Handbook-1_heritage Indistrial Structures

>>> Handbook onSTRUCTURAL ASSESSMENT

OF INDUSTRIAL HERITAGE BUILDINGS

Miroslav Skora et al.

A/CZ0046/2/0013 Assessment of historical immovables www.heritage.cvut.cz

The project is supported by a grant from Iceland, Liechtenstein and Norway through the EEA

Financial Mechanism, the Norwegian Financial Mechanism and the Czech state budget.

HANDBOOK

STRUCTURAL ASSESSMENT OF INDUSTRIAL HERITAGE BUILDINGS

PARTNERSHIP: Czech Technical University in Prague, Klokner Institute:

Assoc. Prof. Jana Markov Prof. Milan Holick Dr. Karel Jung Dr. Miroslav Skora

Norwegian University of Live Sciences, Institute for Mathematics and Technology:

Prof. Thomas Thiis Ing. Andreas Flo Assoc. Prof. Knut Kvaal

Prague/Aas, August 2010

Czech Technical University in Prague, Klokner Institute olnova 7, 16608 Prague 6, Czech Republic ISBN 978-80-01-04609-8 Pages: 155

Foreword

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FOREWORD

The project Assessment of historical immovables Protection, conservation or renewal of historical immovables is becoming an

important task for art historians, architects and civil engineers in many European countries. Inevitable part of a preservation of many historical immovables, including heritage structures such as industrial buildings, bridges and folk architecture, is assessment of their reliability and design of adequate repairs taking into account the actual structural conditions and expected performance.

The research project A/CZ0046/2/0013 Assessment of historical immovables is aimed at developing the general methodology for the complex assessment of heritage structures with a particular focus on industrial buildings and bridges. The main goal of the project is to provide operational tools and background information for decision making concerning the protection, conservation, renewal and extended use of historical immovables. The primary target group includes researchers, designers, practicing engineers, cultural heritage management, local authorities and other specialists interested in preservation of industrial heritage. Outcomes of the project include:

- papers in prestigious journals - active participation in international conferences, - theoretical background documents for standardisation, - handbook, seminar and lectures for life-long education - software products and - project web sites .

In the period 2009-2010 the project is partly supported by the Research Support Fund (EEA Grants / Norway Grants), Czech state budget and by the partners.

Project partners

The project is based on the partnership of the Czech Technical University in Prague - Klokner Institute and the Norwegian University of Live Sciences - Institute for Mathematics and Technology.

Klokner Institute of the Czech Technical University in Prague, leader of the project, is a research institution with an outstanding position in the following fields:

- Theory of structural safety and risk assessment, - Structural diagnostics based on experimental mechanics, numerical analysis and

verification of numerical models, - Material research of concrete, steel, masonry and composite materials,

optimisation of material properties and determination of their functional characteristics,

- Experimental analysis of actual properties of existing construction materials, - Durability of structures, assessment of environmental degradation processes and

optimisation of interventions. So far the researchers of the Klokner Institute have participated (as leaders or co-leaders) in several international projects in the framework of the Copernicus, Leonardo da Vinci, Jean Monnet and Growth programs, in more than 30 projects supported by the Czech Science Foundation, in 4 research plans of the Ministry of Education, Youth and Sports of the Czech Republic and in several research projects of the Ministry of Transport and Ministry of Industry and Trade of the Czech Republic. More than 800 scientific publications have been elaborated and several patents have been registered in the framework of these projects during the last 5 years. Most of the projects and publications have been evaluated by reviewers as outstanding.

Foreword

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The Norwegian University of Live Sciences (UMB) comprises 8 departments. The University is recognised as a leading international centre of knowledge, focused on higher education and research within environmental- and biosciences. Together with other research institutes established at Aas, UMB provides state-of-the-art knowledge based on a broad range of disciplines. These include Applied Mathematics and Statistics, Physics, Spatial Planning, Environment and Natural Resources, Landscape Architecture, Civil Engineering and Building Science.

In total, UMB has about 2 600 students of which about 300 are PhD students. Annually, the University confers about 40 PhD degrees upon successful candidates. Of the 870 University staff, more than half hold scientific positions. Recent research projects include:

- Rural building heritage transformation of old rural buildings, - Building modelling and climatic adaptation of buildings, - Farm buildings in the arctic climatic adaptation of farm buildings.

Handbook The Handbook is focused on the complex methodology for structural assessment of

industrial heritage buildings. The following main topics are treated in particular: - Basis of assessment - Actions - Material and geometric properties - Deterioration models - Reliability analysis - Decision on construction interventions. In addition theoretical procedures are supplemented by several case studies provided

in Annex A. Annex B describes basic statistical concepts and techniques. The Handbook is written in a user-friendly way employing only basic mathematical tools.

A wide range of potential users of the Handbook includes practising engineers, designers, technicians, experts of public authorities and students.

Prague, 2010

Contents

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HANDBOOK STRUCTURAL ASSESSMENT OF INDUSTRIAL HERITAGE BUILDINGS

Page FOREWORD............................................................................................................................ 3 CONTENTS.............................................................................................................................. 5 I INTRODUCTION .............................................................................................................. 9

1. Industrial heritage............................................................................................. 10 2. Criteria for listing industrial heritage ............................................................... 10 3. Importance of protection .................................................................................. 12

3.1 Industrial heritage buildings 12 3.2 Industrial heritage bridges 12

4. Initiatives concerning the industrial heritage ................................................... 13 4.1 Initiatives on an international level 13 4.2 Initiatives in the Czech Republic 14

References .................................................................................................................... 16 II BASIS OF ASSESSMENT ............................................................................................. 17

1. Introduction ...................................................................................................... 18 2. Principles.......................................................................................................... 18 3. Investigation..................................................................................................... 21 4. Basic variables ................................................................................................. 22 5. Structural analysis ............................................................................................ 23 6. Verification ...................................................................................................... 23 7. Assessment in case of damage ......................................................................... 25 8. Report. .............................................................................................................. 25 References .................................................................................................................... 27

III ACTIONS....................................................................................................................... 29

1. Introduction ...................................................................................................... 30 2. Actions and effect of actions............................................................................ 30

2.1 Definition of actions 30 2.2 Effect of actions 30

3. Classification of the actions ............................................................................. 31 3.1 General 31 3.2 Variation in time 31 3.3 Origin 31 3.4 Variation in space 32 3.5 Nature or structural response 32 3.6 Bounded and unbounded actions 32

4. Reference period and distribution of extremes ................................................ 32 4.1 Climatic actions 32 4.2 Imposed loads 33

5. Characteristic values ........................................................................................ 35 5.1 General 35 5.2 Permanent actions 35 5.3 Variable actions 37 5.4 Imposed loads 37 5.5 Snow loads 38

Contents

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5.6 Wind actions 38 5.7 Thermal actions 41

6. Representative values of actions ...................................................................... 41 6.1 General 41 6.2 Combination value of a variable action 42 6.3 Frequent value of a variable action 42 6.4 Quasi-permanent value of a variable action 42

7. Representation of the dynamic actions ............................................................ 42 8. Representation of fatigue actions ..................................................................... 44 9. Probabilistic models of actions ........................................................................ 44

9.1 Permanent actions 44 9.2 Imposed loads 45 9.3 Snow loads 47 9.4 Wind actions 47 9.5 Time-dependency of the climatic actions 48 9.6 Model uncertainties of load effects 48

References .................................................................................................................... 50 IV MATERIALS AND GEOMETRY............................................................................... 53

1. Introduction ...................................................................................................... 54 1.1 Background materials 54 1.2 General principles 54

2. Characteristic values of material properties ..................................................... 54 2.1 General 54 2.2 Determination of the characteristic values 55

3. Estimation of the characteristic and design material properties....................... 56 3.1 Estimation from a theoretical model 56 3.2 Estimation from limited experimental data 59

4. Probabilistic models of material properties...................................................... 63 4.1 Strengths 63 4.2 Model uncertainties of resistance 64

5. Geometrical data .............................................................................................. 65 5.1 General 65 5.2 Probabilistic models of geometrical data 66

References .................................................................................................................... 68 V DETERIORATION ........................................................................................................ 71

