Calculating Fire Resistance

By Ronald L. Geren, RA, CSI, CCS, CCCA, SCIP

If you’ve ever worked on or with construction documents that involve fire-resistive construction, you’re probably familiar with UL design numbers, GA file numbers, or building code prescriptive item numbers. These numbers identify construction assemblies that have been specifically tested for fire resistance. In order to be approved, an ass embly must be installed as tested; any modification will likely be disapproved by the building official. However, in today’s construction, some unique assemblies can’t be identified using a previously tested assembly number; and having a special test performed for a project-specific assembly may break the budget. Fortunately, there are other methods to determine the fire resistance of an assembly without the financial impact and still comply with the building code.

Calculating fire resistance has been a part of model building codes for many years. The Uniform Building Code provided UBC Standard 7-7 which established criteria for calculating fire resistance of steel, concrete, wood, and masonry. The International Building Code has taken UBC Standard 7-7 and integrated much of the document directly into IBC Section 721.

Section 721 provides methods for calculating fire-resistance for individual construction materials and components that are integrated into a fire-resistive assembly. The fire-resistive time periods provided in this code section are based on historical testing of materials using ASTM E 119, which is an essential requirement of allowing alternate methods for determining fire resistance per Section 703.3: “The application of an y alternative methods listed in this section shall be based on the fire exposure and acceptance criteria specified in ASTM E 119.”

Specifically, the IBC provides al ternate methods for calculating fire resistance in concrete, concrete masonry, clay brick and tile masonry, steel assemblies, and wood assemblies. The intent of this article isn’t to explain in detail the various methods of calculating fire resistance for each material and assembly, but, rather, to provide a broad overview of what’s available in the code and to cover some general concepts in calculating fire resistance.

Concrete Masonry

We’ll start first with concrete masonry since it’s a common material used in fire-resistive construction throughout the country, especially in single-story to low-rise type structures. Concrete masonry uses portland cement and aggregates to obtain its fire-resistive qualities; however, it’s the aggregate that has the greatest impact on a concrete masonry unit’s fire-resistance rating.

The IBC divides aggregates into four categories: pumice or expanded slag; expanded shale, clay or slate; limestone, cinders or unexpanded slag; and, calcareous or siliceous gravel. These are listed from lightweight aggregates to normal weight aggregates. As the weight of the aggregate increases, the fire-resistive rating of concrete masonry decreases; thereby, requiring thicker walls to achieve an equivalent fire rating when comparing lightweight masonry to normal weight masonry. Speaking of wall thickness…

The thicknesses provided in the IBC are the “equivalent thicknesses” required, and not the actual thicknesses. Since concrete masonry is considered hollow, the equivalent thickness is the effective fire-resistive thickness determined by dividing the net volume of the unit by the product of the length and width of the unit. For example, a standard 8x8x16 concrete masonry unit, with a net volume of 563 cubic inches, will have an equivalent thickness of 4.73 inches.

With the equivalent thickness and type of aggregate material at hand, consult Table 721.3.2 to determine the fire-resistance rating. If the masonry is made of expanded slag aggregate, the fire-resistance rating would be at least 4 hours. If an equivalent thickness is between two thicknesses in the chart, the fire-resistance rating can be interpolated. In the case of mixed, or “blended,” aggregates, then the ratios of the various aggregates used are app lied to the individual fire-resistance ratings for each aggregate to achieve the overall unit fire-resistance rating.

Brick and Tile Masonry

The fire-resistance of brick and tile (clay) masonry is similar to that of concrete masonry, by utilizing equivalent thicknesses in the calculation. However, because of clay masonry’s limited material composition, the shape of the brick--hollow versus solid, filled versus unfilled--has a significant impact on the fire-resistance rating. In brick terminology, solid really isn’t solid. To be considered solid, the cores of the brick cannot exceed 25% of the surface in the plane containing the cores.

Steel

Steel construction, on the other hand, has limited resistance to fire, although it is considered a noncombustible material. Steel loses its tensile strength very quickly as it absorbs heat from a fire. This has been a major concern investigated by the National Institute of Standards and Technology (NIST) following the World Trade Center collapse in 20011. At 1,022 degrees F., steel will lose approximately 50% of its yield strength; a temperature that can be reached in less than 10 minutes in a fire with a normal fuel load such as that in a typical office, leading to imminent collapse.

To protect steel from this premature collapse, steel members must be insulated from fire. Several methods are provided in the IBC, including concrete encasement, spray-applied fire-resistant materials, masonry protection, and gypsum board protection. The key to calculating steel fire-resistance protection is understanding the weight-toheated- perimeter ratio, commonly referred to as the W/D ratio.

The W/D ratio for a steel shape is determined by dividing the weight per linear foot (W) by the exposed surface area of the steel member (D); the higher the ratio, the greater the member’s fire-resistance, thus requiring less protection when calculating ratings for the various types of protection. The ratio can be increased by using a heavier member, or using a member with reduced exposed area (steel tube versus wide flange). The IBC provides some W/D ratios for common wide-flange shapes, as do some of the manufacturers of spray-applied fire-resistant materials.

Concrete

Like concrete masonry, concrete is a very fire-resistant material, which is why it is used throughout most Type I and II mid- to high-rise structures. The fire-resistance characteristics of concrete includes those of concrete masonry (they’re both cement-based materials) and steel (used for reinforcing). However, concrete has a unique characteristic that isn’t common in most other fire-resistant materials: the ability to shape the material into a variety of precast and cast-in-place shapes. Concrete, used commonly for floor systems and structural frames, has been thoroughly tested in a variety of horizontal applications. For any material, the fire exposure in a horizontal application is more severe than in a vertical condition. Therefore, based on its performance in horizontal fire endurance tests, concrete can be expected to perform equally or better in a vertical, or wall, condition.

Like masonry, concrete’s fire-resistance is based on the type of concrete aggregate used and the equivalent thickness. But unlike masonry, the unique shapes provided by concrete construction require different calculations to determine the equivalent thickness based on the type of shape. These calculations can be somewhat daunting at first, but once you understand the formulas, the equivalent thickness can be quickly and easily derived. After determining the equivalent thickness, the fire-resistance ratings are determined using Tables in the IBC. For walls, the fire-resistance ratings are determined by Table 721.2.1.1, and Table 721.2.2.1 for floors and roofs.
For horizontal construction, the steel reinforcing becomes critically important--just as with structural steel. And, like concrete encased structural steel, the concrete insulates the steel reinforcing from the extreme temperatures of a fire. Therefore, adequate coverage of the steel reinforcing needs to be provided. IBC Table 721.2.3(1) provides the minimum cover for steel reinforcing based on aggregate type, required fire-resistance rating, and restrained versus unrestrained conditions. You can obtain the latter from your structural engineer.