UBC Theses and Dissertations
A decision model for the erection of cable-stayed bridges Chan, Canisius W. L.
Safety during bridge erection has had little consideration, in comparison with the extensive knowledge base on safety for completed structures. During construction, exposure time to various loads is less, but since the full stiffness and geometry of the bridge has not yet been realized, the structure is especially vulnerable. Decisions made at this time require careful consideration of consequences. This situation is illustrated by a case study of an actual cable-stayed bridge proposed for construction. The erection of the bridge is carried out during a short period compared to the service life of the structure. This difference is a ratio of the order of 1 year to 75 years. It is reasonable to expect that the design wind load during construction can be adjusted to account for the lesser likelihood of exposure to an extreme storm event. It is the intention of the author to recommend a rational method for defining the design wind load, taking into account consequence costs. With the proposed method, it is possible to go one step further and integrate the construction-period wind into project-specific decisions regarding scheduling and sequencing. This rational definition could lead to more cost effective designs in cases where the code-prescribed loads are overly conservative. This could also help to distinguish where the code is unconservative as well. The partially-erected bridge deck is subject to large deflections as well as other aerodynamic effects. Different measures can be taken to provide improved stability against wind loading during erection stages. These include the installation of temporary support devices such as cable bracing systems and tuned mass dampers (TMDs). The selection of temporary supports will have an impact on the overall design of the bridge. Each support option is characterized by a set of benefits and drawbacks. One particular drawback of bracing arrangements is their introduction of ship collision hazard to the erection process. Currently, there is no explicit method to assess the risks and merits of a temporary support system, given the many variables that could possibly have an impact on the decision. In light of this fact, a decision model encapsulating the need to address wind loading and vessel collision concerns is proposed. The decision model permits a rational evaluation of the conceptual erection scheme, where traditional techniques fail to capture the unique nature of bridge erection methods. It also facilitates the work of the decision-maker by organizing the decision variables in a logical order, and allowing a formal framework within which engineering judgement can be effectively utilized. In this example, the decision analysis was able to put forth an erection strategy that accounted for wind and ship collision risks, and their associated costs.
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