Anchorage of positive moment tensile reinforcement at a simple support-AS5100.5

[kww2008]

In AS5100-2009, Clause 8.1.8.3 states:

"
(a) At a simple support, sufficient positive moment tensile reinforcement shall be anchored past the face of the support for a length such that the anchored tensile reinforcement can develop a tensile force of 1.5V* at the face of the support, where V* is the design force at a distance d from that face.

(b)At a simple support, of the tensile reinforcement required at midspan, either -

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(i) one half shall extend past the face of the support for a length of 12db, or equivalent anchorage, where d_b is the nominal diameter of the bar, wire or tendon; or

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(ii) one third shall extend past the face of the support for a length of 8d_b plus d/2.

(c) At a support where the beam is continuous ...
"


My question is would meeting only (b) sufficient as I have come across a slide presentation relating to AS3600-2009(8.1.10.4), the equivalent clause to (a) above, but with the tensile force being V*cot(theta_v)/phi and the anchor point being the mid-point of the bearing, which states that the requirement for AS3600-2009(8.1.10.4) is deemed to be satisfied if either
"
>= 1/2 A_st is exended past the face of the support by >= 12 d_b; or
>= 1/3 A_st is extended past the face of the support by >= 12d_b + D/2,
where A_st is the tensile area required at mid-span.
"

My view is that at a simple support of a bridge, both (a) and (b) are to be met, and would like to get your view on this.

 

[rapt]

kw2008,

This is being rewritten in the next version of AS5100.
(A) should always be satisfied for both codes. AS3600 changed to the V cot theta rule in 2009 from the 1.5V* rule in the 2001 and previous versions. AS5100 will be changing to something similar next version.

This will give you a tension force that must be developed. You can then decide how much of your reinforcement and prestress needs to extend whatever distance past the support point (centre of bearing if there is one as in a bridge), remembering that the last 12db of a bar gives no development by itself. So you need Lst (from clause 13.1.2.4) >= 12db for reinforcing and for prestressing the length required from 13.3 >= .1Lpt.

The second rule is some sort of deemed to comply but is useless where Reinforcing and Prestress are combined as Ast at mid span might be 0 with the full tension taken by prestressing steel!