Blast Effects and Mitigation

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6 September 2019 - 14:57, by , in blast mitigation, Comments off

Blast Resistant Design

 

Sources of Explosions

  • Condensed phase explosions (high explosives)
  • Vapor cloud explosions
  • Combustible dust explosions
  • Bursting pressure vessels
  • Boiling liquid expanding vapor explosion
  • Rapid phase transition

 

Who is interested in Explosions?

  • Military
    – Design facilities to resist hostile weapons, terrorist threats
    – Targeting/weaponeering of hostile facilities
    – Munitions storage and handling
  • Industry
    – Oil refining
    – Chemical processing
    – Explosives manufacturing
  • High‐profile buildings
    – National, local governments
    – Developers, owners of high rise or office buildings

 

How often do Accidental Explosions Occur?

  • ~1/day somewhere in the world
  • Explosive loading should be considered a rare event for design purposes, but not an impossible one

 

Some concepts to get used to

  • “Blast resistant” can be a misleading term
    – Better: “Able to resist blast loads of a given intensity”
  • Blast loads are low probability, high consequence
    – Always more difficult to accommodate in design process
  • Blast typically governs structure over conventional design
    – Adds some cost depending on level of threat
  • Components may undergo significant deformation and damage even at its design level
  • Building owner may determine design load and level of acceptable damage
    – Performance based design similar to current seismic design approach
  • No single authoritative, binding code

 

Differences between Conventional and Blast Design

Conventional Blast
High frequency event Low frequency event
 Always compulsory  Often voluntary
 Governed by building code  Design specification often selected by owner
 Loads prescribed by code  Design load related to owner decision
 Enforced by building officials  Exempt from building official review; often not subjected to peer review
 Loads are static  Loads are dynamic (time dependent)
 Response is elastic  Response exceeds elastic limit, can accept significant permanent deformations
 Analysis is static  Dynamic analysis methods are needed
 Material properties are conservative (reduced from nominal)  Material properties are realistic (increased from nominal)
 Relatively precise  Very high degree of uncertainty

 

Steps in the Design Process

  • Identify critical assets
    – Occupied buildings
    – Business interruption
    – Safe havens
  • Select hazard or threat
  • Determine applicable loads on the assets
  • Select allowable damage level (or level of protection)
    – Light, moderate, severe damage (response criteria)
  • Conduct analyses
    – Determine structural response of each component
    – Correlate to project mandated response criteria
  • Correlate component damage to building damage
  • Determine acceptability, iterate as necessary

 

Asset Identification

  • What are you trying to protect?
    – Your facility/buildings
    – Your employees
    – Off‐site personnel and property
    – Your reputation
  • What are they worth?
    – Business interruption cost
    – Replacement cost (of buildings)
    – Liability and damages

 

Threat or Hazzard Definition

  • Typically, need to define the threat
    – Military
    – Munition size, type, and standoff
    – Industry
    –  Assessment of hazards related to processes
    – Occupied buildings
    – Assessment of terrorist threats
  • Multiple threats may apply

 

Design Basis Threat (DBT) and Design Criteria

  • Design criteria consist of the loads and allowable response
    – Design Basis Threat – The capabilities and weapons that a potential assailant may possess
    – Design pressure and impulse for industrial hazards
  • Response levels based on guidance documents – unlike conventional static design, these vary with the use of the building!
    – AT/FP guidelines
    – DoD manuals
    – ASCE guidance
    – Industry publications or practice

 

Hand-Carried Charges: Bulk Explosives

 

 

Damage and Crater from Small Vehicle Bomb

 

Types of Blast Load Specification

Static
Quasi‐static
Pressure and duration (triangular)  
More complex   

 

Damage Levels

  • Two types of damage levels
    – Component
    – Building
  • Allowable damage level depends on criticality of asset being protected
    – Low damage: high‐priority buildings with critical function (e.g., central control room)
    – Medium damage: low‐priority buildings with non‐critical function, but significant populations
    – High damage: sparsely populated or unoccupied buildings

