Keywords: spectrum, time-history, accelogram, seismic force.

This paper will focus on plain plane frame structures and provide an overview of the methods used for obtaining seismic forces [1]. This paper analyses 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 constructed of concrete C25, in line with Eurocode2 [1].

Pseudo-static analysis is made according to Croatian regulations. Frames are also analyzed using spectrum analysis according to EC8. The 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. A non-linear pushover analysis is made for seismic assessment of the structure. Analysis was displacement controlled [1].

Very large numerical differences in relation to the type of analysis are noted. The elastic spectrum analysis has a 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 the very strong earthquake used for generation of synthetic accelogram, a linear time history has the greatest seismic force when the two-story frame base shear factor is 0.5 of the total weight of the structure. As the height increases, the base shear factor decreases for linear time history (for a ten-story frame the base shear is 0.175). Non-linear time history analysis has around 40-50% lesser forces, manly due to a non-linear distribution of forces. Although the cross sections for higher frames were bigger, it is obvious that seismic performance rapidly decreases with the height of the structure.

This paper compared various methods for obtaining seismic forces. It is shown that the pseudo static as the oldest method analysis is not sufficient enough even for this kind of a simple structure. As expected the 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.

1

D. Mestrovic, V. Stanilovic, "Comparison of different methods used for seismic design of reinforced concrete frames", SEE5, 2007.

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