SUGGESTIONS FOR IMPROVEMENTS IN TIMBER PORTAL FRAME DESIGN WITH SEMI-RIGID KNEE JOINTS

Abstract

Timber portal frames assembled on the construction site are the most suitable solution from the point of view of limitations of transportation options. However, the connections of the rafters and columns assembled at the construction site in the knee joints, using bolts, show a certain tendency to show plastic deformations under load. In addition, the transmitted shear forces at the knee joint form different angles with the fiber direction in the rafter and column elements. Service experience of timber portal frames shows that the vertical displacement of the apex point increases, also cracks appear in the rafter elements near the knee joints over time. Unfortunately, code design formats do not present specifications for timber structures incorporating L-shape joints with mechanical fasteners. This article presents the results of studies that confirm the need to supplement the design rules for wooden portal frames. It is recommended to implement the rotational spring constant when define rigidity of L-shape joints for RFEM model of structure. The Hoffman failure criterion has been chosen as the most appropriate of the known ones for evaluating the crack resistance of structural wooden elements, because this criterion includes more important specific properties affecting the development of plastic deformations and the formation of cracks in wood.