 

Alternative to Damage Levels: Levels of Protection

  • Can think of
    LOP  1/Damage
    – High damage  low LOP
    – Low damage  high LOP
  • Used in AT/FP guidelines

 

Response Analysis

  • Apply design loads to structure
  • Analysis can be performed in various ways
    – Look‐up curves
    – Engineering models (SDOF)
    – High‐fidelity models (finite element)
  • Determine response
    – Typically interested in peak response
    – For high‐fidelity modeling, more interested in material stresses and strains

 

 

ASCE Component Response Levels

Damage Level Description
Low Component has none to slight visible permanent damage.
Medium
(Typical Design Objective)
Component has some permanent deflection. It is generally repairable, if necessary, although replacement can be more economical and aesthetic.
High Component has not failed, but it has significant permanent deflections causing it to be unrepairable.
Collapse Component has failed completely.

Ref: ASCE, “Blast Resistant Buildings in Petrochemical Facilities,” Table 5.B.1.B, p. 69

 

 

Response Criteria

  • Typically applicable to engineering models
  • Typically defined in two parallel ways
    – Ductility = measure of extent of plasticity
    – Support rotation = related to deflection and span
    – Must satisfy both criteria (if both apply)

 

 

ASCE Building Damage Levels

 

Damage Level Description
Low Localized component damage. Building can be used, however repairs are required to restore integrity of structural envelope. Total cost of repairs is moderate.
Medium Widespread component damage. Building should not be occupied until repaired. Total cost of repairs is significant.
High Key components may have lost structural integrity and building collapse due to environmental conditions (i.e. wind, snow, rain) may occur. Building should not be occupied. Total cost of repairs approaches replacement cost of building.
Collapse Building fails completely. Repair is not feasible.

Ref: ASCE, “Blast Resistant Buildings in Petrochemical Facilities,” Table 5.B.1.B, p. 69

 

 

Overview of Assessment/Design Process

 

 

What is the Blast Consultant’s Role?

  • Need to be involved in the early stages of the project
    – Incorporate blast resistance in the structural system from the start
    – Far more efficient than upgrading a building after it is built
  • Assist in developing the system, not just the structure
    – Value of trade‐offs
    – E.g., longer distance from operator shelter to process unit
    – Lower blast loads
    – More time spent walking to/from shelter
    – Assist client in prioritizing objectives
    – Assist in selecting design threat, acceptable damage level

 

What is the Client’s Role?

  • Set budgets
  • Assign priorities to buildings
  • Determine damage level for design
  • Maintain realistic expectations
    – Understand roles of each discipline
  • Define operational constraints
    – E.g., retrofits to be applied externally only or building must remain operational

 

 

Blast Loads and Effects

 

What is an Explosion?

“…an explosion is said to have occurred in the atmosphere if energy is released over a sufficiently small time and in a sufficiently small volume so as to generate a pressure wave of finite amplitude traveling away from the source.… However, the release is not considered to be explosive unless it is rapid enough and concentrated enough to produce a pressure wave that one can hear.”
Baker, W.E., Cox, P.A., Westine, P.S., Kulesz, J.J., Strehlow, R.A., Explosion Hazards and Evaluation, Fundamental Studies in Engineering, Vol. 5, Elsevier, Amsterdam, 1983.

 

“The sudden conversion of potential energy (chemical, mechanical, or nuclear) into kinetic energy that produces and violently releases gas.”
National Fire Protection Association

 

 

Types of Waves

  • Two main classes of pressure waves that can be produced by explosions
    – Shock wave
    – Discontinuity in pressure— instant pressure rise
    – More severe loading condition for structures
  • Pressure wave
    – Gradual rise and decay of pressure
    – Less severe loading condition for structures

 

Ideal/Non-Ideal Distinction

  • Helpful to classify explosions into two basic categories
    – Ideal
    – Produces a shock wave
  • Non‐ideal
    – Can produce a shock wave
    – More commonly, produces a pressure wave

