Abstract:
Blast incidents in recent years indicate that most of the terrorist attacks target on high occupancy iconic and public building using vehicle borne incendiary devices. Most of these buildings have been built without considering the vulnerability of such events. If standoff distance is not restricted around a building, terrorists can easily place the explosives as close as possible to the building. Therefore, damage degree and blast vulnerability assessment of a building subjected to near field explosions is necessary to undertake mitigation strategies. So far, the effect of blast load on structure has been done by dynamic analyses method which is time consuming and not suitable for simple hand calculation. Equivalent static analysis method can overcome this problem.
In this context, the present research investigates the response of reinforced concrete (RC) beam and column under near field explosions using finite element method (FEM). A beam and a column located at ground floor of a typical 7-storied building located at Dhaka city have been selected for this purpose. Finite element (FE) software ABAQUS (2014) is used to assess possible damage degree and blast vulnerability of RC beam and column based on residual capacity. Initially, the FE model of RC beam, column and frame considering material and geometric nonlinearity are validated under static load comparing with the experimental and analytical results. Then, the validation has been extended for impact and blast load. FE validation with selected parameters and analyses procedure shows enough confidence in numerical modelling of RC beam and column subjected to blast loading. After validation, parametric study has been conducted on RC beam and column under different parameters such as charge mass of explosive, transverse reinforcement detailing (Ordinary Moment Frame, Intermediate Moment Frame, Special Moment Frame) as per BNBC (Draft, 2017), column cross section, height of burst and effect of axial load ratio under 100 to 2000 kg of TNT.
It is observed that for a particular blast pressure, damage index is least for Special Moment Frame (SMF), highest for Ordinary Moment Frame (OMF). For Intermediate Moment Frame (IMF), it is in between of SMF and OMF but very close to OMF. Hence, the safe standoff distance for a particular blast load is least for SMF, highest for OMF and for IMF, it is close to OMF. Moreover, comparatively lesser damage has been observed in RC column with larger cross-section. Surface burst causes severe damage than that caused by explosion at column mid-height. Increasing the axial load enhances the resistance of the column against localized damage and shear damage and flexural damage depend on the axial load-bending moment (P-M) interactions of the column.
Finally, an equivalent static analysis method of blast load has been developed and from this analysis an empirical equation has been proposed in terms of charge mass of TNT (WTNT) to convert blast pressure to equivalent static pressure. Using this equation, it is possible to calculate the safe standoff distance and blast vulnerability of a building under a particular charge mass using simple hand calculation.
