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!

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.
   

Blog note: Feel free to comment!

A6. Laps of Steel Fabric Reinforcement

Code Compliance & Reference
This detail complies with HKRC2004 Clause 8.7.3.1 regarding the minimum lap lengths of 250mm; references from consultant’s drawings regarding the “minimum lap = mesh spacing +25mm” is also included.

Comments

1. In reality, the steel fabric reinforcement is almost only used in on-grade slabs. At least this is what the HK consultants specified in their drawings; I seldom see consultants specifying fabric reinforcement for other elements such as slabs and walls.
2. According the typical details from ArchSD and HyD, the minimum lap for ‘normal’ on-grade slabs and external driveway on-grade slabs are 300mm and 450mm respectively. (These requirements will be specified in the details of on-grade slabs.) These more stringent requirements will override this 250mm minimum lap. Hence, I highly doubt that this 250mm minimum lap will ever be considered.
3. I once doubted the feasibility of the lapping of fabrics. Unlike bars (which is 1-D), a 2-D mesh will have to lap with at least 2 other fabrics at the corners, which would mean SIX layers of congested steel reinforcement! On second thoughts, since the mesh are mostly used in on-grade slabs, and (as you will see later) the mesh are discontinued at construction joints every 6m, the laps actually occur in one direction, i.e. at most 4 layers of steel.
   

Blog note: This is the end of “laps and anchorage” details. Next up: SLABS. Feel free to comment my details! I would be eager to incorporate any comments in my compilation of completed typical details.

A5. Crank Laps

Code Compliance & Reference
No requirements in HKRC2004. I saw the detail of cranked bars in many consultant’s drawings, but only two of them specifically indicated where to use the cranked bars.

Comments
Purpose of this detail is to specify where to use crank laps. To my understanding, cranked bars are used to avoid congestion of lapping bars. Just a couple of comments:

1. One of the consultant use 50mm as the criteria on bars spacing to determine when to use cranked laps. I like this idea, so I keep it as 50mm. However, as the HKRC2004 states that the minimum clear spacing between bars is 25mm, I was tempted to change criteria to 25mm. However, I decide against it – spacing of 25mm is really small.
2. A consultant specifies the use of crank laps in beams & columns, and non-crank laps in slabs & walls. However, I didn't specify it this way because there can be beams & columns with wider bar spacings, and slabs & walls with smaller bar spacings.

Blog note: Any idea on how troublesome it is to make a crank bar? How slow and how costly? If it is indeed very troublesome I can consider revising the 50mm minimum bar spacing criteria to something like “minimum hagg+ 5mm or bar size” to make contractor’s life easier (and without much compromise on construction quality).

A4. Minimum Transverse Reinforcement for Lapped Splices

Code Compliance & Reference
This detail complies with HKRC2004 Clause 8.7.4 and Figure 8.6 regarding the minimum transverse reinforcement for lapped bars.

Comments

1. I understand from BD that this detail is applicable to all structural elements, including slabs, beams, columns, walls, staircases and transfer plates (only applicable for laps with bar size equal to or more than 20mm).
2. Stirrups in beams, binders in columns and horizontal reinforcement in walls can be regarded as “transverse reinforcement”.
3. There will likely be conflicts between the specified stirrups in beams details (or the like) and the minimum transverse reinforcement UNLESS the designer specifies the location of the laps and put in the increased transverse reinforcement accordingly. However, it may be beneficial to both the contractor and the engineer NOT to specify the locations of the laps (such that the engineer can save time in preparing the drawings, and the contractor can place the lap location to suit site constraints). Hence, the designer should remind the contractor about this detail and the contractor should check and place the minimum transverse reinforcement wherever they place a lap.
4. Slabs, staircase& transfer plates may have a slight problem because the code specifies that the transverse reinforcement have to be placed “between the bar and the surface of concrete”. This could mean an additional layer of bars! Luckily, most slabs and staircases have bar size less than 20mm. As for transfer plates, I think the contractor can/should avoid that additional layer of bar by ensuring that the 'percentage of lapped bars in any one section is less than 25%'
5. The size/spacing of transverse reinforcement is based on the following principle: (i) calculate the required reinforcement according to code and distribute them within the lap length (ii) Always use T10 unless the spacing is less than 100mm; (iii) maximum spacing is 150mm as required in code (hence in some case the provided steel is much more than required); (iv) Round down to nearest 10.