1. Introduction ...................................................................................................... 72 2. Overview of weathering effects on building materials .................................... 73

2.1 Environmental effects 73 2.2 Pollution effects 76

3. Overview of damage functions ........................................................................ 78 3.1 Stone recession 79 3.2 Concrete 80 3.3 Steel 82

References .................................................................................................................... 84 VI RELIABILITY ANALYSIS ......................................................................................... 89

1. General principles ............................................................................................ 90 2. Uncertainties .................................................................................................... 90

Contents

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3. Reliability ......................................................................................................... 91 3.1 General 91 3.2 Definition of reliability 91 3.3 Probability of failure 92 3.4 Reliability index 93

4. Reliability verification ..................................................................................... 93 4.1 Deterministic methods 93 4.2 Probabilistic methods 94 4.3 Probabilistic updating 101

References .................................................................................................................. 103 VII DECISION ON CONSTRUCTION INTERVENTIONS....................................... 105

1. Design of construction interventions ............................................................. 106 2. Target reliability levels .................................................................................. 106 3. Principles of the total cost minimisation........................................................ 107 4. Simplified estimation of failure cost .............................................................. 108 References .................................................................................................................. 110

VIII CONCLUDING REMARKS................................................................................... 113 ANNEX A - CASE STUDIES.............................................................................................. 117

1. Textile mill ..................................................................................................... 118 1.1 Probabilistic reliability analysis 118 1.2 Cost optimisation 119

2. Steel roof ........................................................................................................ 120 2.1 Deterministic verification 120 2.2 Probabilistic reliability analysis 121 2.3 Parametric study 122

3. Steel beam ...................................................................................................... 123 4. Assessment of concrete strength .................................................................... 124 5. Masonry strength............................................................................................ 125

5.1 Motivation 125 5.2 Evaluation of tests 125 5.3 Masonry strength in accordance with present standards 129 5.4 Probabilistic analysis 130 5.5 Concluding remarks 132

References .................................................................................................................. 133

ANNEX B - BASIC STATISTICAL CONCEPTS AND TECHNIQUES ...................... 135 1. Introduction .................................................................................................... 136

1.1 Background materials 136 1.2 General principles 136

2. Population and samples.................................................................................. 136 2.1 General 136 2.2 Sample characteristics 136 2.3 Distribution function 137 2.4 Population parameters 138

3. Selected models of random variables............................................................. 140 3.1 Normal distribution 140 3.2 Lognormal distribution 141

Contents

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3.3 Gamma distribution 144 3.4 Beta distribution 146 3.5 Gumbel and other distributions of extreme values 148 3.6 Function of random variables 152

References .................................................................................................................. 153 Appendix 1 - Probabilistic models of basic variables Appendix 2 - Statistical parameters of functions of random variables

I INTRODUCTION

Chapter I - Introduction

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According to the Convention concerning the protection of the world cultural and natural heritage, adopted in 1972 by UNESCO, the following shall be considered as cultural heritage:

Monuments: architectural works, works of monumental sculpture and painting, elements or structures of an archaeological nature, inscriptions, cave dwellings and combinations of features, which are of outstanding universal value from the point of view of history, art or science,

Groups of buildings: groups of separate or connected buildings which, because of their architecture, their homogeneity or their place in the landscape, are of outstanding universal value from the point of view of history, art or science,

Sites: works of man or the combined works of nature and man, and areas including archaeological sites which are of outstanding universal value from the historical, aesthetic, ethnological or anthropological point of view.

1. INDUSTRIAL HERITAGE

A number of factories, warehouses, power plants and other industrial buildings, built since the beginning of the Industrial Revolution in the second half of the eighteenth century, has been registered as industrial cultural heritage in the Czech Republic and abroad. Such structures are mostly of significant architectural, historic, technological or social value [1]. They often form part of the urban landscape and provide the cityscape with visual historical landmarks. However, insufficient attention seems to be paid to systematic recognizing, declaring and protecting the industrial heritage in many countries including the Czech Republic. This is an alarming situation as the lack of attention and awareness of the industrial structures may gradually lead to their extinction [2].

When out of use, the industrial heritage buildings are degrading and often turning into ruins. Re-use and adaptation of such buildings allow for integration of the industrial heritage buildings into a modern urban lifestyle and help protect cities cultural heritage [2]. These buildings are often adapted to become hotels, museums, residential parks, commercial centres etc. Several examples of successful reconversions are shown in Figs. I-1 to I-3.

The handbook attempts to provide the framework for complex reliability assessment of such structures. In addition the handbook is aimed to increase awareness of the high architectural and cultural significance of the industrial buildings and bridges, indicating positive influence of their preservation on the sustainable development and promoting discussion among experts on sustainable use of the industrial heritage structures.

2. CRITERIA FOR LISTING INDUSTRIAL HERITAGE [6]

According to the Industrial Buildings Selection Guide [6] key issues to address when considering industrial structures for designation include:

The Wider Industrial Context. Industrial structures should be considered in their wider setting as a part of the system where each element (building) plays its role.

Regional Factors. Regional perspective in the selection of buildings and sites should be considered to achieve a representative sample for each sector of an industry. It also requires the identification of regional specialism, and the study of survivals related to these industries.


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Fig. I-1. School building [3].

Fig. I-2. Residential houses in Manchester [4].


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Fig. I-3. Residential houses in Wien [5].

Integrated Sites. If the process to which a building is related involved numerous components, then the issue of completeness becomes overriding. For instance an exceptionally complete site (perhaps with water systems and field monuments as well as buildings) may provide such an exceptional context that it raises the importance of buildings that might otherwise not be listable.

Architecture and Process. An industrial building should normally reflect in its design (plan form and appearance) the specific function it was intended to fulfil.

Machinery. The special interest of some sites lies in the machinery. Where the machinery makes a building special, the loss of this will reduce its eligibility for listing.

Technological Innovation. Some buildings may have been the site of the early use of important processes, techniques or factory systems (e.g. coke-based iron production, mechanised cotton spinning, steam power applied to pumping etc.). Technological significance may also reside in the building itself rather than the industrial process it housed, e.g., early fire-proofing or metal framing, virtuoso use of materials etc. The works of noteworthy wheelwrights or engineers will be of equal importance to major architects.

Historic Interest. Where physical evidence of important elements of industrial history survives well, a high grade may be justified. In some cases historical association with notable achievements may be sufficient to list: much will depend on the force of the historical claims, and the significance of the persons or products involved at the site in question.

Rebuilding and Repair. In assessments for listing, a high level of reconstruction is sometimes the basis for a decision not to list. With industrial buildings, partial rebuilding and repair is often related to the industrial process and provides evidence for technological change that may in itself be significant enough to warrant protection and alteration can thus have a positive value.

Note that specific industries such as engineering works, factories, mines, mills, warehouses may be valuable for different features that have to be taken into account.

3. IMPORTANCE OF PROTECTION

3.1. Industrial heritage buildings Protection (including adaptations and re-use) of the industrial heritage structures is an

important issue of the sustainable development. More specifically, it has been recognised in [1,7-9] that the protection and re-use may positively contribute to the sustainable development by:

Preservation of the cultural values and identity of locations,


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Recycling of all potential resources and avoiding wasting energy, making use of existing infrastructures,

Facilitating the economic regeneration of regions in decline that may provide psychological stability for communities facing a sudden high rate of unemployment.

It follows that the protection has considerable ecological and social contexts that are becoming even more important due to the global shortage of energy, economic crisis and environmental protection. The re-use of the industrial heritage is thus no longer an isolated issue, but is of general significance and may directly represent policy of the sustainable development. The protection of historical sites and the full use of limited resources deserve considerable public attention and participation.

The following critical factors to assure the sustainable use of the industrial heritage buildings include [10]:

Multidisciplinary approach to analysis and decision-making, Iterative, incremental process, Public-private partnerships, Begin with the end (end-user goals and end goals for environment, economy and

society), Integration of the natural, remediated and built environments, Life-cycle economics (assessing long-term costs and benefits and the value of non-

economic benefits).

