Progressive collapse - Revision history

Jan: Add 4 watermarked SVG illustrations from the Expert visual plan

2026-11-02T16:29:36Z

Add 4 watermarked SVG illustrations from the Expert visual plan

← Older revision		Revision as of 17:29, 2 November 2026
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	== How local damage spreads ==		== How local damage spreads ==
	A progressive collapse starts with an initial local failure, for example of a column, a girder or a connection.<ref name="N1-206" /> The loads that this element carried must then move to the elements around it. The United States National Institute of Standards and Technology (NIST) therefore recommends that primary structural systems provide alternate paths for loads in case components fail.<ref name="N1-206" /> This matters most in buildings where a single column or girder supports an unusually large floor area.<ref name="N1-206" /> When the neighbouring elements cannot carry the extra load, they fail in turn and the damage spreads.<ref name="N1-206" />		A progressive collapse starts with an initial local failure, for example of a column, a girder or a connection.<ref name="N1-206" /> The loads that this element carried must then move to the elements around it. The United States National Institute of Standards and Technology (NIST) therefore recommends that primary structural systems provide alternate paths for loads in case components fail.<ref name="N1-206" /> This matters most in buildings where a single column or girder supports an unusually large floor area.<ref name="N1-206" /> When the neighbouring elements cannot carry the extra load, they fail in turn and the damage spreads.<ref name="N1-206" />
			[[File:fse-progressive-collapse-v1.svg\|thumb\|upright=1.5\|alt=Two elevations of the same six-storey frame with one ground-floor column removed: left, the floors above bridge the gap; right, failure spreads up and sideways.\|The same frame after the loss of one column. Left: the loads find an alternate path and the damage stays local. Right: the failure spreads from element to element. Based on National Institute of Standards and Technology (2005), p. 206, and Australian Building Codes Board (2020b), p. 91.]]

	The investigation of World Trade Center Building 7 (WTC 7) identified three ways in which failure travelled from one column to the next. The first was the loss of lateral support after floor failures. The second and third were the forces exerted by falling debris and the loads shed by columns that had buckled before them.<ref name="N1A-xxxvi" /> Failure can travel vertically, from floor to floor, and horizontally, from one column line to the next.<ref name="MAC-121">MacNeill et al. (2008), p. 121.</ref> Floors also brace the columns sideways. In WTC 7, the collapse of the floors around one interior column left it without lateral support over nine storeys, and it buckled.<ref name="N1A-54">National Institute of Standards and Technology (2008), p. 54.</ref>		The investigation of World Trade Center Building 7 (WTC 7) identified three ways in which failure travelled from one column to the next. The first was the loss of lateral support after floor failures. The second and third were the forces exerted by falling debris and the loads shed by columns that had buckled before them.<ref name="N1A-xxxvi" /> Failure can travel vertically, from floor to floor, and horizontally, from one column line to the next.<ref name="MAC-121">MacNeill et al. (2008), p. 121.</ref> Floors also brace the columns sideways. In WTC 7, the collapse of the floors around one interior column left it without lateral support over nine storeys, and it buckled.<ref name="N1A-54">National Institute of Standards and Technology (2008), p. 54.</ref>
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	For the fixed-ended beam, the elastic moment at the supports is <math>\omega L^2/12</math>, so the steel first yields at <math>\omega_y = 12 M_y/L^2</math>.<ref name="FITZ-139" /> With the same factor of 1.5, the design load becomes <math>8 M_y/L^2</math>, while the collapse load is <math>16 \times 1.14\, M_y/L^2 = 18.24\, M_y/L^2</math>. The fixed-ended beam therefore collapses at 2.28 times its design load instead of 1.71 times. The difference is the reserve that moment redistribution provides. NIST lists continuity, together with the strength and ductility of connections, as one of the conventional approaches to preventing progressive collapse.<ref name="N1-206" />		For the fixed-ended beam, the elastic moment at the supports is <math>\omega L^2/12</math>, so the steel first yields at <math>\omega_y = 12 M_y/L^2</math>.<ref name="FITZ-139" /> With the same factor of 1.5, the design load becomes <math>8 M_y/L^2</math>, while the collapse load is <math>16 \times 1.14\, M_y/L^2 = 18.24\, M_y/L^2</math>. The fixed-ended beam therefore collapses at 2.28 times its design load instead of 1.71 times. The difference is the reserve that moment redistribution provides. NIST lists continuity, together with the strength and ductility of connections, as one of the conventional approaches to preventing progressive collapse.<ref name="N1-206" />
			[[File:fse-progressive-collapse-v2.svg\|thumb\|upright=1.5\|alt=Bar chart of design, first-yield and collapse loads for a simply supported and a fixed-ended steel beam; the fixed beam collapses at 2.28 times its design load.\|Design load, first-yield load and plastic collapse load of the same steel wide-flange beam with simply supported and with fixed ends, in multiples of M<sub>y</sub>/L². Based on Fitzgerald (2002), pp. 1-139 to 1-141.]]

