Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

VERIFICATION OF BLAST DEMOLITION
PROBLEMS USING NUMERICAL AND
EXPERIMENTAL APPROACHES
Daigoro Isobe1, Masashi Eguchi2, Kensuke Imanishi2 and Zion Sasaki 2
ABSTRACTOne of the objectives of this study is to perform some blast demolition analyses of framed
structures using a dynamic finite element code, whose validity against impact collapse
problems of full-model large-scale structures has recently been developed and verified.
Another objective of this study is to develop a mimic demolition experimental system thatuses magnetic devices to express member fracture caused by explosion. The magnetic fieldof the devices is controlled by a blast interval controller and power switches, which control
the binding of each connection in the framed structure. The experimental system does not use
explosives or any such dangerous substances, and multicycle use is possible. Someexperiments are carried out for comparison with the numerical results.
KEY WORDS
blast demolition, framed structures, ASI-Gauss technique, finite element analysis, demolition
experiment.
INTRODUCTION
The technology used in the demolition of old worn-out buildings has always been of majorinterest and challenge in civil engineering. Because conventional demolition techniques
using a hydraulic concrete crusher, a concrete cutter or a nonexplosive demolition agent are
lengthy and costly to meet the heavy demand for demolition work, a controlled explosion
technique using high explosives has been developed in recent years. Although the blast
demolition technique increases work efficiency, it poses a high risk of damaging neighboringbuildings, especially in urban areas (Williams 1990). It also requires high levels ofknowledge and experience (e.g., Yarimar 1989, Kinoshita et al. 1989), which are very
difficult for general engineers to master. Although there are some representative demolition
companies in USA and Europe, the blast demolition technique has been only used in a few
cases in Japan; this may or may not be due to the above reasons. To familiarize thedemolition technique among construction companies, numerical assumptions using
computational analysis in devising the blast demolition have become essential in ensuring the
success of this technique.
1
2

Associate Professor, Department of Eng. Mech. and Energy, Univ. of Tsukuba, 1-1-1 Tennodai, Tsukubashi, Ibaraki 305-8573, Japan, Phone +81-29-853-6191, FAX +81-29-853-5207, isobe@kz.tsukuba.ac.jp
Graduate student, Univ. of Tsukuba, 1-1-1 Tennodai, Tsukuba-shi, Ibaraki 305-8573, Japan, Phone +81-29853-5461, FAX +81-29-853-5207, [e011264], [e011258], [z_sasaki]@edu.esys.tsukuba.ac.jp

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June 14-16, 2006 - Montréal, Canada

Thus far, there are few numerical examples showing a high performance in dealing withstructurally discontinuous problems, such as the Distinct Element Method (DEM) (Cundall
1971) or the Discontinuous Deformation Analysis (DDA) (Shi and Goodman 1984), applied
to demolition or seismic damage analyses (e.g., Tosaka et al. 1988, Ma et al. 1995). Althoughthese examples have shown the collapse phenomenon, there are no adequate numerical
procedures to track the demolition process that includes coupled and complicated failure
mechanisms.
The first objective of this study is to perform some blast demolition analyses using a
dynamic finite element code, whose validity against impact collapse problems of full-model
large-scale structures has recently been developed and verified (Isobe and Lynn 2004). The
code is based on the ASI-Gauss technique, a modified version of the formerly developed
Adaptively Shifted Integration (ASI) technique (Toi and Isobe 1993, Isobe and Toi 2000) forthe linear Timoshenko beam element, which computes highly accurate elastoplastic solutions
even with the minimum number of elements per member. The ASI-Gauss technique gains
still a higher accuracy particularly in the elastic range, by placing the numerical integration
points of the two consecutive elements forming an elastically deformed member in such a
way that stresses and strains are evaluated at the Gaussian integration points of the twoelement member. Moreover, the technique can be used to express member fracture, by
shifting the numerical integration point to an appropriate position and by releasing theresultant forces in the element simultaneously. Some blast demolition analyses are performedon framed structures, where the blast conditions are changed to see the difference in the
demolition modes.
We also developed a mimic demolition experimental system that uses magnetic devices
to express member fracture caused by explosion. Two electromagnetic devices are mounted
on both sides of an aluminum pipe, and the pipes are connected via a steel-made connection
with a magnetic field. The magnetic field is controlled by a blast interval controller andpower switches, which control the binding of each connection in the framed structure. The
experimental system does not use explosives or any such dangerous substances, andmulticycle use is possible. Some experiments are carried out for comparison with thenumerical results.
NUMERICAL METHODSIn the ASI technique, the numerical integration point is shifted immediately after theformation of a fully plastic section in an element to form a plastic hinge exactly at that
section. As a result, the ASI technique gives a converged solution with a small number of
elements per member and consequently reduces the calculation cost to some extent. However,the solution in the elastic range is not accurate when the number of elements per member isvery small, due to the one-point integration used in the element for avoiding shear locking.
Thus the technique is modified into an ASI-Gauss technique to improve the accuracy in theelastic range. In this section, general information on both schemes is described.

