Theoretical Framework for the Mechanics of Progressive Collapse in Welded Space Frame Structures Under Combined Fire and Blast Loading Relevant to Nigerian Petroleum Processing Facilities

The mechanics of progressive structural collapse in welded steel space frames exposed to the combined thermal and dynamic loading of hydrocarbon fire and blast events at petroleum processing facilities represent a theoretically complex, multi-physics failure problem that existing single-hazard structural analysis frameworks address inadequately, with the consequence that process facility structural design in Nigeria relies on conservative and decoupled assessment methods that neither capture interaction effects nor provide a rigorous basis for optimised protective design. This research develops a theoretical framework for the mechanics of progressive collapse in welded steel tubular space frames under combined fire and blast loading, with reference to the structural typologies and hazard scenarios characteristic of Nigerian onshore petroleum processing plants in the South South zone. The framework formulates a sequentially coupled thermomechanical finite element methodology in which fire-induced temperature field evolution, thermal softening and creep degradation of structural steel, and dynamic blast pressure loading are combined in a validated nonlinear dynamic analysis procedure. A theoretical model for connection failure under combined elevated-temperature and dynamic moment-shear-axial force interaction is developed and implemented as a user-defined material subroutine in ABAQUS. The framework is applied to three representative process module configurations derived from Nigerian petroleum facility construction standards, and collapse progression sequences are identified under credible explosion and fire event combinations. Parametric studies establish the relative sensitivity of progressive collapse resistance to structural redundancy, connection ductility, and fire protection specification. The framework provides a theoretically rigorous basis for combined hazard structural design in the Nigerian upstream processing sector. Keywords: progressive collapse, combined fire and blast, petroleum processing, welded space frame, thermomechanical analysis.