Blog note: (i) I hope to present the calculations both for the lap length and the minimum transverse reinforcement LATER. (ii) I wonder any consultants (who specified this detail in their drawing) notice whether the contractors really add theses minimum transverse reinforcement… (iii) Any comments on the readability of the table?

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!

A3. Adjacent Laps

Code Compliance & Reference
This detail complies with HKRC2004 Clause 8.7.2 and Figure 8.4 regarding adjacent laps.

Comments
To be frank, after reading this clause several times, and even after looking up the similar clause in Eurocode 2, I still don't fully understand this clause.
Questions & Confusions:

1. Is there a difference between ‘staggered’ lap and ‘adjacent’ lap? Or are they actually the same thing? If adjacent laps need to have a 0.3L distance between the laps, won't it make the lap a ‘staggered’ lap?
2. Then, the clause mentions that if the three conditions (including the “0.3L distance between laps” requirement) are fulfilled, then “the requirement on staggered laps can be waived”. I'm very confused: if there can be no staggered laps, why will there be a requirement on “distance between lap”?? So, either staggered’ laps and ‘adjacent’ laps are two different things (which I can’t figure out yet), or the clause imply that by fulfilling the other two requirement (i.e. the space between lapping bars and space between adjacent bars) the requirement of staggered lap can be waived. I have doubts for either explanation.
3. Why is the minimum clear spacing between adjacent bars only 20mm?? Shouldn’t it be “hagg+ 5mm or bar size”?? This requirement on minimum clear spacing seems meaningless as there is a more stringent requirement on bar spacing in Clause 8.2.
4. The distance between lapping bars can be increased beyond 4d (or 50mm); it can be done by increasing the lap length “by a length equal to the clear space exceeding 4d or 50mm”. Is there a maximum distance that such increase is allowed? What if someone increase the spacing of lapping bars to 1m??
5. Sumarizing the above three: (i) minimum clear spacing of adjacent bars of 20mm is not critical; (ii) spacing between lapping bars of 50mm can be increased by lap length; and (iii) with no staggered laps, there is no horizontal distance of adjacent laps. That means staggered laps can always be waived by increasing lap lengths. Is it truly the intention of this clause?? I think it is intended to reduce the congestion of lap zones; but now I can avoid this just by making the lapping zone longer.
6. The final confusion: is this lapping arrangement required for ALL laps, or is it required only if I want to avoid staggered laps? My first impression is the latter, but the code didn’t explicitly say so (neither did EC2). BD seems to interpret it as “all laps should comply”. But if that’s the case, I wonder why should it bother to mention that “staggered laps can be waived IF…”.
   

Regardless of all these confusion, this detail has to be included in the typical details because of BD’s requirement during previous approvals. SE wants them in the approval plans, and so be it. This detail is almost identical to Figure 8.4 in the CoP except for some additional description for clarity.

Just one additional comment:

• As mentioned above, the distance between lapping bars can be increased beyond 4d (or 50mm) by increasing the lap length “by a length equal to the clear space exceeding 4d or 50mm”. I personally do not like this “leniency” especially when there is no restriction on the maximum increase; and hence I didn’t add this to my typical details.

Blog note: Somehow I still think there must be some rationale behind this detail. I hope someone can give some insight on what this detail is meant for??

A2. Anchorage of Links and Shear Reinforcement

Code Compliance & Reference

This detail complies with HKRC2004 Clause 8.3 regarding the minimum internal bend radius and Clause 8.5 & Figure 8.2 regarding the anchorage links and shear reinforcement.

Comments

1. The anchorage length for 90-degree bend is slightly longer than that specified in BS8110:1985.
2. The size of the longitudinal reinforcement inside the bend is REQUIRED to be equal or larger than that of the links and shear reinforcement. The designer should take note of this requirement in their preparation of beam/column details; specifying such requirement in the typical details can cause confusion to the contractor as there may be conflict between this detail and the beam/column details.
3. I found that in many consultants’ typical details, the anchorage lengths are specified in other details too (i.e. besides THIS detail, i.e. on arrangement of column links). Those anchorage lengths should be removed to avoid complications, and instead those bends should be labeled as “90-degree hook” etc. (and thus the reader is assumed to refer to this detail).

Blog note: You may notice that the first detail is ‘A2’instead of ‘A1’. This is because the detail ‘A1’ is a bit complicated and I would rather start my discussion on an easier detail first!
Feel free to comment on this detail and any other details that follow!! Thx~