Strength Analyses of Wooden I-Beams with a Hole in the Web

Sammanfattning: Wood-based light-weight I-beams are today widely used in the construction industry. An important feature of these beams is that the user can make holes in the web where needed. Today there is no general method used to calculate the reduced strength of these beams with a hole in the web. The calculation methods vary between the manufacturers and are commonly based on empirical results. The aim of this master’s thesis was to create finite element models of this type of beams and with these investigate the stress distribution in beams with holes in the web, where a crack would likely occur and in what direction it will grow. The aim was furthermore to calculate the shear force capacity for beams with holes by use of different models based on fracture mechanics theory, as well as investigate how changes in the material properties influence the shear force capacity, and finally to evaluate the currently used calculation methods and suggest improvements or a new method. Calculations showing the location of the most stressed point and the orientation of the principal stresses in an area surrounding this point were performed for a number of load cases. For load cases dominated by shear force the results indicated diagonal cracking in 45◦ direction. The load cases with pure normal or moment loading indicated fracture in the upper or lower edge of the hole. Furthermore, the calculated stresses indicated that a crack would both initiate and continue to grow along an approximately straight line perpendicular to the edge of the hole. Three methods based on fracture mechanics were used in the finite element calculations of the shear force capacity; the point stress criterion, the mean stress criterion and the initial crack criterion. The calculated shear force capacity from these methods was compared to the shear force capacity gained in a previously performed test study. In this study 11 beam geometries were tested, and to be able to compare the calculations, the same geometries and load cases were used in the present study. The results show that the mean stress criterion and the initial crack criterion are suitable for shear force capacity calculations for beams with holes in the web. The point stress criterion severely underestimated the shear force capacity for some beams. The calculation method used by the manufacturers Swelite and Forestia was evaluated by comparing the results from the test study with the results from using this method. This comparison showed that this method overestimated the real shear force capacity for one beam. A new calculation method can be based on the mean stress criterion, since this method gave values well corresponding to the results from the test study and since this is a fairly easy method to use.