Accidental Loads Research Articles

Enhancement of collapse-resistant capacity of non-seismically designed RC frames using various CFRP strengthening schemes

The existing studies have demonstrated relatively weak robustness of non-seismically designed reinforced concrete (RC) frames against collapse than seismically designed RC frames, causing the demand of efficient strengthening schemes to enhance their collapse-resistant capacity. Therefore, this paper presents an experimental program aiming at strengthening the collapse-resistant capacity of non-seismically designed RC frames using carbon fiber reinforced polymer (CFRP). A total of seven sub-frames were tested, of which the penultimate column or edge column was notionally removed to replicate the initial damage caused by accidental loads. Two sub-frames without strengthening were tested first as reference tests. Similar to existing research outcomes, the referential sub-frames experienced premature rebar fracture at the beam ends near the removed column, resulting in a severe softening in load resistance. Whilst the load resistance could reascend, the fracture of the rebar at the beam ends near the side columns only allowed an insufficient mobilization of catenary action (CA). In addition, the side joint of the referential sub-frame representing a penultimate column removal scenario suffered significant damage. Subsequently, five strengthening schemes were applied to the referential sub-frame, they are designed to increase the compressive arch action (CAA) capacity or CA capacity, or both the CAA and CA capacities through CFRP strengthening. Test results demonstrated that the proposed strengthening schemes in this paper can efficiently increase the load resistance at the CAA and CA stages but failed to mitigate the severe load resistance softening. The strengthening scheme planned to increase the CAA capacity unexpectedly decreased the deformation capacity of the strengthened sub-frame due to premature fracture of beam rebar near the side columns. The strengthening schemes planned to increase the CA capacity or both the CAA and CA capacities could increase not only the CAA capacity but also the CA capacity. The enhanced load resistance at the CA stage was mainly attributed to the continuous CFRP strips attached to the soffits. Unfortunately, the energy method demonstrated that the dynamic load resistance of the tested sub-frames to prevent collapse was achieved at the CAA stage rather than the CA stage because of the severe load resistance softening. Thus, the efforts devoted to increasing the CA capacity by this paper were valid in a real collapse of building scenario. In addition, 113 available test results were collected to compare with the acceptance criteria in existing codes for collapse-resistant design. Based on the test results and comparison, design suggestions were given for the collapse-resistant design.

Challenges to the integrity of calandria, the CANDU PHWR moderator vessel, under serious accident loads and consequences of its failure

This paper summarizes the technical and analytical basis for a more advanced examination of structural integrity of a CANDU PHWR Calandria vessel under certain reactor accident conditions. Of foremost interest is the early Boiling Liquid Expanding Vapour Explosions (BLEVEs) induced challenge to vessel structural integrity following a spontaneous in-core rupture of a fuel channel. A sudden, flashing jet discharge of over 100 kg/s of the 10 MPa, 300 °C saturated primary coolant into its 70 °C subcooled near atmospheric pressure moderator water inventory will cause an explosive, shock wave impact. This results from rapid nucleation and vaporization of the discharged superheated liquid. Consequential pressure pulse, pipe-whip and bundle debris impact will induce damage to adjacent channels and cause widespread failure of fuel channel calandria tube rolled joints. These thin tubes inevitably deform to create serious challenges to vessel integrity. On the other extreme of reactor accidents, is the matter of an in-vessel-retention (IVR) of high temperature molten fuel debris in the reactor vessel after a severe core damage accident, such as one caused by an unmitigated station blackout. This scenario has understandably drawn greater attention after Fukushima and potential IVR in the > 200 mm thick PWR vessels when partly submerged in water is viable.It is highly unlikely that we will see reactor debris melt retention in this 19 to 29 mm thick, stepped shell with a tapestry of welds. This is because partial shell melting, diminished strength beyond 650 °C across 2/3 of the thin shell thickness and severe thermal stresses adds to underlying residual stresses near the welded seams. The result is inevitable catastrophic off-site consequences as energetic interactions upon vessel breach of water with hot debris can cause additional steam & hydrogen explosions, containment failures, and large off-site releases.This paper presents technical arguments to illustrate the problem in accepting faulty assumptions conveniently made in licensing safety assessments to simplistically claim survival of the thin-walled welded structure under these two illustrative accidents. I propose pathways to further investigations. Therefore, the issue of Calandria vessel survival under a wide range of accident conditions must be given a second, thoughtful look. Extending boundaries of a safe operating envelope as pressure tubes age needs to be avoided. We need to be more realistic in our severe accident analysis and consequence assessments. Fukushima taught us that the consequences of continuing to rely on convenient but technically faulty rationale, along with hype of eternal safety, can be very unforgiving.

