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Vulnerability assessment of precast concrete cladding wall panels for police stations:

experimental and numerical investigations

Giannicola Giovino1, Sapienza University of Rome, Carabinieri Corps Pierluigi Olmati2, Sapienza University of Rome Franco Bontempi3, Sapienza University of Rome

ABSTRACT

The purpose of this study is to investigate the behavior of precast concrete wall panels for strategic structures sub-jected to exceptional loads as a consequence of man-made attacks. In particular, the attention is focused on the Ital-ian police stations. Among else, these kinds of station have a widespread distribution on the national territory. This is obviously an advantage for the community. But often for offering this service to the community, these police stations are no other than common civil buildings adapted for police use. Consequently, in the majority of cases, these police stations do not have a structure with adequate performance against man-made attacks. The current guidelines do not provide criteria to comprehensively assess the vulnerability of police stations against man-made attacks. And as stated above, several police stations were former common civil buildings; generally it is not economical sustainable retrofitting these buildings for police use. It is necessary to design and build new police stations providing the adequate resistance performances against man-made attacks. The cladding system is a crucial component of the building for protecting the inside of the police stations against external explosions. For evaluating the resistance performance against man-made attacks of the cladding system of the police stations both experimental and numerical investigations are carried out. The cladding system under inves-tigation is a precast concrete wall panel. Typically the length and the width of these panels are adapted to the specif-ic architecture requirements, but the thickness is approximately of 15 or 20 cm. The steel reinforcement is generally placed in the middle of the cross section. From a design point of view, these walls should protect people and equipment from external attacks. Three precast concrete cladding wall panels subjected to detonations of explosive are investigated both experimen-tally and numerically and the results are presented in this paper. 1 P.E., Ph.D Student, Captain of the Carabinieri Corps, giannicola.giovino@uniroma1.it 2 P.E., Ph.D. Candidate, pierluigi.olmati@uniroma1.it 3 Professor, P.E., Ph.D., franco.bontempi@uniroma1.it

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Vulnerability assessment of precast concrete cladding wall panels for police stations:

experimental and numerical investigations

Giannicola Giovino1, Sapienza University of Rome, Carabinieri Corps Pierluigi Olmati2, Sapienza University of Rome Franco Bontempi3, Sapienza University of Rome

INTRODUCTION

Due to the necessity of improving the performances of the Italian police stations against external explosions, this study presents both experimental and numerical investigations on the assessment of precast concrete wall panels, for using as exterior cladding system, subjected to explosive detonations. The aim of the experimental program is twofold: i) collect data in order to verify and validate both analytical and numerical models, and ii) check with the experimental evidence the necessity of properly design the cladding system of the Italian police stations. A precast concrete cladding wall panel system has advantages over other traditional non-load bearing cladding systems [1]. The first advantage of precast concrete wall panels versus traditional mason-ry cladding (concerning the blast loads considered in this paper) is the increased resistance of the precast system to a blast demand. Precast concrete has shown to provide improved resilience against blast in comparison to traditional steel stud contraction as discussed in [2]. Generally precast concrete has many advantages over the cast-in-place concrete counterpart. The final condition of a concrete product is highly sensitive to environmental conditions dur-ing the curing process. The more finely controlled the environment is (such as humidity, temperature, hydration effects, etc.), the better control over the final condition of the concrete. For this reason, precast components are of-ten more aesthetically pleasing than cast-in-place components. Additionally, precast concrete can often be more economical than cast-in-place concrete. As precast components can be made with the same formwork repeatedly, the cost for constructing the component drops. Finally, precast components will often allow for a more efficient construction process and decrease the total construction time. Precast components are fabricated in the factory, shipped to the work site, and then are placed into position as opposed to cast in place concrete which requires build-ing formwork and time for curing. Moreover, the building life-cycle should be considered when selecting the fa-çade. Concrete cladding can be integrated with other materials improving the response of the panel to environmen-tal attack such as acid rain and chlorine ions. In [3] the behavior of a precast concrete panel with an insulation layer to improve the thermal resistance of the panel is investigated, focusing on the shear ties connecting the two concrete layers confining the insulation layer. The panel and its surface are durable, not requiring expensive maintenance repairs over time. In fact, this kind of cladding system is economically and ecologically sustainable, while maintain-ing all the advantages of traditional precast components. Following the current increase trend of security requirement for strategic buildings and infrastructures numerous works are published in journals papers on both numerical and experimental assessment of structures subjected to explosive detonations. In [4] protected and unprotected concrete slabs are tested with a large hemispherical surface detonation of TNT. The protection of the slabs was made by layer of aluminum foam attached on the blast exposed side of the slabs. Moreover numerical simulations were carried out. In [5] a series of different kind of concrete slabs were tested in order to compare their blast resistance. The slabs under investigation were made by: reinforced concrete augmented with FRP plates; ultra-high performance concrete without reinforcement; and ultra-high per-formance concrete with reinforcement; moreover a normal reinforced concrete slab was tested as control specimens. The amount of explosive in the detonation schedule varied from 1 to 20 kg. The authors conclude that plain ultra-high performance concrete with reinforcement slabs suffered less damage than the normal reinforced concrete slabs when subjected to similar blast loads, which confirms that ultra-high performance concrete is an effective material for blast resisting structure. 1 P.E., Ph.D. Student, Captain of the Carabinieri Corps, giannicola.giovino@uniroma1.it 2 P.E., Ph.D. Candidate, pierluigi.olmati@uniroma1.it 3 Professor, P.E., Ph.D., franco.bontempi@uniroma1.it

