This document summarizes a student's dissertation on the robustness of steel structures. It includes acknowledgments, contents, and an introduction. The introduction discusses the aims of investigating failures in steel structures through analyzing different design methods for robustness. It outlines objectives to examine the effects of robustness through member tying and alternative load paths, and to provide an overview of methodologies for quantifying catenary and membrane mechanisms. The document layout includes chapters on literature review, finite element method, modeling and analyzing simple and large 3D structures, and conclusions.

1. University of
East London
Robustness
of Steel
Structures.
Supervisor : Dr. Jawed Qureshi
Syed Mutayib RIZVI

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Acknowledgment
I would like to take this opportunity to thank Dr Jawed Qureshi who helped me during
my academic year. He made this project a wonderful experience for me to work on by
providing impeccable guidance on the use of structural engineering software.
I would also thank Mr John Walsh who provided the much needed initial guidance to
start the project and make this project a reality.

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Contents
Chapter 1 | Introduction.........................................................................................................................5
Aim......................................................................................................................................................6
Objectives ...........................................................................................................................................6
Layout..................................................................................................................................................6
Chapter 2 | Literature Review ................................................................................................................9
2.1 Introduction ..................................................................................................................................9
2.2 Previous Research | Critical Review of Design guidelines ..........................................................11
2.3 Current Design Procedures | Inadequacy and future improvements ........................................14
2.4 Current Engineering Practices | Structural Robustness..............................................................16
2.4.1 Structural Integrity | IBC Provisions.....................................................................................16
2.4.2 Integrity | ACI Methodology................................................................................................17
2.4.3 Structural Integrity | ASCE Provisions..................................................................................17
2.4.4 Unified Facilities Criteria......................................................................................................18
2.5 Progressive Collapse | Review of Significant Collapses..............................................................18
2.5.1 Ronan Point, London UK, 1968 ............................................................................................18
2.5.2 Alfred P. Murrah Federal Building, USA, 1995.....................................................................20
2.5.3 World Trade Centre, New York USA, 2001 ..........................................................................21
2.5.4 Discussion.............................................................................................................................25
2.5.5 Conclusion............................................................................................................................26
Chapter 3 | Finite Element Method......................................................................................................27
3.1 Introduction ................................................................................................................................28
3.2 Application..................................................................................................................................28
3.3 Material Properties.....................................................................................................................29
3.4 Conclusion...................................................................................................................................29
Chapter 4...............................................................................................................................................31
Modelling and Analysing a Simple 3D Structure...................................................................................31
4.1 Introduction ................................................................................................................................32
4.2 Analysis approach and Objectives ..............................................................................................32
4.3 Experiment..................................................................................................................................33
4.3.1 Introduction .........................................................................................................................33
4.3.2 Analysis of the 3D Frame .....................................................................................................34

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4.3.3 Discussion.............................................................................................................................37
4.3.4 Conclusion............................................................................................................................38
Chapter 5...............................................................................................................................................39
Modelling and Analysing a Large 3D Structure.....................................................................................39
5.1 Introduction ................................................................................................................................40
5.2 Structural Modelling ...................................................................................................................40
5.2.1 Introduction .........................................................................................................................40
5.2.2 Issues Faced .........................................................................................................................42
5.3 Discussion....................................................................................................................................42
5.4 Axial Force...................................................................................................................................44
5.5 Membrane forces........................................................................................................................47
5.6 Member Tying Check ..................................................................................................................49
Chapter 6...............................................................................................................................................51
Conclusion and Recommendation........................................................................................................51
Chapter 7 | References.........................................................................................................................55

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Figure 1: Ronan Point (Photo: UK Crown Copy Right). Ronan Point Collapse of a
concrete structure due to a gas explosion. ................................................................................9
Figure 2 Disproportionate Collapse of the Ronan Point Residential Building, 1968 .........19
Figure 3: Collapse of the Murrah Federal Building 1995 .......................................................21
Figure 4: Progressive Collapse of the World Trade Centre 2001.........................................22
Figure 5: Core structure of the World Trade Centre 1 ...................................................................23
Figure 6: Perimeter tube of the WTC 1...........................................................................................23
Figure 7: Hatch Truss to minimize the sway of the building from wind.........................................24
Figure 8: Floor Slabs consisting of 4inch thick concrete on steel deck supported by a grid of steel
bar trusses connected to the perimeter tube. ...............................................................................24
Figure 9: Dialog box from Robot Structural Analysis showing material properties used...............29
Figure 10: Section Plan of the 3D frame. .................................................................................34
Figure 11: Axial force distribution before column removal.............................................................35
Figure 12: Axial force distribution after column removal from Robot Analysis Software ...............35
Figure 13: Exemplified survival of the floor via catenary action ....................................................36
Figure 14: Side frame of the 3 by 5 bay Structure..........................................................................40
Figure 15: Change in the combination factors of DL1 and LL1.......................................................43
Figure 16: The dialog box showing the combination of static load with the required combination.
........................................................................................................................................................43
Figure 17: Axial force components of the Structure before column removal................................44
Figure 18: Axial force after column removal ..................................................................................45
Figure 19: Axial force after removal of column R1 (corner right)...................................................45
Figure 20: Membrane forces in the structure before column removal.........................................47
Figure 21: Membrane forces after front column removal.......................................................47
Figure 22: Membrane forces when Colum R1 was removed. ........................................................48
Figure 23: Vertical displacement when column was removed.......................................................49

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Chapter 1 | Introduction
The research is an indoctrination of the different strategies involved in structural
engineering which ensure robustness in steel structures. Various incidents in
building structures leading to progressive collapse (term used to describe the
collapse of structures lacking Robustness) are discussed. These collapses will be
further discussed in detail in the literature review.
In order to gain a better understanding of the different aspects that are scrutinized
in the report, it is of utmost importance to define Robustness in its essence.
The definition of Robustness has not been precisely laid down to date [Uwe
Starossek]. Authors tend to use it differently in the discussions on progressive
collapse and there exists no general consensus.
Whenever a key element is removed it results in the load being transferred to other
elements which if not designed in accordance to the standards (each country has
its own set of standard regulations for ensuring robustness) of robustness would
result in a collapse of the structure.
The report does not only debate about different strategies for robustness but also
examines the different changes in regulations that were laid down by engineers
following the incidents of progressive collapse.
A range of structural collapses from the Ronan Point Collapse UK (1968) to the
World Trade centre USA (2001) have been review. The effect that these incidents
had on the building regulations in the respective countries is discussed.
It is known that engineers have always tried to make the life of people better and
safe by making certain that the infrastructure around us is improved over time. It is
this belief that in turn ensures that the new structures built are raised to a better
quality or standard over generations.
World Trade centre USA (2001) collapse, 33years after the Ronan Point Collapse
raises questions whether the standard of modern structures has significantly been
raised over the time or not. Throwing light on other constraints that would have
influenced this collapse can help arrive at a conclusion.

