη συνέχεια, προτείνεται στη διατριβή ένας νέος τοπικός δείκτης βλάβης στον οποίο οι βλάβες από καμπτική, διατμητική και ολισθητική παραμόρφωση λαμβάνονται υπόψη με διακριτό τρόπο. Ο προτεινόμενος δείκτης είναι ικανός να προσομοιώσει με επάρκεια τον αθροιστικό χαρακτήρα που έχουν οι επιμέρους συνιστώσες της σεισμικής βλάβης αλλά και την προσαύξηση της υποβάθμισης από την αλληλεπίδραση μεταξύ των επιμέρους μηχανισμών παραμόρφωσης. Η βαθμονόμηση του προτεινόμενου πεπερασμένου στοιχείου και δείκτη βλάβης υλοποιείται με τον έλεγχο της ανελαστικής συμπεριφοράς έξι πειραματικών δοκιμίων υποστυλωμάτων και τριών πειραματικών πλαισιακών φορέων Ο/Σ. Οι αναλυτικές προβλέψεις από το πεπερασμένο στοιχείο βρίσκονται σε πολύ καλή συμφωνία με τις πειραματικές καταγραφές τόσο σε όρους δυσκαμψίας και αντοχής όσο και σε όρους μετακινήσεων. Επιπρόσθετα, ο προτεινόμενος δείκτης βλάβης αποδίδει με επάρκεια την πραγματική εικόνα βλάβης των εξεταζόμενων πειραματικών δοκιμίων