PSA 2023 - chapter 1.pdf

PROGETTAZIONE STRUTTURALE ANTINCENDIO
A.A. 2022/23
Prof. Ing. Franco Bontempi
Docente di TEORIA E PROGETTO DI PONTI – GESTIONE DI PONTI E GRANDI STRUTTURE –
PROGETTAZIONE STRUTTURALE ANTINCENDIO
Facoltà di Ingegneria Civile e Industriale
Università degli Studi di Roma La Sapienza
franco.bontempi@uniroma1.it

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Progettazione Strutturale Antincendio
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STRUCTURAL DESIGN BASIS

Structural Analysis and Design
• Structural Engineering is the discipline that responsibly deals with designing,
calculating, building, and managing valid artificial works - constructions - to
support the society and its members for the satisfaction of their needs and
desires, based on scientific methods, using appropriate techniques, adopting
correct heuristic evaluations.
• Structural Engineering activities can ideally be divided into activities of:
A) Structural Analysis
B) Structural Synthesis or Structural Design
• A schematic representation of these two activities is illustrated here:
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Structural Engineering Activities
• A) the analysis activity is a linear process, in which we start from the indication of
all the data of the problem, a calculation phase is developed, and results are
obtained.
• B) vice versa, the activity of structural synthesis / design is an iterative process -
initialized by a pre-dimensioning solution (indicated with the index K = 0) - in
which a refinement of the solution indicated by the index K is repeated several
times, until to reach, if possible, a solution, if not optimal, at least satisfactory; the
analysis activity is therefore nested in the overall design process, resulting in an
essential quantitative step to judge and refine the design solution.
• In fact, the structural design can be described more realistically as in Fig. 2, where
it is shown that the various and varied needs related to the implementation of a
work must be merged in a decision-making vortex [2].
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Future
• In addition to the decisional nature of the planning activity, it should be
noted that the main difficulty in this activity is predicting what the
construction will experience in its lifetime: this planning activity is extremely
complex and characterized by significant uncertainty, particularly in the case
of unexpected and extreme situations such as fire, and especially in the case
of extrapolation.
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Prescrittivo vs Prestazionale (1)
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Prescrittivo vs Prestazionale (2)
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FORECAST IN STRUCTURAL DESIGN

Classification of Structural Problems
• In general, Structural Engineering is characterized by the following types of
situations,
a. simple structural problems,
b. complex structural problems.
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Aspects that contribute to making a structural design problem simple or complex
• In the case of a design problem, one of two situations can be faced
depending on the following aspects:
1) intended use of the building, or activities and processes that can be
carried out there,
2) number and particularities of people who may be in the building or in its
vicinity,
3) environmental and social context in which the building is located,
4) dimensions and geometric characteristics of the building,
5) materials and technologies used,
6) actions that may affect the construction.
• The following Table presents some examples and considerations that can
illustrate what is implied by the previous points.
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Simplicity Complexity
1 Use
There are activities that present a regular
development without specific dangerous
characteristics.
Activities are carried out that have intrinsic
characteristics such as to cause or amplify
dangerous situations for people, goods, and
the environment.
2 Person There may be few people present.
A relatively considerable number of people
can be involved.
3 Contest
The construction is partially separated in
terms of space and functionality from the
context.
The construction is strictly connected to the
surrounding environment, in terms of
spatial proximity or functional insertion into
infrastructural systems.
4 Geometry
There are limited dimensions, regularity
characteristics and the presence of
symmetries.
The construction is characterized by large
dimensions, with irregular geometry and
lack of symmetries.
5 Technologies
Materials with well-known characteristics
and proven technologies are used.
There are innovative technological features
and advanced materials.
6 Actions
The actions are defined with certainty or
well characterized statistically, producing
well identifiable load conditions.
The scenarios in which the construction can
be found are not clearly definable or
framed, without an adequate statistical
basis, or there is a high degree of
uncertainty.

Note (1)
• A specific aspect of the fire action concerns its intrinsic characteristics of
accidentality.
• In general terms, the adjective accidental is meant to distinguish a situation
that cannot be excluded from the future during the life of the building but is
not predictable over time and quantifiable in magnitude with accuracy.
• In other words, it cannot be excluded (and in general it is not uncommon)
that the construction undergoes a situation in which, for example, a fire
develops whose temporal occurrence and whose intensity cannot be fully
identified, not even by a statistical point of view.
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Note (2)
• These characteristics are quite different from those of natural actions with
cyclical occurrence, such as wind or snow, and of anthropic ones, that is,
linked to the use of the building, which instead have fixed nominal
characteristics or known or determinable statistical properties.
• In particular, the loads related to their own weights, structural or not, can be
defined with certainty, while even the seismic action does not have
characters of such high uncertainty as accidental actions.
• In fact, in the case of earthquakes, a database and experimental evidence
have been collected over the years that can be framed in fairly settled
theories that allow, albeit with appropriate caution, a certain degree of
prediction in the mechanism and entity.
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General Approch
• In the case of fire, as in the case of all accidental actions, the general
approach of the structural problem must therefore consider this inevitable
great uncertainty in the prediction and quantification of the possible
scenario. In this regard, it may be useful to consider the general framework
represented in the following
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Setting the structural problem in a deterministic or stochastic framework as a function of complexity. HPLC
= High Probability - Low Consequences; LPHC = Low Probability - High Consequences.

