Development of the safety concept in design codes

Transcription

1 Development of the safety concept in design codes Lennart Elfgren Luleå University of Technology Anchor Bolts for Nuclear Power Plant Applications Design and Assessment Energiforsk Seminar, Stockholm, 3 rd November, 2015

2 Outline Introduction Historical Development Safety factors Partial coefficients Reliability Anchorage Future

3 Introduction Safety is a major concern for all of us. From our birth we learn by trial and error, which actions that are safe and which are unsafe (walking, fighting, negotiating ) Our society uses advanced technical systems which influence the whole globe. It is of vital importance to use safe procedures (traffic, energy, climate change )

4 Anchorages for a wall in the Old Town in Stockholm Cephalus, Myntgatan-Storkyrkobrinken-Västerlånggatan

5 Personal Responsibility BC Hammurabi, Babylon, 1772 BC: If a builder build a house for some one, and does not construct it properly, and the house which he built fall in and kill its owner, then that builder shall be put to death. (No 229) Code on diorite stele, Louvre, Paris

6 Allowed stresses before Transport Board (Kungl. Väg- och vattenbyggnadsstyrelsen ) prescribed loads, materials and allowable stresses for steel Steel (Järnbestämmelser, SFS 1919:193) s st,till 0,5 s st, yield 100 á 150 MPa Concrete (Betongbestämmelser, SFS 1926:13) s cc,till 0,2 s cc,crushing 3 á 4 MPa Allowable stresses were ca: 50 % of the yield stress for steel e.g. s st,till = 150 MPa for St 44 (s y = 260 MPa) 30 % of the concrete compressive strength e.g. s cc,till = 12, 5 MPa for K40 Järnbestämmelser SOU 1938:37, Stålbyggnorm 1968, 1970 Betongbestämmelser SOU 1949:64; B5, B6, B7 1968

7 Reliability analysis Eurocode Handbook on Reliability

8 Partial coefficients after 1980 Starting 1979 new codes were introduced based on partial coefficients BBK79, BSK87, BKR 93 Frequency Load Effect E(F) or Sollicitation, S(F) Resistance, R(f) 5 % 5% R, E, S E m E k E d < R d R k R m Characterisitic values on loads F k and strengths f k are turned into design values with partial coefficients g. At failure (ultimate limit state, ULS) we have: F d = g f F k Usually g f = 1,3 for design loads and 1 for dead loads f d = f k /(g m g n ) Ususally g m = 1,2 for steel and 1,5 for concrete. g n = 1,0 1,2 depending on safety class 1,3 1,5 1,2 = 2,34

9 Reliability index b Frequency Load Effect, E (Sollicitation, S) Resistance, R m E R, E G = R E 0 m G = m R m E with s G std deviation b = m G /s G where b = reliability factor b = 4,75 corresponds to a failure safety of p = 10-6 (one failure out of a 1 million)

10 Probability and Reliability index b b Reliability index 1,28 2,32 3,09 3,72 4,2 4,75 5,2 P f Probability T = 50 years ASSESS DESIGN DESIGN BRIDGES The reference period T is of importance. Usually T = 50 or 100 years for structures For T = 1 year the probability will be 1/50 of the numbers for 50 years For swedes with a median life length of 80 years the probability to die before the age of 50 is 5% = 0,05 = the age of 80 is 50% = 0,5 = Probability for me to die from a Traffic accident 10-4 per year = per 50 year Train accident per year = 2, per 50 year Thunder storm 10-7 per year = per 50 year

11 Eurocodes from 2011 The work to create a European code began in Sweden joined in 1989, when the work was transferred to CEN (Comité Européen de Nomalisation) Pre-standards (ENV) were published between Voluntary to use. Early example: Öresund bridge. Definite standards (EN) All major parts are now finalized. National codes may be used during a transition period. SS-EN (Swedish version, generally translated). NA (National Annex, adjustable parameters for each country).

12 Survey of the Eurocodes EN 1990 Eurocode 0: Basis of Design EN 1991 Eurocode 1: Actions on Structures 1-1 Self weight 1-2 Fire Actions 1-3 Snow 1-4 Wind 1-5 Thermal Actions 1-6 Construction Loads 1-7 Accidential Actions 2 Traffic on bridges 3 Loads from cranes 4 Silo loads EN 1997 and EN 1998 Eurocode 7: Geotechnical Design Eurocode 8: Design in seismic areas EN 1992 to EN 1996 Eurocode 2: Concrete structures 2-4 Fastenings Eurocode 3: Steel structures Eurocode 4: Composite structures Eurocode 5: Timber structure Eurocode 6: Masonry structures Assessment of existing structures 59 separate standards approx pages EN 1999 Eurocode 9: Aluminium structures

13 NAVIGATION THROUGH STANDARDS FOR STEEL BRIDGES EN 1990 EN 1990-A2 EN EN EN EN EN Load combination Self-weight EN Safety aspects g G /g Q -values actions Traffic actions EN Wind actions EN Thermal actions EN Imperfections General EN Connections EN Stability of plates design Fatigue Ropes EN Seismic design Bearings EN 1337 EN Materials Prefabrication EN EN Welding execution Site work EN 1337 EN Corrosion protection Tolerances EN EN EN CE-marking Traceability product conformity Inspection EN Maintenance EN

