Issue 15: Required Fire Resistance Ratings for Structural Building Elements
By Arthur J. Parker, P.E., and Jesse J. Beitel
The building codes prescriptively require minimum fire-resistance ratings for structural building elements such as the structural frame, floor construction, walls (interior and exterior) and roof construction. The purpose of providing these minimum fire resistance ratings is to contain a fire to the space of origin and allow for safe occupant egress, permit safe fire fighter entry and operations and provide structural stability.
Listings for products used to provide various hourly fire-resistance ratings for all types of building elements are provided in various fire resistance directories. Two main types of protection systems are listed in the Underwriter's Laboratories, Inc. (UL) Fire Resistance Directory: membrane protection systems and direct applied protection systems. Typical membrane protection systems include:
- Ceiling grid systems;
- Mineral and fiber board products;
- Batts and blankets materials;
- Metal lathe and plaster systems; and
- Gypsum wallboard.
Direct applied systems include:
- Sprayed fire-resistive materials (SFRMs);
- Intumescent paints/coatings;
- Mastic coatings; and
- Masonry and concrete.
For most building applications, the appropriate protection system can be found in a fire resistance directory which provides the required hourly fire-resistance ratings. This is a prescriptive response to the building code requirements that many building elements be subjected to the fire exposure conditions specified in ASTM E 119 (with the exception for hydrocarbon exposure ratings for structural steel columns denoted in the XR listings).
The listings in the directories do not cover all building conditions. A means for making adjustments to the minimum SFRM insulation thickness is provided in the UL Fire Resistance Directory. Initially, SFRM manufacturers conducted standard ASTM E 119 testing of various sized columns, beams and floor/ceiling assembly configurations to determine the minimum insulation thickness required to provide a specific hourly fire resistance rating. Once the testing has been completed, listings are generated which state the minimum steel size, the particular assembly details (inclusion in a floor/ceiling assembly, restrained or unrestrained assembly, etc.), and the minimum insulation thickness for a particular hourly fire resistance rating. These prescriptive hourly fire-resistance ratings are required by the building code and are dependant on the building construction type and occupancy classification.
Empirical data developed from many full-scale fire tests has resulted in the creation of equations which can be used to adjust the SFRM thickness for beam assemblies (restrained and unrestrained) and columns. These equations are located in Sections IV (beams) and V (columns) in the commentary found in the UL Fire Resistance Directory. The SFRM thickness for alternate sized beams may be substituted for the minimum beam sizes contained in restrained and unrestrained designs of the same shape using an equation relating the thickness to the W/D ratio.
The W/D ratio is defined as the weight per unit length (W) of the steel member tested divided by the heated perimeter (D) of the member. Steel members which have a lower W/D ratio than the referenced or tested steel member require an increased fireproofing thickness. Conversely, steel members with a higher W/D ratio than the referenced steel member may have the SFRM thickness reduced.
The UL Fire Resistance Directory identifies several limitations of the beam substitution equation:
- W/D values shall not be less than 0.37 lb/ft2 (8.8 kg/m2);
- Adjusted SFRM thickness shall not be less than ⅜-inch (10 mm);
- Restrained and unrestrained hourly rating shall not be less than 1 hour; and
- The substitution equation is applicable for SFRMs only and not to intumescent or mastic materials. Similar equations are, however, contained in some intumescent and mastic materials listings to adjust the required thickness based on different steel sizes (within specific limits).
For columns, the SFRM thickness can be also be adjusted. The minimum SFRM thickness can be adjusted for wide flange steel sections smaller than specified using an equation that relates the SFRM thickness to the W/D ratio for the specified steel section and the alternate sized steel section. The SFRM thickness is not to be reduced for columns having a larger W/D ratio than the specified steel section; rather it conservatively remains as listed in the specific design. Obviously, this procedure does not account for the conservatism inherent in increasing the steel section W/D ratio and not proportionately reducing the minimum SFRM thickness.
