Fire Resistance Provides Critical Time

Fire resistance is the ability of a structural assembly (walls, floors, roofs, beams, and columns) to successfully withstand a severe fire for a given time without collapse and without exceeding certain temperature limits. The intent of such fire resistive construction is to support or serve as a fire safety barrier in buildings in order to control fire propagation and heat transmission to adjacent compartments without structural failure. Fire resistive construction will often necessitate use of structural fire protection. The furnace fire testing procedures and traditional acceptance criteria for fire resistance in the US have been specified within the ASTM E 119 standard. The resulting time duration during which the construction test assembly exhibits adequate structural integrity, stability and temperature control on the unexposed surface and for any steel elements that may be contained within the assembly is expressed as a fire resistance rating from one to four hours. This rating time quantifies the relative ability of the assembly to withstand the standard fire in comparison with other assemblies or elements. Published listings of generic and proprietary fire resistive assemblies are available for various construction materials (structural steel, concrete, masonry, masonry or wood) and fire protection products. (spray-on materials, gypsum board, intumescent coatings, concrete encasement, and the like) These prescriptive ratings inherently contain the assumptions and limitations of the standard test, which includes a single fire exposure under maximum design loads for a prototype assembly of limited size. Effects of different fires, partial loads, and larger assemblies with structural continuity cannot be evaluated in this manner. Since prescriptive-type fire resistance ratings based on fire test standards are not predictive and may be inappropriate for some design conditions, alternative performance-based engineering methods for demonstrating fire resistance adequacy are available. These are based on more realistic analytical models of heating and structural response to a design-basis fire scenario in lieu of the conventional use of prequalified fire test assemblies or their derivative empirical correlations.

Development and Applications of Fire Resistance Ratings

Picture_Section 1-Sevices_Risk and Hazard Analysis_Modeling_Fire ResistanceJENSEN HUGHES’ has provided technical assistance to numerous Clients to develop or modify various types of fire protection products, systems, and rated construction assemblies, mostly through standard laboratory testing and some through analyses.  This work included not only the basic construction assemblies for fire resistance rated walls, floors, roofs, beams, and columns, but also their supplementary counterparts for firestopping of through-penetrations, joints, and perimeter fire barrier systems. As project consultants, we have recommended use of certain fire resistive assemblies and/or fire protection products in both new and existing buildings based on code and architectural requirements for best functionality, weight and efficiency. In cases where the installed assembly will differ from its referenced fire resistant design, JENSEN HUGHES has conducted many studies to review and assess the differences. When a rating or construction change or a product substitution in a given rated assembly listing can be justified, a written Engineering Judgment with suitable documentation for approval submission request to the authority having jurisdiction is provided.

Code Compliance Reviews

Picture_Section 2-Sevices_Risk and Hazard Analysis_Modeling_Fire ResistanceReviews of fire resistive construction for code analysis and compliance in existing buildings and other structures have been frequently conducted. These needs may arise because of changes in the building code requirements, identified problems or for due diligence purposes during ongoing repairs, upgrades, or property sales. Sometimes, these reviews may evolve into broader forensic investigations of the installed fire protection that transcend visual observations and progress into more in depth analyses and/or testing. Performance-based Design This alternative approach to prescriptive fire resistive design requires use of more advanced simulation of the site-specific natural fire scenario, computation of its heating effects on the structural members and the resulting structural response under gravity load. It may be applied to an entire building, but to date usually has been limited to specific structural elements (such as specific columns, frequently of concrete-filled steel tubular sections), subassemblies (such as floors or trusses), or exposure to exterior fires.  The objective of this type fire resistive analysis is to demonstrate that the critical structural components or framing will not collapse during the worst-case fire exposure expected for the building and its occupancy. Performance-based design typically entails the use of fire modeling, heat transfer and structural analyses.

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