Generic limit state design of structures

Transcription

1 10DBMC International Conference On Durability of Building Materials and Components Generic limit state design of structures Asko Sarja, Author Technical Research Centre of Finland, VTT PL 1803, Kemistintie 4, Espoo FIN VTT Finland ABSTRACT TT3-161 Generic limit state design is aiming at fullfilling the generic requirements and criteria of the life time quality. Lifetime quality means the capability of a facility or structure to fullfill the requirements of owners, users and society over the design life. On generic level the requirements include social (human), economic, ecological and cultural requirements, which in design are transformed into technical and economic level. The extended limit states including the following three classes: 1. Mechanical limit states under static and dynamic loads 2. Durability limit states under physical, chemical and biological loads 3. Usability limit states under obsolescence loads (changes of use or requirements) Traditionally the design is focused on mechanical design, because of the structural safety reasons and high consequences of these types of failure to human beings. However, degradation and obsolescence together are most important loads causing needs for refurbishment or demolition, and therefore have big economic consequences. Extensive analysis of reasons of heavy refurbishments or demolitions of buildings and bridges have shown, that the obsolescence under changing use, demands of users and general requirements of the society are the most dominant reasons and still more common than the degradation. Therefore it is important to develop rational and systematic methodologies and methods for controlling all these three types of limit states in lifetime planning, design and management process of buildings and civil infrastructures. This report is focused on durability limit state design and obsolescence limit state design. There is an analogy between the terms and parameters of mechanical design against static and dynamic loads, and the durability design against degradation, but most of the variables and methods are different. The durability design can be carried out with similar statistical or deterministic safety factor methods as mechanical design. Usability design against obsolescence can be based on statistical risk analysis and other methods of systems engineering, like Risk Analysis, Quality Function Deployment (QFD) or Multi Attribute Decision Analysis (MADA). The obsolescence analysis and optimisation is always made with comparison of planning, design or product alternatives, comparing their response towards changing requirements and supposed changes of user demands. KEYWORDS design, durability, limit state design, safety factors, risk analysis

2 1 INTRODUCTION The current objectives towards sustainable society and construction sector can be interpreted into generic requirements for construction as presented in table 1. Table 1. Generic classified requirements of structures and buildings [1,3]. 1. Human requirements functionality in use safety health comfort 3. Cultural requirements building traditions life style business culture aesthetics architectural styles and trends imago 2. Economic requirements investment economy construction economy lifetime economy in: o operation o maintenance o repair o rehabilitation o renewal o demolition o recovery and reuse o recycling of materials o disposal 4. Ecological requirements raw materials economy energy economy environmental burdens economy waste economy biodiversity When looking at the statistics of demolitions of buildings and structures we can notice the following reasons for refurbishment or demolition: Degradation is the the main reason for refurbishment of buildings in 17 % (Aikivuori, 1994)[15] and in 26 % (steel) to 27 % (concrete) of demolition of bridges (Iizuka, 1988) [16]. In individual cases degradation can be a dictating reason for refurbishment or demolition of the structures, which are working in highly degrading environment. Obsolescence is the cause of refurbishment of buildings in 26 % (Aikivuori, 1994) [15] and the reason of demolition of bridges in 74% of demolition cases (Iizuka, 1988)[16]. In the case of modules or component level renewals of facilities the share of obsolescence is still higher. This means that the obsolescence is the dominating reason for refurbishments and demolitions of facilities and their structures. A conclusion of this, and a challenge for structural engineering is, that we have to include the degradation and obsolescence criteria into the design, as well as into the MR&R (Maintenance, Repair and Rehabilitation) planning of structures. In this use we need new methodology, models and methods for analysis, optimisation and decision making. 2 DESIGN LIFE Integrated life time design is aiming at fullfilling the generic and specific requirements of lifetime quality of a facility or structure. The lifetime quality means the capability of a facility to fullfill the requirements of owners, users and society over the design life. The generic classification of these requirementas are presented in table1. Design life is a specified time period, which is used in calculations. The classification of design life is presented in table 2.1 of the European standard EN1990: 2002 [2].

