Comparative Analyses of Impact Tests with Reinforced Concrete Slabs

Transcription

1 Comparative Analyses of Impact Tests with Reinforced Concrete Slabs C. Heckötter* (GRS), J. Sievers* (GRS), F. Tarallo** (IRSN), N. Bourasseau** (IRSN), B. Cirée** (IRSN), A. Saarenheimo*** (VTT), K. Calonius*** (VTT), M. Tuomala**** (TUT) * Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbh, Cologne, Germany ** Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Fontenay-aux-Roses, France *** VTT Technical Research Centre of Finland, Espoo, Finland **** TUT Tampere University of Technology, Tampere, Finland Abstract: In the framework of the design of nuclear facilities reinforced concrete structures have to be assessed concerning their robustness against impacts of different kinds of either externally or internally generated missiles. Experimental results are required for the validation of analysis methods that are used for the assessment of impact events. Currently medium scaled impact tests are carried out by VTT in the framework of an international research project called IMPACT. Among others, the Technical Safety Organizations (TSOs) GRS and IRSN are IMPACT partners. Scientific advice is provided by TUT. Missile impacts are usually classified as either hard or soft according to the missiles deformability. They may give rise to phenomena of localized target damage like punching as well as to global dynamic flexural response of the target. The present paper describes two representative tests of the IMPACT project, namely one hard missile test with dominant punching target failure mode and one soft missile test with flexural target response. Both tests were analysed by the authors using complex computer codes as well as simplified analytical methods. The comparative analyses show that the used complex analysis methods are suitable to predict the mechanical behaviour of reinforced concrete structures including various damage types like punching, scabbing, penetration, perforation and bending. Scattering of analysis results is a measure for the accuracy to simulate relevant phenomena. Deviations of analysis results to test results are caused by uncertainties of specific modelling parameters. 1 INTRODUCTION The consequences of various impulsive impact loads (drop loads, impacts of equipment after failure, airplane crashes, wind generated projectiles) on civil structures must be analysed when designing or assessing nuclear facilities. In that field, the behaviour of reinforced concrete structures impacted by deformable soft or by not deformable hard missiles is one of the complex issues that must be addressed. Until the 1990s, these studies primarily covered specific hazards involving moderate degrees of energy (light touring airplanes, military aircrafts) and used empirical or analytical methods to study structural behaviour, such as slab perforation. During the past years, those analyses were further developed thanks to the enhanced capabilities of computers and calculation codes. Because of the small amount of available data in that field, an experimental program

2 is carried out by VTT in Finland. Launched in 2004, this international program named IMPACT project, has about 12 partners from various countries, including IRSN and GRS. The second three-year phase of the program began in In order to provide relevant data, this program consists of impact tests on rigid or deformable targets and includes impacts of deformable and hard missiles on reinforced concrete slabs. Analysing the results of the IMPACT project tests gives the opportunity to improve the understanding of the physical phenomena observed and enhances numerical simulation capabilities of impacts on civil structures. Teams from VTT, GRS and IRSN have separately simulated two representative tests of the IMPACT project using complex computer codes. Hereby IRSN used the fast dynamic finite element code LS-DYNA /1/. In the framework of a current reactor safety research project sponsored by the German Federal Ministry of Economics and Technology GRS validates analysis models of impact tests which are solved with the simulation tool ANSYS AUTODYN /2/. The finite element code Abaqus/Explicit /3/ is used by VTT. Furthermore, TUT analyses the impact tests by means of simplified methods such as spring mass systems and empirical formulae. The comparison of those simulations with the tests data makes it possible to draw conclusions on the capabilities of different fast dynamic computer codes and on the advisable methodologies to analyse the consequences of impacts on civil structures. 2 TEST FACILITY A flexible experimental setup, shown in Figure 1, has been developed at the Technical Research Centre of Finland (VTT) for medium scaled impact tests. A detailed description is given by Lastunen et al. /4/. So called force plate tests with soft missiles can be carried out to measure the impact force time history. Hereby, forces caused by the impact are measured at force transducers behind the force plate target structure. Concrete slab tests with different soft and hard missiles and various boundary conditions can be conducted. In slab tests the target response is measured by various displacement and strain sensors and the support reactions can be measured from the back pipes. A pressure accumulator is used to provide the required initial energy for a test and an acceleration tube is used to accelerate the missile to an impact velocity ranging from about 50 m/s to 200 m/s. The mass of the missile can be up to 100 kg. A schematic figure of the test apparatus is shown in Figure 2. At first, soft missile impact tests on reinforced concrete slabs with the bending as the dominating target deformation mode were conducted within the IMPACT project at VTT. Later, also a considerable number of impact tests with hard missiles leading to substantial local target damage have been made. In some tests, the missiles contain a lot of liquid or soft solid material. The experiments have provided information on the force-time functions during impact of soft missiles against a rigid wall, on the damage process of reinforced concrete walls due to impact, and on the spreading of liquid droplets. Furthermore, so called 3D missiles with structural components like small scaled wings and engines resembling the shapes of real aircrafts have been used. 2

