Alternate Load Path Method Research Articles

Progressive Collapse Analysis of SRC Frame-RC Core Tube Hybrid Structure

Steel reinforced concrete (SRC) frame-reinforced concrete (RC) core tube hybrid structures are widely used in high-rise buildings. Focusing on the progressive collapse behavior of this structural system, this paper presents an experiment and analysis on a 1/5 scaled, 10-story SRC frame-RC core tube structural model. The finite element (FE) model developed for the purpose of progressive collapse analysis was validated by comparing the test results and simulation results. The alternate load path method (APM) was applied in conducting nonlinear static and dynamic analyses, in which key components including columns and shear walls were removed. The stress state of the beams adjacent to the removed component, the structural behavior including inter-story drift ratio and shear distribution between frame and tube were investigated. The demand capacity ratio (DCR) was applied to evaluate the progressive collapse resistance under loss of key components scenarios. The results indicate that the frame and the tube cooperate in a certain way to resist progressive collapse. The core tube plays a role as the first line of defense against progressive collapse, and the frame plays a role as the second line of defense against progressive collapse. It is also found that the shear distribution is related to the location of the component removed, especially the corner column and shear walls.

Effects of different steel-concrete composite slabs on rigid steel beam-column connection under a column removal scenario

Trapezoidal and Reentrant are two ordinary deck profiles in modern steel-concrete composite floor system in China. The progressive collapse resistance of rigid steel beam-column connections with these two different composite deck profiles was experimentally investigated. This research addressed progressive collapse behavior of components evaluated by removing columns through the alternate load path method where the connections simulated the behavior of connections from a multi-bay steel moment-resisting frame. Also, collapse resistance of the connection above the removed column and the adjacent connection was considered. The results were compared with a bare steel subassembly, which has the same configuration but without composite slab, the load carrying capacity of the specimen with trapezoidal steel deck is improved by 28% and the specimen with reentrant steel deck is improved by 44%. The type of steel decks influenced the degree of restraint to the beam top flange and resistance of the connection. The specimen with trapezoidal steel deck had a higher plastic rotation capacity than the reentrant one, but the specimen with reentrant steel deck had improved composite behavior in the large deformation range and, therefore, developed more catenary action than the specimens with trapezoidal steel deck. Overall, the specimens with reentrant steel deck had a better performance under the progressive collapse situation, however, some constructional measures must be made to delay the bottom beam flange fracture.

A new method for progressive collapse analysis of steel frames

As a massive explosion happens inside a building, a number of structural members (columns, beams, slabs and so on) are damaged or failed, along side with non-zero initial conditions (displacement, velocity, acceleration and so on), and the progressive collapse of the building structures is most likely to occur. However, limited research works about blast load effect of structural members inside a building can be found. In view of this, based on the substructure model, a new method for progressive collapse analysis of steel frames under blast load is proposed. First, the massive explosion scenario inside a building is introduced. Then, the substructure model within effective areas of blast influence is established. After that, the calculation method of non-zero initial conditions and initial damage for structural members is given, and finally the specific steps of the proposed method are described. By way of example of a steel frame with 5 stories in height, 4 bays in the longitudinal direction, 3 bays in the transverse direction, direct simulation method, alternative load path method and proposed method are all employed to simulate the progressive collapse process, respectively. Through the example analyses, it is shown that blast load effect of structural members cannot be ignored on the ground floor, and it can be ignored on the other floors by the effect of the reinforced concrete slab. The non-zero initial conditions and initial damage of structural members can be predicted well by the substructure model, and the proposed method is also reliable and accurate.

