Seismic pounding and collapse behavior of neighboring buildings with different natural periods ()
Abstract
Seismic pounding phenomena, particularly the collision of neighboring buildings under long-period ground motion, are becoming a significant issue in Japan. We focused on a specific apartment structure called the Nuevo Leon buildings in the Tlatelolco district of Mexico City, which consisted of three similar buildings built consecutively with narrow expansion joints between the buildings. Two out of the three buildings collapsed completely in the 1985 Mexican earthquake. Using a finite element code based on the adaptively shifted integration (ASI)-Gauss technique, a seismic pounding analysis is performed on a simulated model of the Nuevo Leon buildings to understand the impact and collapse behavior of structures built near each other. The numerical code used in the analysis provides a higher computational efficiency than the conventional code for this type of problem and enables us to address dynamic behavior with strong nonlinearities, including phenomena such as member fracture and elemental contact. Contact release and recontact algorithms are developed and implemented in the code to understand the complex behaviors of structural members during seismic pounding and the collapse sequence. According to the numerical results, the collision of the buildings may be a result of the difference of natural periods between the neighboring buildings. This difference was detected in similar buildings from the damages caused by previous earthquakes. By setting the natural period of the north building to be 25% longer than the other periods, the ground motion, which hada relatively long period of 2 s, first caused the collision between the north and the center buildings. This collision eventually led to the collapse of the centerbuilding, followed by the destruction of the north building.
1. INTRODUCTION
In the 1985 Mexican earthquake, many apartment buildings in Mexico City, which was approximately 400 km away from the epicenter (see Figure 1), collapsed due to long-period ground motion [1,2]. Among those collapsed structures, there was a specific apartment structure called the Nuevo Leon buildings in the Tlatelolco district, which had three similar 14-story buildings built consecutively with very narrow gaps and were connected with expansion joints (see Figure 2). Two buildings among them, the north and the center, collapsed completely as a result of the earthquake (see Figure 3). The damage was caused by the impact of the neighboring buildings,which resulted from the change in the natural periods of the buildings from the prior reduction of strength and soil subsidence. An additional effect of the resonance phenomena was caused by long-period ground motion. In the case of Mexico City, extremely soft soil, such as the clay of Lake Texcoco, lies under most parts of the city. This unique subsurface condition resulting from the historical lakebed has distinct resonant low frequencies of
Figure 1. Epicenter of the 1985 Mexican earthquake.
Figure 2. The Nuevo Leon buildings before the earthquake.
Figure 3. Collapse of the Nuevo Leon buildings (south building at the far side, picture by Marco Antonio Cruz).
approximately 0.5 Hz [3]. Therefore, nearly all of the 14-story buildings in the district, which had natural periods of approximately 2 s, were destroyed during the earthquake, as shown in Figure 4.
We investigated the seismic pounding phenomena due to the long-period ground motion by conducting analyses on a simulated model of Nuevo Leon buildings and two neighboring framed structures with different heights. We used a finite element code based on the adaptively shifted integration (ASI)-Gauss technique [4], which provides higher computational efficiency than the conventional code for this type of problem, and enables us to address dynamic behavior with strong nonlinearities, including phenomena such as member fracture and elemental contact. Contact release and re-contact algorithms are developed and implemented in the code to understand the complex behaviors of structural members during the seismic pounding and collapse sequence. In the analysis of the Nuevo Leon buildings, we set the natural period of one building to be 25% longer than those of the other buildings, as a difference in natural periods was observed in similar buildings based on the damage caused by previous earthquakes.
2. NUMERICAL METHODS
The general concept of the ASI-Gauss technique compared with the earlier version of the technique, the ASI
Figure 4. Ratio of the damaged buildings vs. story no. of buildings in the 1985 Mexican earthquake.
technique [5], is explained in this section. In addition, the algorithms considering member fracture, elemental contact, and incremental equation of motion for excitation at fixed points are described.
2.1. ASI-Gauss Technique
Figure 5 shows a linear Timoshenko beam element and its physical equivalence to the rigid bodies-spring model (RBSM). As shown in the figure, the relationship between the location of the numerical integration point and the stress evaluation point where a plastic hinge is formed is expressed as [6]