Long-term cumulative damage model of historical timber member under varying hygrothermal environment

Abstract

A long-term damage cumulative model for the duration of load effect of structural timber is proposed in this paper, which is economical in analysis as well as involving long-term hygrothermal effect. Based on the Miner linear damage cumulative theory, the cumulative damage model is applied to analyze the annual hygrothermal, daily cyclic thermal and daily cyclic relative humidity's effect on load-duration behavior and to calculate the sum of damage in one year. The results indicate that the annual and daily hygrothermal effect should be taken into consideration when calculating the damage accumulation, in which the influence levels from large to small are annual hygrothermal, daily relative humidity and daily thermal effect. Considering both annual and daily hygrothermal variations as external loads the long-term model is determined. Its application to service-life prediction of a historic timber structure verifies the feasibility and high-efficiency of the proposed approach.The strength of structural timber can be affected by both the intensity and the duration of loads, which is often referred to as the duration of load (DOL) effect. It is actually induced by the 'creep-rupture' phenomena of the wood material. Owing to the creep, the deformation of the structural timber under loading induce plastic deformation, which cannot restore and increase continuously until the wood fiber ruptures, thereby causing the reduction in the strength of wood [1] .The DOL effect is a major and difficult issue in the research of wood science. It causes the reduction in the strength of wood material and thereby induces the substantial deformation of the structures constructed by timbers. In some historic buildings the structural timber is repaired or replaced due to the large deformation instead of the lack of material strength. For example in the Tai-he Palace in the Summer Palace, the large flexibility of a timber beam induced the drop of the swallowtail by about 10-11.5 cm [2] . Similar observations are also reported in other historic buildings. Therefore, in the research on the security assessment and residual service-life prediction of historic buildings, the DOL effect is one of the major factors. Many experimental programs have focused on DOL effects in full-size timbers and various cumulative damage models have been proposed. Gerhards et al [3] first applied the Miner linear cumulative damage theory to evaluate the time-related effect of loading on the strength of sawn larch under constant environment. Barrett et al [4] and Foschi et al [5] revised the WANG Xueliang et al : Long-Term Cumulative Damage Model of … 431 model and proposed a threshold ratio for stress level. They believed that when the stress level is less than the threshold ratio, the time-related effect should not be considered. Fridley et al carried out some experiments on sawn larch to study constant temperature [6] , cyclic temperature [7] , cyclic relative humidity [8] and hygrothermal effects [9] on load-duration behavior of timber and obtained correspond...

Investigation of the Effect of the Force Arm on the Bending Capability of Prestressed Glulam Beam

“…Such studies mainly consider the influence of reinforcement form, reinforcement ratio, and prestressing on the bending performance of the beam. Fiber material reinforcement refers to the pasting of fiber cloth or fiberboard at the bottom of a beam, mainly studying the influence of fiber material layers, arrangement, and fiber types on the reinforcement effect [20][21][22].…”