Study of the thermo-mechanical performances of the EU-DEMO Water-Cooled Lead Lithium Left Outboard Blanket segment

The development of a sound conceptual design for the Water-Cooled Lead Lithium Breeding Blanket (WCLL BB) is pivotal to make a breakthrough towards the selection of the driver blanket concept for the EU-DEMO. To this goal, a research campaign has been launched over the last years at the University of Palermo, in close cooperation with ENEA Brasimone, under the umbrella of EUROfusion. In this frame, the analysis of the thermo-mechanical behaviour of the WCLL Left Outboard Blanket (LOB) segment is being performed. In a first phase, the assessment of the segment's overall structural performances was addressed, allowing the investigation of its global response under the selected loading scenarios. On this basis, the local structural analysis of the central region and of the upper and lower regions presenting geometric discontinuities (namely those regions where the stiffeners numbers changes) is presented in this paper, with the aim of assessing in detail their structural behaviour under the nominal BB operating conditions as well as steady-state accidental loading scenarios. Adopting the sub-modelling technique, the displacement field calculated in previous LOB global structural analysis can be mapped and applied at the boundaries of each local model. Moreover, it is possible to include there some structural details missing in the global analysis, like the Segment Box cooling channels. In this way, it is possible to study the thermo-mechanical behaviour of these regions in detail, assuming at the borders the mechanical action of the rest of the structure. The assessment has been performed in compliance with the RCC-MRx code, adopting the set of criteria on the basis of the nature of the considered loading scenario. The obtained results showed a promising structural behaviour of the segment and highlighted the necessity to revise the attachment system layout, which originates excessive deformation leading to the prediction of high stress.

Experimental and numerical response and failure of laterally impacted carbon/glass fibre-reinforced hybrid composite laminates

The paper presents low-velocity impact tests and finite element simulations of carbon/glass fibre-reinforced hybrid composite laminates struck by a pyramidal frustum impactor in order to examine crushing deformations and failure mechanisms. Three mass ratios between the carbon and glass fibre (1:2, 1:1 and 2:1) are considered to compare the influence of the fibre hybridisation on the impact resistance of these laminates. This investigation is an essential step in the design process when ship structural crashworthiness is considered. The experimental results are presented in terms of the force-displacement responses and permanent deformations. Aspects of particular relevance to the deformation and fracture behaviour of hybrid-composite structures subjected to accidental loads are discussed, especially fibre/matrix debonding and fibre damage. Glass fibre shows better impact resistance than carbon fibre, despite the higher modulus of the latter. A series of simulations performed by the LS-DYNA finite element solver are used to calibrate the material failure criteria based on the experimental data, and the sensitivity of the failure criteria to mesh sizes is investigated.

The latent failure probability: A conceptual basis for robust, reliability-based and risk-based design optimization

The safety of structural systems depends on social, political, financial, organizational, behavioral, and other non-structural factors. These factors are related to use of the structure and its environment, malevolent and other unanticipated accidental loading, construction method and experience of construction crews, gross errors, monitoring to prevent abuse, etc. The above can broadly be classified as non-technical factors, involving significant epistemic uncertainty. In this manuscript, we formalize a procedure to handle non-technical factors and/or epistemic uncertainties in Robust, Reliability-based and Risk-based structural Design Optimization (R3DO). We propose that latent failure probabilities, subjective point-estimates reflecting non-technical factors and/or epistemic uncertainties, be added to the nominal member failure probabilities, calculated from objective aleatory uncertainty (technical factors). Latent failure probabilities are subjectively estimated by the analysis team, based on a holistic risk analysis addressing the technical and non-technical factors above. The relevance and impact of latent failure probabilities in R3DO have already been demonstrated elsewhere; these results are briefly recapped herein. We demonstrate that structural systems should be made redundant, not because of objective aleatory uncertainties in loads and material strengths, but to cope with the large impact of subjective epistemic uncertainties related to non-technical factors.

