Objective  The wall wood columns of ancient building are partly or completely wrapped by the wall. It is easy to decay for the contact part between the wood column and the wall, and the detection operation space is narrow. Through the study on the nondestructive testing for defect status of the wall wood columns in Yangxin Hall of the Palace Museum of Beijing, the nondestructive testing method of the defect status and the existence law of the decay for the wall wood columns were explored and revealed, respectively, which provides the basis for the repair work of the wall wood columns in Yangxin Hall, and also provides reference for the nondestructive testing and defect evaluation for the wall wood columns in other ancient wooden buildings.

  Method  68 wall wood columns in the Main Hall, Meiwu Hall, East Hall and West Hall of Yangxin Hall were used as the research objects. First of all, the exposed face, air vent and opening of wood columns were used for field detection, including external defect detection, knocking detection, internal defect detection, moisture content detection and tree species sampling. Then, the data of wood column size, defect types and size, micro drill resistance curve and so on were collected and analyzed to evaluate the defect status of each wood column tested, and the decay distribution law of the wall wood columns was summed.

  Result  The wall wood columns in Yangxin Hall were relatively complete. The results showed that there were different forms for the external and internal defects of the wall wood columns in Yangxin Hall. The main forms of external defects were external decay and material missing, and the main forms of internal defects were internal decay and cavity, and the secondary forms of these two main defects were cracks. For a single wood column, the decay mainly occurred in the area where the wood columns were in contact with the wall at the cross section. In the longitudinal, decay mainly occurred in the bottom of wood column, and the decay decreased with the height. The probability of decay for two-sided exposed wood columns was the lowest and followed by the single-sided exposed wood columns, and the completely wrapped wood columns were the highest.

  Conclusion  The method used in this study is feasible for nondestructive testing and evaluation for defect status of the wall wood columns, and the position, wrapping degree and tree species of wall wood columns have influence on their defect status.