Effect Of Moisture Variations On The Performance Of Bolt Connections In Timber Structures

The behavior of bolt connections in timber structures and the stresses in the woodmembers are affected by moisture variations. Moisture induced stresses cause significantreduction in wood bearing strength. This paper focuses on the moisture induced stressesin a typical double shear bolt connection and compares moisture induced stress with loadinduced stress so that the influence of moisture variations can be figured out.The results of stress analysis show that when the wood member surfaces are exposedto decreasing RH the diameter at middle of the bolt hole (mid-plane of member) shrinks,and when exposed to increasing RH the diameter at the end of the hole (outer surface ofmember) shrinks. Both increase the compression stress in the timber. The moisture varia-tions tend to increase the overall stress level in the connection and make stress distributionalong the thickness of member more ununiform. The influence of moisture is distinguish-ingly evident at the outer surface of the wood members, which greatly increases the localstresses and produces alternating tension and compression. The results are validated bytheoretical derivations and test results from related literature.In the parametric study, the bolt diameter, the wood member thickness and the gapbetween the bolt and the hole are taken as the variables. The results show that as thebolt diameter decreases the stress concentration near the contact areas of wood membersbecomes intenser, and as the members thickness increases the stresses are also changed.Another phenomenon is that when there is no gap between the bolt and the hole the dis-tribution of load induced stress is more uniform than in a gapped connection. However,when moisture induced stress is considered, removing of the gap causes some material tobe subjected to alternating tension and compression stress. It means that the variation ofa particular parameter affects load induced stress and moisture induced stress differently.The same phenomenon exists with the variation of bolt diameter and member thickness.Therefore, when moistrue induced stress is considered, the parameters must be chosenwisely to make an effective connection.