Abstract: This paper focuses on the different conceptual frameworks that govern the structural problem and provides an insight on the results obtained from structural analysis, towards a sound framework for structural design. The interdisciplinary of many aspects is highlighted, considering the developments on the sustainable development and the architectonic design, and the availability of modern technologies that nowadays are integrated in the structural forms. The paper provides significant concepts and case studies (long span bridges, offshore wind turbines, high-rise buildings etc.), studied thoroughly in the last 10 years in the Sapienza University of Rome by the research group on structural analysis and design www.francobontempi.org. Keywords: Structural Engineering, Analysis, Design, Knowledge.

2. DCEE4
Proceedings of the
4th International Workshop on
Design in Civil and Environmental Engineering
Shang-Hsien (Patrick) Hsieh
Shih-Chung (Jessy) Kang
Editors

3. 4th International Workshop on
Design in Civil and Environmental Engineering
October 30TH
-31ST
, Taipei City, Taiwan
Organized by
National Taiwan University
Supported by
Ministry of Science and Technology, R.O.C.

4. Committees
Workshop Chairs
Shang-Hsien “Patrick” Hsieh
Shih-Chung “Jessy” Kang
Organizing Committee
Shang-Hsien “Patrick” Hsieh
Shih-Chung “Jessy” Kang
Hervé Capart
Shih-Yao Lai
Mei-Mei Song
Advisory Committee
Ren-Jye Dzeng
Bing-Jean Lee
Liang-Jenq Leu
Feng-Tyan Lin
Ching-Wen Wang
Pao-Shan Yu
International Advisory Committee
Franco Bontempi
Chris Brown
Tahar El-Korchi
Renate Fruchter
Timo Hartmann
Lotte Bjerregaard Jensen
Adib Kanafani
Giuseppe Longhi
Ashwin Mahlingram
Dominik Matt
Chansik Park
Ser Tong Quek
Mary Kathryn Thompson
Nicola Tollin
Nobuyoshi Yabuki
National Taiwan University
National Taiwan University
University of Rome “LA SAPIENZA”
Worcester Polytechnic Institute
Worcester Polytechnic Institute
Stanford University
Twente University
Technical University of Denmark
University of California, Berkeley
Master Processi Construttivi Sostenibili IUAV
Indian Institute of Technology Madras
Fraunhofer Italia Research
Chung-Ang University
National University of Singapore
Technical University of Denmark
Bradford Centre for Sustainable Environments
Osaka University
National Taiwan University
National Taiwan University
National Taiwan University
National Taiwan University
Tamkang University
National Chiao Tung University
Feng Chia University
National Taiwan University
National Cheng Kung University
National Chung Hsing University
National Cheng Kung University

5. Foreword
Design has always been an essential subject in Civil and Environmental Engineering
(CEE) education and practice but needs more attention as it deserves. Buildings and
civil facilities are meant for a long period of time of use and are greatly related to the
safety and welfare of human society. In recent years, the increasing frequency and
impact of natural disasters resulted from global climate change have demanded the
CEE design to address more on the disaster prevention/reduction and sustainability of
built environments. Obviously, CEE designers and engineers have to think beyond now
and into the future more than ever before.
I am very glad to have the opportunity to organize DCEE 2015 in NTU, Taipei, Taiwan,
following previous successful DCEE workshops hosted by KAIST, South Korea in 2011,
WPI, USA in 2013, and DTU, Denmark in 2014. We planned a pre-conference workshop:
“Sustainable City – A Hundred Years from Now”, facilitated by Prof. Pirjo Haikola
(Finland) and Prof. Mei-Mei Song (Taiwan), in hope to bring on some discussions one
step further into the future and it turned out to be an inspiring event that enriches all
participants’ thinking about our future cities. This year’s workshop features 3 keynote
speeches and 13 technical presentations by researchers from Japan, U.S.A., Denmark,
Italy and Taiwan. The presentations spanned a wide range of studies related to Design
in CEE, from environmental design, structural design, to engineering design education.
A mini-workshop was also organized for discussing the futures of DCEE. The
discussions were facilitated using Futures Thinking tools and fruitful outcomes from
the discussions were reported at the end of this proceedings.
I would like to thank all of the presenters, particularly the three excellent keynote
speakers, Prof. Hideyuki Horii from Japan (Designing Innovation Workshop: i.School
UTokyo), Prof. Eduardo Miranda from USA (Performance Based Design), Mr. Ying-Chih
Chang from Taiwan (Structural design for best integration with Architecture), and the
two professors, Profs. Haikola and Song, for facilitating the pre-workshop and mini-
workshop. My sincere thanks also go to my co-chair, the organizing committee,
international advisory committee, sponsors and all the participants and staff of the
workshop.
Finally, we are very much looking forward to the next DCEE Workshop to be held in
Sapienza University of Rome, Italy in October 6-8, 2016 and hopping that you will join
us for the continuation of important and interesting discussions on all aspects of
design in CEE.