Irregularity index for quick identification of worst column removal scenarios of RC frame structures

Irregular structures are known to be more prone to disproportionate collapse (DC) than structures with regular horizontal and vertical layouts. Despite this, many public, commercial and institutional buildings have to be designed with various irregularities in structural layout due to architectural and aesthetic considerations. While extensive experimental and numerical studies have been carried out to advance the basic understanding of DC behaviors of regular structures, limited research has been carried out on irregular structures. On the other hand, alternate load path method has been recommended by several design codes and standards for DC design, but the large number of potential column removal scenarios involved poses a real challenge for the wide use of the method. This paper presents a quantitative measure referred to as irregularity index, or Π, that can be used to quickly identify the worst column removal scenarios for design check of DC resistance of reinforced concrete (RC) frame structures. Based upon a solid understanding of various competing failure mechanisms and DC resistance of RC frame structures, the irregularity index was derived from the maximum load factors between the yielding load and the limit load at the end of tensile membrane action stage. For verification and validation of the proposed Π, a 3D nonlinear finite element model was developed in OpenSees and calibrated by beam-slab sub-assemblage experiments. Various numerical experiments were carried out to verify the effectiveness of the Π. The very high correlation coefficient between the calculated irregularity indices and DC resistance obtained from finite element analyses showed that the proposed irregularity index can quickly identify the worst column removal scenarios in various irregular structures including setback buildings and large-bottom-space structures. Sensitivity analysis further verified the reliability of the proposed irregularity index.

Study on Structural Robustness of Isolated Structure Based on Seismic Response

The qualitative analysis for structural robustness study subjected to severe earthquakes is unable to meet engineering requirements, and a quantitative analysis method for structural robustness is needed to be proposed. The existing analysis methods, such as Incremental Dynamic Analysis Method and Pushover method, only study the response of the structure directly from the macroscopic view, rather than focusing on the response of a single component on the structure. Especially for the construction of isolated structure, the impact of accidental bearing failure on the isolated structure and the impact of progressive collapse cannot be considered. In this paper, based on the Alternative Load Path Method, the quantitative analysis method for structural robustness analysis under earthquake is proposed. The structural robustness of some different vertical irregular isolated structures under different earthquakes is studied.

A simplified model for alternate load path assessment in RC structures

To assess the robustness of reinforced concrete structures under progressive collapses, alternate load path method by introducing a single column removal has been widely adopted by structural engineers. Numerical analyses of structures under several possibilities of single column removal may be time consuming, especially when non-linear finite element analyses are employed which require high computational costs and modelling skills. Towards this end, a simplified analytical model is proposed in this paper to facilitate engineers in predicting resistance of the affected substructure (frames or frame-slabs above the removed column), and allow them to perform a quick check on the adequacy of progress collapse resistance of the structure to stop or prevent the damage propagation to the remaining structure. The proposed model considers development of different bridging mechanisms in RC structures under a concentrated loading above the removed column, including compressive arch action, catenary action, and tensile membrane action. In addition to validation against test results, applications of the proposed analytical model to predict the bridging capacities of a 2-D two-storey frame and unsymmetrical double-span beams are also presented.

Dynamic response analysis for submerged floating tunnel with anchor-cables subjected to sudden cable breakage

Anchor-cables are critical bearing components of the submerged floating tunnel (SFT). As the accidental cable-breakage incident will seriously threaten the public safety, this paper investigates the global dynamic response of a SFT subjected to an abrupt anchor-cable failure by focusing on the post-breakage behavior. Firstly, an approximate theoretical approach is proposed, in which the analysis model of SFT is simplified and the alternate load path method (AP method) is adopted to simulate the cable-breakage process. Then, the differential equations of the SFT tube are established based on the Hamilton principle, and solved through the fourth order Runge-Kutta method. A finite element analysis in ABAQUS is also performed as a verification of the theoretical results, in which the VUSDFLD subroutine and ABAQUS/Aqua is employed to simulate the stiffness loss of the cable and apply the fluid loads respectively. A good agreement exists between the simplified theoretical model and FE simulation. Finally, the effects of some key parameters are discussed, such as the gravity-buoyance ratio and the damping ratio of the SFT, the breakage time and position of the broken cable, etc. The results show that the structural vibration is intensive after the sudden cable breakage. Also, the remaining anchor-cables close to the cable-loss position are most affected by the cable rupture. The change of gravity-buoyance ratio and damping ratio have notable effects on structural deformation. The SFT is most unfavorable when the cable breakage happens at the mid-span or near the two ends of the tunnel. The vibration amplitude attenuates significantly with the increase of the failure time of anchor-cable.

