Lap Weld Joint

Lap Joint is the rebellious cousin of the Butt Joint. It is the easiest joint to prepare, the most forgiving to fit up, but arguably the most dangerous if used in the wrong application.

From the body panels of a Ford F-150 (Resistance Spot Welding) to the structural fillet welds on a storage tank floor, the Lap Joint is everywhere. But unlike a Butt Joint, which transfers stress in a straight line, a Lap Joint creates an eccentric load path that bends, twists, and shears the weld.

What is a Lap Weld Joint?

Lap joints are used for joining two overlapping plates so that the edge of each plate is welded to the surface of the other. The overlapping portion is called the LAP. In a Lap joint, the width of the Lap is generally 3 to 5 times the thickness of the plate to be welded. Welds are usually run on each side of the Lap.

Lap weld joints are one of the most common and versatile types of welds. Lap welds are created by overlapping two pieces of metal and then welding them along their overlapping edges.

Lap welds are created by welding fillet weld (either continuous or stitch weld types) along the overlapping area of two members. They are designed for shear stresses mainly.

A Lap Joint is formed when two members are placed on top of each other (overlapping) and joined, typically using a Fillet Weld. Unlike a Butt Joint where the edges meet, a Lap Joint relies on the shear strength of the weld metal and the friction between the plates.

Key Geometries

• Single Lap: Welded on one side only. (Low strength, high distortion).
• Double Lap: Welded on both sides. (Higher strength, balanced load).
• Joggled Lap: One plate is offset (bent) so the two surfaces are flush.

Design Rules: The “3t” Overlap

How much should the plates overlap? If the overlap is too short, the joint will peel open like a banana under load.

The Golden Rule (AWS D1.1):

The minimum overlap (L) shall be at least 3 times the thickness (t) of the thinner plate, or 1 inch (25 mm), whichever is larger.

L_min = max(3t, 25mm)

Why? This ensures the stress is distributed over a wide enough area to prevent the base metal from tearing at the toe of the weld.

Lap Weld Symbol

On a blueprint, a Lap Joint is almost always represented by the Fillet Weld Symbol (a triangle). There is no special symbol for lap welds as they are actually a type of weld joint and not a type of weld configuration.

• Triangles on both sides: Double Lap Weld.
• Triangle below the line: Weld on the “Arrow Side.”
• Triangle above the line: Weld on the “Other Side.”

A lap weld is a type of welding joint in which two pieces of metal are joined together by overlapping them and welding along the seam. The most common type of lap weld is the fillet weld, which is created by welding along the edge of the overlap.

Types of Lap Weld

The main types of Lap Weld Joints are:

• Single Welde Lap Joint
• Double Welded Lap Joint
• Joggled or Offset Lap Joint

Lap Weld Joint Design

In Lap weld, the overlaps of joining members are usually from 30% to 80%. Lap joints should not be used on plate thickness above 10 mm or 1/2 inch approximately. In essential weldments lap joints must be welded from both sides.

Lap Weld vs Fillet Weld

Two of the most common types of welding are lap welds and fillet welds. So, what is the difference between these two types of welding?

Lap welds are created when two pieces of metal are placed on top of each other and then joined together using welding. This type of weld is often used for sheets of metal that will be exposed to high levels of stress, as it creates a very strong joint. However, lap welds can be difficult to create as they require a lot of precision.

Fillet welds, on the other hand, are created by welding two members at a right angle to each other. This type of weld is much easier to create than a lap weld, but it isn’t as strong.

Types of Welds on Lap Joints

While the Fillet Weld is the standard, Lap Joints accommodate several specific weld types:

1. Fillet Weld: The triangle weld placed at the edge of the overlap.
2. Plug Weld: A circular hole is drilled in the top plate and filled with weld metal to join it to the bottom plate.
3. Slot Weld: Similar to a plug weld, but an elongated hole (slot) is used for more strength.
4. Spot Weld: Used in sheet metal (Resistance Welding). No hole is drilled; current is passed through the layers to fuse them at a single point.
5. Seam Weld: A continuous spot weld used to make leak-tight cans or tanks.

Lap Joint Strength Calculations (Shear Stress)

This is the most critical engineering concept. Lap Joints do not fail in tension; they fail in shear.

When you pull a lap joint, the force tries to slide the plates past each other, slicing the weld in half. Therefore, you must calculate the Throat Area.

