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.

Description of Column Used in this Study

The column selected for the study is a typical circular column of 600 mm diameter and 4 m high. The column is reinforced with 16f16 longitudinal bars and f10@200 mm c/c as ties. The concrete grade of the column is taken as M30. The clear concrete cover to the ties is taken as 30 mm. The column represents a real life column of one of the reinforced concrete structures in Riyadh. The effect of strengthening has been studied numerically by retrofitting the column with CFRP layers of 1 mm thickness. Two layers of CFRP were thus applied as hoops for enhancement of shear strength and concrete confinement and two layers were provided along the length of the column, with fibers running in the vertical direction, to increase its flexural capacity. The properties of materials used are given in Table 1.

Finite Element Modeling

LS-DYNA (2007), a general purpose finite element program was used to develop the 3-D model of the column. Two cases were considered in the modeling of the column. The first case involved the column to be modeled without any strengthening and the second case involved strengthening of the column with Carbon Fiber Reinforced Polymer (CFRP) sheets. Damping has been ignored, as it has a negligible effect for short duration, impulsive loads.

Finite Element Mesh

Modeling of the column was first completed using ANSYS-Version 11 as it has a very strong graphical user interface and the file was then imported to FEMB (which is a preprocessor for LS-DYNA) database for incorporating the different parts as well as the blast interface and contact segments. A combination of eight and six node solid elements was used to model the concrete volume. The longitudinal reinforcing bars and ties were modeled using 2-node Hughes Lui beam elements. For the modeling of CFRP sheets, 4-node shell elements were employed. Perfect bond was assumed between rebar elements and the surrounding concrete volume and also between the FRP and the concrete substrate. Fig. 1 details the mesh discretization for the concrete elements, the CFRP elements and the reinforcing cage used in the study. The total numbers of elements in the model are 13472.

Material Modeling

The Karagozian & Case (K&C) model (Malvar et al. 2000), designated as Material type 072R3 in LSDYNA, was employed to represent concrete for the column. The model is specially designed for predicting the response of concrete under blast loads. It is a three-invariant model, which uses three shear failure surfaces and includes damage and strain rate effects. It also incorporates many important features of concrete behavior such as tensile fracture energy, shear dilation and effects of confinement. The reinforcement was modeled using material type 024 to model the elasto-plastic response with strain rate dependency. In order to model the CFRP material, material type 054-055 was utilized, which is capable of defining orthotropic material characteristics. The material angles for the circumferential and longitudinal layers were specified as 0° and 90° respectively. The manufacturer’s data sheet for the CFRP material was used for defining different material parameters. The laminated shell theory was used for the purpose of correcting the assumption of a uniform constant shear strain throughout the thickness of the composite shell, thus avoiding very stiff results. The failure criteria of composite material used in the analysis is the one proposed by Chang and Chang (1987) with special features of compression failure governed by the criteria of Matzenmiller and Schweizerhof (1991). A summary of material properties used in the analysis are presented in Table 1.

Erosion

The erosion option provides a way of including failure to the material models. This is not a material or physics-based property; however, it lends a great means to imitate concrete spalling phenomena and produce graphical plots, which are more realistic representations of the actual events. By activating this feature, the eroded solid element is physically separated from the rest of the mesh. This erosion model represents a numerical remedy to distortion, which can cause excessive and unrealistic deformation of the mesh. The application of erosion to the simulated model requires calibration with experimental results; however in the absence of experimental validation, the consequence of possible discrepancy in the erosion specified is limited. This is because the damage level of the concrete material is basically governed by the material model itself. In this study, the concrete elements in the RC column were allowed to erode when the principle tensile strain reached 50% (Lan et al. 2005).

Loading and Boundary Conditions

Fixed boundary conditions were assigned for the top and bottom nodes of the column. The axial load acting upon the column due to dead plus live loads from upper stories was applied as nodal loads at the column top. This axial load was applied as a ramp function over a period of 0.5 s as shown in Fig.2.

