Clarifying 135-degree vs 180-degree stirrup hooks: the 180-degree hook belongs only to plain longitudinal tension bars (straight leg >= 3d); stirrup hooks are >= 90 degrees or >= 135 degrees by member type (no 180-degree tier), with bend inner-diameter calibers for load-bar hooks/bends

Topic Positioning

Clarifying the common confusion "are stirrup hooks 135 or 180 degrees": there is no 180-degree tier for stirrup hooks — by member type, >= 90 or >= 135 degrees; the 180-degree hook is the end detail of plain round bars (HPB300) as longitudinal tension bars. This card consolidates the attribution of the two hook families, straight-leg lengths, and bend inner diameters, for fabrication disclosure and acceptance checks.

Core Conclusions

1. The two stirrup-hook lines (key points): general structural members — bend angle >= 90 degrees, straight leg >= 5 stirrup diameters; members with seismic fortification or special design requirements — >= 135 degrees, straight leg >= the larger of 10 diameters and 75 mm (GB 50666-2011 Clause 5.3.6; GB 50204-2015 Clause 5.3.3, acceptance wording consistent though it omits 75 mm). Neither code provides 180-degree stirrup hooks — "stirrup 180" on site is the tension-bar hook requirement misapplied to stirrups.
2. Attribution of the 180-degree hook (longitudinal tension bars) — GB 50204-2015 Clause 5.3.2, verbatim: "The straight-leg length of longitudinal load bars after bending shall meet the design. Where a plain round bar ends in a 180-degree hook, the hook's straight leg shall be not less than 3 bar diameters." Inspection: no fewer than 3 pieces per shift per bar type from the same equipment; method: steel tape.
3. GB 50666-2011 Clause 5.3.5 to the same effect: "...the straight leg after bending shall meet the design and GB 50010. Where a plain round bar ends in a 180-degree hook, the straight leg shall be not less than 3 bar diameters." Commentary: this covers 180-degree hooks of plain tension bars, bend anchorage of ribbed bars in joints, hooked anchorage, and development of batch-cut bars; only the plain-bar 180-degree hook leg is codified, other details follow the design.
4. Bend inner diameters of load-bar hooks/bends — DB11/T 1832.3-2021 Clause 12.4.7 (Beijing work rule):

• "HPB300 bar ends shall have 180-degree hooks; the bend inner diameter D shall be not less than 2.5 bar diameters d, the straight leg preferably >= 3d. In lightweight-aggregate concrete, D >= 3.5d."
• "HRB400 bars ending in 90- or 135-degree bends: D >= 4d; the straight leg per design."
• "HRB500 bars below 28 mm: D >= 6d at bends; 28 mm and above: D >= 7d."
• At corner bends of beam-top/column-outer longitudinal bars in top-story end joints: D >= 12d for d < 28 mm, D >= 16d for d >= 28 mm; at mid bends of bent-up bars D >= 5d.

5. DB11's stirrup caliber (same as national): "For seismic and torsion members the stirrup hook angle is 135 degrees, the straight leg >= the larger of 10 stirrup diameters and 75 mm" (Figure 12.4.7-4, the 135/135 stirrup fabrication diagram); within column/beam splice zones, stirrup straight legs preferably >= the sum of 2 bar diameters.

Applicable Boundaries

• Applies to judging "what hook angle for stirrups, where the 180-degree hook belongs" in fabrication/tying disclosure and acceptance, and distinguishing the load-bar and stirrup hook systems.
• Full stirrup-hook provisions (circular stirrups, single legs of composite stirrups, wall/slab tie bars, inspection counts) are in the referenced dedicated card; this card focuses on the 180-degree attribution clarification and fabrication calibers.
• DB11/T 1832.3-2021 is a Beijing local standard; its inner-diameter values and "preferably >= 3d" wording are Beijing practice; other regions follow local rules with GB 50666-2011 Clauses 5.3.4/5.3.5 and GB 50010.
• Welded-closed stirrups are outside the hook clauses; where the drawings exceed the code, follow the design.

Further Reading

• GB 50204-2015 Code for Quality Acceptance of Concrete Structure Construction, Clauses 5.3.2 (load-bar hooks) and 5.3.3 (stirrup/tie hooks).
• GB 50666-2011 Code for Construction of Concrete Structures, Clauses 5.3.4 (inner diameters), 5.3.5 (load-bar hooks), 5.3.6 (stirrup/tie hooks).
• DB11/T 1832.3-2021 Construction Technology Specification for Building Works, Part 3: Concrete Structures, Clause 12.4.7 (Figures 12.4.7-1-4).
• GB 50010 Code for Design of Concrete Structures: design basis for anchorage and hook details.