Progressive Collapse Analysis of Latticed Telecommunication Towers under Wind Loads

As the antenna-supporting structures, latticed telecommunication steel towers are considered as critical members of telecommunication infrastructures. It is necessary to perform progressive collapse analysis of lattice telecommunication towers under wind loads. The present study conducts a nonlinear dynamic analysis on 50 m high typical standard latticed telecommunication tripole tower and angle tower by alternative load path method. The finite element models for two towers subjected to design wind loads are developed by ABAQUS. The analysis results show that, for 50 m high standard tripole tower, the member failure in the first three tower sections from tower top would not trigger the collapse of the tower. From the fourth tower section to tower bottom, the member failure at certain wind direction may cause a collapse. For 50 m high standard angle tower, the single member failure in any tower section would not cause the collapse of the tower. A dynamic sensitivity index is proposed to identify the most unfavorable wind direction for tripole tower and angle tower. A progressive collapse fragile curve based on collapse probability of telecommunication tower under wind loads is proposed to assess the anticollapse performance of the towers.

Progressive Collapse Analysis of Reinforced Concrete Structures: A Simplified Procedure

In this paper, a simplified analysis procedure to calculate the column removed point displacement at progressive collapse analysis of reinforced concrete structures is proposed. The energy absorption capacity under the column missing event is used for formulations. The approximate method is simple to utilize, user friendly, yet accurate. For progressive collapse analysis of structures, linear static analysis, nonlinear static analysis, linear dynamic analysis and nonlinear dynamic analysis can be performed. In this paper, the nonlinear static analysis from alternate load path method is used and the reason of initial local collapse has not been considered. In fact, an energy-based method by using load-displacement curve of RC frame and considering the effect of floor slab for the progressive collapse analysis is considered. The accuracy of the proposed method is demonstrated by comparing the results to three experimental and analytical results. Finally, the effects of the spans length, sections dimensions, material properties and the beams reinforcements of column removed spans on substructure behavior is studied, as well.

Progressive Collapse Analysis of Reinforced Concrete Structures: A Simplified Procedure

In this paper, a simplified analysis procedure to calculate the column removed point displacement at progressive collapse analysis of reinforced concrete structures is proposed. The energy absorption capacity under the column missing event is used for formulations. The approximate method is simple to utilize, user friendly, yet accurate. For progressive collapse analysis of structures, linear static analysis, nonlinear static analysis, linear dynamic analysis and nonlinear dynamic analysis can be performed. In this paper, the nonlinear static analysis from alternate load path method is used and the reason of initial local collapse has not been considered. In fact, an energy-based method by using load-displacement curve of RC frame and considering the effect of floor slab for the progressive collapse analysis is considered. The accuracy of the proposed method is demonstrated by comparing the results to three experimental and analytical results. Finally, the effects of the spans length, sections dimensions, material properties and the beams reinforcements of column removed spans on substructure behavior is studied, as well.

Robustness analysis of steel structures with various lateral load resisting systems under the seismic progressive collapse

Abstract Robustness is called to non-sensitivity of a structure to local damage. In the literature of Civil Engineering, the sensitivity of structures to the local damage which can cause a progressive collapse is called Robustness Index. After 9/11, progressive collapse drew the attention of researchers and since that time, increasing the Robustness Index in a structure has been considered as one of the factors in strengthening the structure against progressive collapse. The aim of this study is assessing the Robustness Indices of steel structures having various lateral load resisting systems. So, 8-story, 4-story and 15-story structures with Moment Frames, Concentrically Braced Frames and Eccentrically Braced Frames were modeled and assessed under Robustness analysis. For a progressive collapse analysis, the Alternative Load Path method was used and after emerging local damage, the potential of progressive collapse was assessed using the Robustness Index. First, Robustness analysis was carried out on the basis of Robustness Index assessment using Stiffness and Base Shear methods. Subsequently, it continued by introducing a simple method based on Energy. The results show that despite their simplicity, the methods which are based on Stiffness and the Base Shear, have lots of disadvantages. But, the Energy method has better capabilities for understanding the behavior of structure under seismic loads in progressive collapse scenario. Furthermore, structures which have bracing had a better reaction in progressive collapse scenario.

