The propagation of long cracks under constant amplitude cyclic loading is studied in complex welded box beams made of high strength low alloy (HSLA) steel. The 8 x 1 x 0.7 m box beams were designed to simulate the cellular structure of a double hull ship but the results of the experiments are equally applicable to other box systems such as bridges. These experiments were designed to evaluate the residual fatigue life after a significant fatigue crack has formed. After testing, residual stresses were measured on two box beams using the standardized strain gage hole-drilling method. The experiment results demonstrated the good crack tolerance of cellular structures. The residual life of a box beam (after a welded detail has failed) was significant. The crack driving force was evaluated using finite element modeling. Reasonable correlation between these large scale tests and the fatigue crack growth rate from small compact specimen was obtained only with models that included the effects of crack closure due to residual stresses. Linear elastic fracture mechanics proved to be sufficient to predict the behavior of long cracks in this case.