Description
• The complexity of the design problem is indicated on the abscissa,
complexity that increases in the presence of non-linear manifestations (due,
for example, to mechanical behaviour in materials and structural in the
presence of instability phenomena) and interaction mechanisms (such as
those related to development the fire action as a function of the structural
response, the behavior of the active fire control systems and, last but not
least, the actions of the people present in the building or who came to the
rescue).
• The ordinate indicates the two possible settings of the analysis, namely a
deterministic formulation of the problem, with the precise assumption or
bounded by extremes of the parameters on which the analysis depends, or a
stochastic formulation based on probabilistic distributions of the same data.
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Events
• Furthermore, the previous figure indicates at the top the different regions in
which two categories of named events take place, respectively:
• HPLC - High Probability Low Consequences Events - Frequent Events with
Limited Consequences.
• LPHC - Low Probability High Consequences Events - Rare events with High
Consequences.
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Events HPLC LPHC
Energy release Low High
Number of failures Low High
Number of people involved Low High
Structural behaviour Linear Nonlinear
Interactions Soft Strict
Uncertainty Low High
Breakdown of the event High Low
Predictability of the event High Low

Events
• In general, accidental actions, and therefore also fire, belong to the latter
category. Table 2 summarizes, in broad terms, the peculiarities of the two
categories of events:
a. the first three lines consider the amounts of energy, breakdowns and
people that may be involved.
b. the next three lines describe the problems in terms of event analysis.
c. the last two refer to the ease or difficulty of predicting the dynamics of
the event.
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Intrinsic Nature of LPHC Events
• The intrinsic nature of LPHC events can lead to developments with cascading
effects, in which chains of circumstances follow one another in an
unpredictable way, giving rise to the so-called runaway situation as
illustrated in the following figure: the effect of the action goes beyond the
limits of the predictability framework.
• It is precisely this characteristic, this possible escalation of the event, which
can lead to catastrophic situations (referred to as Chinese syndromes, from
the homonymous film https://it.wikipedia.org/wiki/Sindrome_cinese.
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"China syndrome" is a
fanciful term that describes
a fictional result of a nuclear
meltdown, where reactor
components melt through
their containment
structures and into the
underlying earth, "all the
way to China".

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Possible runaway over time in the event of an LPHC event

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Fire Development in time

Black Swan Events
• In this consideration of unforeseen events and unpredictable effects, the
theme of the so-called Black-Swan Events has recently been added.
• These events are characterized:
A. from being singular events, outside the normal expectation as nothing from
past experience seems to have prefigured their possibility.
B. from having an extreme impact in terms of consequences on people, things,
the environment, the economy.
C. from being, despite their non-prediction and their unexpected occurrence,
once they have occurred, retrospectively conceivable and explainable.
• It is the latter affirmation, which leads to profound changes in the scientific
and technical community, such as that which occurred in Structural
Engineering following the attack on the World Trade Centre in 2001.
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Consequences
• With all these indications, the setting of the structural analysis and the
consequent construction safety checks in the event of accidental actions,
first of all the fire, cannot be fully carried out with similar settings to the
cases of actions related to the normal use or under normal environmental
and natural conditions.
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Setting the structural problem in a deterministic or stochastic framework as a function of complexity. HPLC
= High Probability - Low Consequences; LPHC = Low Probability - High Consequences.

Strategies (1)
• If we return to examine the previous picture, we note how the setting of the
structural problem changes with increasing complexity, in the presence of
actions that cannot be precisely defined such as fire.
• It starts with simple deterministic settings, applicable and effective in the
case of low complexity problems and HPLC-type events, then moving on to
more complicated probabilistic settings, more effective and exhaustive than
the first ones in the case of medium complexity, to finally return to
deterministic approaches.
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Strategies (2)
• The latter, when applied to highly complex problems and / or in the presence
of LPHC-type events, albeit apparently similar to those considered for the
simpler problems, require significant expert judgments or are heavily based
on heuristics accumulated over the years and in multiple situations.
• In the latter case, there is a need to identify, sometimes in a pragmatic way,
the scenarios in which the construction can be found in an accidental event.
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NTC 2005
• The Designer, following the classification and characterization of the actions,
must identify the possible contingent situations in which the actions can
challenge the work itself. To achieve this, it is defined:
I. the scenario: an organized and realistic set of situations in which the
work may be found during the useful life of the project.
II. the load scenario: an organized and realistic set of actions that
challenge the structure.
III. the contingency scenario: the identification of a plausible and
consistent state for the work, in which a set of actions (load scenario) is
applied to a structural configuration.
• For each limit state considered, load scenarios must be identified (ie
organized and coherent sets of actions in space and time) that represent the
combinations of actions that are realistically possible and probably more
restrictive.
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Fire
• In the case of fire action, the contingency scenarios in which the building
may be located can be identified, therefore, for example, through a matrix
that lists, in an orderly way, the different load situations in the columns and
the different structural configurations in the rows: in in this case, it is also
possible to highlight aspects relating to the state of the systems, or to the
functioning of the active control systems for fire action.
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Fire Scenarious
• In these terms, as seen in the following figure it is possible to highlight the
different conceivable scenarios for the construction in question, of which,
through the scrutiny of an expert judgment, the most important can be
selected (in the example considered, with a certain load condition called SIM
and with another one called EMIS, while the structural configurations are
indicated with NS and S).
• It is therefore possible, on the one hand, to try to reach the certainty of
having listed as exhaustively as possible all the possible combinations
(loading system - configuration of the structural system), and, on the other
hand, to have consciously selected those situations that are judged most
important-realistic-plausible for design purposes, reducing the burden of
analysis.
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Setting up contingency scenarios and their selection