3 3 EXTENDED LIMIT STATES OF LIFETIME DESIGN The classes of extended limit states for integrated lifetime design are as follows: Mechanical (static and dynamic serviceability and safety) Durability (degradations) Usability (obsolescence) The serviceability limit states and ultimate limit states of concrete structures in relation to this classification are presented in table 1 [1]. Models of generic specifications of these limit states are presented in tables 2 and 3. Analogical features of terms and variables between mechnical, durability and usability designs are presented in table 4. Table2. Summary of performance and functionality limit states (Sarja, 2003) [1]. A. Performance limit states Serviceability limit states Ultimate limit states 1. Surface cracking 1. Failure under static, dynamic or fatigue loading 2. Surface scaling 3. Deflection 4. Carbonatisation or chemicals (e. g. chlorides) penetration until reinforcement 5. Corrosion of reinforcement B. Functionality limit states 1. Weakened functionality 1. Total loss of functionality 2. Weakened economy of operation 2. Total loss of economy of operation 3. Weakened economy of MR&R 3. Total loss of economy of MR&R 4. Minor health problems in use 4. Severe health problems in use 5. Aesthetic change of surface (abrasion, 5. Total loss of aesthetic acceptability colour changes) 6. Cultural ineligibility 6.Total loss of cultural acceptance 7. Weakened ecology 7. Severe ecological problems or hazard Table 3. Generic mechanical, durability and usability limit states of concrete structures [1]. Classes of the Limit states limit states Mechanical limit states under static and dynamic loads 1. Serviceability limit states 1. Deflection limit state 2. Cracking limit state Durability limit states under physical, chemical and biological degrading loads 3. Surface faults causing aesthetic harm (colour faults, pollution, splitting, minor spalling) 4. Surface faults causing reduced service life (cracking, major spalling, major splitting) 5. Carbonation or chemicals penetration into the concrete cover (grade 1: one third of the cover, grade 2: half of the cover, grade3: entire cover ) Usability limit states under loads causing obsolescence through changes of use and requirements 6. Reduced usability and functionality, but still usable The safety level does not allow the requested increased loads Reduced healthy, but still usable Reduced comfort, but still usable 2. Ultimate 1. Insufficient 2. Insufficient safety due to 3. Serious obsolescence causing

4 limit states safety against failure under loading indirect effects of degradation: heavy spalling heavy cracking causing insufficient anchorage of reinforcement corrosion of the reinforcement causing insufficient safety. total loss of usability through: loss of functionality in use (use of building, traffic transmittance of a road or bridge etc.) safety of use health comfort economy in use MR&R costs ecology cultural acceptance Table 4. Comparison of the terms and variables of limit state methods in mechanical, durability and usability design and control.. Mechanical limit state design 1. Strength class 2. Target strength 3. Characteristic strength ( 5 % fractile) 4. Design strength 5. Partial safety factors of materials strength 6. Static or dynamic loading onto structure 7. Partial safety factors of static loads 8. Service limit state (SLS) and ultimate limit state (ULS) Durability limit state design 1. Service life class 2. Target service life 3. Characteristic service life (5% fractile) 4. Design life 5. Partial safety factors of service life 6. Environmental degradating loads onto structure 7. Partial safety factors of environmental loads 8. Serviceability and ultimate limit states, related to the basic requirements: Human requirements, lifetime economy, cultural aspects and lifetime ecology Obsolescence limit state design 1. Service life class 2. Target service life 3. Characteristic service life (5%fractile) 4. Design life 5. (Partial safety factors of service life) 6. Obsolescence loading onto structure 7. Partial safety factors of obsolescence loading 8. Serviceability and ultimate limit states related to obsolescence in relation to the basic requirements: Human requirements, lifetime economy, cultural aspects and lifetime ecology 4. EXTENDED RELIABILITY OF STRUCTURES The statistical reliability requirements are defined in the European standard EN 1990: 2002 [2]. The reliability requirements are expressed also in terms of statistical reliability index β. P f Ф(-β) (1) where Ф is the cumulative distribution function of the standardised Normal distribution. The requirementsfor the reliability index are shown in Table B2 of the standard EN 1990 for the design of new structures, as well as for the safety of existing structures.

5 5. DURABILITY LIMIT STATE DESIGN WITH SAFETY FACTOR METHOD In practice it is reasonable to apply the lifetime safety factor method in the design procedure for durability, which was first time presented in the report of RILEM TC 130 CSL [8,11]. The lifetime safety factor method is analogous with the static limit state design. The durability design with lifetime safety factor method is always combined with static or dynamic design and aims to control the serviceability and service life of a new or existing structure, while static and dynamic design controls the loading capacity. The design service life is determined by formula (Sarja and Vesikari 1996 [8,11], modified:sarja 2001 [12] and Sarja 2002[3,4]): t L d t Lk / γ tk > t g (2) where t Ld is the design service life, t Lk the characteristic service life γ tk the lifetime safety factor, and t g the target service life. The lifetime safety factor depends on the maximum allowable failure probability. The lifetime safety factor also depends on the form of service life distribution. Figure 1. illustrates the meaning of lifetime safety factor when the design is done according to the performance principle. The function R(t) S is called the safety margin. D Failure probability µ D D max Safety margin µ t L t Ld µ Time t γ L t0 Figure 1. The meaning of lifetime safety factor in a degradation process. An example of calculated lifetime safety factors using sepecific values of standard deviations of parameters is presented in table 5.

