Concrete Detailing for Blast

By Elizabeth Agnew, MS, Shalva Marjanishvili, Ph.D., P.E. & Sharon Gallant, MS, S.E.

Introduction to Blast element’s support. As the weapon is placed farther away from
the structure, the airblast pressure distribution changes resulting
An explosion is a rapid release of energy taking the form
in a flexural response.
of light, heat, sound and a shock-wave. This shock wave is a
condensed air pressure wave that travels outward from the source Blast Detailing ®
at supersonic velocities. When the wave encounters a surface,

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A blast is a localized, highly irregular, and potentially non-
such as a building face, it is reflected and can amplify up to
uniform load. In blast design, it should be assumed that
thirteen times. The duration of this loading can vary depending
structural elements will be loaded beyond their yield strength

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on the geometry of the structure; for example, re-entrant corners
and up to failure. Thus the detailing of reinforced concrete
can further amplify air-blast waves and elongate load duration.
elements is of great importance. Desirable structural element
After the initial pressure wave travels through an area, it is
performance under blast loading can be achieved through the

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followed by a negative wave that creates a vacuum as shown
following general measures.
in Figure 1. This not only causes load reversals on structural
• Limit concrete compressive strengths to 5,000 psi or less,
ght

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elements, it also turns hazardous debris into flyingriprojectiles.
y since elements with higher strength concrete will ex-
PRESSURE (psi)
Cop perience more brittle modes of failure when subjected
to inelastic yielding.

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• Design for load reversals, which can subject elements to
loads for which they were not designed; for example,

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Peak tension in a column due to floor slab uplift. This
Reflected affects both longitudinal reinforcing placement

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Pressure

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and connection designs.

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• Ensure that the ratio of the steel reinforcement’s actual
Peak

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tensile strength to actual yield strength is not less than

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Incident

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Pressure 1.25 for sufficient yield capability.
• Locate lap splices outside of the hinge region of an

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element as predicated by the design airblast threat.

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• Design lap splices as tension splices. With blast,

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TIME (msec)
localized loading locations are unpredictable and hinge
regions could be located anywhere along the length

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POSITIVE
PHASE
NEGATIVE PHASE
DURATION of the member.
DURATION
Reinforced concrete columns and floor systems are the most
Structural Design

Figure 1: Typical blast pressure versus time important structural systems for protecting the building from
collapsing, and are also the most vulnerable to airblast loading.
Explosive pressures are many times greater than any other Figure 2 shows the succession of pressures on a building due to
loads for which a building is designed, so the goals in blast an external weapon.
engineering are modest by necessity. We have to accept that
some building damage and injuries may occur, and the building
may not be useable after an incident. The primary goal for high
population buildings is to save lives. In order of priority, this is
accomplished by:
1) preventing the building from collapsing
discussions on design issues for structural engineers

2) reducing flying debris

3) facilitating evacuation and rescue/recovery efforts 1. Blast wave breaks windows
Exterior wall columns blown in
Evacuation, rescue and recovery efforts can be significantly
improved through effective placement, structural design, and
redundancy of emergency exits and critical mechanical/electrical
systems. Reducing flying debris generated by failed exterior
walls, windows, and other components can be highly effective
in mitigating the severity of injuries and the risk of fatalities.
Concrete detailing is most important for the first priority –
2. Blast wave forces floors upward
preventing the building from collapsing. Ductile detailing of
primary members and connections allows for large deformations
while maintaining load-carrying capacity.
When a weapon is placed relatively close to the structural
element, it may cause shattering of the concrete in the immediate
vicinity – a phenomenon referred to as breach. When the
weapon is placed farther away from the elements, the explosive
3. Blast wave surrounds structure - Downward
effects change from breach to direct shear, and eventually to pressure on roof - Inward pressure on all sides
flexural response. Direct shear occurs when airblast pressures
are highly concentrated in a relatively small area close to the Figure 2: Succession of blast pressure on a building

STRUCTURE magazine 26 January 2007

 • Control the longitudinal reinforcement ratio such that the yield
moment exceeds the cracking moment and the column is not
over-reinforced (see ACI 318, section 21.4.3.1). Good practice
hinge zone is to ensure that column longitudinal reinforcement does not
exceed six percent of the gross area, which also serves to avoid
congestion in the splice zones.
splice zone
• Concrete spalling and crushing decrease the column’s ability to
accommodate large plastic deformation without significant®

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strength degradation. Provide closely spaced hoops for adequate
confinement of concrete. This increases the capacity of
the concrete in compression and helps prevent buckling of

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hinge zone
the longitudinal bars after the concrete crushes.
• Provide closely spaced ties or spirals along the entire column
height when airblast loads are non-uniform as shown in

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Figure 3 (preferably comply with ACI 318, section 21.4.4.1).
h t • Provide spacing of closed-hoop confinement reinforcement
yrig

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Cop
at column hinge regions to comply with ACI 21.4.4.4.
Figure 3: Column subjected to non-uniform air-blast loading • Design lap splices as tension splices and locate them outside

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of plastic hinge regions (see ACI 318, section 21.4.3.2).
Columns It may be more practical for interior short columns to have
splices located in the level above, where longer splice lengths
Failure of a single structural column may have a devastating effect

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can be accommodated. Alternatively, mechanical splices can
on the overall structural integrity of a building. For tall buildings, the

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be used, provided that they have the capacity to develop the
structural columns can carry substantial axial load due to gravity, and

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tensile strength of the spliced bar (see ACI 318, section 21.2.6).