	== Fire as the initiating event ==		== Fire as the initiating event ==
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	The probable collapse sequence began on the east side, where floor beams spanned about 15 m.<ref name="N1A-53">National Institute of Standards and Technology (2008), p. 53.</ref> Heating made these long beams expand more than the other parts of the floor system, and their simple shear connections could not resist the resulting axial forces.<ref name="N1A-53" /> On Floor 13, the expanding beams pushed the girder between Columns 79 and 44 to the west until it walked off its support at Column 79.<ref name="N1A-22">National Institute of Standards and Technology (2008), p. 22.</ref> Floor 13 then collapsed and started a cascade of floor failures down to Floor 5, which was much thicker and stronger.<ref name="N1A-22" /> Column 79, left without lateral support over nine storeys, buckled.<ref name="N1A-54" />		The probable collapse sequence began on the east side, where floor beams spanned about 15 m.<ref name="N1A-53">National Institute of Standards and Technology (2008), p. 53.</ref> Heating made these long beams expand more than the other parts of the floor system, and their simple shear connections could not resist the resulting axial forces.<ref name="N1A-53" /> On Floor 13, the expanding beams pushed the girder between Columns 79 and 44 to the west until it walked off its support at Column 79.<ref name="N1A-22">National Institute of Standards and Technology (2008), p. 22.</ref> Floor 13 then collapsed and started a cascade of floor failures down to Floor 5, which was much thicker and stronger.<ref name="N1A-22" /> Column 79, left without lateral support over nine storeys, buckled.<ref name="N1A-54" />
			[[File:fse-progressive-collapse-v3.svg\|thumb\|upright=1.5\|alt=Plan of a WTC 7 floor corner where expanding beams push a girder off Column 79, and an elevation of Column 79 buckling after the floors fell.\|Collapse initiation in WTC 7. Left: on Floor 13 the heated east floor beams expand and push the girder between Columns 79 and 44 off its support. Right: falling floors leave Column 79 unbraced over nine storeys and it buckles. Based on National Institute of Standards and Technology (2008), pp. 6, 22 and 53–54.]]

	The failure then spread in both directions. Floors failed up to the roof on the east side, and Columns 80 and 81 buckled as well.<ref name="N1A-54" /> These three columns were the only interior supports for gravity loads in the eastern part of the building.<ref name="N1A-54" /> A progression of interior column buckling followed from east to west, after which the exterior columns buckled and the building above moved down as a single unit.<ref name="N1A-xxxvi" /> Above Floor 7, the floor framing was the only load path between the interior columns, and its shear connections were designed to transfer vertical shear only.<ref name="N1A-58">National Institute of Standards and Technology (2008), p. 58.</ref> No bracing or other mechanism could therefore move gravity loads from one interior column to another.<ref name="N1A-58" />		The failure then spread in both directions. Floors failed up to the roof on the east side, and Columns 80 and 81 buckled as well.<ref name="N1A-54" /> These three columns were the only interior supports for gravity loads in the eastern part of the building.<ref name="N1A-54" /> A progression of interior column buckling followed from east to west, after which the exterior columns buckled and the building above moved down as a single unit.<ref name="N1A-xxxvi" /> Above Floor 7, the floor framing was the only load path between the interior columns, and its shear connections were designed to transfer vertical shear only.<ref name="N1A-58">National Institute of Standards and Technology (2008), p. 58.</ref> No bracing or other mechanism could therefore move gravity loads from one interior column to another.<ref name="N1A-58" />
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	The Australian handbook treats collapse in fire as a question of probability. A fire can be stopped at several stages: by the occupants, by automatic sprinklers, by the fire brigade before flashover, or by the fire resistance of the structure.<ref name="ABCB-92">Australian Building Codes Board (2020b), p. 92.</ref> For scenario UF, the analysis only needs the probability that the whole or a major part of the structure collapses.<ref name="ABCB-92" /> The time to structural failure then has a distribution with two peaks, because serious installation errors, such as substituted fire protection boards, add an early peak.<ref name="ABCB-93">Australian Building Codes Board (2020b), p. 93.</ref> For buildings designed to Verification Method BV2, more than one element will probably have to fail before a catastrophic collapse results.<ref name="ABCB-93" />		The Australian handbook treats collapse in fire as a question of probability. A fire can be stopped at several stages: by the occupants, by automatic sprinklers, by the fire brigade before flashover, or by the fire resistance of the structure.<ref name="ABCB-92">Australian Building Codes Board (2020b), p. 92.</ref> For scenario UF, the analysis only needs the probability that the whole or a major part of the structure collapses.<ref name="ABCB-92" /> The time to structural failure then has a distribution with two peaks, because serious installation errors, such as substituted fire protection boards, add an early peak.<ref name="ABCB-93">Australian Building Codes Board (2020b), p. 93.</ref> For buildings designed to Verification Method BV2, more than one element will probably have to fail before a catastrophic collapse results.<ref name="ABCB-93" />
			[[File:fse-progressive-collapse-v4.svg\|thumb\|upright=1.5\|alt=Flowchart from fire ignition through five barriers, each with a yes exit to a consequence from minimal to severe and a final no exit to catastrophic collapse.\|The progression from ignition to catastrophic collapse: each barrier that stops the fire limits the consequences. Based on Australian Building Codes Board (2020b), p. 92, Figure 9.4.]]

