## 1 Introduction  
Traditional buildings have suffered different degrees of damage, mainly due to external loads and material strength degradation [1]. Long-term loading can lead to material damage [2, 3]. Due to an environment suitable for fungi growth, the decay of timber members in traditional buildings is common [4], which in turn leads to a decrease in the strength of wood components [5]. Systematic research on the effect of aging on the mechanical properties of timber has been carried out in Japan during the 1950s and 1960s [6, 7, 8, 9, 10, 11]. The effect of load history was also studied in Europe and the United States in the 1980s and 1990s [12, 13, 14, 15]. More reports [16, 17, 18, 19, 20] on the static and impact strengths and dynamic properties of aged timber have also been published. All of these studies significantly contribute to new knowledge of the material science of timber for the protection of aging traditional heritage timber structures. A timber member in an existing building usually cannot be replicated, especially in aging heritage buildings. A destructive test is usually not applicable to obtain the strength of timber material. On the other hand, the strength of clear specimen samples (without flaw) from the timber member can be easily obtained. Some experimental studies have considered the correlation between the strengths of clear specimens and the corresponding wood logs with the aim to develop a long-term strength model of timber [21, 22]. A relationship between the strengths of clear specimens and wood logs was later established, based on reliability theory [23, 24, 25]. Other researchers investigated the development of wood grading methods and the correlation of results from different methods, which included visual inspection, clear specimen tests, log tests, and nondestructive tests [26, 27, 28]. The parallel-to-grain compressive strength of clear specimens obtained between the 1950s and 2020s are summarized in Figure 1, which is drawn based on the data of new and old materials in Appendix A. The \(Y\)-axis label in Figure 1 is the percentage difference of parallel-to-grain compressive strength between the aged timber (\(\sigma_{\text{O}}\)) and new timber (\(\sigma_{\text{N}}\)), denoted as \((\sigma_{\text{O}}-\sigma_{\text{N}})/\sigma_{\text{N}}\%\). They have been obtained from samples ranging from 50 to 1800 years old (Appendix A). There are many cases in Figure 1 where the parallel-to-grain compressive strength of aged timber is greater than that of new timber. This is not consistent with the common belief that the strength of aged timber is less than that of new timber [26; 29]. This suggests that the parallel-to-grain compressive strength of clear specimens is not representative of the strength of on-site structural members with damage and decay, which accumulates with service life. Although creep increases with the time, the creep rate slows as the amount of accumulated creep increases [30]. Therefore, it is necessary to evaluate the strength of timber members based on the strength of the natural specimen with damage and decay. The important fact that defects are not uniformly distributed across the cross-section of a timber member has long been ignored. The sapwood (SW) is subject to natural decay soon after the tree is felled because of its high moisture content (MC) and rich nutrients that welcome fungi and insects. The heartwood (HW) has a low MC and contains ingredients such as tannins, aromatic oils, and calcium carbonate, which are toxic to fungi [5]. The heartwood also decays, but is more resistant to corrosion than the sapwood. There is a significantly different presence of defects between the SW and the HW, resulting in their different strengths. Therefore, it is necessary to study the strength differences of the heartwood and sapwood. The strengths of specimens from both the HW and SW are experimentally studied in this report. The Chinese fir (_Cumingham lancolata_) is examined using the static test method.