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

-1

s1

0
s

θ1
u1

1
θ2
u2

Linear Timoshenko beam element

r1

θ1

r

θ2

u1

u2
Rigid bodies-spring model (RBSM)
Numerical integration point
Rotational and shear spring connecting rigid bars
(Plastic hinge including effect of shear force)

Figure 1: Linear Timoshenko beam element and its physical equivalent
ASI AND ASI-GAUSS TECHNIQUES
Figure 1 shows a linear Timoshenko beam element and its physical equivalence to the rigid
bodies-spring model (RBSM). As shown in the figure, the relationship between the locationsof the numerical integration point and the stress evaluation point where a plastic hinge isactually formed is expressed (Toi and Isobe 1993) asr = −s .

(1)

In the above equation, s is the location of the numerical integration point and r the location
where stresses and strains are actually evaluated. We refer to r as the stress evaluation point
later in this section. s is a nondimensional quantity, which is defined as z/(L/2), where L is
the length of the element. s takes a value between -1 and 1.
In both the ASI and ASI-Gauss techniques, the numerical integration point is shifted
adaptively when a fully plastic section is formed within an element to form a plastic hinge
exactly at that section. When the plastic hinge is determined to be unloaded, thecorresponding numerical integration point is shifted back to its normal position. Here, thenormal position means the location where the numerical integration point is placed when theelement acts elastically. By doing so, the plastic behavior of the element is simulated
appropriately, and the converged solution is achieved with only a small number of elements
per member. However, in the ASI technique, the numerical integration point is placed at the
midpoint of the linear Timoshenko beam element, which is considered to be optimal for onepoint integration, when the entire region of the element behaves elastically. When thenumber of elements per member is very small, solutions in the elastic range are not accurate
since one-point integration is used to evaluate the low-degree displacement function of thebeam element.
The main difference between the ASI and ASI-Gauss techniques lies in the normal
position of the numerical integration point. In the ASI-Gauss technique, two consecutive
elements forming a member are considered as a subset, and the numerical integration pointsof an elastically deformed member are placed such that the stress evaluation points coincide

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

MemberMember

-1

-1/2

0

1/2

1

-1

1

0

-1 -1

0

1

1

Element 1

r g 0 sg

Element 2

Numerical integration point

-1/√3

0

1/√3

1

-1 -1

sg 0 r g

1

Element 1
Element 2
sg = 1-(2/√3), rg = -1+(2/√3)

Stress evaluation point(a) ASI technique(b) ASI-Gauss techniqueFigure 2: Locations of numerical integration and stress evaluation points in elastic range
with the Gaussian integration points of the member. This means that stresses and strains areevaluated at the Gaussian integration points of elastically deformed members. Gaussian
integration points are optimal for two-point integration and the accuracy of bending
deformation is mathematically guaranteed. In this way, the ASI-Gauss technique takes
advantage of two-point integration while using one-point integration in actual calculations.
Figure 2 shows the locations of the numerical integrations points of elastically deformed
elements in the ASI and ASI-Gauss techniques. The elemental stiffness matrix, generalizedstrain and resultant force increment vectors for the ASI technique are given by Eqs. (2a) ~(2c) and those for the ASI-Gauss technique by Eqs. (3a) ~ (3c).
[ K ] = L[ B(0)]T [ D(0)][ B(0)](2a){∆ε (0)} = [ B(0)]{∆u}(2b){∆σ (0)} = [ D(0)]{∆ε (0)}

