Analysis and improvement for the inherent backing-bar crack problem of electrogas and electroslag welding in box-columns

Electroslag welding (ESW) and electrogas welding (EGW) are frequently used for joining column plate and diaphragm plate in the box-column construction in Taiwan. These two welding methods utilize backing bars to prevent molten weld metal from leaking out. When the molten weld metal solidifies, an "inherent crack" forms between the backing bar and the column plate. When a large-scale earthquake strikes, this kind of inherent cracks might propagate and result in fractures of the EGW or ESW joints. Applying basic fracture mechanics, this study took stress intensity factor as the index for measuring the stress concentration of an inherent backing-bar crack tip. For the inherent backing-bar crack problem in EGW and ESW, the detailed geometric parameter analysis was performed by using finite-element method and least-squares method. The four construction problems which might cause fractures in EGW and ESW joints were analyzed and discussed. An improved method using "corner-cut" backing bars in EGW and ESW was tested in this study. The numerical analysis verified that the improved method could effectively lower the stress intensity factor of the inherent backing-bar crack tip and reduce the risk of crack fracturing. In addition, a non-contact optical experiment was conducted in this study to measure the stress intensity factor variations of the three inherent backing-bar cracks in a cruciform specimen before and after the fillet weld reinforcement. After comparison, the experimental results verified the accuracy of the numerical analysis method utilized in this study.