AS3600 refined development length in precast (insitu stitch)

[Settingsun]

Hi,

In the simple structural system shown in the image, I want to achieve a code-compliant lap in the red slab bottom reinforcement in the shortest length possible. This is the keep the required width of the precast beam to a minimum.

The minimum lap length is 29*db for 500MPa reinforcement (equation 13.2.2; db is the bar diameter). To achieve this, Lsy.t must be less than 29/k7*db = 23.2*db. Put another way, k4*k5 must be less than 0.8 (equation 13.1.2.3).

I think k5 will be 1.0 as there won't be much transverse pressure. The reaction will be at the beam edge under the precast slab panel rather than at the splice location, and the pressure will be fairly low since the reaction is spread over a long length.

The k4 equation only considers the transverse reinforcement (blue stirrup in the image). Generally, I will have fairly light stirrups and won't get down to k4=0.8 using the code equation (unless providing additional stirrups just to reduce this lap length). Say around the k4=0.90 mark.

Can the presence of the precast beam be considered to provide a similar benefit in controlling the split, and the 0.80 value for k4 be justified on this basis? If so, would a calculation be required, or just judgment? The precast beam would typically be >600mm deep in my case and usually 800~100mm.

 

[Agent666]

I'm not familiar specifically with the AS3600 development provisions, but I just wanted to say that usually the transverse reinforcement reduction is for stirrups orientated perpendicular to the lap/splice. I.E. usually only applicable for a bar in or near the corner of a stirrup around the bars being lapped/spliced. It's about how many legs intercept splitting parallel to the lap. So I believe your stirrups don't really do anything to justify the use of the typical transverse reinforcement development length reduction if AS3600 is like any other code.

Does AS3600 allow laps for bars with hooks, that would be your best bet for reduction of the lap length.

On the presumption that there is continuous tension reinforcement in the topping, do you even need to lap the bars on the compression side of the slab? Top reinforcement ties it all together.

 

[Settingsun]

Can you provide some other code references so I can compare? They might have a broader description of the requirements.

AS3600 does require that the transverse bar be located between the longitudinal bar and the concrete tensile edge. From the images and text, this is to control a vertical split parallel to the longitudinal reinforcement as you said. I'm still wondering whether the presence of the precast beam, instead of a free edge as in all the code examples, provides any benefit. The precast has a roughened top surface so will bond with the insitu concrete and maybe restrict this potential crack/split.

For the horizontal split shown in AS3600 (rows 2-4 in the table below), the stirrups from my precast beam look at least as effective as the code examples as they have better anchorage.

The minimum lap length is 29*db and I read this as applying even if there is a cog or hook in the lapped bars.

The reason I want this lap in the bottom reinforcement is because the slab bending moment due to dead load is zero at the supports due to the construction sequence (built as a series of simply-supported spans until the insitu concrete sets). If a lateral load is applied, the sawtooth moment diagram causes maximum tension at the bottom face on one side of the support.

.

 

[Agent666]

Hi Steveh49, sorry I have not replied earlier, I've been out of town and away from my copy of NZS3101 to compare provisions.

NZS3101 the New Zealand concrete standard does have similar provisions and diagrams for A_tr (as you will note below) and probably a similar badly worded explanation of the transverse reinforcement requirement (it always seems to confuse people, but the commentary explanation of why the bars help is quite useful to explain the concept).

One thing I can point out is that in terms of your rows 2-4 in the first figure, depending on the configuration either the horizontal or vertical part of the leg is what crosses the crack (or possibly both), and the vertical/horizontal leg is required to be orthogonal to the splitting/crack direction along the spliced bars.

So the way I see it your vertical and horizontal stirrup leg does not cross the crack/split at all and therefore does not satisfy the following fundamentals:-

1 - the stirrups themselves are not around the bar being developed, like shown in the diagrams (for at least some of the bars), though this isn't neccessarily a prerequisite for the transverse reinforcement to count

2 - nor are they running orthogonal to the bars being lapped (between the bars being lapped and the free surface of the member). The horizontal part of your stirrup is running parallel to the crack, the vertical legs don't intercept the splitting either.