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SCENARIOS IN THE CASE OF FIRE

Contingency Scenario
• In the case of fire, but not only, the contingency scenario is relevant, which
considers the different configurations in which a building may be located:
therefore, transitory situations must be considered, such as those related to
extraordinary maintenance, or those in which part of the plant systems (such
as detection and extinguishing systems) are partially or totally inefficient.
• It is common experience that transitory situations are those that most
frequently favour the fire event, resulting in the construction being more
vulnerable and more subject to dangerous activities in these phases.
Furthermore, attention should also be given to scenarios of chained
accidental actions (e.g. earthquake -> explosion -> fire, or impact -> fire) not
taken into consideration by European and Italian standards but actually
occurred not infrequently.
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Scenarios in case of fire according to ISO 13387.
• The following figure, taken from the Technical Report ISO 13387 on Fire
Safety Engineering, highlights the operational aspects of defining scenarios
in the event of a fire: we see here how, depending on several factors, the
number of possible scenarios grows combinatorically.
• Indeed, a precise assessment of the plausibility and representativeness of
these scenarios is therefore necessary to select a limited sample.
• This activity is called in English pruning, or pruning the tree of scenarios: for
example, of the 24 scenarios in the following figure, only the 7 cases
indicated with A, B, C, D, E, F, G have been extracted. and considered the
most relevant.
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Possible fire diffusion paths according to ISO 13387
• In identifying the fire scenarios, a crucial point is the prediction of what may
be the path of the fire inside the building or its temporal and spatial
development. In this regard, the picture shown in the following, again taken
from ISO 13387, is useful.
• In general, the greater the extent of the fire, the greater its destructive
potential.
• The control of the spread of the fire is therefore essential and can be
understood considering the following four levels:
I. the fire spreads in the room of origin.
II. the fire spreads to the adjacent rooms [a) - g)];
III. the fire spreads to the other floors [h) - m)];
IV. the fire spreads to other buildings [n), o)].
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I. The fire spreads in the room of origin (a)
• The spread of fire within the room of origin depends on the rate of heat
release of the object that begins to burn.
• Vertical and horizontal fire spread will be significantly increased if the room
is lined with combustible materials that are susceptible to rapid flame
spread on the walls and especially on the ceilings.
• The properties of interest are flammability, flame spread, and the amount of
smoke produced; these are often called fire hazard initiation properties or
fire reaction properties; these properties can be improved with the use of
special paints or specific treatments.
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II. Fire spreads to adjacent rooms (cases a,b,c,d,e,f,g)
• The spread of fire and smoke in adjacent rooms contributes significantly to
the development of fires and the spread of its lethal effects. The movement
of fire and smoke is highly dependent on the geometry of the building.
• If the doors are open, they can provide a path for smoke and toxic
combustion products to move from the top layer of the fire room to the next
room or corridor. Keeping the doors closed is therefore essential to prevent
the spread of fire from one room to another. Fire doors must be able to
maintain the containment function of the barrier in which they are located,
both for smoke control and fire resistance. Door latches that work
automatically when a fire is detected, therefore, very effective in significantly
increasing fire safety.
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II. Fire spreads to adjacent rooms (cases a,b,c,d,e,f,g)
• Hidden paths are one of the most dangerous means of spreading fire and
smoke. Hidden cavities are a particularly significant problem in older
buildings, especially if numerous new ceilings or partitions have been added
over the years.
• Fire can also spread to adjacent rooms by penetrating the surrounding walls.
Fire resistant walls must extend through the countertops to the floor or roof
above so that the fire does not spread by traveling through the hidden space
above the wall.
• The wall can be extended above the ceiling line to form a parapet, or the
ceiling can be fireproofed for some distance on either side of the top of the
wall.
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III. The fire spreads to other planes (cases h,i,j,k,l,m)
• Shafts and vertical staircases must be isolated from fire or separated from
the space occupied at each level to avoid producing a path for the spread of
fire and smoke from floor to floor. A particularly dangerous situation can
arise if there are interconnected horizontal and vertical hidden spaces, inside
the building or on the facade.
• This is particularly important in the construction of curtain walls where the
external panels are not part of the structure. Careful detail and proper
installation are necessary to ensure that the entire space is sealed, especially
at corners and intersections, to eliminate any path for the fire to spread.
Gaps like these between structural and non-structural elements are often
closed with suitably deformable fireproof materials to allow for seismic or
thermal movements.
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III. The fire spreads to other planes (cases h,i,j,k,l,m)
• Vertical fire spread can also occur outside the building envelope, through
combustible cladding materials or external windows.
• Vertical spread of fire from the window is a danger in multi-storey buildings.
This hazard can be partially controlled by keeping windows small and well
separated and by using horizontal overhangs that protrude from window
openings.
• Flames from small narrow windows tend to project farther from the building
wall than flames from long, wide windows, leading to a lower likelihood of
fire spreading.
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IV. Fire spreads to other buildings (cases n, o)
• Fire can spread from a burning building to adjacent buildings by contact with
the flame, by radiation from windows or by burning fragments.
• Fire spread can be prevented by providing a fire barrier or by providing
sufficient separation distances. If there are openings in the outer wall, the
likelihood of fire spreading greatly depends on the distances between
buildings and the size of the openings.
• Collapsing exterior walls can be a danger to firefighters and bystanders and
can lead to further spread of fire into adjacent buildings.
• The propagation of fire by contact with the flame is only possible if the
buildings are close enough to each other, while the fire spread by radiation
can also occur at distances of many meters. Finally, fire can also travel large
distances between buildings if combustible vegetation is present.
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The space-time development of the fire
• Being able to predict the space-time development of the fire is, therefore, a
critical aspect in the evaluation of safety. In the analytical and numerical
modelling of the scenarios in the event of a fire, the picture shown in the
following must always be remembered; it concerns the fundamental aspects
of the development of a fire scenario, in which the following are involved:
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i. The development of fire
properly understood as an
uncontrolled combustion
process, which requires in
terms of modelling the use of
numerical fluid dynamics
calculation codes (CFD).
ii. The transfer of heat from the
temperature of the ambient
gases to the structural parts
and within them, with the use
of the laws of heat
transmission.
iii. The evaluation of the
structural response with
thermo-mechanical behaviours
(material non-linearity - NLM /
material nonlinearity - MNL)
and considering large
displacements and large
deformations (ie taking into
account the so-called
geometric non-linearity - NLG /
geometric nonlinearity GNL).
iv. The human
behaviour, in
terms of
evacuation
processes and
opening of
escape routes
or cracking of
the
compartment
alization
system.

