Integrated Buildings & Structures

  • 1.  Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-08-2020 06:57 PM
    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,

    ------------------------------
    Justin Redman Aff.M.ASCE
    Civil / Structural Engineer
    Port of Spain
    ------------------------------


  • 2.  RE: Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-10-2020 09:19 AM
      |   view attached
    Dear
    attached is commentary of section 13.3.3.
    Hoping it will answer your query
    Regards

    ------------------------------
    Ammar Fakhoury P.E., M.ASCE
    Riyadh
    ------------------------------



  • 3.  RE: Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-10-2020 12:10 PM
    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.

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



  • 4.  RE: Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-10-2020 03:43 PM
    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

    ------------------------------
    Richard McGrath A.M.ASCE
    Structural Engineer
    Airdrie AB
    ------------------------------



  • 5.  RE: Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-10-2020 04:19 PM
      |   view attached
    Screen shot from Aci commentary is attached. Hopefuly. It answer the question

    ------------------------------
    Ammar Fakhoury P.E., M.ASCE
    Riyadh
    ------------------------------



  • 6.  RE: Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-18-2020 05:00 PM
    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.


    ------------------------------
    Andres Guzman Ing., M.ASCE
    Associate Professor
    UNIVERSIDAD DEL NORTE
    Barranquilla
    ------------------------------



  • 7.  RE: Anchorage of Positive Moment Reinforcement in Slabs

    Posted 08-13-2020 08:38 AM
    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!!

    ------------------------------
    Muhannad Husain
    Civil Structural Engineer
    Hohai University
    Nanjing
    ------------------------------