Assessment of Semi-Rigidity of Dowel Type Knee Joint between Timber Elements

Abstract

Creation of a moment resisting connection between column and beam elements in knee joints is the core problem in portal frame design of structural timber. The main performance requirement for rigid joints is to avoid any rotation between the members connected. Withstanding this condition by using glued finger joints, the knee joint can be classified as a rigid one and the bending moment produced by external loads can be transferred safely. Nevertheless, the solution of knee joints with mechanical fasteners is an effective way to overcome transportation limitations. Knee joints with mechanical fasteners exhibit a relevant semi-rigidity when some rotational movement develops between beam and column members connected due to the embedment of the fasteners in the wood and their bending deformation. The aim of the current study is to examine the rotational stiffness of semi-rigid joints. The rotational stiffness of the connection was determined in terms of design moment capacity to radians of rotation. The calculation methods for design of semi-rigid structural timber connections developed during recent years were used for the calculation of the bearing capacity. L-shape connection models of softwood lumber assembled by normal strength dowels were tested under a stepwise static load. The analysis of the test data testifies that connections with dowels located around a single circle demonstrate an intensive development of rotational movement. For design purposes the semi-rigidity of these connections may be characterised by the rotational stiffness value Kp = 10 Md /rad. Moment resisting joints of dowels located in double circles behave almost as rigid, and their stiffness may be characterised by rotational stiffness value Kp = 65 Md /rad. It is found from comparison of embedment deformations that dowels located around external circle transfer more than four times bigger reactive shear force than dowels around inner circle.