3 Figure 1: Photograph of the test facility constructed for the IMPACT project. Figure 2: Schematic figure of impact test apparatus. 3 HARD MISSILE IMPACT Local response to hard missile impact is usually analysed in terms of concrete slab damage modes such as spalling, penetration, scabbing and perforation. At this, ejection of concrete pieces from the wall due to impact is called spalling on the impacted side of the protective slab and scabbing from the rear side. Penetration of the missile into the target may lead to perforation, if the contact force surpasses the remaining local load bearing capacity of the slab. Due to missile impact the concrete slab may be broken into smaller pieces or a shear failure cone may be formed. Slab responses to hard and soft missile impacts are usually quite different. The global behaviour of the target slab is more important in the soft missile impact case discussed in chapter 4 than in the hard missile impact case. 3

4 3.1 Description of hard missile test One part of the IMPACT test campaign consists of concrete wall tests with hard missile impact. The impacted targets in these tests are concrete walls of size 2m * 2m supported width and a thickness of 250 mm, as shown in Figure 3. The wall includes different types of reinforcements, namely bending reinforcement and shear reinforcement in form of T-headed bars. In other tests also so-called Dywidag bars are installed, which after casting enable the post-tensioning of the wall both in vertical and horizontal directions. Orbovic et al. /5/ and Tuomala et al. /6/ give more detailed descriptions and analyses of this test series. The selected test considered in the present paper is TEST 699. The hard missile shown in Figure 4 with a mass of about 47 kg hits the slab with a velocity of 100 m/s. Figure 5 shows the damage of the slab. A shallow crater of about 38 mm depth is formed on the front face and the missile rebounds. The scabbed area on the rear face is nearly circular with a diameter of about 700 mm. After the test one quarter is sawn out of the slab. The formation of a typical punching cone is apparent from the section. Figure 3: Principal dimensions of the slab (left) and reinforcement including T-headed bars (right) for TEST 699. Figure 4: Missile for TEST 699 on the launch pad before (left) and after (right) the test. 4

5 Figure 5: Damage of the test slab in TEST 699 due to impact: shallow crater and spalling of concrete cover on front face (top left), scabbing of concrete cover on the back face (top right), sections of one quarter of the slab (down). 3.2 Numerical studies on hard missile test Comparison of analysis models Three dimensional analysis models for different computer codes are employed by IRSN, GRS and VTT to simulate hard impact phenomena. Empirical and semi-empirical approaches are applied by TUT. In the following the different analysis models for TEST 699 are briefly described. IRSN modelling IRSN has simulated TEST 699 using the LS-DYNA fast dynamics finite element calculation code /1/. The model built for that purpose includes about elements, among which solid elements for concrete (eight layers through the concrete wall thickness) and beam elements for reinforcement steel. At each node, elements representing concrete and steel are tied. The boundary conditions are such that the slab is simply supported on its 4 sides. The so-called Winfrith smeared crack material model (LS-DYNA material # 84) is used for concrete. The missile is represented with a rigid material. Figure 6 shows the model of the slab and the missile. Figure 6: LS-DYNA model of IRSN for TEST

