Camber Control in midas Civil

Table of Contents

1. Introduction

2. Understanding Construction Stage DisplacementsOversimplification for FEM  Practitioners

3. How Camber is Calculated? 

1. Introduction

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.
b) 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.

2. Understanding Construction Stage Displacements

Construction Stage Analysis(CSA) is very much required to make real construction safe and serviceable. It is imperative to perform a detailed construction stage analysis for statically indeterminate structures having the loading history included in the analysis. Many factors affect the deflections in construction stages like Construction Process, Creep and Shrinkage, Prestress Losses, Time Delays, etc.

Three displacement results can be obtained from Construction Stage Analysis in midas Civil
           a) Accumulated Displacement
           b) Current Stage Displacement
           c) Stage/Step Real Displacements

Accumulated displacements are the displacement results obtained by adding all stage displacements including the current stage displacements. The discontinuous point of the sawtooth shape exists since each segment upon creation has only the load influence accumulated (the load influence has not been accumulated before the creation of the segments).

In Figure 1, the first element is activated, and then it is deformed due to its self-weight, which means the center node is in a deformed location but in the next construction stage when the second element is activated the third node is activated in the modeled location. As a result, the elements form an angle even though there was no angle when the elements were initially modeled. Next, in construction stage 2, the second element will deform due to its self-weight.

Current Step displacements are the displacement results obtained only due to the loads/boundary changes acting in the current stage.

Tangential displacements 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 3: Cantilever constructed In three stages

 

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).

 

Now the deformation results in each CS Stage are as follows:

Accumulated Displacements:

CS1 --> CS1 deformation = (current step Displ. of CS1)

CS2 --> CS2 deformation = (CS1 deformation) + (current step Displ. of CS2)

CS3 --> CS3 deformation = (CS2 deformation) + (current step Displ. of CS3)

Stage/Step Real Displacements:

CS1 --> CS1 Stage/Step Real Displ. = (current step Displ. of CS1) + (Tangent displacements)

CS2 --> CS2 Stage/Step Real Displ. = (CS1 deformation) + (current step Displ. of CS2) + (Tangent displacements)

CS3 --> CS3 Stage/Step Real Displ. = (CS2 deformation) + (current step Displ. of CS3) + (Tangent displacements)

Note: In midas Civil, If "Initial Tangent Displacement" is turned ON, then boundary conditions will be activated using stage/step real displacements, hence we need to be careful in using deformed or Original position while activating boundary conditions in construction stage definitions.

3. 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(δcamber) 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(δN,pre-camber) is considered when the construction method uses prefabrication/precast techniques (in steel girders and precast concrete girders).