Hi Justin,
That's a definitely a good remark. Such remarks can substantially help you understand more and more how things work and how do concretes behave.
Back to your question, placing rebars in concrete is not only controlled by actions (e.g. bending moments and shear forces), there's what so-called "integrity rebars", i.e. rebars placed to satisfy structural integrity requirements. For example, one effective way to counteract progressive collapse incidents is to provide tie-forces (a form of integrity rebars) to achieve a certain level of ties between structural members and to assure they are well-connected. I believe such code requirement, like in your question, is mainly made into action to address such structural integrity issues. You can google some codes as "GSA 2016" and "UFC3" to understand more about such requirements made solely to address progressive collapse in concrete structures.
Cheers!!
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Muhannad Husain
Civil Structural Engineer
Hohai University
Nanjing
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Original Message:
Sent: 08-08-2020 06:28 PM
From: Justin Redman
Subject: Anchorage of Positive Moment Reinforcement in Slabs
Hi everyone,
Provision 13.3.3 of ACI 318-05 states that "Positive moment reinforcement perpendicular to a discontinuous edge shall extend to the edge of
slab and have embedment, straight or hooked, at least 6 in. in spandrel beams, columns, or walls."
My understanding of structural behaviour is that there would be a hogging moment at supports where the slab is fixed / partially fixed, and in the case of a simply supported slab, positive moments near the support should not be high. I cannot think of a condition where this would be untrue. Can anyone explain the basis for this requirement?
Regards,
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Justin Redman Aff.M.ASCE
Civil / Structural Engineer
Port of Spain
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