 

 

Definition of an Ideal Explosion

  • An instantaneous release of energy
    – Initially stored as internal chemical energy in the explosive
    – Instantaneously (or nearly) converted to heat and pressure through rapid chemical reaction
  • Energy dissipates radially outwards
    – Blast wave
    – Thermal radiation
  • Chemical reaction converts explosive material into detonation products at high temperature, pressure

 

Ideal Best Wave

 

 

Incident vs. Reflected Pressure

 

When blast wave propagation is interrupted by a rigid surface, the pressure increases to values greater than those for the incident blast wave
– Rigid boundary generates 2× reflection factor

Reflection factor for shock waves
– Approaches 2.0 as peak incident pressure decays below 1.0 psi [7 kPa]
– At higher pressures, factor can be as high as 10‐15

 

Loads on Structures

  • Front wall
  • Side wall
  • Roof
  • Back wall

 

 

 

Idealized Internal Explosions

Explosions inside a structure produce loads in two phases

  • Shock
  • Gas, or quasi‐static

Shock similar to exterior blast discussed previously

  • Complicated by presence of numerous internal reflections

Gas pressure

  • Due to confinement of detonation products within a finite volume
  • Function of type of explosive, explosive weight, and room volume
  • Subject to venting

 

Comparison of Shock and Gas Pressure

Shock Gas
Instant rise to peak Slow rise to peak
High magnitude Low magnitude
Short duration Long duration
Dependent on charge location Independent of charge location
Spatially varying (highly dependent on location) Spatially independent (assumed constant throughout room)
Not dependent on openings Venting highly dependent on area of openings

 

Internal Blast Loads

  • Must account for both shock and gas phases
  • Shock loads must include effects from internal reflections
  • Gas loads must include effects of venting

 

Combination of Shock and Gas Pressures

Internal blast loads typically simplified as bilinear pulse

  • Shock pressure idealized as triangle
  • Gas pressure idealized as triangle
  • Design pressure uses envelope of the two triangles

 

 

Effect of Casing on Blast Parameters

  • Typical conventional weapons use steel casing around explosive
  • Casing acts to absorb energy from the detonation and reduce the energy in the blast wave
  • The heavier the casing, the greater the reduction
  • Note: casing fragments (shrapnel) provide additional source of loading and damage to structural components (and lethality to humans) and must be accounted for separately

 

Non-Ideal Explosions Characteristics

  • Characterized by a relatively low detonation or deflagration velocity
  • Low energy density (energy / volume) also creates a non‐ideal explosion
    – Not a point source explosion as HE detonations are generally idealized

 

Examples of Non-Ideal Explosions

  • Vapor cloud explosions (VCE)
  • Fuel/air explosives (weaponized version of VCE)
  • Bursting pressure vessels
  • Dust explosions
  • Boiling liquid expanding vapor explosion (BLEVE)
  • Rapid phase transition (sudden conversion from liquid to gas)

 

Self-Acceleration in VCEs

  • Flame is accelerated by turbulence
  • Turbulence is created by obstacles in the flame path and confinement
  • Flame speed and blast generated is a function of:
    – Congestion
    – Confinement
    – Fuel reactivity
    – Volume of cloud
    – If deflagration, then volume of congested/confined region (i.e., not the total volume of the cloud)
    – If detonation, total volume of cloud

 

VCE Explosions

  • Generally considered the most credible catastrophic explosion hazard on a plant site
    – Especially where hydrocarbons are being processed
  • Typically external, but may also be internal
  • Most commonly result in deflagrations
    – But may undergo deflagration‐detonation transition (DDT) under some circumstances and produce a detonation

 

Consequences of VCE

 

Scene after Fertilizer Plant Explosions

 

Energetic Materials Plant Accident

 