6 GRS modelling Figure 7 shows the analysis model for TEST 699 developed by GRS and solved with the explicit finite element code ANSYS AUTODYN /3/. Missile and target concrete are modelled with solid volume elements while the reinforcement is modelled with beam elements. A slide bearing boundary condition is applied to the front and back face of the slab. The average concrete element length is about 22.5 mm. Eleven element layers through the wall thickness are used. The concrete cover of reinforcement is assumed to be 22.5 mm on each face. The mesh size corresponds to concrete elements. For the concrete part the RHT (Riedel, Hiermaier, Thoma) /7/ material model is employed. This material model is capable to describe the behaviour of concrete subjected to dynamic loading. The RHT input parameters were adjusted to the concrete test data provided by VTT. Figure 7: AUTODYN model of GRS for TEST 699. VTT modelling The impact test is simulated by dynamic and nonlinear finite element analyses with explicit time integration using Abaqus/Explicit version /3/. The finite element model is a threedimensional solid quarter model using quarter symmetry conditions. It includes the concrete wall, steel missile and the reinforcements inside the wall. The model of this two-way simply supported slab is shown without element mesh in Figure 8. In this partly translucent figure, the missile is shown in light blue colour, the wall in grey, bending reinforcement in blue and T-headed bars in red. The wall has 10 elements through the thickness, 40 elements in other two span wise directions and altogether linear solid brick elements. All the reinforcement bars are modelled with linear truss elements. The total number of truss elements is The missile is modelled with 1048 linear brick elements and the element size is approximately half of the one in the wall. Two material models are needed in order to describe both tunnelling and scabbing of the impact loaded concrete. Penetration by tunnelling is modelled by the modified Drucker- Prager material model with so-called cap plasticity. This material is applied to a rectangular section at the impact area shown in green colour in Figure 8. Scabbing is modelled by applying everywhere else an elastic-plastic isotropic hardening material model assuming von Mises yield surface, a tensile failure criteria and element deletion. Element deletion is not applied to the elements in the supporting edges and in the impacted zone. 6

7 Figure 8: Abaqus/Explicit quarter symmetry model of VTT for TEST 699. TUT modelling Hard missile impact was first studied in military applications, and many empirical and semiempirical formulae for analysing penetration, perforation and scabbing were derived from a large number of tests and regression type analyses. The considered range of parameters is usually somewhat different in nuclear industry and its formulae. A good recapitulation of common formulae including the formulae used by TUT is given in the paper of Li et al. /8/. Based on the NDRC formula Chelepati and Kennedy derived a new formula for smaller values of the ratio of penetration depth to missile diameter. At this, also the ultimate compressive strength of concrete was taken explicitly into account. Degen derived a perforation formula on the basis of experiments made in the nuclear industry. Chang proposed a perforation formula and a scabbing formula, which have been used widely in nuclear industry. Berriaud et al. developed a perforation formula, the so called CEA-EDF formula, in which the parameters are the density and the compressive strength of concrete, the mass, the diameter and the impact speed of missile. The UKAEA formula is similar to the NDRC methodology, but also the perforation velocity is predicted by this method. Also the amount of bending reinforcement is included as an input parameter for the perforation velocity formula. Forrestal et al. /9/ derived a new penetration theory primarily for high velocity impacts of hard missiles. This method is based on a cavity expansion theory. The first stage of penetration is the crater formation phase. The projectile may proceed into a subsequent tunnelling phase. Shear cone formation and displacement may lead to perforation. The effects of bending and shear reinforcements can be added to this model in assessing the possible shear cone separation (see Figure 9). 7

8 Figure 9: Penetration and perforation model according to Forrestal et al. /9/ Comparison of selected analysis results IRSN analysis results The deformed view of the model at time of maximum deflection is given in Figure 10. The modelling without element erosion cannot directly predict scabbing. However, the formation of a cone by cracking can clearly be seen on the damage visualization in Figure 11, where cracks with a width higher than 0.1 mm are visualized. Besides that, the cone cracking exhibits an angle of about 45, consistent with the experiment. Figure 10: Mesh deflections of the target (multiplied by a factor of five) in the IRSN calculation of TEST

9 Figure 11: Damage visualization of the concrete cracking in the IRSN calculation of TEST 699. GRS analysis results Contour plots of the RHT concrete material model damage parameter are shown in Figure 12. Totally damaged material corresponds to a damage parameter of unity, i.e. the concrete material exhibits no strength in tensile loading region. The region with a calculated damage parameter of unity on the back face is interpreted as region with scabbing. In the calculation this region is circular with a diameter of about mm. The scabbed region of the test slab is also roughly circular with a diameter of about 700 mm (see Figure 5). Furthermore, a punching cone formation can be seen in the section plot. The punching angle of about 45 is quite close to the experimental observations. The predicted penetration depth of about 40 mm is also in good agreement with the test result (38 mm). Figure 12: Damage parameter in GRS simulation of TEST 699, 60 ms after the impact. VTT analysis results Figure 13 shows the deformed model shape at the instant of maximum deflection, approximately 10 ms after the missile has hit the surface of the wall. Figure 14 shows the deformed reinforcement bars with strain distribution at the same time instant. Some of the centre rebars are broken. There are distinct cracks in the wall and the calculated scabbing radius is approximately mm (i.e. the diameter is about mm) which agrees well with the test result. The punching cone angle is approximately 45. 9

