Dynamic analysis of 3D steel structures equipped with the Seesaw system

The efficiency of the seesaw system when applied to low-rise 3-D steel structures is presented in this PhD Thesis. This efficiency is judged by several seismic response results coming from inelastic seismic time-history analyses of 2-, 5and 8-storey steel structures. These response results involve height wise distributions for peak interstorey drift ratios (IDR), peak residual interstorey drift ratios (RIDR) and plastic hinge formations in frame elements as well as peak forces to spiral strand ropes and viscous dampers of the seesaw system. For the foundation of the steel structures, different soil types are considered in order to assess the effects of soil-structure interaction (SSI). The angle of seismic incidence angle is also taken into account. From the results presented it is concluded that the seesaw system, essentially working always in tension, constitutes a bracing solution that can be potentially proposed by seismic codes as an alternative seismic force resisting system for low-rise steel structures.Στην παρούσα διδακτορική Διατριβή παρουσιάζονται τα αποτελέσματα σεισμικής απόκρισης τρισδιάστατων μεταλλικών κατασκευών χαμηλού ύψους εξοπλισμένων με το σύστημα seesaw. Τα αποτελέσματα αυτά προέρχονται από μη-γραμμικές αναλύσεις χρονοϊστορίας μεταλλικών κατασκευών 2, 5 και 8 ορόφων και περιλαμβάνουν τις καθ’ ύψος κατανομές των μεγίστων σχετικών μετακινήσεων ορόφων, των μεγίστων παραμενουσών σχετικών μετακινήσεων ορόφων καθώς επίσης και τις δυσμενέστερες κατανομές πλαστικών αρθρώσεων. Οι κατασκευές αυτές θεωρούνται είτε πλήρως πακτωμένες στο έδαφος είτε σε αλληλεπίδραση με αυτό. Από τα αποτελέσματα αυτά συμπεραίνεται ότι το σύστημα seesaw μπορεί να αποτελέσει μια ελκυστική λύση για μεταλλικές κατασκευές χαμηλού ύψους παραμερίζοντας τους συμβατικούς συνδέσμους δυσκαμψίας με τα προβλήματα λυγισμού τους

Effectiveness of the Seesaw System as a Means of Seismic Upgrading in Older, Non-Ductile Reinforced Concrete Buildings

This work investigates and discusses the effectiveness of the seesaw system when installed in an older, non-ductile reinforced concrete (RC) building for seismic upgrading purposes. In particular, two different configurations of the seesaw system are assumed in a two-storey 3D RC framed building which was designed according to older seismic provisions and, thus, is susceptible to flexural and shear failures. To check if there is any merit in employing the seesaw system in this RC building, non-linear time-history (NLTH) analyses are conducting using 11 seismic motions. Peak values for inter-story drift ratios (IDR), residual inter-story drift ratios (RIDR) and floor accelerations (FA) are computed, and the sequence and cause (i.e., due to surpass of flexural or shear strength) of plastic hinge formations are monitored. Leaving aside any issues related to fabrication and cost, interpretation of the results obtained by the aforementioned seismic response indices for the RC building under study leads to the conclusion that the seesaw system can be a retrofitting scheme for the seismic upgrading of older, non-ductile RC framed buildings

Seismic Performance of Steel Structure-Foundation Systems Designed According to Eurocode 8 Provisions: The Case of Near-Fault Seismic Motions

The seismic performance of steel structure-foundation systems subjected to near-fault earthquakes was assessed on the basis of response results from nonlinear time-history seismic analyses. The structural results included the maximum values for residual interstory drift ratios, base shears, and overturning moments of the steel structures, as well as the maximum values for residual settlement and tilting of the foundations. In order to reveal the influence of soil-building-interaction on the aforementioned response results, the steel building-foundation systems were designed according to Eurocode 8 provisions, assuming initially fixed and then compliant base conditions. It was concluded that for the case of near-fault seismic motions, good seismic performance of steel building-foundation hybrid systems designed according to European Codes was not guaranteed. A particular thing to note for these systems under near-fault seismic motions was that the seismic performance of the steel structure was most likely unacceptable, while one of the foundations was always acceptable

The Effect of Long Duration Earthquakes on the Overall Seismic Behavior of Steel Structures Designed According to Eurocode 8 Provisions

Premature and simultaneous buckling of several steel braces in steel structures due to the prolonged duration of a seismic motion is one of the issues that must be addressed in the next version of Eurocode 8. In an effort to contribute towards the improvement of the seismic design provisions of Eurocode 8, an evaluation of the overall behavior of some steel building-foundation systems under the action of long duration seismic motions is performed herein by means of nonlinear time-history seismic analyses, taking into account soil–structure interaction (SSI) effects. In particular, the maximum seismic response results—in terms of permanent interstorey drifts, overturning moments and base shears of the steel buildings as well as of the permanent settlement and tilting of their foundations—are computed. It is found that the seismic performance of steel buildings when subjected to long duration seismic motions is: (i) acceptable for the two and five-storey fixed base steel buildings and for the two-storey steel buildings with SSI effects included; (ii) unacceptable for the eight-storey fixed base steel buildings and for the five and eight-storey steel buildings with SSI effects included. In all cases of steel buildings with SSI effects included, the seismic performance of the mat foundation, as expressed by the computed values of residual settlement and tilting, is always acceptable