Steel Fire Resistance — Temperature, Strength Reduction, and Protection

Unprotected structural steel loses strength rapidly at elevated temperatures. At 550 degC (1020 degF), steel retains only about 60% of its room-temperature yield strength; at 700 degC (1290 degF), it retains less than 25%. Fire protection of steel is therefore required by all building codes. The governing standards are AISC 360-22 Appendix 4, EN 1993-1-2, AS 4100 Section 12, and the IBC/ASCE fire provisions.

Steel properties at elevated temperature

Strength reduction factors (per EN 1993-1-2 Table 3.1)

Key observation: steel retains full strength up to 400 degC. The critical range is 400-700 degC where strength drops from 100% to 23%.

Thermal elongation of steel

A 30 ft beam heated to 600 degC expands approximately 2.8 inches. This thermal expansion can cause significant frame distortion and connection forces.

Critical temperature concept

The critical temperature Tcr is the temperature at which the member capacity equals the applied load under fire load combinations. For a member loaded to utilization ratio mu = Ed,fi/Rd:

Tcr = 39.19 * ln(1/(0.9674*mu^3.833) - 1) + 482    [EN 1993-1-2 Eq. 4.22]

Critical temperature by utilization ratio

Lower utilization gives higher critical temperature and less required fire protection. Design for lower mu under fire combinations can save 20-40% on fire protection costs.

Fire load combinations

Section factor Am/V (or Hp/A)

The rate of temperature rise depends on the surface area exposed to fire relative to the volume of steel:

Am/V = heated perimeter / cross-sectional area    [m^-1]

Am/V for common W-shapes (3-sided exposure, column)

Heavier sections heat more slowly (lower Am/V). A W14x730 can often achieve 60 min fire resistance with minimal protection, while a W8x10 needs substantial insulation.

Am/V for hollow sections

HSS sections have higher Am/V than W-shapes of equivalent weight because all surfaces are exposed.

Fire protection systems

Spray-applied fire-resistive material (SFRM)

Cementitious or mineral fiber sprayed directly onto steel. Most common protection method.

SFRM thickness by rating (typical, W14x48 column)

Actual thickness depends on the specific SFRM product and the steel section Am/V. Always verify with manufacturer's UL listing.

Intumescent coatings

Thin-film coatings (15-40 mils dry) that expand 20-50x when heated, forming an insulating char. Provide 1-2 hour fire ratings.

Board systems

Board systems provide clean finished appearance. Used for architecturally exposed steel where SFRM is not acceptable and intumescent cost is prohibitive.

Fire rating requirements by occupancy (IBC Table 601)

Type I-B (2 hour) is most common for mid-rise steel buildings. Type II-B (unprotected) is used for low-rise industrial and parking structures.

Worked example -- fire protection for a W14x48 column

Given: W14x48 column, 1-hour fire rating required, utilization ratio mu = 0.55 under fire load combination.

Critical temperature: Tcr = 39.19ln(1/(0.96740.55^3.833) - 1) + 482 = 39.19ln(5.22) + 482 = 39.191.65 + 482 = 547 degC.

Section factor: Am/V = 180 m^-1.

SFRM thickness: From manufacturer data for Am/V = 180 and 1-hour rating: approximately 7/8" cementitious or 5/8" mineral fiber.

Intumescent alternative: 25-30 mils DFT for equivalent protection.

Worked example -- unprotected heavy column check

Given: W14x311 column, 1-hour rating, mu = 0.25 under fire combination.

Critical temperature: Tcr for mu = 0.25 = approximately 700 degC.

Temperature after 1 hour: For W14x311, Am/V = 55 m^-1. From standard temperature curves (ISO 834 fire), unprotected steel with Am/V = 55 reaches approximately 550 degC after 60 minutes.

550 degC < 700 degC -- the column is OK unprotected for 1 hour. No fire protection needed.

This demonstrates the value of heavy sections: lower Am/V means slower heating, which may eliminate the need for fire protection entirely.

Multi-code comparison

Cross-code critical temperature comparison

All codes produce similar critical temperatures for the same utilization ratio.

IBC Fire Rating Requirements for Steel Structures

The International Building Code (IBC) specifies fire resistance rating requirements based on occupancy type, building height, and structural element type. The following table summarizes requirements from IBC Table 601:

Key IBC provisions for steel structures:

• Section 704: Methods of applying fire protection (SFRM, intumescent, gypsum encasement, masonry)
• Section 703.2: Fire resistance must be determined by ASTM E119 standard fire test or calculated per approved methods
• Section 703.3: Alternative methods include ASCE 29 analytical calculation procedure
• Type II-B and Type III-B: Unprotected steel is permitted for low-rise buildings, but local amendments may impose additional requirements

Steel Temperature vs. Strength Retention

Structural steel loses yield strength progressively as temperature increases. Per AISC 360-22 Appendix 4 and Eurocode EN 1993-1-2, the following reduction factors apply:

Critical threshold: at approximately 600 degC, steel retains only 47% of its yield strength. For a member at full utilization under gravity loads (mu = 1.0), failure occurs near 550 degC. For members at lower utilization (mu = 0.3 to 0.5), the critical temperature rises to 600--700 degC, potentially allowing thinner or no fire protection.