6 Table 5. Central lifetime safety factors γ 0 and characteristic lifetime safety factors γk in the cases, when 0,3 and 0,4. The reliability index values of EN1990: 2002(Table B2) are applied. Lifetime safety factor Reliability Class/ Consequence Class RC3/CC3: High consequence for loss of human life, or economic, social or environmental consequences very great RC2/CC2: Medium consequence for loss of human life, or economic, social or environmental consequences considerable RC1/CC1: Low consequence for loss of human life, or economic, social or environmental consequences small or negligible Safety index β 1 year reference period 1 year reference period 50 years reference period Characteristi Central Characterist c safety safety factor ic safety factor γ 0 factor γk γk Central safety factor γ 0 Ultimate limit states 0,3 50 years reference period 0,4 0,3 0,4 0,3 0,4 0,3 0,4 5,2 4,3 2,56 3,08 2,07 2,42 2,29 2,72 1,80 2,06 4,7 3,8 2,41 2,88 1,92 2,22 2,14 2,52 1,65 1,86 4,2 3,3 2,26 2,68 1,77 2,02 1,99 2,32 1,50 1,66 Serviceability limit states RC3/CC3 No general recommendations. Will be evaluated in each case separately RC2/CC2 2,9 1,5 1,87 2,16 1,38 1,50 1,45 1, RC1/CC1 1,5 1,5 1,45 1, ,45 1, The degradation limit state design procedure is as follows (Sarja&Vesikari 1996 [8], Sarja 2002 [3]: 1. mechanical (static, dynamic, fatique) limit state design with traditional limit state design, applying a relevant norm or standard (e. g. EN 1990: 2002: Basis of structural design) 2. specifying the target service life and design service life 3. analysing environmental loads onto structures 4. identifying durability factors and degradation mechanisms 5. selecting a degradation model for each degradation mechanism 6. calculating durability parameters using available calculation models 7. possible updating the calculations of the ordinary mechanical design 8. transferring the durability parameters into the final structural design documentation

7 6 RELIABILITY UNDER OBSOLECENCE 6.1 Principles Obsolescence means the inability of a facility or a part of it to satisfy changing functional (human), economic, cultural or ecological requirements. Obsolescence can affect to the entire building or civil infrastructural facility, or just some of its modules or components (Sarja 2002 [3], Sarja 1998 [7]). The obsolescence behaviour of a facility is very different in comparison to mechanical and durability behaviour. The principal difference is, that the obsolescence limit states are related to changes in requirements and user demands, while mechanical and durability limit states are related to original requirements, while the changes leading to limit states happen in the properties of the facility or structure itself. The obsolescence control is aiming to guarantee the ability of the buildings and civil infrastructures to maintain their ability to meet all current and changing requirements with minor changes of the facilities. Because the obsolescence process of a facility depends on the development of local conditions, as well as on the general development of society during the service life (or residual service life) of a facility, there is lot of uncertainty involved in obsolescence analyses and control. Like in any uncertainty-filled problem, also in obsolescence situation the case must be structured down to smaller parts, which can be consistently handled. It must be noted that the systematic obsolescence avoidance and control thought should be present in all life cycles of the facility planning and programming: design; construction; operations, maintenance, refurbishment, renewal and reuse. The obsolescence analysis should be performed before the onset of obsolescence, as a part of the facility owning and management strategy (Sarja et al 2004 [14]). The obsolencence control is included in investment strategies and planning, lifetime design and lifetime management strategies and planning, which all aim at minimising the need of early and unexpected refurbishment, renewal or demolition. Examples of the obsolescence are as follows: Functional obsolescence is due to changes of users or changes of functions or other demands of the user of the facility. Examples of these are currently frequent change of users, increased demands of healthy, convenience and accessability etc. Technological obsolescence can be a cause when new products providing better performance and easier maintenance in operation become available with new materials, new technology, new equipments etc.. Economic obsolescence means that operation and maintenance costs are too high in comparison to new facility, systems or products. Cultural obsolescence is related to the changes of demands and criteria of culture, living and working, aesthetic and architectural styles and trends, and imago of the owners, users and society. An example of this are the buildings of mass production time in Europe, especially in eastern Europe, which are not more accepted because of a pure architectural and aesthetic imago. Ecological obsolescence means the inablity of a facility to fullfill the incraeasing ecological and environmental requirements of the society, regarding to energy consumption, pollution, raw materials consumption, waste production or loss of biodiversity or geodiversity. 6.2 Methods for obsolescence limit states control The usability analysis and optimisation against obsolescence is always made with comparison of alternatives, comparing their response towards changing requirements and supposed new demands. For each alternative of strategic or operative process in investment briefing, design of a facility or MR&R strategy or plan, the following obsolescence procedure will be made: 1. identifying the relevant obsolescence factors (changes) 2. analysing relevant obsolescence limit states 3. selecting evaluation methods for the relevant potential obsolescence cases 4. evaluating the characteristic service life against the actual modes of the obsolescence 5. evaluating the required lifetime safety factors for each mode of obsolescence 6. listing the modes of the obsolescence, and the corresponding values of the design service life

