Bearing stiffness in wood-to-wood compression joints

Abstract

In post-tensioned timber frames beams and columns are connected by means of a tendon. The tendon presses the beams against the columns which leads to a stress situation where beams are loaded parallel to the grain and columns are loaded perpendicular to the grain. These kind of wood to wood connections are semi-rigid and can therefore resist bending moments. An important factor influencing the behaviour of post-tensioned timber frames is the connection stiffness.Compression tests perpendicular to the grain have been performed in order to estimate the perpendicular to the grain stiffness of a post-tensioned timber connection. The stiffness is represented by the modulus of subgrade reaction, which is the equivalent of a spring constant distributed over an area and which is a parameter frequently used in soil mechanics to calculate deformations and stresses under a foundation. The modulus of subgrade reaction is also needed to calculate the moment-rotation behaviour of post-tensioned timber connections.The compression tests were evaluated using different analytical models based on spring models and the Boussinesq solution in order to estimate the perpendicular to the grain stiffness, i.e. the modulus of subgrade reaction.The test evaluation showed that the modulus of subgrade reaction can be estimated based on the geometrical and mechanical properties of the column solely as suggested by the authors earlier if the Young’s modulus is estimated according to EN 338. However, if the calculation is based on higher values from literature or tests, the stress distribution in the column has to be accounted for in order to get accurate results for the modulus of subgrade reaction.The tests also showed that a reinforcement of the column is necessary in order to obtain a satisfactory stiffness in the post-tensioned timber connection. Reinforcing the Norway spruce (picea abies) column with European ash (fraxinus excelsior) increases the modulus of subgrade reaction significantly compared to a column made of spruce entirely.