DOUBLE TEE FLANGE – FIRE RESISTANCE
High-performance, precast, prestressed double tees have been the go-to structural floor framing in parking structures for many decades, and there is no reason to think they will not continue to do so into the future. They are easily capable of supporting vehicular automotive live loads while free-spanning common 60’-0” parking modules, thus improving the Level of Service (LOS) and providing a user-friendly parking experience.
Parking structures are assigned Occupancy Group S-2 (Low-Hazard Storage) by the International Building Code (IBC). IBC has numerous parameters to direct the designer in establishing the Construction Type, but the lion’s share of open parking garages comply with Construction Type 2A. For tier areas ≤ 50,000 s.f. and building heights ≤ 10 tiers this results in a 1-hour fire resistance rating for floor construction in Construction Type 2A. Occasionally, however, limited openness, larger tier areas, or a greater number of tiers requires a more restrictive classification such as Construction Type 1A or 1B. These result in a 2-hour fire resistance rating for the floor construction.
For purposes of discussion, this paper focuses on IBC 2012, though the concepts and provisions stated are also applicable to prior IBC versions. Furthermore, the topic is limited to thickness requirements for the double tee flanges. 4” flanges for pre-topped double tees are nearly universal throughout the precast, prestressed industry, and easily support the IBC requirements for 3,000# concentrated loads from automobile jacks. Accordingly, the proposition herein is that a performance-based analysis demonstrates why standard 4” thick double tee flanges should be acceptable in lieu of the IBC 2012 prescriptive-based 4⅝” thickness based upon the following arguments…
- There are two primary and separate criteria of achieving a specified level of fire resistance, the structural end-point and heat transmission end-point fire endurance criteria as outlined in the Design for Fire Resistance of Precast/Prestressed Concrete (MNL-124-11) published by the Prestressed Concrete Institute (PCI), which has been evaluated by the ICC Evaluation Service in ESR-1997 as being in compliance with the Building Code.
- IBC 2012, Table 601 lists a 2-hour fire-resistance rating for Construction Types 1A and 1B, with no additional specificity as to structural vs. heat transmission end-point fire endurance criteria.
- IBC 2012, Table 722.2.2.1 prescribes a minimum concrete slab thickness of 4.6” when using carbonate aggregate to achieve a 2-hour fire resistance rating. Again, there is no additional specificity as to structural vs. heat transmission end-point fire endurance criteria, though it strongly implies conformance with heat transmission end-point since no reference is made to its reinforcement.
- IBC 2012, Table 508.4 obviously does not require a separation when the entire structure occupancy is S-2.
- IBC 2012, Section 510.3.4 does not even require protected openings through floor assemblies between Group S-2 (enclosed parking garages) and Group S-2 (open parking garages).
- IBC 2012, Section 714.1.5 does not require fire resistant joint systems through floors within open parking structures.
Since openings through parking levels (including ramps) do not require protection, and since the joints between the double tee joints do not require protection, one can reasonably infer that allowance of flame passage is permitted. Since passage of flame is permitted and could ignite combustibles on the opposing surface it stands to reason the heat transmission end-point fire endurance rating, which is a function of elevated flange temperatures, is neither a design consideration nor a limiting criterion. The structural end-point fire endurance rating still assures, however, that a more localized event like a car fire will not collapse the structure. This design methodology permits the flanges of the double tees to remain at the standard 4” thickness since the heat transmission end-point fire endurance is not applicable, while the reinforcing in the double tees flanges and stems is proportioned to achieve the 2-hour structural end-point fire endurance rating.
This topic is discussed extensively Chapter 10.8 in the PCI Design Handbook (MNL-120-10, 7th Edition). The following text is an excerpt taken directly from this section to substantiate waiving the heat transmission end-point criterion…
“In general building construction, other considerations for fire resistance are heat transmission and flame passage through the horizontal floor system. Both end point criteria are defined in ASTM E119. Due to the physical characteristics of a parking structure, however, these criteria are generally deemed not applicable to this type of construction. The vast open space on and between floor levels makes compartmentalization of a fire virtually a non-issue, since heat and smoke transmission throughout the building is uninhibited. Further, in the interior of many parking decks, ramp or litewalls provide support for the floor system. These are wall components with large openings to allow for greatly enhanced lighting and, therefore, security for the user. In effect, between the ends of adjoining bays, vertical air shafts are created in the gap between the ends of the double-tees, roughly equal to the thickness of the supporting wall times the distance between stems, less the width of the supporting wall column section. If floor openings such as these are not required to be enclosed it is illogical to require either of the end point criteria discussed previously in the determination of fire resistance; that is generally the basis of their being waived. The code specifically recognizes this in allowing an exception for fire-resistant joint systems in open parking structures (Section 713.1 of IBC 2006).”