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therefore it is prudent to include the effect of axial load in the blast

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analysis. The axial load in reinforced concrete columns increases the
Beams and Slabs

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bending capacity of the column, due to the large imbalance in concrete
tensile and compressive strengths. Axial load reduces stiffness and
strength in tall and slender columns due to the buckling phenomenon,

g a When designing the floor system, consider three possible loading

scenarios: direct airblast loading (including uplift), redistribution of

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which may result in catastrophic failure of the column. loading in the case of a lost element, and the impact of falling debris

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In considering the distribution of airblast pressure along the height from floors above.
continued on next page
of the column, loading is often approximated as uniform by assuming

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a plastic hinge forms at mid-height of the
column. This simplification is inaccurate
for columns located in close proximity to
the explosion. As shown in Figure 3, a non-
uniform distribution of airblast loading could
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result in a plastic hinge located below mid-
height of the column, which could potentially
Seattle, WA
result in a large shear demand at the bottom
Tacoma, WA
of the column. If the deflected shape is Portland, OR

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improperly predicted, this shear demand can Sacramento, CA
be significantly underestimated. San Francisco, CA
The location of the splice and hinge zones Los Angeles, CA
will depend on the expected location of Irvine, CA
the design airblast threat. If the column is San Diego, CA
Phoenix, AZ
located in a parking garage, its threat may
Denver, CO
come from a close-in parked vehicle weapon. St. Louis, MO
For above-grade perimeter columns, the more New York, NY
likely weapon location would be far-range,
depending on the perimeter standoff distance.
Figure 4 illustrates the plastic hinge zones for
a far-range weapon scenario – a loading that
may be approximated as uniform.
Below are column detailing practices
that generally result from airblast loading
analyses.
• Specify ASTM A706 reinforcement,
which has a larger ratio of ultimate
to yield strength, providing more
Fox Tower
ductile behavior at hinge regions.
Portland, Oregon

STRUCTURE magazine 27 January 2007

 The expected location of the weapon will dictate
the floor system design. The roof system is typically
designed for downward pressures as the building
is engulfed by the shock wave from an exterior
airblast event (step 2 in Figure 2). Depending on
the perimeter standoff distance, the outer bay may
require hardening to protect the floor systems from
uplift after the loss of the exterior envelope (step ®
3 in Figure 2). An interior threat, such as one in

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a parking garage, would load the floor below with
significant downward pressures, and the floor above
with upward pressures. A multi-story garage would

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Lo splice zone 2 Lo splice zone Lo
require that each floor system be designed for both
upward and downward loading.

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The primary goal when designing a blast-resistant
floor system is to focus on containing the damage
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to the secondary elements in one bay. This can be
righ

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achieved by ensuring that the floor system design p y
is balanced; that is, the capacity of the secondary
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elements is less than that of the primary elements,
such as the beams and girders along column Figure 5: Beam Subjected to downward air-blast loading
gridlines that serve to brace columns and maintain • Provide confinement reinforcement at areas outside of hinge and

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the building’s stability. More specifically, this means following the splice regions at a spacing no more than half the beam effective
load path from one element to the next and checking that the ultimate

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depth (ACI 21.3.3.4).

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capacity increases correspondingly. • Ensure that both top and bottom longitudinal reinforcing are

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The detailing guidelines below focus on providing ductility to ensure

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continuous throughout the length of beams and slabs.
that the beam’s full capacity can be achieved and the floor system has • Design lap splices as tension splices, locate them outside of plastic

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the integrity to handle all three loading scenarios described above. hinge regions (see ACI 318, section 21.4.3.), and stagger them.
• Provide spacing of closed hoop confinement reinforcement at

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beam hinge regions (see Figure 5) and at lap splice regions based

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on the smallest of the following (ACI 21.3.3.2):
Conclusion

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o beam effective depth / 4 The above enhancements to the structural design through detailing
o 8 x diameter of longitudinal bars would provide robustness, ductility and redundancy for extreme

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o 24 x diameter of hoops loading scenarios such as airblast and progressive collapse. Many of
o 12 inches the suggested ductile detailing measures are derived from the American
Concrete Institute standard ACI 318 and US Department of the Army
TM5-1300, Structures to Resist the Effects of Accidental Explosions. ▪

Elizabeth Agnew is a Project Engineer at Hinman Consulting Engineers,

Inc. in San Francisco, CA, and can be reached at eagnew@hce.com. She
hinge zone specializes in blast resistant design and progressive collapse research.
Shalva Marjanishvili is the Technical Director at Hinman in San Francisco.
He is an expert in the dynamic non-linear response of structures from seismic,
impact, and explosive loadings. Email Shalva@hce.com.
splice zone
Sharon Gallant is a Project Manager at Hinman in San Francisco with over
15 years of seismic design experience. Sharon oversees a variety of project types,
including blast resistant new design, seismic peer reviews, and progressive
collapse analysis. She can be reached at sgallant@hce.com.
hinge zone

References
Hinman, E., Primer for Design of Commercial Buildings to Mitigate Terrorist
Attacks, FEMA 427, December 2003.
splice zone Paulay, T., and Priestley, M. J. N., Seismic Design of Reinforced Concrete and
Masonry Buildings, New York: John Wiley & Sons, Inc., 1992.
Marjanishvili, S., and Gallant, S.M., Analysis of Reinforced Concrete Columns
hinge zone for Airblast Loads, 12th International Symposium on Interaction of the
Effects of Munitions with Structures, 2005.
American Concrete Institute, Building Code Requirements for Structural
Concrete (ACI-318) and Commentary (ACI318R), Farmington Hills,
MI, 2005.
Department of the Army, the Navy, and the Air Force, Structures to Resist
Figure 4: Column subjected to far-field air-blast loading the Effects of Accidental Explosions, TM5-1300 / NAVFAC P-397 / AFR
88-22, 1990.

STRUCTURE magazine 28 January 2007