Seminar on Bomb Blast Resistant Structure by Shantanu Patil

Bomb Blast Resistant Structure________________________________________________________________________

A

SEMINAR REPORT

On

” BOMB BLAST RESISTENT STRUCTURE ”

SUBMITTED BY

Mr. SHANTANU SANJAY PATIL

DEPARTMENT OF CIVIL ENGINEERING

S.S.V.P.S.’s B.S.DEORE COLLEGE OF ENGINEERING,

DHULE- 424 0052014-2015

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Chapter No.1

INTRODUCTION

The design of civilian or commercial buildings to withstand the

effects of a terrorist blast is unlike the design of military installations or

the design of embassy buildings. The objectives of the “Structural

Engineering Guidelines” for the Design of New Embassy Buildings are to

prevent heavy damage to components and structural collapse.

Adherence to the provisions of the guidelines will minimize injuries and

loss of life and facilitate the evacuation and rescue of survivors. The

blast-protection objective of any commercial or public building must be

similar to those of embassy structures, that is to prevent structural

collapse, to save lives, and to evacuate victims.

Architectural and structural features play a significant role in

determining how the building will respond to the blast loading. These

features can include adjacent or underground parking, atriums, transfer

girders, slab configurations, and structural-frame systems. The keep-out

distance is vital in the design of blast resistant structures since it is the

key parameter that determines the blast overpressures that load the

building and its structural elements. The degree of fenestration is another

key parameter as it determines the pressures that enter the structure.

The smaller the door and window openings the Embassies and military

structures occupy secure sites with substantial keep-out distances better

protected the occupants are within the structure.

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1.1 Expected Terrorist Blasts On Structures.

External car bomb

Internal car bomb

Internal package

Suicidal bombs

1.2 Major Causes Of Life Loss After The Blast.

Flying debris

Broken glass

Smoke and fire

Blocked glass

Power loss

Communications breakdown

Progressive collapse of structure

1.3. Goals Of Blast Resistant Design

The goals of blast-resistant design are to:

Reduce the severity of injury

Facilitate rescue

Expedite repair

Accelerate the speed of return to full operations.

Chapter No.2

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DESIGN CONSIDERATION

2. 1 Structural Engineering

Structural engineering, or structural design, is the design of a building's

internal support system. Structural design includes the selection of a framing

method or structural system, as well as the selection and sizing of structural

members, based on loading and architectural requirements. Structural members

include beams, columns, the foundation, floor slabs, connections of these

elements to each other, and other ancillary components.

Building design (structural and architectural) can contribute to

infrastructure security by minimizing the extent and depth of damage in an attack.

Structural integrity can help mitigate blast and fire damage to the building; protect

inhabitants; protect equipment, property, and records; allow critical operations to

function immediately after an attack; and allow rescue operations in and around

the building preserved after an attack.

This section focuses on blasts and fires, describing engineering concepts for

structural integrity and strategies for minimizing damage. The concepts

discussed include:

Blast loads

Blast damage

Progressive collapse

Blast mitigation

The sections of most building codes relating to structural components

address service loads and methods to determine the proper size of structural

members and their connections. Service loads specified in building codes are

based on the location and intended use of the proposed structure, and include:

Minimum dead load: the weight of the structure

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Live load: variable loads such as people, cars, furniture, etc.

Earth load: earth pressure on buried structures, retaining walls,

foundations, etc.

Wind load: pressure applied to the structure by wind

Snow load: the weight of snow on a building

Seismic load: loads induced on structural members during an earthquake

Building codes do not usually address "blast loads"; the force exerted on a

building from the detonation of an explosive device.

Blast loads are different from the usual types of service loads considered

by a structural engineer when designing a building. Service loads are relatively

predictable in their magnitude and placement on the structure. In contrast, blast

loads are much greater in magnitude, are unpredictable in size and placement.

However, there are certain engineering strategies that agencies can use to

enable a building to maintain its structural integrity after some of its components

have been compromised or completely destroyed in a blaisit.

2.2 Blast Loads

A bomb exploding at ground level produces a hemispherical shock wave.

As with other waves, such as sound waves, the shock wave can reflect off

objects, concentrate in confined areas such as tunnels, or change direction. This

is important to understand because once the skin of a building is breached, the

shock wave can travel or ripple through a building's corridors as the energy in the

wave dissipates.

