Content uploaded by Ioan Marginean
Author content
All content in this area was uploaded by Ioan Marginean on Nov 03, 2020
Content may be subject to copyright.
A preview of the PDF is not available
Robustness of moment steel frames under column loss scenarios
Abstract
Buildings, like other components of the built infrastructure, should be designed and constructed to resist all actions that may occur during the service life. When the actions are caused by extreme hazards, such as explosion or impact, the structural integrity should be also maintained by avoiding or limiting the damage. Depending on the type of structural system and class of importance, specific requirements should be met in order to ensure structural integrity. In the case of framed buildings, one such requirement is that after the notional removal of each supporting column (and each beam supporting a column), the building remains stable and any local damage does not exceed a certain acceptable limit. This requirement can be achieved by several means, but a combination of strength, ductility and continuity of the structural system is likely to provide a high level of protection and safety against extreme hazards. Steel frames are widely used for multi-storey buildings, offering the strength, stiffness, and ductility that are required to resist the effects of gravity, wind, or seismic loads. Considered to produce robust structures, the seismic design philosophy has been seen as appropriate for controlling the collapse of structures also subjected to other types of extreme hazards. However, there are specific issues that should be taken into account in order to forestall the localized failures, particularly of columns.
The thesis focuses on the evaluation of the structural response of steel frame buildings following extreme actions that are prone to induce local damages in members or their connections. Extensive experimental and numerical studies were used in order to identify the critical points and to find the structural issues that are
required for containing the damage and preventing collapse propagation. Four types of beam-to-column joints, which cover most of the joints used in current practice, have been investigated experimentally, and the data was used in order to validate advanced numerical models. The findings indicated that catenary action substantially improves the capacity of moment resisting frames to resist column loss, but increases the vulnerability of the connection due to the high level of axial force. The results showed that bolted connections could fail without allowing for load redistribution if not designed for these special loading conditions. The composite action of the slab increases stiffness, yield capacity, and ultimate force but decreases ductility.
Parametric studies were performed so as to improve the ultimate capacity of joints and, implicitly, the global performance of steel frame building structures in the event of accidental loss of a column, without affecting the seismic performance and design concepts. Based on calibrated numerical models, an analysis procedure was developed for evaluating the performance of full-scale structures to different column loss scenarios considering dynamic effects and realistic loading patterns. Moreover, a design procedure was proposed for verification of the capacity of beam-to-column connections to resist progressive collapse, including design recommendations for each connection configuration.
... Main objectives of experimental program were to investigate capacity of bolted T-stub connections under large deformation demands according to Eurocode propositions and to predict response of T-stub components. These tests are already evaluated and explained in (Anwar, 2017) for end plate thicknesses of 10 mm and 12 mm and in (Marginean, 2017) for end plate thicknesses 15 mm and 18 mm, with increasing the bolt distance for each thickness, using 100, 120 and 140mm. General configuration and dimensions of T-stub are shown in the Figure 4. ...
... Increasing distance from 100 mm to 120 mm deformation capacity increases but with reduction of resistance and from 120 mm to 140 mm only the increase in deformation capacity appears without reduction in total resistance. However, the failure is ultimately attained due to the fracture of the bolts in all cases (Marginean, 2017). In the figure suffix C is for cold (room) temperature conditions with strain rate 0.5 mm/seconds replicating quasi-static conditions; while suffix Cs is for cold (room) temperature with high strain rate of 10 mm/seconds. ...
... This master thesis work focuses on examination and evaluation of macro components of T-Stub elements in bolted beam to column connections, their ultimate capacity and ductility under large deformation demands. Existing experimental studies were used to identify ultimate strength and deformation capacity and the data was used to validate numerical models and employed in a parametric numerical study on two main parameters, distance between the bolts and end-plate thickness (Marginean, 2017). The main objectives were the studying of the post yielding behavior for different failure modes, as well as investigation of capacity, ductility and stiffness of T-stub macro components. ...
These days, design process of aerospace and aeronautic industries depends heavily on Computational Fluid Dynamics (CFD) analysis techniques, while it becomes more dominant each year. From CFD point of view, current status of computers is acceptable for solution of most of steady problems. However, when it comes to unsteady calculations, it is not sufficient most of the times for industrial purposes. This issue is more prominent for turbomachinery and aeroelasticity studies where required
calculation effort is extremely high due to simultaneous
contribution of several physical events that are entirely in 3D pattern.
