blast loading & its effect on structures

• Introduction

• Blast Loads

• Blast Waves

• Effects on Structures

• Expected Damage Levels

• Response of Structure

• Blast Resistant Buildings

• Solutions for Blast Resistant Buildings

• Conclusions

• References

• The study of blast effects on structures has been an area of formal technical

investigation for over 60 years.

• The blast explosion nearby or within structure is due to pressure or vehicle bomb or

quarry blasting.

• Due to different accidental or intentional events, the behavior of structural components

subjected to blast loading has been the subject of research effort in recent years.

• Disasters such as the terrorists bombings of the MURRAH FEDERAL BUILDING in

Oklahoma City in 1995, demonstrated the need for examination of behavior of

members subjected to blast loading.

• Structures should be protected from the blast effects, which are likely to be the

targets of terrorist attacks.

A bomb explosion within or immediately nearby a building can cause catastrophic

damage on :

The building's external and internal structural frames,

Collapsing of walls,

Blowing out of large expanses of windows,

Shutting down of critical life-safety systems.

Loss of life and injuries to occupants can result from many causes including :

• Direct blast-effects,

• Structural collapse,

• Debris impact,

• Fire effects,

• Smoke effects

• An explosion is a rapid release of stored energy characterized by a bright

flash and an audible blast.

• Part of the energy is released as thermal radiation (flash); and part is coupled

into the air as air blast and into the soil (ground) as ground shock, both as

radially expanding shock waves.

• Blast loads on structures can be classified into two following main groups on the basis

of the confinement of the explosive charge :

1. Unconfined Explosions, which include free air burst, air burst and surface burst

explosion having unreflected and reflected pressure loads respectively.

2. Confined Explosions, the confined explosions include fully vented explosions,

partially confined explosions, fully confined explosions.

To be an explosive, the material will have the following characteristics :

• Must contain a substance or mixture of substances that remains unchanged under

ordinary conditions, but undergoes a fast chemical change upon stimulation.

• This reaction must yield gases whose volume at the high temperature is much greater

than that of the original substance.

• The change must be exothermic in order to heat the products of the reaction and thus

to increase their pressure.

Common types of explosions include :

• Construction blasting to break up rocks,

• Blasting to demolish buildings and their foundations,

• Accidental explosions resulting from natural gas leaks or other chemical/explosive

materials.

• The rapid expansion of hot gases resulting from the detonation of an explosive

charge gives rise to a compression wave called a shock wave, which propagates

through the air is known as blast wave.

Blast effects on building structures can be classified as :

• Primary effects and Secondary effects.

• Primary effects include ;

Air Blast

Ground Shock

Heat

Fragments

• The blast wave causes a pressure increase of the air surrounding a building structure

and also a blast wind.

• For example, the blast may deflect structural steel frames, collapse roofs, dish-in

walls, shatter panels and break windows.

• An explosive which is buried completely or partly below the ground surface will

cause a ground shock.

• This is a horizontal vibration of the ground, similar to an earthquake but with a

different frequency.

• A part of the explosive energy is converted to heat.

• Building materials are weakened at increased temperature.

• Heat can cause fire if the temperature is high enough.

• Fragments from the explosive source which are thrown into the air at high velocity.

• For example wall fragments of an exploded gas tank.

• Secondary Effects can be fragments hitting people or buildings near the explosion.

• They are not a direct threat to the bearing structure of the building, which is usually

covered by a facade.

• However, they may destroy windows and glass facades and cause victims among

inhabitants and passers-by.

• Minor :

• Non-structural failure of building elements such as windows, doors & cladding.

• Injuries may be expected and deaths are possible but unlikely.

• Moderate :

• Structural damage is confined to a localized area and is usually repairable.

• Structural failure is limited to secondary structural members, such as beams, slabs &

non load bearing walls.

• Injuries and deaths are expected.

• Major :

• Loss of primary structural members such as columns.

• In this case, extensive deaths are expected.

• Building becomes non repairable.

• Blast loading is a short duration load also called impulsive loading.

