Key considerations for fire protection of seismic isolation bearings

Seismic isolation bearing insights

• Seismic isolation bearings require specialized fire protection methods, as traditional fireproofing materials are inadequate due to the lower failure temperature of the rubber components.
• Fire protection methods for seismic isolation bearings, such as flexible blankets, must be tested using standards like ASTM E1725.

Protection of buildings from earthquake damage is a primary goal of structural engineering.  Indeed, ASCE 7: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, devotes more than ten chapters to seismic design.

Structural engineers use a variety of methods to protect buildings against earthquakes, including structural reinforcement, vibration damping and base isolation. Base isolation involves installing devices that allow a building, or portion of a building, to move horizontally with respect to the ground. These devices are known as seismic isolation bearings.

Seismic isolation bearings are designed to allow horizontal movement while limiting vertical movement. Seismic isolation bearings may incorporate alternating layers of rubber and metal, typically lead, in a layer cake orientation.

In addition to resisting seismic forces, many buildings must withstand fire exposure. The International Building Code (IBC) regulates the allowable height and area of buildings based on the construction type and the occupancies in the building. As buildings get larger in height or area, the IBC requires buildings to be noncombustible construction and/or have higher levels of fire resistance.

The IBC generally requires noncombustible building elements, such as steel or concrete, in Type I or II construction. IBC defines a range of construction types (I-V), with the highest degree of protection being Type I. Buildings of Type III, IV or V construction are permitted to have combustible building elements including dimensional lumber, mass timber and composite wood materials.

Fire resistance is measured in units of time, generally hours. The hourly fire resistance rating is a measure of the amount of time that a building element was able to perform in the standard fire test specified in ASTM E119. The term “fire-rated” is a general term for components and assemblies that have passed a standard fire test referenced by the building code.

The IBC requires certain walls and horizontal assemblies, such as floors, to have fire resistance ratings. For example, stairway enclosures and elevator shafts are required to be enclosed by fire-rated walls and horizontal assemblies. Depending on the construction type classification of a building, floors may be required to have a fire-resistance rating. The IBC requires any supporting construction for the walls and horizontal assemblies to have the same fire-resistance rating as the walls or horizontal assemblies that they support. This is so that supporting members do not fail before the elements that they support.

The fire exposure shown in Figure 1 is based on the “fire load concept” that was developed in the 1920s. In addition to specifying a fire exposure, the ASTM E119 standard identifies what constitutes acceptable performance.

Figure 1: A standard time temperature curve based on ASTM E119. Courtesy: Jensen Hughes

The fire resistance is evaluated by constructing a prototype of a building element, such as a floor, beam or columns, in a large furnace. The protection that is used to achieve the required fire resistance is applied to the prototype that will be tested in the furnace. One method of achieving fire resistance includes sprayed fire resistance materials, commonly known as fireproofing, such as fibrous, cementitious or intumescent coatings. Some building elements, such as those made of concrete or mass timber, may achieve a fire-resistance rating on their own without any supplemental material protection.

Acceptable performance for columns in the ASTM E119 standard is judged by the ability of a loaded column to sustain an applied load. Alternatively, the performance of a non-loaded steel column can be deemed acceptable if the average temperature on the surface of the column does not rise by more than 1,000 F and the temperature rise at any point does not increase by more than 1,200 F throughout the test.

The allowable temperature rise permitted by the ASTM E119 fire exposure is based on the ability of a column to support loads. Steel loses approximately 50% of its yield strength at a temperature of 1,000 F, and the modulus of elasticity decreases by approximately 40%.

The IBC requires fire protection of seismic isolation bearings because they are in the load path of building elements required to have fire resistance ratings.

The IBC requires seismic isolation bearings to have the same fire resistance ratings as the assemblies that they support. It also requires the protection (e.g., fireproofing) to withstand the expected movement of the seismic isolation bearings.

Depending on the location of the seismic isolation bearings, fire resistance may not be required. For example, seismic isolation bearings that are installed underneath a building (where a fire could not occur) would not require fire resistance. Conversely, seismic isolation bearings that are installed beneath columns inside a building would require fire resistance.

The rubber components of seismic isolation bearings fail at a lower temperature than steel does. Specifically, the rubber fails at a temperature of approximately 350 F – 400 F. Supplementary material protection is required since the seismic isolation bearings will not achieve a fire-resistance rating without protection.

The typical methods used for fire protection of steel beams and columns (such as spray-applied fire-resistive materials or intumescent paint) are not adequate for the fire protection of seismic isolation bearings. The allowable temperature rise of the protected steel beams or columns at the end of a standard fire exposure is higher than the failure temperature of the rubber components in the seismic isolation bearings.

Flexible blankets vs board products

There are several products available marketed for the protection of seismic isolation bearings from fire. These products generally fall into the following categories: (1) flexible blankets or (2) board products.

Board products are rigid. If board products are used, then they must be installed to remain in place during the anticipated movement of the seismic isolation bearings. Flexible blankets are not rigid, so they can move more easily with the seismic isolation bearings.

Additionally, regardless of which type of product is used, it is important to review how it was tested. If the product literature identifies a fire-resistance rating in accordance with the ASTM E119 or similar standard fire test, then the product may not be suitable for protecting seismic isolation bearings that have rubber components.

Electric cables are a commonly applied protection method for seismic isolation bearings. Cable protection is tested in accordance with ASTM E1725: Standard Test Methods for Fire Tests of Fire-Resistive Barrier Systems for Electrical System Components. The maximum temperature rise at the end of the ASTM E1725 fire test is an average of 250 F, and the maximum peak temperature rise at any single location is 325 F. The allowable temperature rise on the unexposed side in ASTM E1725 is low enough to protect the rubber components in the seismic isolation bearings.

Figure 2: The fireproof detailing of a seismic isolator. Courtesy: Jensen Hughes

A blanket product was used to protect seismic isolation bearings on a recent project shown in Figure 2. Because the seismic isolation bearings supported two‑hour rated construction, a two-hour fire resistance rating was required by the IBC.  The design used was based on an assembly that had been tested in accordance with ASTM E1725.

Fire modeling was used to evaluate the protection method shown in Figure 2 with the fire dynamics simulator model (version 6.9.1-0). The standard ASTM E119 fire exposure was modeled. The blanket product was represented as a one dimensional solid with a heat transfer perpendicular to the surface, but no heat transfer parallel to the surface.  The model also used proprietary material property data provided by the manufacturer.

Figure 3: The temperature at the surface of seismic isolation bearings modeled. Courtesy: Jensen Hughes

The modeling showed that the temperature on the seismic isolation bearings’ surface will not exceed 212 F during two‑hour fire exposure.

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