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In [6] a finite difference analysis model was proposed in order to predict the structural response of simply supported structural members subjected to blast loads. The model was validated by an experimental test and a matching of the predicted displacements with the experimental evidence was found. In [7] normal concrete slabs were subjected to localized impact loads by a drop test machine. The aim of the study was to investigate the effects of both different kinds of slab reinforcement and the applied impact loads on the dy-namic structural response of reinforced concrete slabs. The paper highlights the importance of an experimental in-vestigation for better understanding the behavior of reinforced concrete elements. Moreover the conducted experi-mental tests show that both the amount of steel reinforcement affected the slab failure modes. Finally in [8] experimental impact tests were considered from the work of [9] for validating the numerical simula-tions able to predict the structural response of aluminum and composite panels under such impact events. In this paper a three specimens are tested, and each specimen is subjected to a single detonation. The first specimen (A) is conventionally designed with a minimum amount of reinforcement required (0.15 %), the second specimen (B) is designed to achieve a specific maximum deflection if subjected to a specific blast demand, and the third spec-imen (C) is equal to the specimen (B). The amount of explosive used to conduct the test is the same for the speci-mens A and B, instead for the specimen C is used a greater explosive charge. The experimental texts were conducted at the facility of the R.W.M. ITALIA s.p.a. (www.rwm-italia.com) at Domus Novas (Italy). The R.W.M. ITALIA s.p.a. provided technical and logistical support for conducting the tests. All the specimens are horizontally simply supported and the explosive charge is orthogonally suspended at 1500 mm from the center of the exposed blast side of the concrete panel. The panels are located about 400 mm from the ground and the lateral side of each specimen is close by sandbags for avoiding the diffracting pressure on the back side of the panel that can leads a reduction of the mid-span deflection. Finite Element Analyses (FEAs) are carried out with the explicit Finite Element (FE) code LS-Dyna® [10] for pre-dicting the displacement time history of the precast panels. Solid elements are utilized for modeling the concrete instead beam elements are adopted for modeling the reinforcement. Also a contact algorithm for modeling the Boundary Conditions (BCs) is utilized. Moreover the LS-Dyna® keyword Load Blast Enhanced [10 and 11] is adopted for providing the blast load. A direct match made by the experimental and numerical results validates the adopted modeling technique.

TEST MATRIX

A total of three precast concrete wall panels are tested (specimen A, B, C). All the specimens are made by normal concrete and standard reinforcement. The length and the width of the three specimens is 3500 x 1500 mm. The specimen A is 150 mm thick; instead the specimens B and C are 200 mm thick. All the panels are provided by rein-forcement in both the longitudinal and transversal directions, but the panels do not have shear reinforcement. This single layer of reinforcement is located in the middle of the cross section. The specimen A is longitudinally rein-forced respecting only the minimum reinforcing steel recommended [12], and the longitudinal reinforcement con-sists of seven rebar with a diameter of 8 mm; instead the transversal reinforcement consists of ten rebar with a diam-eter of 8 mm. The specimens B is longitudinally reinforced to achieve a limited deflection, so it is designed for hav-ing a specific resistance against blast loads; the specimen C is equal to the specimen B. The longitudinal reinforce-ment consists of twelve rebar with a diameter of 10 mm; instead the transversal reinforcing are the same of the spec-imen A. Table 1 summarizes the characteristics of the specimens.