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Since, many tragic events have occurred due to the lack of structural robustness,
there is much to learn for structural engineers regarding the robustness of a steel
building’s structural system.
1.1 Aim
To investigate failures in steel structures by analysing the different design methods
for Robustness of Structures.
1.2 Objectives
i. Examine the effect in Robustness of a Steel Structure due to difference in various
different methods namely:
Member Tying: Investigate the tying force generated in a pin steel frame.
Alternative Load Path: Investigate the resisting mechanism in pin steel frames and
compare the results with pin-rigid frame.
ii. Provide a brief overview of the methodologies that have been proposed and
validated via experimental testing for quantifying the catenary and membrane
mechanisms.
1.3 Layout
The dissertation consists of five chapters excluding the introduction chapter. The
different chapters give detailed information about the proceedings, giving
background knowledge of what is to be expected in the next chapter.
Chapter 2 | Literature Review: Standing alone from the rest of the report this
chapter discusses the classes of progressive collapse and promoting features of
collapse. Previous works in this field have been considered. Design methods and
design codes to prevent progressive collapse are reviewed.
This section addresses the following topics:
a. Introduction to progressive collapse and design objectives: A quantitative and
definition of Robustness, with guidance on the use of design objectives in
decision making process.

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b. Previous Research | Critical Review of Design guidelines:
This section talks about the publishing on progressive collapse for the past few
decades which were mainly in response to the catastrophic events of progressive
collapse. The events such as the Ronan Point Collapse (1968) to the World Trade
Centre have been discussed along with the shortcomings that were highlighted in
the design codes in response to these events.
It also discusses in depth the works of the authors that were published after the
respective events and the regulation changes that resulted in the design code of
different countries.
c. Current Design Procedures | Inadequacy and future improvements:
This chapter provides critical analysis on the current Design Procedures while
highlighting the inadequacies in the design. The scope for future improvements in
the design procedures has also been discussed. Improvements related to the
treatment of dynamic effects and non-linear dynamic analysis have been
suggested.
d. Current Engineering Practices | Structural Robustness:
An overview of the summary of the modern provisions for the design codes and
standards has been indoctrinated to give an idea of the modern techniques
adopted by practising engineers.
e. Progressive Collapse | Review of Significant Collapses:
A consultation of the major historical events has been inculcated in this section to
provide a better understanding of such collapses. Ranging from most early events
of the ‘progressive collapse’ to more lately events have been put forward after a
critical review. Also, the effect of these events on the design codes and approaches
from time to time has been mentioned with due precision.
The critical analysis is then followed by a thorough discussion, from which the
conclusion drawn have been penned down.
Chapter 3 | Research Methodology: Finite Element Method

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Following up the literature review, this section comprises of an introduction to FEM
with possible scope and application of the method. Based on the discussion of the
possible advantages and disadvantages of this method conclusions are drawn
regarding the need of accurate modelling.
Chapter 4 | Report of Small 3D Structure Analysis
This section comprises of an introduction to 3D modelling followed by the analysis
approach and objectives of the report. This is followed by an analysis of the 3D
frame and the results are discussed and recommendations based on it are
concluded.
Chapter 5 | Modelling and Analysing a Large 3D Structure
In this chapter, a comparatively large 3D structure is studied and a non-linear finite
element analysis including P-Delta effect is carried out.
This chapter forms the main part of the report and gives a thorough analysis for
achieving the research objectives. The structure is studied under uniform loading
conditions in 3 different scenarios and a thorough discussion is penned down.
A comparative analysis between the 3 conditions helps to achieve the desired
research objectives.
Chapter 6 | Conclusion and Recommendations for Future Work
This section comprises of 3 parts. A meticulous discussion is accompanied by
conclusion and recommendations for future work are suggested.
Chapter 7 | References.

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Chapter 2 | Literature Review:
2.1 Introduction
“Progressive collapse” as evident from its name is an event where the building
collapses progressively to an extent which is disproportionate to its actual cause.
The trigger of this collapse is the failure of one of the key elements (beam, column,
etc.) of the structure which may occur due to an explosion (e.g. Ronan Point
collapse, 1968).
Figure 1: Ronan Point (Photo: UK Crown Copy Right). Ronan Point Collapse of a
concrete structure due to a gas explosion.
To gain a better understanding of progressive collapse, it is vital to mention the
Collapse of Ronan Point (1968).
In the Ronan Point Collapse, the explosion occurred on the 18th floor (due to the
failure of a key element) which resulted the Debris from the 18th to 22nd to fall on
the 17th floor. This caused the floors below to fall progressively down to the 1st floor.

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Therefore, “progressive collapse” can be defined as the disproportionate collapse
of an entire structure or large part of a structure, spreading from an initial local
failure from an element (column or beam) to another.
To prevent such collapse in the buildings, different measures were made to resist
progressive collapse.
EN 1991-1-7 of the Accidental Actions defined “robustness” as
“The ability of a structure to withstand events like fire, explosions, impact or the
consequences of human error, without being damaged to an extent
disproportionate to the original cause”.
‘Progressive collapse’ and ‘Robustness’ are two different things that should not be
confused together. While ‘progressive collapse’ is the issue that needs to be
tackled, ‘robustness’ forms the method in solving this complex issue by
safeguarding the buildings against progressive collapse.
Thus, robustness can also be defined as “collapse resistance” or
“unresponsiveness to local failure in the structure” [Uwe Starossek (2009)].
However, there are two approaches to understand this definition. It can be
understood quantitatively and qualitatively. Unresponsiveness/insensitivity and
local failure are not quantifiable by simple mathematics.
On a case-by-case study, they are more often used in the decision making process
which depends on the design objectives 3 and 4.
Design Objectives
1. Accidental actions (assumable)
2. Case of local failure (assumable)
3. Extent of collapse (acceptable).
4. Other acceptable damages.
5. Applicable partial safety factors and combinations of actions. [ 1]
1
[Uwe Starossek (2009)] “Progressive collapse of structures”.