A bomb or other explosive device produces a blast that creates a blast

load. Explosions cause damage by the generation and propagation of heat,

pressure, and flying debris (shrapnel). An explosion is a rapid, often violent,

release of energy that produces a rapid release of gases and heat. The rapid

release of gases compresses the air immediately around the bomb, creating a

shock wave. This shock wave, or pressure wave, propagates through the air

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outwards from the explosion. When this shock wave encounters an object, such

as a building or a trash receptacle, it exerts a force on that object.

Table Peak reflected overpressure (MPa) with different W-R combination

W R

100kgTNT

500kg TNT

1 tonTNT

2 tonTNT

1m 165.8 354.5 464.5 602.9

2.5m 34.2 89.4 130.8 188.4

5m 6.65 24.8 39.5 60.19

10m 0.85 4.25 8.15 14.7

15m 0.27 1.25 2.53 5.01

20m 0.14 0.54 1.06 2.13

25m 0.09 0.29 0.55 1.08

30m 0.06 0.19 0.33 0.63

The blast load striking a building or other object depends on the amount

and quality of explosive detonated and the distance of the explosion from the

building. Maximizing standoff distances is important; the farther away an

explosion, the weaker its effects. As the shockwave radiates away from the

explosion, the magnitude of the shockwave decreases and the duration of the

shockwave increases

The peak magnitude of the shockwave increases by a reflection factor as

it encounters the face of a building. This increase in magnitude is analogous to

ocean waves rising as they strike a sea wall and the water "piles up" against the

wall. The reflection factor varies with the incident angle (the angle at which the

shockwave hits the building). The increase is maximized when the direction of

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wave travel is perpendicular to the building. This can increase the pressures by

an order of magnitude.

Explosive materials vary in their efficiency (energy released per pound of

material). In calculating blast loads, current practice expresses all explosives in

terms of an equivalent weight of TNT, regardless of the actual explosive material

used. Information for determining blast load magnitudes in relation to building

hardening design is available through the Department of Defense, General

Services Administration, and in other security-related publications.

2.3 Damage from Blasts

The main threat to the structural integrity of a building is blast force,

regardless of whether the explosion occurs inside or outside the building. The

primary vulnerability is the overloading of the structural system by blast loads that

cause the system to fail and the building to collapse.

Figure 2-3. Variation of Explosive Pressure and Duration with Distance from Explosion

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Blast damages are classified as either direct (those that occur in the

explosion) or indirect (those that occur as a subsequent consequence of direct

damage).

Direct Damage

A hole in the ground or foundation.

Localized damage to the building's façade (bricks, windows, etc.).

Damage or removal of a structural member or members (a beam,

column or other structural element) directly caused by the blast.

Indirect Damage

Flying shards of glass: Glass shards thrown from a window can

cause serious injury to people, even if they are several feet from

the window that shattered.

Progressive collapse: If a blast directly destroys a column or beam

locally, other structural members may fail. This can start a chain

reaction of failures that results in damage disproportionate to the

blast and collapse of the entire building.

2.4 Progressive Collapse

The worst-case consequence of blast damage related to structural engineering is

progressive collapse. This is the disproportionately large collapse of a building or

structure from an explosion, caused by the loss of one or more structural

members, resulting in only localized damage. Progressive collapse occurs

because most buildings are designed to carry the required loads, based on the

assumption that all structural members are in place.

Two types of progressive collapse are possible:

Pancaking is the stacking of floors on top of each other. It occurs when an

explosion destroys a structural member or members, causing the floor

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directly above the destroyed members to collapse, which causes the next

floor above it to collapse, and so on.

Cascading is the collapsing of a series of bays (the section of a building

between two rows of columns) from the destruction of one or a few bays.

Cascading occurs when an explosion destroys a bay, or bays, causing the

adjacent bay or bays to collapse in succession.

Net upward pressure on slab

Fig. Progressive collapse

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Chapter No.3

DESIGN TECHNIQUES

3.1 General Description Of Loading Requirements

To resist blast loads, the first requirement in the assessment of a structure

is to determine the threat. While numerous threats exist, the present seminar will

be limited to intentional explosions, such as those caused by terrorist bombings.

