Computer Simulation for Building Implosion Using LS ...

Computer Simulation for Building Implosion Using LS-DYNA Georgios Michaloudis, Steffen Mattern, and Karl Schweizerhof Institute of Mechanics, Otto-Ammann-Platz 9, Geb. 10.30, 76131 Karlsruhe, Germany, [email protected], Steff[email protected], [email protected]

1 Introduction The destruction of buildings by placing explosive material at strategic positions is particularly efficient for very high and large buildings. Controlled demolitions have been subject of the research done by the research group DFG Forschergruppe 500 “Computergestuetzte Destruktion komplexer Tragwerke durch Sprengung” [1] which is supported by the DFG. This contribution describes the continuation of simulations of building implosions with finite elements. These simulations are only part of the general project and serve in principle as verification for simplified models [2].

2 Aspects of Numerical Simulation The finite element program LS-DYNA [5, 6] is used for the generation of the models and the simulations which must consider all the significant phenomena and uncertainties [7–9], though for the latter some series of analysis are needed. In order to apply the so-called Node Split algorithm for modelling local failure a modification of the finite element mesh is necessary. To avoid performing these modifications all by “hand” since no preprocessor supports such a feature, an automatic algorithm has been programmed which at first creates coinciding nodes, so that every element consists of eight unique nodes. Within the algorithm then the element connectivity can be given up depending on some conditions. In the following simulations contact is then checked between the new created surfaces. Some additional modifications are necessary, e.g. applying the proper boundary conditions for the splitted model. The resulting meshes need to be fairly fine to achieve a reasonably good approximation of the geometry of the failure surfaces. All computations were made in parallel on a HP X6000-Cluster of the Karlsruhe Institute of Technology.

W.E. Nagel et al. (eds.), High Performance Computing in Science and Engineering ’10, DOI 10.1007/978-3-642-15748-6 37, © Springer-Verlag Berlin Heidelberg 2011

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3 Simulation Models 3.1 Previous Models Within the research unit FOR500 five reference models are studied with varying parameters. The first model is a simple academic fictional model, which was created in order to test the basic numerical algorithms before applying them to the large, time-consuming models. Both algorithms for the modelling of failure, element-erosion and node-split, were applied in this model [2]. The second model is a storehouse in Thueringen, demolished in 1998, which has been computed previously with the element-erosion-algorithm [10] and is now computed in this contribution with the node-split-algorithm. The third model is a silo-building in Borna, for which up to now only the element-erosionalgorithm has been used [11] with satisfactory results for the entire process. These three first models represent typical cases of blasting strategies which lead to inclination and finally to the collapse of the structure. The fourth reference model is a fictional model which was created in order to test the simulation of the demolition of the fifth large reference model, the Sparkasse building in Hagen. This model incorporated some basic characteristics of the Sparkasse building but was much smaller in size, not having more than 150.000 elements. A similar demolition strategy was applied and the basic algorithms were first tested in this analysis, since applying them directly to the large model would be too time consuming. After certain modifications and when the fictional building was computed [12] with satisfactory results all algorithms were used in the simulation of the large model. Both models (fourth reference model and the building in Hagen) represent vertical collapse scenarios, in which structural failure results mainly from strong compressive forces and shear. The experience gained from these examples showed firstly that the elementerosion-algorithm in the case of non vertical collapse scenarios, in which failure occurs mostly due to strong bending of elements and not due to high pressures, can provide reliable results, at least for the initial collapse kinematics. On the other hand, the erosion scheme should not be applied in the case of a vertical collapse, since as a result of removing elements it does not deliver reliable results. The node-split-algorithm, however, is more time consuming and imposes several changes in the discretization, but is a better alternative which delivers realistic results even in the case of vertical collapse with high pressures and material failure due to strong compression. 3.2 Sparkasse Building in Hagen The demolition of the Sparkasse building in Hagen was one of the most complex blasting processes which ever took place in Germany [13]. The size and especially the height of the building in combination with the limited space available for the final debris demand a sophisticated blasting strategy which

