COST C26 Final Conference: “Urban Habitat Constructions under Catastrophic Events” Naples, Italy, 16-18 September 2010 WG3 - Impact and explosion resistance Task 3 - Vulnerability and damageability of constructions under impact and explosion Florea Dinu Romanian Academy, Timisoara, Romania
Introduction • Impact and explosion resistance (WG3) – to analyse the behaviour of constructions from urban environment under very strong accidental actions, such as gas explosions, bomb blast, or impact from projectiles or vehicles out of control. – these phenomena are characterized by a large amount of energy that is released in a very short period of time – The main research activities: • Assessment of degradation and damage • Modeling MoU, Cost C26 • Structural analysis
Research in WG3, Task 3 • The research focused on tools to assess vulnerability and robustness of structural systems to blast and impact: – Performance expectations for constructions – Threat and risk assessment – Progressive collapse – Available tools and approaches – Furthure developments
Performance expectations for constructions Threat characterization Murrah Federal Building Khobar Tower TNT Equivalent 4,000 lb. TNT Equivalent 20,000 lb. Standoff 15 feet Standoff 80 feet DoD Antiterrorism Standards for Buildings
Performance expectations for constructions Incorporation of prescriptive measures Ufundi House US Embassy formerly 8 stories DoD Antiterrorism Standards for Buildings
Threat and risk assessment • Maintaining adequate stand-off is effective in reducing the risks from blast but may not be enough • Risk mitigation strategies must be aimed at three basic levels where: P(C) = probability of structural collapse due to abnormal load, H, λ H = rate of occurrence of the abnormal load or hazard, P(LD|H) = probability of local damage given that the abnormal load occurs P(C|LD) = probability of collapse given that local damage occurs.
Progressive collapse • The approaches for reducing the risk of progressive collapse are categorized as follows: – Event control – Indirect design – Direct design
Reaction • CEN/TC 250 decides to form an Ad-hoc Group “Robustness” • Improved definition and quantification of robustness • Clearer description of accidental actions needed – EN 1991-1-7 (external blast ?) • Development of sets of material dependant (prescriptive) measurements in dependency of the consequence classes to ensure robustness
Design approach • Structural design strategies for structures designed to resist blast and mitigate progressive collapse, fall into two general categories: – the indirect method – the direct methods • providing specific local resistance for the abnormal load - such an approach provides resistance to only one hazard • developing alternate load paths - is intuitively more attractive because it focuses the attention of the designer on the behavior of the structural system
The indirect method • In the indirect method , it is used a prescriptive approach to increase the overall robustness of the structure. • This is accomplished by incorporating general structural integrity measures (selection of structural system, layout of walls and columns, member proportioning, detailing of connections). • The indirect method is recommended for facilities that are characterized as requiring a very low level of protection.
The indirect method Different types of ties incorporated to provide structural integrity
Specific local resistance • In the specific local resistance method , the designer explicitly designs critical load bearing building components to resist the design level threat, such as blast pressures. • Thus, it is a threat specific approach. • This method is also referred to as key element design • The specific local resistance direct design approach is often the only rational approach when retrofitting an existing building
Specific local resistance • Key elements are defined as structural elements whose notional removal would cause collapse of an unacceptable extent • They should therefore be designed for accidental loads, which are specified in several standards as 34 kPa. Such accidental design loading should be assumed to act simultaneously with 1/3 of all normal characteristic loading: D + L / 3 + Wn/ 3 where D = dead load, L = live or imposed load, and Wn = wind load, The difficulty with strengthening key elements is that it must be done with a specific threat in mind!
Specific local resistance Reinforced concrete column without/with composite wrap
Specific local resistance THE TAIPEI 101 • 8 units are overlapping with each other. • 10 very strong mechanical floors to stop unexpected progressive collapse.
Alternate load path method • The load carried by the lost element must find an alternate load path to the building supports without initiating structural collapse. • Large deformations are permitted before the onset of failure of an element. • This method reduces the risk of progressive collapse by ensuring structural redundancy • The method does not require characterization of the threat causing loss of the element, and is, therefore, a threat independent approach .
Alternate load path method The amplification of the gravity loadings applies only to the section of the structure directly below the failed element!
Alternate load path method Load Application for Alternate Path Analysis (UFC 4-023-03, 2004)
Available tools and aproaches • Vulnerability of urban area configurations to blast effects – Single buildings – Rectangular urban layout • Robustness analysis of constructions to impact – Bridge response to train collision – Response of RC frames to impact loading • Robustness analysis of buildings to blast – Tying force method – The specific local resistance method – The alternate load path method
Assessment of explosion effects in railway stations • Blast waves resulted from the detonation of a solid high-explosive charge (a TNT equivalent). • Waiting hall area about 50×30×12 m, long corridor about 100×10×8 m • The full simulation of the explosion is performed using an Eulerian Geometrical finite element model of structure formulation for the ex- consisting of a main hall and a long corridor plosive and for the fluid representing the air
Structural response at four time instants using the pressure-time functions approach
Top view of death risk contours for the main hall and corridor
Response of RC frames to impact loading • The study aims – To calculate the qualitative changes of the RC framed structures, under impact loading – To evaluate risk level for such abnormal situations • The impactor was modeled as a rigid 197 kg weight of cylindrical body with the density of 7830 kg/m3, with an elastic modulus 159 GPa and RC framed structure Poisson’s ratio of 0.2
Comparison the point of risk level. Indicated to parentheses in RC slab state D 2,1 Point of risk level 1,9 1,7 1,5 1,3 0,1 0,11 0,12 0,13 0,14 0,15 0,16 0,17 0,18 0,19 0,2 Time (ms) D (A) D (B) D (C)
Robustness evaluation of a multistory building in case of column loss • The 26 story designed to withstand seismic forces up to 0.24g • Overstrength conditions for non-dissipative critical members (columns, joints, ) + ductility for dissipative members • Alternate load path to evaluate the robustness of the structure • In order to develop a multilevel evaluation, different extension of damage was considered • Columns are removed one by one and 3D dynamic nonlinear analyses are employed
Structural configuration
Alternate load path Load C5 C4 C3 C2 C6 C1 (1/20)T 10 Time (sec) Column removal locations Application of vertical load in the dynamic analysis
Results 0 Displacement (mm) -20 -40 Linear response C3 -60 C4-5 -80 Nonlinear response -100 -120 0.00 0.10 0.20 0.30 0.40 0.50 Time (sec) Vertical deformation vs. time Plastic hinge & axial force diagram for loss of two columns at a time – case C4-5.
Results 2 1.8 1.6 numerical DIF DOD, 2009 1.4 1.2 1 0 1 2 3 4 5 6 Normalised plastic rotation Dynamic increase factors for different loss scenarios
Further developments • Accounting for accidental actions from man-made hazards (blast, impact) in the codes • Improved requirements for structural robustness (eg. prevention of progressive collapse from terrorist attacks) • Risk oriented design rules for important facilities • Design of new buildings and assessment of existing buildings under blast and impact – methods of analysis (direct and indirect methods) – acceptance criteria – structural detailing for structures subjected to blast, impact (principles, type of connections, tying systems) – possible extension of seismic design principles and details • New materials and construction techniques – unobtrusive and aesthetic facades – nonfrangible glass for facades • Performance based format for multi-threat assessment (eg. blast and fire).
Blast load/ explosions – The Risk Matrix Recommended practice for the design of offshore facilities against fire and blast loading API RP 2FB First Edition 2006
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