Specifying intumescent coating film thicknesses

by sadia_badhon | March 25, 2019 11:00 am

by Bob Glendenning

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Passive fire-protection coatings, or intumescent coatings, have been designed to buy time for building occupants—and the steel-framed structures themselves—during a fire. Applied to structural steel, the coatings react chemically in fire, forming a char that expands with heat exposure, much like the reaction taking place when lighting a black snake firework (Figure 1). The coatings swell to approximately 50 times their dry film thickness (DFT), to a maximum of 50 mm (2 in.), thereby providing a thick layer of insulation ‘char’ that reduces the rate of heat transfer to the structural steel.

Slowing the rate of heat transfer is important, as structural steel under load can quickly lose strength in a fire. Exposure to high heat for a specific period can cause the steel to eventually reach its critical failure temperature. At this state, the steel could start to collapse, possibly bringing down large building sections or even the entire structure.

Intumescent coatings help building owners avoid such catastrophic losses by providing fire resistance while responders work to contain a fire. They offer a theoretical amount of time the coated steel can resist fire before reaching its critical failure temperature, as proven within a furnace per criteria found in various test standards such as:

• the American National Standards Institute/Underwriters Laboratories (ANSI/UL) 263, Standard for Fire Tests of Building Construction and Materials;
• ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials;
• International Organization for Standardization (ISO) 834, Fire-Resistance Tests – Elements of Building Construction; and
• British Standard (BS) 476, Fire Tests on Building Materials and Structures (Figure 2).

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This time period or fire-resistance rating is based on the applied thickness of the coatings. The thickness is determined by the size, weight, and heat exposure of each structural steel member.

In order to assist engineers, architects, and other building professionals achieve a specified fire-resistance rating, ANSI/UL 263 and ASTM E119 list the required coating thicknesses for structural steel members from large to small. However, the specifications do not include every possible size, leaving data gaps especially for very small and large steel sections. This presents challenges for specifying proper coating DFTs.

Dangers of using extrapolated data

To ensure safety as well as compliance with building codes, specifiers must stipulate the appropriate coating thickness to apply to steel sections of various sizes. However, the thickness can be difficult to determine when a particular-sized steel section is not listed in the ANSI/UL 263 or ASTM E119 dataset.

Small, lightweight steel sections need a higher film thickness to achieve the desired protection compared to larger, heavier assemblies. However, as noted above, the heat exposure of each structural steel member also influences the coating thickness required to achieve a specified fire-resistance rating. The thickness is most accurately based on the steel weight (per lineal foot)/fire exposed steel perimeter (W/D) ratio of the steel section (i.e. the ratio of the section’s weight [W] to the total square area in contact with fire [D]). The D value is an important variable, as some steel sections are fully exposed to fire, while others are protected on one or more faces due to their orientation and mounting.

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When UL data is not available for a particular structural steel section, some coatings suppliers make recommendations based on extrapolated data. However, UL has said extrapolated DFT data is outside the scope of its intumescent coating certification program. This noncompliant data is considered unsafe for the following two reasons:

1. The extrapolated data might underestimate the required coating thickness, resulting in a steel section with insufficient coating DFT to achieve the desired fire resistance. These kind of sections are likely to reach their critical failure temperatures sooner than other properly coated areas.
2. Conversely, the extrapolated data might overestimate the required coating thickness, thereby causing the steel section to have a higher coating DFT than needed. In this case, the weight of the expanded char could cause the coating to delaminate and fall off, exposing the steel directly to fire without any protection in place.

Given these potential hazards, it is advisable for specifiers to work with a coatings supplier to find a safe, workable alternative and not rely on extrapolated data when encountering structural steel section sizes outside of UL’s listing.

Why does data extrapolation happen?

Building fire-resistance ratings are expressed in half-hour increments from one to three hours. In the United States, one- and two-hour ratings are common. The specified rating for a building depends on multiple factors, including building codes, type of construction, owner preferences, and insurance regulations.