In recent terrorist attacks, the explosive device was a mixture of Ammonium

Nitrate and Fuel Oil (ANFO). These ingredients and detonating devices can be

purchased relatively easily; however there are many other types of explosive

devices, including TNT, C-4, and Semtex, which are more efficient and must be

considered. To standardize the criteria, the industry refers to the charge weight of

an explosive device in terms of equivalent TNT weight. The relative effect on

pressure and impulse can be scaled to an equivalent amount of TNT.

The threat for a conventional bomb is defined by two equally important

elements, the bomb size, or charge weight, and the standoff distance, the

minimum guaranteed distance between the blast source and the target. As

terrorist attacks range from the small letter bomb to the gigantic truck bomb as

experienced in Oklahoma City, the mechanics of a conventional explosion and

their effects on a target must be addressed. With the detonation of a mass of

TNT at or near the ground surface, the peak blast pressures resulting from this

hemispherical explosion decay as a function of the distance from the source as

the ever-expanding shock front dissipates with range. The incident peak

pressures are amplified by a reflection factor as the shock wave encounters an

object or structure in its path. Except for specific focusing of high intensity shock

waves at near 45° incidence, these reflection factors are typically greatest for

normal incidence (a surface adjacent and perpendicular to the source) and

diminish with the angle of obliquity or angular position relative to the source.

Reflection factors depend on the intensity of the shock wave, and for large

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explosives at normal incidence these reflection factors may enhance the incident

pressures by as much as an order of magnitude.

The duration of the positive-phase blast wave increases with range,

resulting in a lower-amplitude, longer-duration shock pulse the further a target

structure is situated from the burst. Charges situated extremely close to a target

structure impose a highly impulsive, high intensity pressure load over a localized

region of the structure; charges situated further away produce a lower-intensity,

longer-duration uniform pressure distribution over the entire structure. In short, by

purely geometrical relations, the larger the standoff, the more uniform the

pressure distribution over the surface of the target. Eventually, the entire

structure is engulfed in the shock wave, with reflection and diffraction effects

creating focusing and shadow zones in a complex pattern around the structure.

Following the initial blast wave, the structure is subjected to a negative pressure,

suction phase and eventually to the quasi-static blast wind. During this phase,

the weakened structure may be subjected to impact by debris that may cause

additional damage.

While it may be possible to predict effects of a certain charge weight at a

specified standoff distance, the actual charge weight of explosive used by the

terrorist, the efficiency of the chemical reaction and the source location are not

reliably predictable. The most significant observation that one draws from blast-

pressure phenomenology is that the most effective means of protecting a

structure is to keep the bomb as far away as possible, by maximizing the keepout

distance. No matter what size the bomb, the damage will be less severe the

further the target is from the source. The external explosive threat is by no

means the only type of terrorist attack, but for the purpose of the present paper

an uncased bomb at street level is assumed.

Structural hardening should actually be the last resort in protecting a

structure; detection and prevention must remain the first line of defense. As the

cost of protection increases dramatically with the assumed charge weight, to the

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point at which the cost of protection becomes untenable, and since the size of

the potential threat is such an unknown quantity, the structural engineer is put in

a very uncharacteristic role. Rather than designing to a specific charge weight, as

one would a live load or 50-year wind load that has a presumed return period, the

structural engineer must design a structure to exhibit its best behavior in the

presence of a blast loading. The blast loading may originate from any point

around the perimeter of the structure-within the loading dock, the mail room, or

the lobby. The engineer must design and detail specific components to withstand

the various threats such that catastrophic failure and progressive collapse is

avoided and the rescue of victims may proceed unhindered. The recognition of

the localized intensity of the close-in blast and the inability to design the entire

structure to withstand this type of loading is the first step in prescribing the design

forces to be withstood.

3.2 External Treatments

The two parameters that most directly influence the blast environment that

the structure will be subjected to are the bomb's charge weight and the standoff

distance. Of these two, the only parameter that anyone has any control over is

the standoff distance, and this is primarily dictated by the site. Regardless of the

selected charge weight, the maximum attainable standoff or keep out distance

must be secured around the entire perimeter of the building.

For the building under consideration, it is clear that only the pubic sidewalk

around the building can be controlled to limit the standoff distance. Thus the

building is extremely vulnerable to unimpeded hand-delivered or car-bomb

attacks. The most directly affected building elements are the lower-floor facade

and structural members. Therefore, the site parameter and the exterior elements

at the lower floors require special attention.