Multi-physical / multi-disciplinary fire modelling.
i. The development of fire properly understood as an uncontrolled
combustion process, which requires in terms of modelling the use of
numerical fluid dynamics calculation codes (CFD).
ii. The transfer of heat from the temperature of the ambient gases to the
structural parts and within them, with the use of the laws of heat
transmission.
iii. The evaluation of the structural response with thermo-mechanical
behaviours (material non-linearity - NLM / material nonlinearity - MNL)
and considering large displacements and large deformations (ie taking into
account the so-called geometric non-linearity - NLG / geometric
nonlinearity GNL).
iv. The human behaviour, in terms of evacuation processes and opening of
escape routes or cracking of the compartmentalization system.
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Human Behaviour
• This last aspect is a peculiar feature of the fire action. In fact, fire is the only
action whose development over time and space depends on concomitant
human behaviours: think, for example, of the event of the opening of a fire
door which involves the possible spread of compartment-to-compartment
fire, i.e., the influx of oxygen that revives the fire.
• The multiphysics-multidisciplinary nature of the study of a fire scenario is
therefore evident, as well as the intrinsic complexity of its development.
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Perrow
• Precisely on the notion of complexity of any system it is necessary and
appropriate to give further indications. In the following figure a scheme
known in literature [Perrow] is proposed in different forms, which considers
along two orthogonal axes those that can be understood as dimensions of
complexity; these dimensions are represented:
• A. the type of interactions,
• B. the type of connections between the various parts of a system,
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Dimensions of Complexity
• The dimensions represented:
A. the type of interactions,
B. the type of connections between the various parts of a system,
• allow to identify 4 quadrants containing recurring configurations.
• Complexity is considered increasing, both passing from linear interactions
(characterized by proportionality and therefore predictability) to non-linear
interactions, and passing from loose connections (i.e., systems with poorly
connected parts) to tight connections: the systems that can be represented
in quadrant 3 are characterized by (relative) simplicity, while the systems of
quadrant 2 are characterized by maximum complexity.
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Events HPLC LPHC
Breakdown of the event High Low
Predictability of the event High Low

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https://royalsocietypublishing.org/doi/10.1098/rsta.2011.0419

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STRUCTURAL QUALITIES

Qualities (1)
• From a strictly structural point of view, it can be said that the qualities such a
construction must have can be divided into two categories:
I. elementary qualities.
II. systemic qualities.
• This distinction is based, naturally, on the ease with which these properties
occur, and, in part, even earlier on how the structure must be designed to
have them. In simplified terms, to reach the former, reference can be made
to the structure divided into its individual elements, while this fragmentation
is not adequate for achieving systemic qualities.
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Qualities (2)
• A further consideration can be made with reference to qualities that refer to
the structure in its nominal configuration or to a damaged configuration.
• With all these considerations, and always considering the limits inherent in
this attempt at generalization, we can arrive at the picture represented in
the following figure.
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Overall framework of structural requirements

Structural behaviour (intensive characteristic)
• One way to consider the behaviour of a construction is to summarize the
response of the structure as the load increases, by drawing a diagram (load
on the structure - response of the structure).
• The simplest way is to represent how a significant structural displacement
varies as a function of the intensity of the applied load.
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Equilibrium Path
• In the following figure such a diagram is represented, which has a general
character; in fact, one can think of tracing how, for example, the arrow of the
end of a shelf varies according to the applied load, or one can think of
tracing the horizontal displacement of a building (drift) according to the
intensity of the action horizontal which causes it.
• This diagram is called the equilibrium path, because it can be thought of as
obtained by following the pixel on a monitor that follows in real time the
displacement of a point of the structure as a function of the increasing load.
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Elementary Structural Qualities
• We can ideally identify the elementary structural qualities:
1. stiffness (linked to the operating condition - serviceability);
• while the following three qualities are related to the ultimate conditions
(safety)
1. resistance or bearing capacity.
2. stability, noting the presence of a bifurcation point.
3. ductility, represented by the area under the curve even after reaching
the maximum bearing capacity.
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Systemic Structural Qualities
• If these are the elementary structural qualities, the aspect that characterizes
structural qualities such as durability, strength, and resilience as systemic, is
that for their correct analysis it is necessary to spatially and temporally
extend the horizon in which the structure is considered.
• In this regard, with the help of the following figure, the main structural
behaviours related to the three systemic qualities can ideally be introduced:
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Time Horizon of a Structure (1)
• On the vertical axis, the structural integrity is reported, understood as an
overall measure of the ability of a building to withstand the loads to which it
is subject, performing the functions for which it was built: the structural
integrity can therefore also be considered as coincident with the quality of
construction.
• The horizontal plane represents the time horizon; in fact, throughout its life,
a construction experiences two types of events:
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I. The first type considers events that take place continuously and that can
be represented along an axis along whose direction the construction
naturally loses continuously, if you want for thermodynamic reasons,
quality; here the causes of degradation are environmental, due to the
environment in which the structure is immersed (e.g., corrosion), or
anthropogenic, i.e., related to the use of the building (e.g., fatigue); in
these cases, we speak of durability.
II. The second type includes events which, on the other hand, have a discrete
nature: that is, they occur at very precise moments, being linked to
accidents, or accidental actions; in these cases, there are very precise
discontinuities in the structural quality, as these events appear as localized
in time and with special characteristics; in these cases, robustness must be
considered.
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• As can be seen from the previous figure, the generic construction develops a
precise trajectory over time according to the events that the construction
itself experiences.
• In particular, the problem arises over the life of the building, of having to
restore its integrity to an acceptable level: the ability of a building to recover
an adequate level of integrity is represented by resilience.
• This systemic characteristic will not be explored further on, but it is
interesting to note that it takes its cue from the definition given in
psychology, that is the ability of an individual to face and overcome a
traumatic event or a period of difficulty.
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STRUCTURAL ROBUSTNESS

