Part 3: Mechanical response Part 3: Mechanical response

Transcription

1 DIFSEK Part 3: Mechanical response Part 3: Mechanical response 0

2 Resistance to fire - Chain of events Θ Loads 1: Ignition time Steel columns 2: Thermal action 3: Mechanical actions R 4: Thermal response time 5: Mechanical response 6: Possible collapse Part 3: Mechanical response 1

3 How structures react to fire Temperature rise thermal expansion + loss of both stiffness and resistance additional deformation eventual collapse t = 0 θ = 20 C 16 min θ = 620 C 22 min θ = 720 C 31 min θ = 820 C Part 3: Mechanical response 2

4 Damage after Fire Part 3: Mechanical response 3

5 Assessment of mechanical response of structures in fire Purpose to describe structural behaviour under any type of fire condition Means P P Fire tests Design Deflection Load-bearing resistance Standard Fire Time P Time Part 3: Mechanical response 4

6 Basic features related to assessment of mechanical response of steel structures in fire Mechanical loadings under fire situation specific load combination Mechanical properties of relevant materials at elevated temperatures stiffness and resistance varying with temperatures Assessment methods for structural analysis in fire different approaches application domain Specific consideration in fire design of steel and composite structures connections, joints, etc Part 3: Mechanical response 5

7 Mechanical loading combination according to Eurocode (EN1990 and EN ) G k,j + (Ψ 1,1 or Ψ 2,1 ) Q k,1 + Ψ 2,i Q k,i j 1 i 2 G k,j : characteristic values of permanent actions Q k,1 : characteristic leading variable action Q k,i : characteristic values of accompanying variable actions ψ 1,1 : factor for frequent value of a leading variable action ψ 2,i : factor for quasi-permanent values of accompaning variable actions Load level: η fi,t (see presentation of WP1) Part 3: Mechanical response 6

8 Mechanical properties of structural steel at elevated temperatures (EN ) Strength % of normal value Elastic modulus Effective yield strength Temperature ( C) Normalised stress 1 20 C 200 C 400 C C C C C Strain (%) Elastic modulus at 600 C reduced by about 70% Yield strength at 600 C reduced by over 50% Part 3: Mechanical response 7

9 Mechanical properties of concrete at elevated temperatures (EN ) Strength % of normal value Strain (%) 6 Strain ε cu at 100 maximum 5 strength Normalweight Concrete Temperature ( C) Normalised stress C C 800 C 400 C 600 C Strain (%) ε cu Compressive strength at 600 C reduced by about 50% Part 3: Mechanical response 8

10 Thermal expansion of steel and concrete (EN and EN ) 20 L/L (x10 3 ) normal concrete steel temperature ( C) Part 3: Mechanical response 9

11 Different design approaches for mechanical response of structure in fire Three different approaches according to Eurocodes global structural analysis analysis of parts of the structure member analysis (mainly when verifying standard fire resistance requirements) Part 3: Mechanical response 10

12 Different design approaches for mechanical response of structure in fire Member analysis Global structural analysis independent structural element analysis simple to apply generally for nominal fire condition interaction effects between different parts of the structure role of compartment global stability Part 3: Mechanical response 11

13 Three types of design methods for assessing mechannical response of structures in fire Tabulated data composite structural members Simple calculation models critical temperature steel and composite structural members Advanced calculation models all types of structures numerical models based on: finite element method finite difference method Classic and traditional application Advanced and specific fire design Part 3: Mechanical response 12

14 Application domain of different design methods under fire situation Thermal action defined with nominal fires Type of analysis Tabulated data Simple calculation models Advanced calculation models Member analysis Yes ISO-834 standard fire Yes Yes Analysis of a part of the structure Not applicable Yes (if available) Yes Global structural analysis Not applicable Not applicable Yes Part 3: Mechanical response 13

15 Application domain of different design methods under fire situation Thermal action defined with natural fires Type of analysis Tabulated data Simple calculation models Advanced calculation models Member analysis Not applicable Yes (if available) Yes Analysis of a part of the structure Not applicable Not applicable Yes Global structural analysis Not applicable Not applicable Yes Part 3: Mechanical response 14