10 Figure 13: Deformed model shape in the VTT simulation of TEST 699 at 10 ms after the impact. Figure 14: Deformed rebars with strain distribution in the VTT simulation of TEST 699 at 10 ms after the impact. Tensile strains of magnitude over 3% are depicted with orange, red and brown colour. 10

11 TUT analysis results Penetration depth values have been determined with various formulae for TEST 699, i.e. in the case of the impact of a rigid missile with a diameter of d = m and a mass of m = 47 kg on a concrete target with a compressive cube strength of f c = 54 MPa. The results for the penetration depth show a scattering between about 25 mm and 142 mm. The experimental value of 38 mm is included in this range. The best conservative approximation of the test result was achieved with the UKAEA formula. It predicts a penetration depth of about 64 mm. Comparison of selected analysis results Some fundamental results of the simulations are compared with the corresponding test results in Table 1. Penetration depth, diameter of scabbed area and punching cone angle can be approximated quite well. Deviations of analysis results to the test results are caused by uncertainties of specific modelling parameters such as the input parameters of the different concrete material models available in the different computer codes. It can be concluded, that the finite element codes in general are capable to simulate the fundamental phenomena of hard missile impact, such as cone cracking, scabbing and penetration. Tool Penetration depth Scabbing diameter Cone angle IRSN LS-DYNA 14 mm 1100 mm about 45 GRS AUTODYN ca. 40 mm mm about 45 VTT Abaqus/Explicit mm mm about 45 TUT Simplified methods mm - - Test 38 mm 700 mm about 45 Table 1: Overview about basic test and analysis results of TEST SOFT MISSILE IMPACT Structural behaviour and damage mechanisms of a reinforced concrete structure due to an impact by a soft missile are different in comparison with the consequences of a hard missile impact. Soft missiles show considerable deformation during the impact. Also the duration of the loading transient caused by a soft missile is longer. From the structural behaviour point of view the deflection of the target accompanied by concrete cracking and plastification of the rebars may be the dominating damage mechanism compared with punching, penetration or perforation in the case of hard missile impact. 11

12 4.1 Description of soft missile test Another test series of the IMPACT project is dealing with the impact of soft missiles onto simply supported one-way and two-way reinforced concrete slabs. The one-way slabs are simply supported on the two vertical edges while the two remaining edges are free (see Figure 15). The impact conditions are selected such, that a bending vibration without significant punching occurs. A plastic hinge appears in the centreline of the slab that behaves approximately like a beam. In the following a test taken from this series with designation TEST 673 is further considered. The test slab of 2.3 m width, 2 m height and 0.15 m thickness is equipped with bending reinforcement and shear reinforcement in the form of closed shaped stirrups. Among other things the wall deflections during the test were recorded. The setup is shown in Figure 15 and Figure 16. As impactor a thin walled aluminium pipe with a rigid rear made of steel is employed. The total mass is about 50 kg and the impact velocity is 127 m/s. Figure 17 shows the missile before and after the test. From the high speed camera frames shown in Figure 18 the duration of impact may be approximated to be about 12 ms. The missile failure mechanism is a combination of folding and tearing. Figure 15: Photographs of target slab and supporting frame (left) and reinforcement including stirrups (right) in TEST 673. Figure 16: Photograph of the instrumented back face of the target slab (left) and displacement sensor layout (right) in TEST