Introduction
• The recognition of the specific nature of accidental actions and related
structural problems is relatively recent.
• Arguably, the starting point is the dramatic collapse of the 22-story Ronan
Point building in East London on May 16, 1968.
• Here, following a gas explosion on the eighteenth floor, there was the
collapse of a significant part of the building constructed with the assembly of
large, prefabricated panels. This event allowed the scientific and technical
community to reflect on:
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The
Downfall
of
British
Modernist
Architecture
-
Failed
Architecture

Points (1)
1. The concept of structural robustness: a structure possesses this property if
it shows damage proportional to the cause that caused it; in Ronan Point,
an explosion of limited intensity caused noticeably extensive damage to
the structure: the system with the assembly of large, prefabricated panels
was not, in this case, robust.
2. The progressive collapse mechanism: the breaking of an element spread
inside the structure in an uncontrolled way, in a way that is sometimes
called domino: in Ronan Point, the collapse of the floor of a floor caused
the collapse of the floor below and so on, up to the loss of all the floors
below that of the apartment where the explosion occurred.
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Points (2)
3. The impossibility of predicting all forms of damage and therefore the need
to provide forms of defense, that is, alternative and redundant load paths;
in general terms, these measures must preserve structural integrity.
4. The importance of construction details, as the propagation of the collapse
could be, if not avoided, limited by specific measures; this concerns the
conception and sizing of the connections between the different elements
and the different structural parts.
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“fifth amendment”
• These reflections materialized after a few months in 1970 in a heavy
regulatory rethinking in England, with the introduction of measures to make
the buildings robust. In fact, the so-called “fifth amendment” of the U.K.
Building Regulations in 1970, reads:
[it] applies to all buildings over four stories and requires that under
specified loading conditions a structure must remain stable with a reduced
safety factor in the event of a defined structural member or portion thereof
being removed. Limits of damage are laid down and if these would be
exceeded by the removal of a particular member, that member must be
designed to resist a pressure of 34 kN/m^2 (51 lb/〖in〗^2) from any
direction. Of special importance in relation to load bearing wall structures
is that these conditions should be met in the event of a wall or section of a
wall being removed, subject to a maximum length of 2.25 times the story
height.
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Eurocodes
• It is interesting to note that over the course of about twenty years, the
definition of structural strength has expanded to include - alongside
accidental actions such as fire, explosions, and impacts - also the effects
related to human error [12, 13]. In fact, in the basis of the Eurocodes in the
1990s, we find the definition:
"Robustness is 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."
• Furthermore, it is worth underlining that structural strength requires, from
an intensive point of view, a loss of load-bearing capacity that is regular and
proportional to the cause, while, from an extensive point of view, a limited
diffusion of the damage in the structure.
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Italy
• In Italy, from a regulatory point of view, only in 2005, in the so-called
Consolidated Text of Technical Construction Standards referred to in the
D.M. 14 September 2005, for the first time in Chapter 2 - Safety, expected
performance, actions on constructions, the concept of robustness was
introduced and, at the same time, in Chapter 4, accidental actions.
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Accidental vs. Exceptional (1)
• Subsequently, these concepts were represented in the Technical
Construction Standards of 2008, in which accidental actions were renamed
exceptional actions.
• This literal change, due to the fact of wanting to avoid ambiguity with the
Italian custom for which accidental loads were non-permanent loads that
can weigh on a structure (e.g., for a bridge, the load due to the transit of
pedestrians and of vehicles, while the load relative to the bridge's own
weight is permanent), denotes a certain improperness / inconsistency in the
identification of concepts.
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Accidental vs. Exceptional (2)
• In fact, the exceptional term should denote a situation in which an action
presents itself with an unusual value (e.g., a snowfall of one meter in Sicily),
while the adjective accidental should refer to a situation that occurs by
accident or chance ( e.g., the fire of a tanker): the term exceptional should
therefore refer to the intensity of the action, while the term accidental to
the intrinsic mechanism with which the action takes place.
• It is also necessary to reflect on the fact that, while the snowfall of one
meter in Sicily is statistically unlikely (outlier), the fire of a tanker is more
frequent, but occurs in a unique way.
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No redundancy

