Diagnosis method for seismic performance of Japanese traditional timber buildings

First in this paper, the guidelines of diagnosis for the seismic performance of traditional timber buildings are introduced which are under development in a committee of the Agency for Cultural Affairs of Japan. Next, the evaluation method of seismic performance proposed in the diagnosis is summarized. Next, as calculation examples, the lateral loaddisplacement relationships of five temple buildings and ten houses are presented, which are calculated according to the diagnosis method. Finally, the values of natural frequency derived from the load-displacement relationships are compared with those observed in micro tremor measurements on the buildings to confirm the adequacy of the evaluation method. As the result, an adequate correlation was observed between the calculated and observed values of natural frequency both for large buildings and houses. It suggests that the evaluation method proposed in the diagnosis is appropriate to evaluate the seismic performance of Japanese traditional timber buildings. INTRODUCTION So many traditional timber buildings exist in Japan and lately, the number of rebuilt buildings by traditional construction methods is also increasing. However, the seismic performance of these buildings is not well known, and it is necessary to develop a diagnosis method for the seismic performance of these buildings. Therefore, guidelines of diagnosis for important cultural properties are now under development in a committee of the Agency for Cultural Affairs of Japan. There are some problems in the diagnosis. First, the number of these buildings is very large compared with the number of specialists who are familiar with these structures. Next, the required performance level is affected by the usage of the building, the value as a cultural property, and so on. Therefore, the guidelines suggest that the diagnosis includes the first step executed by owners and the step of deciding required performance level according to the usage, cultural value, etc. Moreover, it is very difficult to evaluate the reduction of seismic performance by deterioration. Therefore, the evaluation of load-displacement relationship is divided from the assessment of deterioration, in the guidelines of diagnosis. There is a problem also in the structural modeling of these buildings. Concerning the evaluation of load-displacement relationships, there are a few experimental and/or theoretical studies on column rocking resistance (Ban 1941, Kawai 1996) and effect of clay walls as shear wall (Sugiyama et.al. 1984). Based on the results of these studies, performance evaluation method, main part of which is the estimation of load-displacement relationships, is under discussion. While, the authors have executed micro tremor measurements on many traditional timber buildings (Maekawa et.al. 1998, Uchida et.al. 1998). As the traditional timber buildings are usually valuable cultural properties, we have to employ the non-destructive examination to obtain their dynamic characteristics, and micro tremor measurement is satisfactorily applicable. It is possible to confirm the adequacy of the calculated load-displacement relationships by comparing the values of natural frequency between calculation and micro tremor though it is only in a small displacement area. First in this paper, the guidelines of diagnosis are introduced and the evaluation method of seismic performance in the diagnosis is summarized. Next, the lateral load-displacement relationships of 15 buildings according to the diagnosis method are presented. Finally, the values of natural frequency are calculated using the load-displacement relationships and are compared with those observed in micro tremor measurements to confirm the adequacy of the evaluation method. 1 Senior Research Officer, Nara National Cultural Properties Research Institute, 2-9-1 Nijyo-cho Nara-city, Nara 6308577, Japan 2 Head, Evaluation system division, Building Research Institute, 1 Tatehara, Tsukuba-city, Ibaraki 305-0802, Japan 3 Lecturer, Dept. of Architectural Engineering, The Polytechnic University, 4-1-1 Hashimotodai, Sagamihara-city Kanagawa 229-1196, Japan GUIDELINES FOR DIAGNOSIS The outline of guidelines for diagnosis of the seismic performance of important cultural properties (traditional timber buildings) is as follows. These guidelines give recommendations regarding the standard procedures and methods for owners and others to assess the potential seismic damage and to determine the appropriate counter-measures. The diagnosis is comprised of three steps (see Figure 1). 1) Structural diagnosis by the owner The owner takes on the responsibility of assessing the seismic performance of the buildings, taking into account the local environment, the structural characteristics, and the condition of conservation. As a general rule, the owner will carry out the diagnosis by himself or herself and, if necessary, will make a decision in cooperation with the municipal Board of Education concerned. 2) Basic structural diagnosis This diagnosis is carried out at the owner’s discretion when necessary as a result of the owner’s diagnosis. The owner should seek guidance from the Board of Education and should then leave the diagnosis to an appropriate conservation architect or architectural specialist. This diagnosis will decide whether there is sufficient lateral resistance to maintain the structural integrity of the building in order to protect its value as a cultural property, and will confirm the safety of the building for adaptive re-use. The diagnosis would be based on data from a physical survey and other materials such as geological maps. 3) Structural diagnosis by specialist Based on detailed data such as information obtained during restoration, this structural diagnosis must be carried out with an appropriate method in response to the structural characteristics. The owner should seek guidance from the prefectural Board of Education, and should leave the diagnosis to an appropriate structural specialist. If necessary, a special committee can be formed consisting of specialists such as architectural historians, seismic technicians, conservation architects and others. ESTIMATION OF LOAD-DISPLACEMENT RELATIONSHIP As an important part of the diagnosis by specialists, the estimation of load-displacement is discussed, which considers three types of earthquake resistance column rocking resistance, clay walls as shear walls, and single columns with suspended clay wall(s). In the diagnosis, load-displacement relationship of the building in each story and each direction can be estimated simply as the summation of the effect by these resisting elements. Column rocking resistance For Japanese traditional buildings with thick columns, the lateral resistance due to column rocking is regarded as one of the most important resisting elements. If the assumption of rigid bodies is used, the relationship between lateral load and displacement at the top of the column is expressed approximately by ) ( δ − = a h mg P (1) where, P = lateral load, m = mass on column(s), g = gravity acceleration, h = height of column, a = average of the diameter of column bottom and the width of daito bottom, and δ = displacement at the top of the column. Maximum force (P0 ) is given when δ equals zero. Actually, embedment occurs in the joints at the top and the bottom of the column, and the load-displacement relationship figures a curve as shown in Figure 2. Based on the review of experimental data (Ban 1941, Kawai 1996), a model of load-displacement relationship due to column rocking resistance is derived for diagnosis as Figure 3 shows. In compliance with the request of the owner, the prefectural Board of Education will: *1 provide guidance and advice with the aim of basic structural diagnosis following the opinion of the municipal Board of Education as well as the local committee for the protection of Cultural Properties. *2 provide guidance and advice with the aim of providing a structural diagnosis by a specialist in consultation with a structural architect. *3 if necessary, provide a guidance and advice with the decision of future counter-measures following consultation with the Agency for Cultural Affairs. Figure 1: Flowchart of the procedure for determining the diagnosis approach and consideration of counter-measures. Small-scale structures Structural diagnosis unnecessary General traditional timber structures Large-scale or special types of structures Structural diagnosis carried out by the owner < Take on the responsibility of assessing the seismic performance > Decision Basic structural diagnosis by architectural specialist Structural diagnosis by structural specialist Decisions regarding counter-measures mImprovement of maintenance Another look at adaptive re-use Restoration of the building Improvement of the seismic performance Restoration plan (including structural reinforcement) Decision *1