One way to overcome this problem is to use more suitable mathematical methods that fits the problem and requires less calculation. One of the recent methods for unsteady turbomachinery analysis is to use frequency domain instead of the time domain to improve equation by eliminating time-dependent
parameters. In such methods, equations in frequency domain can be solved similar to steady/ average counterpart to be utilized to obtain unsteady solutions, which are periodic in time. In this study, unsteady inviscid flows inside turbomachines are investigated using three different methods in frequency domain; linearized harmonic, non-linear harmonic and harmonic balance. Harmonic methods are more suitable for considering blade flutter and forced response impacts on of unsteady flow behaviour between
passages. In addition, multi-row analyses are conducted to encounter the interaction of stator and rotor on each other due to relative motion of rotor. The methods are applied for test cases evolving flutter and forced response disturbances and then extended to multi-row configuration in which both distortions
coexist. Finally, results are compared to available experimental or computational results in literature.
... Main objectives of experimental program were to investigate capacity of bolted T-stub connections under large deformation demands according to Eurocode propositions and to predict response of T-stub components. These tests are already evaluated and explained in (Anwar, 2017) for end plate thicknesses of 10 mm and 12 mm and in (Marginean, 2017) for end plate thicknesses 15 mm and 18 mm, with increasing the bolt distance for each thickness, using 100, 120 and 140mm. General configuration and dimensions of T-stub are shown in the Figure 4. ...
... Increasing distance from 100 mm to 120 mm deformation capacity increases but with reduction of resistance and from 120 mm to 140 mm only the increase in deformation capacity appears without reduction in total resistance. However, the failure is ultimately attained due to the fracture of the bolts in all cases (Marginean, 2017). In the figure suffix C is for cold (room) temperature conditions with strain rate 0.5 mm/seconds replicating quasi-static conditions; while suffix Cs is for cold (room) temperature with high strain rate of 10 mm/seconds. ...
... This master thesis work focuses on examination and evaluation of macro components of T-Stub elements in bolted beam to column connections, their ultimate capacity and ductility under large deformation demands. Existing experimental studies were used to identify ultimate strength and deformation capacity and the data was used to validate numerical models and employed in a parametric numerical study on two main parameters, distance between the bolts and end-plate thickness (Marginean, 2017). The main objectives were the studying of the post yielding behavior for different failure modes, as well as investigation of capacity, ductility and stiffness of T-stub macro components. ...
The design of engineering structures takes place today and in the past on the basis of static calculations. The consideration of uncertainties in the model quality becomes more and more important with the development of new construction methods and design requirements. In addition to the traditional forced-based approaches, experiences and observations about the deformation behavior of components and the overall structure under different exposure conditions allow the introduction of novel detection and evaluation criteria. The proceedings at hand are the result from the Bauhaus Summer School Course: Forecast Engineering held at the Bauhaus-Universität Weimar, 2017. It summarizes the results of the conducted project work, provides the abstracts of the contributions by the participants, as well as impressions from the accompanying programme and organized cultural activities. The special character of this course is in the combination of basic disciplines of structural engineering with applied research projects in the areas of steel and reinforced concrete structures, earthquake and wind engineering as well as informatics and linking them to mathematical methods and modern tools of visualization. Its innovative character results from the ambitious engineering tasks and advanced modeling demands.
... Studies [3]- [5] have shown that fulfilling the standard tying requirements does not necessarily guarantee the required collapse resistance to survive such accidental scenarios as the loss of a column. Therefore, emphasis has shifted towards threatindependent approaches that aim at enhancing the structural robustness based on the concept of "alternative load paths" [6]- [9]. ...
Steel frames subjected to a column removal typically exhibit a complex load resisting mechanism characterised by three contributions: i) beam yielding mechanism, ii) compressive arch, and iii) catenary action. The development of compressive arch actions in the beams bridging over the lost column has not been yet thoroughly investigated in steel frames, although such effects proved to significantly contribute to the collapse‐resistance of reinforced concrete frames.
This paper aims at shedding light on the compressive arch actions of steel beams in case of column removals accounting for various boundary conditions. Numerical and analytical methods were employed for quantifying the contribution of these effects to the structural robustness of steel frames. A simplified analytical model that allows estimating the magnitude of arching effects in steel beams with various types of end‐connections and horizontal restraints provided by the surrounding structure was introduced. The agreement between numerical and analytical results suggests that the proposed analytical model allows for accurate predictions. This validates the model's use for practical applications and gives promising prospects for its inclusion in consistent analytical methods for predicting the activated alternative load path in steel frames subjected to a column loss scenario.
