Due to the consideration of time effects and the respective active loads and elements in the construction simulation of bridges with concrete elements structural engineers, in general, understand that the final stage design of a bridge might be quite different from one with all the constructability considerations. Different deformations are caused by creep and shrinkage, so the bridge engineer should design his project to be built so that the final construction stage reaches the predefined position of the planned gradient. Therefore we have to prepare bridge plans with Camber information, i.e., with the expected pre-camber line. If we consider that Camber is the summation of all deformations due to permanent loading cases during the construction stages then the pre-camber is the opposite sign of all these summations and the bridge designer should deliver this data to be checked by the bridge constructor at any time during the construction sequence.   

The Camber analysis is performed with a given starting geometry which is usually the structure in its desired final geometry. Pre-camber is the theoretical stress-less geometry of the bridge that is required for the assembly to reach the final geometry under given loading conditions. As the bridge structure is erected in segments the pre-camber geometry is defined as the required coordinate offsets of the segment beginning and endpoints.

The camber line calculation could follow different philosophies; but, whatever method is used we start with a shape-finding schedule where we use the summation load case to accumulate the displacements and then, in sequence, start a schedule recalculation including pre-camber to achieve the final position with almost zero deflections.

A very easy-to-understand classical example of this type of deformation is the freestanding cantilever. When built in stages the expected deflection changes, as in the image below, from the usually expected load only dependent shape to a time-load non-linear stepped deformation line at a three-stage construction. Despite the load being the same, i.e., the total deformation line is similar, the introduction of the concrete materials C&S between the actual activation of elements causes this non-linearity while time goes by the interval the next stage is not yet performed. The consideration of time-dependent variables adds complications to simple structures and it is usually much more complex for bridge structures that are commonly built in stages.

   

RM Bridge uses the summation load case to add all the permanent deformations and consider the inverse for the pre-camber line. Therefore, the final grade, at a certain predetermined time in the future of the bridge life span, is the bridge alignment.

The Camber-FAQ document has this same example with 5 construction stages and a more complex one typical for Balance Cantilever Bridges built with the Advanced Shoring method, and that is similar to Cable-Stay Bridges and Segmental bridges.