Resilience of tall steel moment resisting frame buildings with multi-hazard post-event fire

Infrastructure resilience is the ability of an infrastructure asset to limit the effect and duration of damaging extreme events. The four main components of the concept of resilience are robustness, resourcefulness, recovery and redundancy most of which are very difficult to be quantified. In addition to this difficulty, a careful study of extreme event cases can demonstrate that it is very common for an extreme event scenario to include cascading multi-hazard events such as blast, floods, earthquake, and fire. This paper studies the resilience of a multi-story steel frame with multi-hazard considerations which include a post-event fire scenario. The initiating extreme event is simulated through the threat-independent alternate load path method of analysis and a post-event fire is considered following the extreme event. The work in the paper combines previous work by the authors on stability-induced collapse of damaged steel structures and closed-form solutions for temperature predictions of wide-flange components during a fire scenario. For the purposes of the fire scenarios, new fire time-temperature curves are developed based on experimental data from the well-known Cardington fire tests. The results show that even when a structure can withstand an extreme event scenario, a post-event fire consideration is highly critical to evaluate the remaining time of survival of the structure before collapse. It is shown that the sequential method of multi-hazard analysis can lead to very short available time periods before the post-event fire leads to the complete collapse.

A new partial-distributed damage method for progressive collapse analysis of steel frames

The alternate load path method has so far dominated the field of progressive collapse of structures; in order to assess the resilience of structural systems, the concept of the removal of a key element is utilized as a means of damage introduction to the system. Recent studies have indicated that the complete column loss notion is unrealistic and unable to describe a real extreme loading event, e.g. a blast, that will introduce damage to more than one elements in its vicinity. This paper presents a new partial distributed damage method (PDDM) for steel moment frames, by utilizing powerful finite element computational tools that are able to capture loss of stability phenomena. Through the application of a damage index δj and the investigation of damage propagation, it is shown that the introduction of partial damage in the system can significantly modify the collapse mechanisms and overall affect the response of the structure.

Improving the accuracy of progressive collapse risk assessment: Efficiency and contribution of supplementary progressive collapse resisting mechanisms

This paper investigates the structural response of RC framed buildings subjected to accidental/abnormal loads (explosion, impact and other hazards). Several existing national or international design codes (GSA 2003, DoD 2009, 2013) provide limited guidelines for the assessment of progressive collapse resistance in the design process and the alternate load path method is widely used in current structural design codes. Since the progressive collapse is a dynamic and nonlinear event the structural components undergo nonlinear deformations before failure. In this study the nonlinear procedures (NSA and NDA) are applied. These types of analyses imply significant computational power and time costs. In addition, the reinforced concrete framed structures are able to develop multiple post flexural resisting mechanisms. These are currently not considered in the structural analyses performed according to GSA2003 and DoD2009 provisions. Thus, the first objective of this paper is to identify by numerical computation the presence and contributions of such supplementary resisting mechanisms. A further objective is to determine what will be the influence on the accuracy of results when, starting from the original structure, the number of its bays is successively reduced; also, what is the efficiency in saving run-time costs when such simplified models are considered in the analysis. The study reveals that in order to resist abnormal loads, RC structures are able to develop supplementary resisting mechanisms beyond the flexural behavior and respectively, important time savings are obtained without significantly affecting the results accuracy.

Comparison between Alternative Load Path Method and a Direct Applying Blast Loading Method in Assessment of the Progressive Collapse

Extensive research has been focused on the progressive collapse analysis of buildings and most of them are based on the alternative path method (APM) with sudden removal of one or several columns. However, in this method the damage of adjacent elements of removed columns under blast conditions was ignored and this issue can lead to an incorrect prediction of progressive collapse. Therefore, in this study to evaluate the alternative load path method in predicting the progressive collapse due to blast loading, a 3-D finite element model of a 7 storey steel building simulated and the behavior of structure was studied using the direct applying of blast load method and alternative load path method. For simulating and applying the blast loading and assessment of their direct effects on structures, a blast load equivalent to 1 ton TNT was considered at a distance of 4 meters from the corner of the structure. The pressures of this blast in 4 loading cases are applied to the adjacent structural members and the structural response has been examined. Finally, the exciting forces in adjacent structural members of blast site in each case have been compared. The results show that in assessment of the potential of progressive collapse occurrence by considering the blast loading as the initial reason of failure, the structure response will be different compared with the alternate load method that in which the initial reason of progressive collapse was ignored.