... Two such effects are the amplification of the gravity loads due to inertial effects (dynamic load amplification) and the changes in the mechanical properties of the materials (strain rate effects). However, in the present study, the strain rate effect will be neglected [28]. ...
The design of building structures is expected to provide sufficient structural robustness in order to reduce the risk of progressive collapse under extreme loading events. One way of checking the robustness is the Alternate Path method, which is achieved if the sudden removal of an element does not lead to progressive collapse. In order to verify that the structure meet this requirement, various methods can be used. Whereas the advanced non-linear dynamic analysis provides most accurate results, in practice, the non-linear static approach is more practical, but requires indirect consideration of dynamic effects using dynamic increase factors. This study investigated the factors that can affect the dynamic response of multi-storey steel frames following the loss of a column. Comparisons with current analytical approaches are also made.
... The present paper, summarizing studies carried out within last years at the Research Center of Mechanics of Materials and Structural Reliability-CEMSIG ( [5][6][7][8]), from the Department of Steel Structures and Structural Mechanics, is intended to provide some new evidence on the previous FEMA 277 conclusions. The goal is to give a response to the following question: Does the application of seismic design principles to steel building frames mitigate the risk of progressive collapse? ...
Multi-story buildings often use steel moment frames as lateral force resisting systems, because such systems would allow architectural flexibility, while providing the strength, stiffness, and ductility required to resist the gravity, wind, and seismic loads. Steel moment frames on which capacity design concepts are applied to resist earthquake induced forces, are generally considered robust structures, with adequate resistance against collapse for other extreme hazards, for example blast or impact. Starting from this point, the present paper summarizes the results of some recent studies carried out in the Department of Steel Structures and Structural Mechanics and CEMSIG Research Center from Politehnica University Timisoara, aiming to evaluate the influence of beam-to-column joints, designed to satisfy seismic design requirements, on the progressive collapse resistance of multi-story steel frame buildings.
... In a scenario where a structure is subjected to an extreme loading and damage occurs, the connections need to assure the transfer of the loads from the damaged area to the undamaged elements. This requires a connection which has adequate levels of strength, stiffness and ductility, as well as a good post-flexural behaviour (Marginean 2017). ...
Any building structural system should be designed as to ensure structural stability and resistance for usual loading conditions but also for abnormal loading situations. In multi-story steel building frames, the second requirement can be achieved by designing the structure such that the structural stability is maintained when a key structural member, such as a column at the ground level, is lost. In this case, the behaviour of elements and their connections should be adequate to avoid structural failure by assuring the development of alternate load paths. In this paper, a study on beam-to-column end-plate connections with four bolts per row is presented. The study assessed the viability and behaviour of such connections under extreme loading situations, where the robustness of the structure is a critical parameter. The study is based on parametric advanced FEM analyses considering the relevance and impact of several geometrical parameters.
... The pressure wave can 'unload' the beams, or even change the sign of the bending moment in the blast phase. The part of the structure subjected to free fall has the same mass for the inertia forces as in the case of notional removal, but also additional forces due to the rebound from the blast pressure (Marginean 2017). Jahromi et al. (2012) reported a similar behaviour. ...
Building structures should be sufficiently robust to resist progressive collapse resulting from localised failures (e.g. due to blast). However, current codes governing the design for robustness are rather generic and have limited provisions ensuring that structures withstand the exposure to such threats. Due to the complexity of the phenomenon (blast pressure, dynamic response, level of damage, residual capacity, propagation of collapse), the experimental validation of full-scale models may still be necessary for the development of numerical or analytical tools. An ongoing national research project, aiming to develop and validate numerical models for predicting the blast response of a steel framed building is under development. The building will be subjected to blasts (TNT or equivalent) with different charge sizes and locations, resulting in different scaled distances. As the scaled distance reduces, the peak overpressure increases, thus causing the shear failure of the elements located in the proximity. The potential for progressive collapse following local damage will be also investigated. The paper presents the result of a numerical study that investigated the structural response of the building for different combinations of charge weights, stand-off distances and levels of gravity load on the building floors. The preliminary validation of the numerical model is done using the results of blast tests, which were performed on similar steel frames within a previous research project.
