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February 24, 2022
In a world aiming toward sustainability and risk mitigation, the traditional prescriptive method to structural fire safety is becoming less and less viable. Fortunately, new industry resources can benefit the AEC sector in this respect.
Structural fire engineering design has the potential to create a building stock that is more resilient and reliable to uncontrolled fire events, while also providing a wide array of opportunities for engineers to add tremendous value to building projects by specifying structural fire safety more intelligently.
HISTORY OF FIRE SAFETY
More than 100 years ago, it was postulated that if the temperature of individual structural members could be maintained below a specific critical temperature, then the entire structural system will fulfill its function during fire conditions.
Within this framework, all that is required from the structural engineer in terms of structural fire safety is consideration of ambient temperature hazards (e.g., hurricanes and earthquakes) if the prescribed level of thermal insulation (fireproofing) is uniformly applied to structural elements. With the purpose of protecting structures from fires, different approaches towards insulating structural elements were developed throughout this century. These approaches are standardized by means of standard furnace testing (ASTM-E-119) and are presented in building codes (notably the IBC) as required fire resistance ratings.
Simply put, the building code specifies the required fire resistance rating, and designers primarily use guides and tables to define the product and required insulation thickness for structural elements. In this context, the skills of structural engineers and fire protection engineers do not appreciably come to bear.
Accordingly, structural systems that have been optimized for ambient design loads are later blanketed throughout with fireproofing without consideration of the relation between the two in terms of actual performance. Unfortunately, project stakeholders may be left wondering if the intended structural fire safety is provided and if a rational use of resources (both physical and intellectual) was employed. This differs from almost all other aspects of building design in which these aspects are taken extremely seriously.
VISION FOR FIRE SAFETY
In response to new and rapidly evolving building construction trends, designers have increasingly sought an alternative to the traditional prescriptive approach. Encouragingly, reliability-based structural design for fire effects (referred to as “structural fire engineering design”) is now establishing itself in the U.S. as a distinct engineering discipline which can fill this void.
Notably, newly developed industry consensus guidance contained within ASCE/SEI 7, ASCE/SEI Manual of Practice No. 138, the SFPE International Handbook of Structural Fire Engineering and the freely available ASCE/SEI Structural Fire Design Guide provide designers the framework to legitimately practice SFED in the U.S., as well as provide building officials a potent set of tools to properly evaluate such designs.
The free ASCE/SEI Structural Fire Design Guide explicitly demonstrates the proper execution and potential benefits of SFED as an alternative to the traditional prescriptive method. This project included the analysis of four real, anonymized buildings by 40-plus structural and fire engineers with a review panel of academic advisors.
The scope of each analysis included the characterization of uncontrolled fire exposure within building spaces, the associated thermal response of structural elements, and the resulting structural system response per the provisions of ASCE/SEI 7. Each design team evaluated the demands on the structural systems under fire exposure, considering fire effects such as induced forces at structural connections due to restrained thermal expansion and contraction.
Furthermore, each design team evaluated the capacity of the structural systems under fire exposure, considering many aspects of fire robustness such as the influence of floor slab compressive-tensile membrane action.
The above-described analyses identified key structural system vulnerabilities under fire exposure, which would not have been revealed by the traditional prescriptive method. The analyses also revealed that in situ fire effects (such as restrained thermal expansion) are significantly more consequential to the provided performance levels than the temperature of structural elements, which is a focal point of the traditional prescriptive method. To provide the targeted level of performance, each design team developed designs with rationally allocated fireproofing and modest structural enhancements, where necessary, to provide robust structural performance under fire conditions.
At the same time, SFED was demonstrated to have the potential to enhance project economics, carbon footprint, aesthetics, quality control, site conditions, and life-cycle maintenance especially when harnessed with performance-specified off-site application of fireproofing, which can reduce construction time. Accordingly, it is envisioned that the emergence of SFED in the U.S. will change the way that project stakeholders view the optimization of a structure.
Kevin LaMalva is principal analyst and director of structural services at Code Unlimited.