Dynamic Response of 15 MW Floating Wind Turbine With Non-Redundant and Redundant Mooring Systems Under Extreme and Accidental Conditions

AbstractThis work focuses on examining the dynamic behavior of large floating offshore wind turbine (FOWT) exposed to extreme (wind and wave with 50-year return period) and accidental (mooring line breakage) loadings. The FOWT considered is 15 MW reference turbine supported on semi-submersible platform stationed using catenary moorings. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (three-line) and redundant (six-line) systems were studied and compared. The coupled multi-body dynamic system was solved using Openfast. When simulating the mooring line failure, both the operating and extreme loading conditions were considered. Failure of one mooring was considered at a time. The response of the coupled system due to breakage of the mooring indicates high displacements in surge and sway directions in comparison to the intact system especially for the non-redundant mooring system. Furthermore, change in platform yaw angle and increased tension in the other intact mooring lines were observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large FOWT systems. In addition, insights into using non-redundant and redundant mooring configurations for such large structures with respect to extreme and accidental loadings were also discussed.

Exploratory study of the EU-DEMO Water-Cooled Lithium Lead breeding blanket behaviour in case of loss of cooling capability

Within the framework of the European Roadmap to the realization of fusion energy, a strong international cooperation is ongoing to develop a Breeding Blanket (BB) system for the EU-DEMO reactor. Although it is still to be decided whether the DEMO in-vessel components should perform any safety function, the pursuing of robust blanket concepts able to handle upset and accidental loading conditions has been always seen as good practice in fusion reactor engineering to enhance the inherent plant safety performances. Amongst the several classes of events that might challenge the BB structural integrity, the large Loss of Coolant Accident is one of the most relevant because it usually leads to a fast loss of cooling capability of the structures. Due to the characteristic of the tokamak assembly, the behaviour of each blanket segment during a sudden loss of cooling capability does not depend only upon distinguishing features of the component itself. In fact, the overall transient can be governed by conditions established in surrounding elements, like adjacent blanket segments and vacuum vessel, as well as by the plasma shutdown strategies adopted to protect the reactor.The scope of the activity herein presented is to make a preliminary assessment of the intrinsic capability of EU-DEMO tokamak architecture to cope with the loss of cooling in the Water-Cooled Lithium Lead (WCLL) BB concept. Evaluation of BB thermal field in short and medium term under simplified, yet conservative, assumptions was carried out for four transient scenarios with the aim of investigating the response of the structure in case of: a) fast or soft plasma shutdown, and b) different blanket cooling schemes. Moreover, the WCLL BB thermo-mechanical response in the most critical time steps has been assessed. The obtained results shall help for future decisions on safety systems/action to be implemented to cope with accidents.

Numerical Analysis of the Impact Resistance of Partially Precast Concrete Beams and Prediction of the Displacement Response

A building can be subject to multiple accidental loads during its service life. Partially precast concrete (PC) beams are primary force-bearing members. When subjected to an impact load, their impact resistance considerably affects the overall safety of the structure. In this study, we performed numerical analyses of the dynamic response of PC beams subjected to impact loading. The related parameters, including the element type, material model, contact type, and hourglass control, were discussed in detail. The simulation and test results were compared. The model's validity is verified from four aspects: energy conversion, failure form, impact-force time history curve, and midspan-displacement time history curve. Based on these findings, the effects of the concrete strength, longitudinal reinforcement ratio, and stirrup ratio on the impact resistance of the components were evaluated. Furthermore, the differences in the equivalent plastic strain cloud diagram, impact-force time history curve, and midspan-displacement time history curve under different parameters were compared. The results show that an increase in the strength of the post-pouring concrete can reduce local damage to PC beams, and improving the overall strength of concrete can reduce overall damage of PC beams. In addition, an increase in the longitudinal reinforcement ratio can ease the failure of the normal cross-section of PC beams, and an increase in the stirrup ratio can effectively inhibit the generation and development of oblique cracks. Then, by fitting the parameters, we established simplified formulas for calculating the peak and residual values of the midspan displacement. According to the formulas, when the external impact mass and height are known, the component's damage degree can then be predicted, which provides a basis for the design of fabricated structures.