Specimen Length Width Thickness Stand-off Explosive weight Reinforcement [mm] [mm] [mm] [mm] [kg TNTeq] Longitudinal/Transversal

A 3500 1500 150 1500 3.5 7 Φ8 / 10 Φ8 B 3500 1500 200 1500 3.5 12 Φ10 / 10 Φ8 C 3500 1500 200 1500 5.5 12 Φ10 / 10 Φ8

Table 1: Test matrix

All the three specimens are loaded by explosive at a stand-off distance of 1500 mm perpendicularly from the center of the panel. The specimen A and B are loaded by 3.5 kg of equivalent TNT, instead the specimen C is loaded by 5.5 kg of equivalent TNT. The explosive provided by the R.W.M. ITALIA s.p.a. is the PBXN-109 composed by the 64.12 % of RDX, the 19.84 % of Aluminum, and the 16.04 % of Binder.

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The compressive resistance of the concrete is estimated by testing six concrete specimens. Also the resistance of the rebar is tested experimentally by traction tests of three rebar specimens for each rebar diameter. The steel used for the reinforcement is the B450C [12]. Table 2 shows the results provided by the R.W.M. ITALIA S.p.a. of the rein-forcing steel and concrete testing.

Specimen Concrete Rebar steel Rebar steel N° Rc [MPa] fy [MPa] ft [MPa]

1 37.46 536 616 2 35.87 540 625 3 35.60 541 626 4 35.19 547 670 5 29.89 549 676 6 31.01 547 672

Average 34.17 543 647

Table 2: Reinforcing steel and concrete testing results

The specimens are simply horizontally supported, and the supports are made by concrete blocks. The explosive charge is suspended at 1500 mm from the panel surface and it is orthogonal with the center of panel surface. The supports are 400 mm high and the lateral open space between the panels and the ground is closed by sandbags. In this way the shock wave would be not able to diffract on the back face of the panels. See both Figure 1 and Figure 2. When the precast concrete panels are installed on a building façade the shock wave is not able to load the back side of the panels, so it necessary to reproduce this scenario. Moreover for the purpose of verify and validation of numerical models both the boundaries and the loading conditions should be known as much is possible. Leaving the shock wave to diffract on the back side of the panel, also interacting with the ground, leads an erroneous estimation of the blast load on the panels consequently the prediction of the structural response would be affected by errors. Generally minor deflections are assessed if the shock wave loads the back side of the panels. Figure 1 and Figure 2 show the aerial view and the longitudinal section of the testing site respectively. The testing site consists on an underground open space without ceiling surrounded by concrete walls. A ramp for entering and exiting is on a side of the testing site, see Figure 1. The walls around the specimens cause the undesirable reverber-ating of the shock wave. This leads an amplification of the blast load and consequently greater displacements are assessed than the ones predicted in the test program design. During the back analysis reported in this paper the im-age charge method is adopted to take account the reverberating effect due to the surrounding walls. However more precise results can be obtained using the Arbitrary Lagrangian Eulerian (ALE) method.

Panel t a b c

[mm] [mm] [mm] [mm] A 150 1550 B 200 1160 1550 C 200 880 1550 2030

Table 3: Thickness of the panels and position of the meter devices (see also Figure 1)

Two kinds of displacement meter are used and provided by the R.W.M. ITALIA s.p.a.: the comb device and the coaxial tube device. The comb device works by the impacting force of the panel on the single tooth of the comb, the tooth is damaged by the panel and after the detonation the maximum deflection is measured by counting the bent teeth of the comb device. Instead the coaxial tubes device works by the impacting force of the panel on the top of the internal tube of the coaxial device, the internal tube is so pushed inside the external tube of the coaxial device and three screws scratch the surface of the internal tube marking the panel deflection. Figure 2 shows the position of the meter devices respect the panel specimen; however the arrangement of their positions is different for each spec-imen and Figure 2 is not to scale. For the specimen A only the coaxial tube is adopted, it is positioned in the mid-span of the panel. For the specimen B both the comb and coaxial tubes devices are utilized, the comb device is in the mid-span and the coaxial tube is longitudinally about 400 mm away from the comb devices. Finally for the specimen C three meter devices are utilized, one of the two comb devices is placed in the mid-span and the second

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one is placed longitudinally about 500 mm away from the first one and the coaxial tube devices is placed at about one third of the span. With reference to Figure 2, Table 3 summarizes the exact positions of the meter devices for all the three specimens. For measuring the maximum pressure on the blast side of the specimens the rupture discs for hydraulic applications are utilized without appreciable results, see Figure 1. Each disc has a hydrostatic failure pressure however the dy-namic characteristics of the disc are unknown. Positioned on the blast side of the panel, the specimen A and B have four rupture discs instead the specimen C has five rupture discs.