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In the decision making process, the design objectives have to be established in
advance which form the bases in defining robustness quantitatively.
In the same way ‘local failure’ equates to design objective 2, ‘collapse resistance’
can be equated to design objective 1 (assumable accidental actions). Design
objective 3 and 4 quantify insensitivity. When the DO 1 progresses to an
unacceptable total damage, a disproportionate collapse is said to have been
occurred.
The research aims to provide guidance on different design strategies for
robustness in buildings by carrying out a series of computational experiments in
relation to the constraints that may affect the design strategies.
2.2 Previous Research | Critical Review of Design guidelines
There have been many attempts of addressing the issue of progressive collapse
and much literature has been published in response to the catastrophic events that
have taken place. Ronan point collapse (1968), Skyline Tower in Virginia, USA,
(1973), attack on US barracks in Beirut(1983), Lebanon, Alfred P. Murrah Federal
Building(1995) and 9/11 World Trade Centre(2001 are some of the incidents of
progressive collpase which have ignited widespread research in regard to
progressive collapse avoidance.
In the UK, the first regulation changes were made after the Ronan Point Collapse
in 1968. A lot of research was started as a result of this incident to make clear the
much un-understood progressive collapse [The Structural Engineer, 1969; ISE,
1969].
Allen and Schriever [1972]
The UK building regulations introduced ‘Tying’ which would provide stability to
structural members in case of an untoward incident causing collapse. In the USA,
Allen and Schriever [1972] also did studies to address this complex issue. In their

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research, a summary of events of progressive collapse that had taken place in
North America (US, Canada) between 1969 to1972 was presented.
Ellingwood [1978]
To reduce the risk of progressive collapse suitable design strategies were
discussed by Ellingwood [1978] using the probabilistic method. Gross [1983]
discussed his analytical model (2D computer-based) after he presented his studies
of progressive collapse. In this discussion, he has explained the Alternate Load
Path (ALP) method which is claimed can ensure robustness. The explanations
were based on his reported structural behaviour in response to the removal of
columns from different locations.
Pretlove [1991]
Pretlove [1991], examined the dynamic effects in progressive collapse. His
experiments showed that local failure (in an element) is capable of inducing
progressive fracture in some of the remaining elements/members. However, the
key factor that is to be noted here is the high probability of a fracture to take place
before a new equilibrium is established.
Owens and Moore [1992]
Abiding by the UK regulations, the tying forces induced in the steel connections
have to be resisted. Therefore, with the aim of inspecting the simple steel
connections’ capacity to the forces, Owens and Moore [1992] presented a series
of experimental data. This proved vital in providing help with design approaches
soon after.
Stefieck[1996]
In 1996, New York City Technology Center a six storey building saw Stefieck report
a methodology to protect the exterior of the building. The designer increased the
ductility of beam-column connection. The moment capacity, size of sprandels and
columns were also increased. This enabled the frame to redistribute the load to the
other undamaged parts of the structure in case of an exterior column being
removed ensuring a higher redundancy.

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[Corley et al, 1998]
A few years after the Alfred P. Murrah Federal Building in the city of Okhlahama
Corley [Corley et al, 1998] investigated the damage caused by the explosion. He
aimed at determining the mechanism responsible for the failure of the structure.
Engineering strategies were reviewed to prvent such damage in the future whih
resulted in the Compartmentalised Construction of all the new Federal Buildings.
Dual System and Special Moment Frame were recommended to improve the
redundancy.
DOD [2002]
Due to the rise in the political influence of USA all over the world, the military and
defence agencies have become a target of violent attacks. In response to the threat
posed by growing attacks, the Department of Defence published a series of Unified
Facilities Criteria (UFC). The departments of the military, the defense agencies,
and Department of Defence field activities applied the series of UFC to design their
building facilities.
Buildings with 3 or more stories were considered to be in a higher risk of
progressive collapse by the UFC (DOD 2002). It is because the number of stories
in a building has a direct effect on the ‘chain reaction’ [Ellingwood and Leyendecker
1977].
Only one adjacent floor has the ability to collapse due to the loss of the local
element on the ground level when less than three stories are present. From this, it
can be inferred that less high structures may be proportionate to the initial failure
which triggers the event.
“…Design the superstructure to sustain local damage with the structural system as
a whole remaining stable and not being damaged to an extent disproportionate to
the original local damage” is the general statement used in the document (DOD
2002) to prevent from progressive collapse.
The framing system should assure adequate levels of continuity, redundancy (for
load paths) as well as the provision of energy dissipating potential to accomplish
the requirements.

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ASCE (2006)
American Society of Civil Engineers (ASCE) addresses structural integrity in a very
broad sense. It requires that the damage to the structural system should not occur
and the building should withstand a local failure (damage). Recommendations
regarding the probability of failure are given.
Some of the key applications laid down by the ASCE are mention in the table
below:
Plan layout System of ties Span directions of
floors
Floor slab catenary
action
Wall’s beam action Avoidance of long
spans
A little more detailed view of the ASCE will be provided in section 2.4
2.3 Current Design Procedures | Inadequacy and future improvements
The probabilistic theory of reliability forms the basis of the most design codes as
well as the verification procedures. Probability density functions represent the
statistically determined actions and resistances which are obtained by
measurements. The next step is the computation of the design values for the
above mentioned actions and resistances. These are computed by using the
probabilistic method after the probability of failure (allowable) has been specified.
Thus, assuring an invariable safety level.
Partial safety factors and schemes of load combination series reflect the
correctness of this mathematically sophisticated approach. The development of
appropriate counter measures to progressive collapse susceptibility is an area
where this approach may fail. The reasons for this have been mentioned below.
a. More focus is laid on local failure as compared to the global. E.g. when structural
safety has to be checked, the equations used for checking the stresses, sectional
forces as such are applied at a local level.
b. Another reason for the inadequacy is the neglect of accidental circumstances.

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c. The specification of an acceptable failure probability is a requirement for the
probabilistic concept. When adopting probabilistic design, it seems that no societal
consensus is necessary. This is because of the fact that previous deterministic
design codes are used for the derivation of design codes for target failure
probabilities.
As discussed earlier, difficulties are faced when the theory is applied to actual
structures. It would be of real significance to observe the response of the structure
when a local failure takes place. The high number of interactions in the structural
components make it extremely difficult to measure these responses. Structural
components separate, displacements and deformations along with the overturning
of the elements are some examples of such interactions within the structure.
However, improvements can be made by treating the dynamic effects in an
appropriate manner. A non-linear dynamic analysis (geometrically and materially)
in time domain can offer a proper treatment of the effects. Unusual loading at joints
or post-tensioning at tendons (not well established) can cause problems when
modelling structures in such scenarios. Computational outcome can see large
deviations if only small errors are made in the modelling assumptions.
To maintain the framework of current design procedures, an attempt to observe
structural response resulting from a local failure could be made on the resistance
side of design equations by adding partial safety factors. Non-robust structures
would have a value of less than 1 as additional partial safety factors and the robust
structures would have a value of 1 as partial safety factors. Non-robust structures
have design values of resistance reduced. This is based not on thorough
probabilistic analysis of a structure but on an engineering judgement.
Parametric analysis of the global failure against safety could form the basis of to
specify the partial safety factors when taking on board structural robustness. This

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has to be so for structures of all kinds covered by respective design codes. Thus.
Structures need to be classified and assigned reduction factors respectively to
each structure. Also, the use of safety factors need to be dispensed. This can be
done by following a fully probabilistic analysis for design task at hand.
The other two problems mentioned previously are reliability based design
challenges. These problems stem from the unknown probabilities of the accidental
circumstances. These unknowns cannot be neglected as they form the input values
for the probabilistic analysis. There would be an important factor missing in a
computation (probabilistic) even if an acceptable probability of a disaster is agreed.
2.4 Current Engineering Practices | Structural Robustness
The various practices in the design codes and standards have been presented in
this section. It presents an overview of the modern design techniques adopted
mainly in response to specific events of “progressive collapse”.
2.4.1 Structural Integrity | IBC Provisions
High rise buildings saw a major change in design code provisions after the
progressive collapse of World Trade Centre Collapse on September 11, 2001. This
event in particular saw a number of changes that were proposed in building design
codes.
IBC, the “2009 International Building Code” (ICC 2009) came up with specific
requirements in case of buildings with more than 3 stories to provide for structural
integrity. Two changes that were seen as a result were related to column splices
and beam connections.
It required for the columns splices to withstand force due to tension imposed by
dead and live loads.
It also required for the beam connections to be designed in such a way that it resists
(as a horizontal load) two-thirds of the vertical load (factored).

