Building Blast Analysiswind engineering & architectural aerodynamics

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dynamic loading produced by the detonation of high-explosive materials is, unfortunately, an increasingly important load case that designers of important structures need to consider for the safety of the people interacting with the structure and integrity of the structure itself.

Tel: +44 20 8614 4400 Fax: +44 20 8943 3224 Email: enquiries@bmtfm.com Website: www.bmtfm.com

in recent years, considerable attention has been given

to explosive loads on landmark structures as a result

of both accidental and intentional events. at the same

time, building designers the world over have been

increasingly pushing the boundaries on design with ever

more demanding architectural and structural forms and

utilizing advanced structural materials and methods.

understanding how these designs and materials are able

to withstand blast loads and maintain structural integrity

is now no longer possible with semi-empirical methods

without making highly conservative assumptions that drive

up asset cost and compromise design.

the balance of compromising design for the safety of the

structure is heavily influenced by the acceptable level of

risk. a risk-based approach to design for blast loading

should be taken where accurate methods are employed to

assess the consequence of an event, and combined with

the likelihood of an event occurring.

Bmt have taken their extensive experience in risk-based

explosion load analysis from the oil and gas industry

and coupled it with class-leading software for non-linear

dynamic structural finite element analysis (Fea), and

unparalleled knowledge of structural environmental

loadings to provide the construction industry a complete

package for blast load design. the approach that Bmt

provide through all stages of design ensures that the risk

posed to building occupants and structural integrity from

terrorist and other explosions is significantly reduced. the identification of key threats is the first stage of any blast

study for a new development. explosion threat scenarios

are typically specified in terms of an explosive charge weight

at a standoff distance from the target structure. Bmt works

closely with security consultants and governmental bodies

to determine the likelihood of a given scenario occurring.

Key services

• identification and assessment of likely threat scenarios

• Blast load Prediction

• non-linear dynamic structural analysis to assess resistance to blast loading

• analysis of blast resistant façade and glazing systems

• advice on blast resistant design and development of mitigation measures

• risk-based assessments

• determination of protection levels

• Building evacuation analysis

Key BeneFits

• risk-based assessment of security related threats

• cost effective structural and façade designs that balance an acceptable level of risk

• interactive solution development

• iterative design optimisation

• visualisations of the blast waves, cladding and structural response to aid design development

Blast loads

threat assessment

Overpressure resulting from a small device (~20kg TNT equivalent) detonated in a built-up urban area. Reflection of the blast wave-front is

shown, resulting in successive cycles of overpressure.

the quantification of blast loads on structures from

explosion events requires the accurate modelling of

peak overpressures and the associated impulse from

the expanding gases of detonation as well as taking into

account the geometric effects of blast wave reflection

and diffraction. the recent developments in the fields

of computational Fluid dynamics (cFd) and non-linear

Finite element analysis (Fea) have meant there have been

significant advances in the prediction of blast loads that

were not possible up until recently. accurate explosion

loads and structural responses can now be modelled using

software that couples these technologies for complex

designs and for a range of threat scenarios.

Bmt use advanced three-dimensional cFd software to

calculate the evolution and movement of the blast pressure

wave through the air and its interaction with surrounding

structures. the end result is an accurate blast pressure

distribution across the structure that is then coupled to the

dynamic structural response model.

Protection of a structure’s occupants by the façade

system is the first line of defence against blast. however,

for extreme events or events where terrorists target weak

points in a structure’s design, the integrity of the structure

is vitally important. Bmt can carry out detailed blast

analysis to ensure that the structure is robust enough

to maintain integrity for an acceptable blast risk event

through the development of alternate load paths. the

analysis of structural elements to resist blast loading

requires a dynamic analysis with non-linear ductile

response where ultimate limit state assessments are

carried out. where complete structural failure is predicted,

Bmt can work with the building designer to develop

alternative load paths and mitigation strategies.

design of structural elements

design of Protected sPaces

design advice & mitigation measures

in some buildings, it is necessary to design protected

spaces that are hardened to resist the effects of external

explosions and protect the occupants inside or a critical

piece of equipment. in the event of a bomb threat,

sometimes it is safer for the building occupants to seek

internal shelter or to evacuate through subterranean exits

due to the high risk of injury from falling glass and other

debris that occurs over a wide area.

Bomb shelter areas (Bsas) are ideally located in

basements using reinforced concrete and away from

areas exposed to high blast and direct fragment hazards.

to ensure that the Bsa does not become a hazard itself to

evacuees, it is vitally important to determine the protection

offered for a range of threats. in other words, its limiting

blast capacity must be quantified.

Bmt are able to determine the limiting capacity of a

Bsa from a blast by modelling the structural integrity of

the building and shelter for a range of blast threats. the

analysis of the modelling results can be used to determine

the optimum location of the Bsa and identify ways for

strengthening the shelter.

in the early stages of a structure’s design, Bmt can

provide advice on good design practice for blast

resistance and mitigation measures to reduce the risk

of damage from bomb blasts. often this requires an

assessment of the buildings external form to identify

areas of re-entrant corners, cantilevered upper floors/set-

backs of the lower floors, recesses, internal courtyards

and arcades which will exacerbate the effects of an

external bomb blast. Bmt then work with the building

designers to reduce these problem areas and introduce

mitigation schemes such as convexly curved or faceted

facades and barrier schemes around the building’s

perimeter.

the proliferation of fully glazed facades in new building

designs means that the importance of glazing system

design to resist blast loading is more important than ever.

the fragmentation and subsequent projection of glass

fragments from bomb blasts is frequently the highest

threat to human life. ordinary annealed or toughened

glass forms elongated shards and small fragments when

subjected to blast loads of sufficient strength that will

travel at high speed and cause severe or lethal injuries

to personnel and extensive damage to fixtures, fittings

and equipment. appropriately supported laminated glass

panels have far superior blast resistance properties that

allow them to absorb energy with little or no fragmentation.

the key to optimum protection from laminated glazing

systems is the laminate composition and the support

system. Bmt’s advanced non-linear Fea tools are capable

of modelling the failure mechanism of glazing laminates

and the supporting system to identify areas of design that

can be optimised for maximum protection. the levels of

protection can then be categorised using standards such

as gsa –ts01, astm F 1642-04, and iso 16933.

design of glazing systems

Fully-coupled blast simulations showing the initial failure of a double

glazed facade element and glass fragmentation after complete failure

Adapted from the US General Services Administration (GSA) standard testing criteria for blast resistant glazing.

Pressure waves travel through the structure during a blast

faster than the wave-front travels through air, resulting in

structural stresses in advance of the main blast impact.

Tel: +44 20 8614 4400 Fax: +44 20 8943 3224 Email: enquiries@bmtfm.com Website: www.bmtfm.com

For further information contact: Tel: +44 20 8614 4400 Fax: +44 20 8943 3224Email: enquiries@bmtfm.com Website: www.bmtfm.com

BMT Fluid Mechanics Limited, 67 Stanton Avenue, Teddington, Middlesex, TW11 0JY, UK