Progressive Collapse Mitigation Research Articles

Robustness evaluation and design of steel and steel‐concrete buildings according to current standards and recent research results

AbstractIn terms of mitigation of progressive collapse, structural robustness is a specific safety consideration, which requires particular attention from all professionals involved in the construction industry, including architects, designers and constructors. The design of buildings considering robustness has evolved considerably over the last two decades, during which different research activities have been developed, leading to new methods and design processes. These practice‐oriented methods and concepts have been recently summarized in a design manual prepared in the framework of the FAILNOMORE RFCS project (grant No 899371), providing to the designer a general design strategy for robustness based on these methods as well as easy to follow worked examples. The proposed design strategy allows to ensure the global stability and the mitigation of progressive collapse of steel and composite structures when subjected to exceptional events, such as vehicle impact, explosion, cascading events involving localized fire or events which are not explicitly specified in the typical structural design process. In this paper, the application of these recommendations is demonstrated by means of realistic worked examples. Typical steel and steel‐concrete composite office buildings, designed according to the current European standards, are evaluated for robustness against identified and unidentified threats. The results obtained with simple and more advanced verification approaches are compared, and possible solutions to improve the robustness of the structures are provided.

Numerical study on upgrading beam-column connections in steel framed buildings for progressive collapse mitigation

This research used intensive numerical analysis to identify weak points and structural issues important to limiting the spread of collapse. As a consequence, four specimens of strengthened and unstrengthened steel beam-column joint assemblies were examined using scaled models. The data were used to validate numerical models. The control specimen was a simple shear joint from one of the four experimental assemblies. The second specimen was a bolted steel beam column joint used as a reference specimen to represent the ideal beam-column joint often used in intermediate moment-resisting frames in seismic zones across the globe. The third specimen, like the control, but it had two side plates welded together to strengthen the joint site, while the fourth specimen had high-strength, hot-rolled steel bars pretensioned inside the joint zone. ABAQUS software was used to create numerical finite element models to evaluate the behavior of steel frame assemblies before and after upgrading. The FEM matrix included 22 specimens with various characteristics, including plate thickness, steel grade, a contact between beam flange-strengthening plates, and a column that was either welded or not welded in the first scheme, number and diameter of hot-rolled steel bars were for the second scheme. The effectiveness of the strengthening techniques was established by comparing the mode of failure and load–displacement characteristics of the investigated specimens.

Numerical Investigation on Progressive Collapse Mitigation of Steel Beam-Column Joint Using Steel Plates.

This research employed extensive numerical analyses to locate the weak areas and determine the structural issues critical to preventing the spread of collapse. As a result, three specimens were tested using scaled models of strengthened and unstrengthened steel beam-column joint assemblies. The data were utilized to verify numerical models. One simple shear joint from the three experimental assemblies was used as the control specimen (unstrengthened joint). The second was a bolted steel beam-column joint utilized as a reference specimen to reflect the ideal beam-column joint generally employed in intermediate moment-resisting frames in seismic zones worldwide. Similar to the control, the third specimen (strengthened joint) had two side plates welded together to strengthen the connection site. Numerical finite element models were developed using ABAQUS (2020) software to extensively investigate the behavior of steel frame assemblies before and after upgrading. The FEM matrix comprised 17 specimens with varying parameters, including plate thickness, steel grade, a joint between the beam flange-strengthening plates, and a column that was either welded or not welded. The effectiveness of the strengthening techniques was established by comparing the mode of failure and load-displacement characteristics of the investigated specimens. The results indicate that the average increase in peak load due to a change in plate thickness for grades A36 and A572 is approximately 22% and 8%, respectively. Plates made of A572 steel increase peak load by 30%. All strengthened specimens attained catenary action, mitigating the possibility of progressive collapse.

