B4. Extra Reinforcement at Corners of Two-Way Slabs at Building Edge

Code Compliance & Reference
This detail complies with HKRC2004 Clause 6.1.3.3 (d) (v) & (vi) regarding the reinforcement at retrained two-way slabs.

Comments

1. I must first say that I’ve only one consultant with this details – MINE. I’ve also seen OVE having similar details in their typical details book, but not in their submission drawings.
2. The idea is simple: add U-bars to the corners of two-way slabs at building edges. The U-bars act as torsional reinforcement to give the two-way slabs a restrained corner.
3. The code requires “the area of reinforcement ... should be three quarters of the area required for the maximum mid-span design moment in the slab”. It is not easy to ‘typical-ize’ this requirement. Hence, to make things simple, I choose to keep these U-bars same size as the mid-span reinforcement and make the spacing increased by 4/3 to that at mid-span.
4. However, for slabs with bar diameter >10mm and bar spacing >225mm, there will be some waste of steel (because I also limited the spacing to a maximum of 300mm). Well, I guess it’d be up to the designer to choose whether to specify the U-bars for each two-way slab at corners/edge of building, or just to accept a small over-provision of steel reinforcement. (Afterall, there isn’t many corners and edges for a two-ways slabs in a building)

Blog Post: Typhoon No. 8 tonight! 3 hours after the No.8 is hoisted, there are still some colleauges working in my office. Crazy.

B3. Edge Reinforcement at Free Edge of Slabs

Code Compliance & Reference
This detail complies with HKRC2004 Clause 9.3.1.6 regarding the reinforcement at free edge for slabs.

Comments

1. The code specifies a U-bar (with cross bars) at the free edge. For slabs without top reinforcement, having bottom reinforcements continue around the free edge can help reduce amount of steel reinforcement (due to omitting a lap).
2. The common locations for free edge of slabs include cantilever slabs and staircase (which span top to bottom). For staircase slabs, the free edge is on the transverse direction; if the code is to be followed strictly, additional cross bars will be required (since the main reinforcement will be outside the bent of transverse bars and cannot act as cross bars).

B2. Anchorage of Slab Reinforcement at Upstand Beam

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. This detail is essentially the same as previous B1 details. The only differences are that both anchorages bent upward and the bottom slab reinforcement shall be crank at 1:12.
2. Why is the bottom slab reinforcement cranked for a upstand-beam connection, while the top slab reinforcement doesn’t need to be cranked for a downstand-beam connection? I don’t have a convincing answer. Maybe it’s because the top slab reinforcement is more important at edge (due to hogging moment) and hence no crank would be better. Maybe it’s because the bottom slab reinforcement at the edge of a slab has a higher chance to be larger than 10mm; it may clash with the beam reinforcement (when the slab bottom bar diameter is larger than the beam’s stirrups), hence it’d be safer to just crank the slab bottom bar up.
   

Blog note: Very busy last two weeks!