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.
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Allen Hulshizer P.E., F.ASCE, FACI
Consulting Structural Engineer
Life Time ASCE Member
Chalfont PA
(215)8872838
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Original Message:
Sent: 05-31-2018 19:00
From: Neil Kazen
Subject: Effect of cambered steel reinforcement in beams
Is the camber in a flexural member that is:
1- Supposed to be flat after all dead loads are applied
Or
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.
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Neil Kazen F.ASCE
Structural Design Manager Transportation Division
Toronto ON
Original Message:
Sent: 05-28-2018 17:45
From: Mohammad Almikati
Subject: Effect of cambered steel reinforcement in beams
What is the effect of using cambered steel reinforcement inside a reinforced beam or even a grillage slab?
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Mohammad Almikati Aff.M.ASCE
Beirut
96176 703098
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