SFRM Thickness Calculation

Sprayed fire-resistant material (SFRM) thickness is determined by the ratio of the required thickness to the tested thickness, adjusted for the actual section factor (W/D ratio):

Per ASCE 29 and UL fire resistance directories, the required SFRM thickness for a given rating can be estimated from tabulated values based on W/D:

These are approximate values from UL design directories. Actual thickness must be specified from the specific UL design number (e.g., UL D982 for restrained beam spray-applied SFRM).

Intumescent Coating Specifications

Intumescent coatings expand when heated to form an insulating char layer. Key specifications for structural steel applications:

Advantages and limitations of intumescent versus SFRM:

Fire Protection Cost Comparison

Budget-level cost comparison for protecting a typical W24x68 beam (24 ft span) with different fire protection methods, assuming a 2-hour rating:

Cost reduction strategies per AISC Engineering Journal research:

• Designing members at 50--60% utilization ratio (rather than 90%+) can reduce required protection by one rating level
• Using heavier sections to increase W/D ratio can reduce SFRM thickness by 30--50%
• Composite construction increases the effective W/D, often reducing protection requirements

Practical tip: reducing fire protection cost

Design for lower utilization ratios under fire load combinations (1.2D + 0.5L per ASCE 7). A column at mu = 0.30 has Tcr = 660 degC versus mu = 0.60 at Tcr = 547 degC. The higher critical temperature may allow thinner SFRM, no protection for heavy sections, or thin intumescent instead of thick SFRM. This optimization can save 20-40% on fire protection costs.

Common mistakes

1. Using room-temperature capacity for fire design. Steel at 600 degC has only 47% of room-temperature yield strength. Fire load combinations must use reduced properties.

2. Ignoring the section factor. Two beams with the same load may need completely different protection thicknesses due to different Am/V ratios.

3. Specifying intumescent for 3-hour ratings. Standard intumescent coatings are typically limited to 2 hours. Board systems or thick SFRM are needed for 3-4 hour ratings.

4. Not checking connections. Connection failure temperatures may be lower than member failure temperatures due to the concentration of forces at bolts and welds.

5. Applying SFRM over contaminated surfaces. SFRM adhesion requires clean, primer-compatible surfaces. Mill scale, oil, and excessive rust prevent proper bonding.

6. Forgetting thermal expansion forces. A beam heated to 600 degC expands 9 in per 100 ft. If the beam is restrained at both ends, this generates significant axial force that can damage connections or supports.

7. Not considering localized fires. The standard fire curve assumes uniform heating. In reality, fires are localized and can create thermal gradients that cause differential expansion and distortion.

Frequently asked questions

At what temperature does steel lose half its strength? Approximately 550 degC (1020 degF). At this temperature, ky,T = 0.63, so the yield strength is reduced to 63% of its room-temperature value.

Can steel be left unprotected? Yes, in certain cases. Type II-B construction (IBC) requires no fire rating. Heavy sections with low Am/V may achieve required ratings unprotected. Parking garages are often unprotected steel.

What is the cheapest fire protection? SFRM (spray-applied) at $3-8/ft^2. It is the most economical but has poor aesthetics and impact resistance.

How thick is intumescent paint? 15-80 mils dry film thickness (0.4-2.0 mm). It expands 20-50x when heated to form an insulating char layer.

What is Am/V? The section factor: heated perimeter divided by cross-sectional area (units: m^-1). Higher Am/V means faster temperature rise. Heavy sections have low Am/V and heat slowly.

Do I need to protect both beams and columns? Yes, if the building type requires rated construction. Columns typically need higher ratings than beams (3 hr vs 2 hr for Type I-A). Unprotected steel is only permitted in Type II-B construction.

What about fire protection for bolts and welds? Bolts and welds lose strength at elevated temperature similar to base metal. However, the connection region typically has more thermal mass and heats more slowly. EN 1993-1-2 recommends protecting connections to the same level as the connected member.

Run this calculation

• Column Capacity Calculator
• Beam Capacity Calculator
• Section Properties Database

Related references

• Steel Grades
• Stress-Strain Curves
• Exposed Structural Steel (AESS)
• Thermal Action on Steel
• Section Properties
• Beam Design Guide
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable fire standard (AISC Appendix 4, EN 1993-1-2, AS 4100 Section 12) and local building code requirements. The site operator disclaims liability for any loss arising from the use of this information.