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Redundancy

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LOAD PATH

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Direct load path

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Undirect load path

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STRUCTURAL SYSTEM

Structural Organization and Breakdown
1. a load collection system, consisting of flat surfaces such as floors.
2. a system for transmitting loads to the ground, composed for gravitational
loads of vertical compressed elements (steel columns and reinforced
concrete pillars).
3. a stabilization system, necessary to absorb horizontal loads and to make
the overall system stable, consisting of bracing elements or walls.
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Note
• Broadly speaking, the stabilization system is the critical part of a structural
system, followed by the ground load transmission system, while the load
collection system is the least delicate. This means that for a steel structure
with a pendular upright resistive scheme, again in general terms and with
obvious exceptions, the most important elements are the bracing elements,
followed by columns and pillars, and finally by the beams.
• Finally, it is necessary to observe that the decomposition of the structural
system into its parts is essential to understand the real functioning of a
structure since, for example, detailed structural components, at small scales,
can significantly influence the overall behaviour of the entire system: this is
especially true for connections.
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Bernoulli regions and diffusive regions
• B-regions: these are regions of the structure in which simple deformation
behaviours occur (and therefore, at least in the linear elastic field, equally
simple stress states); to understand, there are elementary kinematic
hypotheses such as those relating to the theory of the Bernoulli-Navier
beam, in which the section rotates and translates while remaining flat; for
these regions, the analysis and verification procedures are classic and
univocal;
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Bernoulli regions and diffusive regions
• D-regions: these are regions of singularity within the structure, typically
where a) there are abrupt geometric variations (changes of centerline, of
section), b) changes of material (for example, between steel columns and
reinforced concrete beams); c) failure to comply with the hypotheses of
lengthening of the theory of beams (stubby brackets, column foundations,
...); d) application of concentrated loads or constraints; in all these nodal
regions, the state of deformation and that of stress are complex and cannot
be classified univocally in exact analytical theories.
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D-regions
1) these are nodal regions in which different structural elements converge
and are connected: the effectiveness of these connections is obviously
essential for the correct functioning of the construction as a whole, or for
structural integrity.
2) these regions have complex geometric configurations, with a strongly
three-dimensional character; this entails the possibility that the
connections themselves are not made correctly (think, for example, of the
difficulty of installing bolts); even any protections (paints or coatings) can
be difficult to install and in case of imperfections they constitute points of
entry for the heat of the fire which can be transmitted to the rest of the
element, affecting the protective properties
3) specific behaviours are concentrated in these regions, such as the so-called
semi-rigid knots in the case of steel-framed structures.
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Principio di De Saint Venant (DSV)
• I sistemi A e B di carichi applicati sul corpo sono
staticamente equivalenti, ovvero hanno le stesse
risultanti globali.
A B
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Principio di Saint Venant: trave a sezione compatta
A
B
C
b
F
F
C = F• b
F
F
2 • F
x
x
x
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B-/D- Regions (1)
A
B
C
D E
F
G
H
1
2
3
4 5
6
7
8
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B-/D- Regions (2)
A
B
C
D E
F
G
H
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Suddivisione della struttura in B- e D-Regions (1)
• L'analisi di un sistema strutturale complesso difficilmente può
essere condotto in un'unica fase.
• All'interno della struttura sono infatti presenti, generalmente, due
classi di regioni, che presentano comportamenti meccanici
qualitativamente differenti.
• Si possono infatti individuare le cosiddette:
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• B-Regions: regioni dove lo stato di sforzo è conseguente ad un
regime deformativo semplice (con andamenti lineari); la lettera B
deriva da Bernoulli, che individuò insieme a Navier l'ipotesi sul
comportamento delle sezioni delle travi che ruotano, restando
piane;
• D-Regions: regioni dove l'assenza di una cinematica semplice,
comporta stati di sforzo comunque complessi; si hanno quindi
regioni genericamente sedi di stati di sforzo diffusivi, da cui deriva
la lettera D.
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• Le D-regions sono tutte quelle zone di singolarità per la struttura,
ove si verificano discontinuità geometriche o di materiale, o dove
sono applicate forze concentrate, sia carichi che reazioni vincolari.
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Nota
a) Tutte le connessioni si configurano come D-Regions.
b) Il comportamento complessivo strutturale è il risultato
dell'integrazione (risultante a livello macroscopico) del
comportamento locale di tali D-Regions.
c) Il comportamento locale, in particolare la deformabilità locale,
può far emergere comportamenti inaccettabili.
d) Eventuali crisi locali possono mettere in pericolo la cosiddetta
integrità (robustezza) dell'intero organismo strutturale e ciò è
evitato da una accurata progettazione.
124
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STRUCTURAL INTEGRITY

