AWS D1.1 · Section 4.17 · Table 4.5

AWS D1.1 Fatigue Stress Categories Explained — A Through F

AWS D1.1:2025 Section 4.17 with Table 4.5 organize details into eight stress range categories: A, B, B*, C, D, E, E*, and F. Each has a constant C_f and threshold F_TH for allowable cyclic stress range. These are not preheat Category A through G in Table 5.11.

Two “Categories” in D1.1 — do not confuse them: Table 5.11 uses preheat Category A through G (per-process subdivisions of the prequalified preheat lookup; D1.1:2025 added Categories E, F, and G to the original A–D). Table 4.5 uses fatigue stress range Category A/B/B*/C/D/E/E*/F to rate the fatigue strength of joint details. They share letter labels but refer to different code layers. This page is about Table 4.5. Base metal grades are grouped separately into Groups I–V in Table 5.6; see the D1.1 Quick Reference.

Table 4.5 vs Table 5.11 — The Two “Categories” in D1.1

AWS D1.1:2025 reuses the word “Category” for two completely unrelated concepts. The collision causes more confusion than almost any other terminology overlap in the code:

Preheat Category A through G (Table 5.11): per-process subdivisions of the prequalified preheat lookup. Category A is SMAW with other-than-low-hydrogen electrodes; Category B is SMAW with low-hydrogen electrodes plus SAW, GMAW, and FCAW. The original A–D system was extended in D1.1:2025 to A–G covering newer process and steel combinations (see the D1.1:2025 changes guide for the E/F/G addition). Used together with the steel Group I–V classification in Table 5.6 (governed by § 5.3 Base Metal) to look up minimum preheat per Table 5.11, plus filler metals per Table 5.7 and essential variables per Table 5.5.
Fatigue stress range Category A through F (Section 4.17 + Table 4.5 + Figure 4.16): rates the fatigue strength of welded joint details. Category A is plain base metal away from welds. Category E* is the most-penalized configuration. Determines the allowable stress range a detail can carry over the design cycle count.

The two systems do not map onto each other. A572 Gr 50 in steel Group I (Table 5.6) welded with low-hydrogen SMAW (preheat Category B in Table 5.11) at the toe of a transverse fillet weld on a tension flange falls into fatigue Category C in Table 4.5 — three independent letter labels for three independent decisions on the same physical weld. The preheat category drives shop temperature; the fatigue category drives detail sizing and crack-initiation life. This page covers the second — Table 4.5.

How to Find Your Detail's Category

Per Section 4.17.2, the calculated stress range shall not exceed F_SR computed from Formulas (4-12) through (4-19). The procedure:

Identify the detail configuration — what kind of weld, what joint type, where the load applies. Then locate the matching row in Table 4.5. The table has six sections: (1) plain material away from welding, (2) connected material in mechanically fastened joints, (3) welded joints joining components of built-up members, (4) longitudinal fillet welded end connections, (5) welded joints transverse to direction of stress, (6) base metal at welded transverse member connections.
Read the stress category for that row. Some rows have multiple categories depending on a sub-condition (flange thickness, transition radius, weld termination grinding).
Read the constant C_f and the threshold F_TH from the same row.
If the live load stress range is below F_TH, no fatigue evaluation is required (per Section 4.15.1). Otherwise, compute F_SR from Formulas (4-12) and (4-13) using C_f and the design cycle count N.
Verify F_SR is at least F_TH (the formulas have F_TH as a floor).
Plot the operating point against the curves in Figure 4.16 for sanity check.

Calculated stresses are nominal — per Section 4.16.1, “Stresses need not be amplified by stress concentration factors for local geometrical discontinuities.” The category itself bakes in the geometry effect (see Commentary C-4.17.2 on the NCHRP basis).