	Both the Australian handbook and the International Fire Engineering Guidelines stress that fire engineers need structural engineers for this work.<ref name="ABCB-93" /><ref name="IFEG-1.6-8">National Research Council of Canada et al. (2005), p. 1.6-8.</ref> The guidelines note that barriers supported by a failing structural element may fail with it, so the consequences of that failure must be followed through the building.<ref name="IFEG-1.6-8" /> Van Rossum likewise lists structural behaviour as one of the knowledge areas that fire safety engineering draws on.<ref name="VR-1.3">Van Rossum (2027), §1.3.</ref> Every analysis also carries uncertainty. NIST stresses that some facts about WTC 7 could not be established, because the building and the records kept in it were destroyed.<ref name="N1A-xxxv" />		Both the Australian handbook and the International Fire Engineering Guidelines stress that fire engineers need structural engineers for this work.<ref name="ABCB-93" /><ref name="IFEG-1.6-8">National Research Council of Canada et al. (2005), p. 1.6-8.</ref> The guidelines note that barriers supported by a failing structural element may fail with it, so the consequences of that failure must be followed through the building.<ref name="IFEG-1.6-8" /> Van Rossum likewise lists structural behaviour as one of the knowledge areas that fire safety engineering draws on.<ref name="VR-1.3">Van Rossum (2027), §1.3.</ref> Every analysis also carries uncertainty. NIST stresses that some facts about WTC 7 could not be established, because the building and the records kept in it were destroyed.<ref name="N1A-xxxv" />

Mark: FSE wiki: add the conditions of the C/VM2 30-minute option (written by Claude Opus 5.5)

2026-11-01T22:33:40Z

FSE wiki: add the conditions of the C/VM2 30-minute option (written by Claude Opus 5.5)

← Older revision		Revision as of 23:33, 1 November 2026
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	The same handbook defines a fire design scenario for unexpected catastrophic failure, called scenario UF. Its intent is to show that the risk of disproportionate collapse due to fire is no greater than for a reference building that follows the NCC Deemed-to-Satisfy Provisions.<ref name="ABCB-90">Australian Building Codes Board (2020b), p. 90.</ref> The scenario generally applies only to buildings of more than three storeys.<ref name="ABCB-91" /> For buildings with an effective height above 60 m, more extensive structural analysis may be appropriate.<ref name="ABCB-94" /> Its purpose is to address the potential increase in [[societal risk]] that a catastrophic collapse would bring.<ref name="ABCB-94" />		The same handbook defines a fire design scenario for unexpected catastrophic failure, called scenario UF. Its intent is to show that the risk of disproportionate collapse due to fire is no greater than for a reference building that follows the NCC Deemed-to-Satisfy Provisions.<ref name="ABCB-90">Australian Building Codes Board (2020b), p. 90.</ref> The scenario generally applies only to buildings of more than three storeys.<ref name="ABCB-91" /> For buildings with an effective height above 60 m, more extensive structural analysis may be appropriate.<ref name="ABCB-94" /> Its purpose is to address the potential increase in [[societal risk]] that a catastrophic collapse would bring.<ref name="ABCB-94" />

	In New Zealand, the C/VM2 commentary (2013) reproduces clause C6.4 of the Building Code.<ref name="MBIE13-36" /> The clause states that the collapse of elements with a lower fire resistance must not cause the consequential collapse of elements that need a higher one.<ref name="MBIE13-36" /> Verification Method C/VM2 (second edition, 2025) requires that, in buildings with an escape height above 10 m, the load-carrying structure and floor systems resist collapse until burnout.<ref name="CVM2-55">Ministry of Business, Innovation and Employment (2025b), p. 55.</ref> ~~Where~~ an intermediate floor may collapse ~~after at least~~ 30 minutes, its collapse must not cause the collapse of any part of the structure that ~~has to~~ resist burnout.<ref name="CVM2-55" /> In ~~lower~~ buildings, floors and their supports must resist collapse for at least 30 minutes, to avoid an unexpected or sudden collapse that would endanger firefighters inside.<ref name="CVM2-55" />		In New Zealand, the C/VM2 commentary (2013) reproduces clause C6.4 of the Building Code.<ref name="MBIE13-36" /> The clause states that the collapse of elements with a lower fire resistance must not cause the consequential collapse of elements that need a higher one.<ref name="MBIE13-36" /> Verification Method C/VM2 (second edition, 2025) requires that, in buildings with an escape height above 10 m, the load-carrying structure and floor systems resist collapse until burnout.<ref name="CVM2-55">Ministry of Business, Innovation and Employment (2025b), p. 55.</ref>