3 - Usually in these type of provisions there is some minimum spacing of the transverse reinforcement along the lap length that is required (unsure if Australian code has this), but NZ code states that at least 3 separate transverse bars (generally stirrups) are required orthogonal to the bar being developed over the length of the development, and at some minimum spacing between transverse bars which is 8 times the bar diameter of the bars being lapped (which usually governs for typical lap lengths, requiring more than 3 sets of orthogonal bars per lap/development length). I'd be surprised if AS3600 didn't have something similar tied to the development provisions for that reduction factor.

What might make some difference and allow you to apply the reduction is if the bars are lapped inside the beam cage as they appear to be with presumably the longitudinal beam reinforcement underneath, then I see your last case in the first figure as being appropriate, how this affects the calculation I have no idea as I'm not familiar with the equations at play here.

One thing that helps to understand the typical text used in standards is that when it refers to 'transverse reinforcement' it doesn't necessarily limit it to 'stirrups', 'transverse' should really be read as any 'orthogonal' reinforcement. Therefore it is any transverse/orthogonal reinforcement that crosses the potential splitting plane along the length of the splice bar that is between the lapped bar through to the free surface of the member.

NZ requirement is as follows for comparison, hopefully the commentary discussion helps to cement the fundamentals I'm noting above and how it is applied in a first principles approach, AS3600 will be based on a similar approach even though the equations might be a bit different:-
Code - the important things are reinforcement is transverse/orthogonal, outside the bars being lapped, needing at least 3 transverse bars and needing a minimum spacing.

Commentary

 

[Agent666]

One more thing worth mentioning thats quite important in applying the provisions, you might need to assess several types of splitting to come up with the most critical area of transverse reinforcement for the section or individual laps as discussed in the text (no one reads the commentary so always gets it wrong), i.e. comparing 1st and 2nd sections in C8.4, clearly the first one is going to result in less A_t for the same longitudinal bar arrangement and one exterior perimeter stirrup.

The first one with a single perimeter stirrup wouldn't comply with some of the other NZS3101 provisions regarding transverse reinforcement, but that's for another day, I don't write this stuff!

 

[Settingsun]

Thanks Agent666, and no need to apologise. I don't expect you to drop everything to reply to me.

I have concluded that I would need to provide additional bottom reinforcement in the insitu pour to reduce the k4 factor. Aside from what you've said above, I've moved away from the idea that the precast helps similar to reinforcement because it's the lower-quality concrete at the top of a deep pour; and the beam top reinforcement is quite some distance below the lapping slab reinforcement (slab bottom cover plus beam top cover).

I don't quite follow your comment that the vertical legs of the stirrups won't intercept a crack because the stirrup is rotated 90 degrees compared to the typical cases shown in the code sketches (your item #2 from your post 30 October 20:46). See the image below, which is a cross section through the slab. Even though the stirrup isn't bent around the lapping bars, it is still anchored either side of the horizontal crack. To me, it's effectively the same as the last row of the AS3600 table (slab/wall without fitments). With both the blue stirrups and the green additional bottom bars, a crack in any direction is intercepted (including say a 45 degree crack which I didn't draw).

 

[Agent666]

I guess whether or not a crack is intercepted by the vertical leg depends on the direction of the crack. I accept for some cracks it could be intercepted, but it's more critical if it doesn't which is also a possibility (the critical case) without the added green bars. I'm also considering that it was ineffective due to the fact its only two vertical legs either side of the section vs a regular spacing of intermediate legs.

Does AS3600 allow reduction of the length based on the percentage utilisation. If it does adding more bars being lapped to lower the stress in each individual bar would get a reasonable reduction in length. Add enough bars to lower the stress to make it work. If you place saddle bars in the topping to take the negative moment, the stress in the bars at the base of the slab would tend to go right down anyway?

 

[Retrograde]

Does AS3600 allow reduction of the length based on the percentage utilisation.

Yes it does:

 

[Settingsun]

Hi Retrograde, that reduction only applies to development length. Laps must be based on the yield stress.

I am however starting to wonder whether I need a lap or just development. Since the specific situation has a saw-tooth moment diagram, the insitu top reinforcement on one side is in tension rather than the precast panel's bottom reinforcement on that side. Probably a job for strut-and-tie, but I expect that the anchorage requirement using S&T will be more onerous than providing a lap splice.