3.2. Industrial heritage bridges About 350 bridges are included in the Czech Register of the Industrial Heritage. Road

bridges have become an essential part of cities transport infrastructure. However, unfavourable environmental effects and ever-increasing traffic loads may yield severe deterioration of existing road bridges. Therefore, rehabilitation of bridges is presently an urgent issue of bridge engineers and responsible authorities in many European countries. Present expenditures on maintenance and rehabilitation are limited and seem to be inadequate [11].

4. INITIATIVES CONCERNING THE INDUSTRIAL HERITAGE

4.1. Initiatives on an international level The protection of the industrial heritage is a multidisciplinary topic including

historical, architectonic, civil engineering and ecological aspects. First initiatives aimed at research and conservation of industrial buildings started in 1950s on an amateur basis [7]. In 1973 the Society for Industrial Archeology was established and the First International Congress on the Conservation of Industrial Monuments took place. When the third congress was held in 1978, the International Committee on the Conservation of the Industrial Heritage (TICCIH) was founded to promote preservation, conservation, investigation, documentation, research and interpretation of industrial heritage. This wide field includes the material remains of industry - industrial sites, buildings and architecture, plant, machinery and equipment - as well as housing, industrial settlements, industrial landscapes, products and processes, and documentation of the industrial society. Members of TICCIH come from all over the world and include historians, conservators, museum curators, researchers, students, teachers, heritage professionals and anyone with an interest in the development of industry and industrial society.


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The recent cooperation of TICCIH and the International Council on Monuments and Sites (ICOMOS) has resulted in registration of more than 40 industrial sites in the World Heritage List, such as Vlklingen Ironworks in Germany shown in Fig. I-4.

4.2. Initiatives in the Czech Republic In the Czech Republic numerous industrial heritage structures including structures of

railways infrastructures, breweries, sugar factories, and other industrial structures were built in the period from 1870 to 1930. Due to historical reasons, views of Czech architects and civil engineers on protection of the industrial heritage are often considerably different [4] and an important issue may then be to achieve consensus on significance of the heritage value to be preserved. To provide a desired coordinating platform for experts in the fields of historical-structural research, monument conservation, and the reconstruction of the industrial heritage buildings and sites, the Research Centre for Industrial Heritage has been established as an independent research institution of the Czech Technical University in Prague. The Centre - the Czech local representative in TICCIH - compiles findings from various research fields, maintains a database of the Czech industrial monuments (containing more then 10 000 monuments) and seeks for new uses of the industrial heritage buildings.

Fig. I-4. Vlklingen Ironworks in Germany.

To promote discussions among experts from various fields, the international conference Vestiges of Industry (www.industrialnistopy.cz) is organised every two years. On


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the occasion of the 3rd biennial conference held in 2005, the international cooperation relating to the conservation, documentation, promotion and interpretation of a common European industrial heritage was declared. The activities which deserve special attention include:

The promotion of education, knowledge and a deeper understanding of the industrial heritage by conferences, seminars and educational programmes,

The evaluation and conservation of industrial heritage, The conversion of industrial heritage to new uses as a positive form of cultural

potential with the objective of revitalising industrial regions, towns and brownfields in decline.

The declaration is available on the conference web sites. In 2009 the Czech Technical University in Prague and the University of Applied

Sciences in s (Norway) launched the research project Assessment of historical immovables described in the Foreword.


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REFERENCES

[1] TICCIH: The Nizhny Tagil Charter for the Industrial Heritage, Nizhny Tagil: The International Committee for the Conservation of the Industrial Heritage, 2003. .

[2] Luferts, M. & Mavunganidze, J.: Ruins of the past: Industrial heritage in Johannesburg. In Proc. STREMAH XI, ed. C.A. Brebbia, Ashurst Lodge: WIT Press, 2009, pp. 533-542.

[3] de Bouw, M., Wouters, I. & Lauriks, L.: Structural analysis of two metal de Dion roof trusses in Brussels model schools. In Proc. STREMAH XI, ed. C.A. Brebbia, Ashurst Lodge: WIT Press, 2009, pp. 121-130.

[4] Fragner, B.: Pstupy k zchran prmyslovho ddictv v esk republice (Approaches to protection of the industrial heritage in the Czech Republic - in Czech), Stavebnictv Vol. IV, No. 01/2010 (2010), pp. 16-18.

[5] Fragner, B.: Brownfield v souvislostech prmyslovho ddictv (Brownfield in relation to industrial heritage - in Czech), Vesmr Vol. 84, No. leden 2005 (2005), pp. 58-60.

[6] English Heritage, Heritage Protection Department: Industrial Buildings Selection Guide. March 2007, English Heritage, 2007, p. 16.

[7] Zhang, S.: Conservation and adaptive reuse of industrial heritage in Shanghai, Frontiers of Architecture and Civil Engineering in China Vol. 1, No. 4 (2007), pp. 481-490.

[8] Schneider, B. & Osika, K.: Inspired by the past, built for the future conversion of a former substation, a monumental listed building. In Proc. CESB10, eds. P. Hjek, J. Tywoniak, A. Lupek, J. Rika and K. Sojkov, Prague: Grada Publishing, 2010, pp. 5.

[9] Skora, M., Holick, M. & Markov, J.: Advanced assessment of industrial heritage buildings for sustainable cities development. In Proc. CESB10, eds. P. Hjek, J. Tywoniak, A. Lupek, J. Rika and K. Sojkov, Prague: Grada Publishing, 2010, pp. 9.

[10] Brebbia, C. A. & Beriatos, E. (eds.): Brownfields IV, Ashurst Lodge: WIT Press, 2008.

[11] Markov, J., Holick, M., Skora, M. et al.: Assessment of Bridges Registered as Industrial Heritage. In Proc. 5th Int. ASRANet Conf. Anonymous ASRANet Ltd., 2010, pp. 10.

II BASIS OF

ASSESSMENT

Chapter II - Basis of assessment

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1. INTRODUCTION

Assessment of industrial heritage buildings and bridges preceding reconversions and re-use of these structures is becoming a more and more important and frequent engineering task. General principles of sustainable development regularly lead to the need for extension of the life of a structure, in majority of practical cases in conjunction with severe economic constraints. That is why the assessment of industrial heritage structures often requires application of sophisticated methods, as a rule beyond the scope of traditional design codes.

The new European standards EN Eurocodes are intended for the design of new structures. Supplementary rules for the verification of existing structures including those registered as the industrial heritage are still missing. A new Part of the EN Eurocodes on the assessment and retrofitting of existing structures is to be prepared as indicated within a medium-term strategy of the development of the EN Eurocodes [1]. This document is intended to cover the following topics:

Methodology of collecting, evaluating and updating data, Recommendations for the verification applying the partial factor method and/or

using directly the probabilistic methods consistent with EN 1990 [2], Target reliability level of existing structures taking into account the residual life time,

consequences and costs of safety measures, Assessment based on satisfactory past performance, Recommendations concerning intervention and report.

General requirements and procedures for the assessment of existing structures based on the theory of structural reliability are provided in the International Standard ISO 13822 [3]. For practical applications, information on properties of various construction materials may be provided in National Annexes. Supplementary guidance can also be found in ISO 2394 [4], ISO 12491 [5], and in numerous publications [6-9]. Specific issues related to heritage structures are detailed in [10].

2. PRINCIPLES AND GENERAL FRAMEWORK OF THE ASSESSMENT

In general industrial heritage structures are subjected to the reliability assessment before reconversions due to:

Rehabilitation of an existing structure during which new structural members are added to the existing load-carrying system,

Adequacy checking in order to establish whether the existing structure can resist loads associated with the anticipated change in use of the facility, operational changes or extension of its design working life,

Repair of an existing structure, which has deteriorated due to time-dependent environmental effects or which has suffered damage from accidental actions,

Doubts concerning actual reliability of the structure - it has been recognised that many heritage structures do not fulfil requirements of present codes of practice.