Definition
• integrity s. f. [from Lat. integrĭtas -atis]. - 1. Being intact, whole, intact; the
state of a thing that possesses all its parts, its elements and attributes, which
preserves its unity and nature intact, or which has not suffered damage,
injury, quantitative or qualitative decrease: safeguarding the i. of the
national territory; check the i. seals, check that they are intact; return a text
to its i., when it has reached us mutilated or altered; defend the i. of the
language, to preserve it from contamination of foreign words and the like;
observe, apply the laws in their i., entirely, fully, without exceptions or
omissions; the. of a (human) body, being healthy, unharmed, capable of all
its functions; and with reference to the state of virginity of the woman: i. of
the hymen, i. virginal, or absol. integrity. 2. In a moral sense, being whole,
uncorrupted; honesty, absolute righteousness: i. of life, of customs; there. of
judges, witnesses, an official. Also, being intact, devoid of guilt or accusation:
i. of name, fame, honour.
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Strictly
• Structural integrity is the quality of a building of having all its parts organized
neatly and capable of developing their functions to ensure the safety of
people (and the environment) in addition to the performance provided for
the construction.
• By structural integrity we can therefore mean briefly both the set of all
structural qualities, and the single quality when appropriate. Obviously, it
will have to be considered how the quality varies for the structure starting
from its nominal configuration, new or as the Americans say brand new,
during the life of the same
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Structural Checks - A
A. operating conditions (concerning use - "use"), or situations that are
present during normal use and operation of the structure - usually
collected within the so-called Operating Limit States (S.L.E.); in terms of
probability density of occurrence f (D), these conditions occur frequently
precisely because they are connected to the use of the structure.
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Structural Checks - B
B. ultimate conditions (concerning safety - "safety"), or particularly
burdensome cases that can be expected, for example from an exceptional
trial, or from incorrect or even clumsy use of the construction - which flow
into the so-called Ultimate Limit States (ULS); in terms of probability
density of occurrence f (D), these conditions rarely occur.
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Structural Checks - C
C. extreme conditions (concerning integrity - "integrity"), in which the
construction may find itself, scenarios that are sometimes not fully
foreseeable or statistically characterized, in which the structure must
maintain a minimum of structural integrity, or not give rise to collapses
disproportionate to the triggering cause - conditions that converge in what
could be defined as Limit States of Integrity (SLI).
D. Events for which a classical statistical treatment of occurrence loses
significance, are the events referred to as "black swans - black swan
events": as seen previously, these are extremely rare events, which have
enormous consequences and which, in retrospect, appear perfectly
predictable.
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Framework of structural checks according to the distribution of demand D

Summary
A. operating conditions involve the more or less extensive limitation (even
total) of the use of the building, with obvious economic damage but
without consequences for people (except for special structures such as, for
example, hospitals).
B. ultimate conditions involve the crisis (rupture) of parts of the structure or
even of the structure as a whole, with repercussions on the safety of
people.
C. extreme conditions lead to significant collapses, even disastrous ones, with
the possibility of having serious repercussions on the safety of people and
on the environment.
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Crisis levels
• Level I - in one point of the structure the crisis of the material is reached: if
this is not fragile, this crisis can be overcome.
• Level II - in a section, the punctual crises accumulate and organize
themselves to bring a section of an element into crisis; if the system is
isostatic, the crisis of a section is fatal (at least one lability is formed), while if
the system is hyperstatic, a sectional crisis does not involve global collapse,
resulting in the most partial collapse.
• Level III - an element (an auction) of the system can collapse, typically due to
instability: here too, depending on how the structural system is organized, in
particular if it is hyperstatic, this level of crisis can be overcome, presenting
at most the character of a partial collapse.
• Level IV - the structure develops as a whole a generalized collapse or a global
collapse.
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Levels
i. punctual or sectional crisis: reaching a break in a point or section.
ii. local collapse: reaching the breakage of an element within a structure.
iii. partial collapse: collapse of a part (for example, several rods) of a
structure.
iv. generalized collapse (or global collapse): widespread and disastrous
collapse of the entire structure.
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Note
• If this is phenomenologically the sequence of crises which occurs in a
construction as the load increases, it is important to note that the current
regulatory approach, with the desire, even admirable, to simplify the
assessments, conventionally makes the achievement of the Ultimate Limit
State correspond i.e., the achievement of the maximum bearing capacity of
a structure, with the achievement of crisis levels:
I. punctual, if the material is fragile.
II. sectional, if there is a certain ductility.
III. element, in case of instability.
• This is certainly questionable in terms of realistic evaluations in ultimate
conditions, but it is even more so considering extreme situations, such as
those in which accidental actions such as fire are present, in which the
structural integrity must be fully evaluated, possibly residual.
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Furthermore
• A further reflection concerns the fact that even considering the same type of
crisis, for example, the local collapse of an element, this can have different
severity depending on how the supporting system of a building is organized.
• As exemplified, the local collapse of a single element obviously has different
consequences depending on the cases A, B and C: in all three cases, there is
the failure of a member, but the consequences are obviously different and,
as can be guessed, of increasing gravity passing from A to C.
• This point is not always made explicit and clarified in the normative
approaches, and perhaps it could not even be, since it is linked to the
understanding that the Designer must have of the structural system, its
organization, and its functioning, which are peculiar characteristics of the
specific structure.
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General aspects of the structural response

Equilibrium Path
• The relation R (Δ) = P geometrically represents the equilibrium path,
representing all situations of equilibrium between the external load applied
and the internal response provided by the structure.
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Equilibrium Path
• The OABCDE curve represents the mechanical response to the increase of
the generalized displacement Δ which has been chosen as significant:
through this representation, the structure, however complex, is reduced to a
generalized spring with kinematic degree of freedom Δ.
• The structural response is then seen as a recall force R (Δ), represented by
the OABCDE curve reported in ordinate. The external generalized load P
applied on the structure is also represented in ordinate.
• As an elementary case, it can be imagined that P represents a concentrated
force at the end of a shelf and Δ the lowering of this end. In general, there is
no such concrete association between the load parameter, which can simply
by a multiplier λ, and Δ which can be any significant deformation parameter
of the structure.
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Sensitivity to imperfections
• The previous response refers to an ideal structure, devoid of the following
imperfections:
1. global geometric imperfections, such as lack of verticality and lack of
straightness.
2. local geometric imperfections, such as lack of constancy of the overall
sectional characteristics (area, moments of inertia, ...) and local defects,
lack of parallelism between the wings or lack of orthogonality between
web and wings in an I-beam.
3. characteristics of homogeneous and constant materials within the generic
component.
4. absence of initial deformations and self-tensions: the structure is therefore
unloaded and undeformed at the beginning of the loading process
represented by point O.
• It is obvious that these imperfections can only deteriorate the structural response:
this is especially true in the presence of critical bifurcation points.
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Effects of imperfections

