AS3600-2018 - Minimum Core Wall Reinforcement

[HD-111]

Hi All,

I am currently designing a multi-story residential building with a precast core box acting as the main lateral stability element with a ductility of u=2. Looking at Clause 14.6.7 in AS3600-2018, vertical and horizontal reinforcement must be provided in both faces of the wall and must be split equally:

Lateral stability precast walls are typically provided with central dowels with lapped tension bars to transfer the tension forces in the wall to the foundation. Does the new clause require dowels to be provided on each face, regardless of if vertical panel reinforcement is provided on each face within the wall? Do the tension lap bars/dowels also contribute to the minimum vertical reinforcement requirement in Clause 14.6.7 even if they are provided central?

Thanks for your help in advance

 

[HD-111]

Further to this, do you believe that precast panel dowels must comply with the minimum reinforcement requirements of Clause 14.6.7? My initial thinking was that As,dowels must be at least greater than As,wall as per Clause 11.7.5. However upon further inspection, the Clause seems to call up Ast,dowels > Ast,wall. I believe that this Clause is satisfied since the tension lap bars provided in the panels are typically equivalent to the dowels. Is my thinking correct?

 

[li0ngalahad]

On the minimum Ast for dowels, I believe the idea is that dowels always have a higher strenght than the walls they are conencting, so you are sure that they do not contitute a weak point in the wall, in particular in plastic hinge regions..

In regards to the each face requirement, I also wondered the same, and would be good to have some input fro the eng-tips community. Maybe a way to tackle this is to always have grout tubes/dowels slightly staggered along the wall. On the other hand, I am of the idea that vertical reo detailing does not necessarily apply to dowels, which are ofthe provided at much larger centres than vertical reo, and often more than the maximum vertical reo spacing (I have often seen 750-1000mm spacing on precast dowels on lightly loaded walls).. but obvioulsy being common doesnt make a practice necessarily correct.

 

[hokie66]

I think that your reinforcement is required on both faces. The dowels are reinforcement, thus required on both faces. If that makes precast impractical, so be it.

 

[Agent666]

In NZ while the panel has two layers of reinforcement the dowels can be central. Though we also have rules around staggering the laps in potential hinge regions which depending on the wall proportions is several storeys high. So in a ductile design this presents some challenges.

So basically to achieve the stagger the best solution I've seen is where you use grout sleeves for all the main reinforcement on the faces, and then provide some longer dowels in the centre of the wall to effectively ensure any plasticity is forced away from the joint (as its the best crack initiator you'll ever see). Otherwise you'll just concentrate all the plastic elongation of your bars within a short length over the cold joint and fracture the bars. The intent of any code is that the inelastic energy disapation is well distributed via lots of cracks over an effective hinge length, rather than a single crack scenario.

https://engineervsheep.com

 

[blihpandgeorge]

if its a wall that has its panel joint at a floor, then my first thought it that central dowels are ok. If the panel joint is not at a wall, then its two layers of dowels.

when you refer to tension laps to dowels - are you saying you effectively have 3 layers of vertical reo - one on each face acting for out of plane bending / crack control, and then a central layer that is fully lapped from top to bottom?

i believe the requirement for Ast Dowel > Ast wall is to ensure yield doesnt happen at the joint. If have 3 layer of reo, a bending layer each face and then a central tension layer, then having Ast Dowel > Ast.tension (central layer) seems acceptable

i'm interested to hear others views

 

[rapt]

Logical that the dowels have to provide at least minimum reinforcement as that is the reinforcement to provide multiple cracks and improve ductile action. And they have to be at least equivalent to the required wall reinforcement.

Other than for spalling problems, one of the reasons for main reinforcement on both faces rather than central is to stop the use of the idiotic detail with central reinforcement and dowels having to be in the same plane, and the resulting solution some have then proposed not providing horizontal bars over the depth of the dowels as both do not fit in the same space. Once Agent66 gets off the floor after hearing this possibility he might like to comment!

 

[Gishin1]

As per others' comments, I would think that central dowels are acceptable, but you must have the same or more tension steel as required in the wall, to achieve the level of ductility you are designing for, and the dowels should be concentrated at your critical tension zone, to ensure the tension capacity along the cross section is compatible with the each-face reo in the wall.

You need to make sure the dowels have enough shear capacity at the cold joint, accounting for the amplified shear force (over-strength factors). I wouldn't rely on any contribution from aggregate free grout bedding. Also check AS1170.4 minimum shear requirements.

Its also worth noting that if you are using Mu>1, then you cannot use the simplified wall capacities, and must design to section 10. I would take this meaning you have design for the minimum moment at all cross sections, even in the weak axis, therefore at the joints (even at slab level) of the walls, your moment capacity comes from the central dowel to the extreme compression face of the wall. You may need to increase dowel bar size, or stagger to achieve the minimum moment capacity. Also consider that your grout-tube might be 75mm diameter, so the effective depth can be quite small.

If your wall is heavily loaded, and you are relying on the each-face reo for strength, then you likely need your dowels to lap each face to ensure the bars are fully developed at the necessary cross sections in the wall/column.

Section 10 also specifies lap splices to be in accordance with Section 13, which means you likely need to increase the lap splice length by 1.5x space between dowel and vertical bars (CL13.2.2).