Categories A Through F — Per-Category Walkthrough

The eight stress range categories in Table 4.5, in order from least- to most-penalized:

Category	C_f	F_TH ksi [MPa]	Representative configuration
A	250 × 10⁸	24 [165]	Plain base metal away from any welding (line 1.1)
B	120 × 10⁸	16 [110]	Non-coated weathering steel base metal (1.2); continuous longitudinal CJP groove or fillet welds in built-up members (3.1); CJP transverse splices ground parallel to stress and inspected (5.1)
B*	61 × 10⁸	12 [83]	Continuous longitudinal CJP with backing left in place (3.2); continuous longitudinal PJP groove welds (3.2); CJP transverse splices with F_y ≥ 90 ksi at slope transitions (5.2)
C	44 × 10⁸	10 [69]	Members with reentrant corners (1.3); rolled cross sections with ground weld access holes R ≥ 1 in (1.4); base metal at drilled holes (1.5); CJP T- or corner joints with reinforcement not removed (5.4); transverse fillet pair on tension plate, toe crack (5.7); transverse fillet on rolled beam flange adjacent to stiffeners (5.8)
D	22 × 10⁸	7 [48]	Drilled or reamed holes, lower limit (1.5); base metal at ends of longitudinal welds at weld access holes (3.3); CJP butt splices with backing left in place, tack welds inside groove (5.5)
E	11 × 10⁸	4.5 [31]	Base metal at ends of longitudinal intermittent fillet weld segments (3.4); ends of partial-length cover plates (3.5, 3.6 thin flange); longitudinal fillet end connections, plate t ≤ 0.5 in (4.1); CJP butt splices with tack welds outside groove (5.5 lower)
E*	3.9 × 10⁸	2.6 [18]	Cover plates wider than the flange with welds across the ends, flange thickness > 0.8 in [20 mm] (3.6); cover plates wider than the flange without welds across ends (3.7) for thin flanges — not permitted for thick flanges; longitudinal fillet end connections, t > 0.5 in [12 mm] (4.1); base metal at ends of longitudinal welds at weld access holes R < 1 in (3.3)
F	(separate formula 4-14, 4-15)	(see formulas)	Shear stress on the throat of fillet welds — uses Formulas (4-14) and (4-15), a different fatigue exponent than Categories A through E*

The order is not arbitrary. Each two-category step roughly halves the threshold stress range — A→C drops 24 to 10 ksi (58%), B→D drops 16 to 7 ksi (56%), C→E drops 10 to 4.5 ksi (55%). A Category A detail at 24 ksi threshold tolerates roughly an order of magnitude more cyclic stress than a Category E* detail at 2.6 ksi threshold (9.2× ratio) — for the same nominal stress on the cross section.

B* and E* — The Asterisk Categories

The asterisk on B* and E* signals a tightened version of the unmarked category. Both are real categories with their own C_f and F_TH values in Table 4.5, not typographical artifacts.

B* (C_f = 61 × 10⁸, F_TH = 12 ksi [83 MPa]) is the version of B applied when a longitudinal CJP groove weld has backing left in place, or when the joint is a longitudinal PJP rather than CJP. The backing or the PJP root reduces the fatigue strength below clean CJP without backing (which is straight Category B). Same family of details, different sub-condition.

E* (C_f = 3.9 × 10⁸, F_TH = 2.6 ksi [18 MPa]) is the version of E applied when the configuration combines geometric discontinuity, transverse weld termination, and a thicker base metal section. The most common E* triggers: cover plates wider than the flange (lines 3.6 and 3.7), longitudinal fillet end connections on thick plates (line 4.1, t > 0.5 in [12 mm]), and weld access hole reentrant corners with smaller radius (line 3.3, R < 1 in [25 mm]).

For thick flanges with cover plates wider than the flange and no welds across the ends, line 3.7 reads “flange thickness > 0.8 in [20 mm] is not permitted.” The configuration is unsalvageable — the design must change the cover plate width, add welds across the ends (which lifts it from E* to E for thin flanges), or use a different connection.

F_TH Threshold and F_SR Allowable Stress Range

Two values control the fatigue check. They have different meanings and different decision points.

F_TH is the threshold fatigue stress range — the maximum stress range for infinite life. Per Section 4.15.1: “No evaluation of fatigue resistance shall be required if the live load stress range is less than the threshold stress range, F_TH (see Table 4.5).” If the calculated live load stress range stays below F_TH for the detail’s category, the connection has infinite cycle life and the fatigue check is satisfied.

F_SR is the allowable stress range for finite life. When the live load stress range exceeds F_TH, the design must compute F_SR from Formulas (4-12) and (4-13) using the per-category constant C_f and the design cycle count N (cycles per day × 365 × years of design life). The actual stress range must not exceed F_SR.

Formula (4-12) for ksi units: F_SR = (C_f / N)^0.107 ≥ F_TH. Formula (4-13) is the metric equivalent. Figure 4.16 plots these curves graphically for all categories from A through E*.