“Research was performed to evaluate realistic vehicle fire loads (time-temperature or time-heat flux relationships) for precast concrete parking structures, and to investigate the influence of structure geometry and fire characteristics on the resulting fire loading. A typical precast concrete parking structure was analyzed for a series of vehicle fires, and the resulting fire loads at various points in the structure were determined. Fire analyses were run on nine simplified parking garage models. Seven analyses were single-vehicle fires, and two analyses were sequential, multiple-vehicle fires. Analyses were performed using the Fire Dynamics Simulator (FDS), a computational fluid dynamics computer program. Heat flux time-histories from the FDS analyses were used as input to subsequent heat-transfer finite element analyses to determine the temperature rise in the prestressing strand of the double-tee floor components. Results show that the highest steel temperature reached in any of the analyses occurred in analysis 9, which involved a multiple-vehicle fire confined to one floor of the parking structure. In this analysis, the peak gas temperature and peak heat flux at a given double-tee location were similar to a single-vehicle fire, but because more than one vehicle was burned, the duration of the thermal input to any location was longer than for a single vehicle fire. Using the equations for reduction in strength in prestressing steel as a function of temperature, the impact of the fire loading was evaluated. The maximum temperature reached in the concrete at the level of the prestressing
steel corresponded to a strength reduction in the prestressing steel of about 20%. For all of the single-vehicle fire analyses, the concrete temperature at the level of the strand is much lower, with a maximum temperature corresponding to about an 8% reduction in prestressing steel strength.”
It is encouraging to note that Section 722.2.2.1 of IBC 2015 has provided exception for the minimum thicknesses of Table 722.2.2.1 for floors and ramps within open and enclosed parking garages. While not all states or municipalities have adopted IBC 2015, the arguments provided herein, when used in conjunction with the evolution of the International Building Code, should easily justify acceptance by the code official to permit designers to detail standard 4” flange thicknesses. Not only does this take advantage of modular precast construction without the need for expensive form modification, but it reduces the weight of the structure by nearly eight (8) psf and saves nearly three (3) tons per truckload of freight cost.
NICORE™ PLANK – SUPERB FIRE RESISTANCE PROPERTIES
The International Building Code specifies minimum fire resistance ratings for various structural building components. Fire resistance ratings are determined based upon the Construction Type and can vary from 1-hour up to 3-hour, depending upon the occupancy. Unfortunately, many structural components are dependent upon the use of topically applied materials like Type X gypsum, sprayed mineral fiber, or intumescent mastic to achieve the necessary fire resistance rating. But this is precisely where NiCore™ Plank from Nitterhouse Concrete Products, Inc. shows its superior performance without the need for enhancing add-on materials.
Remarkably, there appears to be limited knowledge within the architectural and engineering design community of the foundational factors that influence a fire resistance evaluation of structural precast elements. Architects tend to be familiar with prescriptive fire resistance ratings as summarized in the IBC and tested by the Underwriters Laboratory and listed in their various directories. Increasingly, however, many structural components such as NiCore™ Plank are not being tested by UL. The purpose of this paper is to address these issues and to demonstrate the value in a performance-based analysis for this robust, high-performance structural building system.
NiCore™ Planks utilize an IBC code-compliant calculated fire resistance that is specific to each project’s structural system, span, and loading condition. This has a distinct advantage over UL assemblies that are unable to practically test every possible combination of span and applied loads. Section 703.2 of The 2018 International Building Code (IBC) states that the fire resistance ratings of building elements shall be determined in accordance with the test procedures set forth in ASTM E119 or in accordance with Section 703.3, which in turn permits calculations in accordance with Section 722. One such calculation method is found in ACI 216.1, and another is outlined in Design for Fire Resistance of Precast/Prestressed Concrete (MNL-124-11) as published by the Prestressed Concrete Institute (PCI), which has been evaluated by the ICC Evaluation Service in ESR-1997 as complying with the Building Code. These analytical methods are also summarized in Chapter 10 of the PCI Design Handbook (8th edition).
There are two primary and separate criteria for achieving a specified level of fire resistance, the structural end-point and heat transmission end-point fire endurance criteria as described below. Both of these criteria must be met in order to satisfy the required fire resistance rating.
