Disproportionate Collapse Research Articles

Large panel system dwelling blocks – updating structural assessment guidance

This paper deals with an aspect of the enduring implications of the collapse in 1968 of part of Ronan Point, a large panel system (LPS) built dwelling block. While it is commonly recognised that this incident resulted in the introduction of ‘disproportionate collapse’ as a structural concept and changes to the UK Building Regulations in force at the time, the issues relating to the ongoing management of the remaining population of existing LPS dwelling blocks are perhaps less widely appreciated. There are many high-rise LPS dwelling blocks in the UK that are expected to remain in service for an extended period. Block owners have an ongoing responsibility for their safety, which requires their periodic inspection and structural assessment. The guidance historically used for the assessment of such blocks has become outdated by developments since its publication. This paper summarises a programme of work to develop updated technical evaluation criteria and associated guidance for undertaking structural assessment of LPS dwelling blocks for accidental loads. The programme followed the classic forensic engineering process of learning from an unfortunate event to improve engineering practice in the future.

Robustness of structures

Disproportionate collapse is prevented by ensuring collapse resistance, a property defined as the insensitivity of a structure to abnormal events. Enhancing robustness and reducing vulnerability are two different structural measures for achieving collapse resistance. The vulnerability of a structure is its susceptibility to become initially damaged by abnormal events; robustness is its insensitivity to such initial damage. Another option for preventing disproportionate collapse is the reduction of the exposure of the structure to abnormal events. The design strategy reducing vulnerability aims at preventing failure initiation whereas the strategy enhancing robustness aims at preventing disproportionate failure spreading. These two approaches are compared. Robustness can be achieved by the design methods alternative load paths and segmentation, which are also discussed. Furthermore, measures for the quantification of robustness are discussed.

Protection of structural systems and mechanisms from catastrophic and life-threatening failure caused by unforeseeable events

Structural framing systems and mechanisms designed for normal use rarely possess adequate robustness to withstand the effects of large impacts, blasts and extreme earthquakes that have been experienced in recent times. Robustness is the property of systems that enables them to survive unforeseen or unusual circumstances (Knoll and Vogel, 2009). Queensland University of Technology with industry collaboration is engaged in a program of research that commenced 15 years ago to study the impact of such unforeseeable phenomena and investigate methods of improving robustness and safety with protective mechanisms embedded or designed in structural systems. This paper highlights some of the research pertaining to seismic protection of building structures, rollover protective structures and effects of vehicular impact and blast on key elements in structures that could propagate catastrophic and disproportionate collapse.

Protection of Structural Systems and Mechanisms from Catastrophic and Life-Threatening Failure Caused by Unforeseeable Events

Structural framing systems and mechanisms designed for normal use rarely possess adequate robustness to withstand the effects of large impacts, blasts and extreme earthquakes that have been experienced in recent times. Robustness is the property of systems that enables them to survive unforeseen or unusual circumstances (Knoll & Vogel, 2009). Queensland University of Technology with industry collaboration is engaged in a program of research that commenced 15 years ago to study the impact of such unforeseeable phenomena and investigate methods of improving robustness and safety with protective mechanisms embedded or designed in structural systems. This paper highlights some of the research pertaining to seismic protection of building structures, rollover protective structures and effects of vehicular impact and blast on key elements in structures that could propagate catastrophic and disproportionate collapse.

Evaluation of wind load integration in disproportionate collapse analysis of steel moment frames for column loss

Abstract The design of steel structures, in most cases, depends majorly on the level of wind loads which are prescribed by codes and regulations and are used in the structural analysis due to the fact that steel structures being light and ductile systems are strongly affected from a slight difference in the values of wind loading. During the last decades, disproportionate collapse analysis has become of major interest mainly due to the increasing number of failures occurring in that pattern. Commonly accepted guidelines and methods of analysis have been produced, the most dominating of which being the Department of Defense Facilities criteria or DoD. In the DoD, as well as in other criteria, the event of a column loss is suggested as the modeling scenario which has to be sustained by a structural system in order to be robust. However, all the guidelines so far have disconnected the column loss analysis from wind loads and have only performed it for gravity loading. This paper presents the dynamic time history disproportionate collapse analysis of steel frames, including various levels of wind loading. Interesting aspects are discussed through the parametric analysis of five different numerical examples of moment resisting frames.

