In bridge engineering, camber refers to a pre-fabricated/pre-designed shape that enables the members to maintain a stress-free state for the member to reach a targeted shape at a specified time. Two main prerequisites that need to be set before calculating camber are:
a) Target shape and the time of the target shape. Stress-free state.
For simple bridges, the target shape is considered as the reference state and camber calculations are straightforward. But, for long-span bridges, (like a cable-stayed bridge), the camber calculations are iterative as the camber will affect the deformations and stresses, thus causing a non-linear problem (Camber -> Deformation+Stress -> Camber).
Accurately calculating the camber by the designer is imperative and will serve as crucial data for the construction engineer, enhancing the serviceability and durability of the bridge especially cable-supported bridges.
Understanding Construction Stage Displacements
are the displacement results obtained only due to the loads/boundary changes acting in the current stage.
are the displacements of the elements which will be created in the next stage considering the rotational angles of nodes resulting from each current construction stage. In Figure 2, the initial tangent displacement of node 3 is a result of the deformation of the first element in construction stage 1. Next, in construction stage 2, the second element is deformed such that it has the deformations of its self-weight and also includes the angular displacement created by node 2. In midas Civil, the initial tangent displacements are calculated by using the specified displacement function.
Figure 2. Tangential Displacements
To simulate the above construction stages in midas, we have created three stages as shown below CS1, CS2, and CS3 (green dashed line indicates the construction joint).
Figure 4. Three Construction Stages: CS1, CS2, CS3
How Camber is Calculated:
Now we know the displacement output from FEM analysis software like midas Civil, we as a designer need to determine the camber/pre-camber that needs to be supplied to the construction engineer. To avoid confusion between the designer and the construction engineer, the ‘camber and related terminology’ have been summarized in below Table 1.
The Camber ( is provided when the construction method is using the cast-in-situ concrete (in this case, the fabricated shape and shop form do not exist as everything happens on-site).
The Shop Form ( is considered when the construction method uses prefabrication/precast techniques (in steel girders and precast concrete girders).