Under some circumstances, assessments may also be required by authorities, insurance companies or owners, or may be demanded by a maintenance plan. However, it appears that insufficient attention has been paid by experts to specific issues of the reliability assessment of industrial heritage structures so far. Differences between the assessment of industrial heritage structures and design of new structures that should be considered in the assessment include:

Social and cultural aspects - loss of cultural and heritage values,


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Economic aspects - additional costs of measures to increase reliability of a heritage structure in comparison with a new structure (at a design stage cost of such measures is normally minor while cost of strengthening is much higher),

Principles of the sustainable development - waste reduction and recycling of materials (these aspects may be more significant in case of the assessment),

Lack of information for the assessment - commonly, testing of the properties of materials is difficult, expensive, but also very important due to variability of properties and changes that may have occurred during the working life of a structure (influence of deterioration and damage).

As a consequence, it is often required to minimise construction interventions and use original materials in rehabilitation and upgrades of industrial heritage structures. Effects of the construction process and subsequent life of the structure, during which it may have undergone alteration, deterioration, misuse, and other changes to its as-built (as-designed) state, must be taken into account. However, even though an existing structure may be investigated several times, some uncertainty in behaviour of the basic variables shall always remain. Therefore, similarly as in design of new structures, actual variation of the basic variables describing actions, material properties, geometric data and model uncertainties are taken into account by partial factors or other code provisions.

Two main principles are usually accepted when assessing industrial heritage structures:

1. Currently valid codes for verification of structural reliability should be applied, original codes valid in the period when the structure was designed should be used only as guidance documents.

2. Actual characteristics of construction materials, actions, geometric data and structural behaviour should be considered, the original design documentation including drawings should be used as guidance documents only.

The first principle should be applied in order to achieve similar reliability level as in case of newly designed structures. The second principle should avoid negligence of any structural condition that may affect actual reliability (in favourable or unfavourable way) of a given structure. It follows from the second principle that a visual inspection of the assessed structure should be made whenever possible. Practical experience shows that inspection of the site is also useful to obtain a good feel for actual situation and state of the structure.

As a rule, the assessment need not to be performed for those parts of the existing structure that will not be affected by structural changes, rehabilitation, repair, change in use or which are not obviously damaged or are not suspected of having insufficient reliability.

In general, the assessment procedure consists of the following steps (see Fig. II-1): Specification of the assessment objectives required by the client or authority, Scenarios related to structural conditions and actions, Preliminary assessment, Study of available documentation (design documentation, construction methods and

events that might have altered structural behaviour), Preliminary inspection, Preliminary checks, Decision on immediate actions, Recommendation for detailed assessment, Detailed documentary search, Detailed inspection, Material testing, connections, effects of deterioration, determination of actions,

irreversible deflections, Determination of structural properties,


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Requests/Needs

Specification of the assessment objectives and plan

Scenarios

Preliminary assessment- Study of documents and other evidence- Preliminary inspection- Preliminary checks- Decisions on immediate actions- Recommendation for detailed assessment

Detailed assessment?

- Detailed documentary search and review- Detailed inspection - Material testing and determination of actions- Determination of properties of the structure- Structural analysis- Verification of structural reliability

Further inspection?

Construction- Repair- Upgrading- Demolition

Reporting results of assessment

Judgement and decision

Intervention

Operation- Monitoring- Change in use

- Periodical inspection

- Maintenance

Sufficient reliability?

No

No

Yes

Yes

No

Yes

Fig. II-1. General flow of the assessment of industrial heritage structures.


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Structural analysis, Verification of structural reliability, Report including proposal for construction intervention.

The sequence is repeated if necessary. When the preliminary assessment indicates that the structure is reliable for its intended use over the remaining life, a detailed assessment may not be required. Conversely, if the structure seems to be in dangerous or uncertain condition, immediate interventions and detailed assessment may be necessary.

3. INVESTIGATION

Investigation of an industrial heritage structure is intended to verify and update the knowledge of the present condition (state) of a structure with respect to a number of aspects. Often, the first impression of the structural condition will be based on visual qualitative investigation. The description of possible damage of the structure may be presented in verbal terms like unknown, none, minor, moderate, severe, or destructive. Very often the decision based on such observation will be made by experts in a purely intuitive way.

A better judgement of the structural condition can be made on the basis of (subsequent) quantitative inspections. Typically, the assessment of existing structures is a cyclic process, when the preliminary inspection is supplemented by subsequent detailed investigations. The purpose of the subsequent investigations is to obtain better knowledge about the actual structural condition (particularly in the case of damage) and to verify information required for determination of the characteristic and design values of important basic variables. For all inspection techniques, information on the probability of detecting damage, if present, and the accuracy of the results should be given.

The statement from the investigation contains, as a rule, the following data describing Actual state of the structure, Types of construction materials and soils, Observed damage, Actions including environmental effects, Available design documentation.

A proof loading is a special type of investigation. Based on such tests one may draw conclusions with respect to:

The bearing capacity of the tested member under the test load condition, Other members, Other load conditions, The behaviour of the system.

The inference in the first case is relatively easy; the probability density function of the load bearing capacity is simply cut off at the value of the proof load. The inference from the other conclusions is more complex. Note that the number of proof load tests need not to be restricted to one. Proof testing may concern one member under various loading conditions and/or a sample of structural members. In order to avoid an unnecessary damage to the structure due to the proof load, it is recommended to increase the load gradually and to measure the deformations. Measurements may also give a better insight into the behaviour of the system. In general, proof loads can address long-term or time-dependent effects. These effects should be compensated by calculation.

A diagnostic test may be used to verify or refine analytical or predictive structural models. Diagnostic testing attempts to explain why the structure is performing differently than assumed. The disadvantage of this method, as compared with the proof loading, is that the results are determined for service loads and need to be extrapolated to ultimate load levels.


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When the structural damage is small or it is in the interior of the system, its detection should not be carried out visually. A useful tool is then dynamic testing (e.g. horizontal or vertical vibration testing of structural members or a whole structure) that is based on the fact that the damage or loss of integrity in a structural system leads to changes in the dynamic properties of the structure such as natural frequencies, mode shapes and damping. Dynamic measurements can give information on the position and severity of the damage that occurred. Generally, the eigenfrequencies decrease while the damping increases.

4. BASIC VARIABLES

In accordance with the above-mentioned general principles, characteristic and representative values of all basic variables shall be determined taking into account the actual situation and state of the structure. Available design documentation is used as a guidance material only. Actual state of the structure should be verified by inspection to an adequate extent. If appropriate, destructive or non-destructive inspections should be performed and evaluated using statistical methods.

For verification of the structural reliability using partial factor method, the characteristic and representative values of basic variables shall be considered as follows:

1. Dimensions of the structural elements shall be determined on the basis of adequate measurements. However, when the original design documentation is available and no changes in dimensions exist, the nominal dimensions given in the documentation may be used in the analysis.

2. Load characteristics shall be introduced considering the values corresponding to the actual situation verified by destructive or non-destructive inspections. When some loads were reduced, or removed completely, the representative values can be reduced or appropriate partial factors can be adjusted. When overloading has been observed in the past and no damage has occurred, it may be appropriate to increase adequately the representative values. This may be important in particular for industrial buildings. For industrial heritage structures with significant permanent actions, the actual geometry should be verified by measurements and the characteristic values of weight densities of materials should be obtained from statistical evaluation of test results.

3. Material properties shall be considered according to the actual state of the structure verified by destructive or non-destructive testing. In many cases it may be appropriate to combine limited new information from inspection and tests with prior information available from previous experience (material properties known from long-term production, performance of similar structures with similar exposure levels). Bayesian techniques [5], [6] and [11] provide a consistent basis for this updating. Analytical or predictive approaches used to determine structural resistance may be overly conservative due to neglected system effects, load redistribution etc. In these cases, proof, diagnostic or dynamic load tests may help to update information on structural properties [8]. When the original design documentation is available and no serious deterioration, design errors or construction errors are suspected, the characteristic values given in original design may be used.

4. Model uncertainties shall be considered in the same way as in design stage unless previous structural behaviour (especially damage) indicates otherwise. In some cases model factors, coefficients and other design assumptions may be established from measurements on the existing structure (e.g. structural stiffness). Thus, the reliability verification of an existing structure should be backed up by

inspection of the structure including collection of appropriate data. Evaluation of prior


23

information and its updating using newly obtained measurements is one of the most important steps of the assessment.

