Addressing the Current Practice of Over Estimation of Structural Resistance of Existing Strengthned Structure.

Abstract

Present regional practices for performing a conventional offshore structural analysis erroneously considers strengthening proposed for any additional loads on existing structures to also support the already existing loads which mainly is due to the limitations of the linear analysis approach presently adopted. This method could lead to false perception of code compliance exposing strengthened structures to a higher failure risk. The paper introduces a more accurate approach for the assessment of the structure and the evaluation of the effectiveness of strengthening introduced at different stages through the structure’s lifetime. Through the course of this study, a comparison was carried out between the conventional linear strength analysis vis-a-vis the non-linear finite element analysis technique for a typical offsore structural Bridge. The work involved full simulation of a bridge supported by two offshore structures using the conventional method and assessing the results. Subsequently an integrated finite element model, with critical locations represented through 4-node shell elements was generated. A non-linear finite element analysis was adopted where stage-wise strengthening and load history was simulated. The structural assessment using the non-linear finite element analysis technique has demonstrated that the bridge did not comply with the structural strength requirements of ISO 19902 international standards in as-is condition. This was in contradiction to the conclusion and outcome of the previous design reports. The previous strengthening design had adopted the conventional linear design methods, ignoring the realistic effects of loading and strengthening stages throughout the history of the structure. Ignoring these effects had misled to the perception of effectiveness of the new combined sections, whereas accurate representation had demonstrated that only the new additional loading is distributed to the new combined section and the existing loading is resisted by original sections only. This behavior is applicable if the installation techniques did not include jacking systems to relieve the loading prior to strengthening. The results and conclusions of the study have given an insight to the limitation of the conventional linear structural analysis methods in capturing the accurate structural behavior. It also provided a demonstration of how an incompatible type of assessment may lead to incorrect perception of compliance.