13 Figure 17: Photographs of missile with a mass of 50.3 kg before (left) and after (right) impact. Figure 18: High speed camera frames of the missile failure 2 ms (a), 6 ms (b), 10 ms (c), and 14 ms (d) after the first contact of target and missile. 4.2 Numerical studies on soft missile test Comparison of analysis models Three dimensional analysis models for different computer codes are employed by IRSN, GRS and VTT to simulate soft impact phenomena. A simplified two degree of freedom model is applied by TUT. In the following the different analysis models for TEST 673 are briefly described. IRSN modelling IRSN has simulated TEST 673 using the fast dynamics finite element calculation code LS-DYNA /1/. The model built for that purpose, whose total number of elements exceeds , includes the concrete slab itself, with solid elements for concrete and truss elements for reinforcement steel. The so-called Winfrith smeared crack (LS-DYNA material model # 84) is used for concrete. The metallic supporting structure, with solid, shell and truss elements, provides realistic boundary conditions of the slab supported on two opposite sides. The aluminium missile is represented with shell elements. This model is refined, because the deformable nature of the missile leads to a more complex interaction with the target than in the case of a rigid missile. The values of the concrete parameters are chosen as standard ones, and are not adjusted to the experimental results. This approach leads to a realistic, industrial type, simulation. Figure 19 shows the model of the slab, the supporting structure and the missile. 13

14 Figure 19: Mesh details for the LS-DYNA model of IRSN for TEST 673. GRS modelling The analysis model developed by GRS shown in Figure 20 was solved with ANSYS AUTODYN /3/. Constituent parts of this are reinforcement elements, concrete parts and a representation of the missile. The supporting frame of the target is not modelled, but rather a slide bearing boundary condition is applied to two opposite slab edges. Fundamental geometrical test parameters like slab dimensions, reinforcement arrangement and missile dimensions are considered in the model. For the concrete part the RHT (Riedel, Hiermaier, Thoma) material model is employed, that is capable to describe the behaviour of concrete subjected to dynamic loading. The RHT input parameters were adjusted to the concrete test data provided by VTT. Figure 20: AUTODYN model of GRS for TEST 673. VTT modelling The finite element model for one quarter of the slab used in TEST 673 is shown in Figure 21. The calculations were carried out with Abaqus/Explicit Version /3/. There are 440 fournoded shell elements (Abaqus element S4R) in the model and the number of nodal points is 14

15 483. The bending reinforcement is modelled as smeared layers. Stirrups are not considered, since they cannot be included in this type of shell model. In order to simulate the effect of impact a load time function (see Figure 28, curve C) for the current missile was derived according to the so called Riera method /10/. The loaded area is determined by assuming a load spreading angle of 45 o in the slab thickness direction to the shell mid surface. The Concrete damaged plasticity material model was employed for the concrete. Material parameters to model compression crushing and tensile cracking were predicted according to available material test data. The measured ultimate compression crushing strength of concrete is about 60 MPa. The measured concrete tensile strength was about 3.6 MPa. The fracture energy in modelling tensile cracking was assumed to be 200 N/m. The yield strength of the reinforcement steel is highly strain rate dependent. The dynamic yield strength was taken into consideration by the Cowper-Symonds formula /11/ for uniaxial tension or compression. Figure 21: Abaqus/Explicit shell element model of VTT for TEST 673. TUT modelling Bending and shear failure of a plate or a shell can be modelled at simplest by a two degree of freedom model (TDOF model), such as the CEB model /12/. This model has been used more recently by e.g. Bartera et al. /13/. In Figure 22 spring 1 and mass 1 are connected to the global bending deformation of the plate and spring 2 and mass 2 are used in modelling the local shear behaviour in the neighbourhood of missile impact area. The behaviour of bending spring is shown in Figure 23 and the local behaviour connected with the possible formation of a shear cone (shear spring) is shown in Figure 24. The internal force in spring 2 is composed of the contributions due to concrete, r c, stirrups, r s, and bending reinforcement, r b. Concrete behaves elastically until the displacement difference u 21 =u 2 -u 1 reaches the value u cu. Stirrups are assumed to break when the difference is u 21 =u su. The ultimate displacement connected to concrete deformation u cu is very small but usually a large displacement difference is needed to activate a significant bending reinforcement contribution to the shear spring force. The bending reinforcement breaks at u 21 =u bu. The load time function f(t) is the same as in the VTT shell element model (see Figure 27). 15

16 The local resistance of the slab to impact load is due to concrete, stirrups and bending reinforcement. The resistive force of concrete alone can be determined by assuming a shear cone of Figure 24 with an angle of inclination of α measured from horizontal plane. Figure 22: A two degree of freedom (TDOF) impact model. Figure 23: Local shear strength of TDOF slab model of reference /12/ showing the contributions of concrete, stirrups and bending reinforcement. Figure 24: Assumed shear punching cone in the TDOF method. 16