Vulnerability of structures to progressive collapse and mitigation of the effects of local damages are topics widely discussed. The studies worldwide carried out allowed identifying different design strategies. However, specific knowledge is still limited and this gap is apparent in codes of practice. A European project focusing on robustness of steel and steel and concrete composite structures regarding the effects of impact recently started combining the 2 methods of residual strength and alternate load path. In the framework of the project activities, the Authors aimed their studies on the redundancy of the structure through slab-beam-systems and ductile joint as well as on the local behaviour of the impacted members. For this analytical, numerical and experimental investigations have been planned and executed. The paper describes the experimental activities related to the investigations on the structural redundancy and presents the preliminary test outcomes.
Frame-core tube structure system is commonly adopted in high-rise office buildings and landmarks, which will cause a large range of damage and serious loss of life and property once the progressive collapse occurs. To analysis the progressive collapse resistance of a high-rise frame-core tube practical project, numerical simulation analysis based on alternate path method and explicit dynamics theory was conducted to study the dynamic response of the remain structure after removing a key member. In addition, large-scaled tests for the frame-core tube structure under static and dynamic loads were carried out to study the behavior and damage situation of the structure. The numerical simulation and test results proved that progressive collapse would not occur when local damages occurred in the bottom corner column, and the structure still has a good seismic performance under dynamic actions. And the feasibility and reliability of the selected numerical simulation method of progressive collapse analysis is proved according to the test results.
The different nature and intensity of accidental loads make difficult the development of design requirements for such situations. Therefore, a better strategy is to limit the ex-tent of damage so that the progressive collapse is not initiated. Features like ductility and continuity provide more deformation capacity and redistribution of loads, so that the structure can bridge over damaged/lost elements. Interaction between the steel beams and the concrete slab is also expected to enhance the resistance after the loss of a column. This paper presents the results of an experimental study that aimed at investigating the contribution of the floor system and beam-floor interaction to the load redistribution capacity in case of a column loss. For this purpose, a 3D steel frame structure, with composite beams and extended end-plate bolted beam-to-column connections, was considered. The specimen was tested under mono-tonic loading applied to the top of the central column until complete failure. The results showed the system was capable of developing larger capacity to resist the loss of a column but lower deformation capacity when compared to a bare steel frame tested under similar conditions.
During their lifetime, buildings may be subjected to accidental actions, like blast and impact, which can cause significant local damages. To limit the effects of such unforeseen events, the structure should retain the structural integrity by limiting the extent of dam-age and prevent its progress. Taking advantage of structure’s inherent redundancy and available load paths, seismic resistant steel frames are considered appropriate to resist local damage without collapse. However, there are specific problems, which need to be considered when localized failures, particularly of columns, occur, i.e. large deformations and catenary response of beams. In this study, we investigated the performance of four steel frame beam-to-column connection types affected by the column removal. Acceptance criteria for progressive collapse events were proposed and compared to existing provisions.
The beam-to-column connections of moment-resisting steel frames should exhibit capacities that allow them to transfer the forces that develop under normally expected loading conditions. However, when a column is lost owing to accidental loading, these conditions change, and the forces are redistributed to the adjacent beams and columns. In such cases, the connections must be capable of resisting the combined axial and flexural loads and allow for the redistribution of the loads, so that progressive collapse development is prevented. In this study, we investigated the performances of four types of beam-to-column connections, namely, the welded cover plate flange connection (CWP), the haunch end plate bolted connection (EPH), the reduced beam section welded connection (RBS), and the unstiffened extended end plate bolted connection (EP), against progressive collapse. Two span frames were constructed and tested under a central column removal scenario until failure. The results from the experimental tests were used to validate finite element models. The CWP, EPH, and RBS specimens showed good ductility, with the catenary action making a significant contribution to the ultimate load resistance. Further, the ultimate rotations of the beams were greater than the deformation limit given in the latest Unified Facilities Criteria guidelines for design of buildings to resist progressive collapse. Specimen EP showed the lowest ductility and ultimate load resistance, with the bolts in the rows under tension fracturing before the catenary action could develop. Further, the failure mode for specimen EP indicated that bolt strengthening is necessary for improving its progressive collapse resistance.
