Quantifying the Risk for Progressive Collapse in New and Existing Buildings
March 2003
The terrorism risk has increased the collapse
potential for new and existing buildings, but abnormal loading conditions that
occur without malicious intent have caused significant damage as well, resulting
in attention to the need for additional measures for structural integrity. The
U.S. General Services Administration has published guidelines (2002) for the
progressive collapse analysis and design of new federal office buildings and
those undergoing major modernization. Following these guidelines and implementing
other mitigation and remediation measures can significantly reduce the risk
associated with progressive collapse.
by Nathan
C. Gould, D.Sc., P.E., S.E.
ABS Consulting
With the 1995 bombing of the Murray Federal Building in Oklahoma City and
the terrorist attacks in New York, additional attention has been focused on
the ability of both new and existing structures to withstand significant structural
insult and still maintain a degree of structural integrity sufficient to allow
the occupants of the building to exit safely. Consideration of the potential
for progressive collapse is often associated with analysis and design of important
federal and state buildings; however, given the current security climate, private
companies have recently been focusing more attention on this aspect of structural
design for their critical facilities.
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.
Figure
1
Figure
2
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.
Figure
3
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).
Figure
4
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.
Figure
5
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.
Opinions expressed in Expert Commentary articles are those of the author and are
not necessarily held by the author’s employer or IRMI. This article does not purport
to provide legal, accounting, or other professional advice or opinion. If such advice
is needed, consult with your attorney, accountant, or other qualified adviser.