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  • 1.  Effect of cambered steel reinforcement in beams

    Posted 05-28-2018 09:00 PM
    Edited by Tirza Austin 05-28-2018 09:00 PM
    What is the effect of using cambered steel reinforcement inside a reinforced beam or even  a grillage slab?

    Mohammad Almikati Aff.M.ASCE
    96176 703098

  • 2.  RE: Effect of cambered steel reinforcement in beams

    Posted 05-29-2018 10:00 AM
    ​Cambering means that the center portion of the member is higher than the ends.  Therefore, the steel in the middle of the reinforced beam is higher, reducing its distance to the compression zone.  This would reduce the moment capacity of the beam.  Why would you want to do this?  Cambering the steel reduces the beam's moment capacity and would also be more expensive to fabricate and install.  There is no good reason to do this.

    Dennis Young P.E., M.ASCE
    Young Engineering, LLC
    Omaha NE

  • 3.  RE: Effect of cambered steel reinforcement in beams

    Posted 05-30-2018 10:01 AM
    Edited by Tirza Austin 05-30-2018 10:00 AM
    The point of camber is to balance out dead load deflection so that you have an approximately flat member once your dead loads are in place. I'm not an expert in concrete so correct me if this is off, but don't they normally put chairs in intermittently to keep the reinforcing at the correct elevation along the length of a member? This would support the reinforcing in the correct place, precluding the need for camber to get the reinforcing in the proper location.

    Theoretically, you want the reinforcing to take the shape that correlates to the member shape once all the concrete is in place and still wet, i.e. you want the reinforcing to be in the right place while the member is being cast so that when it hardens, the reinforcing is in the right place. Chairs resting on the bottom of the form will support the reinforcing in this condition if the reinforcing or reinforcing cage is flexible enough (i.e. not too rigid) to deflect to the chairs. I image you wouldn't have this condition unless you have a reinforcing cage.

    If for some reason you don't have chairs and/or the reinforcing (cage) has to span significant distances, you would want to camber the reinforcing so that under its self weight it will deflect into the proper position. It does not support any concrete, so no additional dead load should need to be applied to it.

    Again, I'm not an expert in concrete or its design practices so I don't know what is typically done to account for these conditions, but they are the structural consideration related to reinforcing location I perceive as needing to be satisfied in order to have a member built with the properties it's designed to have.

    Paul Chabot B.S., EIT, A.M.ASCE
    Structural Designer - Forensic Tech
    Grosse Pointe Woods MI

  • 4.  RE: Effect of cambered steel reinforcement in beams

    Posted 05-30-2018 10:02 AM
    Edited by Tirza Austin 05-30-2018 10:01 AM
    For mild reinforcement, I think we would all agree with Mr. Young. Perhaps you're interested in prestressed reinforcement? If so, draping (not to be confused with cambering), when combined with pre- or post-tensioning of the steel to compress the concrete, can be used to selectively increase the flexural capacity of the beam or slab where you need it most. Where the strands or bars are concentric with the neutral axis of the concrete, the effect is uniform compression through the depth (axial prestress). As the steel moves away from the neutral axis, becoming eccentric, the tension in the steel induces an internal moment that can be used to precompress only the tensile face for elements, or regions of elements, that will experience significant flexure (flexural prestress). Care must be taken, however, not to overstress the concrete in tension or compression when the precompression is applied or during construction prior to application of the in-service loading for which the prestress was designed.

    Gregory Harrell P.E., M.ASCE
    Senior Bridge Engineer, HNTB
    Tampa, FL
    (813) 498-5152

  • 5.  RE: Effect of cambered steel reinforcement in beams

    Posted 06-01-2018 09:59 AM
    Edited by Tirza Austin 06-01-2018 09:59 AM
    Is the camber in a flexural member that is:
    1- Supposed to be flat after all dead loads are applied


    2- Supposed to be an arch in its final form?

    For 1: the whole flexural member is cambered to offset predicted deflection due to DL. The reinforcement bars are supported by chairs on the formwork. The formwork is cambered with the reinforcement following it. Such cambers are usually small enough that even the largest reinforcing bars can follow accordingly. Once the whole DL is applied, the soffit is supposed to have become flat and the reinforcing straight.

    In case 2: The rebars are fabricated curved according to the required final curvature of the flexural member. However, in this case a "popping out" effect of the rebars is needed: the tensile force on a curved rebar generates popping out force, towards the centre of curvature, distributed along the length of the rebar. That force needs to be resisted by enough concrete cover. To find out the necessary thickness of concrete cover (for that action) an assumed wedge shaped failure pattern needs to be analyzed and designed. Further, such rebars need to be anchored by ties/ stirrups to the rebars on the compression side.

    Neil Kazen F.ASCE
    Structural Design Manager Transportation Division
    Toronto ON

  • 6.  RE: Effect of cambered steel reinforcement in beams

    Posted 06-04-2018 10:03 AM
    Edited by Tirza Austin 06-04-2018 10:02 AM
    Assumption:  that Cambering of the rebar is planned to offset DL Deflection.

    Cambering of structural steel to off set DL deflection is a fairly straight forward approach and can be reasonably accurate.  The stiffness of the steel is well known.  The dead load is applied to the steel member which has its full capacity available.  If concrete is used as a deck, the load is applied and not set until the beam reaches the applied dead load.

    Contrast Cambering of Concrete: to achieve a DL camber in concrete the beam/slab forms must be cambered.  Cambering the Rebar will not work as the location of the steel is constant with respect to its initial location in the form.
    Consider, that first the stiffness (modulus) of the concrete, for deflection calculations, is a variable for at least a few week and dependent on a number of actors that are assumed and/or not well defined with out a lot of test on the particular concrete mix you "think" will be representative of what will be used at the time the job is underway.  Further,  the set time of the concrete is a variable, although it can be monitored for strength gain, its gain in stiffness in green and early ages cannot be calculated with accuracy by the standard deflection equations relative to concrete whose stiffness is really compatible to the strength at the given time.

    Unless the concrete beam is cast separate from the later cast slab, all the DL is supported by  the forms so that when the forms are removed will determine when the DL is applied to what ever the stiffness of the "System" is present.  Therein is another complication, it is not only the DL deflection of the beam but of the whole beam/slab system and what ever can be determined to be accurately engaged at the time. 

    To obtain a true cambered system in  concrete, it is not just cambering the bottom of the form but also contouring the top to match the camber of the bottom of the system including the rebar at its relative design location, which should be obtainable by the bar support chairs.

    Then comes the matter of truth when the forms are removed, too soon, on time, too late, etc., to find out what the deflected finished floor level contour is like.  Keep in mind that there is some creep in loaded early strength concrete so that form removed too early can continue to deflect, how much????  I have seen, high rise building constructed using Flying Forms, removing the forms in two days or less, resulting in warped floors between columns because of the DL Creep deflection.  What do you do with a warped, non-level 9+/-) floor then??

    If a level floor is absolutely required, I recommend that the floor system be placed using a second topping slab that can be troweled "level" after the major DL has occurred in the main structural system otherwise I believe there are too many variables and trial testings required "in an attempt" to achieve a level system. 

    Allen Hulshizer P.E., F.ASCE, FACI
    Consulting Structural Engineer
    Life Time ASCE Member
    Chalfont PA