17 4.2.2 Selected analysis results IRSN analysis results Two deformed views of the global model and of the deformed missile at time of maximum slab deflection are shown in Figure 25. Due to the complexity of the model several aspects of the impact are reproduced in the simulation, e.g. the shearing-off of the guiding rail. The calculated maximum central deflection is slightly underestimated, but it is still in satisfying agreement with the test result. The concrete slab exhibits a vertical plastic hinge at mid-span, consistent with the test. The post impact vibration frequency is slightly too high (see Figure 30). Figure 25: Mesh deformation during the impact in the IRSN model for TEST 673. GRS analysis results A model representation at 20 ms is shown in Figure 26. Material fracture is not taken into account for the missile material modelling. Therefore, a pronounced folding pattern of the aluminium pipe can be observed. It is apparent from the contour plot of effective plastic strains in the reinforcement that a much localised plastic hinge has formed. The calculated maximum central deflection is slightly overestimated, but it is still in satisfying agreement with the test result (see Figure 30). Relative to the test the post impact vibration frequency is slightly too high, indicating that the degradation of stiffness due to concrete damage may be underestimated. 17

18 Figure 26: Global mesh deformations (left) and plastic strain in rear face reinforcement (right) after 40 ms in the GRS model for TEST 673. VTT analysis results Based on this study it can be concluded that this rather simple shell element model is capable for calculating the deflection behaviour of a reinforced concrete wall loaded by a deformable missile (see Figure 30). Since the wall is damaged by a bending mode, the maximum deflection can be predicted quite well. The frequency of the calculated bending vibration around the permanent deflection is somewhat higher than the one recorded during the test. TUT analysis results All the forces of the TDOF model are shown in Figure 27. In this case the bending mode represented by mass 1 and spring 1 dominates the behaviour of the slab. The external load time function according to Riera is the same as used by VTT and is also shown in Figure 28. The results for the central slab displacement are shown in Figure 30. The TDOF model is an effective tool to predict the maximum slab displacement. 18

19 Figure 27: Forces of TDOF model in analysing test 673. Comparison of selected analysis results It can be concluded, that all models are capable to simulate the basic phenomena of the flexural slab response with a plastic hinge at mid-span. The impact loading in the models of IRSN and GRS is directly simulated by means of contact between missile and target. In Figure 28 the resulting accumulated and smoothed contact forces are compared to the force time function according to the Riera method used in the VTT and TUT analyses. Due to the missile s folding and numerical effects the contact forces exhibit some high-frequency vibration, while the Riera method represents an averaged loading. However, the total momentum transmitted to the target shown in Figure 29, that is the impact force integrated with respect to time, gives quite similar results in all cases. In the area of saturation the Riera curve and the IRSN result are consistent with the initial momentum of the missile of about 6400 Ns, while the transfer of momentum is slightly higher in the GRS model. This is probably due to the straight rebound of the missile with a residual velocity of about 4-5 m/s in this simulation. 19

20 Figure 28: Comparison of forces acting on the target in different analyses. Figure 29: Comparison of momentum transmitted to the target in different analyses. Calculated and measured back side displacements of the slab at the positions of two different displacement transducers are compared in Figure 30 and Figure 31. With the TDOF model of TUT the central slab displacements are calculated. Since the differences relative to the displacements measured at the location of displacement transducer D1 are assumed to be small, the result of this calculation is also shown in Figure 30. It is apparent from both figures, that the maximum deflection can be predicted quite well and that the test result is in 20

21 the range of scattering of the simulation results. Concerning the post impact vibration frequency and damping further differences occur. The most important reasons for that are probably the different features of the used concrete material models in describing the loss of stiffness due to cracking. It should be noted that the measurement of the deflection D1 was not completely successful. The measured deflection curve is flat at the local maximum (see curve A in Figure 30). The higher initial peaks (see Figure 28) in the loading function of the GRS analysis may cause the steeper incline at the beginning of the slab deflection curve C in Figure 30. However, the shape of the loading function does not remarkably change the maximum deflection of the slab, as far as the duration of impact and transmitted momentum are quite similar compared to the other analyses. Figure 30: Comparison of measured and calculated back side target deflection at the position of displacement sensor D1 (see Figure 16). 21

