Response of eight storey peinforced concrete builidng for near-fault earthquakes

Abstract

Anticipating the response of a structure to near fault earthquakes, is an object of great concern to scientists worldwide and have proposed various methods of assessment of this response. The main aim of this master thesis was to assess the vulnerability of an 8-storey reinforced concrete building that subjected to near-field earthquakes, based on the HAZUS methodology and to establish the reliability of the results according to SAP2000 program. Moreover, comparing the maximum top displacement of the building through the sample of near field earthquakes we can conclude the important factors that give larger displacements and larger damages.The HAZUS methodology is a set of components that attempt to estimate losses, operational (probabilistic estimation) and economic, due to an earthquake scenario. Firstly, the building subjected in modal analysis that generated the natural frequencies of it ( T1 = 1.165 sec, T2 = 1.113 sec). Then it subjected in pushover analysis with load distribution according to the first Eigenmode in order to construct the pushover curve (base shear- top-floor displacement).The aim was to construct the fragility curves that defining by HAZUS. These curves classify the structure at four levels of damage (Slight, Moderate, Extensive, Collapse) and describe the possibility to have a certain level of damage to the building. Depending on the type of building and the vulnerability curve we wanted to build was necessary to calculate standard deviations (vds) that take into account uncertainties on the curve pushover, with levels of performativity, with the features of construction, with the pulse of directivity and territorial motion. Eventually the curves defined by following lognormal distribution. To determine the vulnerability of the building, it used an existing sample of near field earthquakes, with range of seismic magnitudes of 6.4 to 7.6, and larger maximum spectral displacement. Earthquakes applied through accelerograms (using SAP2000) in the building and after inelastic time-history analyses, resulted the maximum displacement for each record. Comparing the records in each earthquake we are taking the following fragility curves and conclusions separate for each earthquake. The straight lines on the fragility curves are the maximum top-floor displacement for each record. Table 25 presents the structural damage levels for all earthquake records, which have been obtained using the “HAZUS-Umax” and “Observation” methods, respectively, as those have been described in previous. A general observation is that all seismic motions result in structural damage, while, in the majority of the cases, the damage level is either “moderate” or “extensive”, indicating the detrimental effects of near-fault ground-motions on the seismic performance of the 8-storey reinforced concrete building. Nevertheless, none of the two methods showed that the building will undergo complete damage or collapse during any of the selected seismic actions. Moreover, it seems that the magnitude of the earthquake is not the primary factor that determines the severity of the damage of the analysed structure. It is observed that earthquakes of higher magnitude, such as the Izmit (Mw = 7.4) and Duzce (Mw = 7.1) result in lower damage levels than seismic events of lower magnitude, such as the Kobe (Mw = 6.8) and Northridge (Mw = 6.7) earthquakes. That means that other important factors and characteristics of the ground motion, in combination with the structural properties, determine the overall seismic performance of the building during an earthquake. For example, it is observed that a significant role on the severity of the damage plays the epicentral distance of the seismic recording. In particular, when observing the results in Table 25, we can see that in the case of the Chi-Chi earthquake, the ground-motions with relatively small epicentral distance (TCU065, TCU068 and TCU102) result in “extensive” damage, while the rest of the records from the same event with larger epicentral distances result in “moderate” and “slight” damage. The ultimate goal was to understand how the magnitude of the earthquake, the directivity and the distance of the fault from the recording station affecting the results that have been obtained. Comparing the max|Ux,Top results that analysis gave we can conclude the following: 1. Forward directivity: All the earthquakes that reach the extensive damage limit had forward directivity. Earthquakes with Neutral Directivity gave moderate damage limit even if they had larger magnitudes than others with forward directivity. This gives the result that earthquakes with forward directivity can give larger displacements and bigger damages. 2. Closest distance from fault: For the same earthquake and stations with the same directivity the displacements increase as the distance of the station from the fault. As we can see from Chi-Chi Taiwan earthquake that gave the larger displacement, the records with smaller distance from the fault gave larger displacements. 3. Magnitude of the earthquake: Comparing the results of Tabas (Iran), Izmit (Turkey), Chi-Chi (Taiwan) and Duzce (Turkey) with magnitude 7.1, 7.6, 7.4 and 7.1 respectively we can see that only the Chi-Chi Taiwan earthquake occurs extensive damages. It is known that as the magnitude of the earthquake is increasing the building suffers from larger top displacements and therefore stronger levels of damage. From these records we can conclude that the magnitude is not the most important factor to occur larger displacements. This may be due to the saturation of the ground motion observed in large earthquakes, ie the size grows, but does not increase ground motion. However, larger magnitude with the factors that are written above can be catastrophic. Conclusion of all seismic excitations can conclude that the forward directivity and the closest distance from the fault resulted in large displacements in the building and larger damages. However the displacements are increasing more when seismic magnitude is increasing. We can note that the building did not reach a complete damage despite that it was suffered by very strong earthquakes. The record that occurred largest displacement (0.51 m) was TAK-000 Hanshin Kobe), Japan with magnitude 6.8 and classified the building damages as extensive and gave the larger possibility 40% for complete damage. Knowing the consequences that will appear due to an earthquake, appropriate measures can take in order to reduce them. The proper design of structures and measures can reduce the damages of the structures and the number of casualties.