Experimental Investigation of Wear Effects on the Friction Coefficient of a Curved Surface Slider: Comparison of Small- and Full-Scale Tests

ABSTRACT The current practice to determine friction properties of sliding isolation systems is to perform displacement-controlled dynamic tests where the effects of several loading conditions with different amplitudes, loading velocities, and vertical forces are of concern. The number of cycles in these tests is generally three for most of the loading conditions assumed to be representative of seismic conditions. On the other hand, the isolator may undergo small-amplitude but large-cycles of service motions due to thermal deformations and accidental loads. In that case, the methodology requires to perform cyclic tests on small-scale samples of the slider material (not the isolator) considering very large accumulated sliding distances such as 1000 m for use in buildings, and 10,000 m for use in bridges. However, two unsettled issues are regarded: (i) the suitability of small-scale slider tests to estimate the characteristics of full-scale sliding isolation systems and (ii) the change in seismic performance of a sliding isolation system after it is subjected to a large amount of accumulated sliding distance. In the present study, both a full-scale double concave friction pendulum (DCFP) and small-scale sample composed of the slider of the DCFP are subjected to a total accumulated sliding distance of 1000 m. The corresponding variations in friction properties are comparatively presented, demonstrating that small-scale test results provide reasonable estimates for the friction properties of the tested full-scale DCFP. Moreover, the seismic response of the investigated DCFP remains almost the same regardless of the considered sliding distance.

Numerical and Analytical Investigations of the Impact Resistance of Partially Precast Concrete Beams Strengthened with Bonded Steel Plates

A building may be subjected to a variety of accidental loads during its service life. Partially precast concrete (PC) beams are a primary structural component. Their impact resistance can have a substantial impact on the overall safety of a structure when it is subjected to an impact load. In this study, numerical analyses were performed on the dynamic response of PC beams strengthened with bonded steel plates subjected to impact loading. The model was verified from four aspects: energy conversion, failure form, impact force–time history curve, and midspan displacement–time history curve. The dynamic response eigenvalues of the peak impact force, peak midspan displacement, and residual midspan displacement were compared between the numerical simulations and experimental tests. The relative inaccuracy of the peak impact force ranged from 9.51% to 14.0%, with an average value of 11.9%. The average relative error for the midspan displacement was −0.09%, with the greatest relative errors varying between −0.64% and 0.3%. The residual value errors of the midspan displacement ranged from −0.95% to 2.38%, with an average relative error of 0.94%. On this basis, the effects of the impact mass, impact height, width, and length of the bonded steel plate on the impact resistance of the components were evaluated. Furthermore, the differences in the equivalent plastic strain contours, impact force–time history curves, and midspan displacement–time history curves under different parameters were compared. The results demonstrated that the failure modes and flexural deformations of the test beams were influenced by the impact mass and impact height. The increase in the length and width of the steel plate had no effect on the impact force response, but the peak and residual values of the midspan displacement decreased, which could significantly increase the impact resistance of the beams. Lastly, the impact mass m, the impact height h, the thickness t of the bonded steel plate, the length of the bonded steel plate hs, and the width of the bonded steel plate bs were all taken into account in the fitting formula. These five parameters were used to predict the peak impact force response, the peak value of the midspan displacement, and the residual value of the midspan displacement. The results demonstrated that the fitting formula had small errors and could accurately reflect the dynamic responses of the PC beams strengthened with bonded steel plates under impact loading.