Shang-Hsien (Patrick) Hsieh
Chairman, DCEE 2015 Organizing Committee
Professor, Department of Civil Engineering, National Taiwan University
June 20, 2016

6. The Long Way towards a Sound Framework for Structural
Design: 10 Years of Experience in Rome
Franco Bontempi*1,2
, Konstantinos Gkoumas1
, Stefania Arangio1,2
, Francesco Petrini1,2
,
Chiara Crosti1
franco.bontempi@uniroma1.it, konstantinos.gkoumas@stronger2012.com, stefania.arangio@stronger2012.com,
francesco.petrini@stronger2012.com, chiara.crosti@stronger2012.com,
1
StroNGER srl, Italy
2
Department of Structural and Geotechnical Engineering, Sapienza University of Rome, Italy
Abstract: This paper focuses on the different conceptual frameworks that govern the structural problem and
provides an insight on the results obtained from structural analysis, towards a sound framework for structural
design. The interdisciplinary of many aspects is highlighted, considering the developments on the sustainable
development and the architectonic design, and the availability of modern technologies that nowadays are integrated
in the structural forms. The paper provides significant concepts and case studies (long span bridges, offshore wind
turbines, high-rise buildings etc.), studied thoroughly in the last 10 years in the Sapienza University of Rome by
the research group on structural analysis and design www.francobontempi.org.
Keywords: Structural Engineering, Analysis, Design, Knowledge.
Introduction
Together with the realization of large-scale structural
and infrastructural projects in the last years, structural
design evolved as well in a profound manner. This is
because the complexity of this kind of structures,
related to several aspects, for example, their nonlinear
dynamic behavior, the presence of various sources of
uncertainties - both objective and cognitive - and the
strong interaction between components, necessitate
the necessary attention in the design phase. In the
above sense, the complexity of a system depends on
the number of elements from which it is composed, the
number of interactions among these elements, and the
convolution of the elements and interactions.
An elevated complexity can be identified in a
long span bridge (Arangio and Bontempi 2010
Bontempi 2006; Petrini et al. 2007; Petrini and
Bontempi 2011), in offshore wind turbines (Bontempi
et al. 2008, Petrini et al. 2010), in an industrial hanger
(Gkoumas et al. 2008), in long span parking structures
Crosti 2009), in high-rise buildings (Ciampoli and
Petrini 2012; Petrini and Ciampoli 2012, Milana et al.
2015). The complexity, is not a single outcome of the
structure itself, but an outcome of a system as a whole,
including issues related to performances, lifecycle,
loading conditions etc.
With the above in mind, it became clear in the
civil engineering community that structural design
methods and techniques from the past are no longer
adequate and new improved methods are necessary to
face the challenges of the future.
Aim of this paper is to bring forward, issues,
methods, trends and techniques that the research group
led by one of the authors (www.francobontempi.org)
encountered in the past 10 or more years, in the
structural analysis and design of complex structures.
All these are grouped in a reasoned manner following
the flowchart of figure 1. The correlation between
different aspects can be taken into account by applying
the principles and techniques of System Engineering,
which is a robust approach to the creation, design,
realization, and operation of a complex civil
engineered system (Bontempi et al. 2008).
What comes first (flowchart of figure 1, phase
one) is the general design and optimization, as an
outcome of detailed structural analyses. This is
completed by criteria for new or existing construction
(figure 1, phases two and three). The implementation
of systems developed in recent years helps improving
the reliability of the results and the confidence in the
design (figure 1, phase four). Furthermore, specific
scenarios are considered for tertiary design purposes,
e.g. to test the structural design under severe or
unforeseen events (figure 1, phase 5). Finally, an
aspect worth mentioning is the forensic investigation
of structures, a field in constant growth in the last years
(figure 1, phase six).
The sequence of the different phases is
determined by the sequence of different design needs
(e.g. phase 2: Criteria, rationally follows phase 1:
Theory and methods). However, these are reflected
also in the research activity maturated over the years
by the research group (e.g. phase 6: Forensic
engineering comes as the culmination of the
knowledge acquired in the previous phases) and in the
complexity of the system (phase 5: Scenarios is for the
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7. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
most part referred to complex structural systems, and
goes beyond standard design). In the following
paragraphs, the above mentioned concepts are
presented, using when possible, real case studies.
Adequate references are provided for the reader for
further inquiry.
The discussion of phases 3 and 5 is omitted for
the sake of brevity.
Figure 1. Methods, concepts, issues and techniques for structural design
Means for structural design
The above mentioned issues, methods, trends and
techniques, applied in different case studies, are shown
below.
Theory and methods
The theoretical framework for the design of complex
structural systems should be based on a
comprehensive evaluation of all the performances. In
this sense, the aim of structural engineering is not only
to achieve an ideally good design and a nominal
construction, but also to assure, by means of
appropriate maintenance, the long-term exploitation of
the system as a whole.
Organization and system decomposition
The first step in the process of solving a structural
problem is to hierarchically organize the entire
structural system. This is an important task since the
decisions taken by the designer are based on his
knowledge on the object of study. Figure 2 (from
Sgambi et al. 2012) shows the case of a long-span
suspension bridge where the entire structure is
hierarchically divided into substructures (macro-level),
components (meso-level), and finally (not shown in
the figure), elements (micro-level).