Threat-Independent Column Removal and Fire-Induced Progressive Collapse: Numerical Study and Comparison

Progressive collapse is defined as the spread of an initial failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it. The current progressive collapse analyses and design methods in guidelines and codes focus on the alternate load path method. This method is suitable especially in the case of blast-induced progressive collapse. In this paper, fire-induced and threat-independent progressive collapse potential is numerically investigated in steel moment resisting frames. Affecting parameters such as location of initial failure and number of floors are considered in this study. Two different mechanisms were observed in threat-independent and fire-induced progressive collapse: while in threat-independent column removal alternative load paths play major role, in fire-induced progressive collapse the weight of the structure above the failure region is the most important parameter.

Influence of Sudden Column Loss on Dynamic Response of Steel Moment Frames under Blast Loading

Modeling buildings response to blast and subsequent progressive collapse interested more and more researchers during the past two decades. Due to the threat from extreme loading, efforts have been made to develop methods of structural analysis and design. In this paper, progressive collapse capacity of steel moment frames was first investigated using alternate load path method, then a nonlinear dynamic analysis was carried out to examine the response of the steel moment frames in blast and sudden column loss scenario. The structural response of the building under sudden loss of column for different scenarios of column removal, with or without external blast loading was assessed in detail. According to the results, progressive collapse potential are strongly dependent on location of column loss. Loss of column can affect overall response of structure under external blast loads. The obtained results provide better insight into the influence of sudden column loss on dynamic response of steel moment frames under blast loading.

Evaluation of progressive collapse potential of multi-story moment resisting steel frame buildings under lateral loading

The aim of this study is to investigate whether MRF steel structures that have been designed based on seismic codes, are able to resist progressive collapse with damaged columns in different locations under seismic loading.For this purpose, 3-D and 2-D push-over analysis of structures is carried out. The progressive collapse potential has been assessed in connection with 5 and 15-story buildings with 4 and 6 bays by applying the alternate load path method recommended in UFC guidelines.Member removal in this manner is intended to represent a situation where an extreme event, such as vehicle impact or past earthquake shock or construction error, may cause a critical column, as a result of local or global buckling, to lose a part or whole of its load bearing capacity.In contrast with 3-D models, two-dimensional frames represent a higher sensitivity to base shear reduction and element removal. In the case of middle column removal, the structure is more robust than in a corner column removal situation. The influence of story number, redundancy and location of critical eliminated elements has been discussed.

Numerical Simulation for Progressive Collapse and Protection of a Multi-Storey Building due to an Explosion in its Basement

The progressive collapse of a multi-story building and its protective effect with aluminum foam under an explosion in its basement are numerically investigated in this paper. The three-dimensional coupling model composed of a multi-story frame structure with a basement and the surround soil is modeled using ANSYS/LS-DYNA. The progressive collapse of the entire structure under explosive impact and the protective effect of the aluminum foam are simulated using the proposed three-stage simulation method (TSSM). The result shows that, the aluminum foam as a protective layer can reduce the impact effects very well, and then can prevent progressive collapse effectively. The same model is also simulated by direct simulation method (DSM) and alternative load path method (ALPM). By comparison with the two methods, the TSSM is illustrated practical, accurate and economic for the analysis of progressive collapse.

An efficient compound-element for potential progressive collapse analysis of steel frames with semi-rigid connections

In this paper, the formulation of a novel 1D frame compound-element for the materially and geometrically non-linear analysis of steel frames with flexible connections is outlined. The element is formulated based on the force interpolation concept and the total secant stiffness approach, and implemented in a FORTRAN computer code. The accuracy and efficiency of the formulation are verified through some numerical examples. For steel frames with bolted flush end-plate and extended end-plate connections, a static and dynamic progressive collapse assessment based on the alternate load path (ALP) method is undertaken by employing the developed analytical tool and dynamic load factor (DLF) is estimated. Furthermore, the implications of analyzing semi-rigid steel frames based on the assumption of fixed connections and the effects of the connection details on the global response of a frame during different progressive collapse scenarios are investigated.