Anti-Terrorism Blast Design For Building Engineers

January 2017

Doug Heinze, PE, LEED AP

• Introduction to Blast (Resistant) Engineering• Blast Resistant Facades• Blast Resistant Structures• Strategies for Incorporating Blast Requirements into

Building Design

Agenda

Introduction to Blast (Resistant) Engineering

High Energy (HE) Explosives• Blast shockwave expands

spherically from charge location

• Strength of shockwave decreases with distance

• Standoff distance is the most effective form of protection

Blast loads are immense

Shockwave load-time history

td

Simplified load-time history

tdtd

POSITIVE IMPULSE, iS

tdtd

• Blast Pressure Magnitude >> Environmental LoadsBlast Pressures typically measured in psi: 1 psi = 144 psf

• Blast Pressure Duration << Environmental LoadsBlast Load Duration typically measured in milliseconds:1 msec = 0.001 second

• Upper Bound Static Approach:Required strength = 2 x peak dynamic blast pressure

• Dynamic Analysis Required𝑚 ሷ𝒚 𝑡 + 𝑐 ሶ𝒚 𝑡 + 𝑘𝒚 𝑡 = 𝑭 𝑡

• Inelastic Behavior Encouraged

Just how extreme are they?

• Government ProjectsGeneral Services Administration (GSA): Federal buildingsDepartment of Defense: Military basesDepartment of State: Embassies

• Private DevelopmentsAttractive targets: Iconic, High profile tenant, etcMission criticalCollateral damage

When is blast mitigation required?

• Government projectsThreat defined by standard (often sensitive) criteriaRisk assessment may be required to assign required security level

• Private developmentsSecurity consultant performs Threat and Risk Analysis (TARA)Reverse engineering

Who defines the threats?

Impact of Event

Likelihood of OccurrenceHigh Moderate Low Very Low

DevastatingSevereNoticeableMinor

Very Low

Looking back in history

U.S. Embassy

Beirut, Lebanon

March 1983

Khobar Towers

Dhahran, Saudi Arabia

November 1996

Murrah Fed. Bldg.

Oklahoma City, OK

February 1995

U.S. Embassies

Kenya & Tanzania

July 1998

World Trade Center

New York, NY

February 1993

WTC Attacks

September 2001

Pentagon Attack

September 2001

Corporate Housing

Riyadh, Saudi Arabia

May 2003

• Vehicle Borne Improvised Explosive Devices (IEDs)Unscreened VBIEDS are difficult to quantifyPotential for screening to limit threat sizeMoving or stationary vehicle – Anti-ram barriers required?

• Man Portable IEDsUp to 100 lbs TNT equivalentPotential for screening to limit threat sizeCritical at short standoff distances (near contact)

Threat definition is subjective

Façade failure can occur at standoff distances well beyond the range of fatal blast pressures.

Providing increased standoff distance is the first step, but can only help so much.

Conventional buildings are vulnerable…

Blast shockwaves from exterior explosions load multiple building components:• Façade (direct and indirect)• Roof• Building lateral system• Infill loading

…to large exterior vehicle threats…

Smaller explosions can result in localized direct damage but can lead to disproportionate progressive collapse

…and small satchel threats

Simplified methods are industry standard for building design:• Calculate loads via empirical software• Calculate component response on a case-by-

case basis via Single Degree of Freedom (SDOF) systems

• Idealized elastic, perfectly plastic resistance function

• Utilize strength increase factors• Established inelastic response criteria for

Level of Protection or Level of Damage• Allows for adherence to project schedule

(and typical budgets)

How do we assess the building response?

Ru

Dy

F

D Forc

e-D

ispl

acem

ent

Resi

stan

ce F

unct

ion

SDOF

M

odel

Structural Performance

Component Damage Levels Building LOPDamage

Level

Description of Component

Response

SuperficialUnlikely to exhibit any permanent

deflection or visible damage.

Moderate

Unlikely to fail. Some permanent

deflection. Likely repairable but

replacement may be preferable

for economic or aesthetic

reasons.

Heavy

Unlikely to fail. Significant

permanent deflections. Unlikely

to be repairable.

HazardousThe element is likely to fail and

produce debris.

LOPPrimary

Structure

Secondary

Structure

Non-structural

Elements

Very

LowHeavy Hazardous Hazardous

Low Moderate Heavy Heavy

Medium Superficial Moderate Moderate

High Superficial Superficial Superficial

Multi-Degree-of-Freedom (MDOF) analysis: Captures phasing of multiple components

FE Analysis: Captures phasing, local response and higher order effects

Computational Fluid Dynamic (CFD) analysis: More accurate calculation of blast load

Sometimes more detailed study is warranted

Arena Test

Analytical Model

Arena Test

Analytical Model

Column – Far Field Beam – Near Contact

• The probability of a blast event is extremely low and cannot be precisely defined.