• The MACRO-LEVEL is related to a geometric size
comparable with the entire structure or with a
significant role in the structural behavior. The
different parts considered are identified as macro-
Theory and
Methods
Organization and
system decomposition
Performance-based
design
Optimization and
structural analysis
Criteria
(new construction)
Risk analysis
Resilience
Sustainability
Robustness
Dependability
Safety
Serviceability
Redundancy
Criteria
(existing construction)
Structural assessment
Historic buildings
Systems
Earthquake engineering
Wind engineering
Fatigue
Fire-safety engineering
Structural control
Structural Health
Monitoring
Scenarios
Existing actions
Fire and impact
Explosions
Forensic engineering
Responsibility
Numerical investigations
(historic structures)
Numerical investigations
(contemporary structures)
Back analysis
4
2
6
1
5
3
LP-HC events
Black swan events
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8. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
components. Essentially three systems are
identified:
(a) the main structural system, connected with the
main resistant mechanism, composed of the
following:
(i) supporting conditions: tower foundations,
towers, anchorages;
(ii) suspension system: saddles, main cables,
hangers;
(iii) bridge deck: highway box girders, railway
box girder, cross box girder;
(iv) special deck zones: inner (in proximity to
the towers), outer (at the end of the deck);
(b) the secondary system, related to the structural
parts directly loaded by highway and railway
traffic;
(c) the auxiliary system, related to specific
operations that the bridge can normally or
exceptionally face during its design life:
operation, maintenance and emergency.
• MESO-LEVEL is associated to the geometric
dimensions still relevant if compared with the
entire superstructure but connected with a specific
role in the macro-components; the parts considered
in this manner are identified as structures or
substructures.
• MICRO-LEVEL is linked to smaller geometric
dimensions with specialized structural role: these
are simply components or elements.
In accordance with this point of view, it is
possible to modify each variable and optimize the
structural behavior in order to achieve a required
performance level.
Figure 2. Bridge structural system decomposition
As figure 3 suggests, the essential role of the
structural breakdown is confirmed by the complexity
of the modelling/structural analysis of a cable
supported bridge (Petrini and Bontempi 2011).
Figure 3. Complexity of a structural system due to
nonlinearities, interactions and uncertainties: the case
of a long-span suspension bridge
Performance-based design
The general framework for the design of special
structures can be arranged with reference to the
scheme of figure 4, where the phases necessary for
finding in a positive approach the solution to the
design problem are shown:
Figure 4. Framework for the design of complex
structural systems: the case of a long-span bridge
a) definition of the structural domain, that is, the
bridge geometrical and material characteristics;
b) definition of the design environment where the
structure is located with specific attention to the
specifications of the:
i. environmental actions (principally,
wind/temperature and soil/earthquake);
ii. anthropic actions (related to pedestrian,
highway and train loads);
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9. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
c) assessment of the performances that can be attained
by the current structural design configuration,
resulting from accurate and extensive structural
analysis developed on models, both analytically or
experimentally;
d) alignment of expert judgments and emergence of
decisions about the soundness of the design, first in
qualitative and successively in quantitative terms;
e) negotiation and reframing of the expected
performances, in comparison with what has been
obtained by the analysis and with the knowledge
acquired working on the problem.
This scheme is identified as a Performance-
based design approach. It is worth observing two
features:
1) the influence of the problem formulation by
heuristics and experience and the acknowledgment
of the solution - essentially, only the engineering
deontology is capable to correctly address the
interest of all the stakeholders;
2) the central role of the numerical modeling, as the
exclusive knowledge engine capable of linking
together both the theory and experiment details, in
a truly comprehensive representation of the
problem and of its solution.
In order to quantify with the maximum possible
precision, the performance, and considering the
structural decomposition of figure 2, the meaning of
this subdivision is multifaceted:
a) First of all, the organization of the structure is
naturally connected with the load paths developed
by the structure itself. In this manner, the
subdivision helps the design team identify better
the role of each part of the structure.
b) Parts related to different levels of this organization
require different reliability thresholds. With regard
to structural failure conditions, this decomposition
allows single critical mechanisms to be ranked in
order of risk and consequences of the failure
mechanism.
c) There is a strong relationship between life cycle
and maintenance of the different parts: with
reference respectively to their structural function,
the required safety levels and their repairability,
structures and sub-structures are distinguished in
primary components (critical, non-repairable or
components that their repair may lead to the bridge
being out of service for a long period), and
secondary components (repairable with minor
restrictions on the operation of the bridge).
d) Regarding operative aspects, the entire structural
analysis can be subdivided in coordinated phases
as shown in figure 5, phases that indicate the
connection among different performance levels
and different design variables. The link is
established by efficient modeling, at different
linked structural scales, with the possibility that the
model outcomes at one level become the input for
another model at another scale.
Al these considerations can be summarized in the
scheme of figure 5, referring to the case of a long span
bridge.
Figure 5. Performance and variables from the
structural decomposition of a bridge
Optimization and structural design
For structural systems that show intrinsically
nonlinear behavior, an accurate description of the
response cannot be obtained without entering into the
nonlinear field. Consequently, the reliability
assessment of a structure belonging to such a class of
systems, cannot be definitely assured without
considering its actual nonlinear behavior. In this
context, thought the reliability of the structure as
resulting from a general and comprehensive
examination of all its failure modes, one must pay
attention to the following three aspects which define
the assessment process:
1. available data;
2. nonlinear analysis;
3. synthesis of the results.
That said, let p be a parameter belonging to the
set of quantities which define the structural problem
and  a load multiplier. It is clear that to each set of
parameters corresponds a set of limit load multiplier,
one of them for each assigned limit state. For sake of
simplicity, we can start by considering the relationship
between one single parameter p and one single limit
state defined by its corresponding limit load multiplier
. At first, it is worth noting that, in general, such
relationship is nonlinear even if the behavior of the
system is linear. This is typical of the design process
where the structural properties which correlate loads
and displacements are considered as design variables.
Thus, the nonlinear relationship (p) can be
drawn as in figure 6 (left), which shows that for each
value of p, there is a corresponding value of .