			Intermediate floors in such buildings must also resist collapse until burnout.<ref name="CVM2-55" /> The exception is an intermediate floor with an occupant load of no more than 100 people and an escape height of no more than 4.0 m.<ref name="CVM2-55" /> The area below such a floor must also be open to the firecell.<ref name="CVM2-55" /> The floor may then be designed to resist collapse for not less than 30 minutes, but its collapse must not cause the collapse of any other part of the structure that must resist burnout.<ref name="CVM2-55" /> In buildings with an escape height of 10 m or less, floors and their supports must resist collapse for at least 30 minutes, to avoid an unexpected or sudden collapse that would endanger firefighters inside.<ref name="CVM2-55" />

	In the United States, NIST reported in 2005 that New York City had adopted a rule on progressive collapse in 1973.<ref name="N1-195" /> The rule covered extreme local loads, but only in buildings with precast concrete wall panels.<ref name="N1-195" /> Federal agencies and the United Kingdom had developed guidelines since the towers were built.<ref name="N1-195" /> In 2008, NIST observed that the model building codes still did not require buildings to be designed to resist progressive collapse.<ref name="N1A-60" /> In the Netherlands, the fire safety rules of the Besluit bouwwerken leefomgeving (Bbl) aim to prevent casualties and the spread of fire to neighbouring plots.<ref name="VR-1.2">Van Rossum (2027), §1.2.</ref> Preserving the building itself is not one of those aims, but keeping the load-bearing structure standing is one of the sub-goals through which the Bbl pursues them.<ref name="VR-1.2" />		In the United States, NIST reported in 2005 that New York City had adopted a rule on progressive collapse in 1973.<ref name="N1-195" /> The rule covered extreme local loads, but only in buildings with precast concrete wall panels.<ref name="N1-195" /> Federal agencies and the United Kingdom had developed guidelines since the towers were built.<ref name="N1-195" /> In 2008, NIST observed that the model building codes still did not require buildings to be designed to resist progressive collapse.<ref name="N1A-60" /> In the Netherlands, the fire safety rules of the Besluit bouwwerken leefomgeving (Bbl) aim to prevent casualties and the spread of fire to neighbouring plots.<ref name="VR-1.2">Van Rossum (2027), §1.2.</ref> Preserving the building itself is not one of those aims, but keeping the load-bearing structure standing is one of the sub-goals through which the Bbl pursues them.<ref name="VR-1.2" />

Mark: FSE wiki: align citations and references with the canonical reference list (written by Claude Opus 5.5)

2026-11-01T22:10:19Z

FSE wiki: align citations and references with the canonical reference list (written by Claude Opus 5.5)

Show changes

Mark: New FSE wiki article: Progressive collapse (written by Claude Opus 5.5)

2026-11-01T22:00:39Z

New FSE wiki article: Progressive collapse (written by Claude Opus 5.5)

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'''Progressive collapse''', also called disproportionate collapse, is the spread of local damage from element to element, starting from an initiating event, until an entire structure or a disproportionately large part of it collapses.<ref name="N1A-xxxvi">National Institute of Standards and Technology (2008), p. xxxvi.</ref> The initiating event can be the removal of a column by an impact or an explosion, and a fire can play the same role.<ref name="N1-206">National Institute of Standards and Technology (2005), p. 206.</ref> Fire is a special case because it heats many members at the same time. Heated members expand, sag and weaken, so that they push or pull on the members around them.<ref name="N1A-53">National Institute of Standards and Technology (2008), p. 53.</ref><ref name="N1-144">National Institute of Standards and Technology (2005), p. 144.</ref>

The collapse of World Trade Center Building 7 in New York on 11 September 2001 was the first known total collapse of a tall building caused mainly by fire.<ref name="N1A-xxxv">National Institute of Standards and Technology (2008), p. xxxv.</ref> Its probable cause was a chain of local failures that began with the [[restrained thermal expansion]] of long floor beams.<ref name="N1A-xxxvi" /> Engineers limit the risk with continuous and ductile connections, with local strengthening of key elements, and with alternate load paths around elements that may fail.<ref name="N1-206" /> In Australia and New Zealand, the building codes require that local damage does not lead to a disproportionate or consequential collapse.<ref name="ABCB-91">Australian Building Codes Board (2020), p. 91.</ref><ref name="MBIE13-36">Ministry of Business, Innovation and Employment (2013), p. 36.</ref>

== How local damage spreads ==
A progressive collapse starts with an initial local failure, for example of a column, a girder or a connection.<ref name="N1-206" /> The loads that this element carried must then move to the elements around it. The United States National Institute of Standards and Technology (NIST) therefore recommends that primary structural systems provide alternate paths for loads in case components fail.<ref name="N1-206" /> This matters most in buildings where a single column or girder supports an unusually large floor area.<ref name="N1-206" /> When the neighbouring elements cannot carry the extra load, they fail in turn and the damage spreads.<ref name="N1-206" />

The investigation of World Trade Center Building 7 (WTC 7) identified three ways in which failure travelled from one column to the next. The first was the loss of lateral support after floor failures. The second and third were the forces exerted by falling debris and the loads shed by columns that had buckled before them.<ref name="N1A-xxxvi" /> Failure can travel vertically, from floor to floor, and horizontally, from one column line to the next.<ref name="MAC-121">MacNeill et al. (2008), p. 121.</ref> Floors also brace the columns sideways. In WTC 7, the collapse of the floors around one interior column left it without lateral support over nine storeys, and it buckled.<ref name="N1A-54">National Institute of Standards and Technology (2008), p. 54.</ref>