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Therefore, it can be inferred that a distinguished lack of inherent structural
robustness in buildings has been addressed.
2.4.2 Integrity | ACI Methodology
The Ronan Point UK building (see details of collapse chapter 2.1) that saw a
progressive collapse in 1968 was mainly consisted of pre-caste concrete structure.
Due to its pre-caste concrete build, it relied heavily on gravity loads and bond and
sparked high interest in the concrete industry to get involved in robustness and
collapse resistance [Popoff 1975].
The American Concrete Institute building code requirements explained the
reinforcement and connections in detail stating that tying of structural members
effectively would ensure integrity/robustness for the overall system. This would
further go on to set the standard of minimum requirements in concrete framed
structures providing minimum level of structural integrity. The intention was to
confine or minimise the damage resulting from a failure of a key element, within a
relatively small area by improving redundancy and ductility. This would give the
structure a better chance to ensure its stability.
2.4.3 Structural Integrity | ASCE Provisions
“Minimum Design Loads for Buildings and Other Structures (ASCE 2010)” a
publication by ASCE gives general guidance on providing structural integrity.
Allowance of the forces and moments to transfer between members helps provide
a continuous load path as intended by the provisions.
In its commentary, the ASCE considers the safety of public. It discusses the
structures housing higher number of people which may be a subject of attack and
states that “….more rigorous protection should be incorporated into designs than
provided by these (above mention) sections.”
The definition of untoward incidents should be considered at design stages. ASCE
does not take the responsibility of defining these events.
Two design methods for robustness that have been discussed in the commentary
are the direct and the indirect methods.

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2.4.4 Unified Facilities Criteria
Owing to the growing risk of terrorist threats Department of Defense (DoD) and
Department of Defense (GSA) formed guidelines for resisting collapse due to
explosions. The guidelines were provided in the form of the Unified Facilities
Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse (DOD
2005).
This document is a generic guide providing information in issues related to
robustness of structures. To quote an example, the redistribution of loads from a
damaged part of the structure to other undamaged parts by the use of tie forces
has been talked about.
Analysis of alternate load paths in a structure and use of amplification factors when
designing structures is also provided.
2.5 Progressive Collapse | Review of Significant Collapses
Consultation to the literature of historical events can lead to a better understanding
of the mechanism of progressive collapse. The following are the few but more
important incidents of progressive collapse which have occurred in the last 5
decades. The investigation of the causes of such collapses helps to get a better
understanding of the mechanism of “progressive collapse”.
All the three mentioned buildings have suffered progressive collapse.
The research makes an attempt to investigate the cause and mechanism of the
collapse by providing a critical analysis of all the constraints related to progressive
collapse. Special attention is given to the causes of the collapse of the World Trade
Centre (2001) which paved way for further research in this field (presented in
section 2.5.2).
2.5.1 Ronan Point, London UK, 1968
The partial collapse of the Ronan Point was one of the first incidents of progressive
collapse which ignited a series of researches, specifically in the UK. [The Structural
Engineer, 1969; ISE, 1969]

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A chain reaction of failure happened due to a gas explosion on the 18th floor of the
residential apartment (Ronan Point). The exterior wall panels were blown off due
to the explosion which in turn gave way to the chain reaction of failure that
propagated horizontally and vertically down to the 1st floor.
In the Ronan Point Collapse, the explosion on the 18th floor (due to the failure of a
key element) resulted in the Debris from the 18th to 22nd floor to fall on the 17th floor.
This caused the floors below to fall progressively down to the 1st floor. Pre-
fabricated panels (provided to withstand wind pressures) were used for the
construction of the building.
Figure 2 Disproportionate Collapse of the Ronan Point Residential Building, 1968
At the outside edge, the upper floor slabs failed as they were not supported by the
cladding. The vertical load path lost its continuity due to the fall of the upper floor.
Simultaneously, massive overload was caused at the 17th floor due the debris from
the floors. It should be noted that the debris from the 22nd to the 18th floor had
caused this massive load which couldn’t be resisted by the floors below in
succession due to a lack of ALP (alternate load path). As a result, the floors in
succession collapsed down to the 1st floor.
UK code of practice writers have done a lot of work after the collapse. A number of
recommendations were made following this collapse to guard buildings against

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progressive collapse. In the UK, the recommendations were adopted covering
continuity, loading and ductility. A vertical element may be removed without
causing unacceptable damage in structures with a certain number of stories where
it is not possible for the ties to meet the requirements [DETR, 1994; ODPM, 2004].
If in any case, the vertical members can’t be removed then it should be able to
withstand a specified force in any direction [HMSO, 1991; HMSO, 1992; DETR,
1994; ODPM, 2004].
Clearly, to account for accidental loading and ‘tying’ structural members together
were reconsidered by the following Building Regulations [HMSO, 1970; HMSO,
1976].
2.5.2 Alfred P. Murrah Federal Building, USA, 1995
A large vehicle bomb was detonated approximately 5m from the north face of the
nine storey Murrah Building in Oklahoma City.
Around 168 fatalities were caused due to this incident [Corley et al, 1998; Corley,
2004]. A considerable damage was sustained to the Murrah Building.
The bomb exploded about 5 meters far on the north side of the building (see figure
3). In this explosion, the RC slab and column construction was damaged. The
columns G16, G24 failed in shear after the G20 column was destroyed. From east
wall to the column G12 the girder (transfer girder) lacked support. The frame was
left with 3 columns missing (G16, G20, G24) and could not support itself. The
calculations to support this claim were made by [Corley et al, 1998]. The two bays
on the south side, 8 bays on the northern part of the building.
Three possible mechanisms were discussed by Corley in 2004. The main cause of
the collapse was concluded to be a lack of continuity in the reinforcement in
structures:
1. Transfer girder or
2. Base of the column.

