Introduction

Bayon temple, Angkor, built in late 12th century to early 13th century consists of sandstone-dry-masonry structures. It has been deteriorated possibly by rain, plant intrusion, settlements, and so on. Strong wind of more than 20 m/s was observed in the rainy season (JSA 1995), which may vibrate the structures either directly or via ground motion and eventually cause damage on them.

The temple consists of a main tower, sub-towers, and two libraries as shown in Figure 1. The main tower is more than 30 m high, located at the top of the man-made mound of about 15 m high. Sub-towers stand from several different height levels of the mound, which are interconnected via the inner and the outer corridors. The libraries are located between these corridors.

Vibration characteristics of the main tower and sub-towers in Bayon temple have been studied by micro-tremor measurements since 2003, which is a preliminary study for constructing analytical models to evaluate the effects of the wind excitation. Though the towers are merely a pile of stones, they definitely have vibration modes with predominant frequencies of 2 Hz to 6 Hz according to their heights, with damping factors of 2 to 3%, similar to modern ordinary

Figure 1. Plan and section of the Bayon temple.

reinforced concrete structures (Sugiura et al. 2004, Maeda et al. 2005, Maeda et al. 2007).

Equivalent continuum models of FEM for the towers were constructed by simulating experimentally obtained base-fixed natural frequencies of horizontal vibration modes with adjusted elastic constants. The simulated equivalent Young's modulus ranged from 1/27 to 1/9 of the laboratory test. This extremely low modulus may be attributable to spring effects of some material existed between the interfaces of sandstone blocks, or to reduced contact areas at the interfaces.

In 2006, we had a chance to measure micro-tremor at the libraries. While the main tower and sub-towers are basically stonewalls, the libraries are relatively small frame-like structures. Comparing the equivalent elastic modulus of these different types of structures, insight on the underlying common physical phenomenon of sandstone-dry-masonry may be found.

Our final objectives are the evaluation of bearing capacity for these dry-masonry structures. Since the extremely low continuum-equivalent elastic modulus may reveal incapableness of continuum mechanics in simulating rupture processes of dry-masonry structures, we have applied the Discontinuous Deformation Analysis (DDA) (Shi 1993) to the evaluation of the safety margin of the library against the wind load as an illustrative example.

Figure 2. Appearance of the northern library (upper panel) and the southern library (lower panel).

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