DoD Research and Criteria for the Design of Buildings to Resist Progressive Collapse

The collapse of conventional/nonhardened structures was a concern of the U.S. Department of Defense (DoD) for years before the collapse of the World Trade Center (WTC) towers during the terrorist attacks on September 11, 2011 (9-11), owing to the bombings of the Murrah Federal Building in Oklahoma City, the U.S. embassies in Africa, and the U.S. Marine barracks in Lebanon. Since 9-11, motivated by the lack of any meaningful U.S. progressive collapse design requirements, DoD has worked with the civilian community on a number of significant efforts to improve the design of buildings to resist disproportionate collapse. The DoD efforts have included laboratory and field experiments, numerical simulations, and development of design requirements. Synergy and coordination with the civilian community resulted in combined programs with the General Services Administration, guidance and feedback provided by the ASCE Structural Engineering Institute (SEI) Committee on Disproportionate Collapse Standards and Guidance (DCSG) and its members, and adoption of some European civilian approaches to progressive collapse design. A significant result of the DoD effort was the creation of Unified Facilities Criteria (UFC) 4-023-03, Design of Buildings to Resist Progressive Collapse. The approaches employed in UFC 4-023-03 are currently being evaluated and modified for civilian applications by the SEI DCSG committee. The development and underlying approaches used in UFC 4-023-03 are briefly summarized in this paper, as are the previous DoD laboratory and field tests and numerical simulations.

Probabilistic Robustness Assessment of Pre-Northridge Steel Moment Resisting Frames

This paper investigates the robustness of moment-resisting steel frames that are typical of building construction in seismic regions before the 1994 Northridge earthquake against progressive (disproportionate) collapse. Uncertainties in the collapse demands and the resisting capacities of the connections in the frames are modeled probabilistically. The dominant connection failure mode, which involves fracture of the weld connecting the beam and column flanges under scenarios involving sudden column loss, is developed using a J-integral formulation of fracture demand and is characterized probabilistically. The connection behavior model is validated using connection test data from the SAC Project on steel frames conducted following the Northridge earthquake. The robustness of two three-story steel frames designed in the SAC Project is assessed by utilizing (a) the requirements in the new Unified Facilities Criteria (UFC), and (b) a system reliability analysis. This analysis reveals that steel moment frames with connections similar to those found in pre-Northridge building construction may not meet the UFC requirements for general structural integrity following notional column removal.

Upper-bound yield-line rigid-plastic plane-stress analysis of core coupling-beams in tall buildings

Cores in tall buildings sub-divide and fire-separate vertical riser services such as lifts, stairs, air-conditioning systems, utilities and rooms requiring access to plumbing. This paper addresses the structural design of load-bearing concrete cores as distinct from framed structural steel cores fire-protected by gypsum plasterboard. Concrete cores are much more resistant to abnormal events such as earthquake, terrorist attack and disproportionate collapse, and also provide huge increases in structural stiffness and damping. Concrete cores seem to be universal for tall buildings in Australia perhaps because of local experience in the construction of wheat silos. A decision for a concrete core leaves open the decision to use concrete or structural steel for floor and façade framing. Concrete core-walls are penetrated by vertical families of openings for doors to stairs, lifts and other spaces. These openings separate the core as a whole into a number of sub-cores linked by coupling-beams (the residual strips of concrete core-wall above and below openings). Coupling-beams can be thought of as large-scale shear connectors providing composite action between distinct sub-cores. First yield will often occur in coupling-beams and spread vertically. The span/depth ratio of these beams is determined by architects and lift engineers and is often well into the ‘deep-beam' range prone to brittle behaviour. American seismic design practice, based on experimental research at the University of Canterbury, New Zealand, requires complete ‘X-reinforcement' consisting of tied rebar cages (as for columns) on both diagonals of coupling-beams. X-reinforcement can definitely improve the ductility of otherwise brittle elements. It does, however, create practical construction problems: ties are closely spaced (for Bauschinger buckling) and the crossing X-cages together with the ‘basketing' reinforcement (to avert injuries from falling concrete chunks) can amount to six to eight (or ten) layers of reinforcement across the thickness of a wall, which may also require 40 mm cover for fire-rating. Walls of 250 mm thickness are barely possible and 300 mm thick walls present difficulties. For regions of lower earthquake risk, such as the UK and Australia, one wonders whether there are reasonable alternatives to X-reinforcement, particularly when the beam size provided by lift engineers is large and only minimal rebar is required. Minimal rebar is not much more than the basketing reinforcement required with X-bars, and in some cases may be ample without large reliance on ductility. In New Zealand, more recent codes do permit buildings of limited ductility with stronger elements. In any case, the rigid-plastic collapse mechanisms provided and discussed here are quite capable of including X-bars and the present approach just provides the simplest first-pass treatment. The analyses presented here seem to agree with the conclusion reached by the late Professor Tom Paulay from experimental work at Canterbury beginning in 1967: that the bending ‘compression' rebars may be highly stressed in tension and this does reduce strength and ductility. The paper studies a range of in-plane yield-line mechanisms, starting with an almost conventional bending mechanism and then considering three shear mechanisms before arriving at a combined (bending/shear) mechanism which seems to be ‘exact'. One aim is to exemplify the variability of upper-bound analysis where, in this case, the highest bound is 2·6 times the ‘exact' result.