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Figure 3: Collapse of the Murrah Federal Building 1995 [Corley 1998]
Some of the changes that were recommended/introduced for design in federal
buildings after the Oklahoma city bombing in US were compartmentalised
construction, special moment frames and Dual systems. Such systems [Corley et
al, 1998] would help increase the stability and toughness of the structures if brought
under a sudden loading. This would also allow the building to reduce the probability
of a collapse and help it act in a better manner by the provision of strength and
extra mass.
2.5.3 World Trade Centre, New York USA, 2001
The 110 storey high World Trade Centre towers on September 11, 2001 saw two
hijacked airplanes collide in to it. In the history of the United States, this turned out
to be the worst disaster in which around 3000 people lost their life [FEMA 2001].
Immediately after the attacks, the Structural Engineering Institute of the ASCE and
Federal Emergency Management Agency (FEMA) formed a team of engineering
specialists. It was concluded that the impact had caused localised damaged. The
building finally saw a total collapsed when the steel frame was weakened by the
heat of the fire.

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Figure 4: Progressive Collapse of the World Trade Centre 2001
The collapse was described by Corley [Corley, 2004] as:
“Once the collapse began, potential energy stored in the upper part of the structure
during construction was rapidly converted into kinetic energy. Collapsing floors
above accelerated and impacted on the floors below, causing an immediate,
progressive series of floor failures, each punching in turn onto the floor below. The
collapse of the floors left tall, freestanding portions of the exterior wall. As the
unsupported height of these freestanding exterior wall elements increased, they
buckled at the bolted column splice connections and also collapsed. The process
was essentially the same for both Tower1 and Tower2”.

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Figure 5: Core structure of the World Trade Centre 1
Figure 6: Perimeter tube of the WTC 1

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Figure 7: Hatch Truss to minimize the sway of the building from wind.
Figure 8: Floor Slabs consisting of 4inch thick concrete on steel deck supported by a grid of
steel bar trusses connected to the perimeter tube.
The WTC had a unique design and was considered an extraordinary load carrying
structure. The engineers claimed that the structure had a high redundancy but the
progressive collapse of such a phenomenal structure left the engineers with lots of
an answered question.
A lack of understanding of this form of collapse (progressive collapse) was
underlined by the WTC events.
It was concluded that there is not a good understanding of disproportionate
collapse following a report from the MMC (Multi-hazard Mitigation Council).

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More research was needed in this area to be put into existing research. For the
prevention of progressive collapse in the future, a need for the development of a
National Standard for progressive collapse was outlined [MMC, 2003]
2.5.4 Discussion
Progressive collapse is a topic that has been widely debated over the last few
decades. Researches started as early as 1968. Some of the early works may be
listed as follows: [HMSO, 1968; The Structure Engineer, 1969; ISE, 1969; Allen
and Schriever, 1972; Popoff, 1975].
[HMSO, 1968; HMSO, 1970; BSI, 1972; HMSO, 1976; BSI, 1985; BSI, 1990;
HMSO, 1991; HMSO, 1992] are some of the drafted rules on disproportionate
collapse by the UK in response to the Ronan Point Collapse.
[BSI, 2000; Way, 2003; SCI 98/99] are three stages that of design procedures that
were implemented by the UK to avoid disproportionate collapse.
A minimum tying force is specified in the direct design procedure for minimum tying
strategy so that the structural members are tied in horizontally by ensuring the
required minimum force. Many countries have adopted this design code as it is an
acceptable solution for design against progressive collapse.
An issue which has been under studied is whether these recommendations provide
adequate protection against the collapse discussed. Since, there has not been
much research conducted in this area it wold recommended to do so.
After the 9/11 attack, the understanding of the progressive collapse was
questioned in that whether it was well understood or not. However, the WTC
collapse (2001) was not the first terrorist attack in the USA. Oklahoma City bombing
of the Alfred P. Murrah building also saw a major structural failure due to
progressive collapse in 1995. Progressive collapse was related to nearly 80% of
168 fatalities caused in this incident rather than the explosion [Corley et al, 1998].
The building heavily relied on a few elements (such as transfer girders) rather than
relying on more than on (more than one) load transfer path.
The future Federal Buildings in the US are built as per the new design
recommendation such as the Compartmentalised construction and special frame

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system. The aim of such construction was to increase the structural redundancy of
the buildings.
The design approaches which came in to being following such incidents have
proved useful in improving the behaviour of structures under accidental conditions.
2.5.5 Conclusion
One of the most significant developments of the Ronan Point Collapse in the UK
was the highlighted importance of tying members together. As the focus of the
engineers diverted to the tying member strategy, UK was the first country which
established the minimum tying force values. It also specifically drafted rules for
tying members horizontally and vertically. The rules also mentioned when to tie
some semi-rigid joint stiffness some semi-rigid joint stiffness horizontally, vertically
or both horizontally and vertically (usually depending on the number of storeys).
The method of tying members proved vital as it helped establish integrity in the
structure as well as prevent progressive collapse. Many countries even accepted
the strategy in the formation their design guidance for prevention of progressive.
The influence of the UK tying strategy had particular impact in many European
countries. This is evident in the British and the European Standard as many
similarities between the two can be seen in the design of key elements with the
‘member tying’ strategy being one of the more significant.
The collapse of the WTC had a major impact on progressive collapse strategies.
An extensive research was ignited after the collapse of such a high rise and high
profile structure. As a result, many reports were published after the events and a
doubt related to the adequacy of the use of ‘tying member’ strategy alone to
preventing progressive collapse was expressed.
All the complexities of the collapse of the structures that have been mentioned
before in the report indicate that ‘progressive collapse’ is a dynamic approach and
thus, a static approach such as the minimum tying forces seems inadequate. The
report as such aims at giving a broad understanding of the constraints (force
generated) in damaged steel framed buildings and arrive at a conclusion by
comparing the values to the ones given in the UK design code.

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Chapter 3
Finite Element Method

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3.1 Introduction
To study the behaviour of structures under progressive collapse a specific method
was to be chosen in order to full fill the objectives of the research. Therefore, it was
decided to use the FEM (Finite Element Method). The FEM is a highly
sophisticated and able tool for examining structures’ response under damage (local
failure in this case).
In this section, the advantages and applications of the FEM would be considered.
The FEM analyses a smaller region of a more large and complex system into
simpler parts minimizing errors to provide a solution.
The structural components’ formulation would be briefly re-evaluated. Also, the
implementation of such a formulation in the research would be examined s
required.
3.2 Application
As discussed earlier, the research focuses on the behaviour of steel frame
structures when subjected to failure in a key element. Examination of a structure
in case of being damaged by an accidental loading makes it very difficult to predict.
Static analysis as compared to the dynamic analysis of a structure is less
complicated in general. When an attempt is made at analysing the structure
dynamically, time-related damping and inertia have to be taken into consideration
[Clough, 1975]. It would therefore be of utmost importance to choose the right FEM
software (analysis code).
The FE code requirements to reach the established objectives should be covered.
The FE code should be able to model complex events and be able to combine
loads such as the self-weight with loads that would result when a key element is
removed.
The parameters such as the P-8 (P-Delta) effect [Gupta, 1999] in non-linear
deflection modelling should also be satisfied as they form the pillars of such a
complex analysis.
The modelling of geometrical behaviour non-linearly should also be a pre-requisite
for such an analysis.

