The Long Way towards a Sound Framework for Structural Design: 10 Years of Experience in Rome

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

[email protected], [email protected], [email protected],

[email protected], [email protected], 1StroNGER srl, Italy

2Department 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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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-baseddesign

Optimization and structural analysis

Criteria (new construction)

Risk analysis

Resilience

Sustainability

Robustness

DependabilitySafety

Serviceability

Redundancy

Criteria (existing construction)

Structural assessment

Historic buildings

Systems

Earthquake engineering

Wind engineering

Fatigue

Fire-safety engineering

Structural control

Structural Health MonitoringScenarios

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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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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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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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 ]...[ 21x , and m assigned limit states,

the following objective function is introduced:

m

i

iiF1

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

HPLCHigh Probability –

Low Consequences

LPHCLow Probability –

High Consequences

ComplexityNon linear issues and

interaction mechanisms

Des

ign

ap

pro

ach

:

Sto

chas

tic

Det

erm

inis

tic

QUALITATIVE RISKANALYSIS

PROBABILISTICRISK ANALYSIS

PRAGMATICANALYSIS OF

RISK SCENARIOS

Secondary

design

Primary

design

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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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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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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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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

parametersStructural parametersIntensity Measure ( )IM IP SP

EXCHANGE ZONE

z

y

x,x’

z’

y’

Waves

Mean wind

Current

P

(t)vP

(t)w P

(t)uP

Turbulent

windVm(zP)

P

H

h

vw(z’)

Vcur(z’)

116


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

117


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

118


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

119


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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The Long Way towards a Sound Framework for Structural Design: 10 Years of Experience in Rome

Engineering

Transcript of The Long Way towards a Sound Framework for Structural Design: 10 Years of Experience in Rome