B1. Anchorage of Slab Reinforcement at Support

Code Compliance & Reference
This detail complies with HKRC2004 Clause 9.3.1.3 regarding the reinforcement at end supports for solid slabs (which include a reference to 9.2.1.7 regarding the anchorage of bars at a simply-supported end of a beam).

Comments

1. There are three requirements in clauses 9.3.1.3 and 9.3.1.4 on the minimum amount of steel reinforcement to be anchored into the support. (i) “[at end support] Half the calculated span reinforcement should be anchored into the support (for pin support;) (ii) “[at end support]… an amount of top reinforcement capable of resisting at least 50% of the maximum mid-span moment… should be provided at the support; (iii) “At an intermediate support of a slab 40% of the calculated mid-span bottom reinforcement should be continuous through the support”. Some consultants have added such requirements on the typical details. However, I am against adding such requirements because it will likely have conflicts with the slab details. I think details including amount of slab reinforcement and positions of curtailment should only be placed in the slab details instead of typical details.
2. Some consultants kept the “simplified detailing rules” for beams and slabs from the BS8110 in the typical details. I am also against adding these rules in the details. Besides the conflict with the slab details, these simplified detailing rules are intended for elements with UDL and with equal spans ONLY. I definitely prefer adding the curtailments according to the design moments.
3. The code specified half of d (effective depth) for the start of the anchorage length. However, in view that the contractor may not have the information about the effective depth of slab/beam (e.g. they may not know which side (top/bottom/both) is the tension side), I change d into D (overall slab thickness). Such change in anchorage length is negligible. In most slabs it would be D/2 being the critical case.
4. There is a provision that if the shear stress is small (less than half vc), a short straight bar is sufficient for anchorage. This can help reduce slab steel especially if the building is narrow and have many edges. However, as it involves design of individual slabs, such reduced anchorage should be specified in individual slab details instead.
   

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