Once a large part of a building starts to move downward, the loads become dynamic. In the twin towers of the World Trade Center, the falling upper section released a large amount of potential energy. This energy far exceeded the capacity of the intact structure below to absorb it through deformation.<ref name="N1-146">National Institute of Standards and Technology (2005), p. 146.</ref> As each storey below failed, the falling mass increased and the demand on the next storey grew.<ref name="N1-146" /> The storeys below the level where collapse began therefore offered minimal resistance.<ref name="N1-146" />

== Redundancy and ductility ==
Structures differ in their ability to carry loads around a damaged part. Fitzgerald explains that statically indeterminate structures, such as continuous beams and rigid frames, are inherently stronger than statically determinate ones.<ref name="FITZ-139">Fitzgerald (2002), p. 1-139.</ref> The extra capacity comes from the redistribution of bending moments from highly stressed sections to sections that still have reserve.<ref name="FITZ-139" /> When a steel section has yielded completely, it has reached its fully plastic moment. The moment there cannot increase any further, so any additional load must be carried by other parts of the member.<ref name="FITZ-140">Fitzgerald (2002), p. 1-140.</ref> A statically determinate structure collapses when the first such plastic hinge forms, whereas an indeterminate structure needs two or three hinges before it becomes a mechanism.<ref name="FITZ-141">Fitzgerald (2002), p. 1-141.</ref>

Ductility is the second ingredient. A ductile structure deforms considerably before it collapses, and this deformation warns occupants of the danger.<ref name="FITZ-141" /> Brittle failure and elastic instability give little warning, so structural engineers try to build as much ductility into their designs as possible.<ref name="FITZ-141" /> The Australian handbook on the Fire Safety Verification Method makes the same point for fire design. It asks that ductile failure modes are provided, so that premature brittle failures such as shear failure do not occur.<ref name="ABCB-94">Australian Building Codes Board (2020), p. 94.</ref>

=== Worked example: the reserve from continuity ===
The value of continuity can be shown with the plastic collapse loads that Fitzgerald derives for a uniformly loaded steel beam. The cross-section and the span stay the same, and only the supports change. For a simply supported beam, a single hinge at midspan is enough for collapse, which occurs at the load<ref name="FITZ-141" />
<math display="block">\omega_u = \frac{8 M_p}{L^2}</math>
When both ends of the same beam are fixed, collapse needs three hinges, two at the supports and one at midspan. After the hinges at the supports have formed, the moments redistribute towards midspan until a third hinge forms there.<ref name="FITZ-140" /> The collapse load is then twice as high:<ref name="FITZ-140" /><ref name="FITZ-141" />
<math display="block">\omega_u = \frac{16 M_p}{L^2}</math>

{| class="wikitable"
! Symbol !! Description !! Unit
|-
| <math>\omega_u</math> || uniformly distributed load at which the beam collapses || kN/m
|-
| <math>\omega_y</math> || uniformly distributed load at which the steel first yields || kN/m
|-
| <math>M_p</math> || fully plastic moment of the cross-section || kN·m
|-
| <math>M_y</math> || moment at which the extreme fibres first yield || kN·m
|-
| <math>L</math> || span of the beam || m
|}

Fitzgerald gives two values for steel wide-flange beams. The ratio of fully plastic moment to yield moment, called the shape factor, averages 1.14.<ref name="FITZ-139" /> The design load usually has a factor of safety of 1.5 over the load that causes first yield.<ref name="FITZ-139" /> For the simply supported beam, the collapse load is then 1.50 × 1.14 = 1.71 times the design load.<ref name="FITZ-139" />

For the fixed-ended beam, the elastic moment at the supports is <math>\omega L^2/12</math>, so the steel first yields at <math>\omega_y = 12 M_y/L^2</math>.<ref name="FITZ-139" /> With the same factor of 1.5, the design load becomes <math>8 M_y/L^2</math>, while the collapse load is <math>16 \times 1.14\, M_y/L^2 = 18.24\, M_y/L^2</math>. The fixed-ended beam therefore collapses at 2.28 times its design load instead of 1.71 times. The difference is the reserve that moment redistribution provides. NIST lists continuity, together with the strength and ductility of connections, as one of the conventional approaches to preventing progressive collapse.<ref name="N1-206" />

== Fire as the initiating event ==
Fire differs from most initiating events because it can affect a whole floor area and many members at once. Traditionally, the response of a structure to fire was analysed one member at a time.<ref name="MILKE-235">Milke (2002), p. 4-235.</ref> In a frame with many members, however, load transfer or [[Tensile membrane action|membrane action]] can let a steel member keep its integrity at temperatures above those usually associated with failure.<ref name="MILKE-235" /> Load transfer lets stronger members carry loads that heated, weakened members can no longer carry, and capturing it requires an analysis of the whole frame.<ref name="MILKE-235" />