Disproportionate collapse analysis of cable-stayed steel roofs for cable loss

Disproportionate collapse has been identified lately as a real cause of failure for structural engineering projects. Rare and unexpected, the phenomenon of disproportionate collapse usually results to many fatalities and thus, its analysis and mitigation is deemed necessary. This work describes the analysis of a cable-stayed steel roof under the scenario of a cable loss. The event of a cable loss is assumed to be brittle, while relevant recent recommendations suggest the application of a scaled equivalent static force at the points of the anchorage of the cable but in the opposite direction of the original cable force. In this paper, three different conditions have been considered in order to study the effect of the cable loss into the overall structural response of a typical cable-stayed roof; the level of the equivalent nodal load in the opposite direction of the original cable force varies. The steel structure of the roof, in its complexity, closer to responding as a cable-stayed bridge rather than a steel roof provides a useful template for conclusions; several topics regarding disproportionate collapse and cable losses are discussed.

A case study of the structural responses of a tall building in Singapore subjected to close‐in detonations

AbstractThe response of tall buildings has been a major concern in metropolitan cities, especially with the recent surge in extreme activities targeted at structures with viable commercial values. This paper discusses a study carried out on the structural behaviour of a 2D frame, modelled to represent a tall building with ABAQUS. The model frame was subjected to a charge of the equivalent weight of 1 ton of TNT but placed at two varying cases of 5 and 10‐m standoff distances. Plane‐strain elements that incorporate the feature of material nonlinearity were utilized to model the structural components of the building and the simulated blast overpressures were obtained from the CONWEP software. The effects of large deformations of beams and columns corresponding to the short time loading duration depicted by the explosions were analysed from a local perspective. The extent of the damage is based on a local index defined as the ratio of curvatures. These local indices are consequently used to determine the possibility of disproportionate collapse of the frame from a global perspective. Finally, the provision of more ductile structural detailing is recommended to enhance the structural integrity of the building, increasing its resilience against blast attacks. Copyright © 2009 John Wiley & Sons, Ltd.

A mathematical programming computational model for disproportionate collapse analysis of steel building frames

Disproportionate collapse analysis aims to assure that frames, a common structural system of buildings, can survive unforeseen local events and a central modeling tool of such abnormal deterioration is the concept of column loss. This paper formulates an appropriate computational model on the basis of mathematical optimization, using the collapse load analysis problem of steel frames with pre-existing damage. A respective collapse load robustness measure is proposed. The model has the ability to consider both full and partial column/node removals. It renders to be a linear programming model, if the US steel design regulations are used. A practical example is presented and several aspects are discussed.

Steel moment frames column loss analysis: The influence of time step size

This paper applies two well-known structural dynamics computational algorithms to the problem of disproportionate collapse of steel moment frames applying the alternate load path method. Any problem of structural dynamics strongly depends on the accuracy and the reliability of the analysis method since the parameters involved in the selection of the appropriate algorithm are affected by the nature of the problem. Disproportionate collapse is herein simulated via a time history analysis used to “turn off” the effectiveness of an element to the structure. For this kind of problem the time step size of the computational algorithm is of major importance for the accuracy of the method and thus, remains a variable throughout the present analyses. Two plane steel moment frames are used for the numerical examples, while all the analyses are performed independently. Firstly the β -Newmark method is applied and secondly the linear Hilbert–Hughes–Taylor a-method is applied and the respective results are compared and discussed in the last part of the paper.

Disproportionate collapse performance of partially restrained steel frames with bolted T-stub connections

Current design and regulatory documents require the assessment of certain buildings with regard to their vulnerability to disproportionate collapse through notional load-bearing element removal followed by an alternative path analysis. While several recent studies have examined the robustness of steel building frames with fully moment-resistant connections and their ability to sustain local damage to load-bearing elements, only limited research is available regarding the robustness of steel frames with partially restrained (PR) connections. This paper investigates the performance of steel frames with partially restrained connections fabricated from bolted T-stubs following damage to load-bearing columns. A macro-model of a bolted T-stub connection for use in nonlinear analysis is presented, verified through experimental data, and incorporated in a nonlinear finite element model to evaluate the performance of PR frames. Two prequalified T-stub connections are considered, one of which is intended to develop the full moment capacity of the beam. The evaluation is conducted for various floor plans of typical office buildings. The analysis indicates that the frames with strong T-stub connections can resist collapse in damage scenarios involving notional removal of one first-story column, while the robustness of the frames with weak T-stub connections is questionable.