Numerical Investigation of Beam-Column Connections in Precast Concrete Buildings

The progressive collapse of structures has been the concern of many studies. Efforts have been made to develop methodologies of analysis and design for the mitigation of progressive collapse. As precast structures lack of structural continuity in the load paths, they are more responsive to progressive collapse than the cast-in-situ monolithic reinforced concrete (RC) buildings. In this paper, Analytical study has been conducted to develop a nonlinear finite element FE model using ABAQUS 6.12 software to investigate the behavior of precast non-prestressed reinforced concrete beam-column connection in case of sudden column removal. The FE model consider the nonlinear behavior of concrete and steel. In addition, it takes into account the contact properties between surfaces at joints. A calibration was performed between the numerical and the experimental results, using three half-scale specimens tested under mid column lose, to verify that the FE model could be used for simulating precast beam column connections. There was a good agreement between experimental and numerical results.

Numerical Investigation of Beam-Column Connections in Precast Concrete Buildings

The progressive collapse of structures has been the concern of many studies. Efforts have been made to develop methodologies of analysis and design for the mitigation of progressive collapse. As precast structures lack of structural continuity in the load paths, they are more responsive to progressive collapse than the cast-in-situ monolithic reinforced concrete (RC) buildings. In this paper, Analytical study has been conducted to develop a nonlinear finite element FE model using ABAQUS 6.12 software to investigate the behavior of precast non-prestressed reinforced concrete beam-column connection in case of sudden column removal. The FE model consider the nonlinear behavior of concrete and steel. In addition, it takes into account the contact properties between surfaces at joints. A calibration was performed between the numerical and the experimental results, using three half-scale specimens tested under mid column lose, to verify that the FE model could be used for simulating precast beam column connections. There was a good agreement between experimental and numerical results.

Rational horizontal tying force method for practical robustness design of building structures

This paper presents a rational method for the horizontal tying of multi-storey building structures aimed at robustness design and the mitigation of progressive collapse. Unlike current prescriptive tying force approaches, the new method benefits from i) a rational association with local damage scenarios consisting of the sudden loss of a column or a load-bearing wall, ii) the explicit consideration of the system ductility limit and dynamic amplification, iii) the systematic combination of various types of loading and sources of tying, and iv) supplementary methods for assessing the interaction of the affected floor system with the surrounding structure. Importantly, these relative benefits are still realised within a practical application framework that is comparable in simplicity to prescriptive tying rules, making the proposed tying method suitable for codification and as a replacement for prescriptive tying force requirements in the next generation of robustness design codes. Following a detailed exposition of the method formulation and parameters, the paper presents several studies which demonstrate the effectiveness of the proposed tying force method, concluding with an example that illustrates its application to a typical reinforced concrete floor system.

Static and dynamic tests on steel joints equipped with novel structural details for progressive collapse mitigation

This paper presents static and dynamic tests on nominally-pinned steel joints equipped with novel structural details for progressive collapse mitigation. The proposed structural details utilise the exceptional ductility and strength of stainless steel pins to enhance both the tie force and the rotational capacity of a vulnerable steel joint. The stainless steel pins along with additional supporting elements are installed in the joint region without interfering with the design for gravity loads, and they can be used for both new designs and to retrofit existing steel buildings. A static test on a vulnerable industry-standard steel fin-plate connection is first presented followed by two static tests on the same connection retrofitted with the proposed structural details. The retrofitted connections were subsequently tested under dynamic conditions with increasing imposed loading using a test setup that simulates a sudden column loss scenario. The test results showed that nominally-pinned joints equipped with the proposed structural details can achieve the required tie force capacity while undergoing rotations larger than 0.2 rad. Analytical equations based on simple joint equilibrium are used to validate the results of the static tests. An analytical method based on the energy conservation principle is also proposed and comparison with the dynamic tests shows very good predictive capability when the assumed loss of energy is 22%.