The same interaction between members can also trigger failure. When [[steel in fire]] heats up, it expands, and if columns or other members resist this expansion, forces develop that can buckle beams or break connections.<ref name="N1A-21">National Institute of Standards and Technology (2008), p. 21.</ref> In WTC 7, this happened at temperatures at or below about 400 °C, well below those used to set fire resistance ratings.<ref name="N1A-21" /> One criterion of the standard [[fire resistance test]] ASTM E119 is the time at which the average temperature of a steel column exceeds 538 °C, or that of a floor beam exceeds 593 °C.<ref name="N1A-21" /> NIST concluded that this test does not capture the behaviour of structural systems, such as the effect of expanding or sagging floor beams on girders, connections and columns.<ref name="N1A-59">National Institute of Standards and Technology (2008), p. 59.</ref>

Fitzgerald makes a related point about design based on standard tests. Ratings from such tests ignore the extra strength of continuous construction.<ref name="FITZ-142">Fitzgerald (2002), p. 1-142.</ref> A beam then receives the same rating whether it is simply supported or continuous, although the two may behave quite differently in a real fire.<ref name="FITZ-142" /> A rational design for fire instead uses the properties of materials at high temperature, natural fire curves and the heat transfer through insulation.<ref name="FITZ-142" />

Prescriptive rules are written mainly to protect lives. NIST notes that the intent of current practice based on prescriptive codes is life safety, not the prevention of collapse.<ref name="N1A-xxxviii">National Institute of Standards and Technology (2008), p. xxxviii.</ref> Its key premise is that buildings should not collapse in infrequent, worst-case fires.<ref name="N1A-xxxviii" /> In such fires, sprinklers are absent, out of service or overwhelmed.<ref name="N1A-xxxviii" /> Ordinary combustibles burn and spread from item to item, broken windows supply more air, and fires burn on several floors at once.<ref name="N1A-xxxviii" />

== The collapse of WTC 7 ==
WTC 7 was a 47-storey office building just north of the main World Trade Center complex.<ref name="N1A-xxxv" /> It was a [[high-rise building]] in the sense used by fire protection engineers: nominally taller than 25 m, so that external rescue from fires is not possible.<ref name="N1A-xxxv" /> Debris from the collapse of the north tower at 10:28 a.m. damaged the structure and ignited fires on at least ten floors.<ref name="N1A-xxxv" /><ref name="N1A-xxxvi" /> The fires on Floors 7 to 9 and 11 to 13 grew and spread without any suppression.<ref name="N1A-21">National Institute of Standards and Technology (2008), p. 21.</ref> The sprinklers on those floors depended on the city water main, which the collapsing towers had damaged.<ref name="N1A-xxxvii">National Institute of Standards and Technology (2008), p. xxxvii.</ref> The building collapsed at 5:20:52 p.m., 6 hours and 52 minutes after the north tower fell.<ref name="N1A-xxxv" />

The probable collapse sequence began on the east side, where floor beams spanned about 15 m.<ref name="N1A-53">National Institute of Standards and Technology (2008), p. 53.</ref> Heating made these long beams expand more than the other parts of the floor system, and their simple shear connections could not resist the resulting axial forces.<ref name="N1A-53" /> On Floor 13, the expanding beams pushed the girder between Columns 79 and 44 to the west until it walked off its support at Column 79.<ref name="N1A-22">National Institute of Standards and Technology (2008), p. 22.</ref> Floor 13 then collapsed and started a cascade of floor failures down to Floor 5, which was much thicker and stronger.<ref name="N1A-22" /> Column 79, left without lateral support over nine storeys, buckled.<ref name="N1A-54" />

The failure then spread in both directions. Floors failed up to the roof on the east side, and Columns 80 and 81 buckled as well.<ref name="N1A-54" /> These three columns were the only interior supports for gravity loads in the eastern part of the building.<ref name="N1A-54" /> A progression of interior column buckling followed from east to west, after which the exterior columns buckled and the building above moved down as a single unit.<ref name="N1A-xxxvi" /> Above Floor 7, the floor framing was the only load path between the interior columns, and its shear connections were designed to transfer vertical shear only.<ref name="N1A-58">National Institute of Standards and Technology (2008), p. 58.</ref> No bracing or other mechanism could therefore move gravity loads from one interior column to another.<ref name="N1A-58" />

Two findings show that fire acted as the trigger rather than as the direct cause. First, none of the critical columns was significantly weakened by heat, because their temperatures did not exceed 300 °C.<ref name="N1A-54" /> Second, an analysis for NIST removed a section of Column 79 between Floors 11 and 13 without any fire or debris damage.<ref name="MAC-121" /> The whole building still collapsed.<ref name="MAC-121" /> NIST counted a structural system that was not designed to prevent fire-induced progressive collapse among the factors behind the failure.<ref name="N1A-xxxvi" />

