Effects of pretension in bolts on hysteretic responses of moment-carrying timber joints

Abstract

The adoption of a concept similar to the prestressing technique used in laminated wood decks of bridge structures might increase the initial stiffness or ultimate resistance of dowel-type timber joints by applying pretension to their bolts. This study investigated the effect of pretension in bolts on hysteretic responses and ultimate properties of momentcarrying timber joints with steel side plates. A pretension of 20 kN that yielded a prestress level of 1600 kPa or about 90% of the allowable long-term end-bearing strength of spruce species was applied to the bolts of prestressed joints. The superiority of the prestressed joint over the non-pre-stressed joint was proved by very high hysteretic damping, equivalent viscous damping ratio, and cyclic stiffness. At any given rotation level, hysteretic damping reduction and moment resistance decrement due to continuously reversed loads were found to be small because bolt pretensioning minimized the pinching effect. This study showed that the hysteresis loop of the prestressed joint can be obtained by adding the frictional hysteresis loop due to pretension force into the hysteresis loop of the non-pre-stressed joint. Despite a great increase of initial stiffness, only slight increments in ductility coefficient and ultimate moment resistance were found in the prestressed joint.

Beam to Column Timber Joints with Pretensioned Bolts

“…Table 1 summarizes the equivalent viscous damping ratio at 0.04 rads for four different pretension forces where it is clearly indicated that increase of hysteretic damping due to pretension in bolts significantly increases the damping ratio of the joints. The damping ratio shown in Table 1 is more or less comparable with our previous report [10] in which an equivalent viscous damping ratio of 0.32 was achieved due to bolt pretension of 20 kN. …”

“…Figure 4 shows the experimental hysteresis loops of connection between steel side plates and vertical member for the four different pretension forces. Very high initial stiffness (or joint resistance) due to bolt pretensioning is shown in Figures 4.b Figure 5 shows the relation between pretension in bolts and frictional force, which was derived based on an assumption that frictional moment resistance can be replaced by a couple of frictional force [10]. A linear relation between the pretension and joint frictional force is well indicated in Figure 5 with a friction coefficient of 0.366.…”

“…Improved static and dynamic response of steel-totimber joints due to axial pretension in bolts has been shown in our previous report [10]. The pretension is simply obtained by fastening the bolts, for example by using a torque wrench device, so that the steel plates compress the wood member in its transverse direction.…”

“…An improved joint performance, both strength and ductility ratio, was also reported by gluing highembedding strength materials at joint shear plane, the interface between two adjacent members [4][5][6][7]. The main reason for this local reinforcement is to prevent timber splitting, which in most cases leads to failure of the connection.…”

Exploratory study on fatigue behaviour of laterally loaded, nailed timber joints, based on a dissipated energy criterion

“…Some studies of timber joints with slender fasteners have been focused on the characterisation of mechanical response parameters like hysteretic damping (i.e., dissipation of energy during cycling), viscous damping ratios, cyclic stiffness and loading frequency under displacement controlled loading sequences (Dolan et al 1996 ;Foschi 2002 ;Kent et al 2005 ;Awaludin et al 2008 ). The purpose of such characterisation was mostly due to prescriptive input data on joints as components within system level analysis of wood building superstructures, subjected to the effects of seismic forces.…”