Performance of RC beam-column assemblies during and after elevated temperature to resist progressive collapse

In practice, fire is one of the accidental loads that can result in the progressive collapse of buildings but the commonly used alternate load path (ALP) approach often ignores the impact of fire. Therefore, numerical models based on the finite element software ABAQUS are employed to investigate the performance of reinforced concrete (RC) beam-column assemblies during and after high temperature to resist progressive collapse. The thermal–mechanical sequential coupling method is used for analysis. The effects of fire durations, boundary conditions (middle or penultimate column removal) and side column subject to fire are studied. Previous test results demonstrated that compressive arch action (CAA) and catenary action (CA) are developed in sequence to prevent progressive collapse for the assemblies at ambient temperature. However, the numerical results show that the in-fire assemblies could not develop CA owing to severe degradation of rebar elongation and thus, the load resistance of in-fire assemblies controlled by CAA capacity. Further increasing fire duration to 90 mins, the CAA capacity of the assemblies decreases over 70% compared with that at ambient temperature. Since the rebar can recover most of its mechanical properties after cooling down, the assembly after fire (i.e., exposed to fire then cooled down) can also successively develop flexural action (FA), compressive arch action (CAA) and catenary action (CA) to resist progressive collapse, which is similar to that of intact RC frame at ambient temperature. Under the penultimate column removal scenario, the exterior side column of the post-fire specimen suffers large eccentric compression failure, which causes the highest collapse risk.

Structural assessment of the EU-DEMO water-cooled lead lithium central outboard blanket segment adopting the sub-modelling technique

The development of a sound conceptual design of the Water-Cooled Lead Lithium Breeding Blanket (WCLL BB) is pivotal to make a breakthrough towards the selection of the driver blanket concept for the EU-DEMO. To achieve this goal, an intense research campaign has been performed at the University of Palermo, in cooperation with ENEA Brasimone, under the umbrella of EUROfusion. In this paper, structural analyses of different poloidal regions of the WCLL BB Central Outboard Blanket (COB) segment are reported. In particular, starting from the results of the thermo-mechanical analysis of the whole WCLL BB COB segment, the sub-modelling technique has been applied to the most significant poloidal regions, located at the top, middle and bottom of the segment. The aim is to focus on the stress field locally arising under purposely selected steady-state nominal and accidental loading scenarios. The nominal BB operating conditions, as well as steady-state scenarios derived from both the in-box LOCA and Vertical Plasma Disruption accidents have been considered. Thanks to the sub-modelling approach, the deformative action of the entire segment can be imposed at the boundaries of each local model to realistically assess its structural performances. Moreover, each local model reproduces structural details not included in the global one, such as the Segment Box (SB) cooling channels. Then, the structural behaviour of the selected regions has been assessed in compliance with the RCC-MRx code. The obtained results highlighted that the structural behaviour predicted by the whole segment analysis is similar to that predicted by sub-modelling calculations within the Stiffening Plates, whereas the application of the sub-modelling is a must to investigate in detail the SB structural performances. In addition, results indicate that the BB attachments should be revised, as they contribute to produce the WCLL COB large deformation originating excessive stresses, mainly within the equatorial region.

On the Resistance of Cruciform Structures during Ship Collision and Grounding

This paper presents an experimental, numerical, and analytical study of a novel specimen subjected to local in-plane load, to investigate its crushing deformation and resistance. The specimen was designed and fabricated to simplify the cruciform structure in double-hulled vessels subjected to external loads during collision and grounding incidents. The study results will provide reliable insights into grounding scenarios as well as side collision scenarios of double-hulled vessels. A quasi-static indentation test and related numerical research showed good agreement regarding deformation mode and force–displacement response. On the basis of the experimental and numerical results, an analytical method is proposed to derive the deformation energy, the instantaneous resistance, and the mean resistance of the deformed structure. The analytical method was verified with recorded test data and a nonlinear finite element analysis. It enables a rapid assessment of the response of the structure under accidental loads, which is a guideline for the design of crashworthy hull structures and the assessment of their crashworthiness.

