D3. The different construction phases 

D3. The different construction phases 

Studying the phases of construction has two objectives:

There are various effects related to assembling. They are due to:

Taking these effects into account can be relatively complex and, in the most difficult cases, it may be essential to use software that can model the evolution of the structure step by step.

However, it is often possible to proceed by superimposing various linear analyses.

Effect of the construction phases in the case of the keying of a cantilevered bridge span

The main issues are related to the effects of the delayed deformation of concrete. Indeed, how can one evaluate the final state midspan moment in the above example? In the case of a single keying point, it is possible to use the method known as the "coefficients method" (Figure 6). This approach is based on the following arguments:

Final state = (E(t0, t1)/E(t0, t)) x Final state not keyed + (1-(E(t0, t1)/E(t0, t))) x Final state without phasing.

with t0 the duration of load application, t1 the duration of the keying, t the time considered for the final state, and E(t0, t1) the concrete modulus for obtaining the deformation of the concrete at time t1 for a load applied at t0.

This method, in the case of a single keying point, outputs the exact theoretical final state. However, it is difficult to extend it to the case of multiple keying points and can lead to absurd results.

The unitary case used for the coefficients method

It is best to externalize the effects of changes in the static state as follows (Figure 7) :

This technique can be extended more easily to cases where the static configuration of the structures is modified many times.

Unitary cases used for the superposition method

It is worth remembering that, under the effects of delayed deformation, concrete reacts with an apparent deformation modulus called relaxation modulus, which is lower than the corresponding creep modulus. If the classical ratio between the moduli of steel and concrete, including creep effects, is about 18, in the case of an imposed displacement, this ratio increases up to 24. This tends to make the adjustments by support elevation and actuators less efficient when the latter leads to imposing a deformation on the structure.

From the point of view of FE simulations, the large number of intermediate states that need to be processed multiplies the risk of errors. The verifications must concern:

In construction studies, one must also remember that the actual creep of concrete can deviate strongly from the theoretical formulations. The model must therefore be constructed such that it is easily adaptable to restore the deformations appearing in the first phases, and thus improve the prediction of the following ones.

 


Révision #1
Créé 19 September 2023 10:00:16 par Paul Terrasson Duvernon
Mis à jour 19 September 2023 10:02:52 par Paul Terrasson Duvernon