Parametric Analysis and Simplified Design Method for Multi-Layer Composite Confinement Structures Under Internal Blast Loading

Multi-layer composite confinement structures (MLCCSs) are critical for containing accidental internal explosions, yet conventional design methods cannot capture their three-layer load-redistribution mechanism. Based on a validated fluid–structure interaction (FSI) model, this study investigates four key parameters (the foam-concrete yield strength, interlayer thickness, inner-layer reinforcement ratio, and inner-layer thickness) under 3 and 5 kg TNT charges and develops a simplified design method for MLCCSs with a foamed-concrete interlayer under internal blast. The foam yield strength is found to be the most critical parameter controlling the Mode I/II transition: increasing it reduces the inner-layer displacement but increases the risk of premature outer-layer yielding, challenging the conventional “stronger-is-better” design philosophy and establishing both upper and lower bounds for the foam yield strength. The method is derived from a rigid–plastic inner-layer model and the load-controllability mechanism of the foam interlayer, yielding closed-form expressions for the inner-layer maximum displacement, the minimum required interlayer thickness, and the outer-layer peak hoop strain. The inner-layer maximum displacement is quantitatively validated, with a mean absolute error of 9.50%, while the three remaining outputs are checked for consistency with the numerical results; because the Mode I/II threshold is calibrated from the same numerical dataset, this consistency check is an internal self-consistency test rather than an independent validation of the strain prediction. A step-by-step design procedure incorporating the Mode I/II criterion is proposed and demonstrated on a prospective design example whose configuration is not among the calibration cases. The study provides a rapid screening tool for the preliminary design of MLCCSs.