충격량에 대한 응답을 고려한 폭발강화격벽 설계 방법 연구

Blast Hardened Bulkhead (BHB) is an important measure that can increase the ship’s survivability as well as protect the lives of the crew by mitigating the damage extent caused by an internal explosion in the ship. In particular, both the pressure and the shock impulse should be considered when designing the BHB against reflected shock waves having a high pressure with a short duration. This study proposes a design method for BHB that considers both the pressure and the shock impulse generated during the internal explosion. In addition, analysis and design concepts for accident loads such as explosion, fire, and collision of NORSOK and DNVGL, one of the international design guidelines for the curtain plate type blast hardened bulkhead type applied by the Korean Navy, are utilized. If this method is applied, it is expected that it can be used as a design concept for the pressure as well as the shock impulse of the explosion load of the curtain plate.

Global Structural Behavior and Leg Strength for Jack-Up Rigs with Varying Environmental Parameters

In the mobile jack-up unit, the leg supporting the hull is a very important structure, and it is important to closely examine the changes in accident load, environmental load, and seabed ground during jack-up operation. Generally, jack-up rigs are three-legged structures with a triangular hull that comprises several movable legs used to raise the hull above the sea surface. They can be operated in shallow water at less than 120 m, while large jack-up rigs, which have a structure that can withstand severe environmental loads, can be employed at depths ranging from 150 m to 200 m. However, a complex process is required to finalize the structural design of a jack-up rig, and the influence of various parameters must be comprehensively considered. In other words, the rig will encounter variable environmental conditions with variations in parameters such as wave height, wave period, wind speed, air gap, and so on. A unified procedure is proposed to review the structural strength of legs, hulls, and cantilevers, and different models and analyses can be configured so that it can be solved within a unit flow-chart. Through this process, we can expect that engineering time and cost can be reduced. From survey results, it was possible to determine the inputs to examine the effects of variables, and a large jack-up rig operating under extreme environmental conditions was modeled. In the present study, the jack-up rig was operating in the North Sea, and leg length and water depth were 160 m and 100 m, respectively. The basic environmental characteristics included wave height (20 m), wave period (10 s), wind speed (30 m/s), and air gap (22 m). A parametric sensitivity analysis was performed with varying environmental parameters. Through sensitivity analysis of environmental characteristics, the significance and sensitivity of the effect of each environmental parameter on leg strength was clarified. It is expected that this will be very useful guidance about the effect of parameters during the conceptual design stage of jack-up rigs.

INNOVATIVE COMPOSITE STRUCTURAL SYSTEMS FOR MODULAR TALL BUILDINGS

AbstractModular or offsite construction is believed to shape the future of the construction industry as it possesses significant benefits over traditional onsite construction methods. However, most of its application are limited to steel or concrete buildings. Although steel‐concrete composite structural system has many merits over the steel and concrete systems, its application in modular buildings is very limited. This paper explores recent developments of composite systems for modular high‐rise buildings. They include modular units for resisting vertical gravity loads and lateral structural systems for resisting horizontal forces from wind and earthquake loadings and progressive collapse due to accidental loads such as fire, explosions and impact. Various inter‐module joining methods developed in the literature will also be reviewed. Finally, a case study of the most efficient connection is presented to explore its applicability to high‐rise modular buildings.

Impact resistance-compressive strength relationship of steel fiber-reinforced concrete at ambient and high temperatures

Despite the great scientific developments in fire resistance technologies and construction materials industry, fires threaten human lives and negatively impact the economy every day. One of the possible scenarios when a structure is subjected to accidental fire is the fragmenting of structural elements or falling of heavy objects from higher stories on fired concrete elements, which imposes a dual accidental loading. The aim of this research is to spotlight the relationship between the compressive strength and impact energy of concrete subjected to the dual action of high temperatures and repeated impacts. Cube and disk specimens from three mixtures with 0 %, 0.5 %, and 1.0 % of steel fibers were cast to investigate the compressive strength and repeated impact strength, respectively. Groups of specimens from each mixture were subjected to temperatures of 200 °C, 400 °C, and 600 °C in addition to the reference unheated specimens. The results showed that compressive strength and impact energy behave differently under the effect of high temperatures. However, regardless of the exposure temperature, the impact energy increases with the increase of compressive strength. This increase is more evident for failure impact energy compared to cracking impact energy. Simple linear and second order formulas were found suitable to describe these relations with R2values ranging from 0.71 to 0.96.