16 Tabulated data (steel and concrete composite members) Composite beams Composite columns Slab Concrete for insulation Part 3: Mechanical response 15

17 Tabulated data and relevant parameters (composite columns EN ) A c e w b e f A s u s u s h Standard Fire Resistance R30 R60 R90 R120 Minimum ratio of web to flange thickness e w /e f 0,5 1 Minimum cross-sectional dimensions for load level ηfi,t 0, minimum dimensions h and b [mm] minimum axis distance of reinforcing bars us [mm] minimum ratio of reinforcement A s /(A c +A s ) in % 2 Minimum cross-sectional dimensions for load level ηfi,t 0, minimum dimensions h and b [mm] minimum axis distance of reinforcing bars us [mm] minimum ratio of reinforcement A s /(A c +A s ) in % 3 Minimum cross-sectional dimensions for load level ηfi,t 0, minimum dimensions h and b [mm] minimum axis distance of reinforcing bars us [mm] minimum ratio of reinforcement A s /(A c +A s ) in % Standard fire rating Load level Section dimension Reinforcing steel Concrete cover Part 3: Mechanical response 16

18 Critical temperature method (only steel members and certain composite beams) Beams (steel and composite) Columns Part 3: Mechanical response 17

19 How to apply critical temperature method FIRE RESISTANCE Action in fire E fi.d Design resistance at 20 C : R d or design action at 20 C : E d Load level in fire: η fi = E fi,d R d Utilisation level: µ 0 = η fi,t γ M,fi γ M Critical temperature: θ cr Part 3: Mechanical response 18

20 Critical temperature method θ cr = ln µ Strength % of normal value Elastic modulus Effective yield strength Temperature ( C) Part 3: Mechanical response 19

21 Why direct and iterative critical temperature method (case of steel column) Short column without instability N b,fi,t,rd = A k y,θmax f y 1 γ M,fi Strength reduction factor k y.θ.max at θ a,max Column with risk of buckling N b,fi,t,rd =χ(λ θ )Ak y,θmax f y 1 γ M,fi Reduction factor of buckling χ(λ θ ) depends on: strength stiffness As a consequence, simple iterative procedure is needed to find the accurate θ a,max in case of instability problem Part 3: Mechanical response 20

22 Simple calculation model (steel and composite members) Beams (steel or composite) Columns Part 3: Mechanical response 21

23 Simple calculation model (composite beam) - plastic resistance theory S 1 Concrete slab Connector Steel section S 1 Section S F c + F t + D + Section geometry Temperature distibution Stress distribution Moment resistance M fi,rd + = F + t D + Part 3: Mechanical response 22

24 Simple calculation model (composite column) - buckling curve P A ai Z 1.0 χ(λ θ ) L fi A cj Y 0.5 A sk Effective section 0 Appropriate buckling curve λ θ Load capacity: N fi.rd = χ(λ θ ) N fi.pl.rd χ(λ θ ) strength and rigidity of effective section + column buckling length L fi Part 3: Mechanical response 23

25 Software AFCC Fire design of Composite Columns Part 3: Mechanical response 24

26 Software AFCB Fire design of Composite Beams Part 3: Mechanical response 25

27 Advanced calculation model for any case (steel and concrete composite cellular beam) Deflection (mm) test calculation time (min) Calculation vs test Tested failure mode Simulated failure mode Part 3: Mechanical response 26

28 Fire design by global structural analysis General rules necessary to use advanced calculation model choice of appropriate structural modeling existing boundary conditions loading conditions appropriate material models restrained condition in relation with unmodeled parts of the structure analysis of results and check on failure criteria review of untreated features in direct analysis (consistency between numerical model and constructional details) Part 3: Mechanical response 27

29 Material strength during cooling phase Steel recovers its initial strength during cooling phase Concrete during cooling phase Temperature of concrete θ max 1.0 Strength 300 f c,θ max 0.9 f c,θ max 20 t max For example if θ max 300 C f c,θ,20 C = 0.9 f c,θ max Time Linear interpolation applies to f c,θ for θ between θ max and 20 C 0 t max Time Part 3: Mechanical response 28