• It is generally accepted that damage can and will occur.• Potential goals:

Improve life safety during / following a blast eventEmergency evacuation?Protection of assets?Mission continuity?

Keep blast design goals in context

• Establish a secure perimeter• Mitigate debris hazard (typically from the damaged façade)• Prevent progressive collapse• Isolate internal threats from occupied spaces• Protect emergency services

Basic strategies for implementation

Blast Resistant Facades

Glass debris hazard is the #1 vulnerability

https://www.youtube.com/watch?v=owqXsJOWMnY

Conventional glass is both very brittle and forms extremely hazardous debris

Glass hazard is defined differently than other components – typically based on flight distance instead of ductility or end rotation.

The Goal: Reduce hazard

High Hazard

Hazard Rating Scheme

Hazard Mapping

• Laminated glass consists of multiple layers of glass bonded together by a plastic interlayer (typically PVB)

PVB holds glass shards together post-breakPVB develops membrane resistance to provide ductilityThickness of laminated glass may not be governed by blast requirements

• Alternate #1: Anti-shatter filmPrimarily for retrofitsDaylight application: Holds shards together onlyAttached application: Increases window capacity

• Alternative #2: Fully tempered glass

The Solution: Laminated glass

The Result: Hazard mitigated

Laminated / Film Only Blast Resistant

Well designed blast resistant window systems will break but not fail.

A goal is to dissipate as much energy as possible in plastic deformation of components to reduce forces transferred back to building.

Acceptable failure

Equally important to glass is design of framing

Bite Capture• Exterior glazed systems typically

preferred• Large membrane deformation =

cinching at supports• Structurally glazed systems

preferred

• Mullion Types• Aluminum / Steel• Cables• Glass Fin

• Plastic behavior• Design for blast load or glass

capacity (balanced design)?• Magnitude of load imparted on

structure

Window framing systems

Façade blast reactions often an order of magnitude (or more) larger than wind load reactions

Reactions vary with applied blast load AND mullion properties

Equivalent static reactions only an estimate until façade systems are finalized by Contractor

Consider load path back to structure

F1

F2

FF

Support reactions are heavily dependent on component resistance

Elastic range: Reaction varies with applied load

Plastic range: Reaction limited by component maximum resistance

Blast load vs component resistance

FF

Ru

DyD

ELASTIC PLASTIC

Blast reaction may change simply by changing member resistance

Example: Increasing mullion span may yield larger or smaller reaction depending on elastic or plastic response

Blast demands can be counter intuitive

For a simply supported beam:Ru = 8*Mn / LV = peq*L < 0.5* RuL

Example: Simple Span Beam (or mullion)peq

Consider a Longer Beam:For beam length increased to 2L, Ru = 8*Mn / 2L

Consider Fixed Supports:For fixed end boundary conditions, Ru = 16*Mn / L

Energy absorbing systems

• Reinforced concrete/masonry systems:Heavy = significant inertial resistanceDuctile detailing vs post-installed connectionsPrestressed / Post-tensioned systems are less ductile and not preferred

• CFMF systemsLow mass + strict response limits = less efficient designCladding must provide local resistance

• Metal panelsTypically insulated: Difficult to quantify resistanceConnections are problematic

Other types of blast resistant façade systems

Blast Resistant Structures

Far range exterior threats• Façade pushes on building structure / lateral

system• Exposed structure loaded directly• Infill blast pressures load interior structure

(in event of façade failure)• Blast overpressure applies downward force

on roof

Near contact threats• Extreme loads result in breach and other

local effects to primary structure

How is the structure affected?

Equivalent static blast base shears consider flexibility of lateral system, inertial resistance and acceptable global ductility

Sum of reactions considered at façade connections to structure >> blast-induced story shears

Local vs Global effects

x

m

c,k

F(t)

LocalEffects

F1

Global Effects

F1

Exposed or perimeter columns, walls and unbraced spandrel beams may be subject to lateral blast loads.

Consider column lateral reaction at beam-column connection, particularly in rebound.

Consider shear and bending forces at column splices.

Exposed/perimeter structure

Façade is often not designed for large local threats

Infill blast loads may load interior floor systems in net uplift

Consider lateral bracing requirements for columns vs allowable floor failure

Infill blast pressures

Interior walls are often hardened to protect life safety systems or separate screened from unscreened occupancies.

Hardened walls may be heavy and will generate large lateral reactions.

