Prying Action — Definition, Bolt Force Amplification & Design
Prying action is a phenomenon in bolted connections where flexural deformation of the connected plate or flange amplifies the tension force in the bolts beyond the directly applied external load. When an external tension force is applied, the connecting plate bends away from the connected surface. The plate edges, restrained against the connected member, act as a fulcrum — the bolt becomes the pivot, and the lever action creates an additional contact force (the prying force Q) that further increases bolt tension.
Prying can amplify bolt forces by 20% to over 100%, easily exceeding the bolt's tension capacity if not accounted for. It is the governing limit state for many T-stub connections, end-plate moment connections, and hanger-type connections where plate bending flexibility allows prying to develop.
Mechanism of Prying
PRELIMINARY — NOT FOR CONSTRUCTION. All content is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.
Consider a T-stub bolted to a rigid base with two bolts. When tension T is applied to the stem:
- The flange bends about the bolt line
- The flange tip contacts the base at the edge, creating a compressive reaction (prying force Q)
- The bolt force becomes: Tb = T + Q (external force + prying force)
- The ratio Q/T is the prying amplification factor
The prying action is analogous to a lever: the bolt is the pivot, the applied load is between the bolt and the web, and the prying reaction is at the flange tip.
Parameters Controlling Prying
| Parameter | Effect on Prying | Mechanism |
|---|---|---|
| Flange thickness (tf) | Strongest parameter — thicker flange = less bending = less prying | Stiffness controls deformation |
| Bolt gage (g) | Larger gage = more lever arm = more prying | Moment arm to tip |
| Edge distance (a) | Larger a = more prying | Longer lever for prying force |
| Bolt diameter | Larger diameter = higher bolt stiffness, different failure mode | Bolt vs plate failure sequence |
| Material yield strength | Higher Fy = higher moment capacity at plastic hinges | Plate plastic mechanism |
Design insight: The most effective way to reduce prying is to increase the plate/flange thickness. Doubling the thickness increases bending stiffness 8x (I ~ t^3), dramatically reducing prying deformation.
AISC 360 — Prying Action Design
AISC 360 does not provide a specific prying formula in the Specification. Instead, AISC Manual Part 9 provides the design procedure based on the T-stub analogy.
Available Tensile Strength Including Prying
Per AISC Manual Part 9, the available tensile strength per bolt is:
T_avail = min(B, T_bolt)
Where:
- B = available tensile strength of the bolt per AISC 360 J3.6 (phi = 0.75, Rn = Fnt * Ab)
- T_bolt is determined from the plastic mechanism of the T-stub flange
The prying force Q is not computed directly — instead, the plate bending strength is checked to ensure it can resist the prying moment without excessive deformation.
T-Stub Plastic Mechanism
The plate flange forms plastic hinges at:
- The bolt line (negative moment — bolt pulling up)
- The web/flange junction (positive moment — flange bending)
The plastic moment capacity of the flange per unit width:
Mp = Fy * t^2 / 4 per unit width
The bolt force at which the plastic mechanism forms depends on:
- Flange thickness t
- Bolt gage g (distance from bolt center to web face)
- Edge distance a (distance from bolt center to flange tip)
- Tributary width p per bolt
Alternative: No-Prying Design
If the flange is thick enough, prying does not develop. The "no-prying" thickness is:
t_min = sqrt(4.44 * T / (p * Fy))
Where T is the applied tension per bolt and p is the tributary length. If the actual flange thickness exceeds t_min, prying forces are zero and the bolts carry only the directly applied tension.
Worked Example — Prying Force Calculation
Problem: A T-stub cut from a W12x40 section has flange thickness tf = 0.515", gage g = 3.5" (distance from stem face to bolt line), edge distance a = 1.5". Two 3/4" A325 bolts (Fnt = 90 ksi). Tributary width per bolt p = 4.0". Applied tension per bolt T = 25 kips. Fy = 50 ksi.
Step 1: Check if prying develops
No-prying thickness: t_min = sqrt(4.44 * 25 / (4.0 * 50)) = sqrt(111 / 200) = sqrt(0.555) = 0.745"
Actual tf = 0.515" < 0.745" — prying WILL develop.
