--- title: "Plug and slot weld design concept" subject: "Design of Steel and Timber Structure" chapter: 4 key: "semester_6/design_of_steel_and_timber_structure:plug-slot-weld" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/plug-slot-weld site: BCT Engineering --- # Plug and slot weld design concept Design of Steel and Timber Structure · Chapter 4: Connections in Steel Structures - Asked in: 2073 Magh, 5 marks - Video: [Plug and Slot Weld | Welded Connections | Steel Structures - GATE](https://www.youtube.com/watch?v=V5EHKJZOGg8) — Ekeeda GATE & ESE - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/plug-slot-weld - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-4 ## Question Explain the design concepts of plug and slot weld and its requirement for the connection of members. ## Short answer Plug and slot welds fill circular or elongated holes to transfer shear in lapped plates. They resist shear on the nominal faying area; design strength fwd = fu/(sqrt3 gamma_mw). Requirements: hole width ≥ max(3t,25 mm), slot ends rounded radius ≥ max(1.5t,12 mm), spacing and edge distance ≥ max(2t,25 mm); weld must fuse thoroughly. ## Answer #### Plug and slot welds A **plug weld** is made by filling a circular hole in one lapped plate with weld metal so that it fuses with the plate beneath. A **slot weld** is the same in an elongated (slot) hole. They transfer shear between overlapping plates when the edge fillet length is not enough, and they also stop lapped plates from buckling or separating. ```text top plate =====[ O ]===== plug (circular hole filled) bottom plate =============== top plate ===[ ====== ]=== slot (elongated hole) ``` #### Design concepts - The weld is assumed to resist **shear on the area of the hole/slot** in the plane of the faying (contact) surface: $\text{strength}=A_w\,f_{wd}$ with $f_{wd}=\dfrac{f_u}{\sqrt3\gamma_{mw}}$, where $A_w$ is the nominal area of the hole or slot. - The weld is not designed for tension or bending across the plate. #### Requirements (IS 800:2007, slot and plug welds) 1. **Hole/slot size:** width of the slot or diameter of the hole not less than $3t$ ($t$ = thickness of the plate containing it) or 25 mm, whichever is greater. 2. **Slot ends:** corners at the enclosed ends of slots rounded with a radius not less than $1.5t$ or 12 mm, whichever is greater. 3. **Spacing and edge distance:** distance between the plate edge and a slot or hole, and between adjacent slots or holes, not less than $2t$ and not less than 25 mm. 4. **Effective area:** the nominal area of the hole or slot in the plane of the faying surface. 5. **Filling:** in thin plates (up to about 16 mm) the hole is filled completely; thicker plates may be filled to at least $t/2$ (not less than 16 mm). 6. **Fusion:** the weld must penetrate and fuse with the underlying plate; good welding preparation is required. 7. Plug/slot welds are used mainly for lapped joints, built-up members and to stiffen thin plates, and are not used alone in primary tension members. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a plug weld? A. A fillet along the edge of a plate B. A butt weld across two plates C. A weld made by filling a circular hole in one lapped plate so it fuses with the plate below D. A tack weld used only for alignment Answer: C. A plug weld fills a circular hole in an overlapping plate with weld metal fused to the underlying plate; a slot weld uses an elongated hole. (Key point: Definition) 2. How is a plug or slot weld assumed to resist load? A. In tension across the plate B. In bending of the plate C. In compression only D. In shear on the area of the hole at the contact surface Answer: D. Strength = A_w x f_wd, with the weld area taken at the faying surface; it is not designed for tension or bending. (Key point: Design basis) 3. What is the minimum centre-to-centre spacing of plug welds? A. 4 times the hole diameter B. 1 time the hole diameter C. 2 times the plate thickness D. 10 times the plate thickness Answer: A. Plug welds are spaced at not less than four times the hole diameter. (Key point: Spacing) 4. Why are plug and slot welds used? A. To carry primary tension in main members B. To transfer shear between lapped plates and prevent them buckling or separating C. To replace all fillet welds D. To increase plate thickness Answer: B. They add shear transfer and hold lapped plates together; they are not used alone in primary tension members. 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