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CHAPTER 4: THE CONSTRUCTION PROCESS OF SEGMENTAL BRIDGES - page 35 / 47

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Chapter 4: The Construction Process of Segmental Bridges

191

After having considered the loads and restraints, i.e. all actions, two elements remain to be discussed. The geometry of the structural system itself is developing continuously with progress of construction depending in a manner that depends on the erection method and needs to be considered. Incomplete structures, e.g. the cantilever beam prior to midspan closure for continuity are inherently weaker than in the finished state because of less structural redundancy. All these different construction stages need to be analyzed as a combination of load cases and the resistance of the structure at that construction stage for structural safety.

Apart from that structural resistance the material resistance may also be weaker than in the final state. Especially for cast-in-place segmental bridges the still young concrete usually has not developed its full specified strength when it is being prestressed and loaded with more segments for quick erection. The structural resistance and material resistance can also be understood as the two components of structural safety, namely strength and stability, as outlined in Section 3.1.3.

Summarizing, stresses induced by construction loads may be higher than those from service loads as the incomplete structural system is mostly different and weaker than finished structures, concrete has not gained full strength, and the boundary conditions may be different from the service state. In other words, the great importance of construction stages lies in the criticality that results from the still low structural and material resistance, while loads may be actually more adverse and boundary conditions different.

4.3.1.1 The Zilwaukee Bridge

Underestimating the construction loads and their effects on the unfinished structure caused several accidents and failures of bridge structures under construction in the past. An example for an accident of a bridge under construction is provided by Anon. (1988), who describes construction of the Zilwaukee Bridge in Michigan. The new bridge provided a replacement an old drawbridge on Interstate I-75 that was necessary because of growing traffic. According to Anon. (1988, p69), the Michigan Department of Transportation (MDOT) made plans for both

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