Important Know-How on Typology of Progressive Collapse

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Progressive collapse of a structure is defined as the spread of an initial local failure of a structural element that eventually results in the collapse of the entire structure or a major portion of it. This type of collapse occurs out of proportion; hence it is also called disproportionate collapse.

The two main reasons that trigger progressive collapse are design or construction errors and extreme loading conditions. The progressive collapse of structures is primarily categorized into five different types based on the triggering event and the resulting collapse of the structure.

This article explains different progressive collapse modes; zipper, domino, instability, pancake, section-type, and mixed type collapse categorized based on the type of the structure and the initiating event.

Different Types of Progressive Collapse

Even though progressive collapse is understood as a failure that affects the whole system due to disproportion, the final collapse condition of the structure can occur in different ways. The different types of progressive collapse are:

1. Pancake-type
2. Zipper-type
3. Domino-type
4. Section-type
5. Instability-type
6. Mixed-type

1. Pancake-Type Collapse

A famous example of the pancake-type collapse is the progressive collapse of the World Trade Center (WTC) towers. The impact of the airplanes on the structure led to fire that initiated local failure in the areas of impact. The loss of vertical bearing capacity happened to a few stories, which later extended over the entire cross-section of the tower.

With time, the upper part of the structure started to move downwards and accumulated kinetic energy. The collision of the top portion of the building with the lower part of the structure that was still intact resulted in large impact forces that were larger than the reserve capacities of the structure. This finally resulted in the complete loss of the bearing capacity in the area of impact. This failure progressed and resulted in total collapse.

Hence, the features of pancake-type collapse are:

1. Pancake-type collapse is triggered by the initial failure of the vertical-load bearing elements.
2. The components of the building may undergo partial or complete separation and fallin a vertical rigid-body motion.
3. During this collapse, the potential energy is converted into kinetic energy.
4. The falling or the separation of structural elements affects the remaining structure—the impact loads sue to falling of structural elements results in the failure of the vertical load-bearing elements.
5. Together the above effects result in the progression of collapse in the vertical direction.

2. Zipper-Type Collapse

Zipper-type collapse is triggered by the initial failure of one or more tension elements of a structure. This failure is common when the sudden rupture of the cables of a bridge or failure of one or more anchors of a retaining wall occurs.

A zipper-type collapse exhibits the following features:

1. Zipper-type collapse is triggered by the initial failure of one or more structural elements.
2. Any failure of element is accompanied by redistribution of forces to the remaining structural elements.
3. The initial failure of structural elements creates impulsive loading. The remaining structure shows a dynamic response towards this impulsive loading.
4. The combined action of static and dynamic effects results in the force concentration of elements.
5. The force concentration results in the failure of elements similar in type and function to the failure of adjacent elements or elements that failed in the vicinity.
6. The collapse progresses in a direction that is traverse to the principal forces present in the failing elements.

Some cases that exhibit zipper-type collapse are:

1. The failure of a continuous girder during bending leads to overloading and failure of adjacent spans.
2. In a continuous girder supported by two slender columns, when one column buckles the adjacent column is overloaded. This results in the buckling of the adjacent column.

3. Domino-Type Collapse

Domino-type collapse has a distinguishing feature from the above-mentioned types of collapses and that is the overturning of the individual elements, which tends to cause the next element to fail by a propagating action.

The important features of domino-type collapse are:

1. Domino-collapse starts with the overturning of one of the elements.
2. The element falls off in an angular rigid-body motion around a bottom edge.
3. In this collapse, the potential energy is converted to kinetic energy.
4. This collapse results in a lateral impact of the upper edge of the element on the side face of an adjacent element. This horizontal pushing force results in tilting and motion of the impacting element. Hence, this impact has a static and dynamic origin.
5. The horizontal loading from the impacting element results in the overturning of the adjacent element. This causes progressive collapse in the overturning direction.

The cascading collapse of towers of overhead transmission lines is one of the common examples of domino-type collapse.

4. Section-Type Collapse

Section-type collapse can be explained by considering an example of a beam in bending or a bar under axial tension. When this element undergoes a cut, the internal forces from this area are transmitted to the remaining cross-section. It is a type of fast fracture and is not a type of progressive collapse. But it is included to study its contributions, similarities, and analogies. In zipper-type collapse observed in cable nets, membrane structures, or cable bridges, the analogy between progressive collapse and section-type collapse is more applicable.

5. Instability-Type Collapse

Instability-type collapse are occurred due to small imperfections or transverse loading. Structures are normally designed to get rid of instability issues.

For example, the failure of a bracing element can trigger the whole system to collapse. These are more common in beam or truss structures where bracing systems are used. Another common example is the failure of a plate stiffener that leads to local instability and failure of the offset plate. This would result in a global collapse.

The important features of instability-type collapse are:

1. The initial failure of the elements initiates instability-type collapse. These are elements that stabilize the load-carrying elements in compression.
2. This initial failure is responsible for the instability of elements in compression. These small deformations cause sudden failure of these destabilized elements.
3. These failures finally account for failure in progression.

6. Mixed-Type Collapse

Mixed-type collapse is a combination of different types of collapses explained before. In structures like buildings, progressive collapse can occur by combining pancake-type, domino-type, zipper-type, and instability-type collapse. Here, the buckling of columns in a continuous frame structure due to overloading is a zipper-type collapse. As the failure progresses, it can result in destabilization of elements, which is a feature of instability-type collapse.

In general, zipper-type and section-type collapse can be categorized under redistribution class, while pancake-type and domino -type collapse transform potential energy into kinetic energy and come under the category of impact class. Mixed-type and instability-type collapse have their own features and do not show any features similar to the other types.

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