The December 26th, 2004 tsunami killed many people and caused serious damage to civil engineering structures in the western costal of Thailand. To study and reduce the damage of buildings, this study analyses damage patterns, load distribution in structural members and the lateral resistance of buildings. In this research, the reinforced-concrete buildings are analyzed under tsunami loads. The building models are treated as 3-dimensional frames, and fiber models are used to capture non-linear behaviors of structural members. The building models are calibrated with results of the lateral field load test conducted on the one-story reinforced-concrete building which is the former office of Thai Meteorological Department in Phang-Nga. From the calibration, the errors of stiffness are about 2% - 28%, and the errors of the maximum deformation are about 1% - 30%. This research studies behaviors of the Kamala nursery building by treating as 2-dimensional frames. Two frames, which are parallel and perpendicular to the tsunami flow frames, are considered. The frame parallel to the tsunami flow is analyzed with and without masonry infilled walls. It is found that the maximum lateral resisting force of the frame without masonry infilled walls is 16 kN while that with masonry infilled walls is 360 kN. It is interesting to note that the masonry infilled walls resist 95% of lateral force. The resistance of the frame perpendicular to the tsunami flow is 33 kN. The force predicted from the guideline by Department of Public Works and Town Planning is found to be 93 kN which is larger than the transverse resistance of the frame.