Integrated Buildings & Structures

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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Dear
attached is commentary of section 13.3.3.
Hoping it will answer your query
Regards

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Ammar Fakhoury P.E., M.ASCE
Riyadh
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A simple span beam is the most conservative analysis assumption for bottom steel in a slab.
Negative bending according to the ACI coefficients or by analysis would require hooked top bars.

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Jonathan Price P.E., M.ASCE
Green Cottage Industries, LLC
Pottstown PA
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Hi Justin,

I used to use BS8110 and I think there's a similar requirement.
The requirement might have more to do with shear and tie forces than moment resistance. Also if the slab is loaded with a point load near the support, there is a steep drop in the moment between the position of the load and the centroid of the support. So there could be at least some small moment--in theory--over the face of the support.

Best

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Richard McGrath A.M.ASCE
Structural Engineer
Airdrie AB
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Screen shot from Aci commentary is attached. Hopefuly. It answer the question

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Ammar Fakhoury P.E., M.ASCE
Riyadh
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Hi everyone, it is just me, or the attachments are not being posted in this thread? Two participants reported screenshots, but I can not see them.


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Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
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Original Message:
Sent: 08-10-2020 03:47 PM
From: Ammar Fakhoury
Subject: Anchorage of Positive Moment Reinforcement in Slabs

Dearscreen shot from Aci commentary is attached. Hopefuly. It answer the question

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Ammar Fakhoury P.E., M.ASCE
Riyadh
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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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