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Fig. 1. Sparkasse building in Hagen—Computational model and blasting strategy

results in triaxial collapse kinematics. The structural system of the building consists of frames and shear walls; it is 93 m high (20 floors) and its surface has dimensions 37,20 m × 18,60 m. The blasting strategy, which consists of three phases, is shown in Fig. 1. The finite element model consists of 392481 8-node hexahedral solid elements. At first, the CAD geometry was created from the building drawings. Then, using the preprocessor Hypermesh [14], the finite element mesh was constructed and imported into LS-DYNA. At this final stage, all the numerical algorithms were applied, such as material model, element formulation etc. In the computations to be presented a rather simple, piecewise linear, plasticity model is used (LS-DYNA MAT24) for efficiency reasons. However, some limitations have to be mentioned resulting from the fairly simple failure criterion. In addition to that, it must be noted that also in the case when Node-Split is applied, a rather high resolution of the model is necessary in order to capture sufficiently the entire cracking process. Furthermore, by using continua for the modelling of reinforced concrete the effect of the steel bars is “smeared” in the continuum. E.g. the presence of steel bars, after the mechanical failure of the concrete, would in reality prevent structural parts from flying away from the structure. However, in the simulations when the continua fail, a complete dissolution is obtained. This results in general in an overly brittle behaviour and some elements are flying away from the structure, In the simulations we try to damp this motion which is not a perfect solution. The ground plate is simulated as a rigid body and the building collapses under the effect of gravity. The results shown in the Fig. 2 are from a first analysis in which the element erosion algorithm was applied. As expected modelling material failure with erosion of elements leads to the removal of almost the entire structure and, due to that, the final results do not-by far-have the required accuracy and reliability.

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Fig. 2. Sparkasse in Hagen—Evolution of collapse kinematics, element erosion as failure mechanism

Fig. 3. Sparkasse in Hagen—Evolution of collapse kinematics, Node-Split as failure mechanism vs Real demolition, t = 0.0, 2.5, 3,5, 5.0, 7.0, 7.9 seconds

In Figs. 3 and 4 the results of the simulation when applying the NodeSplit algorithm are presented and compared with photos taken during the real demolition. When the Node Split algorithm is used, each element consists of eight nodes which do not belong to any other element. Nodes with the same positions get merged by certain constraints to enable separating them easily if the failure criteria is reached. The chosen failure criteria checks for the volume weighted plastic strain in adjacent elements. If the given limit for

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Fig. 4. Sparkasse in Hagen—Evolution of collapse kinematics, side view, t = 0.0, 2.5, 3.5, 5.0, 7.0, 7.9 seconds

the plastic strain is reached not all connections of the element are broken, but only the nodes of neighboring elements, which carry this plastic strain, are disconnected, while the other tied connections of the element remain unchanged. The first three figures show the beginning of each of the three phases of the demolition. As expected, this approach provides a much more realistic simulation of the entire process, compared to the results previously shown. All contact conditions are captured and well described since no structural parts are removed due to failure. This allows a good prediction of the collapse kinematics throughout the demolition as well as a reliable prediction of the final rest position at the end of the collapse. It is shown that for a vertical collapse scenario results are achieved which are by far closer to reality than with the erosion algorithm. The computation was performed on the HP XC6000-Cluster of the KIT using the MPP-Parallel version of LS-DYNA on 4 processors. The computational time for this problem of approximately 9.500.000 unknowns, 663807 extra constraint conditions (for the modelling of failure), computed time of 7.9 seconds with timestep size of approximately 6·10−6 seconds (1.310.000 time steps) reaches 43000 minutes. Although the time step size does not change dramatically throughout the analysis the necessary numerical effort is increasing as the problem evolves. This is due to the large deformations and to the fact that more and more elements are splitted due to the failure algorithm. Then e.g. in the last phase of the analysis (computed time 7.0 seconds until 7.9 seconds) the computation for 0.1 seconds of the problem takes almost 15 hours. For the first five seconds of the analysis in order to compute 0.1 seconds the process did not need more than 8 hours. This occurs mainly because of the extreme increase in the number of the contact surfaces due to the splitting of elements. New contact interfaces are created and included in the search contact algorithm which is very time consuming and as a consequence slows down the entire computation. In order to remove the major part of the artificial energy which was added to the model during the application of the initial