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Stand Off

The keep out distance, within which explosives-laden vehicles may not

penetrate, must be maximized and guaranteed. As we all know, the greater the

standoff distance, the more the blast forces will dissipate resulting in reduced

pressures on, and impulse imparted to, the building. Several recommendations

can be made to maintain and improve the standoff distance for the building.

1. The public parking lot at the corner of the building must be secured to

guarantee the prescribed keepout distance from the face of the structure.

Securing this parking lot means that all vehicles must be cleared, i.e., employee

owned or visually inspected, such as delivery trucks. Preferably, the parking lot

should be eliminated.

2. Street parking should not be permitted on the near side of the street, adjacent

to the building. However, the city typically gains large revenue from street parking

and might require annual fees from the owner to compensate for the losses.

3. An additional measure to reduce the chances of an attack would be to prevent

parking on the opposite side of the street. While this does not improve the keep

out distance, it could eliminate the "parked" bomb, thereby limiting bombings to

"park and run," drive-by, and suicide bombers. Unfortunately, as was the case

with the Oklahoma City Bombing, the truck laden with explosives was only

parked for approximately two minutes. Even in the most security-aware

environments, this may not be long enough to draw the attention of security

officers.

Note that the practical benefit of increasing the standoff depends on the charge

weight. If the charge weight is small, this measure will significantly reduce the

forces to a more manageable level. If the threat is a large charge weight, the

blast forces may overwhelm the structure despite the addition of nine or ten feet

to the standoff distance, and the measure may not significantly improve the

survivability of the occupants or the structure.

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Lower Floor Exterior

The architectural design of the building of interest currently calls for

window glass around the first floor. Unless this area is constructed in reinforced

concrete, the damage to the lower floor structural elements and their connections

will be quite severe. Consequently, the injury to the lower floor inhabitants will be

equally severe, especially at these short standoffs. In general, three sizes of

charges can be discussed.

1. To protect against a small charge weight, a nominal 300 mm (12 in.) thick wall

with 0.3% steel doubly reinforced in both directions might be required.

2. For intermediate charge weight protection, a 460 mm (18 in.) thick wall with

0.5% steel might be needed.

3. Finally, a large charge weight at these small standoffs will likely breach any

reasonably sized wall at the lower levels. Therefore, precautions have to be

taken and adjustments made for the design of the entire structure.

3.3 Glazing

Glazing has been described as the first weak link. It should be assumed

that all glazing on the target structure will fail for most realistic car bomb threats,

particularly on the side of the building facing the bomb. Commonly used

annealed glass behaves poorly when loaded dynamically. The failure mode for

annealed glass creates large sharp edged shards, resembling knives and

daggers. Historically, failed window glazing due to the direct pressures produced

by an explosion has resulted in a considerable proportion of the injuries and

casualties. For the window assemblies to behave properly, the glazing, mullions,

and anchorage must all be capable of resisting the blast pressures and transfer

the loads to the adjacent structure.

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While typical annealed plate glass is only capable of resisting, at most, 14

kPa (2 psi) of blast pressure, there exist several other types of glazing that can

resist some modest blast pressures. Thermally Tempered Glass (TTG) and

Polycarbonate layups can be made in sheets up to about l-in. thick and can resist

pressures up to about 200 to 275 kPa (30 to 40 psi). The true abilities of each of

these window types is highly dependent on the actual dimensions. The greatest

benefit of TTG is the way that it fails. Unlike annealed glass, TTG breaks into

rock-salt sized pieces that will inflict less injury on the occupants. TTG is used for

the side and rear windows of automobiles. The failed Polycarbonate glass

unfortunately remains in one piece, similar to the windshield of a car, and can

cause injury similar to a large flying object. Equally important to the design of the

glass is the design of the attachments. For the window to fail properly, it must be

held in place long enough to develop the proper stresses that cause failure. Short

of that, the window would dislodge from the housing intact and cause serious

damage or injury. Unfortunately, as the pressures on the window get large, the

thickness of the supporting wall also drastically increases.

Attaining that delicate balance between adequate protection and properly

distributing blast-construction funds can become challenging. The cost

associated with blast-resistant walls and glazing can be expensive, despite the

fact that it is unlikely the windows will survive any sizable terrorist blast.

3.4 Facade And Atrium

As the building's exterior is its first real defense against the effects of a

bomb, how the facade responds to this loading will significantly effect the

behavior of the structure. The facade is comprised of the glazing and the exterior

wall. The glazing, a pressure sensitive element, is the first building component

likely to fail in response to the initial blast pressure that engulfs the building.