5. STRUCTURAL ANALYSIS

Structural behaviour should be analysed using models that describe actual situation and state of an existing structure. Generally the structure should be analysed for ultimate limit states and serviceability limit states using basic variables and taking into account relevant deterioration processes.

All basic variables describing actions, material properties, load and model uncertainties should be considered as mentioned above. The uncertainty associated with the validity and accuracy of the models should be considered during assessment, either by adopting appropriate factors in deterministic verifications or by introducing probabilistic model factors in reliability analysis.

When an industrial heritage structure is analysed, conversion factors reflecting the influence of shape and size effect of specimens, temperature, moisture, duration-of-load effect, etc., should be taken into account. The level of knowledge of the condition of structural members and their connections should be also considered. This can be achieved by adjusting the assumed variability in either the load carrying capacity of the members or the dimensions of their cross-sections, depending on the type of structure.

When deterioration of a structure is observed, the deterioration mechanisms shall be identified and a deterioration model should predict changes in structural parameters due to foreseen structural loading, environmental conditions, maintenance practices and past exposures, based on theoretical or experimental investigation, inspection, and experience. Examples of unfavourable environmental effects and defects of structures due to degradation, accepted with modifications from [12], are listed in Tab. II-1.

6. VERIFICATION

Reliability verification of an industrial heritage structure shall be made using valid codes of practice, as a rule based on the limit state concept. Attention should be paid to both the ultimate and serviceability limit states. Verification may be carried out using partial safety factor or structural reliability methods with consideration of structural system and ductility of components. The reliability assessment shall be made taking into account the remaining working life of the structure, the reference period, and changes in the environment in the vicinity of the structure associated with an anticipated change in use.

The conclusion from the assessment shall withstand a plausibility check. In particular, discrepancies between the results of structural analysis (e.g. insufficient safety) and the real structural condition (e.g. no sign of distress or failure, satisfactory structural performance) must be explained. It should be kept in mind that many engineering models are conservative and cannot be always used directly to explain an actual situation.

The target reliability level used for the verification can be taken as the level of reliability implied by acceptance criteria defined in design codes. The target reliability level shall be stated together with clearly defined limit state functions and specific models of the basic variables.


24

Tab. II-1. Examples of unfavourable environmental effects and defects of structures due to degradation.

Defects of structures due to degradation Unfavourable environmental

effects Concrete Structural steel, aluminium, iron Masonry Timber

Erosion Cracking Fatigue cracking Scaling,

spalling and delamination

Splitting

Abrasion Reinforcement corrosion Fracture cracking Falling-out of units Decay

Scour Honeycombing Corrosion Cracking Deterioration

of impregnants

Weathering Scaling Friability Elongated bolt holes

Wetting Spalling Disintegration of mortar

Corrosion of metallic

connectors Leaks Delamination Detachment

Efflorescence Disintegration Corrosion of

metallic connectors

Vegetation Alkali-silica reaction Peeling of

mortar coating

Freeze-thaw Breaking-away Deformation

Deterioration of protective coatings Deflections

Damage to mortar coatings Stratification Deformation Deflections

The target reliability level can also be established taking into account the required performance level for the structure, the reference period and possible failure consequences. In accordance with ISO 2394 [4] the performance requirements for assessment of existing structures are the same as for design of a new structure. Lower reliability targets for existing structures may be used if they can be justified on the basis of economical, social and sustainable consideration (see Annex F to ISO 13822 [3]).

An adequate value of the target reliability index should be, in general, determined considering appropriate reference period [3]. For serviceability and fatigue the reference period equals the remaining working life, while for the ultimate limit states the reference period is in principle the same as the design working life specified for new structures (50 years for buildings). This general approach should be, in specific cases, supplemented by detailed consideration of the character of serviceability limit states (reversible, irreversible), fatigue (inspectable, not inspectable) and consequences of ultimate limit states (economic consequences, number of endangered people).

If the structure does not satisfy the reliability requirements, the construction interventions may become necessary. Decision-making concerning construction interventions may be based on a cost-benefit analysis. Note that particularly in case of heritage structures, the use of original materials is the preferable in design of rehabilitation or repairs.


25

A common approach is a step-by-step assessment from a crude visible inspection to a detailed structural reliability analysis. Several levels of numerical assessment can be distinguished:

Linear analysis and verification, Non-linear analysis, System reliability analysis.

It is recommended to start from a very simple level with a little effort and crude assumptions, and systematically increase the level of detail. If the structure did not pass the requirements of the codes, a more advanced numerical assessment method has to be applied.

7. ASSESSMENT OF A DAMAGED STRUCTURE

For the assessment of a damaged structure, the following stepwise procedure is recommended:

1. Visual inspection. It is always useful to make an initial visual inspection of the structure to get a feel for its condition. Major defects should be reasonably evident to an experienced eye. In the case of very severe damage, immediate measures (like abandonment of the structure, immediate structural interventions) may be taken.

2. Explanation of observed phenomena. In order to be able to understand the present condition of the structure, one should simulate the damage or the observed behaviour, using a model of the structure and the estimated intensity of various loads or physical/chemical agencies. It is important to have available documentation with respect to design, analysis and construction. If there is a discrepancy between calculations and observations, it might be worthwhile to look for design errors, errors in construction, etc.

3. Reliability assessment. Given the structure in its present state and given the present information, the reliability of the structure is estimated, either by means of a failure probability or by means of partial factors. Note that the model of the present structure may be different from the original model. If the reliability is sufficient (i.e. better than commonly accepted in design), one might be satisfied and no further action is required.

4. Additional information. If the reliability according to step 3 is insufficient, one may look for additional information from more advanced structural models, additional inspections and measurements or actual load assessment.

5. Final decision. If the degree of reliability is still too low, one might decide to: accept the present situation for economical reasons; reduce the load on the structure; repair the building; demolish the structure.

The first decision may be motivated by the fact that the cost for additional reliability is much higher for an industrial heritage structure than for a new structure. This argument is sometimes used by those who claim that a higher reliability should be generally required for a new structure than for an existing one. However, if human safety is involved, economical optimisation has a limited significance.

8. FINAL REPORT AND DECISION

The final report on structural assessment and possible interim reports (if required) should include clear conclusions with regard to the objective of the assessment based on


26

careful reliability assessment and cost of repair or upgrading. The report shall be concise and clear. A recommended report format is indicated in Annex G to ISO 13822 [3].

If the reliability of an existing structure is sufficient, no action is required. Otherwise appropriate interventions should be proposed. Temporary intervention may be recommended and proposed by an engineer if required immediately. The engineer should indicate a preferred solution as a logical follow-up to the whole assessment in every case.

It should be noted that a client in collaboration with a relevant authority should make the final decision on possible interventions, based on engineering assessment and recommendations. The engineer performing the assessment might have, however, the legal duty to inform a relevant authority if a client does not respond in a reasonable time.


27

REFERENCES

[1] CEN/TC 250 Structural Eurocodes & JRC (Joint Research Centre): The Eurocodes and the construction industry (medium-term strategy 2008 2013). January 2009.

[2] EN 1990: Eurocode - Basis of structural design, Brussels: CEN, 2002.

[3] ISO 13822: Bases for design of structures - Assessment of existing structures, Geneve, Switzerland: ISO TC98/SC2, 2003.

[4] ISO 2394: General principles on reliability for structures, 2nd edition, Geneve, Switzerland: ISO, 1998.

[5] ISO 12491: Statistical methods for quality control of building materials and components, 1st edition, Geneve, Switzerland: ISO, 1997.

[6] Diamantidis, D.: Probabilistic Assessment of Existing Structures, Joint Committee on Structural Safety, RILEM Publications S.A.R.L., 2001.

[7] Institution of Structural Engineers: Appraisal of existing structures, 2nd edition, Institution of Structural Engineers, 1996.

[8] Bucher, C., Brehm, M. & Srensen, J. D.: Assessment of Existing Structures and Life Extension. Working documents of SAFERELNET, 2005, p. 25.

[9] Finnish Ministry of the Environment, Housing and Building Department: Probabilistic Calibration of Partial Safety Factors (Eurocode and Finish proposal). 2000.