Seismic progressive collapse mitigation of buildingsusing cylindrical friction damper

The occurrence of progressive collapse induced by the removal of the vertical load-bearing element in the structure, because of fire or earthquake, has been a significant challenge between structural engineers. Progressive collapse is defined as the complete failure or failure of a part of the structure, initiating with a local rupture in a part of the building and can threaten the stability of the structure. In the current study, the behavior of the structures equipped with a cylindrical friction damper, when the vertical load-bearing elements are eliminated, is considered in two cases: 1-The load-bearing element is removed under the gravity load, and 2-The load-bearing element is removed due to the earthquake lateral forces. In order to obtain a generalized result in the seismic case, 22 pair motions presented in FEMA p 695 are applied to the structures. The study has been conducted using the vertical push down analysis for the case (1), and the nonlinear time-history analysis for the second case using OpenSEES software for 5,10, and 15-story steel frames. Results indicate that, in the first case, the load coefficient, and accordingly the strength of the structure equipped with cylindrical friction dampers are increased considerably. Furthermore, the results from the second case demonstrate that the displacements, and consequently the forces imposed to the structure in the buildings equipped with the cylindrical friction damper substantially was reduced. An optimum slip load is defined in the friction dampers, which permits the damper to start its frictional damping from this threshold load. Therefore, the optimum slip load of the damper is calculated and discussed for both cases.

Mitigation of progressive collapse in steel structures using a new passive connection

If an alternative path would not be considered for redistribution of loads, local failure in structures will be followed by a progressive collapse. When a vertical load-bearing element of a steel structure fails, the beams connected to it will lose their support. Accordingly, an increase in span\'s length adds to the internal forces in beams. The mentioned increasing load in beams leads to amplifying the moments there, and likewise in their corresponding connections. Since it is not possible to reinforce all the elements of the structure against this phenomenon, it seems rational to use other technics like specified strengthened connections. In this study, a novel connection is suggested to handle the stated phenomenon which is introduced as a passive connection. This connection enables the structure to tolerate the added loads after failing of the vertical element. To that end, two experimental models were constructed and thereafter tested in half-scale, one-story, double-bay, and bolted connections in three-dimensional spaces. This experimental study has been conducted to compare the ductility and strength of a frame that has ordinary rigid connections with a frame containing a novel passive connection. At last, parametric studies have been implemented to optimize the dimensions of the passive connection. Results show that the load-bearing capacity of the frame increased up to 75 percent. Also, a significant decrease in the displacement of the node wherein the column is removed was observed compared to the ordinary moment resisting frame with the same loads.

Strengthening of precast RC beam-column connections for progressive collapse mitigation using bolted steel plates

Being one of the most critical scenarios in extreme events such as blast attacks, the progressive collapse of reinforced concrete (RC) structures has attracted the attention of structural engineering community. As precast concrete buildings are deficient in structural continuity, they are more vulnerable to progressive collapse than cast-in-situ RC buildings. Hence, effective rehabilitation techniques to upgrade beam-column joints in existing precast RC buildings for progressive collapse mitigation are needed. The goal of this study is to investigate the effectiveness of using bolted steel plates on the behavior of precast beam-column connections under sudden column-loss scenario. This study presents experiments involving one half-scale precast RC beam-column assembly, which represented the most prevalent types of existing precast beam-column joints in Saudi Arabia. One monolithic test specimen having continuity of top and bottom beam rebars was used for the sake of comparison. Another precast specimen similar to the control one was strengthened using bolted steel plates within the connection region. The progressive collapse scenario was simulated by removing the central column support and applying a sudden vertical load on this column at a rate of 100 mm/s until failure. The collapse load of both monolithic and strengthened specimens was predicted using a simplified section analysis procedure. The analysis was then used for some useful parametric studies in which the effect of different steel plate parameters on the response of test frames under middle column-loss scenario was investigated.

Scheme for beam progressive collapse mitigation

Latest recommendations have made provisions to improve structure resistance to progressive collapse in the event of interior support collapse. This paper defines a mitigating scheme to repel progressive collapse in reinforced concrete beams due to damaged interior supporting member by providing a new path to transfer the loads to other supporting members. The mitigating scheme suggests the use of unbounded unstressed externally installed fibre reinforced plastic cables with straight profile and three deviators. An open source finite element package for structural analysis (ZEUS-NL) using the fibre element approach is adopted to numerically model the mitigated beam. The proposed numerical model evaluates the progressive collapse of such beams using a push-down analysis to simulate column removal. It assumes that the anchorage and deviator locations of the external cables act as rigid arms that connect the external cables to the beam. The numerical results demonstrate that the defined mitigating scheme increases the beam resistance to progressive collapse due to interior column failure.

