Analysis, hysteresis loops:

A basic building block in the study of the dynamic response of wood structures is an understanding of the hysteresis loops for connections. These are characterized by the nonlinear behavior of the fasteners and the wood medium supporting them, and the pinching which develops as slackness develops during cyclic loading. Since the process is history dependent, it must be adjusted to different excitations. Characterization of hysteresis loops is normally done by adjusting a numerical model to a test result for a connection under a selected experimental history. However, the approach taken in this project is to calculate the hysteresis loops from basic properties of the fasteners and the wood medium, and embeding this calculation into the dynamic analysis of the complete structure. Although more computational intensive, the approach permits automatic adjustment to any excitation history, as well as to changes in connection geometry and mechanical characteristics. 

A general hysteretic subroutine, HYST, has been developed for the calculation of the loops for a single fastener in one or more layers of wood medium. In the case of multiple layers, some of these can displace relative to the others. Input for HYST requires elastic and plastic properties of the fastener and the embedment characteristics of the wood medium.

The following figures show examples of calculated loops for 1) a single fastener with a enforced displacement history at the head, and 2) a single fastener joining three layers, where the middle layer displaces with respect to the outside ones.

An extension of HYST, including friction between the shank of the fastener and the surrounding wood medium, required for pull-out predictions, has also been developed under this project.

HYST model details can be found in:
Foschi, R.O. 2000.  "Modeling the hysteretic response of mechanical connections for wood structures", Proceedings, World Timber Engineering Conference, Whistler, B.C. Canada, July 31-August 4, 2000.