A1. Anchorage of Longitudinal Reinforcement

Code Compliance & Reference

This detail complies with HKRC2004 Clause 8.3 regarding the minimum internal bend radius and Clause 8.4 regarding the anchorage of longitudinal reinforcement.

Comments

1. In fact, the HKRC2004 doesn't specify the minimum length (i.e. 4d) beyond the bend. It only states that “bending stress inside the bend must be checked” if the anchorage require a length beyond 4d from the bend. Hence, strictly speaking, such 4d is not required.
2. In other codes (e.g. BS8110 or EC2) the minimum length of 4d beyond bend is required only when the designer wish to regard such standard hook/bend as an 'equivalent' effective anchorage length (thus reducing the required length of anchorage). For example, a 90degree bend (with 4d) is regarded as equivalent to a 12d anchorage. However, in HKRC2004, there is no such provision, so the minimum length beyond bend is not necessary.
3. Nevertheless, in view that the short extra length can contribute to the integrity of the bend, the requirement of a minimum 4d beyond the bend is kept in the typical details.
4. In reality, for wind-resisting beams, it is unlikely that the width of the support for is large enough for the tension anchorage length such that the anchorage can terminate at 4d beyond the bend. In such cases, either the bearing stress inside the bend need to be check, or a cross bar of same or larger size is required. For example, for grade 45 concrete, the tension anchorage length is 30d. Meanwhile, the length of standard bend (+4d) is only about 10d (depends on bar size). So, unless the width of the support is increased by 20d x 2 = 40d beyond the minimum support width, either checking of bearing stress or cross bar will be required for anchorage of wind beam. Client won’t allow increase of support width. On behalf of engineers, we don’t want to check all bending stresses. So I will definitely choose adding cross bars!
5. However, I didn’t add such cross-bar requirement on my typical details, because (i) it may conflict with other details, (ii) in Grade 60 concrete the bearing stress is sufficient for even full tension anchorage length, and (iii) there must be some engineers who have the time for bearing stress calculation. So, designers should be aware of such add-cross-bar-or-calculate situation and design accordingly.

Blog note: I wonder why the HKRC2004 doesn't allow the use of effective anchorage length for hooks and bends. All other codes have similar provision. Hope it will be added in the next revision.

P.S. Hope I can stick with posing a new details every other working days!