Scheme for beam progressive collapse mitigation

Latest recommendations have made provisions to improve structure resistance to progressive collapse in the event of interior support collapse. This paper defines a mitigating scheme to repel progressive collapse in reinforced concrete beams due to damaged interior supporting member by providing a new path to transfer the loads to other supporting members. The mitigating scheme suggests the use of unbounded unstressed externally installed fibre reinforced plastic cables with straight profile and three deviators. An open source finite element package for structural analysis (ZEUS-NL) using the fibre element approach is adopted to numerically model the mitigated beam. The proposed numerical model evaluates the progressive collapse of such beams using a push-down analysis to simulate column removal. It assumes that the anchorage and deviator locations of the external cables act as rigid arms that connect the external cables to the beam. The numerical results demonstrate that the defined mitigating scheme increases the beam resistance to progressive collapse due to interior column failure.

Progressive collapse analysis of a typical RC high-rise tower

The world has recently witnessed tremendous increase in terrorist activities. This led to the requirement of blast resistant design of structures. The progressive collapse of structures, being the most severe consequence of blast generated waves, has been the subject of several studies. Although structural engineers are developing methodologies for the mitigation of progressive collapse, there is a lack of adequate tools that can be employed for simulating and predicting the progressive collapse response of structures with acceptable confidence. An attempt has been made in this paper to develop a practical and acceptable procedure for the progressive collapse analysis of reinforced concrete (RC) framed structures. The adequacy of the procedure has been demonstrated by studying the progressive collapse behavior of a typical RC framed high-rise building in Riyadh when exposed to blast generated waves.

MACRO-ELEMENT MODEL OF A STEEL MOMENT FRAME SUBJECTED TO FIRE-INDUCED COLUMN LOSS

A progressive collapse mitigation strategy is to ensure load redistribution when a column fails due to fire. The study seeks to understand whether welded unreinforced flange-bolted web (WUF-B) moment connections can effectively redistribute loads in a structural system subjected to fire when a critical column is lost. A component (or macro-element) model was derived to simulate the WUF-B connection and validated against experimental tests and high-resolution finite element (FE) models of subassemblies at room temperature and at elevated temperature. The component model was then utilized in a 2D macro FE model of a ten-story steel-framed building subjected to the loss of a column during long fire exposure. This paper presents the collapse mechanisms and quantifies structural performance based on acceptance criteria. A parametric study on location of column loss and fire occurrence is also included.

Enhanced external progressive collapse mitigation scheme for RC structures

This paper discusses enhancement of an external mitigating scheme to maximise the resistance of reinforced concrete continuous beams to progressive collapse due to interior support failure. The mitigating scheme using external unbounded fibre reinforced plastic straight cables and a relevant numerical model were presented by the authors in a published companion paper to predict the progressive collapse resistance of the mitigated beams. This study extends the previous work to efficiently enhance the resistance of mitigated beams by inspecting different mitigating scheme setups. Straight and deviated cable profiles are considered, different cable deviator locations are examined, and different cable deviations are accounted for to compare the corresponding mitigated beam resistance enhancements. The mitigating scheme setup that provides maximum increase in the beam resistance is considered the enhanced setup that efficiently maximises the increase in the mitigated beam resistance and could economically prevent progressive collapse of beams due to interior column failure.

Progressive collapse mitigation approaches for steel-concrete composite buildings

This paper investigates the robustness of steel-concrete composite frames under sudden column loss. Various lateral load resisting systems, including contributions from bracing systems, composite slab and joints are taken into account in analysing buildings against progressive collapse. Robustness of ten-storey steel buildings with (1) diagonal braces at corners, (2) centre core wall, (3) rigid moment joints are analysed for corner, perimeter and internal column loss. The analyses conclude that simple braced frames are more susceptible to the progressive collapse compared with the moment resisting frames which has higher redundancy to redistribute the loads arising from sudden column loss. Various mitigation approaches are proposed to improve the progressive collapse resistance of the simple braced frames. End-plate beam-to-column connection is proposed instead of fin-plate connection due to greater rotational stiffness and higher moment resistance. Vierendeel truss is proposed at the top floor level in a multi-storey building to redistribute the load due to sudden column loss. In addition, modified fin-plate connection is proposed in which a plate is welded to the bottom beam’s flange to increase joint’s rotational restraints helping to reduce beam’s vertical displacement in the event of sudden removal of column.

