Numerical Study for Retrofitting of RC Columns for Blast Loading

H. Abbas, Y.A. Al-Salloum, S.H. Alsayed, H.M. Elsanadedy and Rizwan I. Iqbal, Specialty Units for Safety and Preservation of Structures, College of Engineering, King Saud University, Riyadh, Saudi Arabia

The paper presents the effect of blast loads generated as a result of explosive charges on the existing exterior reinforced concrete (RC) circular column of a typical building. A wide range of parametric studies have been performed as part of this investigation to examine the effects of stand-off distance, charge weight and the presence of Carbon Fiber Reinforced Polymer (CFRP) retrofitting on the level of damage to the column. The nonlinear finite element analysis was carried out using LS-DYNA software with explicit time integration algorithms. Different charge weights at varying stand-off distances were considered. Results described in this paper indicate that CFRP strengthening could be an effective solution to limit the damage caused by moderate explosions. The stand-off distance was found to play a vital role in mitigating the adverse effects of a blast.

Introduction

Structures all over the world have recently become susceptible to the threat of terrorist attacks, accidental explosions and other unthought-of explosion related failures. Buildings and critical infrastructures vulnerable to explosions include government buildings, embassies, financial institutions, densely populated commercial structures, and other buildings of national heritage or landmarks. Consequently, a number of concerns have been raised on the vulnerability and behavior of these structures under extreme loadings.

In a study carried out by Lan et al. (2005), design techniques for reinforced concrete (RC) columns, which are capable of protecting them from the effects of close-in detonation of a suitcase bomb have been described. LS-DYNA software (2007) was used for the finite element analysis of column using solid elements for concrete and beam elements for reinforcing bars. The elements were allowed to erode at a principal tensile strain of 50%. It was shown that the tie spacing plays an important role in the post-blast residual load capacity of columns. Muszynski et al. (1995, 2003) reported results from explosion experiments on RC columns strengthened with GFRP and CFRP. However, during the tests, a previously tested wall became detached and collided with the retrofitted columns, shearing the top and the bottom. The reason for the spoiled tests was a higher than predicted pressure from the explosive. Crawford et al. (2001) conducted experiments for studying the blast vulnerability of a 350mm square column of a four-storey office building. The column failed mainly in shear and the rupture of longitudinal rebars accounted for the majority of the displacement. An identical column was retrofitted with six layers of horizontal CFRP wraps for shear enhancement and three vertical layers for flexural enhancement. The retrofitted column under the same blast loading appeared to be elastic with no permanent noticeable deformation. The static load testing of identical columns was found useful in simulating the blast tests. Crawford et al. (1997) conducted numerical analyses of 1.10 m diameter circular RC column from a multi-storey building retrofitted with CFRP composites to determine its vulnerability to terrorist attacks. DYNA3D, a lagrangian FE code, was used to assess the performance of the column against 682 and 1364 kg TNT charges at 3.05, 6.1 and 12.2 m stand-off distances. Modeling challenges highlighted were the effect of confinement on the concrete of the strength and ductility, strain rate effects, direct shear response and determining loading on many structural members. An explosive loading was applied and a pressure at the top of the column was used to simulate the upper stories. The concrete volume was modeled with 8-node brick elements; reinforcement bars were represented with truss elements and shell elements were used for the floors and joists. All results showed that composite retrofits could have a beneficial effect on the performance of the columns and therefore prevent progressive collapse. The retrofitting was shown to reduce the lateral displacement considerably.

Riisgaard et al. (2007) presented experimental and numerical results of two polymer reinforced compact reinforced concrete (PCRC) columns subjected to close-in detonation. PCRC is a fiber reinforced densified small particle system (FDSP) combined with a high strength longitudinal flexural rebar arrangement laced together in the out of plane direction, using polymer lacing to avoid shock initiated disintegration of the structural element. The two columns were subjected to 7.6 kg of Penta-Erythritol Tri-Nitrate (PETN) HE (85/15) at a stand-off distance of 0.4 m. For both the columns, the concrete matrix was damaged and both columns suffered from bending failure. The amount of aramid lacing was found to have a positive effect on the performance. Berger et al. (2008) performed blast testing on scaled reinforced concrete columns to study the behavior of different types of strengthening of Steel Reinforced Polymer (SRP), CFRP and SRP/CFRP hybrid combination. It was observed that the SRP strengthened columns were quite similar to those strengthened with CFRP. The SRP was found to be effective external strengthening material for increasing the resistance of concrete components, providing similar performance to CFRP wraps at potentially lower cost.

It is seen that the research carried out in the area is mostly qualitative and the behavior of FRP-strengthened structures under blast loading is not well understood and no proper design guidelines are available. The lack of understanding is primarily due to the complexity of the problem where too many variables exist and experiments alone do not lead to effective design methods. Instead, an in-depth understanding of the structural behavior and accurate modeling of the dynamics of the structure under blast waves is required. In order to assess the possibility of progressive collapse of the building, it is extremely important to study the effects of blast loadings on the columns of the structure independently. The present study aims to analyze a RC circular column for investigating the role of CFRP in improving the collapse behavior thus avoiding the progressive collapse. The study considers different charge weights at varying stand-off distances. The RC circular column considered for the present study was selected from a real building located in Riyadh. A close inspection of the building premises revealed that the stand-off distance was virtually zero, which allowed the placement of explosive at a minimum stand-off distance of 1 m.

NBM&CW June 2011

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