--- title: "Failure mechanisms of bolted connection" subject: "Design of Steel and Timber Structure" chapter: 4 key: "semester_6/design_of_steel_and_timber_structure:bolted-failure-mechanisms" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/bolted-failure-mechanisms site: BCT Engineering --- # Failure mechanisms of bolted connection Design of Steel and Timber Structure · Chapter 4: Connections in Steel Structures - Asked in: 2079 Chaitra, 3+3 marks - Video: [Bolted Connections Failure Modes - Steel and Concrete Design](https://www.youtube.com/watch?v=PkXBtE6Ylbo) — Civil Engineering with Tanya J. Laird - 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/bolted-failure-mechanisms - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-4 ## Question Explain different failure mechanisms of bolted connection. Also discuss about combination of stresses developed in different type of eccentrically loaded bolted connection. ## Short answer Bolted connections fail by bolt shear, bearing, tension, plate shear-out, net section rupture, gross yielding, block shear, or prying. For eccentric load in-plane, bolts resist direct shear plus torsional shear; resultant checked against bolt shear strength. For load out-of-plane, bolts resist direct shear plus tension, checked with (V/V_db)^2 + (T/T_db)^2 ≤ 1.0. ## Answer #### Failure mechanisms of a bolted connection 1. **Shear failure of bolt:** the bolt shears across one plane (single shear) or two planes (double shear). Strength $V_{dsb}=\dfrac{f_{ub}}{\sqrt3\gamma_{mb}}(n_nA_{nb}+n_sA_{sb})$. 2. **Bearing failure of bolt/plate:** the plate elongates or tears in front of the bolt. $V_{dpb}=2.5k_bdtf_u/\gamma_{mb}$. Prevented by adequate end and pitch distances. 3. **Tension failure of bolt:** the bolt breaks in the threaded part under direct tension, $T_{db}=T_{nb}/\gamma_{mb}$ with $T_{nb}=0.9f_{ub}A_{nb}\le f_{yb}A_{sb}\gamma_{mb}/\gamma_{m0}$. 4. **Tearing of plate at the edge (shear-out):** if the end distance is too small, the plate shears out behind the bolt. 5. **Tension (net-section) rupture of plate:** the plate fails across the net section through the bolt holes, $T_{dn}=0.9A_nf_u/\gamma_{m1}$. 6. **Yielding of gross section** of the plate: $T_{dg}=A_gf_y/\gamma_{m0}$. 7. **Block shear failure:** a block of plate tears out along shear and tension planes at the bolt group. 8. **Bolt bending (long grip):** prevented by limiting the grip to $8d$ (with reduced shear strength when the grip exceeds $5d$). 9. **Prying failure:** extra tension in bolts due to flexible connected plates. 10. **Slip** in HSFG joints when the friction is exceeded. #### Combined stresses in eccentric connections - **Load in the plane of the bolt group (eccentricity in the plane):** bolts get *direct shear* $P/n$ plus *shear due to torsion* $T r/\Sigma r^2$; the vector sum of both is the resultant shear on the critical bolt, $R=\sqrt{(F_x)^2+(F_y)^2}$, checked against $V_{db}=\min(V_{dsb},V_{dpb})$. - **Load perpendicular to the plane of the bolt group (bracket on column flange):** bolts get *direct shear* $V=P/n$ and *tension due to moment* $T=My/\Sigma y^2$. Combined check for bearing-type bolts (IS 800:2007 cl. 10.3.6): $$ \left(\frac{V}{V_{db}}\right)^2+\left(\frac{T}{T_{db}}\right)^2\le1.0 $$ For HSFG (friction-type) bolts (cl. 10.4.6) the interaction is $\left(\dfrac{V_{sf}}{V_{df}}\right)^2+\left(\dfrac{T_f}{T_{df}}\right)^2\le1.0$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which failure mechanism is prevented by adequate end and pitch distances? A. Shear failure of bolt B. Bearing failure or tearing of the plate C. Prying D. Slip in HSFG Answer: B. Adequate end and pitch distances prevent bearing and shear-out of the plate. (Key point: Bearing failure) 2. What is the strength formula for net-section rupture of a plate? A. Ag fy / gamma_m0 B. 0.9 An fu / gamma_m1 C. 2.5 kb d t fu / gamma_mb D. fub/sqrt3 gamma_mb Answer: B. T_dn = 0.9 An fu / gamma_m1. (Key point: Net section rupture) 3. In an eccentric connection loaded in the plane of the bolt group, the critical bolt force is: A. Only direct shear P/n B. The vector sum of direct shear and torsional shear T r/Σr^2 C. Tension from moment D. Only torsional shear Answer: B. Direct shear P/n and torsional shear combine as a vector sum. (Key point: In-plane eccentric load) 4. In a bracket loaded perpendicular to the bolt plane, which check is used? A. V/Vdb + T/Tdb = 2 B. V <= T C. (V/Vdb)^2 + (T/Tdb)^2 <= 1.0 D. Only V <= Vdb Answer: C. Bolts have shear V = P/n and tension T = M y/Σy^2, checked by the squared interaction equation. 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