The University of Sheffield and

Transcription

1 The University of Sheffield and Buro Happold Specialist Consulting STRUCTURAL FIRE ENGINEERING ASSESSMENTS OF THE MOKRSKO FIRE TESTS An Engineering Prediction Anthony Abu, Berenice Wong, Florian Block and Ian Burgess

2 Introduction Advanced analysis of structures in fire is only as good as the modelling. The programs, the model, used should included all relevant physical features and should be validated. However, the modelling should also be validated as it is at least as important as the model. Two full scale fire tests were conducted in 2008: the FRACOF fire test and the fire test in Mokrsko. A priori modelling of the of both test has been undertaken based on pre-released data and engineering assumptions. After the tests more detailed modelling was done. 2

3 The Mokrsko Fire Test Czech Technical University of Prague S A B C Steel and concrete composite office building consisting of four bays with a size of 9m x 6m each. Tested three different floor systems: Angelina composite beams developed by Arcelor-Mittal with elongated web openings, 3 Meteorog. stanice Skládaný plášť Okenní otvor Ocelobetonová deska nad prolamovanými nosníky Okenní otvor Ocelobetonová deska nad nosníky s vlnitou stojinou Duté předepnuté panely Sádrové tvárnice Beams with corrugated webs made from thin steel plates, Precast hollow-core panels. 1 +4,00 Dveře Sendvičové panely Betonová stěna Mechanické zatížení pytli se štěrkem Meteorog. stanice +0,00 Požární zatížení hranicemi dřeva 3

4 Steel Frame 4

5 Beams Angelina Beams are an Arcelor-Mittal product based on a sine wave cut from an IPE270 with a total new depth of 395mm. The beams with the corrugated webs were 500mm with a web thickness of only 4.5mm. 5

6 Connection Details All beam connections connected only the top flange and a small part of the web of each beam. The bases of the columns were constructed as pinned. 6

7 Composite Slab 120mm composite slab CF60 metal decking using a smooth mesh (196mm2/m) and 10mm bars in each rib. 7

8 View from inside 8

9 View from above 9

10 Cladding 10

11 Mechanical Loading Imposed load was 3.0kN/m2 and the self-weight was 2.6kN/m2. 11

12 Fire Load S 3 A B C Timber cribs with a density of 35.5 kg/m 2 generated a total fire load of about 620MJ/m POŽÁRNÍ ZATÍŽENÍ +4,00 +0,00 POŽÁRNÍ ZATÍŽENÍ 12

13 Timber cribs 13

14 Ventilation The two openings were 2.54 m height and 4.00 m wide each This resulted in a Opening Coefficient of O = m 1/2. 14

15 Instrumentation Frame 15

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17 Thermocouples 17

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21 Test predictions A priori Only the 3 bays with the composite slab were modelled. The Angelina beams and the corrugated-web beams were represented using an effective web thickness approach. The beam connections were modelled as rigid. 21

22 Design Fires As with a normal SFE project a number of parameters were varied in order to test the robustness of the solution. The fire was altered to produce a short-hot fire (1) and a cooler-longer fire (3). The real fire (4) burned cooler than predicted (2) Gas Temperature [ C] Time [minutes] 22

23 Results of a priori modelling Much earlier increase in deflections than the experimental results as the parametric fire curves represent post-flashover fires, and should be moved by about 15min to give a realistic representation of the fire. Vertical deflection [mm] Time [minutes] 3 No indication of collapse but the vertical deflections are larger than span/15, which would normally result in an increase of reinforcement to limit the vertical deflections. All beams framing into columns would be protected in a robust design for fire. 23

24 Test day 18/09/

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29 Test Video 1 29

30 Test Video 2 30

31 1 st Structural Collapse of a Large Scale Fire Test 31

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35 Crack Pattern of Slab 35

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37 Composite Slab Corner 37

38 Compartment Temperatures - Back Teplota plynu [ C] Průměr změřených teplot plynu Okenní otvory Termočlánek TG01 Termočlánek TG07 TG07 TG Čas [min] 38

39 Compartment Temperatures - Front 1000 Teplota plynu [ C] Termočlánek TG11 Okenní otvory TG11 TG Termočlánek TG Průměr změřených teplot plynu Čas [min] 39