• Mathematically blast loading is treated as triangular loading.

• The ductility and natural period of vibration of a structure governs its response to an

explosion.

• Ductile elements, such as steel and reinforced concrete, can absorb significant amount

of strain energy, whereas brittle elements, such as timber, masonry, and monolithic

glass, fail abruptly.

• In the investigation of the response of a building structure to bomb blast,

the following procedures are followed :

The characteristics of the blast wave must be determined;

The natural period of response of the structure must be determined;

The positive phase duration of the blast wave is then compared with the natural

period of response of the structure.

The response of structure due to blast loadings can be as follows :

• Impulsive

• Quasi-static

• Dynamic

o Impulsive

o If the positive phase duration of the blast pressure is shorter than the natural period

of vibration of the structure, the response is described as impulsive. In this case, most

of the deformation of the structure will occur after the blast loading has diminished.

• Quasi-Static

• If the positive phase duration of the blast pressure is longer than the natural period of

vibration of the structure, the response is defined as quasi-static. In this case, the blast

will cause the structure to deform while the loading is still being applied.

o Dynamic

o If the positive phase duration of the blast pressure is close to the natural period of

vibration of the structure, then the response of the structure is referred to as dynamic.

In this case, the deformation of the structure is a function of time and the response is

determined by solving the equation of motion of the structural system.

• Explosions and blasts can produce, in a very short time, an overload much greater than

the design load of a building.

• Nothing can be guaranteed to eliminate all risks; but if the following blast resistant

design features were to be incorporated, many lives could be saved and many

structures and businesses would survive.

• Floors :

must be prevented from ‘falling off' their supports. If pre-cast concrete planks are used

they should have sufficient bearing; but they should not depend on bearing and gravity

to stay in place, they should be made continuous with rebars between adjacent planks

and preferably be made continuous with the supporting beams.

• Joists :

should be made continuous themselves, through every main beam and wherever they

coincide with outer columns.

• Main Beams :

should be continuous across the structure and should have connections to the outer columns

which exceed the plastic capacity of the main beam.

This means that in the case of overload the beams deform, forming hinges, absorbing

energy and taking time. Blast or shock loads will diminish in a very short time.

• Main Outer Columns :

should remain elastic and strong enough to carry likely loads even when main beams

attached to them form plastic hinges.

• The ground to first floor columns carry the heaviest loads. They are always more

vulnerable to attack. They are almost always longer than columns on other floors. So

special care has to be taken: they need to be stronger; to have barriers to protect

them; to have continuity at footings level with ground beams or slabs.

• If all this continuity is achieved, even if a column or two are cut or deformed, the

grillage of beams and joists and slabs at each floor throughout the building will

continue to carry the loads.

• It is not economical to design all buildings for blast loading.

• Public buildings, tall structures and city centers have to be designed against terrorists

attacks and sudden explosions.

• It is recommended that guidelines on abnormal load cases and provisions on

progressive collapse prevention should be included in the current Building Regulations

and Design Standards.

• Requirements on ductility levels will also help to improve the building performance

under severe load conditions.

• Evaluation of the results due to blast loading under several conditions have to be

included in the design procedure to get into the correct evaluation of the stress

characteristics of the material under consideration.

• T. Ngo, P. Mendis, A. Gupta & J. Ramsay, "Blast Loading and Blast Effects on Structures – An

Overview", The University of Melbourne, Australia, EJSE Special Issue: Loading on Structures

(2007).

• N. Munirudrappa, Professor, Civil Engineering department, Dayananda Sagar College of

Engineering, , Bengaluru, Blast Loading and Its Effects on Structures A Critical Review.

• http://www.nbmcw.com/articles/miscellaneous/others/29110-blast-loading-and-its-effects-

on-structures.html

• http://www.slideshare.net/sitaramayya/blast-resistant-structures

• http://www.docstoc.com/docs/73460939/BLAST-LOAD-AND-EFFECTS-OF-BLAST-ON-

STRUCTURES#

• http://www.reidsteel.com/information/robust_resilient.html