Technical Note. Flat Plate Voided Concrete Slab Systems

Transcription

1 ENGINEERING ETN-B-2-16 Technical Note Fire Resistance of Flat Plate Voided Concrete Slab Systems Introduction Concrete and steel are noncombustible materials; that is, they will not ignite or burn when subjected to fire or heat. Because reinforced concrete assemblies and systems are inherently fire resistant, no additional fire protection is required to meet common minimum fire ratings prescribed in the building codes. These inherent material properties reduce fire risk and require minimum ongoing maintenance over the lifetime of a building. Flat plate voided concrete slab systems, which have been used for many years in Europe and other parts of the world, are becoming increasingly popular in the U.S. because of many inherent benefits that can be provided. Descriptions of the various types of flat plate voided concrete slab systems are given in the next section. Information and guidelines on how to determine the fire resistance for these systems are also provided as are results from fire tests. The data shows that the design of flat plate voided concrete slab systems for fire protection is the same as that for other reinforced concrete structures and that a ¾ in. cover to the bottom reinforcing bars will provide a minimum 2-hour fire-resistance rating, which meets minimum required fireresistance ratings for floor assemblies in common occupancies. Flat Plate Voided Concrete Slab Systems Various types of voided concrete slabs have been used throughout the years, including one-way Sonotube systems and two-way joist (waffle slab) systems that are constructed using dome forms. Contemporary systems, pioneered primarily by BubbleDeck and Cobiax, utilize hollow, plastic balls made out of high-density, recycled polyethylene (HDPE), which are commonly referred to as void formers (see Figure 1). These are usually spherical or ellipsoidal in shape and are positioned within wire support cages to create modular grids (cage modules), which are locked between the upper and lower reinforcement layers in the concrete slab (see Figure 2). The void formers reduce the weight of the slab significantly compared to a solid slab of the same thickness; they are judiciously located in zones where concrete is not needed and where the flexural strength and load transfer to supports are not compromised. Typical site installation of a cast-in-place, flat plate voided concrete slab is as follows (Figure 1b): 1. Formwork is erected and bottom reinforcing bars are placed. 2. Cage modules that contain the void formers are placed and tied perpendicular to the top layer of bottom reinforcing bars. 3. Top reinforcing bars are placed directly on and tied to the cage modules. 4. First layer of concrete is cast, which covers the bottom reinforcing bars and the lower portion of the cage modules. This layer of concrete essentially anchors the cage modules and prevents possible upward movement due to buoyancy when the second layer of concrete is placed. 5. Second layer of concrete is cast to the required overall thickness of the slab system. Another type of voided slab system utilizes a reinforced precast concrete layer, which essentially replaces the horizontal formwork of the cast-in-place system described above. Installation of this system is as follows (Figure 1a): 1. Cage modules with top and all of the bottom structural reinforcement are precast into 3-in.-thick panels. 2. The panels are set on shoring that is spaced 7 ft on center. 3. On site, a layer of concrete is cast to the required overall thickness. In applications where 30 to 50 foot spans are required, the overall depth of the slab can range from 10 to 21.5 in. It is common at column locations that void formers are omitted so that a solid concrete section is

2 (a) (b) Figure 1 Void formers (a) BubbleDeck and (b)cobiax USA (photos courtesy of BubbleDeck and Cobiax USA). available to resist two-way shear stresses (see Figure 3). Where additional shear strength is required, shear reinforcement, such as headed shear stud reinforcement, can easily be accommodated, as shown in the Figure. Flat plate voided concrete slabs systems are designed and detailed in accordance with requirements of ACI 318 (ACI 2014) just like any other two-way slab system. It is important to note that the void formers do not contribute to the nominal flexural and shear strengths of the slab system; their only role is to provide voids in the slab. Stiffness and shear modification factors are provided for deflection and shear strength calculations, respectively, which are used in design to take into account the presence of the voids within the slab. No modification factors are required for flexure because the voids do not have an impact on flexural strength; nominal flexural strength is based on compression in the concrete and tension in the reinforcing bars, just as in any other two-way slab system. Additional design and detailing information, including a worked-out design example, can be found in CRSI (2014). In addition to reduction in dead load, there are other noteworthy benefits that can be realized by utilizing a voided slab system, including economical long spans without forming and casting of beams, low floor-to-floor heights, and vibration resistance. See CRSI (2014) and manufacturers literature for more information. Fire-Resistance Ratings In general, fire-resistance rating (or, fire rating), is the period of time (usually expressed in hours) a building element, component, or assembly maintains the ability to contain a fire, continues to perform a given structural function, or both. Fire ratings are determined by tests or by the methods prescribed in Section of the International Building Code (IBC) (IBC 2015). Required Fire-Resistance Ratings Required fire-resistance ratings for elements in buildings are given in Table 601 of the IBC based on the construction type (I though V). Types I and II are types of construction where the building elements are of noncombustible materials, which includes reinforced concrete. The minimum fire-resistance rating for floor elements in Type I construction is 2 hours. Test Methods to Determine Fire-Resistance Ratings IBC Section permits the test procedures in ASTM E119 (ASTM 2016a) and UL 263 (UL 2015) for determining fire-resistance ratings of building elements, components, and assemblies. Standard fire tests are conducted by placing an assembly in a furnace and subjecting it to a fire that follows a standard time-temperature curve. Fire-resistance rating of an assembly is determined by the duration of the test until one of the following end-points is reached: Fire passage end-point (Walls, partitions, floors, and roofs) Cotton waste ignites as a result of flames or hot gases passing through holes, cracks, or fissures in the assembly. Heat transmission end-point (Walls, partitions, floors, and roofs) Temperature of the unexposed surface of the assembly rises an average of 250 F above its initial temperature. 2 Fire Resistance of Flat Plate Voided Concrete Slab Systems [ETN-B-2-16]