However, from figure 6 (right) it is also clear that the
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10. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
response interval [min max] corresponding to [pmin
pmax] cannot be simply obtained from (pmin) and
(pmax).
Figure 6. Relationship between a structural
parameter p and a limit state load multiplier  (left)
and interval of the limit state multiplier 
corresponding to an interval of the parameter p (right)
The problem of finding the interval response
can be instead properly formulated as an optimization
problem by assuming the objective function to be
maximized as the size of the response interval itself. In
particular, for the general case of n independent
parameter p, collected in a vector
T
nppp ]...[ 21x , and m assigned limit states,
the following objective function is introduced:
 

m
i
iiF
1
min,max,)( x
A solution x of the optimization problem which
take the side constraints into account is developed by
genetic algorithms, which are heuristic search
techniques which belong to the class of stochastic
algorithms, since they combine elements of
deterministic and probabilistic search (Michalewicz
1992). The search strategy works on a population of
individuals subjected to an evolutionary process where
individuals compete between them to survive in
proportion to their fitness with the environment. In this
process, population undergoes continuous
reproduction by means of some genetic operators
which, because of competition, tend to preserve best
individuals. From this evolutionary mechanism, two
conflicting trends appear: exploiting of the best
individuals and exploring the environment. Thus, the
effectiveness of the genetic search depends on a
balance between them, or between two principal
properties of the system, population diversity and
selective pressure. These aspects are in fact strongly
related, since an increase in the selective pressure
decreases the diversity of the population, and vice
versa (Biondini 1999).
Criteria (new construction)
Criteria for new construction, include attributes related
to the dependability of the structural system. After a
brief introduction of the term below, a number of them
are reported. Before that, an introduction to aspects of
risk analysis and of the system redundancy are
introduced.
Risk analysis
Nowadays civil engineering structures always bigger
and more complex are designed and build, making use
of particularly innovative methods and materials. The
innovation in all the phases of construction, the
uncertainty from the use of new and often non-
thoroughly tested materials, and the increasing
concern from the society regarding the risk involved
with these civil engineering infrastructures, calls for
an extensive risk analysis act. In fact, one can think of
no greater hazards and risks to society than the threats
to the functionality and survivability of critical
infrastructures, and the associated potential
catastrophic consequences (Haimes 1999).
A major contributing aspect for risk analysis
demand descends from the evolution of the society and
the tolerance of death: nowadays, there is a demand for
mortality risk reduction (e.g., risk at a construction
yard is simple unacceptable).
One particular aspect is the consideration of
complexity. As figure 7 suggests, for less complex
systems, a qualitative risk analysis is sufficient. As
complexity grows, the need of more adequate methods
is evident. This is also the case for HPLC (High
Probability/Low Consequence) events, which are
usually associated with a probability.
Figure 7. Design, complexity, and risk analysis
However, for very complex systems, where the
inherent complexity is large and the uncertainties are
many, a more appropriate method may be the
identification of pragmatic risk scenarios, especially
for LPHC (Low Probability/High Consequence) for
which it is impossible to associate a probability to their
occurrence. What stated above, is important also in the
design phase. QRA (Quantified Risk Analysis) and
PRA (Probabilistic Risk Analysis) are important in the
primary design, while, the consideration of pragmatic
risk scenarios is important in the secondary design
(Bontempi 2005)
Redundancy
Redundancy in structural design focuses mainly on the
human behavior (i.e. to the soft side of a general
p

p
p

HPLC
High Probability –
Low Consequences
LPHC
Low Probability –
High Consequences
Complexity
Non linear issues and
interaction mechanisms
Designapproach:
StochasticDeterministic
QUALITATIVE RISK
ANALYSIS
PROBABILISTIC
RISK ANALYSIS
PRAGMATIC
ANALYSIS OF
RISK SCENARIOS
Secondary
design
Primary
design
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11. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
solution process), rather than to material aspects (i.e.
to the hard aspects). In this sense, a robust numerical
solution can be achieved by working with different
solvers in parallel. The outcomes obtained by the
different solvers are then compared by a so-called
“elective” system, to converge to the final solution.
This attitude extends to consider different
persons, theories, and tools as automatic codes: for
example, figure 8 shows in a concise way the
distribution of use of the main different commercial
codes adopted for the structural analysis of a long span
bridge (Bontempi 2008a).
Figure 8. Use of commercial codes for the structural
analysis of a bridge
In this scheme, passing from left to right, there is
an increase of the specialization of the kind of analysis,
while the sizes of the circles are proportional to the
amount of use of the code. The comparison among
different codes and among different structural
configuration brings confidence to the design
structural configuration.
Dependability
Dependability is concisely defined as the grade of
confidence on the safety and on the performance of a
system. This is a qualitative definition that
comprehensively accounts for several properties,
which, even though interconnected, can be examined
separately. Adapting the conceptual organization
scheme conceived for the electronic and systems
engineering field (Avizienis et al. 2004) in the
structural engineering field, dependability can be
illustrated by dividing it in three different conceptual
groups (Arangio et al. 2011, Sgambi et al. 2012).
The first group deals with the properties that a
dependable structure should possess, commonly
referred as dependability attributes, related both to the
safety and the serviceability. The second group
concerns the external or internal threats that can harm
the dependability level of the structure. Finally, the
third group includes the dependability means, i.e. the
strategies and methods that can be followed in order to
achieve and maintain a dependable system.
As can be seen, dependability embraces several
issues, usually considered separately in the structural
design (figure 9), including safety and serviceability.