Although the wall may be hardened to resist the loading, the options available for

the glass are much more limited. Consequently, the windows will break easily

and the blast will enter the building causing additional damage and injury. There

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exists a direct correlation between the degree of fenestration and the amount of

blast that is allowed to enter the occupied space. Limiting the amount of

fenestration will limit the blast effects.

Atriums are common in prestigious office buildings. This architectural

feature enhances the building's appearance not only by providing a grand

entrance, but also by bringing natural light into the work space and providing

impressive function spaces and balcony elevator lobbies. Atriums are inviting

targets; all the broken glass gives the appearance of extensive damage and

exposes many internal structural elements to blast loading. In the building

described above, the atrium has a large window at the building's exterior that

cannot be designed to withstand the blast pressures. Not only do fragments from

the failed glazing create a hazard, but the failed glazing allows the blast

overpressures to enter the interior of the building. Vented the blast deep into the

structure will result in multiple reflections that will increase the damage area and

cause more injury. If the atrium is extensive, providing a significant open space

relative to the size and shape of the structure, the blast pressures may even

attempt to split the building apart.

Exterior of Atrium

It is not reasonable to harden the exterior walls of the structure and leave

the atrium's exterior wall as an inviting target.

Therefore, these walls of glass must be redesigned to protect occupants

against a small charge or completely eliminated to protect occupants against a

moderate-size charge.

Interior of Atrium

Once within the structure, the blast waves will both dissipate with distance

and reflect off of the internal surfaces. The pressure distributions resulting from a

small charge opposite the plate-glass atrium facade exceed the nominal 14 kPa

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(2 psi) capacity over its entire height, though, depending on the charge weight,

there will be a height above which the pressure is diminished to approximately

70-100 kPa (10-15 psi), a magnitude associated with a reduced likelihood of

human fatality. At this elevation and above, the overpressures that enter the

structure through the shattered glass facades will be reduced sufficiently to pose

a diminished threat to the occupants.

A moderate-size explosion opposite the glass facade, however, produces

dangerously large pressures in excess of 200 kPa (30 psi) over the entire height

of the structure. The shock waves entering the structure, and subsequent internal

reflections, would destroy internal partitions facing the atrium and pose a serious

threat to the occupants. The protection of the occupants from these pressures

can only be achieved by a reinforced concrete wall along the face of the building.

It must be reiterated that the small charge weight, as well as the medium charge

weight, do not correspond to a realistic threat scenario, and therefore decisions

to retain the glass facades would be made in spite of the historical precedent of

recent terrorist car bomb attacks.

To summarize, the presence of an atrium along the face of the structure

will require two protective measures. On the outside of the structure, the glass

and glass framing must be strengthened to withstand the loads. On the inside,

the balcony parapets, spandrel beams, and exposed slabs must be strengthened

to withstand the loads that enter through the shattered glass.

3.5 Floor Slabs

The reinforced-concrete flat-plate structural system supports the gravity

loads within the building. It is an economical solution, which provides for

maximum use of vertical space, particularly for buildings in areas with height

restriction. This allows for mechanical systems to pass unobstructed, and permits

easy forming and fabrication of the slab system. Note that the building, as

designed, does not require the use of drop panels or column capitals. The live

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load requirements is typical throughout the building. Therefore, with the

exception of localized areas, the slab thickness and reinforcement requirements

remain the same for both interior and exterior bays of the building.

If this building is subjected to a blast load, the flat slab construction will be

exposed to a large dynamic pressure load. Punching shear and softening of the

moment-resisting capacity of the slabs will reduce the lateral-load-resisting

capacity of the system. Once the moment-resisting capacity of the slabs at the

columns is lost, the ability of the slab to transfer forces to the shear walls is

diminished and the structure is severely weakened. This may result in several

possible modes of failure, such as the following:

1. The slab itself may experience localized failure .

2. The loss of contact between the slab and the columns may increase the

unsupported column lengths, which may lead to the buckling of those columns.

epicts the case in which the slab column connection is weakened and the

unsupported length of the column is therefore significantly increased.

