Baltimore bridge collapse: the importance of robustness
One of the requirements required by current technical standards is that of robustnessi.e. the capacity of the structure to limit damage in the event of exceptional actions, such as explosions or impacts. The structural safety of buildings is based on the satisfaction of checks against different combinations of operating actions, such as permanent, variable and seismic, referring to the intended use and nominal life, with a level of safety dependent on the importance of the construction (in terms of crowding and economic damage).
Furthermore, some constructions of a strategic nature that cover a obvious importance on public safetysuch as i bridgesthey must also possess sufficient capacity to avoid disproportionate damage with respect to the extent of exceptional actions (with low probability of occurrence), not included in the usual combinations of operating loads, but sufficient to undermine the stability of the entire infrastructure with devastating consequences in terms of human lives and impact economic.
In the case of the Francis Scott Key bridge, a localized impact was sufficient, albeit with a powerful impact if we consider the moving mass of the cargo ship, to trigger a sudden collapse domino effect caused by the loss of support.
The measures that could have avoided the collapse
In addition to the necessary safety analyzes regarding the residual resistance and in planning adequate structural maintenance plansone of the critical issues for existing infrastructures designed decades ago consists inupdating of risk scenarios not contemplated in the original design: greater difficulty in supporting higher traffic flows, more frequent hydrogeological disruptions due to climate change, terrorist acts, accidental collisions, just to give a few examples.
In the past, design criteria did not adequately consider the concepts of structural robustness; on the contrary, the infrastructures were characterized by a limited structural redundancy to optimize construction costs. They were safe structures from a static point of view, but with extremely fragile structural schemes if an exceptional event had involved even just one load-bearing element of the structure. So it should not be surprising that the loss of one support is enough for the entire structure to collapse in a few seconds.
In the case of the Francis Scott Key bridge it would have been useful, for example, to foresee some barriers to be positioned around the pillars to counteract any ship impacts or reduce the effects of accidental impact, which among other things is not that rare in the history of bridge collapses, increasing the level of structural redundancy.
One of the methods is also to predict the compartmentalizationthat is, the segmentation of areas that are not structurally connected to each other, to prevent localized damage from involving the entire structure just as happened to the Francis Scott Key bridge.
The basic principles for a correct structural conception
The concept of robustness it has now been introduced into almost all national and international technical regulations, providing prescriptive design and sizing criteria.
In the Italian regulatory landscape, the CNR-FT 214/2018 Instructions for evaluating the robustness of buildings constitute a valid technical support in robustness analysis «starting from the definition of the exceptional actions to be considered as possible project scenarios, to the strategies for risk reduction, to the operating methods of the structure which must deploy all possible resistance reserves before collapse, in a non-linear field for geometry and material.»
In particular, the following are illustrated basic principles of a correct structural conception for limit the risk of disproportionate collapse:
- the method of local resistance to avoid local damage to the elements whose collapse would lead to uncontrolled propagation of the damage;
- the method based onidentification of alternative load routes so that the structure is able to redistribute the loads carried by the collapsed elements following local damage to the intact structural elements;
- the method based on compartmentalizationwhich aims to limit the extent of disproportionate collapse due to local collapse by isolating the collapsed structural part from the remaining structure.