For additional details regarding the means to a
dependable design, the reader is referred to Bontempi
et al. (2007).
Figure 9. Dependability framework for structural
design
Safety
Concerning safety, the first problem arises from the
definition of the term, which is either referred to the
safety of people or to the integrity of the structure
(Bontempi et al. 2007). It is clear that the achievement
of such different goals (the first aiming to avoid people
injuries, the second focusing on the structural behavior
of the structure), requires to pursue completely
different means for the design conception. It seems
therefore more appropriate to define the term safety by
counterpoising it to that one of risk, the latter
quantitatively evaluated as the product between the
probability of occurrence of an event and the resulting
damage (Schneider 1997).
In the above sense, the safety of a structure is
intended as the quality of providing service with an
acceptable level of risk. It is important to observe
though that a probabilistic definition of the safety
requirement is not optimal when dealing with very rare
accidental circumstances potentially associated with
very severe consequences. These circumstances are
commonly associated with LP-HC events, such as
impact, explosion, fire and other malevolent attacks or
extreme natural disasters. Under these circumstances
either the assessment of risk associated to the event
and the definition of an acceptable level of risk can be
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12. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
challenging (Starossek 2009) and the consideration of
a broader set of system properties seems necessary in
order to evaluate the structural response. It appears
therefore appropriate to refer to the more
comprehensive concept of structural dependability and
define, for a dependable system, a set of attributes
related to the safety requirement as:
• INTEGRITY: the attribute is referred to the
absence of structural failure. This attribute
concerns therefore the structural state, in the sense
that the maximum grade of structural integrity is
related to the nominal configuration of the structure,
i.e. the undamaged one.
• RELIABILITY: the attribute is defined as the
probability that the structure will perform as
expected against environmental or anthropic
actions.
• SECURITY: the term is commonly related to the
vigilance and surveillance system, but in this
context, is more generally referred to the grade of
confidence on the structure with respect to
malevolent (intentional) attacks.
• ROBUSTNESS: the attribute refers to the ability of
a structure to maintain localized an initial damage
and avoid the propagation of failures in the system
(or, as defined in Starossek 2009, “insensitivity to
local failure”).
• COLLAPSE RESISTANCE: the attribute indicates
the ability of the structure to undergo exceptional
actions with the whole system remaining stable (or
as defined in Starossek 2009, “insensitivity to
accidental circumstances”).
• DAMAGE TOLERANCE: the term is referred to
the ability of the structure to absorb, continuously
in time, local damage of small severity, such as due
to material degradation or corrosion.
Serviceability
Complementing the safety performance, the
serviceability performance of the structure is intended
as the ability to provide correct service. The
serviceability of special structures such as a bridge is
also important for the duration of transitory situations,
e.g. during ordinary or extraordinary maintenance.
The following attributes can be considered:
• AVAILABILITY: is intended as readiness for
correct serviceability. This is a very important
property for structures with more than one
serviceability levels (e.g. a long span bridge, object
of this study).
• MAINTAINABILITY: is the ability to undergo
repairs and modifications. It is intended as the ease
with which maintenance can be performed in
accordance with the prescribed requirements.
• SURVIVABILITY: is intended as the ability of the
structural system to provide basic service in
presence of a failure. It is particularly important for
critical infrastructures and transportation networks
and for special structures such as military
constructions, power generation plants etc.
The above-mentioned attributes are non-
exhaustive since the dependability provisions are
referred to a system in operation, i.e. are related to the
function each structural system is meant for.
Robustness
Absence of catastrophic consequences and fault
tolerance are guaranteed by structural robustness
(Starossek 2009, Bontempi 2008b). This is the
capacity of the construction to undergo only limited
reductions in its performance level in the event of
departures from the original design configuration as a
result of:
(a) local damage due to accidental loads;
(b) secondary structural elements being out of
service for maintenance purpose;
(c) degradation of their mechanical properties.
Within a robust structure the damage is a
bounded damage and has no propagation, i.e. the entity
of damage is proportional to the amplitude of its cause.
Figure 10 suggests the different robustness response of
two different structural systems. System A is more
resistant then the system B when integer, but it is less
robust, since when the structures are damaged
structure B shows a lower decrement of the ultimate
resistance with respect to the structure A.
Figure 10. Qualitative robustness slopes of robust (b)
- non robust (a) structures
In general terms, the following
recommendations apply:
 appropriate contingency scenarios shall be
identified, i.e. scenarios of possible damage
together with suitable load scenarios;
 analyses shall be conducted in order to explore and
to bound structural safety and performance levels
of the structure in these conditions.
Sustainability
In the recent years, the construction sector is more and
more oriented towards the promotion of sustainability
in all its activities. The goal to achieve is the
optimization of performances, over the whole life
cycle, with respect to environmental, economic and
social requirements.
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Sustainability issues are wide-ranging, but the
main focus in the building industry is the reduction of
energy consumption in construction and use.
When evaluating the sustainability of structures,
the life cycle approach is required (Biondini et al.
2006), taking into account all phases of a building's
life, including material production, transportation to
the construction site, construction, operation,
demolition or deconstruction, and end of life.
One of the evocative structural design solutions
for sustainable tall buildings is embraced by the
diagrid (diagonal grid) structural scheme. Diagrid,
with a perimeter structural configuration characterized
by a narrow grid of diagonal members involved both
in gravity and in lateral load resistance, has emerged
as a new design trend for tall-shaped complex
structures, and is becoming increasingly popular due
to aesthetics and structural performance. Since it
requires less structural steel than a conventional steel
frame, it provides for a more sustainable structure. A
diagrid structure is modeled as a vertical cantilever
beam on the ground, and subdivided longitudinally
into modules according to the repetitive diagrid pattern.
