What is the meaning of the Pre-cambering

What is the meaning of the Pre-cambering

Pre-cambering
 Pre-cambering reduces the deflection under the loads, being one of the requirements of deflection checking. For usual simple supported girders, the problem is relatively simple, more complex is the pre-cambering problem at continuous and frame structures, especially in bridges where different construction phases and also positions of the convoy have to be considered. Camber in a beam can be designed to compensate for either
    1. A certain percentage of the dead load deflection
    2. The full dead load deflection 
    3. The full dead load deflection as well as a  percentage of the life load deflection 
  1. Camber is usually designed to compensate for deflections caused by pre-composite dead dead loads
Advantage of the Cambering
  1. Supporting beams will deflect under a load of concrete being placed.
  2. This deflection can be exaggerated in a composite floor system where the full strength of the system is not achieved until the concrete has cured.
  3. Cambered beams should deflect to a straight line if load and deflection are predicted accurately and camber equals deflection.
    1. This allows the floor slab to be flat while maintaining a consistent thickness.
  4. If Beams are not cambered the deflection under the load of the wet concrete will result in a ponding effect in the concrete.
  5. To create a flat floor in this situation the concrete will need to be thicker at the center of the bay where the deflection is the greatest.
  6. The volume of concrete used will typically be 10-15% more than if the floor is a constant thickness.
Disadvantage of the Cambering
  1. The use of cambered beams will, to a certain degree, be limited by other aspects of the design for a structure.
  2. Due to the complexity in detailing, fabrication, and fit-up associated with moment connections, the camber should not be used in moment connected beams.
  3. Beams with simple framing connections ma be cambered because the end rotational resistance of a simple connection is a small comparison to that of moment connection.
  4. The processes used to create camber in beams, as well as the actual deflections under a load of cambered beams, are not exact.
  5. Care needs to be taken in the specification and fabrication of camber to ensure that a beam, once in place and under load, will perform within tolerances.
  6. Levelness and consistent floor thickness can be a problem.
  7. The diagrams above show two possible results of chambered beams not deflecting as predicted under the load of the wet(Plastic) concrete.
    1. Stud heads are exposed
    2. Top of slab elevation out of tolerance

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