Saving Time, Costs, and Materials with Engineering-Based Structural Fire Protection

To view the figures and tables associated with this article, please refer to the flipbook above.

The effort to optimize building construction costs is a slippery slope—one that requires balancing the vision of the structure’s design against the realities of its budget. Securing sound footing requires the consideration of numerous variables, including the building’s design elements, the weight of its steel and, perhaps surprisingly, the type and amount of fire resistive materials (FRMs) used on the building’s structural steel to protect it from collapse during a fire.
Finding the optimal combination of variables was once a major chore for building owners, architects and engineers, as any adjustment to the weight or size of steel profiles means a potential adjustment to the applied thickness of FRMs—not to mention the entire building’s cost structure. Today’s advanced software programs simplify and streamline that process by showing the real-time cost increases or savings associated with any design adjustment. However, truly optimizing the entire process requires thinking outside the traditional box of “prescriptive” fire-based design.

The traditional, prescriptive building method assumes full structural loading conditions and follows a worst-case fire scenario, thereby erring on the side of applying thicker FRM coats than may actually be necessary. Those FRM thicknesses can be reduced when using a calculated approach to structural fire engineering design (SFED) that considers the building’s actual structural design loading conditions. Yet, engineers can do even better by combining the principles of the two approaches. This combined method assumes the structure has 20% reserve strength availability, meaning the thickness of FRMs applied to individual steel members can be further reduced while achieving the same level of fire protection.

Adopting the more realistic combined method for building designs often leads to the unexpected result that a building using heavier (and, sometimes, more costly) steel can be less expensive overall because it requires a lesser amount of FRMs to be applied to steel to maintain the intended fire rating. That’s because the additional material and labor costs associated with applying the coatings thicker on lighter steel profiles may otherwise exceed the material savings realized by using that lighter steel.

This article will review how combining the prescriptive and calculated SFED approaches can lead to optimized building designs that save owners time, costs and materials.

The traditional prescriptive approach to engineering buildings and their required fire protection uses numerous assumed and worst-case conditions, many of which do not realistically occur in an actual fire event. Therefore, many of these assumptions can be reconsidered when using today’s more advanced fire protection design methods.

Devised based on the development of the “Standard Fire Curve” (Fig. 1), which was established in the 1917 version of ASTM E119, the prescriptive method of fire engineering became a helpful basis for creating a perceived safe and generally conservative (i.e., deemed-to-satisfy) building fire protection solution. It offers a reproducible and comparable basis for testing FRMs, so manufacturers can compare product performance against the same test criteria and conditions.
This standardized curve and another version developed in the 1980s for hydrocarbon fuel sources (Fig. 2, as published in ASTM E1529 and UL1709) represent what are referenced in code as nominal fires, for use in prescriptive or standard fire-resistance design (SFRD), as referenced in ASCE's Manuals and Reports on Engineering Practice No. 138 (ASCE MOP 138).

Both curves show the typical temperature rise when materials burn, with a continued temperature increase over time in the case of cellulosic fires or a plateau at around 2,000F (1,100C) for hydrocarbon fires. However, no standard fire continues to burn indefinitely, nor at a consistently elevated temperature, meaning the two curves do not represent real fires. As a result, FRM thickness calculations based solely on using these curves as part of the prescriptive building design method favor using more fireproofing material than may be needed.

In North America, the advanced fire safety engineering design approach is referenced in the 2021 International Building Code (IBC) Section 703.2.3, ANSI/AISC 360-22 (Appendix 4) as well as ASCE/SEI 7-22 (Appendix E). However, confusion often arises with the term “Performance-Based Structural Fire Design” (PBSFD), which encompasses all relevant advanced techniques, including those beyond structural steel protection. These processes include the use of design fires (also known as real fires), which are different than the nominal fire curves discussed earlier to test FRMs.

In the context of a full holistic building review, the “Cardington Fire Tests” performed between 1994 and 1997 in the United Kingdom provided a greater understanding of how fire behaves in structures featuring composite steel beams with reinforced concrete floor slabs. The tests helped fire engineers recognize they could reduce or even eliminate the application of FRMs on some steel members for economy without compromising safety. Instead, they can include FRMs on only the essential positions required to ensure structural stability for the building’s designated fire condition.

Further development of fire protection design standards has led to a calculated approach to SFED, as referenced in ASCE MOP 138. In this context, individual structural steel members are evaluated in the standard fire condition, considering the actual structural design loading conditions defined by the structural engineer, not assumed loading conditions as in the prescriptive design method. This SFED approach more accurately reflects the structural behavior of the supporting steel members (for demand and capacity).

In this SFED-based approach, the goal is not to eliminate FRMs but rather tailor the exact level of FRM thickness to be applied to each steel member. The engineer of record (EoR) can calculate an individual member’s “critical temperature” at which it will begin to lose its ability to carry the actual applied structural design loading.