(2c)

[(
)] [D(− 1 + 2 / 3 )][B(1 − 2 / 3 )]{∆ε (− 1 + 2 / 3 )}= [B(1 − 2 / 3 )]{∆u}{∆σ (− 1 + 2 / 3 )}= [D(− 1 + 2 / 3 )]{∆ε (− 1 + 2 / 3 )}T

[K ] = L B 1 − 2 / 3

(3a)

(3b)
(3c)

Here, {∆ε}, {∆σ} and {∆u} are the generalized strain increment vector, generalized stress(resultant force) increment vector and nodal displacement increment vector, respectively. ∆denotes the increment.
The plastic potential used in this study is expressed by
2

2

2

2

⎛ M ⎞ ⎛ My ⎞ ⎛ N ⎞ ⎛ Mz ⎞
⎟ +⎜
⎟ +⎜
⎟ −1= f y −1= 0.
f ≡ ⎜⎜ x ⎟⎟ + ⎜⎜M ⎟ ⎜N ⎟ ⎜M ⎟M
⎝ x0 ⎠ ⎝ y0 ⎠ ⎝ 0 ⎠ ⎝ z0 ⎠(4)Here, fy is the yield function, and Mx, My, N and Mz are the bending moments around the xand y- axes, axial force and torsional moment, respectively. Those with the subscript 0 are

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

s=1
s=sg

Plastic hinge

s=sg

s=0

Node
(a) Elastic stages=0
Fractures=0

Numerical integration point(b) Plastic stage(c) Member fracture

Figure 3: Locations of numerical integration points in ASI-Gauss techniquevalues that result in a fully plastic section in an element when they act on a cross section
independently. The effect of shear force is neglected in the yield function. In this study, we
use a consistent mass matrix, which is assumed to be unaffected by the shifting of numericalintegration points. Newmark’s β method is used as a time integration scheme and the
conjugate gradient (CG) method is used as a solver to reduce the need for memory resources.
MEMBER FRACTURE AND ELEMENTAL CONTACT

A plastic hinge is likely to occur before it develops into a member fracture, and the plastic
hinge is expressed by shifting the numerical integration point to the opposite end of the fully
plastic section. Accordingly, the numerical integration point of the adjacent element forming
the same member is shifted back to its midpoint where it is appropriate for one-point
integration. Figure 3 shows the locations of numerical integration points for each stage in the
ASI-Gauss technique. A member fracture is determined, in this study, by examining axial
and bending strains in two elements forming a member, using the following conditions.
⎛ε ⎞
κy
κx
− 1 ≥ 0,− 1 ≥ 0, ⎜ z ⎟ − 1 ≥ 0
⎜ε ⎟
κ fx
κ fy
⎝ fz ⎠(5)

Here, κx, κy, εz, κfx, κfy and εfz are the bending strains around the x- and y- axes, the axial
tension strain and the critical values for each strain, respectively. When fracture is judged tooccur, a new node is created at the fractured section and the numerical integration point of
the element is shifted back to its midpoint. The resultant forces exerting on the fractured
section are released instantly at the next step after fracture has occurred. A blast at a section
is expressed by inducing member fracture at the relevant section. The nodal mass is equallyredistributed at each separated node after the step.
The contact between the elements is determined using geometrical relations, and isexpressed by connecting the corresponding nodes with two types of gap elements; (1) a hinge
element with only axial stiffness, used between column elements at blast points, and (2) abeam element with axial and bending stiffnesses, used between other sets of elements.