- The structural end-point criterion calculates a reduced nominal flexural strength due to the prestressed strands being weakened during a fire event. Even though the surrounding concrete provides an excellent thermal insulation barrier for the prestressed strands, during prolonged events requiring higher fire resistance the temperature of the prestressed strands increases, thus lowering their strength. For a 1¾” strand height the prestressed strands have approximately a 49% residual capacity after a 2-hour fire, and approximately a 32% residual capacity after a 3-hour fire as compared with a 70oF baseline. Therefore, a longer fire rating might require extra reinforcement to achieve the calculated fire resistance. Since it is not always possible to increase the number of prestressed strands due to manufacturing limitations it is more common to list allowable reduced loads in published load tables. For example, on the attached 8″ NiCore™ Plank load tables using seven (7) ½”Ø prestressed strands on a 28.0’ span, the allowable live load is 137 psf for a 1-hour fire resistance rating but reduces to 113 psf for a 2-hour fire resistance rating. A comparison of the allowable loads for various fire resistance ratings can be found at https://nitterhouseconcrete.com/technical-info/load-tables/nicoretm/.
- The heat transmission end-point criterion for the fire resistance of concrete slabs is a function of the thickness of the concrete and the primary type of aggregate. As noted in the PCI Design Handbook and Design for Fire Resistance of Precast/Prestressed Concrete, the equivalent solid thickness of a hollow-core slab is obtained by dividing the net cross-sectional area by the slab width. It is common practice to determine the equivalent thickness of the entire assembly of the NiCore™ Plank, grouted keyway between individual pieces, and cast-in-place topping (when present) to calculate the fire rating of the entire system as a whole assembly. For example, 8″ NiCore™ Plank without topping has a thermal heat transmission rating of 2.3 hours. If a 2” nominal cast-in-place composite topping is applied the thermal heat transmission rating increases to 4.4 hours!
It is worth noting that the structural and heat transmission end-point criteria are determined based on service loads using a strength reduction factor of unity. The nominal moment capacity under fire conditions is given as ɸMnƟ, where ɸ = 1.0. This value must equal or exceed the demand load of 1.2 D + 0.5 L + 0.2 S per ASCE 7-16, Section 2.5.2.1 for a capacity evaluation of load combinations for extraordinary events. Simply stated, in the example given above with a 28.0’ span and a 2-hour fire resistance rating the allowable live load was shown as 113 psf. If at the end of a standard 2-hour fire a load of 114 psf is present the structural system may fail. This extra 1 psf is the straw that broke the proverbial camel’s back. This is not to imply that structural failure is certain because fire-engulfed buildings might not actually sustain a “standard” fire, loads are not known with precise certainty, components are manufactured and constructed with tolerances, and computations are approximate. Nonetheless, this example serves to provide an explanation of the meaning of the terms. The rationale behind this thinking is to provide a structure that is sufficiently safe to still carry the design loads for the prescribed fire duration, allowing time for the firefighters to battle the fire while giving occupants time to vacate the premises.
On a somewhat related note, the architect and structural engineer of record (EOR) must give a proper evaluation as to whether the members have unrestrained vs. restrained end conditions. Suffice it to say that the NCP load tables conservatively assume unrestrained end conditions. The assumption of a restrained end condition must not be concluded arbitrarily. There are several design and construction variables that influence this assessment and must work in unison under the right combination of conditions to safely achieve a restrained end condition, a consideration of which is beyond the scope of this paper.
Furthermore, it should not be overlooked that the 1¾” strand height of NiCore™ Plank offers a distinct advantage over competing hollow-core slabs. Other systems have their strands located at strand heights of 1½” or even 1¼”. Prestressed strands located at 1½” are much more vulnerable to fire because of the reduced protective concrete cover provided. For example, at a 1½” strand height the ultimate flexural capacity only increases by about 5%, while a 2-hour fire event results in a 25% reduction in allowable load capacity. Ouch! This is quite a penalty that requires serious consideration under elevated fire resistance requirements in Construction Types 1A and 1B, or if a 3-hour fire separation is required in a mixed-use occupancy.
Generally, joints between floor and roof members must be protected to the same fire endurance as that required for the members, with several exceptions noted in Section 714.1 of IBC 2015. Section 7.4 of Design for Fire Resistance of Precast/Prestressed Concrete states, “Joints between hollow-core slabs are filled with a sand and cement grout to approximately the depth of the slab. Therefore, the joints have at least the same endurance as that provided by the equivalent thickness of the hollow-core slab.”
Nitterhouse Concrete Products, Inc. in Chambersburg, PA, is a family-owned company serving the construction industry since 1923. Give us a call at 717-267-4505 or contact us online for information on more quality precast, prestressed products to meet your design and construction needs.
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