3. The lateral load resisting system, which consists of the shear walls, the

columns, and the slab diaphragms that transfer the lateral loads, may be

weakened to such an extent that the whole building may become laterally

unstable.Depicts a case in which the lateral load resisting system is dependent

on the presence of the slab. When the flat slab becomes unstable, the lateral

load resisting system will be degraded.

To avoid such calamity, the following improvements for conventional flat-

plate design must be considered:

1. More attention must be paid to the design and detailing of exterior bays and

lower floors, which are the most susceptible to blast loads .

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2. Spandrel beams, which are included in the building but are not mandatory,

must be included to tie the structure together and enhance the response of the

slab edge.

3. In exterior bays/lower floors, drop panels and column capitols are required to

shorten the effective slab length and improve the punching shear resistance .

4. If vertical clearance is a problem, spearheads embedded in the slab will

improve the shear resistance and improve the ability of the slab to transfer

moments to the columns.

5. The ductility demands and shear capacity required to resist multiple-load

reversals often force the engineer to provide beams to span over critical sections

of the slab. The inclusion of beams will enhance greatly the ability of the framing

system to transfer lateral loads to the shear walls.

6. The slab-column interface should contain closed-hoop stirrup reinforcement

properly anchored around flexural bars within a prescribed distance from the

column face.

7. Bottom reinforcement must be provided continuous through the column. This

reinforcement serves to prevent brittle failure at the connection and provides an

alternate mechanism for developing shear transfer once the concrete has

punched through.

8. The development of membrane action in the slab, once the concrete has failed

at the column interface, provides a safety net for the postdamaged structure.

Continuously tied reinforcement, spanning both directions, must be detailed

properly to ensure that the tensile forces can be developed at the lapped splices.

Anchorage of the reinforcement at the edge of the slab or at a structural

discontinuity is required to guarantee the development of the tensile forces.

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In all, the slab should be designed to prevent a punching shear failure that

in turn develops into a progressive collapse. Although Hawkins and Mitchell have

shown that punching shear failures at interior columns are more likely to result in

a progressive collapse than a failure at an exterior column, for the external bomb

threat, the external bay around the perimeter of the structure must be hardened

at all intersecting columns.

3.6 Columns

The columns of the typical building were primarily designed to resist

gravity loads in which no special accounting for ductility demands has been

taken. For blast consideration, the distance from the explosion determines, to a

great extent, the characteristics of the loading on the structure. Two different

scenarios can be considered.

1. Buildings situated at a substantial distance from a protected perimeter

approximately 30 m (100 ft) or more will be exposed to relatively low pressures

fairly uniformly distributed over the facade. This 30 m (100 ft) enforced standoff,

keep-out distance, is one of the basic tenets of the DOS-FBO guidelines

2. Buildings that are situated less than 30 m (100 ft) from the curb will be

exposed to more localized, higher intensity blast pressures. In an urban setting,

such as the typical building under consideration and the Oklahoma City incident,

where large keep-out distances are unattainable, this is the prevalent scenario.

It is clear that special treatment of the columns is required if this building it is to

resist blast effects. The direct blast pressure will result in severe bending of the

column, in addition to the axial loads that it supports. The column will require

sufficient ductility to sustain the combined effects of axial load and lateral

displacement.

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Furthermore, the blast pressures that enter the structure through the

shattered windows and failed curtain walls will load the underside and

subsequently the top surfaces of the floor slabs along the height of the building.

Both the delay in the sequence of loading and the difference in magnitude of

loading will determine the net pressures acting on the slabs. Consequently, there

will be a brief time for which each floor will receive a net upward loading. This

upward load requires that the slab be reinforced to resist loads opposing the

effects of gravity. Furthermore, it is possible that the uplift, the net upward load

on the slab, will result in a brief tensile force in the columns. Conventional

reinforced concrete columns, such as those in this building, are not designed to

resist the combined effects of bending and tension and may therefore be prone

to damage under these conditions. shows a schematic representation of this

uplift phenomenon.

Given all the limitations of the conventionally designed columns,

adaptation of one or more of the following recommendations will improve the

blast-resisting mechanism:

1. The potential for direct lateral loading on the face of the columns, resulting

from the blast pressure and impact of explosive debris, requires that the lower-

floor columns be designed with adequate ductility and strength .

2. The perimeter columns supporting the lower floors must also be designed to

resist this extreme blast effect .