The general equations for columns cannot be used to reduce the minimum SFRM thickness for tubular steel or pipe shapes. Equations included in the UL listings for tubular steel and pipe shape protection are applicable for adjusting the SFRM thickness for these particular shapes.
Adjustments that can be made to the minimum insulation thickness or membrane protection system are limited. The minimum intumescent material thickness values cannot be extrapolated to steel sections having W/D ratios (or cross-sectional area divided by heated perimeter – A/P) larger or smaller than described in the listings. With gypsum wallboard, the proper type of gypsum wallboard (Type X or C) must be used in the design. Dramatically different assembly performance ratings can be achieved when comparing one type of wallboard to the other. The orientation of the wallboard also has an effect on the fire-resistance rating of a system. The fire-resistance rating of a vertical gypsum wallboard assembly (e.g., wall) may be reduced for the same assembly installed horizontally (e.g., floor/ceiling assembly).
For unique assemblies, the minimum insulation thickness may be required to be determined analytically using finite element method (FEM) heat transfer models. Modeling can also be used to asses the impact of non-rated thermal shorts attached to protected structural steel members, evaluate complex shapes where calculating the W/D ratio may not appropriate for a specific configuration or the ratio is outside the bounds of the listing, determine the protection requirements for connections, or evaluate in-situ assemblies with existing protection systems. The FEM models predict the thermal response of the assembly and insulation material to a particular fire exposure condition. Inputs include a detailed description of the assembly, the insulation material, and the exposure conditions. Figure 1 shows the representative output of a three-dimensional FEM model showing the predicted the temperature gradients from a thermal short.
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| Figure 1. Three-dimensional FEM model output showing thermal short temperature gradients |
Full-scale test data is used as the basis to determining the necessary thermal properties, such as thermal conductivity, density and specific heat capacity. The full-scale test data is typically proprietary to the manufacturer or the test sponsor and may be difficult to obtain unless a close working relationship has been established. Thermal properties over the temperature ranges necessary to simulate the exposure are deduced by modeling the tested geometry and the insulation/steel interface temperature profiles. This is sometimes referred to as "calibrating the model." Figure 2 shows a typical graph of measured temperature data and the model predicted temperature profile.
Thermal properties developed using full-scale test data should always be checked against any other available thermal property data or additional test data to verify the accuracy. Once adequate thermal properties have been established, the performance of other geometries, insulation thicknesses or exposure conditions can then be predicted. The limitation of this procedure is that the thermal properties are based on a limited set of test data for a specific test condition and assembly construction. Modifications to any of these variables will introduce additional uncertainties into the modeling results.
Material thermal properties are typically required over the temperature range which would be encountered during a fire exposure, up to nominally 2,000°F (1,000°C). In some cases, the temperature range of the insulation system contained within the assembly is not subjected to direct fire exposure and the critical temperature range is lower. Generally speaking, accurate thermal parameters are able to be obtained at lower temperature than compared to the higher fire exposure temperatures. Insulation materials which are stable and undergo minor physical or chemical changes, such as ceramic fiber insulation or mineral wool batt insulation, have thermal properties that are relatively well documented. Documenting the thermal properties for materials which experience phase changes, such as intumescent coatings, mastics, concrete or gypsum wallboard, is more difficult. The effects of moisture loss, thermal decomposition, loss of mechanical or physical properties, changing thickness (such as concrete spalling or intumescents with variable char layers) and density changes create a dynamic process. These effects increase the difficulty in obtaining accurate thermal properties.
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| Figure 2. Tested and predicted average unexposed surface temperature |
As with any product, limitations in the product usage exist that must be understood by the design professional, general contractor and installer. A lack of attention to the application details for a specific fire protection product can mean the difference between a system or assembly which provides the code required fire-resistance rating and one which may fail prematurely, unnecessarily endangering the lives of building occupants and fire department personnel.
Arthur J. Parker and Jesse J. Beitel are with Hughes Associates.
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