Different charge weights of 100, 200, 500 and 1000 kg equivalent weight of TNT at stand-off distances of 1, 4 and 15 m were considered in the study. Both the un-strengthened and CFRP- strengthened columns were subjected to these blast loads. The blast loads impinging on the contact segments of the column were calculated by the software using ConWep (1990). The contact segments of the blast were the solid elements of the front face of the column which were taken to be in contact with the blast. The vertical height of the charge was taken as 1.0 m above the base of the column because the explosive is assumed to be carried in a vehicle. Thus, the shock transmitted to the column through ground gets diminished due to which it has been ignored in the analysis. The blast loading was set to trigger at 0.5 seconds as shown in Fig.2.

Solution strategy

LS-DYNA uses explicit time integration algorithm for solving the problems, which is less sensitive to machine precision than other finite element solution methods. The benefits of this are greatly improved utilization of memory and disk. An explicit FE analysis solves the incremental procedure and updates the stiffness matrix at the end of each increment of load (or displacement) based on changes in geometry and material.

Analysis Results

The peak lateral and permanent displacements of column for the blast scenarios considered in the analysis are given in Table 2.

1. The charge weights of 500 and 1000 kg at a stand-off distance of 1.0 m completely destroyed both the columns with and without CFRP strengthening. So it can be assumed that the columns within the focus of a blast of this magnitude would be totally destroyed and may not be protected by retrofitting.
2. The retrofitting of column reduces the peak lateral displacement considerably. The retrofitting of column reduces the peak displacement by 21% when the damage to the column is almost negligible i.e. when the intensity of blast is least severe (100 kg charge weight at 15 m stand-off distance). A study of all blast cases considered indicates that the reduction of peak displacement varies from 8% for 100 kg charge weight at stand-off distance of 4 m to 79% for 500 kg charge weight at a stand-off distance of 4 m.
3. There is exponential increase in peak lateral displacement as well as the permanent displacement with the reduction in the stand-off distance.
4. At 15 m stand-off distance, the blast of even 1000 kg charge weight does not cause any significant damage to the column even without retrofitting. Considering 30 mm as the acceptable permanent lateral displacement for the column, at 4 m stand-off distance, blast of 100 kg charge weight may be resisted by the column even without retrofitting, whereas, 200 and 500 kg charge weights may be resisted by the column after retrofitting. Higher charge weight of 1000 kg could not be resisted by the retrofitting considered in the study. At a stand-off distance of 1 m, the blast of even 100 kg may not be resisted by even the retrofitted column. The increase in the number of layers of CFRP may however help the column to resist it.

State of Stress and Consequent Damage

Tables 3 to 7 report the results of analysis for some of the critical cases of blast for column with and without CFRP strengthening. The results of analysis for 15 m stand-off distance have not been reported in these tables because of almost insignificant damage to the column. In addition, the results of 500 and 1000 kg explosive at 1.0 m stand-off distance have not been listed as the column (with and without CFRP strengthening) has been completely destroyed. The final deflected shapes of the columns are also included in these tables. As seen from these tables, it is clear that the displacement experienced by the retrofitted columns is much lower compared with the un-strengthened columns. This demonstrates that CFRP strengthening might be a valuable tool in protecting the service integrity of RC columns especially when the blast charge weights are smaller.

Conclusion

The major conclusions derived from the present study of un-retrofitted RC column and the lightly retrofitted column using CFRP are given in the following:

1. The retrofitting of column reduces the peak lateral displacement considerably, which varies from 8% for 100 kg charge weight at stand-off distance of 4 m to 79% for 500 kg charge weight at a stand-off distance of 4 m.
2. There is exponential increase in peak lateral displacement as well as the permanent displacement with the reduction in the stand-off distance. Thus, the stand-off distance plays a very important role in mitigating the adverse effects of a blast.
3. The charge weights of 200 and 500 kg at 4 m stand-off distance may be resisted by the column after retrofitting. However, the increase in the number of layers of CFRP may help the column to resist even slightly more intense blasts.
4. A comparison of the retrofitted RC column with un-retrofitted column cases reveals that even a light retrofitting considered in the study provided considerable resistance to blast loads, and thus contributed greatly to impeding the onset of progressive collapse for moderate blasts. The nature of the failure for CFRP-wrapped columns was also less explosive, thereby protecting loss of human life and property.

References

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Acknowledgement

This article has been reproduced from the proceeding of 'National Conference on Repair & Rehabilitation of Concrete Structures' organized by ICI western U.P Gaziabad, IA Sructural Engg, and Association of Structural Rehabilitation, with the kind permission of the organisers.