Category F — shear stress on the throat of fillet welds — uses Formulas (4-14) and (4-15) with a different fatigue exponent (0.067 instead of 0.107). The shear failure mode has different statistics from the tension failure mode that drives Categories A through E*.

Why E and E* Are the Most Penalized

Category E and E* details share three penalty conditions that together produce the steepest stress-range allowables in Table 4.5:

Geometric discontinuity at the weld termination: an abrupt change in cross section, such as the end of a cover plate or the termination of a longitudinal fillet weld. The discontinuity concentrates stress at the toe.
Tension loading across the discontinuity: the stress concentration drives crack initiation from the weld toe or the weld termination into the base metal.
Thicker base metal section: thicker sections constrain stress redistribution. A thin flange can yield locally and shed stress; a thick flange cannot. This is why line 3.6 splits E (flange thickness ≤ 0.8 in [20 mm]) from E* (flange thickness > 0.8 in [20 mm]) for the same physical cover plate detail.

The fatigue framework is consistent with metallurgy: cracks initiate at the highest stress concentration, propagate under cyclic load, and lead to brittle fracture if the section cannot redistribute or arrest the crack. The category constants in Table 4.5 were derived from the NCHRP test program on real specimens with realistic geometries (per Commentary C-4.17.2) — they encode the actual fatigue performance, not theoretical predictions.

AASHTO Detail E' = D1.1 E*

AASHTO LRFD Bridge Design Specifications use the symbol E' (with prime/apostrophe) for the most-penalized fatigue detail category. AWS D1.1:2025 Table 4.5 uses E* (with asterisk) for the same allowable stress range curve. Same constant C_f = 3.9 × 10⁸, same threshold F_TH = 2.6 ksi [18 MPa].

The two notations are interchangeable. AASHTO and AWS publish parallel documents with consistent fatigue framework, so a comment citing E' (AASHTO context, typically bridge work governed by D1.5) maps directly to E* (D1.1 context, typically structural building work). When in doubt about which notation a source is using, the C_f and F_TH values are the canonical identifiers.

Cross-Reference: Table 4.5 (Design) vs Table 8.1 (Inspection)

Table 4.5 is a design table. It controls the allowable stress range a detail may carry over its design life, computed before fabrication. The decision point is whether the detail’s F_SR is sufficient for the load case — if not, the design changes (different detail, larger section, fewer cycles via load shedding).

Table 8.1 is an inspection table. It controls visual acceptance criteria (cracks, undercut, porosity, profile, fusion) for finished welds, separated by statically vs cyclically loaded structures. The decision point is accept/reject after fabrication.

Practitioners commonly conflate the two because both reference “cyclic loading” and both have separate static-vs-cyclic criteria. They are different layers of D1.1 and they apply at different times in the project lifecycle:

	Table 4.5 (Section 4.17)	Table 8.1 (Section 8.9)
Code layer	Design of welded connections (Clause 4 Part C)	Inspection (Clause 8)
Decision point	Pre-fabrication: detail selection + section sizing	Post-fabrication: accept or reject the weld
Who applies it	Engineer of Record at design stage	CWI / qualified inspector after welding
Output	Allowable stress range F_SR; required detail category	Pass/fail per item: cracks, undercut, porosity, profile, fusion, weld size
Cyclic vs static	Cyclic: full Table 4.5 framework. Static: framework does not apply (no fatigue check).	Cyclic: tighter limits per Item 7 (undercut), Item 8 (porosity), Item 1 (cracks zero either way). Static: looser limits.

Linked detail-side coverage of the inspection layer: undercut, cracks, porosity, profile, visual inspection.

“Two tables, both with cyclic-vs-static splits, both with letter labels. Confusing them is the most common Part C exam mistake. Table 4.5 is what the engineer reaches for during design; Table 8.1 is what the CWI reaches for at the inspection station. Same code, different decisions.”
— CWI exam preparation, structural welding instruction

Welding Across an I-Beam Flange — Applied →

Related Standards Guides

Frequently Asked Questions

What is the difference between AWS D1.1 preheat Category A through G and fatigue stress range Category A through F?