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Fig. 5. Sparkasse in Hagen—Validation of results by video overlapping, t = 1.0, 3.0, 5.0, 6.0, 7.0, 7.5 seconds

gravitation with explicit time integration, the system had to be damped for 0.1 second, otherwise large vibrations were leading to error terminations. It proved to be particularly important to avoid the “inversion” of solid elements leading to negative volumes and finally to error terminations, as a result of large element deformations. In order to handle this problem additional contact checks were added which consider contacts between interior surfaces of the hexahedral elements to avoid an inversion. Nevertheless, in the results the limitation in the applicability of the Node Split algorithm is observed from the very strong dissolution of element connections due to material failure, which creates for some parts unrealistic phenomena (structural parts flying e.t.c). The latter would not be possible with reinforcing steel which mostly prevents a complete dissolution of parts. The validation of the results of the finite element simulations is an essential issue because it defines the accuracy of the performed computations. In Fig. 5 the outcome of the validation procedure is presented, in which the video of the simulation is overlapped with the video of the real demolition. It is shown that the results are not only realistic but they also describe sufficiently the entire collapse kinematics. Although there is a small delay at the beginning of the simulation, soon this disappears. Furthermore, even though some elements after the dissolution with their neighboring elements, seem to fly in a distance from the main structure, the final rest position is predicted with an acceptable accuracy and the resulting debris on the ground appears in the simulation as well. 3.3 Storehouse in Thueringen The second example is a seven storey storehouse in Thueringen with a height of 22 m and surface dimensions 22 m × 12 m. It is the second reference model of the research unit FOR500. Two explosions initiate the collapse, as shown

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Fig. 6. Storehouse in Weida—Blasting strategy

Fig. 7. Storehouse in Weida—Initial collapse kinematics, Node Split strategy (top) vs Element Erosion (bottom), at t = 4.6, 5.0, 5.6 seconds

in Fig. 6. The numerical model consists of 82867 hexahedral elements and the node-split-algorithm is chosen in order to model material failure. In Fig. 7 the initial collapse kinematics are presented, which have good correlation with the results of a first analysis in which the element-erosionalgorithm was applied. The node split strategy seems to have a physically better result. 18 hours on 16 processors were necessary for this computation. Simulations with parameter variations are still running for this model. Computing several buildings and different collapse scenarios aims at gaining

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important informations about the whole process and providing these informations to the engineering offices. For each model a large number of simulations is performed in order to define the influence of some algorithmic values as well as process parameters (value of failure criterion, hourglass stabilization, blasting strategy etc) in the final result. This is necessary for a reliable judgement of the results. Testing all these aspects provides the possibility to define general limitations of the algorithms as well as to find the suitable tools for each different case, e.g. that the element-erosion-algorithm has strong limitations and is not suitable for vertical collapse scenarios. Then robust computations can be performed that solve problems which the engineering consultants face very often, e.g. testing different blasting strategies, rearranging the time intervals between explosions etc. Particularly, in the case of vertical collapse scenarios aspects such as the location of placing the explosives or the time difference between the explosions become very significant. In the case of the fourth and fifth reference models the blasting strategy imposes kinematics which combine first inclination of the upper part of the building and then vertical collapse. Delaying the second explosion even for one second can lead to complete different dynamical behaviour and even to a failed demolition. This entire process involves several problems which must be solved. The finite element meshes must be fairly fine, mostly for the modelling of structural failure. Using structural elements (e.g. beams) instead of continuum elements would be insufficient. Due to that, the computations become very time consuming and another difficulty is imposed from the fact that all the preprocessors do not support mesh refinement with meshes for the node-split-algorithm. Hence, it was important to find a reasonable balance between accuracy and computational effort by creating meshes which consist of the minimum number of necessary elements. Even when the mesh is ready, the computation of the model is not straight forward, since a lot of details must be defined in order to ensure a robust simulation. Appropriate contact algorithms, hourglass parameters [15], values for material failure, are some of these details which influence strongly the final result. In the Node Split scheme the value of failure denotes the plastic strain for which elements are disconnected, but this does not necessarily mean that the elements have mechanically failed. In order to define a realistic value for this failure criterion several computations have been performed which were then validated with the video of the real demolition. Experience is gained through the large number of computations and several numerical algorithms have been improved or substituted by others, more efficient ones. For example, the first general contact algorithm is substituted from three more specialized algorithms. One for describing the contact between structural parts and the ground plate (CONTACT AUTOMATIC SURFACE TO SURFACE), one for the contact just between structural parts (CONTACT ERODING SINGLE SURFACE) and an additional check for preventing solid elements to get inverted (CONTACT INTERIOR), which otherwise would be avoided with finer meshes. Similarly, the parameters which control the very

