Earthquake Engineering Research - HKU Libraries - The University ...

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NEURAL NETWORKS
Development
In the case study, the UCSD PC bridge bent analyzed previously was selected as the
reference structure. A nonlinear finite fiber element model of an enlarged PC bridge is
composed as the target structure to be modeled, employing the nonlinear fiber frame
element of the developed software. A total of 86 cases of the FE model subjected to the
ground motion of the 1940 El Centro earthquake were analyzed using the developed
nonlinear FE software. The analytical results were then used as the training and testing
data for the ANNs.
The magnitude and orientation of the horizontal ground motion (1940 El Centro
earthquake) were selected as the input parameters of the ANN. The maximum biaxial
bending moments of columns and girder and the maximum horizontal displacement of
the girder were selected as the outputs of the ANN. Seven MLP networks with different
number of hidden layers and hidden neurons were first tried. An augmented form of MLP
was used to improve the modeling accuracy.
The magnitude of the earthquake was represented by the scale factor for the ground
acceleration record of El Centro earthquake. Scale factors ranging from 1.0 to 3.1 were
used, which corresponds to a PGA level ranging from 0.32g to 0.99g. The orientation of
the earthquake was represented by the inclination angle between the longitudinal axis and
the incident direction of the ground acceleration, as the a angle. Considering the
symmetry of the structure, a range of a angle from 0° to 90° was investigated. Thus, 86
cases of fiber element analysis were conducted, the outcomes of which were summarized.
The bi-axial maximum member-end moments of the piers and the girder and the
maximum horizontal node displacements of all 86 cases were also summarized
It was found that the bending moment at the bottom end of the piers was generally
greater than that at the top end of the piers. Hence, only the bending moment at the
bottom of the piers was chosen as one of the output parameters of the ANNs. Also, the
maximum bending moments of the two piers were very close; thus, only one set of the
maximum bi-axial bending moments M y , M z of the two piers was chosen as the
representative critical bending moments of the bridge piers.
Similarly, a pair of the maximum bi-axial bending moments of the girder was
chosen from the maximum bending moments at the two ends of the girder; however,
since the minor-axis bending moment M y of the girder was generally very small, only the
major-axis bending moment M 2 was chosen to be investigated by the ANNs. The
maximum value of the displacements at the two nodes was selected as the representative
maximum horizontal displacement of the girder. Therefore, there were 4 output
parameters altogether of the ANNs, that is, the maximum bending moments M y and M 2 of
the piers, the maximum bending moment M z of the girder, and the maximum horizontal
displacement of the girder.