Quantifying the Risk for Progressive Collapse in New and Existing Buildings

What Is Progressive Collapse?

Although there is no single, uniform definition of progressive collapse in the structural engineering design community, the American Society of Civil Engineering (ASCE) standard ASCE 7-02 defines "progressive collapse" as "the spread of an initial local failure from element to element that eventually results in the collapse of an entire structure or a disproportionately large part of it." In less technical terms, the idea of progressive collapse is often thought of as the domino effect.

Currently, the trend in selected governmental agencies is to keep the analysis and design for progressive collapse analysis as non-threat specific. That is, the analysis is not performed for a blast of specific intensity, but rather the analysis determines the structural integrity of the structure after the removal of specified primary building elements. The idea is to provide enough redundancy and alternate load paths to enable the structure to remain viable after the removal of a primary structural element.

There are several fundamental issues to consider relative to progressive collapse.

• Structures are typically designed for probable loadings for efficiency and economy.
• Standard design codes do not account for abnormal loads or the possibility that the structure may require load redistribution.
• Structural responses to abnormal loads have exposed inadequacies in certain types of construction to redistribute loads to avoid catastrophic collapse.

While much of the current attention is focused on malicious explosions, it was actually an explosion in the Ronan Point apartment building in the United Kingdom that first focused attention on this subject. An accidental gas explosion on the 18th floor of this 24-story, pre-cast building resulted in the loss of an exterior bearing panel that in turn led to a loss of support for floors above causing collapse upward to the roof. The weight and dynamics of the collapsing floors above then led to a propagation of the collapse downward to the ground level.

Examples of abnormal loading conditions that may require additional measures for structural integrity include the following.

• Accidental impact. Automobile striking a key member in the structure
• Faulty construction practice. Premature removal of supporting forms
• Foundation failure. Erosion or cratering from an explosion
• Violent change in air pressure. Boiler failure, gas explosion, or bomb

The two figures below show a simplified representation of what happens to a horizontal structural concrete member (beam) when a primary vertical load-bearing member (column) is removed.

The top figure represents the behavior of the beam under typical vertical load conditions. The bottom figures represent the behavior of the same beam under the new support conditions (column is removed). In addition to the increased distance that the beam must now span, the steel reinforcements in the beam (shown as the thick, dark horizontal lines) are now in the "wrong" locations to properly resist the applied loads.

Design for Gravity Loads

Response to Column Loss

Current General Services Administration Guidelines

The U.S. General Services Administration (GSA), which is responsible for the design and administration of federal buildings in the United States, has published guidelines for the progressive collapse analysis and design of new federal office buildings, as well as existing federal buildings that are undergoing major modernization.

The GSA Progressive Collapse Guidelines, published in 2000, provide a method to quantitatively measure the ability of a building to retain structural integrity after the loss of a primary vertical structural load-bearing member. The Guidelines focus on the use of a stepwise elastic analysis to minimize the time and financial commitment relative to a more rigorous non-linear, threat-specific analysis. This analysis method, often referred to as the "missing column" or "alternate load path" approach, is also contained in a similar U.S. Department of Defense document related to antiterrorism/force protection construction standards.

Progressive collapse literature, including the GSA guidelines, recognizes that buildings located in high seismic regions that are designed with an increased level of structural ductility, should perform better than similar structures located in low seismic regions. While there are many similarities between seismic and progressive collapse analysis, there are important differences including the application of loads to the members of interest.

Progressive collapse analysis procedures will often require the use of a finite element building model such as the one shown below (left). After removal of the selected element, typically at the perimeter of the building, the remaining structure is analyzed to determine if enough residual capacity or alternate load paths exist to prevent the further propagation of the member failures (progressive collapse). Many structures respond similarly to the figure shown below (right) where the floors above the removed member will try to "hang" from the framing above.

Risk Mitigation

If progressive collapse is deemed to be an issue, there are several means to mitigate the risk. These include the following.

• Increased standoff (distance between defended perimeter and structure)
• Defended perimeter (bollards, planters and retaining walls)
• Structural measures (redundancy, alternate load paths, ductility)

Examples of some of the methods used to defend the perimeter are shown below (bollards and cable restraint in the photo on the left, planters in the photo on the right).

For existing buildings, structural remediation measures may take many different forms depending on the structural material (concrete or steel), building use, and historic designation (if any). A common remediation measure is to increase the capacity of existing connections as shown below.

Summary

Consideration of the potential for progressive collapse is becoming more prevalent in the analysis and design of new and existing public and private buildings. Progressive collapse analysis may take the form of a stepwise linear-elastic procedure to examine if a structure has sufficient redundancy or alternate load paths to successfully redistribute the applied loads.

If a building is determined to have a high potential for progressive collapse, mitigation and remediation measures can be used to significantly reduce the risk associated with progressive collapse.