They are two completely different concepts that share letter labels in AWS D1.1. Preheat Category A through G lives in Table 5.11 and subdivides the prequalified preheat lookup by welding process — Category A is SMAW with other-than-low-hydrogen electrodes, Category B is SMAW with low-hydrogen plus SAW/GMAW/FCAW. The original A-D was extended to A-G in D1.1:2025. Used together with the steel Group I-V classification in Table 5.6 to look up minimum preheat. Fatigue stress range Category A through F lives in Section 4.17 with Table 4.5 and Figure 4.16, and rates the fatigue strength of joint details — Category A is plain base metal away from welds, Category E* is the most-penalized weld detail. The two systems do not map onto each other. A572 Gr 50 in steel Group I welded with low-hydrogen SMAW (preheat Category B) at the toe of a transverse fillet weld on a tension flange falls into fatigue Category C in Table 4.5 — three independent letter labels for three independent decisions.

What is F_TH and what is F_SR?

F_TH is the threshold fatigue stress range — the maximum stress range for infinite life. Per AWS D1.1:2025 Section 4.15.1, no evaluation of fatigue resistance is required if the live load stress range is less than F_TH. The F_TH value depends on the detail's stress category and is read from Table 4.5: Category A is 24 ksi [165 MPa], Category B is 16 ksi [110 MPa], Category C is 10 ksi [69 MPa], Category D is 7 ksi [48 MPa], Category E is 4.5 ksi [31 MPa], Category E* is 2.6 ksi [18 MPa]. F_SR is the allowable stress range for finite life and is computed from Formulas (4-12) and (4-13) using the per-category constant C_f and the design cycle count N. F_SR must always be at least F_TH. Figure 4.16 plots the stress-range vs cycles curves graphically for all categories.

Why is Category E* the most penalized?

Category E* details combine three penalty conditions: a geometric discontinuity at the weld termination (which concentrates stress), tension loading across that discontinuity (which drives crack initiation from the weld toe or termination), and a thicker base metal section (which limits the geometry's ability to redistribute stress). The constant is C_f = 3.9 × 10⁸ and the threshold is F_TH = 2.6 ksi [18 MPa] — roughly an order of magnitude lower allowable stress range than Category B. E* configurations include cover plates wider than the flange with welds across the ends and flange thickness greater than 0.8 in [20 mm] (line 3.6), longitudinal fillet end connections on plates thicker than 0.5 in [12 mm] (line 4.1), and base metal at ends of longitudinal welds at weld access holes with R less than 1 in (line 3.3). On thick-flange details, cover plates wider than the flange without welds across the ends are not permitted at all.

Why does AWS D1.1 not amplify stress for the weld geometry?

Per Section 4.16.1, calculated stresses and stress ranges are nominal — based on elastic stress analysis at the member level. Stresses need not be amplified by stress concentration factors for local geometrical discontinuities. The Commentary C-4.17.2 explains why: the stress range cycle life curves in Formulas (4-12) through (4-23) and Figure 4.16 were developed by the National Cooperative Highway Research Program (NCHRP) on actual specimens that incorporated realistic geometrical discontinuities. The category constant C_f already bakes in the stress concentration of the joint geometry, so applying a separate notch factor would double-count the effect. This is why two physical welds with the same nominal stress can have wildly different fatigue lives — the category captures the geometry, the nominal stress captures the load.

How is AASHTO Detail E' related to D1.1 E*?

AASHTO LRFD Bridge Design Specifications use the symbol E' (with prime/apostrophe) for the most-penalized fatigue detail category. AWS D1.1:2025 Table 4.5 uses E* (with asterisk) for the same allowable stress range curve. Same constant C_f = 3.9 × 10⁸, same threshold F_TH = 2.6 ksi [18 MPa]. The two notations are interchangeable — they refer to the same fatigue strength curve. AASHTO and AWS publish parallel documents with consistent fatigue framework, so a comment citing E' (AASHTO) maps directly to E* (D1.1).

CWI Exam Tip: On the CWI Part C open-book exam, candidates lose time hunting for “Category C” in Table 5.11 when the question is about fatigue, or in Table 4.5 when the question is about preheat. The two tables are in different clauses and use the same letter labels for unrelated concepts. Read the question stem carefully — if it says “preheat,” go to Table 5.11. If it says “base metal” or “steel grade,” go to Table 5.6. If it says “fatigue,” “stress range,” “cyclic load,” or names a detail (cover plate, transverse fillet, weld access hole), go to Table 4.5. See CWI Exam Prep for the open-book navigation discipline.