Lap length for torsion reinforcement

[bugbus]

First time post, so excuse any lack of etiquette etc.

I am designing a RC beam that is subject to significant torsion and requires torsion reinforcement around the perimeter of the cross-section. Because of its large size [1400 mm (D) x 1650 mm (W)], and to allow for tolerance in construction to achieve adequate cover at the sides of the beam, I am considering providing two closed rectangular stirrups (four vertical legs) that overlap by a certain amount.

For torsion, it's necessary to splice the horizontal legs of the stirrups to achieve continuity around the perimeter of the cross-section. My question is pretty simple: if we treat the corners of the rectangular stirrups as standard cogs, are we allowed to make any reduction in the lap length of the horizontal legs of the stirrups? In AS3600 and AS5100, the provision of a standard cog reduces the development length of a bar by 50%, but it isn't exactly clear how this might apply to a splice. My feeling is that this is not permitted, and I would have to provide the full lap length of straight bars, or simply use one large stirrup around the entire perimeter of the cross-section.

 

[Trenno]

For your information, there is a sub-forum for AS/NZS code questions.

Link

Have you read CL 8.2.12.4 of the 2014 AS3600 commentary? It may help with your query. I'm not quite sure whether it's explicitly prohibited or just an undesirable detail.

 

[Lomarandil]

Oh, took me a little bit, but I understand your stirrup configuration. Sort of like this?

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*(please forgive the lazy ascii sketch)

Interesting question -- I'm curious to see what the greater minds will say.

My inclination is that lapping a bar with another bar, and developing a bar into concrete are cousins, but different. I'd be worried about the higher concrete stresses you'd be implicitly creating by relying on the shorter cogged length. If you think about in strut and tie terms, you're putting the same load into fewer concrete struts. But it's not my area, so I could be wrong.



Out of curiosity, is vertical shear demand driving the 4 vertical legs? Otherwise, I assume you'd just lap two open stirrups to handle tolerances.

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The name is a long story -- just call me Lo.

 

[Settingsun]

I think you want AS3600-2018 clause 8.3.3b. Short answer: No, the laps have to be longer than usual.

 

[bugbus]

The four vertical legs is required for maximum stirrup leg spacing (across the section) of 600 mm.

Thanks steveh49 for the tip, it’s good to know that’s there.