22 Figure 31: Comparison of measured and calculated back side target deflection at the position of displacement sensor D2 (see Figure 16). 5 SUMMARY, CONCLUSIONS AND OUTLOOK The comparative simulations of impact tests performed at VTT presented in this paper are an example of joined efforts made by the TSOs to improve the quality of analysis methods used in their safety assessments. The evaluation of analysis results shows that the used complex analysis methods are suitable to predict the mechanical behaviour of reinforced concrete structures including damage phenomena like bending, punching and penetration. The scattering of the analysis results among the different analysis models is a measure for the accuracy to simulate relevant phenomena. Furthermore, certain simulation results are quite sensitive to specific modelling parameters and numerical control settings. In this context the uncertainty concerning specific input parameters of the different concrete material models has to be mentioned. To investigate this, more sophisticated concrete testing techniques and further numerical studies will be carried out in the second phase of the IMPACT project. As already mentioned, each computer code yields a scatter band of results in the framework of parametric studies, i.e. sensitivity studies are required during the process of model validation. Anyhow, one finding is that good agreements between test results and numerical analyses can be achieved concerning selected specific quantities. Therefore, the uncertainties attached to natural phenomena (e.g. concrete cracking) and to the numerical simulations must be considered when assessing the behaviour of civil structures submitted to impulsive loads. Simplified methods are especially amendable for parametric studies. In addition, it should be mentioned, that the experiments indicate possible scattering of test results in repeated tests. Repeatability of tests is another issue to be examined in the second phase of the IMPACT project. Recently, the working group IAGE (Integrity and Ageing of Components and Structures) of OECD-CSNI launched a multinational benchmark, named IRIS_2010, for Improving 22

23 Robustness Assessment Methodologies for Structures Impacted by Missiles, in which 20 partners participate. In that benchmark, the main interest of each participant lies in the opportunity to compare its modelling tool and methodology with the best international practice on such matters. This paper includes the lessons learned from comparative simulations of two impact tests performed at VTT. The future conclusions of IRIS_2010, together with the proceedings of the IMPACT project, will give an overview about the state of the art and more information on the advisable methodologies for the analysis of the consequences of impacts on civil structures. 6 REFERENCES /1/ LS-DYNA Keyword User s manual, version 970, LSTC, 2003 /2/ Ansys Inc. 2009, ANSYS AUTODYN Explicit Software for Non-Linear Dynamics Version i, and AUTODYN Theory Manual /3/ Abaqus Theory Manual Version 6.7. Abaqus Inc. /4/ Lastunen, A., Hakola, I Impact test facility. Transactions of SMiRT-19, August 2007, Toronto, Canada. /5/ Orbovic, N., Elgohary, M., Lee, N. and Blahoianu, A Tests on reinforced concrete slabs with pre-stessing and with transverse reinforcement under impact loading. Transactions of SMiRT-20, August 2009, Espoo, Finland. /6/ Tuomala, M, Calonius, K, Saarenheimo, A, Välikangas, P, 2009, Numerical studies on pre-stressed impact loaded concrete walls, Transactions of SMiRT-20, August 2009, Espoo, Finland. /7/ Riedel, W. 2009, Numerical assessment for impact strength measurements in concrete materials. International Journal of Impact Engineering. Vol. 36, Issue 2, pages /8/ Li, Q. M., Reid, S. R., Wen, H. M. and Telford, A. R Local impact effects of hard missiles on concrete targets. International Journal of Impact Engineering. Vol. 32. P /9/ Forrestal, M. J., Frew, D. J., Hickerson, J. P. and Rohwer, T. A Penetration of concrete targets with deceleration-time measurements. International Journal of Impact Engineering. Vol. 28. P /10/ Riera, J. D., Riera, J.D., On Stress Analysis of Structures Subjected to Aircraft Impact Forces, Nuclear Engineering and Design, Vol. 8, 1968, pp /11/ Jones, N, Structural Impact, Cambridge University Press, 1989 /12/ CEB Bulletin d Information no Concrete Structures under Impact and Impulsive Loading /13/ Bartera, F., Combescure, D., Jason, L. and Guilbaud, D., Evaluation of RC slab response under soft impact by means of various models. CONSEC 07, Concrete under severe conditions, F. Toutlemonde et al. (eds.), pp , France,