Structural failure and the Twin Towers. Dr Barbara Lane CEng

Transcription

1 Structural failure and the Twin Towers Dr Barbara Lane CEng

2 What lessons from the WTC event apply to other more typical building forms?

3 Everyone loves a good conspiracy theory..

4 Is there any scientific validity to the claims of 9/11 controlled demolition conspiracists about the collapse of the World Trade Center buildings?

5 5 WTC 1 & 2

6 6 WTC 1 & 2

7 7

8 8 WTC 7

9 9 WTC 7

10 10 WTC 7

11 Aftermath FEMA Initial Study NIST Investigation ( ) - (1) How & Why did WTC 1, 2 & 7 collapse? - (2) Why were fatalities so high or low depending on location? - (3) How were WTC 1,2 and 7 designed, constructed, operated and maintained? - (4) What needs to change in building & fire codes, standards and practice? 11

12 The numerical analysis of steel frame structures in fire had become of interest in the UK only in the 90s.

13 Broad gate fire Steel structure: - Partially unprotected 14 storey Fire: - developed in construction phase of 1st floor building - duration 4 hrs, max temp 1000 C 13

14 What did it show? Despite some large deflections, the structure behaved well and there was no collapse of any of the columns, beams, or floors. Detailed studies were carried out to determine the effect of the elevated temperatures on the mechanical properties of the steel. It was concluded that the materials showed no major loss in strength, though the maximum temperature of the structural members was around 600 C. It was sought to explain why these temperatures had not been reached and what alternative mechanisms had occurred to explain the structural deformation that had taken place. 14

15 History Cardington Tests Step change in fire resistance - Driven by steel industry in the UK - New stability mechanisms in fire case basis for reduced fire proofing

16 Thermal expansion

17 Thermal expansion What is it? The coefficient of thermal expansion is generally defined as the fractional increase in length per unit rise in temperature. All materials have their own unique increase in length And therefore when combined together cause mechanical displacement

18 Thermal expansion think of a bi-metallic strip

19 Thermal expansion important consequences Thinking of a floor in a building steel beams with shear studs connecting into the concrete slab a layer of steel and a layer of concrete when heated trying to expand in length at different rates with a resulting desired lateral displacement (sagging floor)

20 Thermal expansion in buildings Beams and columns and floors are NOT free to expand They are connected together Expansion happens in steel from temperatures above ambient; steel beams can buckle at 120C

21 Significance? A whole new set of forces to be dealt with by the structure as a result of restraining thermal expansion If you also now contemplate the materials as they cool again another change in shape occurring as well as a change in strength So contrary to popular belief that structure heats, melts, falls down: it s a highly complex change in state (expanding, but also weakening)

22 Back to post 9-11

23 NIST Findings WTC 1 & 2 (2005) Why did WTC1 & WTC 2 Collapse? - The WTC towers likely would not have collapsed under the combined effects of aircraft impact damage and the extensive, multi-floor fires if the thermal insulation had not been widely dislodged or had been only minimally dislodged by the aircraft impact. 23

24 NIST Findings WTC 1 & 2 (2005) Why did WTC1 & WTC 2 Collapse? - The WTC towers likely would not have collapsed under the combined effects of aircraft impact damage and the extensive, multi-floor fires if the thermal insulation had not been widely dislodged or had been only minimally dislodged by the aircraft impact. 24

25 They did not include thermal expansion in their numerical analysis and could not replicate the collapse mechanism

26 Arup/University of Edinburgh Investigations Global Structural Fire Behaviour Local failure mechanisms in fire Understanding fire behaviour in modern fire compartments Defining that realistic thermal exposure as a structural load

27 3D Modelling Strategy Quarter Floor Single Storey Half Floor Single Storey Quarter Floor Multi-Storey Simple Models Single Column Multiple Columns Cross-Section Model

28 Large inward bowing of columns at failure Spandrels undergo significant buckling Short-span double trusses see smaller deflections than longer spans Quarter Floor: Collapse Mechanism

29 Global Structural behaviour Numerical Modelling of undamaged WTC like structure exhibited global collapse mechanism Generation of a clear plastic hinge in the column Contraction of floors results large moment in the façade and rapidly increasing deflections of perimeter columns

30 A Conclusion: Structural mechanics in fire far more dominant than applied standard fire protection/traditional fire resistance ratings

31 31 Fires in Large Compartments

32 Fires in Large Compartments In large compartments only a portion of the floor is on fire Structure is exposed to very high local temperatures and cooler remote temperatures Burning travels through room resulting in longer exposure than traditional design fire

33 NIST, 2008 NIST, 2008 Arup

34 Understanding the Fire Design Fires Developed for Historic Small Rooms Large Open Plan Warehouse floor plate

35 Realistic Design Fires for Structural Assessment Realistic fire behaviour Defined Thermal Load for Structure Structural Heat Transfer

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37 2 nd Conclusion: The fire spread and growth rate over time and as it moves around a typical large space, is far more dominant than the standard fire resistance test for small spaces we rely on today

38 Design Today complex tall buildings Realistic Design Fires Structural Fire Design Evacuation Modelling Enhanced Systems & Coverage Threat & Risk Assessments Blast Analysis Management Strategies & Training 38

39 Application to other building forms

40 Storage compartment TGDB Area and volume limits Normal hazard Area m2 Volume Unlimited High Hazard Area -1000m2 Volume Unlimited 40

41 Storage buildings Period of fire resistance for elements of structure Typically 60/90 minutes for large single volume warehouses Compartmentation Compartment walls should achieve a fire resistance equivalent to period of fire resistance set out in table A2. Compartment walls require rating from each side separately rating based on performance criteria set out in conventional furnace test 41

42 Warehouse Fires Standardised racking system Fire spread test of racking system

43 Warehouse Fires Property protection in warehouses under split tenancy Code requires fire separation between tenancies for life safety (concrete encased steel structure with masonry infill panels) Warehouse structure not provided with applied fire protection, as not required for life safety Arup Client concerned that an uncontrolled fire in an adjacent tenancy might impact on their mission critical assets leading to major financial penalties Assessment undertaken to allow discussion with Client s Insurer to allow a realistic assessment of fire risk to be undertaken with respect to insurance premiums

44 Warehouse Fires Portion of Warehouse to be taken by the Client

45 Potential collapse scenarios Full compartment fires Local fires (i.e. single rack or early stages of a travelling fire) Sway failure mode - Connection failure in roof structure at interface between warehouse sections leads to damage being limited to the firewall gridline only. Single purlin/line of purlins failing - High forces transferred between warehouse sections leading to significant damage to client area

46 Potential collapse scenarios Rack structural failure leads to impact damage on the separating wall. Likely to breach the fire rated wall between demises as lateral impact loads are not typically assessed

47 Warehouse fires - conclusions This demonstrated that a fire in the adjacent tenancy could have a significant effect on the Clients structure The building was designed on the basis of relatively light wind loading and not the high horizontal loads that could result from a collapsing steel structure in severe fires. The bracing on the roof has been located and designed to resist horizontal (wind) loads transferred from the facades, not to resist fire induced loads Therefore a severe fire is likely to lead to a local or global collapse of the structure

48 9-11 was an extreme event but caused substantial analysis and deepened understanding of real fire behaviour and real structural behaviour that applies to anything we work on today

49 Thank you