Each module is defined by a single level of diagrids
that extend over multiple stories. Being the diagrid a
triangulated configuration of structural members, the
geometry of the single module plays a major role in
the internal axial force distribution, as well as in
conferring global shear and bending rigidity to the
building structure.
In a recent study (Milana et al. 2014), it has been
shown and quantified the way in which diagrid
structures lead to a considerable saving of (steel)
material compared to more traditional structural
schemes such as outrigger structures. Different diagrid
structures were considered (figure 11), namely, three
geometric configurations, with inclination of diagonal
members of 42°, 60° and 75°. These configurations, in
addition to allowing a considerable saving of weight,
guarantee a better performance in terms of strength,
stiffness and ductility.
Figure 11. Different diagrid FEM models
Resilience
The concept of resilience is present since the 70’s in
fields of study such as psychology and ecology. In the
civil and architectural engineering field, resilience is
present through the notions of “resilience of urban
areas” and “resilient community”, as introduced by the
Multidisciplinary Centre for Earthquake Engineering
Research - MCEER (MCEER 2006).
The approach has the potential to provide a
considerable contribution in lowering the impact of
disasters, and is implemented through the Resilience-
Based Design (RBD) for large urban infrastructures
(buildings, transportation facilities, utility elements
etc.), conceived as a design approach aiming at
reducing as much as possible the consequences of
natural disasters and other critical unexpected events
by developing actions that allow a prompt recovery
(Bruneau et al. 2003).
On this basis, Ortenzi et al. (2013) present and
apply a framework for the resilience assessment of
urban developments (figure 12).
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14. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
Figure 12. Resilience assessment framework
Systems
It is widely recognized that the most rational way for
assessing and reducing the risks of engineered
facilities and infrastructures subject to natural and
man-made phenomena, both in the design of new
facilities and in the rehabilitation or retrofitting of
existing ones, is Performance-based design.
Performance-Based design, nowadays typical in the
seismic design of structures and infrastructures, has
been extended in other engineering fields, in
particular:
- wind engineering;
- fire safety engineering;
- hurricane engineering.
Specific applications and methods are reported
below, together with provisions for fatigue
performance, and issues related to structural control
and monitoring.
Earthquake engineering
Bontempi (2008c), assess the safety and serviceability
performance under seismic action of a long span
suspension bridge, by means of detailed FEM models,
accounting for the asynchronous seismic action and
the possibility to have crustal displacements between
the pylons. Figure 13 shows a global frame model with
local shell model for the stress analysis (related to the
crustal displacements) and details of the deck.
Figure 13. Global frame model with local shell based
refined modeling for the stress analysis
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15. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
Wind engineering
Wind Engineering has appeared of great potential
interest for further developments of Performance
Based Design. In fact, PBWE - ‘‘Performance-Based
Wind Engineering’’was introduced for the first time in
2004 in an Italian research project, coordinated by Prof.
Ciampoli - must be tackled in probabilistic terms, due
to the stochastic nature of both resistance and loading
parameters. Uncertainties regard the environment, the
exchange zone and the structure (figure 14, left). The
environmental model can be extended also to account
for the wind-wave interaction in the case of offshore
structures (figure 14, right).
Ciampoli and Petrini (2012) apply the PBWE
procedure to the assessment of the comfort
requirement and the structural reliability for a 74
storey building.
The same authors carry out probabilistic
calculations of the structural response in frequency
and time domains, and calibrate the parameters of the
wind velocity field based on the time-histories of the
global floor forces derived by experimental tests on a
rigid 1:500 scale model of the building. The results of
numerical analyses suggest the use of a tuned mass
damper to enhance the building performance.
Petrini et al. (2012) propose a multi-level
approach for the design of offshore wind turbines.
They inquire on the effects on the structural response
induced by the uncertainty of the parameters used to
describe the environmental actions and the finite
element model of the structure, and adopt a shell FEM
model of the blade (figure 15) in order to obtain the
detailed load stress on the blade/hub connection.
Figure 14. Sources of uncertainty in Wind Engineering
Figure 15. Detailed FEM model of the blade
Structural Health Monitoring (SHM)
In recent years, structural integrity monitoring is a
paradigm that has become increasingly important in
structural engineering and in the construction
management field. It represents an influential and
effective tool for the structural assessment of existing
structural systems, integrating - in a unified
perspective - systems engineering and performance
based-design. Structural integrity monitoring issues
include performance, design environment and
structural breakdown, sensor systems and their
optimal placement, data transmission arrangement,
advanced signal processing techniques, state
identification methods and numerical model updating.
Bontempi et al. 2008 identify in a single chart
different phases of the SHM problem, extruded in a
third dimension, to take into account the complexity.
In this way, the various planes represent different
complexity levels (Z axis), while on the X and Y axis
are represented the phases of the lifespan of the
structure and the different implementations of the
monitoring process (figure 16, next page).
The interpretation of the data coming from the
monitoring process, i.e. the system symptoms, in order
to detect and diagnose a system fault is a complex task.