Numerical modeling for assessing progressive collapse risk of RC buildings exposed to blast loads

The safety of strategic structures against accidental loading (such as blast-generated waves) has recently received much attention from practicing structural engineers. Terrorist attacks in the proximity of urban environments may bring blast threats close to the perimeter of the buildings, thereby making them exposed to progressive collapse. This paper aims at investigating the progressive collapse possibility of an existing RC (reinforced concrete) building against blast-generated waves. Data was collected for a typical RC building constructed in a congested area in Riyadh city. A simple NLD (nonlinear dynamic) analysis approach was carried out to examine the progressive collapse possibility of the building when exposed to blast threat events. In this regard, 3D nonlinear two-stage FEA (finite element analysis) was conducted. In the first stage, a local model was created for a typical individual RC column. The local model was validated utilizing the published results of an RC column subjected to blast load. The goal of the local analysis is to evaluate the individual column response against blast load and to come up with the critical stand-off distance at which the column gets severely damaged. In the second stage, a global model (comprising shell and beam elements) was created for the whole building. The goal of the global analysis is to examine the global behavior of the building owing to the removal of the columns identified from the local model stage. The progress of damage caused to different structural elements and the state of stress in these elements was studied. The results enhanced the knowledge about the progressive collapse potential in RC buildings and will be helpful in the development of mitigation strategies.

МОДЕЛИРОВАНИЕ УДАРНЫХ ВОЗДЕЙСТВИЙ НА СТАЛЬНЫЕ РАМЫ ЗДАНИЙ ПРИ ДЕФОРМИРУЕМОМ ОСНОВАНИИ

The article focuses on the currently relevant problem of assessing the safety of steel frame systems on a deformable soil base under accidental impacts. The article considers a case when impact actions can be random, i.e., such parameters as application point, direction and intensity of impact are not determined preliminarily. It is supposed that for the existing design solution the occurrence of such impacts should not lead to progressive collapse, and the structure as a whole should have the property of robustness. In order to estimate this property, it is suggested to carry out a number of calculations for the stress-strain state under the most dangerous random loads and then to carry out a verification analysis in the dynamic statement. For the steel frame, a search problem is solved according to the criterion of minimization of integral safety margin of structural elements. The calculations account for the possibility to prevent the frame buckling. The quasi-static analysis is performed on the basis of the models of the deformation theory of plasticity, and the dynamic analysis is done with regard to the associated law of steel yielding. The proposed procedures allow designing steel frame structures which are resistant to random accidental loads. A shock load is represented in the form of a force impulse which is statically equivalent to the dynamic effect of an inelastic impact of a rigid body on a structural system. An example of design and performance evaluation of a steel frame structure of a two-story building is considered.

Experimental and Numerical Analysis of Impact Strength of Concrete Slabs

The topic of this paper is experimental and numerical analysis of the impact strength of unreinforced concrete slabs. Impact strength of concrete is significant in case of some accidental loads during exploitation. Impact strength can be determined experimentally, using Drop-weight test, Charpy test, Projectile impact test, Explosive test, etc. In this research, a numerical model for determining the impact strength of concrete slabs based on Finite element method (FEM) and high-end engineering software has been proposed. Modelling approach to this problem was applying the Explicit dynamics FEM analysis. Thereat, two different existing material models for concrete were enforced: Concrete damage plasticity model – CDP (implemented in ABAQUS/Explicit software), and the Riedel-Hiermaier-Thoma model – RHT (implemented in ANSYS Workbench software). Analysis parameters for both material models, necessary as input data, have been determined through a series of FEM analyses and validated by performed experiments, using drop-weight test. Results of the numerical analyses have been compared with the experimental ones, as well as mutually. Advantages and drawbacks of both material models are highlighted, as well as the reliability of the proposed numerical models. The proposed numerical FE models, confirmed by experiments, can be successfully used for determining impact strength of concrete slabs in further research.