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

Displacement δ (mm)

80

60

40

20

100

100
2 elements/member
4 elements/member
8 elements/member
16 elements/member

80

60

40

20

0.1

0.2

0.3

0.4

0.5

Time (s)(a) Conventional scheme

2 elements/member
4 elements/member
8 elements/member

80

Displacement δ (mm)2 elements/member
4 elements/member
8 elements/member
16 elements/member
32 elements/member
64 elements/member

P,δ

Displacement δ (mm)

100

60

40

20

0.1

0.2

0.3

0.4

0.5

0.1

0.2

0.3

0.4

0.5

Time (s)Time (s)(b) ASI technique(c) ASI-Gauss techniqueFigure 4: Elastoplastic responses of a space frame
NUMERICAL EXAMPLESElastoplastic response analysis

An elastoplastic response analysis is performed to evaluate the convergency of the schemes.
A load of 12 kN is applied to a space frame as shown in Fig. 4. The conventional schemeshows a very slow convergence and sixteen-element modeling is necessary to obtain theconverged solution. Although the ASI technique gives comparatively better results than theconventional scheme, a difference in the vibration mode can be observed in fewer-element
modeling. On the other hand, the ASI-Gauss technique shows a very fast convergence andnearly converged solutions are obtained even when the number of elements per member istwo. A higher reliability of the ASI-Gauss technique is thus confirmed in structural response
analyses.
Blast demolition analyses

The ASI-Gauss technique is applied to the blast demolition analyses of a 5-story framed
structure, with different blast sequences as shown in Fig. 5. The main properties of thestructure are as follows: each span length 6.0 m (beam), 3.6 m (column), 30 cm*30 cm Htype beams, 30 cm*30 cm box-type columns, with Young’s modulus of 214 GPa andPoisson’s ratio of 0.3 (steel), critical values of bending strains 3.2*10-4 rad/mm (beam),
5.3*10-4 rad/mm (column) and tension strain 0.17. The mass of floor slabs and walls is taken
into account by summing the corresponding quantities into the densities of each member. The
analytical conditions are as follows: time increment 0.1 ms, numerical damping applied(Newmark’s β method, β=4/9), structural damping not considered. The blast sequence ofCase 1 is designed to start from the central core towards the outer structure, to accomplish
the demolition successfully by collapsing all the structural members inward. The blast
sequence in Case 2, however, assumes the case when the explosives in one of the series ofcolumns have completely failed to set off. Inevitably, a difference appears in the demolition
modes as shown in Fig. 6. The structure is successfully brought down in 3.6 s with nofragments dispersed around the region in Case 1, while in Case 2, the right-hand side of thestructure where the explosives are unsuccessfully installed, stands unstably for a while andslowly topples down to the outer region.

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

7
6 1**6 6** ******1 1******1 6 *6 7 * 7* 5 *5 1****1 1 ***1 * 5 * 6
6** 4 ** *** 5 ** 6 *5** 1*****1 1****1 *4 * 55*4*** 4 3 *3 1*****1 1 *****1 4* 3 5* 4
4*
* ** *** 3** 4 *3**2 *1 ****1 1****1 *2 * 3
3*2** ** 2* 3**
7

7
6 1**6 6** ******1 1******1* 5 *5 1****1 1 ***1
6*
* 4 ** ***5** 1*****1 1****1
5*4** 4 3 *3 1*****1 1 *****1
4** ** ***
3**2 *1 ****1 1****1
3*
2*
* **
7

7
**6
*
6*
*5
5*
*4
4*
*3
3*
7

Blast intervals: 0.25 s(a) Case 1(b) Case 2
Figure 5: Blast sequences in demolition analyses