3. Encasing these lower-floor columns in a steel jacket will provide confinement,

increase shear capacity, and improve the columns' ductility and strength. An

alternative, which provides similar benefits, is to embed a steel column within the

perimeter concrete columns or wall section.

4. The possibility of uplift must be considered, and, if deemed likely, the columns

must be reinforced to withstand a transient tensile force.

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5. For smaller charge weights, spiral reinforcement provides a measure of core

confinement that greatly improves the capacity and the behavior of the reinforced

concrete columns under extreme load.

3.7 Transfer Girders

The building relies on transfer girders at the top of the atrium to distribute

the loads of the columns above the atrium to the adjacent columns outside the

atrium. The transfer girder spans the width of the atrium, which insures a column-

free architectural space for the entrance to the building.

The presence of transfer girders in a blast-designed building must be

handled with utmost care. Transfer girders typically concentrate the load-bearing

system into a smaller number of structural elements. This load-transfer system

runs contrary to the concept of redundancy desired in a blast environment. A

blast that might adversely affect the transfer girder may end up affecting the

whole building in a progressive collapse mechanism. The column connections,

which support the transfer girders, are to provide sustained strength despite

inelastic deformations. If transfer girders are to be used in the building , the

following recommendations must be met:

1. The transfer girder and the column connections must be properly designed

and detailed, using an adequate blast loading description.

2. A progressive-collapse analysis must be performed, particularly if the blast

loading exceeds the capacity of the girder

3.8 Internal Explosion Threats

The blast environment could be introduced into the interior of the structure

in four vulnerable locations: the entrance lobby, the basement mechanical rooms,

the loading dock, and the primary mail rooms . These are areas where

unsecured packages can be delivered with little forewarning. Specific

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modifications to the features of these unprotected spaces can prevent an internal

explosion from causing extensive damage and injury inside the building. The

following structural modifications to the building under consideration are

recommended:

1. Walls and slabs adjacent to the lobby, loading dock, and mail rooms must be

hardened to protect against the hand delivered package bomb, nominally a 10-20

kg (25-50 lb) explosive. Hardening these internal spaces will help confine the

explosion and prevent the spread of extensive internal damage or injuries. This

hardening can be achieved by redesigning the slabs and erecting cast-in-place

reinforced-concrete walls, with the thickness and reinforcement determined

relative to the appropriate threat.

2. The basement must be similarly isolated from all adjacent occupied office

space, including the floor above, from the threat of a small package bomb.

3. Any other unsecured spaces in which package bombs may be detonated must

be similarly isolated. This would include protecting any occupied office space,

mechanical rooms, or utility feed rooms located adjacent to an unsecured

underground parking garage.

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Chapter 4

FIRE PROTECTION

While accidental fires may occur, fires resulting from an attack may have a

different kind of impact. For example, an accidental fire usually starts at one

location and often, but not always, spreads relatively slowly. On the other hand, a

fire from arson is often strategically set in multiple locations to maximize the rate

of spread and damage. An arsonist may also sabotage the fire protection system.

An incendiary bomb that produces a fireball or intense heat (as opposed to a

bomb that produces only a shock wave) ignites a large area and can cause

substantial damage, including local damage to the fire suppression system.

Well-established design and construction practices for protecting structural

members from fire are particularly important in case of an attack. Although not all

structural materials will "burn," all structural members, regardless of their material

composition, will lose a percentage of their original strength when subjected to

intense heat. Excessive heat is the principal cause of a fire's detrimental effects

on a structure. Therefore, upgrading or hardening the automatic sprinkler system

is of tremendous benefit in mitigating the effects of fire on a structure.

Additionally, many of the mitigation measures for blast impacts apply to fire

management as well, such as isolating vulnerable areas to prevent the spread of

fire and avoiding progressive collapse This section discusses the effects of fire

on four major structural construction materials: steel (structural steel), reinforced

concrete, pre-stressed concrete, and timber.

4.1 Steel

At high temperatures, unprotected steel looses its strength. For this reason

structural steel members used in building construction are protected

(fireproofed). Fireproofing methods to protect steel members from heat insulate

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the steel from the fire. This increases the time required for heat to transfer from

the fire to the steel.

There are several insulating methods for steel members:

Concrete encasement. Encasing steel members in concrete

provides excellent insulation to the steel. Lightweight concrete

provides better insulation than standard concrete. The selection of

concrete type depends on several design factors that are beyond

the scope of this document. This method is well suited to

insulating columns. It may also be used to insulate floor beams

supporting a concrete floor slab. However this can be expensive

due to complicated forming and increased dead load.

