Progressive Collapse of Structures: A Robustness Interpretation of Intact Vs. Damaged Frames

Progressive Collapse of Structures: How to Reduce the Risk of Disproportionate Damage Through Proper Design and Use of Mechanical Properties of Materials

ALESSANDRO CALVI¹

alessandro.calvi84@gmail.com¹

ABSTRACT

This paper deals with the critical issue of structural collapses and the importance of the correct quantification of the actions in the design phase and the choice of materials with correct characteristics and properties.

The document analyzes the main strategies to be taken into account in the design phase which contemplate the quantification of external actions and the exploitation of the mechanical properties of materials in favor of safety in order to avoid disproportionate damage to the source.

PROGRESSIVE COLLAPSES: LITERATURE REVIEW

The most historically famous case of structural collapse following exceptional action is represented by the event that occurred on 11 September 2001 at the World Trade Center in New York City, when some terrorist attacks first caused the crash of planes against the Twin Towers and subsequently their collapse.

In this case the damage spread from top to bottom causing with a domino effect the loss of bearing capacity of all the floors of the buildings up to the complete vertical collapse.

The main theory with a posteriori analysis of the damage event is probably the one represented by Prof. Z.P. Bazant [1]: according to his interpretation, the initiation and propagation of the collapse occurred due to the heat given off by the fire caused by the aircrafts' fuel, which irremediably reduced the bearing capacity of the steel columns, where the potential energy, directed downwards from the upper floors, could not be absorbed by the plastic moment of the pillars, thus transforming itself into kinetic energy.

Other examples of unfortunately famous episodes of structural collapse involved buildings such as the Ronan Point Tower (1968) and the Murrah Federal Building (1994) where, respectively due to a gas leak and a terrorist attack event, there was the collapse of important portions of the buildings causing loss of life.

The events discussed above lead to the necessary introduction of a characteristic that all buildings should possess in order to guarantee resistance to exceptional events such as terrorist attacks, explosions, impacts and collisions, fires: the structural robustness [2].

The concept of structural robustness, already introduced in various calculation codes, is a fundamental requirement of structures for their ultimate resistance in the event of damage, even minimal, without manifesting consequences or collapses disproportionate to the cause/action.

In other words, it is desirable to ensure, starting from the design stages, that the structure is able to absorb a certain amount of extra load redistributing it or dissipating it in such a way as to exploit the ductility characteristics of the components and materials as much as possible, without inducing trends with fragile behavior.