Blast Analysis and Mitigation Techniques

Overview of Methods

  • Look‐up tables
  • P‐i curves
  • Single degree of freedom (SDOF) models
  • Multiple degree of freedom (MDOF) models
  • Finite element analysis (FEA)

 

Pressure-Impulse Curves

  • Simple but powerful tool
  • Allows rapid assessment of a structure or component

 

 

Prerequisites for Developing Pressure-Impulse Curves

  • Define loading waveform shape
    – Right triangle, isosceles triangle, etc.
    – Include or exclude negative phase
    – Many other options possible
  • Select a predictor of structural response
    – SDOF model
    – FE model
    – Test data
    – Accident data
  • Select a level of response
    – P‐i curves are iso‐response curves (“iso” = equal)

 

Development of P-i Curves

If the response levels correspond to damage criteria, then the zones between curves represent damage levels

  • < 1 in = Low
  • 1 – 2 in = Medium
  • 2 – 5 in = High
  • > 5 in = Collapse

 

 

SDOF Analysis

  • Simplest possible dynamic model
    – “Dynamic” because it calculates a time‐dependent response to a time‐dependent loading
    – Simplest because it only allows one degree of freedom
  • Requires numerous simplifying assumptions
    – Must assume response mode
    – 99% of the time, assume it is first‐mode flexure
    – Must assume load distribution
    – 95% of the time, assume it is uniform
    – Must simplify load‐displacement characteristics of structure

 

Equivalent SDOF System

 

  • Response of actual structural component to blast load can be determined by calculating response of an “equivalent” SDOF system
  • The equivalent SDOF system is a spring‐mass system with properties (M, K, Ru) equal to the corresponding properties of the component (modified by transformation factors)
  • The deflection of the spring‐mass system will be equal to the deflection of a characteristic point on the actual system (i.e., the maximum deflection)
  • Based on kinematic equivalency (equal displacement, velocity, and acceleration for the equivalent and actual system)
  • Properties of the equivalent system are derived from energy relationships

 

Finite Element Analysis

  • High fidelity numerical models are widely used in engineering analysis, focusing on:
    – Solids and structures
    – Fluids
    – Heat transfer
  • Use of the modern finite element method has become widespread as computers have become more powerful
  • Finite element analysis (FEA) has proven effective and widely applicable in engineering practice

 

Classes of FEA

  • Structural (Lagrange)
    – Explicit solver
    – Best for impulsive loadings, transient events
    – Requires very small time step
    – But result is inherently stable
  • Implicit solver
    – Best for steady‐state loads (e.g., gravity, equivalent static seismic and wind, etc.)
    – No minimum time step required
    – But convergence is not guaranteed (particularly problematic for heavily nonlinear problems)
  • Fluids (Euler)
  • Fluid‐structure interaction

 

When to use FEA?

Whenever the problem does not meet the limitations of the SDOF idealizations

  • Inclusion of multiple response modes in single problem
  • Irregular structural geometry
  • Inclusion of higher‐order effects (e.g., buckling, contact)
  • Non‐uniform loading distribution
  • Nonlinear, rate‐dependent material properties
  • Large displacement effects
  • Structural system with multiple interacting components
  • Failure predictions
  • Realistic boundary conditions
  • Need to generate “pretty pictures”

 

Design of Door within a Door

 

Post-Test Photos Compared to Window Catch System in LS-DYNA

 

LS-DYNA Analysis of Polyurea Window Catch System

 

ATFP and Progressive Collapse Requirements

  • Design Requirements
    – Blast design for major modernization of several existing buildings and design of new buildings
    – Assessment and upgrades to existing structural system during demolition, construction, and new operation loads (change of use)
    – Exterior façade (walls, glazing, etc) evaluation and upgrades for Anti‐Terrorism and Force Protection (AT/FP)
    – Progressive collapse prevention evaluations, designs and upgrades
  • Project Challenges
    – Determining as‐built information and assumptions for 100 year old structures
    – Historical/heritage preservation requirements
    – Need for solution with minimal impact on existing construction (i.e., minimal additional loads on building from new construction)
  • Solutions
    – Use of non‐destructive evaluation methods (such as ground penetrating radar scans) to determine exiting reinforcement layouts
    – Upgrade glazing, window frames, doors, and anchorage to meet blast requirements and match appearance of historic components or mitigate by catching debris
    – Combining façade upgrades for blast with interior architectural wall renovations to eliminate changes to visual appearance of building
    – FRP application to address progressive collapse and increase in design floor loads without increasing dead loads on structure
    – Innovative Products for Close Range Bomb Threats