Blast side

3500

150

0

1750

3100

750

10804800

Explosive charge

Concrete support

Panel

500

Not to scale

Wall

137

0

Sandbags

800

Entry and exit ramp

Rupture discs

NORTH

SOUTH

EA

ST

WE

ST

Figure 1: Aerial view of the testing site

Concrete support

Panel

Explosive charge

400

t15

00

3500

3100

1080 4800

Comb device

Coaxial tubes device

Ground

Wall

Not to scale a

b

c

EA

ST

WE

ST

Figure 2: Longitudinal section of the testing site

The hydrostatic failure pressure of the discs is: 4.5, 5, 5.5, 6, 7.5, and 8 MPa. With reference to Figure 1 the speci-men A has four rupture discs, from the left one of 4.5 MPa to the right one of 6 MPa; also the specimen B has four rupture discs, but from the left one of 6 MPa to the right one of 4.5 MPa; finally the specimen C has five rupture discs, from the left one of 5.5 MPa to the right one of 8 MPa. Figure 3 shows details of the testing site. In Figure 3 (a) is in view the specimen A. The panel, the meter device, and the sandbags are positioned, only the explosive charge has to be positioned like in Figure 3 (h). Figure 3 (b)

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shows the operations for positioning the specimen A, and is visible the truck just at the end of the entry ramp of the testing site (see Figure 1). In Figure 3 (c) is a zoom of the specimen A ready to be tested, instead in Figure 3 (d) is a large view of the specimen A with the explosive just armed. Figure 3 (e) shows the coaxial tubes devices, instead Figure 3 (f) shows the comb devices. Finally in Figure 3 (g) is a rupture discs.

(a) (b)

(c)

(d)

(e)

(f)

(g) (h)

Figure 3: Detail images of the R.W.M. ITALIA S.p.a. testing site

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EXPERIMENTAL RESULTS

The experimental test took place the July 22 and 23, 2013. The result data is in terms of both maximum and residual deflection. The rupture discs do not provided a good measure of the maximum pressure, so they are considered un-reliable and not suitable for measuring blast pressure at least in the adopted arrangement. Moreover the crack pat-terns of the specimens are marched and photographed. In the following sections the results of the experimental in-vestigation are reported. The results concerning the rupture discs are not reported because meaningless. Specimen A The specimen A is designed with the minimum reinforcement for a concrete cladding wall panel [12]. The deflec-tion of the specimen A reached the full scale value of the coaxial tubes device. The panel during the deflection im-pacted the external tube of the coaxial tubes device and the panel stops its deformation. The maximum and residual deflection of the panel is so 108 mm. In Figure 4 (a) is shown the panel in contact with the external tube of the co-axial tubes device, and in Figure 4 (d) is the scratch that such tube made on the concrete panel during the impact. Figure 4 (b) shows the measurement of the residual displacement. Finally Figure 4 (c and e) show the flexural crack pattern of the specimen A.

(a) (b) (c)

(d) (e)

Figure 4: Detail images of the specimen A

Specimen B This panel is designed to achieve a specific performance under a blast load, so the specimen B is designed for blast; the amount of explosive for the specimen B is the same of the specimen A. The maximum and the residual deflection achieved by the specimen B is of 70 mm and 35 mm respectively. The specimen B shows a ductile failure with a diffuse crack patterns on the central one third of the panel span; the major

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cracks are 3 mm width. However as shown in Figure 5 (b) some radial crack patterns are present, this is due to the short stand-off distance [11], and however the specimen B develops a flexural mechanism as designed. In Figure 5 (a) is the view both the comb and coaxial tubes device after the detonation; the bend of the comb teeth and the ex-cursion of the internal tube of the coaxial tubes device are appreciable in this figure. Finally in Figure 5 (c and e) is shown the crack patterns and crack width along the mid-span of the panel.

