Recently, the weld fractures of column-diaphragm joints were reported to disrupt the cyclic loading tests on steel welded beam-to-column connections in Taiwan. This paper analyzes the root cause leading to the weld fractures. To confirm the effects of electro slag welding (ESW), the hardness, toughness and microstructure are investigated for a SN490B steel column-diaphragm joint. The result shows that the ESW process may give the weld a relatively low hardness. In addition, the heat affected zone (HAZ) can absorb much less energy than the parent material. In detail, the 30-mm thick parent material absorbs energy more than 105J, satisfying the code requirement of 27J at 0oC in the Charpy impact tests. However, the welding heat causes the microstructure of HAZ to be notably coarse, reducing the Charpy value to 21 J. To prevent the weld fractures, it is recommended to refine the grain sizes of HAZ, improving the toughness. It is also suggested to develop and use the compatible welding materials.