|Required Fire Resistance Ratings for Structural Building Elements|
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
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:
Direct applied systems include:
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:
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
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.
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
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.
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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