The primary objective of the present paper is to experimentally examine the buckling collapse characteristics of fusion welded aluminum-stiffened plate structures under axial compression until and after the ultimate limit state is reached. The secondary objective of the paper is to study a nonlinear finite element method modeling technique for computing the ultimate strength behavior of welded aluminum structures. A set of aluminum-stiffened plate structures fabricated via gas metal arc welding is studied. The test structure is equivalent to a full scale deck structure of an 80 m long high speed vessel. The plate part of the structures is made of 5383-H116 aluminum alloy, and extruded stiffeners are made of 5083-H112 aluminum alloy. Welding induced initial imperfections such as plate initial deflection, column type global initial deflection of stiffeners, sideways initial distortion of stiffeners, welding residual stresses, and softenng in the heat-affected zone are measured. The ANSYS nonlinear finite element method is employed for the numerical computations of the test structure’s ultimate strength behavior by means of a comparison with experimental data. Insights and conclusions developed from the present study are documented.