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3.3 Material Properties
Non-linearity of the material and geometry are included in this study. The non-linear
geometry and material properties of S275 steel used in the analysis were as
follows:
Figure 9: Dialog box from Robot Structural Analysis showing material properties used
3.4 Conclusion
It is inferred that an in depth grasp of the component formulation is essential to
assure a proper use of the FEM. It can solve complex problems of structural steel
behaviour and satisfies the requirements of the research objectives. It is vital to
mention again the purpose of the research which is to gain a broad and
encyclopaedic apprehension of behaviour steel frame structures undergoing
progressive collapse.
To achieve the objectives, simulation of progressive collapse is essential. This
would be done by seeing through that the modelling of the elements (beam,
columns, floor and connections) is apprehended in order to ensure an accurate
modelling.

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A satisfactory research of the FEM analysis at an early stage paved way for a better
understanding of more complex structural analysis to be followed in the next
chapters (4 and 5) providing the analysis of 3D structures.

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Chapter 4
Modelling and Analysing a Simple 3D
Structure

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4.1 Introduction
A number of introductory studies were carried out to gain a better understanding
of how to model structure. In this part of the research, a small 3D structure is
introduced and its behaviour examined.
The response of the structure (such as the moments, axial forces, etc.) is computed
and analysed. These values are then compared in contrast to the results obtained
during the analysis of the structure when a column was removed. The resisting
mechanism of the structure during the column removal is debated and a report
based on the analysis is presented.
4.2 Analysis approach and Objectives
As discussed earlier, it is always not easy to predict the response of a structure
during collapse. Whenever, a structure is subjected to a localised failure in its
element, it would either collapse or attain an equilibrium position.
Take an example of a dynamic load on a structure such as an earth quake. During
an earthquake, the structure tries to establish a new equilibrium position. However,
if this is not possible (due to the structure inability to redistribute force) then the
structure is bound to collapse. In the research, it is desired to study the behaviour
of the structure when a static equilibrium is disturbed. As stated earlier, this may
be due to an externally applied force which causes a localised failure in the
structure’s load bearing elements. Thus, the research aims to investigate the ability
of a structure to maintain its equilibrium. The resisting mechanism in this case is
investigated. In case the building collapses, the failure mechanism is studied.
The main reason behind progressive collapse of a structure is accidental loading
or blasts. However, the research attempts to model the damaged structure after
the load has been applied rather than the modelling of the cause itself.
The structure was preloaded and the forces arising in the damaged frame were
then simulated after the removal of a vertical element (column). The axial forces
and the moments that were recorded before the removal of the column were then
compared to the results obtained after the removal of the column. The results gave

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a realistic and helpful comparison between the pre-removal and the post-removal
forces imposed on the structure. This helped to determine the change in the load
path and the redistribution of the forces in an easy manner. Even though the
prediction statically may not be as accurate as in a dynamic approach, it helps gain
a better understanding of the mechanism of the behaviour of a structure in a
progressive collapse.
4.3 Experiment
4.3.1 Introduction
A small 2 storey building with section plan given below was studied. Early study of
such a small scale structure helps to gain a better understanding of the more
complicated structure in next chapter.
To attain the static required the static equilibrium in the structure, it was designed
as per the BS5950.
Swaying was the main problem faced during the design. The member sizes were
not adequate to prevent this to happen. The problem was then dealt by an increase
in the member size and decrease in span. The achieved plan of the building is
given here. This also helped to enhance the lateral stiffness. The results found
were really helpful to understand the alternate load path but the need for gaining a
better understanding of the above was fulfilled in the study of the larger structure
in chapter 5.
The primary beams (B1, B2) are along the x direction, and tie beams (B3, B4) are
located along the y axis. The cladding is not included in this study. The geometry
details are shown in Figure 10

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Figure 10: Section Plan of the 3D frame.
The 3D structure was kept as simple as possible to make sure that good
understanding of the progressive collapse is gained.
4.3.2 Analysis of the 3D Frame
For the prevention of progressive collapse in the case of a localised failure, the UK
building regulations require the buildings to be tied together.
The structure was loaded with values of 1.O gk+0.5gk. After loading the above
mentioned values, the analysis was carried out which gave out the axial force and
bending values.
Figure below provide the required values that were obtained following the analysis
Figure 11 and Figure 12 respectively.
The data output reveals a significant value of bending. With the tensile forces being
less significant bending resistance appears to be more important than the tying
action.
The BS 5950 advocates catenary action to play a more important part in the
resisting mechanism. Taking into consideration the case in hand (3D frame), it

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should be noted that the members are connected using a pin-rigid joint (support).
This means that large rotations are not possible and thus, minimising the tensile
force giving way to the vierendeel action.
Thus, it can be inferred that the Vierendeel action acts as the resisting mechanism
in this case.
As soon as the column was removed from the building, the change in the forces
(such as the axial force) was clearly noticeable. The diagrams below make it clear
to understand the discussion.
Figure 11: Axial force distribution before column removal
Figure 12: Axial force distribution after column removal from Robot Analysis Software

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As evident from the example above, the structure redistributes the forces in order
to achieve a new equilibrium position from its initial static position. The forces from
the damaged part are bridged to the undamaged part with the help of the tie beams
through the beam-column connection.
However, it is not easy for a structure to achieve such an equilibrium in a short
span of time (explosion causing the sudden failure of a column) when a collapse
occurs. This ability of a structure to maintain or achieve a new state of equilibrium
is considered to define the redundancy of the structure. The higher the probability
of the structure to achieve the new state of equilibrium (also taking into account the
removal time of a key element) in a short period of time the more redundant or
robust is the structure.
There are many different ways to reach a new equilibrium in a damaged case and
these are highly debatable. The damaged frame, in response to the failure of a
vertical element may develop catenary action or Vierendeel action.
In the diagram shown above, the catenary action develops as soon as the column
was removed. The catenary action comes into play as the corner columns were
pulled in. The corner columns on the front were taking an initial axial force of around
17 units (see figure 11 and 12 shown earlier) before the removal of the column.
The diagram shows that after the removal (of the column), the corner columns took
an axial force of around 28 units, which is a substantial amount of increase.
Figure 13: Exemplified survival of the floor via catenary action

