Bottom Plate Lap Joints: What Is a Joggle Joint and When Is It Mandatory?

What Is a Lap Joint and Why Tanks Use It

A lap joint connects two steel plates by overlapping their edges and fillet-welding the overlap. It's one of the simplest joint configurations and requires minimal surface preparation compared to a butt weld.

Example: You have two 6mm shell courses that need to be connected at the bottom. Instead of cutting a neat edge and butt-welding them (which requires beveling, precise gap control, and full-penetration inspection), you overlap them by 30–50mm and run a fillet weld along both sides of the overlap. The joint is complete.

Lap joints are common in bottom plates because they're fast to fabricate and cost less than annular-ring construction. The trade-off: they require careful dimensional control and they must be designed with shell bearing in mind.

Spacing and Overlap Rules

Minimum circumferential spacing between laps: 300mm along the tank circumference. This ensures that lap joints don't cluster or create local weak zones. If your tank diameter requires you to fit 16 bottom-plate courses around the circumference, and you have more than 16 lap joints, you're violating the spacing rule.

Minimum overlap: 5 times shell thickness. Example: 6mm shell → 30mm minimum overlap. The overlap length is measured along the radial direction (from the center of the tank outward). This ensures the fillet weld has sufficient area to carry the hoop stress that would have been carried by the removed material at the joint.

Maximum overlap: Typically 50mm, though this is a fabrication and inspection preference rather than a code-hard limit. Very long overlaps are difficult to weld completely and are hard to inspect (ultrasonic inspection of a thick overlap is challenging).

In practice: Most designers specify 30–40mm overlap for 6–8mm shells, balancing the code minimum with fabricator experience.

What Is a Joggle Joint (Break-Over)?

A joggle joint is an offset or step cut into one of the overlapping plates, creating a distinct height difference where the two plates meet. This offset is intentional — it's not a fabrication error; it's a required design feature for shell bearing.

Example: Two bottom plates, each 6mm thick, are to be lap-jointed. Without a joggle, they overlap in a simple stack. The inner plate (the one underneath) projects 30mm radially inward from the lap edge. When the shell sits on this bottom, the shell edge sits on a sloped or unsupported surface, creating a stress concentration.

With a joggle: the inner plate is stepped down (or relieved) by 3–6mm in the overlap zone, creating a distinct shoulder. The shell rests on this shoulder, providing a proper bearing surface. The load distributes across the shoulder, not at a sharp edge.

Fabrication of a joggle: One of the plates is partially cut (milled or oxy-cut) in the overlap region to create the step. The depth of the step typically matches half the shell thickness or less. This requires additional machine work but is essential for proper shell support.

When Joggles Are Mandatory

Rule: When the shell must bear on the bottom plate, a joggle is required.

The shell-bearing surface must be flat and continuous. If a lap joint is used without a joggle, the edge of the upper plate creates a ridge where the shell would rest, or the shell falls into a gap between the overlapped plates. Both conditions create unacceptable stress concentrations.

Shell bearing occurs when:

• The tank has no annular ring (the shell rests directly on the bottom plate)
• The bottom plate is not supported by a continuous ring or ring-like structure
• The shell weight transfers to the bottom plate through simple resting contact

When shell bearing is NOT required: If the bottom plate has an annular ring welded to its inner edge, the shell may bear on that ring instead of on the flat bottom-plate surface. In this case, the lap joints connecting the bottom plates don't require joggles — the shell isn't resting on them.

Weld Sizing at Lap Joints

The fillet weld at a lap joint must be sized to carry the hoop stress that the overlapping plates would carry if the joint were not there.

Typical sizing: Fillet weld leg size = 1.25 × shell thickness. For a 6mm shell, that's a 7.5mm fillet (rounded to 8mm in practice).

But the code limits this — the weld size cannot exceed the thinner of the two plates being joined. If you have a 6mm shell and a 8mm bottom course, the maximum fillet is limited to 6mm (the thinner plate).

Single-side vs. double-side weld: Some lap joints are welded only on one side (the outside face), while others have fillet welds on both sides (inside and outside). Double-sided welds are preferred for lap joints because they distribute the load and provide better access for inspection. Single-sided welds are acceptable only if the joint is deemed non-critical or if access constraints force the limitation.

Common Mistakes

Mistake 1: Specifying lap joints without joggles when shell bearing is required. This is the most common oversight. The designer lays out lap joints but forgets to specify that joggles are needed. The fabricator notes the ambiguity, but by then the delay has cascaded. Always explicitly call out joggle requirements on the drawing.

Mistake 2: Mis-sizing the joggle offset. If the joggle is too small, it doesn't provide adequate bearing area. If too large, it removes too much material and weakens the bottom plate. Typical guidance: joggle depth = 0.5–0.7 × shell thickness, but this should be detailed on engineering drawings.

Mistake 3: Forgetting to specify lap direction on the drawing. Lap joints can be oriented radially (laps extend from center outward) or circumferentially (laps extend around the tank perimeter). The direction affects fabrication layout and must be clear. "Radial lap" vs. "circumferential lap" should be specified.

Mistake 4: Not accounting for the reduced cross-section at the lap when calculating shell stress. Some designers treat the lap as a full-strength joint, but the overlapping geometry creates a reduced load path. Code calculations account for this, but sloppy design assumptions can miss it.

Mistake 5: Using lap joints at the large-diameter tank perimeter without confirming weld skip spacing. If you have a 30m-diameter tank with bottom-plate laps, the circumferential distance between laps can be very large. If you don't coordinate with the fabricator, you might end up with only a few laps around the entire circumference, violating the 300mm spacing rule.

Practical Tips

• Draw lap-joint details explicitly on your design drawing. Show the overlap length, joggle depth (if required), weld size on each side, and a typical section view. Ambiguous drawings lead to fabrication errors and rework.
• Confirm lap spacing fits your tank perimeter. For large tanks with many courses, the lap count and spacing must be verified against circumference. A quick calculation: circumference / 300mm = maximum number of laps that can fit with 300mm spacing.
• Get fabricator input on lap vs. annular-ring trade-off. For some tank sizes, an annular ring (which eliminates lap-joint complications) may be cheaper than detailed lap-joint fabrication with joggles and tight tolerances.
• Specify the lap orientation (radial vs. circumferential) and sketch it on the drawing. This is non-obvious to the fabricator and must be explicitly shown.
• If shell bearing is required, explicitly note on the drawing that joggles are mandatory. Don't assume the fabricator will infer this — state it clearly.
• For retrofit or inspection projects, inspect lap-joint welds carefully for cracks at the toe of the fillet. Lap-joint stress concentrations can be a fatigue risk if the tank undergoes thermal cycling or pressure cycling.

Related reading: Annular Plate Design, Bottom Plate Design, and Shell-to-Bottom Connection.