Behaviour of Mid-Rise Rc Structures under Blast Loading Using Fem Software

Abstract

This study investigates the dynamic response of mid-rise reinforced concrete (RC) buildings under blast loading, driven by the increasing threat of terrorist attacks that cause severe structural damage. The objective is to evaluate the behavior of bare moment-resisting frames (comprising columns and beams), focusing on displacement, load-carrying capacity, and damage patterns. Using SAP2000, a parametric study was conducted on buildings of varying heights (G+6, G+9, and G+12), including a G+6 irregular building. Time history analyses were performed for explosive charge weights of 10?kg, 20?kg, and 30?kg at standoff distances of 15?m, 30?m, and 45?m. The results revealed peak displacements of approximately 1100?mm in the bare frame structures. The frame, acting like a mesh structure, experiences reduced abrupt loading due to blast wave diffraction. However, these deformations still indicate potential vulnerabilities in conventional frame systems under high-intensity blast loads. To enhance blast resistance, various retrofitting techniques were investigated, including single diagonal bracing, double diagonal bracing, and steel jacketing. These methods significantly improved energy dissipation and overall structural stiffness. The study provides valuable insights into strengthening strategies for improving the resilience of urban infrastructure against explosive threats.

Introduction

In modern structural engineering, blast resistance has become crucial due to rising threats from accidental and intentional explosions, especially in mid-rise reinforced concrete (RC) buildings. These buildings, common in urban settings, are vulnerable to progressive collapse under blast loads due to their vertical layout and load concentration.

Blast loads differ significantly from conventional forces: they are high-intensity, short-duration, and dynamic, involving complex parameters like peak overpressure, impulse, and stand-off distance. Their effects are influenced by structural geometry, material properties, and boundary conditions. Thus, advanced numerical tools such as SAP2000 and LS-DYNA are required for accurate simulation and analysis.

Significance of Study

• Addresses the growing risk of explosions in civilian and industrial environments.

• Aims to improve safety, inform building codes, and support ethical rebuilding in conflict zones.

• Highlights the importance of blast-resistant design to protect lives and infrastructure.

Blast Phenomenon and Structural Impact

• Explosions generate a shock wave with a Friedlander waveform, causing rapid pressure changes that can severely damage structures.

• Critical blast parameters: peak overpressure, positive phase duration, impulse, and standoff distance.

• Structural damage may range from local effects (e.g., spalling, cracking) to global failures like collapse.

Methodology

• Focus: Evaluate how retrofitting improves blast resistance in mid-rise RC buildings.

• Four configurations tested:

  1. NB (Non-Braced) – baseline model

  2. Single Diagonal Bracing

  3. Double Diagonal Bracing ("X" shape)

  4. Steel Plate Jacketing around columns

• Models: G+6, G+9, G+12 heights, with a 20?m × 20?m plan and standard RC dimensions.

• Blast scenarios: Surface explosions of 10, 20, and 30 kg TNT at 15, 30, and 45 meters.

• Key output: Maximum displacement under blast loading, used to assess and compare retrofitting effectiveness.

Conclusion

Explosions occurring in proximity to buildings can inflict significant structural damage. The destructive impact may stem not only from the direct blast wave but also from the resulting collapse of structural components, flying debris, fire, and smoke. In this study, surface blast loads were calculated and applied to a series of structural models using SAP2000, a widely adopted software for dynamic analysis. Blast loads were modelled as time-history functions derived from established empirical equations available in the literature. A parametric analysis was conducted for building models of varying heights G+6, G+7, and G+12 under different explosive charge weights (10 kg, 20 kg, and 30 kg) and standoff distances (15 m, 30 m, and 50 m). The nonlinear dynamic analysis in SAP2000 allowed for the evaluation of maximum joint displacements under each scenario. Results demonstrated a clear trend: as the standoff distance increased, the resulting structural displacements consistently decreased, indicating a reduction in blast impact with distance. To enhance the structural performance under blast loads, retrofitting techniques were applied to the G+9 model at a charge weight and standoff distance. The techniques included: 1) Local Steel Jacketing of columns 2) Single Diagonal Bracing 3) Double Diagonal (“X”) Bracing Among these, bracing systems significantly reduced displacement due to their ability to absorb and redirect dynamic forces. However, it was observed that local jacketing, although less effective in reducing displacement compared to bracing, offers localized reinforcement and enhanced ductility, thereby improving structural integrity during a blast event. In conclusion, while bracings provide superior displacement control, local jacketing remains a viable and practical solution for protecting mid-rise buildings against surface explosion threats, especially in retrofitting existing structures.

References

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Copyright

Copyright © 2025 Vishal B. Patil, Dr. Sachin P. Patil. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.