Protection of critical MEP systems

Satchel threats

• Public areas, loading docks, mailrooms and below grade parking are all potential locations for satchel threats

• Interior satchel threats result in multiple shockwave reflections and build-up in gas pressures, which increase total impulse on surrounding structure

• May lead to disproportionate collapseHow best to mitigate collapse?Threat Dependent vs Threat Independent Approach

Progressive / disproportionate collapse example

Failure scenario: satchel threat fails ground floor column

Failure typically assumed to propagate vertically

Consider a progressive collapse scenario

Progressive collapse mitigation strategies

Threat Dependent:Specific Local Hardening

Threat Independent:Alternate Path Method

Which building materials are preferred?

• Both concrete and steel structural systems are able to efficiently withstand blast loading

• Masonry and precast structures require more attention to detailing to achieve necessary load paths and ability to withstand load reversals

• Wood and CFMF structures are less common due to lower resistance but can be used

Concrete systems have significant inertia but are susceptible to shear failures.

Steel systems have inherent ductility but are locally vulnerable open sections and connections.

Combination of steel and concrete is ideal.

Different materials have different pros/cons

Anti-ram barriers

Barriers tend to have deep continuous foundations• Shallow foundation systems popular in urban

areas to avoid utilities• Barriers may be mounted directly to

structure…performance criteria?

Operable Bollards

Stationary Anti-Ram Barriers

Strategies for Incorporating Blast Requirements into

Building Design

Blast loads are typically combined with gravity loads and load factors are set equal to 1.0.

Use a more realistic guess at day-to-day live load.

While blast loads are dynamic, in some circumstances it makes sense to use equivalent static blast loads for design.

Blast as a separate load case

From ASCE 7:

Add two more (per ASCE 59-11):1.0B + 1.0D + 0.5L

1.0B + 1.0D + 0.2W

Dynamic Load Factor (DLF) may be applied to peak blast pressure to determine equivalent static load.

However, this varies with ratio of td/T

When are equivalent static loads practical?

P

td

Even greater variability is added in when allowing for inelastic response.

Conclusion: Equivalent static loads are not generally applicable for flexural design of systems with ductile, flexible components

Exception: Equivalent static blast-induced base shears are useful given complexity and limited cases.

When are equivalent static loads practical?

Look at connections

• Components in connections are typically stiff (relative to the supported elements) and respond at a high frequency.

• Many connection components are brittle and cannot achieve a plastic response.

• However, connections do not see the blast pulse directly –they experience a different load pulse.

0.1Ttd

Elastic response of supported member

Look at connections

Plastic response of supported member (upper bound)

• Flexible and/or ductile membersDesign dynamically utilizing inelastic response criteria

• Connection componentsDesign using equivalent static blast reactions

B = DLF x Rb

where DLF = 1.0Rb = peak dynamic blast reaction

Blast engineer provides “B” to structural / façade engineer for

comparison to other static load cases

Blast reactions higher than expected for plastic response due to dynamic strength increase factors

Division of responsibility

• Blast upgrades to baseline structural or façade systems should be shown in relevant disciplines drawings

• For performance based systems, such as curtainwall or roof joists, blast performance requirements should be incorporated into project specifications

There is variable familiarity with blast design requirements from trade to trade.More Experienced: curtainwall and precast subsLess Experienced: CFMF, masonry and steel joist subs

Implementation of blast requirements

• Façade design is typically performed to a proof of concept level with performance specifications provided for Contractor use

• How to design and detail supporting structural components for blast reactions that may change when façade is finalized?

Provide “adequate” safety factor on blast reaction forces.

Indicate on drawings what blast reactions have been considered.Does not affect lateral system, which is typically based on assumption of rigid façade.

Hiccups in the process

• Similar to façade components, blast reactions for structural members may be controlled by member resistance

• Changes to the member sizes may increase the blast reactions even though the blast load doesn’t change

• Changes to member boundary conditions may also increase blast reactions

Ru of pinned end beam = 8*Mn/LRu of fixed end beam = 16*Mn/L

• Can lead to shear issues

Hiccups in the process

Ru

DyD

ELASTIC PLASTIC

More detailed analysis may yield benefits (at the expense of time).

In some cases, local failure may be acceptable if it doesn’t

compromise blast design intent.

Blast forces are big

• Ductile = Good / Brittle = Bad• Flexible systems are better for blast• Blast reactions will likely control design• Involve the Blast Engineer early and often to achieve

solutions that account for all requirements and to avoid surprises later on

• Build time into schedule for Blast Engineer to perform review – expect an iterative process

Parting thoughts

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© Thornton Tomasetti Inc. 2017