When designing buildings, engineers and architects must comply with fire-resistance codes defined in ANSI/UL 263 and ASTM E119, as well as any requirements set forth by local municipalities, state governments, building owners, and insurance firms, while minimizing costs. Therefore, lightweight steel sections are usually employed. However, design professionals might not realize some sections are smaller than those tested and listed in ANSI/UL 263 and ASTM E119. Due to lack of data, such sections will not have a defined DFT for applying intumescent coatings to achieve the required fire-resistance rating. In such cases, the parties involved may unwisely consider extrapolating data to determine the appropriate thickness.

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UL has advanced its position on discouraging the use of extrapolated data in recent history, publishing stronger language against the practice. In its 2014 “The Fire & Security Authority” publication and its category code number publications—BXUV and CDWZ—that provide guidance for 263-compliant fire-resistance ratings compliant with ANSI/UL 263, UL said the average thickness of an intumescent coating “should not exceed the maximum thickness published in the individual [steel section] designs.” In 2017, UL published the following fire-resistance guideline as part of the ANSI/UL 263 BXUV guide emphasizing the need to avoid extrapolating data:

Extrapolation of member size and/or material thickness shown in the individual designs[5] has not been investigated and would be considered to void the existing certified assembly.

Finally, in its strongest language yet, UL stated the following as part of its “Best Practice Guide for Passive Fire Protection for Structural Steelwork,” published in October 2018:

Extrapolated thicknesses that are beyond the scope of the published UL design without additional supporting test data are not considered acceptable. Additionally, extrapolated material thicknesses that are beyond the published UL design are not recognized by UL and are considered outside the scope of the UL Certification.

When engineers or architects find data is unavailable for a steel section, they have two options:

• consider an alternate steel size or profile; or
• use advanced structural engineering design principles from the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7-16, Minimum Design Loads For Buildings and Other Structures, Appendix E guidelines.

This article only focuses on specifying different steel sections as the ASCE/SEI guidelines—that consider the steel strength supporting the structure and the reserve strength available to resist fire—deserve a stand-alone article.

Understanding steel section data

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UL publishes a variety of categories for rating the fire resistance of steel sections, with each one having different test and pass criteria. Additionally, UL’s coating DFT requirements differ for various steel member shapes and orientations. Due to both factors, it may be unsafe to use the published maximum intumescent coating thickness in one UL category for another listing.

Each steel type (e.g. beam, column, or brace) listed in UL’s database has a ‘section factor’ to help determine the required intumescent coating DFT for meeting various fire-resistance ratings. This value is a ratio that differs depending on the style of the steel section as well as its exposure to fire. The section factor ratio W/D covers I-beam (or W-profile) sections, while the ratio A/P represents hollow structural sections (HSS) such as cylindrical (pipe) columns. The variables refer to:

• W: The weight of the section (in pounds/foot);
• A: The cross-sectional area of all sides of the HSS (in inches); and
• D and P: The heated perimeter of the section (in inches) or the total square area that would be in contact with fire.

See Figure 3 for an example of a fully exposed W-profile. Here, D represents the section’s entire surface area because it would all be in contact with direct heat from a fire. However, in Figure 4, D only includes the surface area of the steel that is not in contact with the concrete ceiling/slab above the beam. The ceiling—or any material in contact with or partially encasing a steel section—serves as a heat sink in a fire, taking heat away from the coated steel section and offering some fire resistance. In Figure 5, A is the entire surface area of an HSS section, minus any areas in contact with heat sinks.

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The W/D and A/P ratios represent how quickly a steel section will heat up in a fire. One can calculate these ratios by dividing the weight (W) or area (A) by the heated perimeter (D or P). The A/P ratio can then be converted into a W/D ratio to directly compare W-profile and HSS sections.

The amount of fire protection (intumescent coating DFT) required for a steel section is inversely proportional to its section factor ratio. A larger ratio indicates less fire protection is required, and a smaller one requires the application of thicker fire protection. The examples shown in Figures 3, 4, and 5 reveal the smaller W/D and A/P ratios at the top require a greater coating DFT. They also demonstrate coating DFT requirements increase with longer fire rating durations.

