Effect Of Moisture Variations On The Performance Of Bolt Connections With Inserted Steel Plate

Bolt connections are one of the most commonly used joints in modern woodstructures, of which connections with inserted steel plate are often preferred for its fineappearance and simple structure. However, due to some special material properties ofwood, performance of bolt connections in wood structures will be different from otherstructures. When in service, temperature and moisture variations will induce stress inwood. Thus, investigating stress induced by temperature and moisture variations will bevery useful to design of bolt connections with inserted steel plate.The constitutive models of wood and theory of heat and mass transfer are reviewedin this paper. A user defined UMAT was chosen to simulate the behavior of wood in thebolt connections. The stress of wood in the connections was computed in the three cases:with load only, with moisture variations only and with both load and moisture variations.Besides, the influence of the thickness of wood member, bolt diameter and the period ofvariation of the relative humidity was investigated. The results from finite elementanalysis show that moisture variations due to change of relative humidity will induceconsiderable stress in wood, especially in area close to the surface. The deformationmode of bolt connections with inserted steel plate was determined by the thickness ofwood member and bolt diameter. If the period of relative humidity variation is long, thestress induced in the connection will become more significant. Therefore, the influenceof relative humidity variations needs to be considered in design of the connections, anddimensions of bolt and wood member need to be designed reasonably.In addition, the load distribution to each bolt of a multi-bolt wood-steel connectionwill be different. If the unequal distribution of load is not considered in the connectiondesign, the ultimate resistance of connection will be improperly estimated. Accordingly,a bolt-modification factor is introduced into both Eurocode5and NDS1997to calculatethe load capacity of multi-bolt connections, whereas such group-bolt effect is neglectedin Code for Design of Timber Structures GB5005-2003. In this paper, finite elementmodel of the multi-bolt connections was developed to analyse the load distribution inmulti-bolt connection, and a method to estimate the load carried by each bolt waspresented.