Blast Design Oswald

r Structural Engineers Charles J. Oswald, Ph.D., P.E.

r

DoD Antiterrorism Requirements • UFC 4-010-01 “DoD Mimimum Antiterrorism ” • Minimu Minimum m stando standoff ff dist distanc ances es from from con contro trolle lled d perimet perimeter er and parking to “inhabited” buildings • Wi Windo ndow w and and windo window w fram frame e desi design gn sta standa ndard rdss • Pr Progr ogres essi sive ve col colla laps pse e avoid avoidanc ance e stand standard ardss

• A lies to new buildin s and renovations or repairs > 50% of building replacement cost • Minimum standoffs can be reduced if justified y as ana ys s an or u ng ar en ng

STEER Meeting, March 22, 2007

Government Building Requirements • Interagency Security Committee (ISC) Security • Antiterrorist design requirements for U.S. government buildings

• Blast design requirements usually apply only for large federal buildings (e.g. courthouses, FBI buildin s • Building structural member and windows must resist specified external blast loads depending on required level of protection • Progressive collapse avoidance requirements • Requirements for vehicle barriers around building


Explosive Storage and Manufacturing • State and federal safety requirements for • Protect against accidental explosion scenario

• Typically requirements satisfied with prescribed standoff distances • Otherwise, personnel protection from blast, , designed • Personnel may be in same building or in nearby building • Design to contain explosion effects in explosion room • Design of surrounding buildings if explosion not contained


Industrial Building Requirements • Many industrial processes may cause accidental • Petrochemical and chemical manufacturing processes

• OSHA 1910 requires blast siting analysis • Consequences must be estimated from credible explosion scenarios

explosion effects • Typically building occupants more at risk than personnel in open ue o n ury rom a e u ng componen s • Buildings may require blast resistant structural upgrades to provide required protection


General Blast Design Approaches • Single-degree-of-freedom (SDOF) approach • • Combines explicit consideration of non-linear dynamic response with design level simplicity

• Equivalent static load (ESL) approach • Used for connections and other stiff members including bracing and shear walls subject to reaction loads • Most accurate if dynamic load application is slow compared to component response time, or natural period (i.e. no inertial effects)

• Dynamic finite element analyses • Non-linear dynamic analysis


Basic Blast Design Assumptions • Material yield strengths increased for strain• Fast response causes high strain-rate and greater yield strength (10% to 20% increase)

design • Yielding of component in ductile response • Strengthen non-ductile response modes so they don’t control response

•

es gn ase on a owa e e ec on ra than allowable stress • Typical design allows one-half of failure deflection STEER Meeting, March 22, 2007

er

SDOF Design Procedure • Calculate dynamic load • • Use dynamic reaction load from cladding on framing components

• Calculate maximum dynamic deflection • Use equivalent SDOF properties of structural component and blast load to calculate maximum deflection with SDOF procedure • Usually ductile flexural response is assumed

deflection criteria


SDOF Design Procedure (Cont’d) • Must check for any instability or brittle failure • Strengthen brittle/instable failure mode so it does not control ultimate strength • Alternatively, reduce allowable deflection and ultimate capacity

• Must follow load path to foundation • Connections • Diaphragms • Rigid body motion of building


Shock Wave Applying Blast to Building PRESSURE TIME AMBIENT PRESSURE

PEAK PRESSURE

Pso

Pr Time = T

T

T

T


Building

Pb _ < Pso

T

T

Typical Idealized Blast Load Shape • Minimum of two blast load parameters needed e r u s

Peak Pressure (P)

e r P

Impulse, (i)

0

t

Time d


Interior Explosion Blast Loads


Blast Walls • Usually not that effective for reducing blast • oc wave s n qu c y e n as wa s


-

- -


SDOF Equation of Motion Lm

c

′′

c

′

Mc

c

=

c

= mass of blast-loaded component c R c(u(t)) = resistance of blast-loaded component Cc = viscous damping constant of blast-loaded component K Lm = load-mass factor (accounts for fact all mass and load on component does not move through u(t)) u′′ t = acceleration of the mass u′(t) = velocity of the mass u(t) = displacement of the mass = max component deflection


Ductile Component Resistance

Midspan Deflection


Component SDOF Properties • Mass based on component weight and load• Resistance based on dynamic moment capacity, end conditions and load distribution • Design moment capacity varies based on component type and assumed strain-rate effects • Effect of axial stresses ma need to be considered • Tension membrane can be included for low strength members (i.e. cold-formed members) in more detailed anal sis

• Stiffness is the ratio of resistance to component midspan deflection STEER Meeting, March 22, 2007

Response Criteria • Maximum deflection defined in terms of

=

μ =

x m xe

xm = maximum component deflection e STEER Meeting, March 22, 2007

−1

⎛ 2 xm ⎞ ⎝ L ⎠


Available Blast Design Guidance • TM 5-1300 “Structures to Resist the Effects of Accidental Explosions” • Most comprehensive manual for blast design • Probably most conservative design guidance

“ Petrochemical Facilities” • Concise though lacks detail contained in TM 5-1300 • No consideration of high explosives • Design/analysis guidance provided for three damage levels

• • Blast design and blast damage assessment computer programs and detailed methodology manuals


Available Design Guidance (Cont’d) • AISC Design Manuals (ASD and LRFD) • properties and ESL design • ASD contains plastic design section (Chapter N) • LRFD tables for compression members and bolts

• UFC 03-340-01 DAHS Manual for Government and Gov’t Contractors • Intended primarily for military structures • Less conservative than TM 5 – 1300 •


Blast Design Oswald

Recommend Documents