1. Butt welds

The force transmission of the butt weld is direct, smooth, and without significant stress concentration, thus it has good stress performance and is suitable for connecting components that bear static and dynamic loads. However, due to the high quality requirements for butt welds and the strict gap requirements between welded components, they are generally used in factory manufactured connections. Butt welds, also known as bevel welds, are formed by machining the edges of the plate into a suitable form and size of bevel (as shown in the figure below) during welding to provide necessary space for the welding rod to operate and ensure sufficient penetration depth inside the butt weld. The basic forms of bevel can be divided into I-shaped, single-sided V-shaped, V-shaped, X-shaped, U-shaped, and K-shaped. The groove form varies with the thickness of the plate and the welding method. When using manual welding, when the plate thickness t ≤ 10mm, an I-shaped seam without a groove can be used, with a gap of only 0.5-2mm. When t ≤ 5mm, single-sided welding can be used; When the thickness of the plate is t=10-20mm, a V-shaped or semi V-shaped groove is used; For thicker plates t ≥ 20mm, X-shaped, U-shaped, or K-shaped should be used. For the roots of V-shaped and U-shaped seams, it is necessary to remove the welding roots and perform repair welding. If there are no conditions for root cleaning and welding repair, a pad should be added in advance. When using automatic welding, due to the high current and deep penetration, V-shaped groove is only used when t ≥ 16mm. In connections where the width or thickness of the weldment varies, in order to reduce stress concentration, a slope with a gradient of no more than 1:2.5 should be made on one or both sides of the plate as shown in the figure, forming a gentle transition. If the difference in plate thickness is not more than 4mm, a slope can be omitted.

2. Fillet welds

(1) The form of fillet weld can be divided into side fillet weld parallel to the direction of force application, front fillet weld perpendicular to the direction of force application and oblique fillet weld intersecting the direction of force application, and circumferential fillet weld according to their length direction and direction of external force application. The cross-sectional forms of fillet welds are divided into ordinary type, flat slope type, and deep penetration type. HF in the figure is referred to as the weld leg size of the fillet weld. The ratio of the welding foot edge of the ordinary cross-section is 1:1, which is similar to an isosceles right angled triangle. The bending of the transmission line is more severe, resulting in severe stress concentration. For structures that directly bear dynamic loads, in order to ensure smooth force transmission, the front corner welds should adopt a flat slope type with a ratio of 1:1.5 between the two welding corner edges (the long side should follow the direction of internal forces), and the side corner welds should adopt a deep penetration type with a ratio of 1:1.

(2) Construction requirements for fillet welds

1) The minimum fillet weld size is related to the thickness of the welded part. When the welded part is thick but the fillet weld size is too small, the internal structure of the weld will be quenched due to rapid cooling, reducing plasticity and easily forming cracks. Therefore, the minimum weld foot size of the fillet weld should meet the requirement of being the thickness of the thicker weld (in millimeters). For automatic welding, due to heat concentration and large melting depth, it can be reduced by 1mm; for single-sided welds with T-shaped connections, the performance is poor and should be increased by 1mm; when the thickness of the weld is equal to or less than 4mm, it should be the same as the thickness of the weld.

2) The welding foot size of large fillet welds is too large, which can easily cause "over burning" phenomena such as burns and burn through of the welded parts, and generate significant welding residual stress and welding deformation. Therefore, the maximum weld leg size of the fillet weld should meet the requirement of the thickness of the thinner welded component. To prevent the occurrence of "undercutting" during welding, the fillet weld on the edge of the welded component should be welded  

3) When the thickness difference between two welded components is significant and using equal welding leg sizes cannot meet the requirements of the maximum and minimum welding leg sizes, unequal welding leg sizes can be used, which means that the welding leg in contact with the thicker welded component meets the requirements.  

4) When the size of the fillet weld foot is large but the length is too small, the minimum calculated length of the weld seam will cause severe local heating of the welded part, and the arc craters where the weld starts and stops are too close together. In addition, other possible defects may also make the weld seam unreliable.  

5) The maximum calculated length of the side fillet weld is that the shear stress distribution along the length direction of the side fillet weld is very uneven, with larger ends and smaller middle, and the difference becomes larger as the ratio of the weld length to its weld leg size increases. When this ratio is too high, cracks will first appear at both ends of the weld, and at this time, the middle of the weld has not fully utilized its load-bearing capacity, making this stress concentration phenomenon even more unfavorable under dynamic loads. Therefore, the calculated length of the side fillet weld should not exceed 60HF (when subjected to static or indirect dynamic loads) or 40HF (when directly subjected to dynamic loads). When it exceeds the above limit, the excess part will not be considered in the calculation. If the internal force is distributed along the entire length of the side weld seam, its calculated length is not limited by this, such as the flange and web connection weld seam of I-shaped section beams or columns.  

6) When the ends of the plate are only connected by corner welds on both sides (as shown in the figure), in order to avoid excessive bending of stress transmission and uneven stress distribution in the component, the length of each side weld should be greater than the distance between them.  

7) In lap joints, the lap length shall not be less than 5 times the smaller thickness of the welded component and shall not be less than 25mm to reduce residual stress caused by weld shrinkage and additional stress caused by force eccentricity.  

8) In the connection of secondary components or secondary welds, if the stress on the weld is small and continuous welds are used, intermittent fillet welds can be used when the calculated weld foot size hf is less than the minimum allowable value. The length of the intermittent fillet weld segment shall not be less than 10HF or 500mm. The clear distance between each segment (for compression members) or (for tension members) shall be maintained to prevent local bulging of the plate, which may be detrimental to stress or cause corrosion due to moisture intrusion.  

9) When the end of the fillet weld is at the corner of the component, in order to avoid the occurrence of arc defects in this stress concentration area, it is advisable to perform a 2-hf-long fillet weld, and the corner must be continuously welded without arc interruption.