Whichever solution adopted for increasing structural robustness has to be associated with a prescribed performance objective, as stated in the previous section. In this sense, the idea proposed by Lind (1996) of “damage tolerance”, intended as the capacity of the system to be able to sustain some damage without failure, fits well the general approach to structural integrity. Following that, different strategies are possible.
Event Control Event control refers to avoiding or protecting the construction against an inci- dent that might lead to its disproportionate failure. This approach does not increase the inherent resistance of a structure, as reported by Starossek and Haberland (2010), but limits the possibility of occurrence of the event. Examples of event control measures are: (i) planning of the geographical location of the building, provision (ii) of stand-off perimeter, (iii) for surveillance systems such as alarm and security, (iv) prohibiting the storage of explosives, (v) placing fenders around the columns to prevent vehicle impact, (vi) placing barriers around the ground area, (vii) gas detectors and automatic cut-off devices for gas, (viii) control or limiting of fire ignition sources, (ix) limiting fire loads, (x) fire suppression systems, (xi) installation of smoke detectors and alarms, (xii) use of Structural Health and Monitoring Systems, (xiii) quality control during construction, maintenance and repair activities. Some of these measures require a specific design and have to be maintained throughout the whole life of the construction (Diamantidis and Vogel, 2011).
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Specific Load Resistance If sufficient strength is provided to structural elements, they would be able to resist overloads. This is the principle at the base of the Specific Load Resis- tance strategy for robustness. The members have to be classified with respect to their importance for the survival of the structure and the key elements have to be identified in such a way that a failure of one of them does not exceed the performance objectives. Examples of key elements can be found in Starossek and Haberland (2012): piers in continuous bridges, cables in suspended bridges, and so forth. Sorensen and Chris- tensen (2006) proposed to increase the capacity of key elements in order to account for the overload required in case of damage of the structure. Anyway, increasing the capac- ity of the elements is more expensive than preventing the initiation of the damage. That is why the strategy can be applied in those cases in which the key elements are few in number and easy to identify.
Alternate Load Path The strategy consists in providing alternatives for a load to be trans- ferred from the point of application to a point of resistance, namely the foundations. Provided that the alternative paths are sufficiently strong, this enables redistribution of forces originally carried by failed components to prevent a failure from spreading. In order to achieve this requirement, the remaining structural elements must be strong enough, collectively, to resist the loads corresponding to the situation after the event. The resistance of the elements must be associated with a proper capacity in deforma- tion without loss of resistance. In any case, it is necessary that, after the failure, the overall stability is guaranteed. An important issue has to be addressed in these cases in which the redundancy of the structural scheme is increased in order to achieve robust- ness requirements: multiple load-paths may sometimes involve brittle situations or limit deformations with negative consequences. These situations must be avoided (Knoll and Vogel, 2009).
Alternate Load Path strategy is effective in case of both hazard-specific and non-hazard- specific situations because the notional damage to be considered in the application of the alternative-paths method is non-threat-specific (Diamantidis and Vogel, 2011). Because of that, the approach is useful in dealing with Black Swan situations.
In order to implement the strategy, a set of structural components that have to be al- ternatively removed, or that are able to loose their load bearing capacity, have to be considered. The remaining part of the structure has to be able to support the loads with a given reliability for the time, t, required for the reparation or the evacuation of the people (JCSS, 2011). That is
Pr (C < Ain time t | one/more element(s) removed) < pt, (4.2)
where C represent the strength of the residual part of the structure, A are the external forces acting during the period of time t. The target reliability, pt, depends essentially
on the safety requirements for the building and the length of the period for repara- tion/evacuation.
The probability that an element is removed by some cause depends on the sophistication of the design procedure and on the type of structure. In the case of non-hazard-specific design, the Alternate Load Paths strategy starts with the assumption of reasonable sce- narios of initial damage. The structure is then designed such that the spread of this local initial damage remains limited to an acceptable extent (Starossek and Haberland, 2012). Consequence reducing measures In order to ensure the safety of the occupants of a con- structions, alternative measures can be implemented. These are not necessarily linked to structural aspects of the construction. In many cases, technological equipment (like sprinkler for fire, alarms, . . . ) can be installed in order to prevent the damages on the construction. Emergency planning, as well, represents a way for ensuring the safety of the occupants. Structurally speaking, an effective way for reducing the consequences of events are the isolation of parts of the structure in order to prevent the spreading of the damages (Starossek, 2007a). Structural segmentation has been demonstrated to be effective in various cases. For example, consider the collapse of part of the New Termi- nal 2E at Paris – Charles De Gaulle Airport on May 23, 2004. The design of the roof of the terminal was unusual with a shell consisting of curved concrete sections. The causes of failure are pretty unknown. Someone supposes that there was a sum of cir- cumstances that lead to the event: the high flexibility of the structures under dead load, the external actions increased by cracking, a lack of robustness and redundancy able to transfer loads away from local failure, the high local punching stresses where the struts were seated in the concrete shell, and weakness of the longitudinal support beam and its horizontal ties to the columns (Wood, 2005). Only six modules were interested by the failure, due to the compartmentalisation of the structural scheme (Starossek, 2007a). Obviously, in the design phase, one may choose one or more strategies for robustness. For example, consider the terroristic attacks of 9/11 in which the Pentagon Building in Washing- ton, D.C. was stroke by an airplane. The extension of the damages created by the airplane and its burning fuel, i.e. 50 damaged columns at the first level, was relatively reduced if compared with the source. As extensively outlined by Mlakar et al. (2003), this fact is mainly due of the contemporarily occurrence of various structural situation: (i) redundant and alternative load paths of the beam and girder framing system, (ii) short spans between columns, (iii) substan- tial continuity of beam and girder bottom reinforcement through the supports, (iv) design for overloads, (v) significant residual load capacity of damaged spirally reinforced columns and (iv) ability of the exterior walls to act as transfer girders.
Considering the analytical expression of the risk, Eqn. (4.1), the following considerations can be made:
• event control strategies act on the events and on the possibility the hazard interests the construction. In other terms, such strategies involve a reduction of term p (Hi)of
Eqn. (4.1);
Chapter 4 - A robust structure - 65
takes place. In this sense, the more efficient the precautions, the less probable the pos- sibility of local damage. This strategy acts directly on the term p (Dj|Hi)of Eqn. (4.1).
• alternate load path strategies work as stopper for the propagation of the damage from a local to a global extent. That is why the act on term p (Sk|Dj)of Eqn. (4.1).
• consequence reducing measures are strategies that tend to limit the social/physical costs involved by the event on the structure. Therefore, the strategy is linked with term C (Sk)
of Eqn. (4.1).