The Use of Refined Analysis Tools to Evaluate Structure Response of Buildings Subjected to Blast Loading

Accurate evaluation of the blast-resistant capacity of structures in petrochemical facilities is critical to effectively screening occupied buildings as part of facility siting. Several methodologies are used to predict building performance that range from empirical to rational analysis approaches. The first approach might be based on data for similar building characteristics (e.g. construction type, etc.); while the latter approach often includes a stepped process which is carried out on a component level basis to address structural components (e.g. beams, columns, etc.) and non-structural components (e.g. windows, doors, cladding, etc.).The Single Degree of Freedom (SDOF) is the most common methodology used for component level analysis and design of blast-resistant structures but typically results in overly conservative results.

Since the early 60’s the SDOF methodology has been continually validated against test data for an ever expanding realm of structural response applications. Even with the recent strides in Finite Element Modeling, SDOF analysis remains the first approach used by many engineers for the design of blast resistant structures. The wide acceptance of the SDOF methodology relies on the simplicity in the formulation as well as in the numerical solver. The approach is based on several key assumptions including: ideal loading cases, pre-defined response and failure mechanism(s) (including plastic response), ideal support conditions, and exclusion of interactions with surrounding components. Comparisons with test data show that when appropriately selected these assumptions provide designs conservative in nature.

A more refined level of analysis can be achieved using the Multi Degree of Freedom (MDOF) methodology for systems with distributed mass and flexibility. This approach follows a similar formulation used in the SDOF methodology but addresses some of the inherent conservatism by including the interaction of supported and supporting components. Individual member responses can be captured as they behave in dependent groups, and non-ideal connections and complex support conditions can be modeled such as soft or spring supports. Since the interaction of various degrees of freedom in the system are addressed concurrently, the results of the MDOF analysis are more accurate than those obtained by analyzing each component separately using the SDOF approach. This is more evident for more flexible structures such as metal buildings as opposed to more rigid concrete and masonry buildings.

The use of the MDOF analysis approach may be most relevant when screening existing buildings for occupancy in facility siting. In cases where SDOF analysis incorrectly indicates an existing building is not suitable for occupancy and requires extensive retrofit or replacement, refinement of the analysis with MDOF methods can remove conservatism and may indicate sufficient capacity for safe occupation. The authors have developed a simplified MDOF formulation and tool for the evaluation and design of structures subjected to blast loading. The results in some cases significantly reduce conservatism when compared to the traditional SDOF approach. This paper intends to provide a comparison of the methodologies for blast-resistance adequacy based on analysis of structural rather than components basis. P-I diagrams are developed and shown for common cases.