14 Principal loads on anchorages Tension Shear Tension + Shear Shear lever arm fib Bull 58 (2011) Design of anchorages in concrete

15 Failure types Tension Steel failure Concrete cone failure Pull-out failure Combined concrete and pull-out failure Concrete splitting EN Fastenings Concrete blow-out

16 Convenor Rolf Eligehausen Stuttgart fib Group on Fastenings Abisko 2010

17 Test of Bonded Anchors SP, Borås,

18 Failure of bonded anchors after cyclic loading 18

19 Test Set-up [mm] Bonded Anchors Concrete Weight Concrete Foundation L750xW450xH

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21 Failure modes (a) Anchor steel rupture, (b) Anchor steel corrosion rupture, (c) Bond failure with shallow concrete cone (d) Bond failure with shallow concrete cone and splitting

22 Creep to failure curves for tests in Borås 23, 46 and 69% of No

23 Oskarshamn 3, BWR, Built 1985, 1200 MW -> 1450 MW Tests program sponsored by OKG AB and Vattenfall AB in collaboration with Westinghouse, SWECO and Inspecta

24 Step 1: Global seismic analysis Step 2: Pull-out (static & dynamic) Step 3: Updated global seismic analysis incl piping and anchor stifness F F a d t

25 Cracking and Seismic Loading When the first Swedish Reactors were built we did not consider Cracked concrete which reduces the capacity Seismic loads whichmay reduce the capacity

26 Capacity for Concrete Cone Failure fib Guide Part II. Expansion, undercut, screw and bonded expansion anchors Section Upgrade N Part IV. Cast-in headed anchors, Section k 1 = k cr = 8,9 cracked concrete k 1 = k uncr = 12,7 uncracked concrete h

27 Influence of surface reinforcement An investigation by Nilsson & Elfgren, LTU Univ. (2009) on behalf of Westinghouse Electric Sweden AB, OKG AB and Forsmarks Kraftgrupp AB. P code = K f cc 0.5 h 1.5 P test = Ψ s K f cc 0.5 h 1.5 Fastening without surface reinforcement Fastening with surface reinforcement 27

28 Possible parameters influencing the load capacity Amount of surface reinforcement (ρ = %) Pre-cracked & un-cracked concrete (w mm ) Dimensions of slab (length, width and thickness) Headed studs Anchor size: φ30 mm Anchor s head = φ45 mm Concrete Concrete class C25/30 28

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30 Test programme Variables Dimensions of slab (length, width and thickness) 1.2 m 1.2 m 0.3 m 2.2 m 2.2 m 0.3 m 1.2 m 1.2 m 0.6 m 2.2 m 2.2 m 0.6 m

31 Test programme Variables Crack through fastening Concrete cover No 30 mm (60 tests) Yes ( mm wide) 50 mm (6 tests) c

32 Boundary Conditions Larger Ring

33 Load [kn] Load [kn] Load [kn] Load [kn] Cracked Type 2 Ø16 #100 bottom, , cracked 0% (0,72) OKG003 OKG007 OKG Type 11 Ø12 #300 top & bottom, , cracked 0.15% 1,40 OKG037 OKG038 OKG039 OKG040 OKG041 OKG042 small slab Reinf. % P test /P code Deformation [mm] Deformation [mm] Type 13 Ø16 #150 top & bottom, , cracked 0.77% 1,47 OKG025 OKG026 OKG027 OKG028 OKG029 OKG Type 1 Ø16 #100 top & bottom, , cracked 1.16% 1,23 OKG004 OKG008 OKG012 OKG013 OKG014 OKG Deformation [mm] Deformation [mm]

34 Assessment When assessing an existing structures you know or may determine the actual - material properties - geometry - strains and stresses for specific loads These parameters are often uncertain when a new structure is designed. It may thus be conservative to apply codes developed for design when assessing existing structures. This was one of the reasons to start Sustainable Bridges Challenges for structural engineers dealing with existing structures: - Higher live loads - Fatigue safety and serviceability - Accidental actions - Durability - Interventions

35 Design vs Assessment - Reliability Analysis

36 European Work A special Eurocode for the assessment of structures is now under preparation by CEN/TC 250/WG2.

37 1910 TITLE Test to failure of a bridge in Kiruna due to extension of the mine situated under it Text here Deformation zone Fracture zone Bridge Collapse zone Open pit Mining level Hanging wall Iron ore m below surface Footwall 37

38 Pre-stressed five span bridge tested in Kiruna

39 Outline Ductile bending failure 39

40 Bridge beeing demolished, September 2014

41 Climate Change the biggest safety challenge? The probability for disastrous climate change is 1 (b 0) if we do not stop CO 2 emissions. However, we have no experience of it and it is not in front of our noses, so we do not really act. This is due to our brains (developed to help us survive other types of problems some years ago in our African past). Safety of fasteners is an important issue but climate change is a matter of life and death for the next generation. So, take command of your mind! Act now!

42 Thank you for your kind attention Division of Structural Engineering at Luleå University of Technology Chaired Prof. Dr. Andrzej Cwirzen