A numerical modeling approach is proposed to assess the effectiveness of automatic submerged arc welding procedures for steel plates with thickness larger than 50 mm. The scope of study includes partial joint penetration butt welds, their numerical analysis, and the subsequent development of welding recommendations for thick steel plates, with the ultimate objective to reduce the related industrial losses. The proposed approach consists of a heat-transfer numerical model that is integrated with a stress analysis model that was validated with measurements obtained from two heavy steel assemblies that were welded in a fabrication shop. The first consisted of two 75 mm thick plates welded in the flat position, for which temperature measurements were recorded. The proposed model shows good agreement with these measured temperature results. The second consisted of a built-up box column that also utilized 75 mm thick plates. This assembly experienced cracks near the welds after the completion of the welding procedures. Through a comprehensive investigation of the material properties of the steel plates it was confirmed that the material met the specifications in terms of minimum fracture strain elongation and fracture toughness. The model was used to successfully predict the crack initiation due to thermal stresses that were developed near the welds. The proposed model can potentially be employed to assess the defect limits found in current specifications for welded steel assemblies that utilize thick plates.