Step 2: Compute plastic moment of flange
Mp = Fy * tf^2 * p / 4 = 50 * (0.515)^2 * 4.0 / 4 = 50 * 0.2652 = 13.26 in-kip
Step 3: Compute bolt force with prying
The bolt force including prying: Tb = T + Q
Where Q is the prying force, determined from equilibrium:
Q = Mp / a = 13.26 / 1.5 = 8.84 kips
Step 4: Check bolt tension capacity
Total bolt force = T + Q = 25 + 8.84 = 33.84 kips
Bolt tensile strength: phi*Rn = 0.75 * 90 * 0.334 = 22.55 kips (for 3/4" A325, Ab = 0.442 in^2)
Wait — the bolt force (33.84 kips) exceeds the bolt capacity (22.55 kips). The connection FAILS by bolt tension rupture under prying. Solutions:
- Increase flange thickness to t_min = 0.745" (W12x53 has tf = 0.575" — still insufficient)
- Use 7/8" A325 bolts (Ab = 0.601 in^2, phi*Rn = 0.75 * 90 * 0.601 = 40.6 kips — OK)
- Add stiffeners to reduce the effective gage distance
Prying in Common Connection Types
| Connection Type | Prying Critical? | Why |
|---|---|---|
| T-stub hanger | Yes — very critical | Thin flange, large edge distance |
| End-plate moment connection | Yes | Flange bending under bolt rows |
| Split-tee brace connection | Yes | Eccentric load, thin flanges |
| Angle seat connection | Moderate | Angle leg bending under beam reaction |
| Column base plate (tension side) | Yes | Base plate bending induces prying on anchor rods |
| Shear tab (single plate) | No | Shear governs, not tension prying |
| Bolted flange plate (BFP) | Moderate | Check if plate is thin relative to bolt gage |
Code Comparison
| Code | Section | Approach |
|---|---|---|
| AISC 360 / Manual | Part 9, Tables 9-2 to 9-4 | T-stub plastic mechanism, pre-calculated tables |
| EN 1993-1-8 | Clause 6.2.4, Table 6.2 | Three T-stub failure modes (bolt, flange, combined) |
| AS 4100 | Section 9.3, ASI design guide | Similar to AISC, bolt group tension design |
| CSA S16 | Clause 21 | Similar to AISC T-stub approach |
EN 1993-1-8 explicitly defines three failure modes for T-stubs:
- Mode 1: Complete flange yielding (four plastic hinges) — ductile, prying included
- Mode 2: Bolt failure with flange yielding (two hinges + bolt fracture) — mixed
- Mode 3: Bolt failure alone (thick flange, no prying) — brittle
Preventing Prying — Design Strategies
| Strategy | Effectiveness | Trade-off |
|---|---|---|
| Increase flange/plate thickness | Most effective | Weight and cost increase |
| Reduce bolt gage (bring bolts closer to web) | Effective | May reduce access for tightening |
| Reduce edge distance (a) | Moderate | Limited by AISC minimum edge distances |
| Use larger diameter bolts | Effective for bolt failure mode | Higher cost, larger holes |
| Add stiffeners between bolts and web | Very effective | Added fabrication cost |
| Use thicker end plates | Effective | Small weight penalty |
| Design for no-prying (t >= t_min) | Ideal — eliminates prying entirely | Requires thicker plate |
Frequently Asked Questions
What is prying action in bolted connections? Prying action is the amplification of bolt tension forces caused by bending deformation of the connected plate. As the plate flexes under load, its edges bear against the connected member, levering additional force into the bolt. The total bolt force equals the applied tension plus the prying force Q.
How much does prying increase bolt forces? Prying amplification (Q/T) typically ranges from 20% to 100% or more. Very thin flanges with large bolt gages can produce Q/T > 1.0 (more than doubling the bolt force). Connections designed to the no-prying thickness have Q = 0.
How do I avoid prying in my connection design? Use sufficiently thick plates/flanges (t >= t_min per AISC Manual), reduce the bolt gage distance, add stiffeners, or use larger bolts. The AISC Manual Part 9 design tables automatically account for prying when the connection geometry falls within the table limits.
Does prying affect HSFG (slip-critical) bolts? Yes. Prying increases the bolt tension force, which reduces the available pretension for slip resistance. For slip-critical connections, the bolt tension including prying must not reduce the clamping force below the required pretension. HSFG bolts are not immune to prying — they are subject to the same mechanics as bearing bolts, though their higher pretension provides more margin.
Related Terms and Pages
- Shear Lag — Definition & Design Effect
- Block Shear — Definition & Failure Mode
- Web Crippling — Definition & Design Check
- Bolt Capacity Table — AISC Reference
- Bolted Connection Calculator — Free Online Tool
- End Plate Connection Design
- Brace Connection — Gusset & T-Stub Design
- Connection Design Workflow
Educational reference only. Prying action must be checked per the governing design standard (AISC Manual Part 9, EN 1993-1-8 Clause 6.2.4) by a licensed Professional Engineer for all construction applications.
Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.