This paper describes the numerical formulation of a state-of-the-art coupled computational fluid dynamics (CFD) and computational structural dynamics (CSD) methodology, to the simulation of steel case fragmentation and explosively loaded reinforced concrete structures. ASI (Applied Simulations Inc.) have developed a numerical methodology that couples state-of- the-art CFD and CSD methodologies [1]. The flow (CFD) code solves the time-dependent, compressible Euler and Reynolds-Averaged Navier-Stokes equations. The CSD code solves the large deformation, large strain, solid dynamic equations on an unstructured grid composed of bricks and tetrahedral elements. VMS (variational multi-scale) stabilization is utilized to improve the robustness and stability of the numerical CSD solution. The codes are coupled via a ‘loose coupling’ approach which decouples the CFD and CSD sets of equations and uses projection methods to transfer interface information between the CFD and CSD domains. Both codes are parallelized using a hybrid MPI/OpenMP methodology. The final presentation will describe in detail the implementation of the concrete fracture, weapon fragmentation and contact algorithm on the mentioned MPI/OpenMP parallelization frame, which allows spectacular simulation speed-up for real life applications. An improved first principles fragmentation scheme, which enforces mass conservation and compare very well qualitatively and quantitatively with experimental results (fragments size distribution), will be shown too. Finally, a steel case weapon simulation inside a reinforced concrete structure is shown to demonstrate the overall scheme. The simulation addresses the steel case fragmentation, air blast and fragments impact on the concrete structure, structural response, structural failure, debris launch, and propagation of air blast to the far field. The predicted structural disassembly agrees well with the high-speed photography. The predictions exhibit similar failure mechanisms, failure locations and times of failure. The initial structural debris, the weapon fragmentation size distribution, and the weapon fragment velocities follow the experimental ones too. The far field pressures exhibit similar decay with range as the experimental data.