Keywords: complex structures, nonlinear analysis, performance-based design, progressive collapse, robustness, suspension bridge, uncertainties.

This work deals with the robustness of long span suspension bridges inside the general framework of the performance-based design. Due to the inherent complexity in the design process of long span suspension bridges, one needs to define a new design philosophy, the so called performance-based design [8]. For this kind of complex structures, it is more important to take into account all structural requirements [6,7]; in particular, in this work one has considered the robustness. In this general framework, all uncertainties are examined in order to evaluate a robust design, whose aim is to obtain a design insensitive to variations of design variables within a feasible region [1,10].

The robustness implies that a structure shall not be damaged by accidental loads, like fire, explosions, airplane impact or consequences of human errors, to an extent disproportionate to the severity of the triggering event. The robustness concept is closely related to accidental actions, consequences of human error and failure of equipment. These situations are denoted as hazardous circumstances or briefly hazards.

The structures shall incorporate robustness through the consideration of the effects of all hazards and their probabilities of occurrence to ensure that consequent damage is not disproportionate to the cause. Damage from an event with a reasonable likelihood of occurrence shall not lead to complete loss of integrity of the structure. In such cases the structural integrity in the damaged state shall be sufficient to allow a process system to close down and/or a safe evacuation. Framing patterns that provide alternative load paths are preferred. Robustness is achieved by considering accidental limit states that represent the structural effects of hazards. Ideally, all such likely hazards should be identified and quantified by means of rational analyses. The starting point of a robust design is a hazard that is more unlikely to happen than the usual design situations, but not unlikely enough to be neglected. If there is no hazard then there is no requirement in relation to robustness.

A consequence of a local failure is the progressive collapse [9]. Structures shall be designed so that the risk of progressive collapse is slight. This may be accomplished by designing and detailing buildings either in such a way that they can withstand accidental actions or in such a way that primary damage is limited. Such damage shall not give rise to progressive collapse and severe destruction in any part of the structure. Special measures need to be taken in structures in which the risk of serious accidents due to progressive collapse is high.

The influence of progressive collapse on the modelled long span suspension bridge for several contingency scenarios is considered in this work [2,3,4,5].

1

G. Augusti, A. Baratta, F. Casciati, "Probabilistic Methods in Structural Engineering", Chapman and Hall, 1984.

2

F. Bontempi, L. Catallo, L. Sgambi, "Performance-based design and analysis of the Messina Strait Bridge". ASRANET'04 Conference, Barcelona, Spain, 2004.

3

F. Bontempi, L. Catallo, L. Sgambi, "Structural analysis and design of long span suspension bridges with regards to nonlinearities, uncertainties, interactions and sustainability", IABMAS '04 Conference, Kyoto, Japan, 2004.

4

L. Catallo, L. Sgambi, M. Silvestri, "General aspects of the structural behavior in the Messina Strait Bridge design", Proceedings of the Second International Conference on Structural and Construction Engineering, ISEC-02, September 23-26, Rome, Italy, 2003.

5

L. Catallo, "Progettazione prestazionale nei sistemi complessi: fidatezza strutturale" (in italian). Ph.D. Thesis in Civil Engineering, University of Rome "La Sapienza", Rome, April, 2005.

6

D.M. Frangopol, K. Imai, "Reliability of long span bridges based on design experience with the Honshu-Shikoku Bridges", Journal of Constructional Steel Research, Elsevier, 60(3-5): 373-392, 2004. doi:10.1016/S0143-974X(03)00117-2

7

J. Schneider, "Introduction to Safety and Reliability of Structures", Structural Engineering Documents, 5, 1997.

8

I. Smith, "Increasing Knowledge of Structural Performance", Structural Engineering International, 12(3): 191-195, 2001. doi:10.2749/101686601780346931

9

U. Starossek, G. Sauvageot, "Discussion on Bridge Progressive Collapse Vulnerability" by A. Ghali, G. Tadros, Journal of Structural Engineering. Vol. 124, No 12: 1497-1498, 1998. doi:10.1061/(ASCE)0733-9445(1998)124:12(1497)

10

G. Taguchi, "Introduction to Quality Engineering: Designing Quality into Products and Processes", Asian Productivity Association, 1986.

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