(a) (b)

(c) (e)

Figure 5: Detail images of the specimen B

Specimen C The specimen C is equal to the specimen B but the blast demand is greater to leads significant damages to the panel without reach a failure. The specimen C would test the blast resisting range of the panel over the limit of his specif-ic design; for this purpose the amount of explosive is increased at 5.5 kg of equivalent TNT. The maximum and the residual deflection are of 123 mm and 82 mm respectively. Figure 6 (a) shows the comb devices after the detonation measuring the maximum deflection; instead the Figure 6 (b) shows the measurement of the residual deflection of the specimen. Heavy crack patterns are assessed. Along the mid-span of the panel diffuse cracks are present with significant width until 10 mm; in Figure 6 (c) the crack patterns at the mid-span are in view, and in Figure 6 (d) the view of a 10 mm crack is proposed. Moreover some cracks at the mid-span pass through the panel cross section thickness as visible in Figure 6 (e); fur-thermore Figure 6 (f) shows the maximum width of the crack passing through the panel cross section thickness of Figure 6 (e).

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(a) (b)

(c) (d)

(e) (f)

Figure 6: Detail images of the specimen C

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NUMERICAL INVESTIGATION In order to reproduce the experimental tests numerically the explicit Finite Elements (FE) code LS-Dyna® is adopted [10]. To simulate physic phenomena many numerical solution techniques can be utilized, the most relevant are: the “Lagrangian”, the “Eulerian”, the “Eulerian-Lagrangian” meshes [13, 14], and the “Smoothed Particle Hydrodynam-ics” method [14, 15]. Furthermore two methods exist to take account the interaction between the blast load and the structural component: a coupled and an uncoupled approach [16]. In this study a “Lagrangian” mesh is adopted and the uncoupled approach is preferred [17], thus the blast load is computed and applied independently from the struc-tural response of the concrete wall panels. The FE models have constant solid stress elements for the concrete, and beams elements for the reinforcement [10]. To bond the beams and solid elements, the LS-Dyna® keyword Constrained Lagrange in Solid [10] is used. For reducing the computational effort the model of the specimens are only a square part of the panel, so opportune boundary conditions are provided. Figure 7 shows a detailed view of the finite element model of the specimen B and C. The concrete supports of the panels are explicitly modeled and the contact between the panel and the support is provided by the LS-Dyna® keyword Contact Automatic Surface to Surface. Furthermore, in order to take account correctly the clearing effect [11] the boundary conditions for blast are provided; a rigid surface modeling the other three quarter of the panel is added, see Figure 7. The material constitutive law of the reinforcement is the kinematic hardening plasticity model [10] and the strain rate effects is accounted for by the Cowper and Symonds strain-rate model [10], the parameters selected for this model are: D=500 s-1 and q=6. Furthermore the steel Young’s modulus is 200 GPa, the Poisson coefficient is 0.3, and the yielding stress is 543 MPa, see Table 2. The concrete utilizes the Continuous Surface Cap Model (CSCM), MAT159 in LS-Dyna® [10]. The yield stresses are defined by a three-dimensional yield surface based on the three stress invariants. The intersection between the failure surface and the hardening cap is a smooth intersection. The softening behavior of the concrete is taken into account by a damage formulation that affects both the concrete strength and a reduction in the unloading/loading stiffness. The increase in concrete strength with increasing strain rate is taken account by a visco-plastic formulation. The Dynamic Increase Factor (DIF) relation used for the con-crete is shown in Figure 8 (a); instead in Figure 8 (b) the input data for the concrete model are shown.

Support

Blast load BC

Panel

Figure 7: The finite element model

The LS-Dyna® keyword Load Blast Enhanced is used for providing the blast load [18], the load surface is shown in Figure 1; moreover also the gravity load is taken account. Due to the walls delimiting the testing site multiple reflections of the original shock wave occurred; consequently the blast load on the specimens is greater than the blast load on a specimen tested in an open space. For taking ac-count the phenomenon of the reverberated shock waves [19] the ALE method is the most appropriate method, but it is very computationally expensive. Using the uncoupled approach [16] the image charge method [19] provides ac-ceptable results without increasing the computational effort. The image charge method predicts the pressure pulses from a reverberating shock wave. The image charge method consists in taking account the pressure pulse from a reverberating shock wave by a pressure pulse due to a spherical free-air detonation of a fictitious (image) charge with the same weight of the actual charge but located at a stand-off distance from the target equal to the full path length (see for example both the paths B and C in Figure 9) of the

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shock wave to the reflecting wall and then to the target; and hitting the target with the angle of incidence of the re-verberating shock wave. Figure 9 is adapted from [19] and it shows an elementary scenario of reverberating shock waves. The path A is the direct shock wave path; instead the paths C and B are the reverberating shock wave paths on the target.

