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)
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]
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
Homes require timely maintenance to prevent wall leakages in the future. Contact us now, our service is ready to help you.
Source: Baker Engineering & Risk Consultants, Inc.