Sprayed on mineral fiber coatings. Mineral fiber coatings are easy

to apply, and they provide excellent protection when applied

correctly. However, these coatings are easy to scrape off, and

explosive blasts may damage portions of the insulation. Protection

of the insulation is discussed at the end of this section.

Cementitious material coatings. Cementitious coatings form a

continuous coating around the steel. However, during a fire, they

can spall (chip or flake on the surface), and there is a history of

problems with lack of adhesion to the steel.

Intumescent paints and coatings. Intumescent coatings swell

when heated, thereby insulating the steel and retarding the effects

of the flames and high temperatures. These coatings work well to

protect the steel from heat. Exposure to flames can damage or

destroy this type of coating and therefore should only be applied

to components unlikely to be directly exposed to flames.

There are several concerns when selecting a method to fireproof steel, including

method of building construction, and installation and maintenance costs. During a

blast, it is likely that the fire proofing on the steel in the immediate vicinity of the

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blast will be damaged. However, the fire that may result (and spread) will have

an effect similar to conventional fires. Assuming the progressive collapse

considerations were used in design, protection of the remaining steel members

will be effective.

4.2 Reinforced Concrete

Concrete is often used as an insulating material. Although concrete structures

rarely collapse from fire damage, the strength of concrete and reinforced

concrete members is reduced by exposure to high temperatures. Type of

aggregate and moisture content are the principal factors that determine

concrete's sensitivity to heat.

Type of aggregate is the most significant factor. Lightweight aggregates such as

vermiculite and perlite are used in lightweight concrete. Lightweight concrete, in

addition to having better insulating characteristics, has better strength retention

when exposed to intense heat.

The amount of moisture in a concrete affects the member's resistance to heat.

The moisture is trapped in the small capillaries within the concrete. As heat

energy is absorbed, the water in the concrete vaporizes, which locally helps

maintain the concrete's strength until the moisture is burned off. However, voids

left by the vaporized moisture weakens the area. Structural engineers should

consider this when fire is a concern for concrete members.

4.3 Pre-Stressed Concrete

The relevance of aggregates and moisture content for pre-stressed concrete are

similar to those for reinforced concrete. The concrete used for pre-stressed

concrete members is usually stronger than the concrete used for reinforced

concrete members and has better fire resistance, but tends to spall and expose

the reinforcement.

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Pre-stressing steel is the principal concern when exposing pre-stressed

members to intense heat. High carbon-cold drawn steel used in pre-stressing is

more sensitive to intense heat than low carbon, hot rolled steel used in reinforced

concrete. Also, the loss of strength in pre-stressing steel is permanent and not

regained upon cooling. For example, the pre-stressing steel is initially under

great tension. Over time this tension decreases, as the steel tends to creep

(continually deform or lengthen). This is taken into account during the design

process; however exposure to high temperatures, exacerbated by the spalling

concrete, accelerates this "creeping" process. Engineers should consider this

when considering fire effects on building hardening.

4.4 Timber

Unlike steel and concrete, wood will burn. The principal factors that determine

how timber responds during a fire are the size of the timber member and its

moisture content.

As wood burns, a charcoal layer forms on the wood's exterior. This char layer is

an insulator and as the layer thickens, it slows down the rate of burning. The

unburned interior wood retains its strength. Buildings constructed with large

timber members can maintain their integrity for a long time during a fire, providing

an opportunity for the fire to be extinguished before structural failure occurs. As is

in all cases, but especially for timber construction, a hardened sprinkler system is

important. Fire retardants can slow combustion and delay ignition of wooden

members

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CONCLUSION

After studying the behaviour of building subjected to a blast loading,

different structural and architectural features of the building were analyzed, and

their vulnerability to blast loading was presented. Several techniques were

presented for each of the identified features. The implementation of these

techniques will greatly improve the blast – resisting capability of the building.

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REFERENCES

1. Civil engineering journal (oct. 1995)

2. Fire engineering journal (nov. 1995)

3. Structural design and construction journal (nov. 1996)

4. American concrete institute American society of civil engineering

Committee 352 (1985).

5. www.structuremag.org

6. www.berkley.edu

7. www.sciam.com

8. www.civil.usyd.edu

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