0

2

4

6

8

0.001 0.1 10 1000

DIF

[-]

Strain-rate [1/sec]

CompressiveTensile

Density 2.248 lbf/in

4 s2 2.4*103 kg/m3

fcm 4060 psi

28 N/mm2 Cap

retraction active

Rate effect

active

Erosion none

AB

C

Reflecting surface

Reflecting surface

(a) (b) (c)

Figure 8: Concrete model input data Figure 9: image charge ap-proximation, figure adapted

from [19]

In Table 4 are summarized the locations of the image charges for all the three specimens. The target point is the center of the panel; α is the angle made by the orthogonal projection of the stand-off distance of the image charge (full path length of the reverberating shock wave) on the panel surface; instead the image charge side is which side the image charge is located. A total of four image charges plus the real charge load the specimens.

Image charge side

Stand-off α [m] [degrees]

West 6009 27 North 4705 35 South 4705 35 East 13505 13

Table 4: Image charge positions

Figure 10 shows the results of the numerical simulations. The time history of the mid-span deflection is plotted to-gether with the experimental results. In Figure 10 δmax and δres are the maximum and residual mid-span deflections assessed experimentally. For the specimen A the results cannot be compared because the specimen impacted the external coaxial tubes devices and it stopped its deformation; instead for the specimen B and C the experimental and numerical results can be discussed. For both the specimen B and C the maximum deflection computed numerically is about 10 mm less than the meas-ured experimentally, so the uncertainty due to the multiple reverberating shock waves is sufficiently achieved by the image charge method; however the residual deflections are not predicted with enough accuracy. The predicted re-sidual deflection is about 20 mm greater than the measured experimentally. A possible cause can be due to the dif-fracting of the shock wave on the back side of the panel not completely avoided by the sandbags. The kind of sand-bag utilized was not enough strength to resist at the blast pressure and avoiding the diffracting of the shock wave on the back side of the panel. Looking at the Figure 4 (a), Figure 5 (a and b), and Figure 6 (b) the disruption of the sandbags is evident; so probably the sandbags reduced the diffraction of the shock wave on the back side of the pan-els but probably without totally avoiding this phenomenon. The ALE method is probably the best method in order to simulate correctly the conducted experimental tests, be-cause both the reverberating and the diffracting shock waves can be taken account together with the disruption of the sandbags.

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0

40

80

120

160

200

240

280

0 0.05 0.1 0.15 0.2 0.25

δ[m

m]

time [sec]

Experimental

Numerical

Specimen A

δmaxδres

0

10

20

30

40

50

60

70

80

0 0.05 0.1 0.15

δ[m

m]

time [sec]

NumericalExperimental

Specimen B

δmax

δres

(a) (b)

0

20

40

60

80

100

120

140

0 0.05 0.1 0.15

δ[m

m]

time [sec]

Numerical

Experimental

Specimen C

δmax

δres

0

40

80

120

160

200

240

280

0 0.05 0.1 0.15 0.2 0.25

δ[m

m]

time [sec]

Specimen A

Specimen B

Specimen C

(c) (d)

Figure 10: Experimental and numerical mid-span displacement

The US Army Corps developed some Component Damage Levels (CDLs) [19], based on the building level of pro-tection, which are correlated with two response parameters of the single component: the support rotation angle (θ) and the ductility ratio (µ). These parameters are defined in Equations (1) and (2).

θ = arctan �2δmaxL �

(1) μ = δ���

��

(2)

Where δmax is the maximum deflection and δe is the equivalent yield deflection of the panel. In the U.S. antiterror-ism performance-based blast design approach [19], there are five component damage levels (CDLs) considered, listed in order of decreasing damage: Blowout, Hazardous Failure, Heavy Damage, Moderate Damage, and Superfi-cial Damage. The thresholds corresponding to these CDLs are defined in terms of the response parameters θ and µ. For a non-structural concrete cladding wall without shear reinforcement, neglecting tension membrane effects, the CDL thresholds are those reported in Table 5 below.