0.9 s

1.8 s

2.7 s

3.6 s

4.5 s

6.0 s

(a) Case 1

1.5 s

3.0 s

(b) Case 2
Figure 6: Difference in demolition modes
EXPERIMENTS

A mimic demolition experimental system is developed in this study to enable quantitative
comparison with numerical analyses. Two electromagnetic devices (KANETEC, KE-4RA,
max. attach force = 110 N for beams, KE-4B, max. attach force = 400 N for columns) are
mounted on both sides of an aluminum pipe as shown in Fig. 7, and the members areconnected via steel connectors with the magnetic field generated by the electromagnetic
devices. Power is supplied to these devices through a 4-pin connector jack and a telephonecord. Blast intervals and on/offs of the magnetic field are controlled by the blast interval
controller and power switches, which are connected to a PC and a power supply. The
magnetic field of the device at the blast point is sequentially released by the PC, to express
member fracture caused by explosion.
The properties and conditions used for the analysis are as follows: each span length 34
cm (beam), 28 cm (column), 3 cm*3 cm box-type beams, 4 cm*4 cm box-type columns, with
Young’s modulus of 70 GPa and Poisson’s ratio of 0.345 (aluminum), critical values ofbending strains 2.4*10-4 rad/mm (beam), 3.1*10-4 rad/mm (column) and tension strain

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada

2.7*10-4 (beam), 3.0*10-4 (column), time increment 0.01 ms, numerical damping applied andstructural damping not considered.

PC, D/A converter

Steel connector

Blast interval controller

Connection part
Electromagnetic device

Frame model
Connector jackPower supply
Power switches

Column and beam members

Figure 7: Outline of blast demolition experimental system●●●●
Blast sequence
●
●●
● :1
●
●
●
●
●● ●
●●
● ●
●:2
●
●
●●● ●●
●● ●
●:3
●
●●
● ●●
Blast interval : 0.2 s●●(a) Blast conditions(b) Experimental setup

Figure 8: Blast conditions for a 6-story frame model

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Joint International Conference on Computing and Decision Making in Civil and Building Engineering
June 14-16, 2006 - Montréal, Canada(a) Experimental result
0.0 s

0.2 s

0.4 s

0.6 s

0.8 s

1.0 s

1.2 s

1.4 s

(b) Numerical result
Figure 9: Comparison of demolition modes
A 6-story frame model is constructed using the developed devices. The blast conditions
are shown in Fig. 8, where the blast points and the blast sequence are carefully selected toenable the floor sections to fold in one after another from the upper floors, to avoid fragments
dispersing to a wider area. Figure 9 shows the comparison of demolition modes between theexperimental and the numerical results. The interval between each frame is 0.2 s. The

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June 14-16, 2006 - Montréal, Canada

expected mode is observed in either result, and both results are practically in good agreementwith each other.
CONCLUSIONSIn this paper, numerical and experimental approaches are taken to verify blast demolition
problems. The ASI-Gauss technique is applied to numerical analyses, and the experimental
system using electromagnetic devices is newly developed to compare the results
quantitatively. Both results showed almost the same demolition mode, which indicates thevalidity of both the numerical code and the experimental system.
Some demolition experiments with a larger model are scheduled to verify thepracticability and reliability of both approaches.
ACKNOWLEDGMENTS

This work is partially supported by a Grant-in-Aid for Scientific Research, No.16206055,
Japan Society for the Promotion of Science.
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on Computational Methods in Applied Sciences and Engineering ECCOMAS 2004,
Jyvaskyla, Finland.
Isobe, D. and Toi, Y. (2000). “Analysis of Structurally Discontinuous Reinforced Concrete
Building Frames using the ASI Technique.” Computers & Structures, 76, 471-481.
Kinoshita, M., Hasegawa, A., Matsuoka, S. and Nakagawa, K. (1989). “An experimental
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Ma, M.Y., Barbeau, P. and Penumadu, D. (1995). “Evaluation of active thrust on retaining
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Shi, G. H. and Goodman R. E. (1984). “Discontinuous deformation analysis.” Proc. of 25th
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Toi, Y. and Isobe, D. (1993). “Adaptively Shifted Integration Technique for Finite Element
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Tosaka, N., Kasai, Y. and Honma, T. (1988). “Computer Simulation for Felling Patterns of
Building.” Demolition Methods and Practice, 395-403.
Williams, G.T. (1990). “Explosive demolition of tall buildings in inner city areas.” Municipal
Engineer, 7(4), 163-173.
Yarimer, E. (1989). “Demolition by Controlled Explosion as a Dynamical Process.”
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