[10] ISO 13822: Bases for design of structures Assessment of existing structures. Annex I Heritage structures, Draft compiled on 17 October 2008, Geneve, Switzerland: TC98/SC2/WG6, 2008.

[11] JCSS: JCSS Probabilistic Model Code, Zurich: Joint Committee on Structural Safety, 2006. .

[12] COST 345: Procedures Required for the Assessment of Highway Structures. Final Report, Reports of Working Groups 1-6, COST 345, 2004,.

III ACTIONS

Chapter III - Actions

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1. INTRODUCTION

This chapter provides principles for specifications of different types of actions applied commonly in assessment of industrial heritage structures. In general, the characteristic, design and representative values are defined as fractiles of appropriate theoretical models, taking into account their variation in time. The procedure to obtain the characteristic values of permanent and variable actions including wind, snow and temperature is described in detail.

Basic principles and rules for specification of representative and design values of actions and their effect on existing structures including those listed as the industrial heritage are given in ISO 13822 [1]. Supplementary guidance can also be found in EN 1990 [2], ISO 2394 [3] and in the Designer's Guide to EN 1990 [4]. Methods for obtaining representative values of various types of actions are given in different Parts of EN 1991 devoted to actions and effects of actions [5-8]. Additional information can also be obtained from the CIB documents on actions on structures and from the material oriented Eurocodes EN 1992 to EN 1999. Probabilistic models of actions can be found in the JCSS Probabilistic Model Code [9]. For guidance on load combinations reference is made to EN 1990 [2] (deterministic combinations) and to the document [9] (probabilistic combinations).

2. ACTIONS AND EFFECT OF ACTIONS

2.1. Definition of actions EN 1990 [2] defines actions as:

1. Set of forces (loads) applied to the structure, as for example the self-weight of structure itself or the wind pressure on a surface (direct action).

2. Set of imposed deformations or accelerations caused for example by temperature changes, moisture variation, uneven settlement or earthquakes (indirect action).

In general, an action is described by a theoretical model; in most cases a single scalar variable is sufficient to represent the action, which may have several representative values. For example the imposed load on a floor is described by a vertical uniform load expressed in kN/m2 or the wind actions are represented by forces applied on a vertical surface expressed in kN. For some actions a more complex representation of actions may be necessary, for example an action with fatigue effects must be represented by the number of cycles, the mean value of action and its amplitude.

The actions on a structure may be mutually correlated and may also be correlated with resistance variables. When actions are originated from different sources, they often can be taken into account as independent. However, in some cases the dependence of actions is significant and it should be considered. For instance, self-weight and resistances are always correlated through dimensions, but commonly this is less important. Other example could be the case when the maximum wind actions are seasonal, occurring for instance in summer. Thus, it could make no sense to combine the maximum wind actions with snow loads.

The actions that can be assumed to be statistically independent in time and space of any other action acting on the structure are called single actions.

2.2. Effect of actions The effects of actions (or action effects) are the internal forces, moments, stresses,

strains etc. of structural members, or their deflections, rotations etc., caused by the actions on the structure. Each limit state needs to be described quantitatively by the formulation of the


31

actions and resistances in comparable terms. It means that the actions and resistances are expressed both as the forces or moments applied to the structure. The action effect is related to the action and the properties of the structure.

In general the design action effect can be expressed as:

Ed = E(F,iFrep,i, ad) (III-1) where F,i = partial factor for the action Fi; Frep,i = relevant representative value of the action Fi; and ad = design value of relevant geometric dimension(s).

In the case of non-linear analysis (i.e. when the relationship between actions and their effects is not linear), two simplified rules may be considered:

1. If the action effect is increasing more progressively than the action, the partial factor should be applied to the action. This occurs in most cases.

2. If the action effect is increasing less than the action, the partial factor should be applied to the action effect that corresponds to the representative value of the action.

3. CLASSIFICATION OF THE ACTIONS

3.1. General The actions on structures are classified according to different aspects related to the

design situation considered in reliability verification. Actions are classified by: Variation in time (section 3.2), Origin (3.3), Spatial variation (3.4), Nature and/or the structural response (3.5), Bounds (bounded or unbounded actions - section 3.6).

3.2. Variation in time The most important classification of actions is referred to the time when the action is

acting compared with the reference period or an anticipated working life. The actions are classified as:

Permanent G, those likely to act throughout a given reference period and for which the variation in time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value; e.g. self-weight of structures, fixed equipment and road surfacing, and indirect actions caused by shrinkage and uneven settlements,

Variable Q, those likely to act throughout a given reference period for which the variation in magnitude with time is neither negligible nor monotonic, e.g. imposed loads on floors, wind actions or snow loads,

Accidental A, usually of short duration, that is unlikely to occur with a significant magnitude on a given structure during the working life, but its consequences might be severe, e.g. earthquakes, fires, explosions, or impacts from vehicles.

The concept of reference period will be explained later.

3.3. Origin As already mentioned in Section 3.1, two classes are distinguished: direct actions

consisting of forces or moments applied to the structure and indirect actions consisting of imposed deformations or accelerations caused, for example, by temperature changes, moisture variation, uneven settlement or earthquakes.


32

3.4. Variation in space When an action has a fixed distribution and position over the structure or structural

member so that the magnitude and direction of the action is determined unambiguously for the whole structure or structural member, then it is considered as a fixed action. If the action may have various spatial distributions over the structure, then it is a free action.

3.5. Nature or structural response The static actions are those that do not cause significant acceleration of the structure

or structural members. The dynamic actions cause significant accelerations of the structure or structural members. In most cases for dynamic actions it is enough to consider only the static component that may be multiplied by an appropriate coefficient to take account of the dynamic effects.

3.6. Bounded and unbounded actions In some cases, an upper (or lower) bound of the action can be found and then one of

these can be established as a representative value. For instance for the load due to water in a tank; the water weight is limited by the height of the tank, and, therefore also the maximum load will be bounded. In other cases a possible bound could be found, but it should be much higher than the load obtained by statistic assessment and therefore not suitable as representative value, for instance the load given by the material of maximum density stocked up to the maximum possible height.

4. REFERENCE PERIOD AND DISTRIBUTION OF EXTREMES

The definitions given for permanent and variable actions include the term reference period that is the time used as a basis for the statistical assessment of actions and time-varying resistances.

For each type of action, depending on its characteristics, the whole working life of the structure may be split in several reference periods T, of the same or different (random) length. In each of these reference periods the action varies following a more or less similar pattern and, therefore the same independent, identical distributions can be accepted for the action in such a period. This means that the set of extremes coming from the extreme of one of each of such periods will form a sample of extremes from which an extreme distribution function can be derived. The extreme in any period will correspond to an independent realization of such a distribution of extreme values.

The adequate reference period for defining the characteristic value will depend on the type of variable action.

4.1. Climatic actions For these actions snow, wind, thermal actions etc. - a period of a year is generally

adequate; i.e. it can be assumed that the annual extremes are mutually independent. If the distribution function of extremes to related one year is known, the distribution function of maxima in the whole working life, T, assuming the same distribution function for each reference period is given by:

FQ,max(x) = [FQ(x)]T or FQ,min(x) = 1 - [1 - FQ(x)]T (III-2)


33

where FQ,max(x) = distribution function of working life maxima; FQ,min(x) = distribution function of working life minima; and FQ(x) = distribution of annual extremes. Simplification is illustrated in Fig. III-1.

4.2. Imposed loads For these actions, a reference period corresponding to the change of owner or the

change of use of a structure or part of it is generally accepted. In [9] an average value of 5-10 years for reference periods is indicated. This means that about 5 to 10 changes of use may be commonly expected during a 50-year working life as shown in Fig. III-2.