Mitigation of Progressive Collapse by the Activation of the Elasto-Plastic Catenary Behaviour of R.C. Slab Structures

Mitigation of progressive collapse was highlighted in 1968 with the collapse of the Ronan Point building in the United Kingdom. Technical standards followed suit with increased requirements and recommendations to encourage the design and construction of more robust buildings. The intent has been to establish a design process that recognizes and considers the potential that buildings could experience abnormal and extreme loads or events that seriously compromise one or more critical load-carrying elements. This article aims to show that if the main goal of the design is to protect human lives in these extreme conditions, even sacrificing the building functionality, simple measures such as the activation of the elasto-plastic catenary behaviour of the slab reinforcement, could be very effective to increase the building robustness without substantially increasing the cost of a structural system.

Effect of seismic design level on safety against progressive collapse of concentrically braced frames

In this research the effect of seismic design level as a practical approach for progressive collapse mitigation and reaching desired structural safety against it in seismically designed concentric braced frame buildings was investigated. It was achieved by performing preliminary and advanced progressive collapse analysis of several split-X braced frame buildings, designed for each seismic zone according to UBC 97 and by applying various Seismic Load Factors (SLFs). The outer frames of such structures were studied for collapse progression while losing one column and connected brace in the first story. Preliminary analysis results showed the necessity of performing advanced element loss analysis, consisting of Vertical Incremental Dynamic Analysis (VIDA) and Performance-Based Analysis (PBA), in order to compute the progressive collapse safety of the structures while increasing SLF for each seismic zone. In addition, by sensitivity analysis it became possible to introduce the equation of structural safety against progressive collapse for concentrically braced frames as a function of SLF for each seismic zone. Finally, the equation of progressive collapse safety as a function of bracing member capacity was presented.

Numerical studies on dynamic behavior of tubular T-joint subjected to impact loading

Joints play an important role in resisting impact loading in Tubular structures. In this paper, a finite element model validated by experimental results is developed to numerically study the failure modes and energy dissipation mechanism of tubular T-joint impacted by a drop hammer with the initial velocity of 7–10 m/s. The resistant mechanism is investigated based on the dynamic responses of the joints under impact loading. Strain, displacement and the failure modes of the T-joints are also predicted. Global and local deformations of the tubular joints are distinguished using an equal area axis method, which helps to discover the failure mechanism of the joints. Using the yield line theory, an equivalent impact force estimation method is also proposed based on the impact load versus displacement relationship. The numerical analysis and the simplified method provide a basis for impact resistance evaluation and progressive collapse mitigation of steel tubular structures in design practice in the future.

Mitigation of Progressive Collapse in Multi-Storey Buildings

This paper describes recent developments in performance-based design of multi-storey buildings against progressive collapse, where particular emphasis is given to practical implementation within the structural design process. The benefits of sudden column loss as an event-independent scenario are highlighted, and its computational merits as well as its correlation with actual extreme dynamic events are emphasised. It is proposed that the robustness limit state should be based on avoidance of failure in the upper floors above the damaged column, where it is proposed that the design envelope should be stretched from the conventional strength limit to the ductility limit so as to achieve economic design for robustness. In addressing the dynamic effects of sudden column loss, load factor approaches that have been recently proposed for use with some new usa design codes are shown to have serious shortcomings, potentially leading to unsafe estimation of dynamic amplification. These shortcomings are effectively addressed by the ductility-centred approach recently developed at imperial college london, which offers a practical multi-level assessment framework for progressive collapse under sudden column loss. Key lessons are drawn from the application of the ductility-centred approach to steel-concrete composite multi-storey buildings, where the inadequacy of prescriptive and load factor approaches is clearly demonstrated, and where the benefits of such factors as connection ductility, additional connection reinforcement, axial restraint, infill panels and rate-sensitivity are highlighted.