== The collapse of the World Trade Center towers ==
The twin towers show how load redistribution can delay a collapse but not always prevent it. In the north tower, the hat truss at the top of the building moved the loads of the columns severed by the aircraft impact to other columns.<ref name="N1-144" /> As the fires heated the floors, they sagged and pulled inward on the exterior walls, until the south wall bowed inward.<ref name="N1-144" />

Column splices and spandrels in that wall then failed, and the instability spread across the whole face.<ref name="N1-145">National Institute of Standards and Technology (2005), p. 145.</ref> The loads from this wall moved to the heat-weakened core through the hat truss and to the side walls through the spandrels.<ref name="N1-145" /> When the remaining columns could no longer carry the gravity loads, the upper section began to move downward.<ref name="N1-145" /> The weakened structure in the impact and fire zone could not absorb the energy of the falling section, and global collapse followed.<ref name="N1-145" />

NIST also found that the towers did have the capacity to redistribute loads from damaged components to undamaged ones.<ref name="N1-206" /> Their core columns, however, lacked sufficient redundant paths for carrying gravity loads.<ref name="N1-206" /> At the time the towers were designed, building codes contained no explicit structural integrity provisions to limit progressive collapse.<ref name="N1-195">National Institute of Standards and Technology (2005), p. 195.</ref>

== Design approaches ==
NIST describes three conventional approaches to preventing progressive collapse.<ref name="N1-206" /> Indirect design relies on continuity and on the strength and ductility of connections. Direct design means the local hardening of key elements. The third approach provides redundant, or alternate, load paths around any element that may fail.<ref name="N1-206" />

In design, the initiating event may be the removal of one or two columns at the bottom of the structure.<ref name="N1-206" /> NIST considers this a very limited treatment. It recommends that initiating events at several locations, and events that affect other key components, be analysed in proportion to the risks.<ref name="N1-206" /> For fire, it adds a new recommendation to evaluate the fire resistance of the structural system as a whole under worst-case design fires, with active fire protection assumed to fail.<ref name="N1A-xxxviii" /> Features that need particular attention are long-span floors of 12 to 15 m, connections not designed for thermal effects, asymmetric floor framing and composite floors.<ref name="N1A-xxxviii" />

For WTC 7, NIST named capabilities that could have prevented the collapse, had they been in place. They include connections and framing designed to resist thermal expansion and better insulation to keep the steel cooler.<ref name="N1A-60">National Institute of Standards and Technology (2008), p. 60.</ref> Structural systems expressly designed to prevent progressive collapse are on the same list.<ref name="N1A-60" /> It also named [[sprinkler system]]s with independent and reliable water supplies and better [[fire compartmentation]] in tenant areas.<ref name="N1A-60" /> The Australian handbook lists a similar set of measures, from more reliable sprinklers and automatic alarm to the fire brigade to higher fire resistance levels and greater structural redundancy.<ref name="ABCB-95">Australian Building Codes Board (2020), p. 95.</ref>

== Requirements in building regulations ==
Explicit requirements differ between countries, and many are recent. In Australia, the ABCB handbook (2020) reproduces Performance Requirement BP1.1(a)(iii) of the National Construction Code (NCC).<ref name="ABCB-91" /> It requires that a building can sustain local damage while the structural system as a whole remains stable.<ref name="ABCB-91" /> Verification Method BV2 checks this by the notional removal of any supporting column, any beam that supports columns, or any segment of a load-bearing wall as long as the wall is high.<ref name="ABCB-91" /> The building must remain stable, and the collapse must not extend beyond the storeys immediately adjacent to the removed element.<ref name="ABCB-91" /> If one component carries more than 25 % of the total structure, a systematic risk assessment is required.<ref name="ABCB-91" />

The same handbook defines a fire design scenario for unexpected catastrophic failure, called scenario UF. Its intent is to show that the risk of disproportionate collapse due to fire is no greater than for a reference building that follows the NCC Deemed-to-Satisfy Provisions.<ref name="ABCB-90">Australian Building Codes Board (2020), p. 90.</ref> The scenario generally applies only to buildings of more than three storeys.<ref name="ABCB-91" /> For buildings with an effective height above 60 m, more extensive structural analysis may be appropriate.<ref name="ABCB-94" /> Its purpose is to address the potential increase in [[societal risk]] that a catastrophic collapse would bring.<ref name="ABCB-94" />

In New Zealand, the C/VM2 commentary (2013) reproduces clause C6.4 of the Building Code.<ref name="MBIE13-36" /> The clause states that the collapse of elements with a lower fire resistance must not cause the consequential collapse of elements that need a higher one.<ref name="MBIE13-36" /> Verification Method C/VM2 (second edition, 2025) requires that, in buildings with an escape height above 10 m, the load-carrying structure and floor systems resist collapse until burnout.<ref name="CVM2-55">Ministry of Business, Innovation and Employment (2025), p. 55.</ref> Where an intermediate floor may collapse after at least 30 minutes, its collapse must not cause the collapse of any part of the structure that has to resist burnout.<ref name="CVM2-55" /> In lower buildings, floors and their supports must resist collapse for at least 30 minutes, to avoid an unexpected or sudden collapse that would endanger firefighters inside.<ref name="CVM2-55" />