Formula:

Strength = 0.707 x Leg Size x Length x Allowable Shear Stress

Lap Joint Advantages vs. Disadvantages

Advantages	Disadvantages
No Edge Prep: You don’t need to bevel the edges (V-Groove). Just cut and weld.	Eccentric Loading: The forces are not aligned. The joint tries to rotate/bend under tension.
Forgiving Fit-Up: Gap management is easier. You can slide the plates to adjust length.	Crevice Corrosion: Moisture gets trapped between the overlapping plates (the “crevice”).
Mixed Thickness: Easily joins thin sheets to thick plates.	Wasted Material: The overlap itself is wasted steel that adds weight but not length.
Cost: Cheaper preparation time.	NDT Limits: You cannot Radiograph (RT) a lap joint effectively.

“Never use a Single Lap Joint in a corrosive environment. If you weld only one side, rain or chemicals will enter the gap between the plates. The liquid will stagnate, causing rapid Crevice Corrosion or “Rust Jacking,” where the expanding rust actually pries the weld apart.”

• Fix: Always Seal Weld (weld all around) if corrosion is a threat.

Lap Weld Preparation

In order to create a strong and reliable lap weld, proper preparation is essential.

Before welding, the two pieces of metal to be joined must be cleaned of all dirt, grease, paint, or other contaminants. This can be done with a wire brush, sandpaper, or solvent cleaner.

Once the surfaces are clean, they should be positioned so that there is a small gap between them – this gap will allow for proper penetration during welding.

Proper alignment is also important for creating a strong lap weld. The two pieces of metal should be positioned so that they overlap by at least 1/2″. Once the pieces are properly aligned and positioned, welding can begin.

Lap Weld Applications

Lap weld joints are commonly used in the construction of both steel and aluminum structures. They are particularly well-suited for joining thin sheets of metal, as the lap provides good support for the weld pool.

When properly executed, lap welds can be very strong and have a high resistance to fatigue.

Lap Joint advantages and disadvantages

Lap joints are one of the most popular and commonly used methods for joining two pieces of metal together. There are many advantages to using lap joints, including their ease of assembly, strength, and versatility.

However, there are also some disadvantages to using lap joints that should be considered before deciding if they are the best option for your project.

One of the biggest advantages of lap joints is their ease of assembly. Lap joints can be assembled quickly and easily with minimal tools or training required. This makes them an ideal choice for projects where time is a factor, or for those who are not experienced in welding or fabricating.

Another advantage of lap joints is their strength. When properly welded, lap joints are very strong and can handle a lot of stress without failure.

The main disadvantage of Lap joints is that their load-bearing capacity is usually limited only to shear stress loading. Also, in a Lap weld, some metal is wasted on the lap.

Common Defects & Troubleshooting in Lap Weld Joint

A. Distortion (Angular)

Because the weld is on the surface, as it cools, it shrinks and pulls the plates towards the weld face.

• Fix: Clamp the joint rigidly or pre-set the plates at a slight opposing angle (camber) before welding.

B. Lack of Fusion (Root)

In a lap joint, the “root” is the corner where the top plate meets the bottom plate. If you don’t aim the arc deep into that corner, you get a “bridge” weld that has zero penetration.

• Fix: Aim the electrode at a $45^{\circ}$ angle into the root, but favor the bottom plate slightly if it is thicker.

C. Melt-Through (Burn Through)

Common when welding thin sheet metal laps.

• Fix: Use a “Heat Sink” (copper backing bar) or switch to a faster process like MIG (GMAW) Short Circuit.

❓ FAQ: Lap Joint vs. Butt Joint

Q: Which is stronger- Lap Weld Joint or Butt Weld Joint?

A: A Butt Joint (Full Penetration Groove) is stronger. It has 100% joint efficiency and aligns the forces perfectly. A Lap Joint is limited by the shear strength of the fillet and the bending moment caused by the overlap.

Q: Can I use a Lap Joint for pressure piping?

A: Generally No. ASME B31.3 restricts lap joints (like Slip-On Flanges) to lower pressure/temperature classes because they are harder to inspect and weaker in fatigue.

Q: Do I need to clean the steel between the lap?

A: YES. This is a rookie mistake. Even though you can’t see the inside surfaces, oil or paint trapped in the overlap will vaporize when the arc heats it up, blowing out as porosity through your weld. Always clean the mating surfaces before assembly.

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