Web Crippling — Definition, Design Formula & AISC 360 Check

Web crippling is a localized instability limit state in steel beams where the web buckles under a concentrated transverse compressive force. When a beam reaction or point load is applied to the top flange, the force must travel through the web to reach the rest of the beam. If the web is too slender (high h/tw ratio) and the bearing length is too short, the web buckles outward or inward near the load point. This is distinct from web local yielding (J10.2), which is a strength/yielding check — web crippling (J10.3) is a buckling/stability check.

Web crippling is most critical for beams with thin webs (plate girders, light W-shapes), short bearing lengths (knife-edge supports), and unstiffened load application points.

Physical Mechanism

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.

When a concentrated force P is applied to the flange over a bearing length N:

1. The force spreads through the flange and fillet at approximately 1:1 slope (45-degree dispersion)
2. At the web, the force acts over an effective length of N + 2.5k (k = fillet dimension + flange thickness)
3. The web acts as a compressed plate element with one edge restrained (by the flange)
4. If the web is slender enough, it buckles locally at or near the force

The failure is characterized by localized web buckling and permanent deformation at the load point. It is typically ductile enough to be noticed during loading but can propagate and reduce beam capacity if not corrected.

AISC 360-22 Section J10.3 — Web Crippling

The nominal web crippling strength Rn depends on whether the force is applied at an interior location (more than d/2 from the member end) or at an end location (within d/2 of the end). End locations are more critical because the web has less restraint.

Interior Condition (force > d/2 from end)

Rn = 0.80 * tw^2 * [1 + 3*(N/d)*(tw/tf)^1.5] * sqrt(E*Fy*tf/tw)

Applicable when:

• The concentrated force is applied at a distance >= d/2 from the member end
• N/d <= 0.2 (bearing length relatively small)
• If N/d > 0.2, the equation may be conservative

End Condition (force <= d/2 from end)

For N/d <= 0.2:

Rn = 0.40 * tw^2 * [1 + 3*(N/d)*(tw/tf)^1.5] * sqrt(E*Fy*tf/tw)

For N/d > 0.2:

Rn = 0.40 * tw^2 * [1 + (4*N/d - 0.2)*(tw/tf)^1.5] * sqrt(E*Fy*tf/tw)

Design Strength

phi * Rn, where phi = 0.75 (LRFD)
Rn / Omega, where Omega = 2.00 (ASD)

Key Observations from the Formula

Critical difference from web local yielding: Web crippling capacity scales with tw^2 (buckling governed by plate bending stiffness ~ tw^3), while web yielding scales linearly with tw. For thin webs, crippling governs over yielding.

Worked Example — Web Crippling Check

Problem: A W18x35 beam (A992: Fy = 50 ksi) has a concentrated reaction of 45 kips at the end support. Bearing length N = 3.5" (bearing plate). Verify web crippling at the end.

W18x35 properties:

• d = 17.7 in, tw = 0.300 in, tf = 0.425 in
• k_des = 0.827 in (k = distance from outer face of flange to web toe of fillet)

Step 1: Determine condition Force at end (support): distance from end = 0 < d/2 = 8.85 in. End condition applies.

Step 2: Check N/d ratio N/d = 3.5 / 17.7 = 0.198 <= 0.2. Use first end-condition formula.

Step 3: Compute web crippling strength

Rn = 0.40 * tw^2 * [1 + 3*(N/d)*(tw/tf)^1.5] * sqrt(E*Fy*tf/tw)

tw/tf = 0.300 / 0.425 = 0.706
(tw/tf)^1.5 = 0.706^1.5 = 0.593
3 * (N/d) * (tw/tf)^1.5 = 3 * 0.198 * 0.593 = 0.352

1 + 0.352 = 1.352

sqrt(E*Fy*tf/tw) = sqrt(29000 * 50 * 0.425/0.300) = sqrt(29000 * 50 * 1.417)
                  = sqrt(2,054,650) = 1433

Rn = 0.40 * (0.300)^2 * 1.352 * 1433
   = 0.40 * 0.090 * 1.352 * 1433
   = 0.036 * 1.352 * 1433
   = 0.0487 * 1433 = 69.7 kips

Step 4: Design strength

phi * Rn = 0.75 * 69.7 = 52.3 kips
Applied reaction = 45 kips < 52.3 kips — OK

The web crippling check passes. But let's also check web local yielding (J10.2):

Rn_yielding = (2.5*k + N) * Fy * tw = (2.5*0.827 + 3.5) * 50 * 0.300
            = (2.07 + 3.5) * 50 * 0.300 = 5.57 * 15.0 = 83.6 kips
phi * Rn_yielding = 1.00 * 83.6 = 83.6 kips

Web crippling governs (52.3 kips < 83.6 kips), as expected for this thin-web section.

Web Crippling vs. Web Local Yielding

For beams with heavy webs (h/tw < 25), web yielding usually governs. For beams with slender webs (h/tw > 50), web crippling typically governs. The two checks are independent and both must be satisfied.

Web Crippling Capacity — Common W-Shapes (End Condition, N = 3.5")

Note: For sections lighter than W18x35, web crippling capacity is quite limited (< 25 kips for N = 3.5" at the end). Bearing stiffeners or longer bearing plates may be required.

Code Comparison

EN 1993-1-5 is the most detailed, defining separate checks for web crushing (localized yielding near flange), web crippling (localized buckling), and web buckling (global web panel buckling). The three checks correspond roughly to AISC J10.2, J10.3, and G2 respectively.

Prevention Strategies

Bearing stiffener design (AISC J10.5): Full-depth stiffeners in pairs are the most effective web crippling prevention. They must be attached for a snug bearing at the loaded flange and extend at least half the web depth. Stiffener design follows column provisions (effective length = 0.75h, effective area = stiffener pair + 12tw each side of stiffener).

Frequently Asked Questions

What is web crippling? Web crippling is a localized buckling failure of the beam web under a concentrated compressive force. It occurs most commonly at beam supports and concentrated load points. Unlike web yielding (a material strength limit), web crippling is a stability (buckling) limit state governed by plate bending stiffness.

How is web crippling different from web local yielding? Web local yielding (J10.2) is a strength check — the web material yields under compression. Web crippling (J10.3) is a stability check — the slender web plate buckles. Yielding capacity scales linearly with tw; crippling capacity scales with tw^2. For thin webs, crippling governs. Both must be checked independently.

How do I increase web crippling capacity? The most effective strategies are: (1) increase bearing length N (longer bearing plate), (2) add web stiffeners (bearing stiffeners per J10.5), (3) select a section with thicker web (lower h/tw), or (4) use a bearing plate to increase effective N. Increasing tw has a quadratic effect on crippling capacity, making it highly sensitive to web thickness.

When should I check web crippling? Check web crippling at: beam support reactions, concentrated point loads on top flange, crane runway beam wheel loads, column base plate bearing on beam top flange, and any location where a concentrated force enters the web through the flange. The check is less critical for distributed loads or when stiffeners are present.

Related Terms and Pages

• Block Shear — Definition & Failure Mode
• Prying Action — Definition & Bolt Force
• Shear Lag — Definition & Design Effect
• Compact Section — Definition & Limits
• Beam Web Design — Shear & Crippling Reference
• Beam Capacity Calculator — Free Online Tool
• Beam Design Guide — AISC 360
• Connection Design Workflow

Educational reference only. Web crippling must be checked per the governing design code (AISC 360 J10.3, AS 4100 5.13.3, EN 1993-1-5 6.2) by a licensed Professional Engineer for all construction applications.