In the United States, NIST reported in 2005 that New York City had adopted a rule on progressive collapse in 1973.<ref name="N1-195" /> The rule covered extreme local loads, but only in buildings with precast concrete wall panels.<ref name="N1-195" /> Federal agencies and the United Kingdom had developed guidelines since the towers were built.<ref name="N1-195" /> In 2008, NIST observed that the model building codes still did not require buildings to be designed to resist progressive collapse.<ref name="N1A-60" /> In the Netherlands, the fire safety rules of the Besluit bouwwerken leefomgeving (Bbl) aim to prevent casualties and the spread of fire to neighbouring plots.<ref name="VR-1.2">Van Rossum (2027), §1.2.</ref> Preserving the building itself is not one of those aims, but keeping the load-bearing structure standing is one of the sub-goals through which the Bbl pursues them.<ref name="VR-1.2" />

== Analysis and uncertainty ==
Predicting a progressive collapse in fire is a task of [[structural fire engineering]] that requires a model to follow failures as they happen. For WTC 7, the analysts used a 16-storey model in the program ANSYS to calculate fire-induced damage, and a model of the whole building in LS-DYNA to follow the collapse.<ref name="MAC-xxxv">MacNeill et al. (2008), p. xxxv.</ref> LS-DYNA was used because it can model sequential failures, falling debris and debris impact, together with thermal softening and thermal expansion of the materials.<ref name="MAC-xxxv" /> The model was a nonlinear dynamic [[finite element analysis]], built mainly from shell and beam elements.<ref name="MAC-xxxv" /> In 2005, NIST noted that a validated capability to assess the reserve capacity of whole structures did not yet exist.<ref name="N1-206" /> It recommended developing one for routine design practice, covering multiple hazards including fire.<ref name="N1-206" />

The Australian handbook treats collapse in fire as a question of probability. A fire can be stopped at several stages: by the occupants, by automatic sprinklers, by the fire brigade before flashover, or by the fire resistance of the structure.<ref name="ABCB-92">Australian Building Codes Board (2020), p. 92.</ref> For scenario UF, the analysis only needs the probability that the whole or a major part of the structure collapses.<ref name="ABCB-92" /> The time to structural failure then has a distribution with two peaks, because serious installation errors, such as substituted fire protection boards, add an early peak.<ref name="ABCB-93">Australian Building Codes Board (2020), p. 93.</ref> For buildings designed to Verification Method BV2, more than one element will probably have to fail before a catastrophic collapse results.<ref name="ABCB-93" />

Both the Australian handbook and the International Fire Engineering Guidelines stress that fire engineers need structural engineers for this work.<ref name="ABCB-93" /><ref name="IFEG-1.6-8">Australian Building Codes Board (2005), p. 1.6-8.</ref> The guidelines note that barriers supported by a failing structural element may fail with it, so the consequences of that failure must be followed through the building.<ref name="IFEG-1.6-8" /> Van Rossum likewise lists structural behaviour as one of the knowledge areas that fire safety engineering draws on.<ref name="VR-1.3">Van Rossum (2027), §1.3.</ref> Every analysis also carries uncertainty. NIST stresses that some facts about WTC 7 could not be established, because the building and the records kept in it were destroyed.<ref name="N1A-xxxv" />

== See also ==
* [[Critical steel temperature]]
* [[Fire resistance]]
* [[Parametric fire curve]]
* [[Standard fire curve]]
* [[Strength reduction at elevated temperatures]]

== Notes ==
<references />

== References ==
* Australian Building Codes Board. (2005). ''International fire engineering guidelines'' (2005 ed.).
* Australian Building Codes Board. (2020). ''Handbook: Fire safety verification method''.
* Fitzgerald, R. W. (2002). Structural mechanics. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall, Jr., & J. M. Watts, Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 1-132–1-143). National Fire Protection Association.
* MacNeill, R., Kirkpatrick, S., Peterson, B., & Bocchieri, R. (2008). ''Global structural analysis of the response of World Trade Center Building 7 to fires and debris impact damage'' (NIST NCSTAR 1-9A). National Institute of Standards and Technology.
* Milke, J. A. (2002). Analytical methods for determining fire resistance of steel members. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall, Jr., & J. M. Watts, Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-209–4-238). National Fire Protection Association.
* Ministry of Business, Innovation and Employment. (2013). ''Commentary for Building Code clauses C1–C6 and Verification Method C/VM2''.
* Ministry of Business, Innovation and Employment. (2025). ''C/VM2 Verification Method: Framework for fire safety design'' (2nd ed.).
* National Institute of Standards and Technology. (2005). ''Final report on the collapse of the World Trade Center towers'' (NIST NCSTAR 1).
* National Institute of Standards and Technology. (2008). ''Final report on the collapse of World Trade Center Building 7'' (NIST NCSTAR 1A).
* Van Rossum, E. (2027). ''Introduction to Fire Safety Engineering''. Fire Safety Academy.

[[Category:Structural Fire Engineering]]