If the distribution function of the imposed loads on one reference period in buildings with similar use is known (for instance, by a survey of imposed loads at a point in time), assuming that the distribution function does not change with time for the following reference periods, this distribution function can be accepted for all the different periods included in the working life. It is assumed in [9] that the duration is exponentially distributed, and that then the number of changes in the working life has a Poisson distribution. With these assumptions, the distributions of the maxima and minima related to the working life are obtained as:

FQ,max(x) = exp{-T [1 - FQ(x)]} or FQ,min(x) = 1 exp[- T FQ(x)] (III-3) where = average rate of changes in use per year; and T = working life. The product T thus represents the expected number of changes of use during the working life. From these expressions, the characteristic lower and upper values, corresponding to a 5% and 95% of not being reached or not being exceeded, respectively, as function of the fractiles of the distribution of Q(x), are given in Tab. III-1 as a function of the mean number of changes.

Tab. III-1. Probabilities of the fractiles corresponding to the characteristic values. T 5 7 10

Qk,inf 0.010 0.007 0.005 Qk,sup 0.990 0.993 0.995

The fractiles in Tab. III-1 mean that the characteristic value to be accepted, for

instance, Qk,sup, will correspond to the fractile 0.993 of the distribution in each reference period, assuming an average numbers of changes of 7.

Assuming a normal distribution of Q(x), the characteristic values can be obtained from the mean and standard deviation (or coefficient of variation) as follows:

Qk,max = Q + kQ = Q(1 + k VQ) or Qk,min = Q - kQ = Q(1 - k VQ) (III-4) where Q = mean value of Q; Q = standard deviation of Q; VQ = coefficient of variation of Q; and k = coefficient obtained from the fractiles of the standardized normal distribution. Examples of the values of k as function of the mean number of changes in the working life are given in Tab. III-2.


34

Fig. III-1. Model and distribution for a variable climatic action.

Fig. III-2. Model and distribution for an imposed load.

Tab. III-2. Variation of the coefficient k with the mean number of changes T. T 5 7 10 k 2,32 2,44 2,57

As an example it is considered a building for which it is foreseen that changes of use

will likely modify the imposed load given by the weight of non-structural members (heavy partitions) and equipment. This part of the imposed load is assumed to have the normal distribution with the mean value of 1,0 kN/m2 and coefficient of variation of 0,15. For the mean number of changes during a working life equal to 7, the characteristic value follows from (III-4):

Qk,max = 1,0(1 + 2,44 0,15) = 1,36 kN/m2 which is 36 % higher than the mean value Q and 9 % higher than the 95% fractile of the imposed load in one reference period.


35

G

1,64G 1,64G

Gk,inf Gk,sup Gm

G

5 % 5 % G

fG

Fig. III-3. Characteristic values of permanent actions.

5. CHARACTERISTIC VALUES

5.1. General Basic representative value of an action F is the characteristic value Fk. When

sufficient data to assess the characteristic value on the statistical basis are available, the characteristic value corresponds to a prescribed probability of not being exceeded (for unfavourable effects) during the reference period. Otherwise a nominal value or a value given in the project documentation may be considered provided that it is consistent with methods given in EN 1991.

5.2. Permanent actions The characteristic value of a permanent action G shall be assessed as follows:

If the variability of G during the working life can be considered small, one single value Gk equal to the mean value may be used;

If the variability of G is not small, two values shall be used: an upper value Gk,sup and/or a lower value Gk,inf.

Generally the 5% fractile is accepted for Gk,inf and the 95% fractile for Gk,sup as indicated in Fig. III-3. Commonly normal distribution is assumed for G. With these assumptions Gk,inf and Gk,sup can be obtained from:

Gk,sup = G + 1,64G = G(1 + 1,64VG) or Gk,inf = G 1,64G = G(1 1,64VG) (III-5) where G = mean value; G = standard deviation; and VG = coefficient of variation of the distribution of G.

According to ISO 13822 [1] permanent actions on existing structures should be determined considering actual structural dimensions and material properties. In addition foreseen modifications should be taken into account. When the original documentation is unavailable or does not provide sufficient information for load specification, permanent actions should be verified on the basis of site surveys and measurements (densities, dimensions). Characteristic values should be then determined using statistical methods.


36

For n samples g1, g2,, gn, sample mean mG and sample standard deviation sG, the characteristic value Gk of the permanent action is assessed using the following relationships:

;1

)(;

2

==

nmg

sng

m GiGi

G Gk = mG kn sG (III-6) Variation of the coefficient kn on the sample size is indicated in Tab. III-3 given in the Czech National Annex to ISO 13822 [1]. For determination of unfavourable effect of Gk the positive sign is applied, the negative sign is used otherwise.

Tab. III-3. Variation of the coefficient kn with the sample size. Sample size n Coefficient kn Sample size n Coefficient kn

5 0,69 15 0,35 6 0,60 20 0,30 7 0,54 25 0,26 8 0,50 30 0,24 9 0,47 40 0,21 12 0,39 >50 0,18

For intermediate values of n the coefficient kn can be interpolated. The coefficient kn is given assuming a normal distribution of the permanent action.

At least 5 measurements are needed according to ISO 13822 [1]. In cases of a lower

sample size, it is recommended to critically compare an estimated sample standard deviation sG with previous results. In these cases statistical assessment cannot be made directly and an estimated characteristic value is bounded by the minimum value taken as the maximum test value for an unfavourable permanent action.

As an indication for the assessment of industrial heritage structures EN 1991-1-1 [5] gives in its Annex A values of densities (actually unit weights) for the most common materials to be used to calculate the mean value of the permanent loads. In some cases, when the density is considerably dependent on the conditions of the material (e.g. effects of humidity), a range is given instead of a single value. Values of the coefficient of variation are not given in this Eurocode; indicative values can be found in the Probabilistic Model Code [9]. The mean value of the self-weight of one member is calculated on the basis of the nominal dimensions of the member and its mean density.

As an example it is considered a beam of normal weight of concrete: the mean density can be taken as = 24 kN/m3 as given in EN 1991-1-1 [5]. In common situations the characteristic value of the permanent load due to the self-weight of the beam is obtained by multiplying this value by the nominal dimensions of the cross section. That is:

gk = 24ab kN/m where a and b are the nominal dimensions of the cross-section in metres.

Consider now that, due to any circumstance, the structure is very sensitive to its self-weight and it is therefore necessary to take into account the lower and upper characteristic values. In [9] a coefficient of variation of 0.04 is recommended for the density of concrete. It then follows from (III-5):

gk,sup = 24ab (1 + 1,64 0,04) = 25,6ab kN/m or gk,inf = 24ab (1 - 1,64 0,04) = 22,4ab kN/m


37

5.3. Variable actions For variable actions, the characteristic value Qk shall correspond to either:

A higher value with an intended probability of not being exceeded or a lower value with an intended probability of being achieved, during a reference period,

or: A nominal value which may be specified in cases where a statistical distribution is

not known. For the characteristic value of climatic actions (wind, snow, etc.) a year reference

period is generally chosen with an annual probability of exceedence taken as 0,02. This is equivalent to the mean return period of 50 years. It is worth noting that this return period is not related with a generally accepted 50 years as working life of common structures. The return period just gives a probability of exceedence. That is, on the average, every 50 years there will be an action exceeding the characteristic value.

Assuming that the actions in all the years are independent and identically distributed, and considering a return period R and working life T (in years), the probability r of exceedence during the working life is given by:

r = (1 1/R)T (III-7) For R = 50 years and T = 50 years the probability of exceedence is r = 0,63.

5.4. Imposed loads In EN 1991-1-1 [5] two different types of imposed loads are considered:

Uniformly distributed load for global effects, Concentrated load acting alone for local effects.

The characteristic values of the uniformly distributed imposed loads qk and concentrated loads Qk are given in Tables 6.2 and 6.4 of EN 1991-1-1 [5] for various categories of intended use (Category A - residential, B - offices, C - areas where people can concentrate, D shops etc.).

For the assessment of the imposed load affecting one horizontal structural member in a storey-distributed load, a free uniform distributed applied action is considered in the unfavourable part of the influence area of the action effect for this member. This can lead to the need of studying a large number of load cases in complex structures. As a simplification for the assessment of the action effects for this member, the load coming from other stories may be treated as fixed uniform load.