Imperfections
• In fact, now two bifurcation points, F and H, meet: secondary equilibrium
path branches off from them. These critical points can be imagined as
crossroads on the equilibrium path of the structure as the load varies: F is a
bifurcation point in the linear field, a case of Eulerian instability, while H is a
bifurcation point in a nonlinear field, or a case of instability not Eulerian.
• It is interesting from of an engineering point of view to consider that it must
always be assumed, in the presence of imperfections and in favor of safety,
that the structure in its progressive deformation follows the response that
has the least bearing capacity: in point F, the significant response therefore
follows the OFG path and not the OFA path, with a maximum response equal
to at most P_cr,, with respect to the P_max level.
• For the same considerations, if F did not exist, the structure would go up to
H, to develop the OAHI response, failing instead to realize the OAHB
response.
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Equilibrium Path in case of Fire
• The equilibrium path discussed so far refers to a situation in which, other
things being equal, only one load parameter increases.
• In the case of fire, on the other hand, the load is generally to be considered
as fixed and the temperature to which the structure is subjected to be
variable.
• It is therefore a quasi-static evolutionary situation, since, at least before the
final stages relating to the possible actual collapse, the situation does not
require considering inertial aspects.
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Equilibrium Path in case of Fire
• on the horizontal plane it is represented in an ideal form as the temperature
T to which a structure is subjected varies; therefore, we have the function T
= T (t);
• on the other hand, the resistance of the structure is shown vertically: for
t=0,T=T_0 (indicating the ambient temperature with T0), the resistance is the
“cold” resistance, i.e. of the structure at room temperature; this resistance
decreases along the temporal development of the fire due to the
degradation of structural materials with high temperatures. In this way, we
ideally have a parametric curve:
R(t,T)= R(t,T(t))=R(t).
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Evolution of structural resistance in terms of resistance to temperature variations

Note
• This representation is, in fact, significant: it indicates how the structural
resistance varies over time, as the fire progresses, and shows, in particular,
how the same resistance can decrease even after reaching the maximum fire
temperature: this indicates that in general, it is possible that the structural
collapse also takes place in the cooling phase of the structure or in the phase
of fire decrease.
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• in the resistance domain,
requiring that the resistance at the generic instant t be greater than the effect of the loads

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Failure

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• in the time domain,
requiring that the duration of fire resistance is greater than the duration of the fire event

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• in the temperature domain, requiring that the temperature reached (of "design") T_d by the structure
is lower than the critical temperatureT_cr of the same

Nature of the collapse
• In addition to the intensive character of the structural response, the
extensive character of the structural behaviour must be evaluated. This
concerns how the structure behaves as the load increases, or as the
temperature develops in the event of a fire, in spatial terms. We therefore
consider how the progressive deterioration of the structure develops, how
the various structural parts yield and how, in the end, the overall crisis
occurs. This review is part of the structural robustness assessments.
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Nature of the collapse
• In general terms, there are two categories of collapse: consider the case of in
which it is imagined that for the same load level, two structures can collapse
in the two ways illustrated.
• The first way involves an implosion of the structure within its perimeter, or
with parts of the structure that collapse exclusively on themselves.
• The second way involves the collapse occurring beyond the perimeter of the
structure, resulting in an unconfined process that can extend outside the
collapsing structure, involving other structures, objects, people, possibly
present to extinguish or control the fire, and finally the environment.
• The first way is a good collapse, the second bad.
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- Different types of structural collapse: implosive, i.e. confined, and explosive, i.e. unconfined.

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Windsor Hotel
• In this regard, we can consider the case of the fire of February 12, 2005, that
occurred in the Hotel Windsor in Madrid.
• The building consisted of a central core in reinforced concrete housing the
services, an external curtain of steel columns on the façade, and a free plan
consisting of a coffered attic. In the building there were two technical floors,
one at the base and one at mid-height, characterized by sturdy high beams
in reinforced concrete, which divided the building in two in height.
• The fire that developed in the upper part of the hotel caused the instability
of the steel columns of the external facade, and the consequent progressive
collapse of the floors. This progressive collapse, called pancake type, was
stopped by the technical floor at half height of the building: this technical
floor was characterized, as mentioned, by full height reinforced concrete
beams, without openings as visible, which acted as a barrier to the
downward progression of the collapse.
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FIRE SAFETY OF INDUSTRIAL HALL

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Controventi a croce di Sant’Andrea

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Controventi a K e a portali sovrapposti

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Controvento a talai con nodi rigidi

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Soluzione A

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Soluzione B

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Soluzione C

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Soluzione D

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Soluzione E

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https://constructalia.arcelormittal.com/en/solutions_by_performance/fire_safety

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https://www.cesdb.com/luca.html

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Structure and compartmentalisation of storage buildings

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STRUCTURAL DESIGN BASIS

PSA 2023 - chapter 1.pdf

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PSA 2023 - chapter 1.pdf