Component damage levels θ [degree] µ [-]

Blowout >10° none Hazardous Failure ≤10° none

Heavy Damage ≤5° none Moderate Damage ≤2° none Superficial Damage none 1

Table 5: Component damage levels, and the associated thresholds in terms of response parameters

Table 6 shows the summary of the results for each specimen reporting both the maximum and the residual deflec-tions of the experimental and numerical investigations. Moreover the support rotation θ is shown for both the exper-imental and numerical investigations. Looking at the maximum support rotations experimentally assessed: the spec-imen B goes over the Moderate Damage CDL but does not exceed the Heavy Damage CDL; and also the specimen C does not exceed the Heavy Damage. Thus the limits for the response parameters of the CDLs provided in [19] have a good correspondence with the kind of structural response of the specimens assessed experimentally.

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Finally Figure 11 shows the simulated crack patterns of the three specimens; in view is the brittle damage parameter in the range from 0.95 to 1 [10 and 20]. Since only one quarter of the panels is modeled the Figure 11 is obtained mirroring the plot of the crack patterns of the model. Generally the crack pattern of the three specimens is similar, but the specimen A shows more concentrated cracks that the specimens B and C. The simulated crack patterns match with the experimental evidence showing major cracks at the panel mid-span, confirming the developing of the resisting flexural mechanism of the panels without suffering a shear failure. In all the specimens and particularly in the specimen C some radial cracks are present, see Figure 5 (b) and Figure 6 (a and c); also this kind of cracks are partially reproduced by the numerical simulations, see Figure 11 (a).

Specimen Experimental Numerical Experimental Numerical

δmax [mm] δres [mm] δmax [mm] δres [mm] θmax [deg] θres [deg] θmax [deg] θres [deg]

A 108* 108* 244 240 4.0* 4.0* 8.9 8.8 B 70 35 58 50 2.6 1.3 2.1 1.8 C 123 82 114 106 4.5 3.0 4.2 3.9

* Full scale value

Table 6: Summary of the results

Specimen C Specimen BSpecimen A

(a)

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Specimen C

Specimen B

Specimen A

(b)

Figure 11: Crack patterns of the specimens: (a) back view, (b) longitudinal view

CONCLUSIONS

With the aim to investigate how improving the blast resistance of the cladding system for Italian police stations, three precast concrete cladding wall panels have been tested in the testing site of the R.W.M. ITALIA s.p.a. (www.rwm-italia.com). Furthermore numerical simulations with the explicit finite element code LS-Dyna® have been carried out. Only the prescribed minimum amount of reinforcement has been used to design the first panel (specimen A) for showing the failure of ordinary precast concrete cladding wall panels if subjected to detonation of explosive due for example to man-made attacks or accidental explosions. The second panel (specimen B), that is designed for blast, is subjected to the same detonation of the first panel (specimen A) and a good flexural structural response with limited deflection has been assessed. The third panel (specimen C) instead is subjected to a detonation of a greater amount of explosive for assessing the structural resistance of the panel to detonations not considered in the design; it shows a good flexural structural re-sponse without go over the Heavy Damage Component Damage Level. The conducted experimental program has also the aim to provide a benchmark for verifying and validating both nu-merical and analytical models. For this purpose, finite element analyses of the three specimens have been carried out. The complex blast boundary conditions got place to reverberating shock waves and the image charge method has been adopted to correctly load the panels. Good results have been obtained in terms of maximum deflection, but the residual deflection is predicted with less accuracy maybe due to the diffracting shock wave on the back side of the panel, phenomenon not avoided completely by the sandbags that were disrupted by the blast pressure. A future development from the numerical point of view is to use the Arbitrary Lagrangian Eulerian method in order to take account the complex boundary conditions of the blast load and the diffracting shock wave on the back side of the panels together with disruption of the sandbags. Instead form the experimental point of view different kind of cladding panels, like the composite panels, can be tested for both far-field [2] and close-in detonations [11].

ACKNOWLEDGEMENT

The authors would like to acknowledge the R.W.M. ITALIA s.p.a. (www.rwm-italia.com) for supporting the design, the logistic, and the financing of the experimental program. The authors would like to acknowledge also the Carabinieri Corps of the Italian General Command (45 Romania Avenue, Rome), and both Dr. Francesco Petrini and Dr. Konstantinos Gkoumas (Sapienza University of Rome) for the appreciate discussions on the experimental and numerical investigations.

For Public Release

For Public Release

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