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efficient hourglass stabilization method have been updated since the previous values proved appropriate for the general contact algorithm and the elementerosion-technique, but caused instabilities when the new contact schemes were used and mostly in combination with the huge number of constraints imposed from the application of the node-split-algorithm. This is because of the fact that the elements are not removed and undertake very large deformations due to compression. Thus, in order to apply the Belytschko-Bindemann hourglass control form, the hourglass coefficient was reduced by a factor of 10, followed by appropriate changes in other coefficients as well. Other hourglass control types were tested as well (e.g. Flanagan-Belytschko), which in order to deliver reliable results imposed the reduction of the timestep size by a factor of 2.5. As a conclusion, the Belytschko-Bindemann hourglass control was taken, as it proved more efficient and more effective.

References 1. DFG Forschergruppe FOR500 www.sprengen.net 2006. 2. Hartmann D, Breidt M, Nguyen vV, Stangenberg F, H¨ ohler S, Schweizerhof K, Mattern S, Blankenhorn G, Michaloudis G, M¨ oller B, Liebscher M (2008) On a fundamental concept of structural collapse simulation taking into account uncertainty phenomena. NATO Advanced Research Workshop 983112, Sarajevo, Bosnia and Herzegovina. 3. Mattern S, Blankenhorn G, Breidt M, Nguyen vV, H¨ ohler S, Schweizerhof K, Hartmann D (2006) Comparison of building collapse analysis simulation from finite element and rigid body models. In: Eberhard P (Ed.), IUTAM Symposium on multiscale problems in multibody system contacts, IUTAM Bookseries, Springer, Berlin. 4. Mattern S, Blankenhorn G, Schweizerhof K (2006) Numerical analysis of building collapse—Case scenarios and validation. Proceedings of the NATO Advanced Research Workshop, PST.ARW981641, ed.: A. Ibrahimbegovic and I. Kozar. 5. Hallquist JO (2006) LS-DYNA Theory Manual. Livermore Software Technology Corporation. 6. Hallquist JO (2006) LS-DYNA Keyword User’s Manual. Livermore Software Technology Corporation. 7. M¨ oller B, Beer M (2004) Fuzzy Randomness—Uncertainty in civil engineering and computational mechanics, Springer, Berlin. 8. Hartmann D, Breidt M, Nguyen vV, Stangenberg F, H¨ ohler S, Schweizerhof K, Mattern S, Blankenhorn G, M¨ oller B, Liebscher M (2008) Structural collapse simulation under consideration of uncertainty—Fundamental concept and results. Computers and Structures 86, 2064–2078. 9. M¨ oller B, Liebscher M, Schweizerhof K, Mattern S, Blankenhorn G (2008) Structural Collapse under Consideration of Uncertainty—Improvement of Numerical Efficiency. Computer and Structures 86, 1875–1884. 10. Mattern S, Blankenhorn G, Schweizerhof K (2007) Numerical simulation of controlled building collapse with finite elements and rigid bodies—Case studies and validation. ECCOMAS Thematic Conference in Structural Dynamics and Earthquake Engineering, Rethymno, Crete, Greece.

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11. Blankenhorn G, Mattern S, Schweizerhof K (2007) Controlled building collapseAnalysis and validation. Proceedings LS-DYNA Anwenderforum, Frankenthal. 12. Michaloudis G, Blankenhorn G, Mattern S, Schweizerhof K (2009) Modelling structural failure with finite element analysis of controlled demolition of buildings by explosives using LS-DYNA. High Performance Computing in Science and Engineering ’09, In: W.E. Nagel, D.B. Kr¨ oner, M.M. Resch (Eds.), pp. 539–551, Springer, Berlin. 13. Hartmann D, et al. (2010) Computersimulation f¨ ur Bauwerkssprengungen—ein Einblick in laufende Arbeiten. Sprenginfo 32, Nr 01, 27–35. 14. HyperMesh Users Manual, Version 8.0. 15. Belytschko T, Bindemann LP (1993) Assumed strain stabilization of the eight node hexahedral element. Computer Methods in Applied Mechanical Engineering 105, 225–260.