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
- A certain percentage of the dead load deflection
- The full dead load deflection
- The full dead load deflection as well as a percentage of the life load deflection
- Camber is usually designed to compensate for deflections caused by pre-composite dead dead loads
Advantage of the Cambering
- Supporting beams will deflect under a load of concrete being placed.
- 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.
- Cambered beams should deflect to a straight line if load and deflection are predicted accurately and camber equals deflection.
- This allows the floor slab to be flat while maintaining a consistent thickness.
- If Beams are not cambered the deflection under the load of the wet concrete will result in a ponding effect in the concrete.
- 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.
- The volume of concrete used will typically be 10-15% more than if the floor is a constant thickness.
Disadvantage of the Cambering
- The use of cambered beams will, to a certain degree, be limited by other aspects of the design for a structure.
- Due to the complexity in detailing, fabrication, and fit-up associated with moment connections, the camber should not be used in moment connected beams.
- 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.
- The processes used to create camber in beams, as well as the actual deflections under a load of cambered beams, are not exact.
- 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.
- Levelness and consistent floor thickness can be a problem.
- The diagrams above show two possible results of chambered beams not deflecting as predicted under the load of the wet(Plastic) concrete.
- Stud heads are exposed
- Top of slab elevation out of tolerance