This paper presents an experimental and computational study on the behavior of two composite subassemblies under a column removal scenario. The two specimens, designed as beam-joint-beam (B-J-B) subassemblies with reinforced concrete slabs on top of steel beams, were extracted from a prototype steel frame building with composite floor systems. One subassembly with the joint above the removed column was loaded under sagging deflection, and the other with the joint adjacent to the removed column was loaded under hogging deflection, simulating a center column removal scenario at a two-span beam-column subsystem. Detailed finite element models were also developed and analyzed for the two composite subassemblies. The observed failure modes were captured by the numerical models, and the computed load-versus-displacement curves agreed reasonably well with the measured data. To investigate slab effect, test results of the test specimens and steel subassemblies similar to the test specimens but without slab were compared. It showed that the load carrying capacities of the composite subassemblies were > 63% higher than the steel subassemblies. Under sagging deflection loading, the composite subassembly showed a greater initial stiffness than the steel subassembly. Unlike the steel subassemblies, notable compressive axial forces were developed in beams of the composite subassembly subjected to sagging deflection at the early loading stages, indicating arching action contributed to the load resistance at small deformation as well as the initial stiffness. Contributions to the load capacity by resistant mechanisms, e.g. flexural action, arching action and catenary action were characterized and discussed.
Following the loss of an inner column, a moment resisting frame initially carries the upper load by the flexural mechanism with potential contribution from the catenary mechanism as the beams connected to the failed column deflect. The effective utilization of the catenary mechanism depends on the behaviour of the beam-to-column connections in the affected span. Previous studies have demonstrated that the types and details of the moment connections determine their behaviour, especially the failure mode, in the column removal scenario. Two typical failure modes, the beam-end continuous failure and the beam-end interrupted failure, have been studied in the literature. A third failure mode, identified as the column-wall failure, is investigated in this paper through a full-scale laboratory test and finite element (FE) simulations of an I beam-square hollow section column connection with inner diaphragms inside the column. The present experimental observation and FE analysis indicate that a column-wall failure starts with the separation between the bottom inner diaphragm and the column’s inside wall, before fracture takes place in the column wall near the two ends of beam’s bottom flange. The crack extends inwards along the bottom flange and finally upwards, at which point the flexural mechanism is fully replaced by the catenary mechanism. The column-wall failure mode is more desirable than the beam-end (or beam-section) continuous failure mode under the column removal scenario due to the former’s ability to develop an effective catenary mechanism to bridge over the member failure of the column itself. In fact, the column-wall failure mode appeared to develop a more effective catenary mechanism than even the beam-end interrupted failure mode, although further research is required to ascertain the relative merits between the two modes and associated connection methods.
When a reinforced concrete (RC) frame subjected to an edge-column-removal scenario, its floor system exhibits a complicated mechanism against progressive collapse due to the interaction between beams and slabs and the two-way load transfer characteristics. In this study, laboratory tests of five 1/3-scaled RC frame substructure specimens, including four beam-slab specimens and one beam specimen without a slab, are reported. The effect of critical structural parameters (i.e., the beam height, slab thickness and seismic reinforcement) on the collapse resistance was investigated by analyzing the applied loads, structural deformations and material strains. The RC slabs contribute to an increased collapse resistance of the control beam-slab specimen by 146% under small deformations (i.e., the beam mechanism) and 98% under large deformations (i.e., the catenary mechanism) compared to the beam specimen. The resistances of the beam-slab specimens were mainly provided by the slabs and the beams in the direction along the free edge, more significantly by the portion close to this edge where the largest deformation was developed. Increasing the seismic reinforcement in the beams resulted in a much larger collapse resistance under both beam and catenary mechanisms. In contrast, increasing the slab reinforcement by expanding the slab thickness marginally improved the collapse resistance under the catenary mechanism but contributed little to that under the beam mechanism. Increasing the beam height also largely improved the collapse resistance under the beam mechanism, but had limited impact to that under the catenary mechanism.
The collapses of Murrah Federal Building, Oklahoma City, in 1995 and Twin Towers of World Trade Center, New York City, in 2001 demonstrated that mass casualties and economic loss can be attributed to the collapse of buildings rather than the initial blast pressure or shock. Thus, designing buildings to prevent progressive collapse has become an imperative in the professional engineering community and standard-writing group in recent years with the increase of terrorist activities. However, it is uneconomical to design structures to resist progressive collapse purely relying on the flexural strength, as progressive collapse is an inherently low-probability event. Fortunately, existing studies indicated that there are some secondary load-resisting mechanisms neglected in conventional structural designs. These secondary mechanisms, depending on the locations of missing columns and types of structures implicated, can be utilized to mitigate the vulnerability of structures to collapse. This paper provides an overview of the advance of understanding in possible load-resisting mechanisms (Vierendeel action, compressive arch action, compressive membrane action, tensile catenary action, tensile membrane action, and dowel action) to resist progressive collapse of RC structures (frames and flat plate/slab).