Verification and Optimization of a Knockdown Factor Formula for Thin-Shell Structures

Abstract

Shell structures are widely favored due to their ability to bear substantial loads with minimal thickness, aligning with contemporary aesthetic sensibilities. This thesis investigates the buckling behavior of thin-shell structures with the aim to refine a knockdown factor formula. Authored by Yuanxi Zhao under the guidance of Dr.ir. P.C.J. Hoogenboom, Ir. A.C.B. Schuurman, and Dr.ir. F.P. van der Meer, this research addresses the critical issue of load-carrying capacity reduction due to shape imperfections leading to buckling.

A complete range of shell shapes and loading has been studied. The shells buckle in ring mode (1-3, 3-3, 2-1, 2-3), column mode (2-2), mixed column-ring mode (1-1) and in-extensional mode (3-2, 3-1, 1-2) (page 19).

Linear buckling analyses were conducted to explore how parameters such as height, boundary conditions and model size influence the buckling load factor. Geometrical nonlinear analyses were conducted using SCIA Engineer, introducing different imperfection amplitudes to simulate real-world conditions. The knockdown factor was calculated as nonlinear buckling load over linear buckling load. This knockdown factor was compared to a knockdown factor obtained from a formula.

The knockdown factor does not depend on the curvature ratio kyy/kxx or the membrane force ratio nxx/nyy. It only depends on the imperfection amplitude and d/t (page 65) and the slenderness a/t (page 6). The knockdown factor formula produces reasonable values. However, the formula is not accurate (page 62). It is recommended to derive a new knockdown factor formula.