30 Global analysis of steel and concrete composite floor under localised fire Standard part of the floor system Composite slab Steel deck: 0.75 mm 3.2 m 4.2 m 15 m 10 m 15 m 10 m 10 m 15 m Part 3: Mechanical response 29

31 Choice of structural model 2D composite frame model 3D composite floor model Limited membrane effect no load redistribution Between parallel beams Simple calculation so more rapide to realise and more efficient Membrane effect over the whole floor Load redistribution between parallel beams Better evaluation of load bearing capacity More realistic to apply the 3D composite floor model Part 3: Mechanical response 30

32 Validity of 3D composite floor model Test 3D calculation model Vert. Disp. (mm) Hori. Disp. (mm) Time (min) Test Cal. 3D Cal. 2D Time (min) Part 3: Mechanical response 31

33 Strategy of 3D composite floor modelling Fire area Global structure without composite slab Detail of numerical modelling Part 3: Mechanical response 32

34 Mechanical response of the structure Total deflection of the floor and check of the corresponding failure criteria 60 min 230 mm Deflection (mm) mm L/20 = 500 mm Secondary beam Main beam Time (min) 280 mm L/20 = 750 mm Part 3: Mechanical response 33

35 Mechanical response of the structure Check of failure criteria: elongation of reinforcing steel 1.4 % 5 % 1.3 % 5 % Strain of reinforcing steel // slab span Strain of reinforcing steel slab span Part 3: Mechanical response 34

36 Beaviour of slab during test Part 3: Mechanical response 35

37 Behaviour of concrete slab during Cardington test Part 3: Mechanical response 36

38 Specific consideration in fire design of steel and composite structures Constructional details Joint details (steel and composite) Connection between steel and concrete Connectors Reinforcing steel Behaviour during cooling phase under natural fire Joint Part 3: Mechanical response 37

39 Construction details shall be respected in order to consistent with numerical models Reinforcing bars between slab and edge columns φ12 in S500 Maximum gap of 15 mm between beam and column and between lower flange of the beam gap gap 15 mm Part 3: Mechanical response

40 Fire test Damage after cooling Part 3: Mechanical response 39

41 Fire test Damage after cooling Part 3: Mechanical response 40

42 Construction details for connection between concrete and steel (EN ) Connection between steel profile and encased concrete φ r 8 mm φ s 6 mm welding a w 0,5 φ s l w 4 φ s h ν studs d 10 mm h ν 0,3b φ r 8 mm Welding of stirrups to the web b Welding of studs to the web Part 3: Mechanical response 41

43 Windsor Tower Madrid Failure in the fire compartment provoked failure in neighbour compartments. The chain failure is stopped by the Strong floor Part 3: Mechanical response 42

44 Fire is not only to be considered for steel structures!!!!!! Part 3: Mechanical response 43

45 Sudden failure of non exposed column due to dillatation of the exposed structure Part 3: Mechanical response 44

46 Failure in a non exposed compartment provoked failure of whole building Commercial building in Gent (B) 1974 Concrete structure 50x50m (on site pouring) How to ensure the structural stability of non affected compartments? Part 3: Mechanical response 45

47 Parking in Switzerland 7 death firemen Sudden failure No former deformation Fire already under control Complete failure of slab after 90min Punching of columns trough the slab Loading higher than designed No verification in Eurocodes regarding this type of failure Part 3: Mechanical response 46

48 Industrial building in Charleroi Concrete Structure : drop of masonry outwards Steel Structure: drop inwards Part 3: Mechanical response 47

49 Industrial building in Spain Part 3: Mechanical response 48

50 Heavy Façade drop inside Part 3: Mechanical response 49

51 A good fixation between facade elements and the structure is essential Part 3: Mechanical response 50

52 Heavy Façade with insufficient connection to the structure Part 3: Mechanical response 51

53 No Failure during fire due to appropriate Fire engineering approach Part 3: Mechanical response 52