Arangio et al. (2011) provide a reference framework
for the SHM and structural identification of civil
structures. In particular, they adopt and implement a
monitoring process using soft computing algorithms,
ENVIRONMENT
Structure
Non environmental
solicitations
STRUCTURE
Structural (non-
environmental)
system
Site-specific
environment
Wind site basic
parameters
Other
environmental
agents
Wave site basic
parameters
Wind, wave
and current
actions
Aerodynamic and
Aeroelastic
phenomena
Hydrodynamic
phenomena
1. Aleatoric
2. Epistemic
3. Model
Types of uncertainties
1. Aleatoric
2. Epistemic
3. Model
1. Aleatoric
2. Epistemic
3. Model
Propagation Propagation
Interaction
parameters
Structural parametersIntensity Measure ( )IM  IP  SP
EXCHANGE ZONE
z
y
x,x’
z’
y’
Waves
Mean wind
Current
P
(t)vP
(t)wP
(t)uP
Turbulent
wind Vm(zP)
P
H
h
vw(z’)
Vcur(z’)
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16. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
with reference to a long span suspension bridge (figure
17).
Figure 16. The individuality of the monitoring
process
Figure 17. Identification of the possible damaged
elements and location of the measurement points
Structural control
The mitigation of extensive vibration or motion has
been a concern in the civil engineering field since the
early conception of complex structural systems (high-
rise buildings, long span bridges), characterized by
nonlinear behavior. The issue is to control how input
energy (from strong wind, earthquake, etc.) is
absorbed by a structure, by means of different methods
and techniques, as an alternative to conventional
design methods based on ductile response.
Most commonly implemented methods for
structural control include installing isolators or passive
energy dissipation devices to dissipate vibration
energy and reduce dynamic responses. In addition to
these, with the advent of advanced computational
methods (hardware and software), it is now possible to
alter the structural configuration for mitigating the
induced energy.
Bontempi et al. (2003) test and compare both
active and passive control systems for a Benchmark
Problem for controlled cable-stayed bridges on a long
span cable-stayed bridge with a central span of 350.6m
and lateral spans of 142.7m. The concern was to
explore seismic excitation in relation to bridges. In
particular, three different schemes of active control
were compared with each other, and their performance
was also compared with the two most widely used
passive control systems which summarize present
energy dissipation practice.
The response variables explored were the shears
and moments at the base of the central towers and of
the lateral piers, and the horizontal displacements of
the deck.
The authors conclude that for the specific bridge,
a passive system seems to be the most convenient
among the investigated solutions. This system supplies
values of internal action similar to the active system
and its realization is easier. In particular, the
availability of electric power supplies is not necessary,
and the use of electric power is not required during the
phase of control. This latter aspect also implies that the
supply of electric power to the system is ensured, even
during an earthquake.
Fatigue
Fatigue is a major issue for steel structures. In
particular, wind and traffic induced vibrations are the
main causes of fatigue damage in the cables and
hangers of suspension bridges. Due to the high
flexibility and reduced weight (in relation of the whole
dimension of the structure), the suspension cable
system of these type of bridges can experiment a great
number of tension cycles with significant amplitude
during their lifecycle.
Figure 18. Fatigue damages due to: (a) wind actions
(Vm=15 m/s) and (b) transit of freight train
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17. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
Petrini and Bontempi (2011) perform a fatigue
assessment on a long span suspension bridge,
considering the concurrent application of the
stochastic wind action and the train-bridge interaction.
One of the possible interaction mechanisms is directly
related to the interaction between fatigue due to wind
and fatigue due to train transit. Simply put, the sum of
fatigue damage due to wind and train is not equal to
the damage evaluated considering concurrently both
causes of fatigue (figure 18, previous page).
Fire safety engineering
The problem of structural fire safety in the recent years
has gained a predominant role in the engineering
design. This is because nowadays, always bigger and
more complex structures are designed and build,
making use of particularly fire sensitive materials such
as steel, and also, because there is an increasing belief
that structures not only have to resist to the design
loads, but to maintain a minimal performance in
accidental situations as well.
The use of fire safety engineering methods
significantly enhances the design process by adding
flexibility to the design parameters used in the project
such as occupant egress facilities, ventilation
requirements and material selection. Although at
present there is no internationally agreed definition of
Fire Safety Engineering (FSE), it can be defined as the
application of engineering principles, rules and expert
judgment based on a scientific understanding of the
fire phenomena, of the effects of fire and of the
reaction and behavior of people, in order to:
• save life, protect property and preserve the
environment and heritage;
• quantify the hazards and risk of fire and its effects;
• evaluate analytically the optimum protective and
preventative measures necessary to limit, within
prescribed levels, the consequences of fire.
In a FSE complying strategy, a number of
objectives are identified (safety of life, conservation of
property, continuity of business operations,
preservation of heritage, etc.). These (qualitative)
objectives must be characterized by setting specific
performance criteria. Regarding in particular safety of
life, the principal aim is to ensure the necessary time
for the safe evacuation.
The performance of the structure under fire can
be assessed with the implementation of analytical and
computational tools, tools that require a very good
understanding of the fire phenomenon.
Petrini (2013) discusses issues related to the
application of the PBFD in complex structures and
performs, by means of nonlinear FEM analysis, the
fire safety assessment of a helicopter hangar,
considering three different fire scenarios (figure 19).
Consequently, damage measures of the scenarios for
structural components of different hierarchies are
calculated.