 

Hazard Levels – ASTM F1642

 

Facade Upgrades on Interior Surface

  • Masonry Wall Upgrades
  • Window System Upgrades

 

Glazing Catch System and Shields

  • Innovative Catch System
  • Polycarbonate Shield

 

Development of Polyurea Catcher System

  • Test in Shock Tube
  • LS-DYNA Analysis

 

E-Glass Retrofit to CMU Wall

 

 

Slab Uplift and Progressive Collapse Upgrades

  • Progressive Collapse Retrofit
  • Blast Uplift on Slabs

 

R/C Slab in Uplift: Test Results

  • Test SP4 (for Control Slab)
    – 10.3 psi, 75 psi‐ms [71 kPa, 520 kPa‐ms]
    – Severe damage
    – Peak deflection of 9.5 inches [240 mm]

 

R/C Slab in Uplift: Test Results (cont’d)

  • Test SP5 (for SF‐1 scheme)
    – 17.6 psi, 127 psi‐ms [121 kPa, 876 kPa‐ms]
    – Moderate damage
    – Peak deflection of 3.25 inches [83 mm]

 

Concrete Column Carbon Wrap System

 

High-Performance Micro-reinforced Concrete

  • Resistant to high grade explosives
  • Adjustable properties, behaves similar to steel
  • No Spalling, No Fragment projectiles
  • Ideal for close range or contact blast
  • Extremely ductile with high degree of deformation
  • Highly durable, waterproof and fire resistant

 

Wall Retrofits & Shield for Close Range Threats

 

Retrofits & Shields for Close Range Threats

 

Case 2 – Industrial Facility – Existing Control Room

  • Objective is to protect building occupants
  • Blast Upgrades with Minimal Interruption to Building Function

 

Design Support from Concept to Completion

  • Design Requirements
    – Determine design blast loads on buildings based on possible industrial release scenarios
    – Generate blast mitigation concepts for a moderate level of damage
    – Conduct site inspection to document deviations from existing drawings
    – Develop detailed drawing package and specifications necessary for construction
    – Review vendor submittals for doors, windows, etc.
    – Provide construction administration services to address issues that arise during construction

 

Upgrades with Minimal Interruption

  • Challenges
    – Structures Must Remain Fully Functional
    – Interruption of services would result in substantial financial ramifications
    – Retrofits to structure that function as an integral part of facility operations and not compromise safety requirements (fire, toxicity, etc..)
    – Upgrades around sensitive equipment
    – Design modification for equipment that cannot be relocated
  • Existing Construction
    – Conventional construction, brittle materials, poorly maintained, more than 50 years old where blast and seismic design was not a consideration

 

Unreinforced CMU Wall Response

 

Steel Post Upgrade to CMU Wall

 

Test of CMU Wall After Upgrade

 

Example Masonry Wall Upgrade

 

Examples of Post Upgrades in the Field

 

Upgrade with Minimal Interruption

  • Solutions
    – Use of dynamic analysis for optimum solution
    – Relocate retrofits to exterior building surface to minimize impact on occupants or existing equipment
    – Use exterior retrofits techniques that been validated through testing

 

Control Rooms with Exterior Obstruction

 

Metal Building Upgrades

 

Example of Roof Strengthening

 

Door Strengthening 

  • Typical Existing Door
  • Retrofitted Door

 

 

Source: Baker Engineering & Risk Consultants, Inc.

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