engleski

Seismic analysis of plane frame structures

An earthquake is a phenomenon that results from a sudden release of stored energy in Earth's crust that creates seismic waves. Earthquakes are related to the tectonic nature of the Earth. Earth's lithosphere is made of plates which move very slowly but constantly due to the heat in Earth's mantle and core. Plate boundaries grind past each other, creating frictional stress. When frictional stress exceeds the critical value, a sudden failure occurs. When failure at the fault plane results in a displacement of Earth's crust, the elastic strain energy is released and seismic waves are radiated, causing an earthquake. The severity of an earthquake is described by magnitude and intensity. Magnitude characterizes the size of an earthquake by indirectly measuring the energy released (Richter scale). Intensity indicates the local effects and potential for damage produced by an earthquake on Earth's surface as it affects humans, animals, structures and natural objects. Software and computer power are rapidly increasing, thus making unimaginable calculations nowadays performable (time history and pushover) in quite a fast and accurate manner. The question remains, namely, as to how accurate the model of structure (2D, 3D) and input data (earthquake) is? This article will focus on plain plane frame structures and provide an overview of the methods used for obtaining seismic forces. This article analyzes simple two-story, five-story and ten-story reinforced concrete buildings, situated in a seismically active region in Croatia. Each structure is modeled as a simple plane frame. Each frame is made of concrete C25, in line with Eurocode2. Pseudo-static analysis is made according to Croatian regulations. Frames are also analyzed using spectrum analysis according to EC8. Ground type is A (rock or other rock-like geological formation). Damping is assumed to be 5%. Linear and non-linear time history analysis is made by using the synthetic accelogram representing earthquakes in this region. Two well known earthquakes were selected according to their magnitudes. Non linear pushover analysis is made for seismic assessment of the structure. Analysis was displacement controlled. Very large numerical differences in relation with type of analysis are noted. Elastic spectrum analysis has base shear factor ranging from around 0.16 for two-story frame, 0.126 for five-story frame and 0.1 for ten story-frames. Pseudo static analysis has 0.1 base shear factor for all types of frames. Due to very strong earthquake used for generation of synthetic accelogram, a linear time history has the greatest seismic forces (for the two-story frame base shear factor is 0.5 of total weight of structure). As the height increases, the base shear factor decreases for linear time history (for ten-story frame base shear is 0.175). Non-linear time history analysis has around 40-50% lesser forces, manly due to non-linear distribution of forces. Although the cross sections for higher frames were bigger, it is obvious that seismic performance is rapidly decreasing with height of the structure. This article compared various methods for obtaining seismic forces. It is shown that the pseudo static as the oldest method analysis isn’ t sufficient enough even for this kind of a simple structure. As expected time history (both linear and non-linear) resulted in maximal forces for each type of frame, indicating that should enough input data be available, this kind of analysis should be applied.

spectrum; time-history; accelogram; seismic force

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