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Anchorage of positive moment reinforcement at a simple support

Anchorage of positive moment reinforcement at a simple support

(OP)
Hi

I got a question/proposal similar to the one posted recently in thread744-371141: Anchorage of positive moment tensile reinforcement at a simple support-AS5100.5 about the requirements of AS3600-2009 and trying to bring some simplification to them.

I think it is sufficient to anchor at least V*/(Cot(theta_v)*phi) at the centre of bearing with a minimum of 30% of Ast required at mid-span developed at the face of the support.

I reckon that would be a simplification of the roundabout why 8.1.10.4 requires it. For theta_v I think 45deg can be used so it would be similar to the 1.5V* of previous codes.

RE: Anchorage of positive moment reinforcement at a simple support

Asixth,

No, 45 degrees should not be the default. The value of theta used for the shear design at the critical section should be used. In most cases 30 degrees will be the design theta value unless the member is very heavily loaded. So 30 degrees should be the default value if you are not going to calculate the actual value.

Otherwise I do not see how you have simplified it that much!

Also, the definition of when to consider it a simple support needs to be considered. This requirement actually needs to be satisfied at any point of contra-flexure, so should apply to a member with a small end moment, even though not technically simply supported. Hopefully the next version of AS3600 (do not ask me when!!) will sort this out along the lines of the new rules in the AS5100 draft.

RE: Anchorage of positive moment reinforcement at a simple support

(OP)
It's a transfer member and I have absolutely loaded it up with shear reinforcement.

The second part of the requirements says that 50% of the reo should be extended 12*db past the face of the support. I have generalized the development length as being roughly 40*db. So the equivalent anchorage I am going to provide is 30% of the midspan steel developed at the face of the support.

RE: Anchorage of positive moment reinforcement at a simple support

Asixth,

45 degrees would be ok in that case, if it was the angle used in the design, but your initial discussion above seemed to be a general rule interpretation for all cases. You did not specify only for it was for a very heavily loaded beam!

In reality, as long as the V cot theta force is supplied at the support centre line, that should satisfy the truss analogy and the other rules about minimum % of reinforcement developing past a point are unnecessary!

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