Blast and residual capacity analysis of reinforced concrete framed buildings

Abstract

Iconic and public buildings have become a universal target of bomb attacks from terrorists. Most of these buildings have been or are built without consideration for their vulnerability to such events. Planning and building control authorities have begun to recognise the risks of these events and have introduced provisions in planning guidelines for mitigation of such impact. This paper is a study of the impact of near field explosions on the structural framing system and key elements such as columns and describes the component material response. This information can be used in planning strategies to mitigate potential catastrophic and progressive collapse of the structure. Reinforced concrete framed buildings have been selected for this study. A two stage finite element modelling (FEM) and analytical technique has been used to interrogate the structural framing system and components for global stability and local residual strength capacity in the linear elastic and non-linear plastic response regimes. The first stage involved linear time history analysis carried out using SAP 2000 to verify the response of the complete framing system and its ability to restore global frame stability and to enable iterative interrogations. An explicit rigorous analysis accounting for strain rate effects of the reinforced concrete elements was carried out in stage two using LS DYNA code to investigate the non-linear response of vulnerable elements identified in the first stage. The damage mechanisms and the extent of damage have been studied using principal stress plots along with plastic strain diagrams and used to assess the residual strength capacity of key elements that can cause catastrophic failure of large sections of the building and propagate progressive collapse. Numerical analysis is based on techniques that have been established in previous research work and the models have been calibrated with similar work by others. The method used in this research work can be used for assessing vulnerability, damage and residual strength capacity of building frames and component elements subjected to near field blast events.

Introduction

Bomb explosions against iconic and public buildings have become hazardous due to widespread terrorist activities in various parts of the world [1]. Loss of lives and millions of dollars of property damage are the consequence following a successfully targeted bomb attack. While initial casualties are due to direct over pressure released by the explosion, falling of structural elements may extensively increase the total figure. Most buildings which are likely to be the target of a terrorist bomb have been constructed without considering their vulnerability, or their potential to mitigate the impact. However, planning and building control authorities are currently identifying the risks associated with such events in the present environment of global terrorism [2]. It is therefore important to carry out vulnerability and damage assessment of buildings subjected to blast loads to provide mitigation strategies. This paper discusses an evaluation of the response of a reinforced concrete (RC) framed building to a near field blast event. Evidence of recent blast events in major cities shows that explosion at more than 20 m distance from the buildings generally cause damages to their facades with some impact on the structural integrity. Recent examples are the Jewish Community Centre, Argentina 1994 [3], and the WTC twin tower attack, Sri Lanka, 1997 [4]. On the other hand, the near field events such as the Murrah building bombing, USA 1995 [3], the Central Bank bombing, Sri Lanka 1996 [5] and, Khobar Towers bombing, Saudi Arabia, 1996 [6] have had the potential to damage and destroy the structural integrity of the buildings. In some incidents, this has been accompanied by a large number of casualties. Recent incidents such as the Lahore city bomb blast 2009 [7] and, the Baghdad bomb blasts on foreign embassies 2010 [8] illustrate the importance of investigations of the response of building structures to a near field blast in order to protect lives and property. The work described in this paper focuses on the near field blast events that are likely to cause damage on the structural integrity of the buildings and set off catastrophic or progressive collapse.

Research and investigations have been conducted for RC structures to study their vulnerability when subjected to blast loads [9], [10], [11], [12] in order to implement retrofit measures for preventing damage to the structure and its components [5], [13], [14]. Some of these have examined the progressive collapse mechanisms of the building frames and recommended retrofit methods to avoid catastrophic failure of the entire structure while the rest assessed the individual structural elements.

However, limited research has focused on the studies to identify the effects of blast load on the RC framed buildings, but they have focused on either the stability of a sub frame or the response and the damage to a single structural component [15], [16], [17], [18]. An extensive investigation on the progressive collapse of a steel framed multi storey building has been carried out [19] and a similar study is needed for RC framed buildings too. This paper presents a two phase finite element analysis procedure to interrogate the structural framing system and components for global stability followed by a rigorous analysis of key structural components for damage evaluation in the linear elastic and non-linear plastic response regimes respectively. In the first phase, SAP2000 is used to carry out the global linear elastic analysis of the entire building frame under blast loading. The outcome of this analysis will provide information on the damage sequence of the frame system, the critical regions which require rigorous analysis and the capacity of the framing system to restore global frame stability. LS DYNA will be used in the second phase to carry out a rigorous non-linear elasto-plastic analysis of a sub frame embodying the critical members identified in the global analysis. The outcome of the second phase will provide information on the damage extent of the critical members and their post blast capacities. The combined assessment can be used in evaluating the global stability and the structural integrity of the building frames in order to protect against catastrophic collapse.

An explosion detonated at ground level will have a critical impact on the lower storeys of the building structure [20]. Catastrophic failure of the structure will initiate at the critically damaged zone due to reduction of the load carrying capacity of the load bearing key structural components such as columns [21]. Response of framed structures when columns are removed and the possible progressive collapse have been investigated [10], [22]. An assessment tool derived from an appropriate analytical approach is needed to recognise the response and quantify the damage to load bearing key elements of the building. Current knowledge on quantifying and predicting explosion damage of RC framed building subjected to blast effect is incomplete and limited although it is necessary [14]. The establishment of the integrity of the columns and beams with the lateral load resisting framing system is important as the Murrah building bombing incident revealed that the entire collapse occurred due to failure of four columns and loss of integrity of the framing system [1].

A reinforced concrete framed building with base dimensions 32 m×35 m consisting of 10 storeys, (having an overall height of 41.6 m) has been selected as a case study in the present investigation.