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What is more important to notice is the change of axial force in the corner columns
of the floor (floor 1/storey 2) above the floor where the column was removed. This
sudden increase in the column is responsible for the catenary action and the
bending moment to develop. If the new equilibrium is not achieved, these sudden
change in forces can have a higher effect and cause the structure to collapse
progressively. It should also be noted that such collapse can also be encountered
successfully if continuity is ensured at the design stage of the structure. Transfer
girders can also form an important element when alternative load path
methodology is applied.
4.3.3 Discussion
The research aims to examine the tying force generated and the resisting
mechanism of pin joint steel frames. A frame was analysed and the critical forces
generated were studied to gain a better understanding of the behaviour of such
frames.
Attempts at bracing the small 3D frame were made for the provision of lateral
stability. An external support to provide lateral stiffness was also made. This was
not finalised as it was found that such a support would cause extra loading and
effect the results.
For the analysis of the non-linear static analysis of the load path method, the load
factors used are:
2.0 [(0.9 or 1.2) Gk + (0.5Qk or 0.2Sk)] + 0.2Wk
An addition of the dynamic load factor of 2 was accounted because of the static
nature of the analysis.
The strategy of the survival of the structure was discussed. The tension steel would
only take the axial load while relying purely on catenary action mentioned in the
previous section. The analysis carried out also suggests that columns above help
support catenary at the mid-span vie the tie forces. The complexities in the
mechanism of the response in the collapse make it difficult to form accurate
calculations but an approximate/ crude calculation helped suggest collapse or
survival.

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A time-history analysis was also attempted to achieve a more realistic response of
the small structure. These attempts however were not conclusive but proved really
vital as it formed the basis for the research of a more complex structure in the next
chapter.
4.3.4 Conclusion
This chapter presented the modelling and analysis of a 3 storey 3D small scale
structure. The analysis carried out helped to understand the resisting mechanism
of a steel frame when a column was removed.
The analysis suggested that bending moment acts as the primary support
mechanism. The catenary action developed also forms a key resisting mechanism.
The final judgement made was that the supporting mechanism of the steel frame
structure in this case is a combination of bending moment (vierendeel action) and
catenary action.
It was also concluded that to get a more detailed and realistic behaviour of steel
frame structures, analysis of a larger 3D structure shall be carried out. This would
also help to address the issue with lateral stiffness providing bracing to the
structure. The analysis of the structure is presented in the next chapter.

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Chapter 5
Modelling and Analysing a Large 3D
Structure

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5.1 Introduction
The Robot Structural Analysis Software uses the Finite Element Method to model
and design the structures. It provides a friendly user interface for the study of
behaviour of real structures in various complex situations.
The structural collapse of a 5 bay by 3 bay 3D structure is modelled and analysed.
The study includes the investigation of dynamic effects. The steel frames of the
structure at hand were
5.2 Structural Modelling
5.2.1 Introduction
A 5 bay (along y axis) by 3 bay (along x axis) was examined.
Design details: The structure was designed as per the requirements of BS5950-1:
2000 [BSI, 2000].
Figure 14: Side frame of the 3 by 5 bay Structure

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Frame action incorporating moment resistance provides lateral resistance along
the X direction. Along the Y axis, the frame is braced. This provides lateral stiffness
and takes care of the swaying issue as discussed in the previous section.
A statically indeterminate structure is formed by the combination of each structural
component providing lateral stiffness in the continuous frames. As these complex
calculations required for the analysis of this 3D structure are not possible, therefore,
a combination of linear static analysis cases was carried out with the help of a
software [Robot Structural Analysis Software (Autodesk)].
Frames are currently classified as ‘non-sway’ in the steel design code. This is so if
the P-Delta effects are neglected whereas if the effects are not neglected, frames
are classified as sensitive to sway. Elastic critical load helps to determine the
Second Order Effects.
The P-delta effect was not neglected in this case. If the P-delta effect was ignored
this would mean that large member sections would be required. Therefore, in order
to choose the normal size of members for the sake of practicality [Brown, 2002], it
was decided to include the P-Delta effect.
Since the members in this case are designed to resist moments it is acceptable to
approximate for P-Delta effects related to sway (movement). The front facing
frames in figure 17 (pin-frame structure) are the sway frames. As discussed earlier,
it is not possible to use simple tools to analyse a complex behaviour of
‘disproportionate collapse’, the use of Robot Structural Analysis from Autodesk
provides vital guidance to achieve the research objectives. In the previous chapter,
the use of Robot Structural Analysis Software (Finite Element Method) for
modelling and calculations was witnessed to give a realistic behaviour of response
from structures under-going progressive collapse.
The structure was analysed under three different conditions. Firstly the axial forces,
membrane actions on the bars were recorded under normal design conditions (the
building was designed as per the BS5959). Secondly one of the columns on the
front frame was removed and change in the forces were was observed. Thirdly, a
corner column was removed and the behaviour of the building studied and
analysed.

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The changes in the forces are discussed in the next chapters.
5.2.2 Issues Faced
The 3D structure is formed of pin-pin frames. One of the issues that was faced due
to this is the detailed modelling of primary beams to avoid buckling.
Pre-cast units in reality can easily solve this issue by providing lateral restraint. The
use of Pseudo beams can however solve this issue effectively with an increase in
the second moment of area. This also helped to retain the right area of cross
section.
The research is concerned about the resisting mechanism of the damaged frame,
therefore, the effect of pseudo beam section on bending resistance would not affect
the research. Also, the load distribution path would not be affected by this.
5.3 Discussion
The 3 bay by 5 bay structure was designed as per the BS 5950 regulations, details
of which are provided in the appendices A.
A statically indeterminate structure is formed by the combination of each structural
component providing lateral stiffness in the continuous frames. As these complex
calculations required for the analysis of this 3D structure are not possible, therefore,
a combination of linear static analysis cases was carried out with the help of a
software [Robot Structural Analysis Software (Autodesk)] which gives the following
results for axial force and membrane forces. A statically indeterminate structure is
formed by the combination of each structural component providing lateral stiffness
in the continuous frames. As these complex calculations required for the analysis
of this 3D structure are not possible, therefore, a combination of linear static
analysis cases was carried out with the help of a software [Robot Structural
Analysis Software (Autodesk)].
A loading level of 1.0gk and 0.5qk was used. The combination of DL1, LL1 and
ACC was used for the non-linear analysis static analysis.

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Figure 15: Change in the combination factors of DL1 and LL1
Figure 16: The dialog box showing the combination of static load with the required
combination.

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5.4 Axial Force
Figure 17: Axial force components of the Structure before column removal
After carefull observation of the figure 17, 18 and 19, mapings of the axial forces,
it is seen that the case where no column is removed, the axial forces as obviously
are evenly distributed. The mid columns take the most of the axial load with two
sets of columns adjacent (on the right and left) taking less and lesser respectively.