When the influence area for the studied member is large, it is not likely that all the area may have a high imposed load at the same time. In order to take account of this, a reduction factor A due to the size of the area and a factor n with respect to a number of stories can be used. The factor A is given by: A = 0 5/7 + A0 / A 1 (III-8) where 0 = combination factor given in EN 1990 [2] (commonly 0.7); A0 = reference area equal to 10 m2; and A = loaded area.

For vertical members (columns, walls) the imposed load on the upper floors may be considered as uniform distributed load acting in the area affecting the member, reduced by the factor n given by: n = [2 + (n 2) 0] / n (III-9) where n = number of stories (> 2).


38

The simultaneous use of the -factors is not allowed. Also, when the imposed load enters in a combination of actions as an accompanying action being multiplied by the corresponding -factor, then the simultaneous reduction by an -factor is not allowed. 5.5. Snow loads

The characteristic load on a roof due to snow s is given in EN 1991-1-3 [6] by the relationship:

s = i Ce Ct sk (III-10) where i = roof shape coefficient; Ce = exposure coefficient; Ct = thermal coefficient; and sk = characteristic value of the snow load on the ground.

The exposure coefficient takes into account the situation of the referred roof to accumulate snow or not. It is generally taken as Ce = 1,0. Only when the roof is on an open terrain windswept, it is considered as Ce = 0,8. When the roof is sheltered by higher buildings, the coefficient increases to Ce = 1,2.

The thermal coefficient describes the possibility snow melting due to the heat transmission from the building. The usual value is Ct = 1,0. In case of poor thermal isolation such as for glass roofs, the lower value may be reduced according to the guidance provided in ISO 4355 [10].

The distribution of snow load on the roof is generally not uniform. The wind, even of low velocity, causes the snow drift from more exposed to more sheltered parts of the roof. EN 1991-1-3 [6] gives values of the snow load shape coefficient i depending on the angle of the roof and the different shapes: mono-pitched or pitched, single-span or multi-span, cylindrical or roofs abutting and close to taller structures. Consideration has to be given in the cases where there are systems to sweep the snow or where snow fences exist.

In Annex C of EN 1991-1-3 [6] the European snow maps for various climatic regions are provided including Alpine Region; Mediterranean Region; Norway, Sweden & Finland; UK & Republic of Ireland; Central East; Central West; Greece; and the Iberian Peninsula. Each Region map indicates the snow load at the sea level for each of the zones. Moreover, expressions given for these regions make it possible to determine the load at the altitude level of building site from the snow load at the sea level. These expressions are functions of region, the zone corresponding to the site in the map and the site level. It should be noted that, based on fifty year return period, new CEN Member States have developed their snow maps which are included in their National Annexes to EN 1991-1-3 [6].

Information is also given for special cases where the standard information is not sufficient, e.g. for the purpose of taking account of the local effects due to the snow overhanging at the edge of the roof. Indications are also given on how to deal with exceptional snows or exceptional accumulations of snow causing unfavourable local effects on the structure.

5.6. Wind actions EN 1991-1-4 [7] deals with the effects of wind on structures. The scope of this

standard covers buildings of height up to 200 m for the common effects on all parts of the building: components, claddings and fixings, etc. Other effects, as thermal effects on winds, vibrations where more than a relevant fundamental mode needs to be considered, the torsional vibrations due to transverse winds, etc. are not covered. Three models of response are given: the quasi-static response, the dynamic and the aeroelastic.

The effect of the wind on the structure (i.e. the response of the structure) depends on the size, shape and dynamic properties of the structure. Wind fluctuates with time and this


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fluctuation can originate different effects depending on the building characteristics. For most buildings, only a quasi-static response of structure needs to be considered. Dynamic structural responses are needed to be considered only in the cases with very low natural frequency (lower than 1 Hz) and low damping. Aeroelastic response should be considered for flexible structures such as cables, masts, chimneys and some bridges.

The quasi-static response is treated here only. The wind acts directly as a pressure on the external surfaces of enclosed structures and, because of the porosity of the external surface it also acts indirectly on the internal surfaces. It may also act directly on the internal surface of open structures.

Pressures act on areas of the surface resulting in forces normal to the surface of the structure or of individual cladding components. Additionally, when large areas of structures are swept by the wind, friction forces acting tangentially to the surface may be significant.

The quasi-static action of the wind is represented by a simplified set of pressures or forces whose effects are equivalent to the extreme effects of the turbulent wind. The fundamental value of the basic wind velocity vb,0 is the main variable used to determine the wind in a site. It is defined as the characteristic 10-minute mean wind velocity at 10 m height on a terrain category II. The terrain category II is defined as an area with low vegetation such as grass and isolated obstacles (trees, buildings) with separations of at least 20 obstacle heights.

EN 1991-1-4 [7] does not provide maps of fundamental wind velocity; they are given in the National Annexes of CEN Member States to be operative in design procedure. From the fundamental wind velocity the basic wind velocity is derived vb as: vb = cdir cseason vb,0 (III-11) where cdir = directional factor; and cseason = seasonal factor, taking into account that wind in some directions may be reduced and that temporary structures spanning a few months might have a lower probability of high winds. These two factors are usually taken as the unity.

The basic wind pressure qb is derived from the basic wind velocity as: qb = vb2 / 2 (III-12) where = density of the air taken as 1,25 kg/m3.

The basic wind pressure represents the mean value of the pressure on a building placed at a site in a terrain category II and with a reference height of 10 m. The transformation of this value to the pressure at a building at the actual terrain category in reference height is carried out by the mean wind velocity at the relevant height by:

vm (z) = cr(z) co(z) vb (III-13) where vm (z) = mean wind velocity at z reference height; cr(z) = roughness factor; and co(z) = orography factor.

The orography factor takes into account the fact that for buildings placed on elevations, valleys, etc. the wind could be increased. Usually, it is considered as the unity.

The roughness factor is derived as:

cr(z) = kr ln (z/z0) for z zmin (III-14) where kr = 0,19(z0/z0,II)0,07 is the terrain factor; z0 = roughness length given for a terrain category; and zmin = minimum height given for a terrain category. Five terrain categories are defined in Annex A of EN 1991-1-4 [7].

The value of peak velocity represents the effect of the average velocity in 10 minutes and the effect of the gust as shown in Fig. III-4. It is obtained from the mean wind velocity by multiplication by the gust factor G:


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vp(z) = G vm (z) (III-15)

Fig. III-4. The peak and mean velocities.

where: ( )

+=

0o

I

log

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zzzc

kG for z zmin; and kI is the turbulence factor.

The peak velocity pressure in the relevant height becomes:

qp(z) = cr2(z) co2(z) G2 qb (III-16) In common cases c0(z ) = kI = 1 may be taken.

The wind forces can be determined on the basis of pressure or force coefficients. In the first approach the force on the whole structure is determined by the vectorial summation of the external, internal and friction forces on all the surfaces of the building:

refsurfaces

edsew, AwccF = (III-17) ref

surfacesiiw, AwF = (III-18)

Ffr = cfr qp(ze) Afr (III-19) where Fw,e = external force; Fw,i = internal force; Ffr = friction force; cscd = structural factor; we = external pressure on a surface; wi = internal pressure on a surface; Aref = reference area for a surface; Afr = area of the external surface parallel to the wind; qp(ze) = peak velocity pressure at the reference height ze.

In the second approach based on the force coefficients, the force on the whole structure or on one member can be calculated from the relationship:

refelements

fdsew, AcccF = (III-20) Guidance on specification of the coefficients for common types of buildings or

members is provided in EN 1991-1-4 [7].


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5.7. Thermal actions EN 1991-1-5 [8] deals with thermal actions which are classified as variable and

indirect actions. The load-bearing structural members shall be checked to ensure that thermal movement will not cause overstressing of the structure, either by the provision of movement joints or by including the effects of thermal actions in the assessment.

The fundamental quantities for thermal actions are extreme (maximum and minimum) air temperatures in the shade at a building site. The thermal actions on a structural member can be split in three basic quantities:

1. A uniform temperature component, given by the difference between the average temperature T, in summer or winter (or due to operational temperatures) of the member and its initial temperature T0,

2. A linearly varying temperature, given by the difference between the temperatures of the external and internal surfaces of a cross section or layers,

3. A temperature difference p between

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