An energy-based partial pushdown analysis procedure for assessment of disproportionate collapse potential

A disproportionate (or progressive) collapse is triggered by localized structural damage that propagates throughout a large portion of a structural system. The current guidelines issued by the US Department of Defense use the alternative path method to assess the vulnerability of a structural system to disproportionate collapse. In this method, the capability of a structure to sustain local damage is evaluated by notionally removing primary load-bearing elements and checking whether the local damage can be absorbed. The assessment can be performed using linear or nonlinear static structural models or a nonlinear dynamic model. Although nonlinear dynamic analysis gives the most accurate results, it is computationally intensive and requires considerable skill to implement properly. In this paper, the vulnerability of three steel frames to disproportionate collapse is assessed using an energy-based nonlinear static pushdown analysis. The predictions are sufficiently close to the results of a nonlinear dynamic time history analysis that the method would be useful for disproportionate collapse-resistant design of buildings with regular steel framing systems.

Disproportionate Collapse: Terminology and Procedures

A variety of terms are used to describe structural characteristics and concepts in the context of disproportionate collapse of structures. Some of these terms, namely collapse resistance, robustness, and vulnerability as well as redundancy, continuity, ductility, and integrity are discussed in this paper. It is shown that they are often used differently by different writers and that there is not always a general agreement today as to their precise meaning. This paper distinguishes these terms and the associated structural characteristics and suggests working definitions. Based thereupon, a general performance-based framework for preventing disproportionate collapse is presented and the methods available for enhancing the collapse resistance of a structure are discussed.

Avoiding Disproportionate Collapse of Major Bridges

Avoiding disproportionate collapse following some small triggering event is an important aspect in the design of major bridges. A general approach for designing structures against disproportionate collapse is outlined and applied to bridges. Compared with buildings, bridges are primarily horizontally aligned structures with one main axis of extension. The provision of alternative load paths, therefore, is often not only more difficult but also less important. It is found that continuous girder bridges are preferably made collapse resistant by segmentation—which can require the insertion of joints or hinges—or by reducing the probability of key element failure. In cable-stayed or suspension bridges, the stay cables or hangers are the key elements that are particularly vulnerable. A collapse triggered by the loss of a cable is prevented by designing the bridge for loss-of-cable load cases; corresponding nonlinear dynamic analyses are presented. This measure should be complemented by protecting the cables against vehicle impact and malicious action. The robustness of suspension bridges can be raised by the methods of segmentation and alternative paths. The protection of the components of the suspension cables against malicious action deserves particular attention. Arch bridges have similarities with suspension bridges and much of the respective statements apply.

Plastic disproportionate collapse at lost corner columns

This paper deploys an existing method for the simple rigid-plastic hand calculation of slab yield-line mechanisms to address the wider problem of disproportionate collapse at lost columns in multi-storey buildings. Floor systems will be treated as grillages combining torsion-free (Hillerborg) slabs and torsion-free beams. The aim is to achieve an understanding that transcends reliance on non-linear finite element software but does not compete with it. This paper deals with lost corner columns, which are the columns most at risk; the approach can be extended to other columns.

Avoiding Disproportionate Collapse of Tall Buildings

Accidental circumstances must not result in disproportionate collapse. Strategies for achieving this goal are studied and corresponding design concepts are developed. The focus is on large and slender buildings with a high degree of significance and exposure. The study is based on an examination, one by one, of five general approaches: nonstructural protective measures, specific local resistance, alternative paths, isolation of collapsing sections, and prescriptive design rules. Departing from the specific-local-resistance approach, an arrangement of independent primary and secondary load transfer systems is arrived at where the primary system consists of a compact reinforced-concrete tube forming the vertical spine of the building. Based on the isolation approach, a vertical segmentation accomplished by strengthened floor slabs is suggested where a commencing pancake-type collapse is arrested by dissipating energy in shock-absorbing devices. These two design concepts maximize the tolerable accidental circumstances. The alternative-paths approach can be used for secondary systems or when a set of limited accidental actions is agreed upon.

Disproportionate collapse: a pragmatic approach

Probability-based design methods face certain problems in making structures sufficiently collapse resistant and thus preventing disproportionate collapse. These problems are addressed and a discussion is given to outline how they might be overcome both within and outside a probabilistic framework. Definitions for the terms ‘robustness’ and ‘collapse resistance’ are proposed, the former being a property of the structure alone, the latter including possible causes of initial local failure. A pragmatic approach for designing against disproportionate collapse is suggested and a set of corresponding design criteria is presented. Design strategies based on preventing or presuming local failure are compared. In addition to the better-known design methods providing specific local resistance or alternative paths, an approach based on isolation by segmentation is examined. It is found that the applicability of the various design methods depends on the design objectives and on the type of structure.

Discussion of “Practical Means for Energy-Based Analyses of Disproportionate Collapse Potential” by Donald O. Dusenberry and Ronald O. Hamburger

Discussion of “Practical Means for Energy-Based Analyses of Disproportionate Collapse Potential” by Donald O. Dusenberry and Ronald O. Hamburger