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Figure 18: Axial force after column removal
Figure 19: Axial force after removal of column R1 (corner right)

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In the second case, the axial force increases from 38 to 44 units after the removal
of the column. The most important thing to notice is that even when the axial force
increases, the force in the column right above the removed column decreasesd
from 17 units to around 4. This means that the force is now distributed to the
adjacent columns to make that the frame remains intact. The column that is braced
takes an inreased load of around 10 units while as the columns on the righttakes
an increased laod of 8 units. The use of one transfer girder in this case helps to
distribute the load in the undamaged parts of the frame. We also see that the
increase in forces was more significant in the pin joint frame rather than the other
adjacen columns.
In the third case wher the right cornner column (R1) was removed, the axial force
distribution is is achieved differently. When the column was removed, the increase
on the load was seen on the bracing right next to it but the columns soon after that
remained unaffected. Even after the columns on the left of the bracing grid remain
unaffected but the effect was seen on the bracing grid on the left corner of the
buildings front frame.
As seen in the two diagrams above the column adjacent to the left bracing grid
changes from light green to drak green signifying the increase in axial force. The
exact measure can be seen on the larger scale above.
Also, the bracing at the right corner take equal amount of load after the column
removal whereas they differ in the first case.

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5.5 Membrane forces
Figure 20: Membrane forces in the structure before column removal.
Figure 21: Membrane forces after front column removal.
In the first diagram of this section, it can be seen that the membrane forces are
evenly distributed on both sides with a maximum of around 40 units at each corner.
The membrane forces are negative at the centres. However, when the column in
the front frame was removed, the membrane forces saw an increase of around 20
% at the corners and 25% at the middle. This allowed the building of a catenary

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action to support the frame but the catenary action is not as significant as the
bending moment (Vierendeel action) that we noticed in the previous chapters. The
bending moment saw a higher percentage increase as compared to the membrane
forces.
When the column R1 was removed (depicted in the diagram below), the membrane
forces were seen to increase by a higher percentage. The membrane forces at the
corners saw a release in the forces whereas at the centre of the frame, it went
down from -11 to -21 units.
Figure 22: Membrane forces when Colum R1 was removed.

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5.6 Member Tying Check
The 3D structure was then enlarged to a 7 by 5 bay structure with the same plans.
This was done so as to make a tying check for the BS5950 and to check the
adequacy of the provisions.
Figure 23: Vertical displacement when column was removed.

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Chapter 6
Conclusion and Recommendation

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As seen in the report the tying force generated for this case was much higher than
the required minimum value of 222KN according to BS 5950. The tensile resistance
of the cross section of the beam was beam was higher, but it is the beam-column
connection that are designed resist the design tying force. It was noticed that the
there is a high probability of beam-column breaking up in case the capacity is close
to target design value.
UK design regulations make sure that the ultimate limit state forces are resisted by
the connections whereas the tie beams don’t fail.
When the structure in the research was designed, it was made sure that the design
complies with the recommendations provided by the codes. However, the structure
is still susceptible to collapse if only the minimum requirements were met. The tying
force (minimum) required by BS 5950-1 (2000) does not provide an adequate
provision for the safeguarding against progressive collapse.
The results showed that progressive collapse is dependent on time. If the column
is removed in a short span of time, it was found that the structure was susceptible
to a higher deflection and also large forces generated.
It was also concluded that if more routes are provided for the transfer of load path,
the probability of collapse can be reduced significantly.
Resisting Mechanism: It was found that a combination of bending moment and
catenary action acts as the resisting force in the 3D steel frame.
The continuity in the connections helps the catenary action to develop and support
the frame when a local failure occurs.
The catenary action is the resisting mechanism for this pin 3D frame structure that
was studied in the report. It was further concluded that beam-column joints don’t
give way to bending moment. With about 3 times higher in magnitude, the tying
force can help the frame to resist the collapse. These facts help conclude that the
progressive collapse is a dynamic event.

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The fact that rotational stiffness in connections is witnessed when dealing with real
frames also indicates that bending moment and catenary action act as the resisting
mechanism.
In general it was found that:
i. BS 5950 specified design tie force is very small than the actual forces generated
in the frame studied.
ii. Structural stiffness has the ability to influence stiffness in the joints. The tying force
can be reduced by an increase in the stiffness (rotational)
iii. A pin frame can only be provided the resisting mechanism in reality by a
combination of catenary action and bending moment.
Some suggestions for future work that can be carried out are presented below:
 A real tying value for the rupture of a column can be calculated. As connection
failure was neglected, the real value that breaks a connection is not known. This
force may be identified by numerical analysis. This can be really helpful for the
designers while they choose a connection or damaged structures.
 A numerical investigation of buildings with composite slabs can be studied in
future. Only steel frame with pre-caste units were considered in this research.
This would be really important for buildings in case of fire. This is because
composite slabs can offer a very higher resistance to fire when compared to steel.
Steel usually has a coating sprayed over it to prevent it against melting. However,
this coating may not protect the steel at high levels of heat as in the case of the
WTC (2001).The steel may not melt itself but can become weak and lose its
strength which may result in a collapse.
 An in-situ concrete structure could be studied and the checks done in this report
could be used in an in-situ concrete structure.
 The design provided in the chapter 5 for the 5 by 3 bay building can be used as a
check for the collapse of the Alfred P. Murrah Building as the structure presented
has a fairly similar type of the plan. It could also help to check whether the use of

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a transfer girder in the Murrah Federal Building could have been used to prevent
progressive collapse.
 A study based on the different typologies of the collapse [Uwe Starossek (2009)]
can carried out to give required design values for different typologies. It could also
include the different classes of buildings provided in the modern design codes to
serve as a guidance.

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Chapter 7 | References
1. Uwe starossek (2009). Progressive collapse of structures. London, UK: Thomas
Telford Limited. 32-34.
2. Christopher M. Foley et al, MS (2007). Robustness in Structural Steel Framing
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3. A G J Way MEng, CEng, MICE (2004). Guidance on meeting the Robustness
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4. P G Cobb CEng MICE (2010). Practical guide to structural robustness and
disproportionate collapse in buildings. 3rd ed. London: The Institution Structural
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5. Byfield, Michael et al. and others (2014). A review of progressive collapse research
and regulations. “Proceedings of the Institution of Civil Engineers”. Structures and
buildings. Volume 167. Issue SB8. ICE Publishing
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8. BS EN 1991-1-7: 2006: Eurocode 1: Actions on structures – Part 1-7: General
actions -Accidental actions. London: BSI, 2006 and NA to BS EN 1991-1-7: 2006:
UK National Annex to Eurocode 1 – Actions on structures – Part 1-7: Accidental
actions. London: BSI, 2008
9. BS EN 1990: 2002: Eurocode: Basis of structural design. London: BSI, 2002
10.T.D.G. Canisius, J. Baker, D. Diamantidis et al, COST Action TU0601 Robustness
of Structures, STRUCTURAL ROBUSTNESS DESIGN FOR PRACTISING
ENGINEERS
11.British Standards Institution (2001). “BS 5950-1:2000: Structural use of steelwork
in building, Part 1: Code of practice for design – Rolled and welded sections,”
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12. Izzuddin B. A., Tao X. Y., and Elghazouli, A. Y. (2004). “Realistic Modelling of
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