Janusch Töpler defended his doctoral thesis with distinction

On 11 November 2025, Janusch Töpler presented the outcome of his doctoral research titled " Load-bearing capacity of imperfection-sensitive timber members under combined bending and compression" with distinction in front of the doctoral committee. Many congratulations to Janusch Töpler on this outstanding achievement!

Doctoral Committee 

Presentation of Janusch Töpler during defence.

Abstract Doctoral Research

This thesis discusses the load-bearing behaviour and capacity of imperfection-sensitive timber members under combined bending and axial compression. It combines the results of three research projects: DIBt - ZP 52-5-13.194, RP 7-1 of the Cluster of Excellence IntCDC, and IGF No. 21285 N. 

The main objectives were to establish the basis for a comprehensive understanding of the buckling behaviour of timber members and to formulate mechanically sound, consistent verification methods for the buckling of timber members with combined bending and axial compression. The focus was on the lateral torsional buckling phenomenon. However, as there is a fluent transition between the stability phenomena of flexural buckling and lateral torsional buckling and a separation is not necessarily mechanically reasonable, the stability phenomena were always studied holistically.

For understanding the real stability phenomena and generation of a database, imperfection measurements on 242 timber beams and 57 timber columns in 23 buildings, flexural buckling tests on 27 beech LVL GL75 columns, and lateral torsional buckling tests on 19 GL 24h beams were conducted. Through analytical investigations, the knowledge of the theoretical background of the stability behaviour at bending and axial compression was broadened. For the detailed study of the influences of structural and geometrical imperfections and combined bending and axial compression, numerical models were developed. Based on these, the experimental results were also extended by extensive numerical parameter studies.

By combining experimental, analytical, and numerical results, the nonlinear N_x,c-M_y,1-interaction of the k_c-k_m-method was validated, and consistent equivalent imperfections were derived for the design of imperfection-sensitive timber beam-columns. The results additionally demonstrated that the shear and compressive plasticising and the cross-sectional warping due to shear have a significant influence on the load-bearing capacity of (imperfection-sensitive) timber members and should be taken into account in the design.

On this basis, proposals were developed for a modification and extension of the verifications of imperfection-sensitive timber members in FprEN 1995-1-1:2025, which allow for a more reliable, consistent, and economical design. Parts of the proposals, such as the equivalent imperfections and the nonlinear N_x,c-M_y,1-interaction of the k_c-k_m-method, have already been incorporated into FprEN 1995-1-1:2025.

In this thesis, for the first time, the flexural buckling and lateral-torsional buckling of timber members are linked on the basis of extensive experimental, analytical, and numerical calculations in consistent analysis and design models. This in-depth understanding provides a sound foundation on which subsequent research, e.g. on double-tapered beams and other aspects, can build on and expand.