IMPERFECTION MEASUREMENTS ON TIMBER MEMBERS AT RISK OF BUCKLING
With the increasing use of hardwoods and innovative hardwood products with significantly higher strengths, slimmer structures with larger spans can be realized in timber construction. However, this also means that individual timber members are more sensitive to in-plane flexural and lateral torsional buckling and thus more sensitive to imperfections. The current imperfection approaches according to DIN EN 1995-1-1 vary greatly depending on the verification method (e.g. L/288 to L/1100 for BSH) and are not verified by measured values for members made of hardwood and members made of softwood that are subjected to bending.
An extensive measurement program in cooperation with the Institute for Photogrammetry (IfP) of the University of Stuttgart on real members is intended to create a database with geometric imperfections of hardwood members (especially laminated veneer lumber, LVL) and members made of softwood subjected to bending (especially glulam). The measurements of geometric deviations (curvatures, inclinations, …) are carried out by means of laser scanning on the struc-ture of wooden buildings. 15-20 timber buildings and at least 400 members are to be measured.
Together with assumptions on structural imperfections supported by literature, the measured geometric imperfections serve as input values for systematic analytical and numerical calcula-tions on the load bearing capacity of imperfection-sensitive timber members. The comparison of the numerically determined equivalent imperfections with the approaches of the standards allows an evaluation of the existing stability verification methods and the derivation of recom-mendations for improved differentiated approaches.
Finally, recommendations for the equivalent imperfections and buckling curves of imperfection-sensitive timber members, especially made of hardwood, will be given, which allow an economic design with verification methods of EC5 at a uniform safety level. Due to the active participation of the applicant in the standardization work, the results will also be included in the discussion on the revision of EC5.
Prof. Ulrike Kuhlmann
Institute of Structural Design (KE), University of Stuttgart
Deutsche Institut für Bautechnik (DIBt)