It is helpful to make some direct comparisons to understand why it is impossible to use W/D and DFT data from one UL category listing to the next even when the steel section is of the same size.

In Figure 4, the required intumescent coating thickness for a two-hour fire rating for the W10x39 Beam N section is 4 mm (161 mils) DFT (marked in red). When looking at the same-sized W10x39 Column Y section in Figure 3 (marked in red), one will find a smaller W/D ratio that equates to the higher DFT requirement of 5 mm (198 mils) for the same two-hour rating. The Column Y section DFT is 23 percent greater than the Beam N requirement, even though the two sections are of the same size. This difference is because Beam N is in contact with concrete on one face, making its heated perimeter much smaller.

Further, the coating DFT requirements are drastically different for HSS columns requiring significantly higher intumescent coating DFTs because of their structural profile. In Figure 5, the A/P ratio has been converted to W/D for simpler comparison. The 10 x 10 x ¼ HSS Column Y has the same column section factor and a similar size to the W10x39 Column Y in Figure 3 (see red text for both). Both sections are 250 mm (10 in.) deep and have a similar weight per foot. Yet, the HSS Column Y section requires an 8-mm (309-mils) DFT for a two-hour fire rating. This value is 92 and 56 percent greater than the W10x39 Beam N and Column Y DFT requirements, respectively.

Why extrapolated data does not add up

The data limitations on the lower and upper ends of steel member sizes listed in ANSI/UL 263 and ASTM E119 are a result of UL either not testing such steel sections or determining they are unable to be protected with intumescent coatings. As mentioned, extrapolating data beyond the published limits could result in specifying an intumescent coating DFT that is either too low or high, both could result in insufficient fire protection. However, UL has offered some flexibility in specifying coating thicknesses for steel sections in between the established lower and upper size limits.

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UL considers the following two scenarios acceptable in its revised fire-resistance rating guidance documents.

1. Using the minimum listed coating DFT for a particular beam size (specific W/D) on a larger steel section (greater W/D) that has a greater heat sink than the listed steel section. (The heat sink would provide some fire resistance, allowing the minimum DFT to provide a similar fire-resistance rating.)
2. Substituting a steel member for a heavier weight (greater W/D) section using the same specified coating thickness. (The heavier steel section could actually require less coating thickness for the same fire-resistance rating but using the same DFT will certainly provide the same or better protection.)

Conversely, UL lists the following scenarios as unacceptable because the applied coating may be too thick, creating the potential for delamination from the steel during a fire:

• using a coating DFT specified for a larger steel section to cover a smaller steel section with a lower W/D than is listed; and
• substituting a steel member for a lighter weight (lower W/D) section using the same specified coating thickness.

Figures 6 and 7 demonstrate these points for engineers and architects specifying the intumescent coating DFT for a given steel section W/D ratio. In both diagrams, one simply needs to stay within the green areas and stay out of the red sections. In this author’s experience, any point on or below the blue lines is fine.

In both figures, the blue lines represent the maximum allowed data points in ANSI/UL 263 and ASTM E119. The blue line in Figure 6 terminates at a W/D ratio of 0.40, the lightest steel listed in ANSI/UL 263. The required coating DFT is plotted on the X-axis in relation to the steel section’s W/D ratio on the Y-axis. For example, the blue dot shows a steel section with a W/D ratio of 0.40 requires a 6-mm (230-mils) DFT for a two-hour fire-resistance rating. The left-hand green dot shows a steel section with a 0.55 W/D ratio would require a minimum DFT of  5 mm (200 mils). However, the right-hand green dot reveals the 0.55 W/D ratio section could be coated up to a 6-mm DFT without worry. The DFT cannot exceed 5 mm for either of these steel sections because the data is not included in UL’s listing. Therefore, per UL guidelines, specifiers cannot extrapolate the data to a lower W/D ratio (red X) or to a higher DFT (orange X).