40 Compartment Temperatures - Left Teplota, C TG09 TG08 TG TG TG TG10 Průměr z TG TG07,TG08,TG09,TG TG Čas, min 40

41 Compartment Temperatures - Right Teplota, C TG05 TG04 TG06 TG TG TG06 Průměr z TG01,TG04,TG05,TG06 TG TG Čas, min 41

42 Steel Temperatures Angelina Beams Teplota [ C] AS AS2 AS5 AS6 AS5 AS4 AS AS Čas [min] 42

43 Vertical Deflections Čas [min] V5 V V3 V1 V7 V5 V7 V Deformace [mm] 43

44 Temperatures of Corrugated Web Beams TC23 TC Teplota [ C] TC TC TC TC23 TC Čas [min] TC22 TC79 TC21 44

45 Deformed Shape of Corrugated Web Beams 45

46 Connection Temperatures TC71 TC70 TC69 TC Teplota [ C] TC64 TC66 TC67 TC63 TC71 TC69 TC67 TC64 TC TC Čas [min] 46

47 Block shear failure of beam web 47

48 Concrete Spalling 48

49 Concrete Spalling 49

50 Concrete Spalling 50

51 Laser mapping of spalling on back wall 51

52 Concrete Slab Temperatures Temperature [ C] TC32 TC33 TC34 TC35 TC36 TC37 TC Time [min] 52

53 Concrete Slab Temperatures Temperature [ C] TC33 TC34 TC35 TC37 TC Time [min] 53

54 Vulcan Modelling directly after the Test Deflected Shape 54

55 Vertical deflection comparison The deflection curves show that when the real temperature data is used the vertical deflections are represented accurately up to about 44 minutes. The difference between prediction and reality for the beams with corrugated webs (V7) can be explained by the observed shear buckling of the thin webs. Vertical deflection [mm] V3 V1 V Time [minutes] V7 V1 V3 55

56 Horizontal movement of the mid column Due to the very flexible beam connections, therefore the connections were modelled as pinned. In these cases the Vulcan models stops around 43 minutes. The horizontal displacement at the top of the edge column connected to an unprotected Angelina beam. Horizontal deflection [mm] H Time [minutes] H1 + 56

57 New modelling Exact cause of failure of the structure - unknown Possibly due to: Failure of middle column Compression failure of slab Failure of connections Unzipping of slab from edge beams Buckling of back edge beam Reversal of crack in Angelina bay Determine: Approximate magnitudes of tensile forces in concrete Approximate magnitudes of compressive stresses in concrete Connector force distribution along the beam edge Axial forces in the back edge beam Effects of spalling on failure Failure initiation and eventual collapse of the structure 57

58 New Vulcan Models 3 bays, excluding hollow core bay effective stiffness approach include reinforcement in ribs test temperatures pinned column bases pinned (torsion-fixed) beam-to-beam and beam-to-column connections shear connectors for all internal beams realistic model of Angelina beams include cross-bracing 58

59 Angelina Beam - Models Comparison of models to find equivalent model for use in Vulcan Initial analyses (comparisons) with bare steel beams ABAQUS Angelina Beam Effective web thickness Vierendeel girder Truss Composite beam comparisons with Vulcan Effective web thickness Vierendeel girder 59

60 Modelling of Angelina beams Beam A Beam B Beam B2 60

61 Abacus deflected shape Beam A 61

62 Abacus deflected shape Beam B1 62

63 Abacus deflected shape Beam B2 63

64 Abacus deflected shape Effective web thickness 64

65 Abacus Deflection comparison Ambient Temperature 160 Angelina beam (ambient) 140 Applied loading (kn) Beam A Beam B Beam B2 Solid beam (eff. thickness) Truss mid-span displacement (mm) 65

66 Abacus Deflection comparison Elevated Temperature 0 Steel Temp vs Displacement (applied load =70kN) mid-span displacement (mm) Beam A Beam B Beam B2 Solid beam (eff. thickness) Truss Steel Temp 66

67 Angelina Beam - Vulcan Ambient 15kN/m2 applied load Elevated 7.5kN/m2 applied load Standard Fire Uniform heating beam Non-uniform heating - slab A B A B Section A Section B 67

68 Angelina Beam Vulcan Ambient Temperature Midspan deflection [mm] Load [kn/m2] EWT Beam B Beam B2 68

69 Angelina Beam Vulcan Elevated Temperature Deflection [mm] Temperature [ C] Beam B Beam B2 EWT 69

70 Main Analysis - Assumptions Beam B2 used for initial analysis large model long runtime Effective web thickness approach used for most analyses Protection material Promatech H (15mm thick board 870kg/m3, 920J/kgK, 0.21W/mK) Reinforcement - S500 (mesh 5mm 100/ mm bar in ribs) Concrete, f cu = 34MPa Steel fy = S235 (corrugated beams = S320) 19mm diameter shear connectors (3 per 1m) Beam-to-beam, and beam-to-column connections = pinned Effective stiffness approach for the slab Bracings Test temperatures gas temperature 70

71 New Vulcan Models 71

72 New Vulcan Models 72

73 Important points Realistic fires should be considered including the cooling phase. Integrity failure of the floor slab should be controlled by either deflection/curvature limits or finite cracking modeling. Reinforcement mesh in the slab must be sufficiently lapped to form a full tension 1000lap. All edge beams should be composite and 800 the slab should be tied to the beams. All columns should be tied in by protected 400 beams. 200 Connections should be designed to be ductile. Temperature ( C) Time (minutes) 73

74 Conclusion Conservative overall predictions of the response of composite structures to fire using sophisticated FE programs could be made. It was not possible to predict the failure mode or time prior to the tests but Vulcan could model the overall behaviour of both fire tests accurately when the correct input data was used. The tests showed that failure of structures is often caused by details! Therefore, robust construction details should be used until computer modelling can include these phenomena. Everyone who predicts the behaviour of structures in fire using FEA should validate their modelling against simple and well documented experimental data as well as full scale tests. Further modelling required to find cause of failure. 74