8 7. moving the results into the general planning, design or MR&R planning procedure The following methods can be applied in obsolescence analysis, control and optimisation: Quality Function Deployment method (QFD) (Sarja 2004, Lifecon Deliverables D2.3 and D5.1) [14] Multiple Attribute Decision Aid (Sarja et al. 2004, Lifecon Deliverable D2.3) [14] Risk Analysis (RA) (Rissanen 2004, Lifecon Deliverable D2.3) [14]) Life Cycle Costing method (LCC) [Miller et al 2004: Lifecon Deliverable D5.3) [14] Simulations Detailed descriptions of the applications of these methods are presented in (Lifecon, 2004) [14]. 7. REFERENCES [1] Sarja, Asko 2003, Lifetime performance modelling of structures with limit state principles. in: Proceedings of 2 nd International SymposiumILCDES2003, Lifetime Engineering of Buildings and Civil Infrastructures, Kuopio, Finland, December1-3, Association of Finnish Civil Engineers pp [2] EN 1990: 2002: Eurocode - Basis of structural design. CEN: European Committee for Standardisation. Ref. No. EN 1990:2002 E. 87 pp. [3] Sarja, Asko 2002, Integrated Life Cycle Design of Structures. 142 pp. Spon Press, London. ISBN [4] Sarja, Asko 2002, Reliability based life cycle design and maintenance planning. Workshop on Reliability Based Code Calibration, Swiss Federal Institute of Technology, ETH Zurich, Switzerland, March 21-22,. [5] ISO/DIS , Buildings-Service life planning-part 1 General Principles. Draft. [6] Sarja, Asko 2004, Generalised lifetime limit state design of structures. Proceedings of the 2 nd International Conference, Lifetime-Oriented Design Concepts, ICDLOC, pp Ruhr-Universität Bochum, Germany, ISBN [7] Sarja, Asko, Sarja, Asko (ed) 1998, Open and industrialised building. London. E & FN Spon. 228 pp. [8] Sarja, Asko & Vesikari, Erkki (Editors) 1996, Durability design of concrete structures. RILEM Report of TC 130-CSL. RILEM Report Series 14. E&FN Spon, Chapman & Hall,. 165 pp. [9] EN Concrete-Part, 2000: Specification, performance, production and conformity. CEN European Committee for Standardisation. REf. No EN 206-1:2000 E. 72 pp. [10] JCSS Model Code. Joint Committee of Structural Safety. [11] Sarja, Asko, 1999, Environmental Design Methods in Materials and Structural Engineering. RILEM Journal: Materials and structures, Vol. 32,, pp [12] Iselin, D.G., Lemer, A.C., (Eds) 1993., The Fourth Dimension In Building: Strategies For Minimizing Obsolescence. National Research Council, Building Research Board. National Academy Press, Washington, D.C. 59 pp. + Appendix 42 pp. [13] Sarja, Asko 2000, Development towards practical instructions of life cycle design in Finland. RILEM Proceedings PRO 14, Proceedings of the RILEM/CIB/ISO International Symposium: Integrated Life-Cycle Design of Materials and Structures, ILCDES. pp. 1-5 [14] Sarja, Asko (Co-ordinator) et al Life Cycle Managemnt of Concrete Infrastuctures, Lifecon, Deliverables D1-D15. [15] Aikivuori, Anne 1994, Classsification of demand for refurbishment projects.acta Universitatis Ouluensis, Series C, Technica 77.University of Oulu, Department of Civil Engineering. ISBN pp.+7 Appendices. [16] Iizuka, Hiroshi 1988, A statistical study of life time of bridges. Structural Eng./Earthquake Eng. Vol. 5 No1, pp , April Japan Society of Civil Engineers.