Enhancing Aesthetics and Protecting Against Corrosion

Per building fire-resistance codes defined in the American National Standards Institute/Underwriters Laboratories (ANSI/UL) 263, Standard for Fire Tests of Building Construction and Materials, and ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, and enforced by local municipalities, state governments, building owners, and insurance companies, all structural steel members of a building must be coated with the appropriate intumescent coating thickness based on final specifications. Coatings can be applied either in the field or offsite in a controlled facility. After the coatings have cured, applicators will use an electronic gauge to determine the resulting DFT. If insufficient, they will apply additional coating material to reach the specified thickness. Following the application of intumescent coatings on exposed structural steel, architects often choose to apply a topcoat for a more aesthetically pleasing or colored finish. Nonexposed steel sections may also need to be covered with a protective coating in areas where durability is a concern (e.g. areas prone to corrosion due to exposure to weathering or wet/dry cycling). It is important to note the addition of a thick topcoat or too many layers can prevent the intumescent coating underneath from activating in a fire. This is not a concern for new construction, but it must be considered when recoating steel. Engineers, architects, and coating suppliers need to carefully plan to mitigate this potential situation.

When working in the other direction and looking at stronger/heavier steel sections, the same principle holds true, as shown in Figure 7. Here, the blue dot represents the lowest W/D listed in ANSI/UL 263 and ASTM E119. The two-hour DFT requirement at this W/D ratio of 1.74 is 2.5 mm (98 mils). Per UL guidelines, it is acceptable to coat sections with a greater W/D ratio. For example, 1.8 (green dot) with the same minimum 2.5-mm DFT. However, UL does not permit specifiers to extrapolate a reduced DFT for stronger steel sections (orange X) because it has not tested sections beyond the 1.74 W/D ratio. Additionally, UL does not allow specifiers to extrapolate data for lighter steel sections (red X). Specifiers must instead follow the blue line up to match a lower W/D ratio with the correct minimum DFT.

Conclusion

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When a steel-framed building catches fire, the fate of the structure—and the safety of occupants in, on, or around the structure—may come down to a layer of passive fire-protection coatings. To ensure the steel has the appropriate fire-resistance rating, engineers and architects must specify the intumescent coating DFT for each steel section. This specification should only rely on UL’s published data, and not extrapolated data, to avoid the risk of applying too little or too much coating material. As such, UL is reminding coating suppliers, subcontractors, structural engineers, architects, and fire engineers to avoid extrapolating its data. UL first published an updated position on the issue in 2014 and added language to the ANSI/UL 263 and ASTM E119 specification in 2017. Last October, UL said extrapolated thicknesses beyond published UL designs are not considered acceptable without additional supporting test data and are therefore outside the scope of the UL certification.

Intumescent coatings are universally accepted for passive fire protection applications. However, when UL data is unavailable for a particular structural steel section, one must be aware of the recommendations to employ an intumescent coating thickness based on extrapolated data. It is advisable to follow UL guidelines and find a safe, workable alternative to arrive at the proper DFT specifications. In many cases, advanced fire engineering design principles can help to resolve issues associated with unlisted steel sections. However, the applicator and the manufacturer must make the ultimate decision on the applied DFT for a given steel section.

Intumescent coatings are not the only option for passive fire protection. Alternative building systems include cementitious fire-resistive materials and fire-rated boards, either of which may be used in combination with intumescent coatings. These passive materials work differently than coatings by providing a physical barrier of cement or gypsum, respectively, to slow down the transfer of heat to the steel substrate underneath the materials. These materials present their own specification challenges.

Bob Glendenning is a structural engineer and the global fire engineering manager for the fire engineering and estimation team at Sherwin-Williams Protective & Marine Coatings that supports the specification of engineered fire-protection solutions based on simple and complex calculations, as well as inputs from building information modeling (BIM) software. Glendenning spent more than 20 years in the steelwork industry before joining Sherwin-Williams to lead its fire protection team 17 years ago. He can be reached at bob.glendenning@sherwin.com[10].

Source URL: https://www.constructionspecifier.com/specifying-intumescent-coating-film-thicknesses/

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