Introduction to Welds

Welding is a fundamental technique used to join separate pieces of material into a single, cohesive unit. This process involves heating the materials to a point where they melt or soften, allowing the metals to fuse together and form a strong bond. In structural welding, a filler material from an electrode is often added to reinforce the weld joint and enhance its strength.

1. Key Functions of Welds

• Force Transfer: Welds transfer shear, tension, and compression forces within structural joints and from one part of a built-up member to another.
• Component Assembly: They stitch together component parts of an assembly and seal the edges of contact surfaces against moisture.
• Tacking: In shop assembly, welds are used for tacking main and detail material in place before final bolting or welding. Tack welds, although nonstructural, can impact structural performance.
• Temporary Shipping Welds: Temporary welds are sometimes used for shipping purposes, made small enough for easy removal in the field. 

(Refer Chapter 4 ‘Basic Detailing Conventions – Welding’, of the ‘Detailing for Steel Construction manual’ by AISC) 

2. Joint Prequalification

A joint designated as prequalified does not automatically make it suitable for all applications. Good joint design considers various factors:

• Type and magnitude of loading
• Thickness and specification of base material
• Welding access and position
• Equipment available for handling
• Welding process available
• Proper material cleaning
• Edge preparation
• Control of distortion

(Refer Chapter 4 ‘Basic Detailing Conventions – Welding’, of the ‘Detailing for Steel Construction manual’ by AISC) 

3. Challenges in Welding

• Weld Shrinkage: Can create stresses in the through-thickness dimension of material, potentially leading to lamellar tearing.
• Fatigue: Welded members are subject to severe restrictions in nominal strengths when subjected to cyclic loading. Abrupt changes or discontinuities can create stress concentrations.
• Distortion and Shrinkage: Distortion and shrinkage of parts being joined can affect the shape or strength of a member. Fabricators develop procedures to control and minimize these effects.

Weld Types

Welds are identified by their profile or cross-section. The two most significant types of structural welds are fillet welds and groove welds, which include complete-joint penetration (CJP) and partial-joint-penetration (PJP) groove welds.

Fig 1: Types of weld

Common Weld Types

• Fillet Welds: Commonly used in structural applications to join two surfaces at an angle.
• Groove Welds: Includes both CJP and PJP groove welds. These are used for deeper weld penetration.
• Back Welds: Used with single CJP groove welds to ensure complete weld penetration at the weld root.
• Plug, Slot, and Flare Welds: Limited in application but useful for specific purposes.

In summary, understanding the various types of welds, their applications, and the challenges involved is essential for effective welding in structural applications.

In this blog post, we will mainly be focusing on fillet welds.

Basic Welding Symbols

It’s important to understand the distinction between a welding symbol and a weld symbol. A weld symbol specifies the type of weld, while a welding symbol encompasses the weld symbol and additional elements.

(Refer Chapter 4 ‘Basic Welding Symbols’, of the ‘Standard Symbols for Welding, Brazing, and Nondestructive Examination manual’ by AWS) 

1. Basis of Reference

In the current system, the joint serves as the basis of reference. The arrow side is the side  of the joint to which the arrow of the welding symbol points. The other side is the side of the joint opposite the arrow side.

Arrow side

Other side

Fig 2: Basis of reference 

2. Weld Symbols

Weld symbols are drawn in contact with the reference line. The following table shows the weld symbol used for each type:

Fig 3: Weld symbols

3. Welding Symbols

A welding symbol can consist of several elements, but only a horizontal reference line(the foundation of the welding symbol) and an arrow(points to the location of the weld) are required. Additional elements can be included to convey specific welding information. This information can also be provided through drawing notes, specifications, standards, codes, or other drawings.

The tail of the symbol is used for additional information such as specifications, processes, strength, filler metal, peening, backgouging, or other relevant operations.

4. Welding Symbol Placement

The arrow of the welding symbol must point to a line, location, or area that clearly identifies the joint or area to be welded.

Below is a standard diagram showing the location of the elements of a welding symbol:

Fig 4. Welding Symbols

Fillet Welds

Fillet welds are characterized by their triangular cross-section, typically used to join two surfaces at approximately right angles in lap, tee, and corner joints. They are also employed in conjunction with groove welds for reinforcement in corner joints. The cross-section of a typical fillet weld forms a right triangle with equal legs, where the size of the weld is designated by the leg size. The intersection point of the legs is known as the root, and the throat size is the perpendicular distance from the root to the weld face. The effective throat area is defined by the plane that passes through the throat and root lines, and the effective length of a fillet weld is measured from end to end of the full-size fillet along the root line. For curved fillet welds, this length is measured along the centerline of the throat.

Key Features of Fillet Welds

• Triangular Cross-Section: Joins two surfaces at right angles in lap, tee, and corner joints.
• Reinforcement in Corner Joints: Used in conjunction with groove welds.
• Leg Size: Designates the size of the weld.
• Root: The intersection point of the weld legs.

Fig 5: Fillet Welds

• Throat Size: Perpendicular distance from the root to the weld face.
• Effective Throat Area: Plane passing through the throat and root lines.
• Effective Length: Measured from end to end of the full-size fillet along the root line, or along the centerline of the throat for curved fillet welds.

Fig 6: Fillet weld shear failure.

The included angle of the weld deposit can vary from 60° to 135°, and sometimes unequal leg welds are used. In most cases, using the normal throat size for strength calculations is conservative. However, for included angles significantly greater than 90°, the effective throat size should be determined from actual dimensions.

Convexity is when the weld face curves outward, while concavity is when the weld face curves inward. Both convexity and concavity are usually present in all fillet welds and are not necessarily defects.

In summary, fillet welds are versatile and widely used in various structural applications due to their ease of implementation and effectiveness in joining materials at right angles. Understanding their features, including the considerations for their use in holes and slots, is crucial for ensuring strong and reliable welds.

Symbols for Fillet Welds

1. General

• Dimension Location: Fillet weld dimensions are shown on the same side of the reference line as the weld symbol (as shown in all the below examples)
• Double Fillet Weld: For fillet welds on both sides of a joint, dimensions must be specified for each side, whether they are identical or different.

Fig. 7: Equal double-fillet weld sizes

Fig 8: Unequal double-fillet weld sizes

2. Fillet Weld Size

Location:  The size of the fillet weld, denoted as “S,” is specified to the left of the weld symbol.

Fig 9: Single-fillet weld size

Fig 10: Unequal leg fillet weld size

3. Unequal Legs

When specifying fillet welds with unequal legs, the size “S” is still placed to the left of the weld symbol. The orientation of the unequal legs must be shown on the drawing for clarity, as the symbol alone does not indicate this.

Fig 11: Unequal legs

Fillet welds are the most commonly found welds on structural joints. They do not require edge preparation and hit that sweet spot in-terms of cost vs complexity. However, it is imperative that the symbols are placed correctly depicting all surfaces that require welds. The most common mistake in fillet welds is forgetting an edge which requires the weld. Another common mistake is using an incorrect size of weld. Both can cause serious failures. If there is a gap between the materials to be joined, the fillet weld size must be increased accordingly. We at OM Steel Solutions have been detailing drawings with fillet welds for 20 years. We are vigilant and ensure our clients always get the right details.