Figure 19. Compared configurations of three
different fire scenarios
Gentili et al. (2013) investigate the
characteristics of the structural system that could
possibly reduce local damages or mitigate the
progression of failures in case of fire. They use a steel
high rise building as case study and they investigate
the response of the building up to the crisis of the
structure with respect to a standard fire in a lower and
in a higher story. Comparing the fire induced failures
at the different height allows highlighting the role
played in the resulting collapse mechanisms by the
beam-column stiffness ratio and by the loading
conditions.
Figure 20. Analyses outcomes
Figure 20 depicts deformed configurations after
90 min of fire at the 5th (top left) and 35th floor (top
right), the evolution of the axial force in the heated
column (column 15) and yield crisis (center left), the
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18. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
displacements of the mid-span of the heated column
(point L) at the 5th and 35th floor (center right), the
horizontal displacement (bottom left) of the top of the
external column (point N) and vertical displacement
(bottom right) of the top node of the heated column
(point M).
Forensic engineering
Forensic engineering is the application of engineering
principles to the investigation of failures or other
performance problems (ASCE, 2015). Forensic
engineering also involves testimony on the findings of
these investigations before a court of law or other
judicial forum, when required.
A forensic engineer should be able to identify
and explain the causes of the structural failures,
intending with “failure” not only the catastrophic
collapses that may even result in loss of life, but
including all those situations where there is an
unacceptable difference between the expected and
observed performance.
Responsibility
A useful tool for the investigation of an event is the
analysis of its timeline (see for example Arangio et al.
2013). For each phase, the attention is focused on
different aspects and different people are involved:
1. during the administrative practices, the technicians
of the public administration should verify the
feasibility of the intervention from the point of
view of the existing city plan;
2. during the design phase (considering both
architectural and structural design) the attention is
focused of the conception of the work;
3. during the realization phase, the attention is
focused both on the people that materially make the
works and on the managers of the site.
Starting from a timeline of events (figure 21
refers to the collapse of a historic building) it is
possible to arrive to the root of the collapse.
Figure 21. Example of the responsibility profile
Back-analysis
Back-analysis is an approach commonly used in
structural and geotechnical engineering.
Sebastiani et al. (2015) apply a general
procedure of back analysis, considering uncertainties,
aiming at identifying the causes of earthquake damage
patterns of bridges, on a viaduct damaged during the
April 6th
2009 L’Aquila earthquake. The bridge
consists of two distinct concrete decks that are
continuous over the piers, each with 12 spans for a
total length of about 460 m. The bridge was built at the
end of 70’s/start of 80’s and was not equipped with
seismic protection devices, so the decks are simply
supported by steel cylindrical bearings.
After the earthquake, the damage consisted of
bearings failure, with roller dislocation, up to complete
expulsion, breaking of deck joints and damage to the
concrete supporting blocks with significant permanent
displacements, and the transverse breaking of devices
that were not designed to resist those horizontal forces.
Drift displacements between the top of the piers and
deck reflecting the moment repartition, were
calculated, together with the hysteresis curves of the
pier nonlinear links.
Numerical Investigations (contemporary structures)
Crosti and Bontempi (2013) perform a forensic
investigation on the collapse of a temporary metal
structure for the entrainment industry, and highlight
issues that lead to the collapse: the inadequate
structural design and the improper construction
procedure.
Numerical Investigations (historic structures)
Forlino and Arangio (2015) perform a forensic
investigation for the assessment of a masonry building,
collapsed during the demolition of an adjacent
building. During the first steps of the demolition, the
adjacent buildings experienced large damages due to
the modification of the global structural behavior of
the aggregate. Despite the damages, the works
continued and at one point one of the damaged
building collapsed. The different stages of the
demolition are simulated by means of nonlinear FEM
analysis (figure 22).
Figure 22. Non-linear FEM analysis (left) and
collapse assessment (right)
Conclusions and prospects
This paper focuses on how the development of new
approaches in structural engineering based on
performance-based design, system engineering and
structural health monitoring, combined with the use of
new technologies and new software, allows the
responsability
time
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19. Fourth International Workshop on Design in Civil and Environmental Engineering, October 30-31, 2015, NTU
behavior of complex structural systems to be
appropriately assessed with a lower degree of
uncertainty.
The considerations made, are the outcome of the
experience obtained by the research group of Prof.
Franco Bontempi at the Sapienza University of Rome,
and therefore, provide a first-person personal
experience on the structural design. Through the
experience gained in structural design in all these years,
the group evolved and reached a maturity that lead, in
November 2012, to the creation of a research spin-off
by five of its senior members. This led to the
application of additional concepts. In particular,
nowadays the group is involved in two large-scale
research projects on the innovative topics of:
 Energy harvesting (using piezoelectric materials),
together with the ESA (European Space Agency)
 Vulnerability assessment of historic buildings (by
means of IT technologies) together with BIC
(Business Incubator Center) Lazio.
Furthermore, the group is extending its
competencies in recent trends in structural design, for
example:
 the implementation of Building Information
Modelling (BIM) in the structural design;
 crowd simulation for fire evacuation purposes;
 antrifragility as an extension to robust and resilient
design.
The group is committed to innovation in civil,
structural and environmental engineering in the years
to come.
Acknowledgements
This study presents methods, considerations and
results, developed in the last years principally by the
research group www.francobontempi.org. It is
partially supported by StroNGER s.r.l.
(www.stronger2012.com) from the fund “FILAS -
POR FESR LAZIO 2007/2013 - Support for the
research spin-off”. Furthermore, former group
members and the numerous undergrad and graduate
students of the research group are acknowledged for
their contribution to the group growth.
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