Of note, higher critical temperature values result in lower FRM thicknesses being required, as a member that is not using all its strength to resist the structural demand has reserve resistance to fire. Therefore, if the resulting critical temperature value would exceed tabulated limits cited in a prescriptive design approach, the thickness of applied FRMs could be reduced when using the SFED approach. Of course, the opposite may be true, with select members requiring a greater FRM thickness. The goal is to optimize that thickness for the actual thermal load conditions for each steel member to ensure safety.

True fire protection cost balance optimization is possible when combining the prescriptive and SFED approaches to building design. Calculations have shown that this optimization usually comes from having thicker steel members with thinner applied FRM thicknesses (Fig. 3). Following the SFED approach, specifiers would look to published data for steel member sizes to determine the appropriate FRM thicknesses and follow those guidelines. However, the combined approach to fire protection design would recognize there is additional “reserve strength” (assumed at about 20% reserve strength) baked into the thicker steel and therefore allow for further reductions of FRM thicknesses on individual steel members—while achieving the same intended level of fire protection. Of course, other considerations, such as the potentially larger size of the building's foundation to accommodate the heavier steel, will be part of the cost balance optimization and should also be calculated.

Combining the reserve strength revelation with structural fire engineering principles, which allow for the actual applied loading conditions, can help building owners and designers optimize designs for cost, efficiency, and sustainability or a combination of all three. It is recommended for building designers to work with FRM suppliers and engineers to finetune these details. The earliest collaboration possible during the building design process is always the preferred option and may be initiated from multiple directions. Connecting the FRM suppliers with the main contractor, designer, fire engineer, and other key parties during that early window—and before specifications are finalized—will allow for greater optimization benefits.

The following simple examples demonstrate the benefits of the combined fire protection design approach:

First, a W10x22 (22 pounds/foot) four-sided exposed column is the baseline, and it is compared to the prescriptive (standard) design method for 2-hour fire resistance when using a FIRETEX epoxy FRM from Sherwin-Williams. Table 1 compares this size column to a slightly heavier W10x26 (26 pounds/foot) four-sided exposed column. Each example assumes using the typical supply cost of material, application waste and labor rates, and the market rate for bare steel.
This example demonstrates the drop in cost that’s enabled by the combined “help” from the thick steel’s inherent reserve strength. This leads to a reduced coating thickness and lower applied costs since fewer coatings and less labor are required.

The next example in Table 2 looks at tubular steel. Tubular FRM is notoriously costly, particularly when using epoxy FRMs. The table compares a common 8x8-inch square tube, again with a 2-hour epoxy FRM, when using various tube wall thicknesses of ¼-, 5/16-, 3/8- and ½-inch. The examples again assume using the typical coating and steel material costs, as well as application waste and labor rates.

As demonstrated, designers focused on reducing building costs will often arrive at the surprising conclusion that thicker steel can lead to a less expensive build. The counterintuitive result—based on using a combination of the prescriptive and calculated SFED building design approaches—stems from the material and labor costs associated with applying FRMs to protect the building’s structural integrity during a fire event. Fewer FRMs and labor are required with thicker steel, thereby reducing costs. Of course, a balance exists where it may become more costly or simply unfeasible to use thicker steel, in which case the building design can be optimized below that level of steel thickness.

To create a building design that optimizes time, costs and materials, software tools that can adjust costs on the fly based on different designs and different FRM thicknesses are necessary. Designers should work with coatings providers using software featuring integrated BIM and 3D electronic transfer technology to refine their designs, so the team can seamlessly synchronize a steel 3D model with the software and calculate coating thickness values and volumes along the way toward optimization. ■

About the Authors

Bob Glendenning is a structural engineer and the retired Global Fire Engineering Manager for the Fire Engineering and Estimation Team at Sherwin-Williams Protective & Marine, which supports the specification of engineered fire-protection solutions based on simple and complex calculations, as well as inputs from Building Information Modeling (BIM) software. (bob.glendenning@sherwin.com)

Nestor Iwankiw, Ph.D, PE, is a consultant for Jensen Hughes with extensive experience in various aspects of structural and fire protection engineering. He uses both analytical and experimental methods to develop practical solutions for unique issues in performance-based design, forensic investigations, new and existing construction problems, onsite inspections, engineering peer reviews and judgments, research development, and product acceptance for fire resistance. (niwankiw@jensenhughes.com)

References

[1] ASTM E119 – Standard Test Methods for Fire Tests of Building Construction and Materials

[2] ASTM E1529 – Standard Test Methods for Determining Effects of Large Hydrocarbon Pool Fires on Structural Members and Assemblies

[3] ANSI/UL1709 – UL Standard for Safety, Rapid Rise Fire Tests of Protection Materials for Structural Steelwork

[4] ASCE Manuals and Reports on Engineering Practice No. 138 – Structural Fire Engineering

[5] 2021 International Building Code (IBC)

[6] ANSI/AISC 360-22 Specification for Structural Steel Buildings

[7] ASCE/SEI 7-22: Appendix E – Performance-Based Structural Fire Design

[8] ANSI/UL263 – Fire Tests of Building Construction and Materials