3 Figure 2 Flat plate voided concrete slab system. S tructural end-point (All assemblies and members) Test specimen is unable to sustain the applied loading (collapse). The results from fire tests on flat plate voided concrete slabs systems are summarized below. Reinforced concrete meets the requirements of being a noncombustible material that are set forth in ASTM E136 (ASTM 2016b), and reinforced concrete assemblies are generally classified as restrained assemblies in accordance with ASTM E119. Restrained assemblies perform better during a fire compared to assemblies that are unrestrained. Calculations to Determine Fire-Resistance Ratings Section of the IBC permits the use of calculations performed in accordance with Section 722 as one of the acceptable ways of determining fire-resistance ratings for structural members. The fire-resistance ratings provided in that section are based on ASTM E119 fire tests. For reinforced concrete, values of minimum member thickness/size and minimum concrete cover over reinforcement are provided for various fire-resistance ratings. Thus, in order to satisfy a required fireresistance rating based on Type I or II construction, reinforced concrete thickness/size and cover to the reinforcement must be specified that are at least equal to the values in the appropriate tables in Section 722. Requirements for minimum thickness of reinforced concrete floor and roof slabs and for minimum concrete cover over reinforcement in slabs are given in IBC Sections and , respectively. Generally, reinforced concrete slab systems that are proportioned in accordance with the provisions in Chapter 8 of ACI 318 and that have minimum cover to the reinforcement Figure 3 Void formers omitted at column locations (photo courtesy of Cobiax Technologies AG). as specified in Section 20.6 easily satisfy minimum fire resistance requirements. The minimum slab thicknesses provided in IBC Table are for slabs with a uniform thickness, such as a typical one-way or two-way slab system. A method to determine the thickness of slabs with ribbed or undulating soffits to be used for fire-resistance ratings is given in IBC Section This method is applicable for systems with the cross-sectional profiles illustrated in IBC Figure (see Figure 4). The slab thickness for this type of construction to be used in determining the fire resistance ratings in accordance with IBC Table is determined as follows: For s 4t, thickness = t For s 2t, thickness = te For 4t > s > 2t, thickness = t + (4t/s-1)(te-t) where s = spacing of ribs or undulations t = minimum thickness te = equivalent thickness of the slab calculated as the net area of the slab divided by the width, in which the maximum thickness used in the calculation shall not exceed 2t. In the case of hollow core prestressed slabs where the cores are of constant cross-section throughout the CRSI Technical Note 3

4 t d (a) s s s Calculation Per 1 Square Grid of Void Formers Center To Center Measures: (b) Figure 4 Equivalent thickness for (a) ribbed construction and (b) undulating construction. length, the equivalent thickness shall be permitted to be obtained by dividing the net cross-sectional area of the slab including grout in the joints, by its width (see IBC Section ). In a similar fashion, an equivalent thickness for a voided slab system can be determined by dividing the net volume of concrete by the respective floor area. Figure 5 shows examples of such calculations for both BubbleDeck and Cobiax systems with spherical void formers. The overall slab thicknesses t in the figure are the minimum available for both systems. Equivalent thicknesses obtained by this method are greater than 5 in., which is the minimum slab thickness that provides a 2-hour fire-resistance rating for concrete mixes with siliceous aggregate (see IBC Table ). Equivalent thicknesses for voided slab systems with overall slab thicknesses greater than the minimum thicknesses considered here are greater than the corresponding values in the figure. It is shown below that concrete cover to the reinforcing bars on the the fire side is the controlling parameter in the determination of the fire resistance for voided slab systems. The calculations presented here support those findings. The minimum thickness of concrete cover to the positive flexural reinforcement of reinforced concrete slabs with flat undersurfaces is given in IBC Table (1). This table is applicable for (1) solid or hollow core oneway slabs or two-way slabs and (2) slabs that are either cast-in-place or precast. It is evident from the table that a typical concrete cover of ¾ in. provides a fire resistance of 4 hours for restrained assemblies regardless of the unit weight of the concrete or the type of aggregate used in the concrete mixture. Floor Area = s 2 Solid Slab Volume = t s 2 Void Former Volume = 4 3 Equivalent Thickness = Samples: VOID FORMER DIMENSIONS (in.) Fire-Resistance Tests on Flat Plate Voided Concrete Slab Systems Numerous fire tests have been performed on BubbleDeck systems and Cobiax systems in accordance with the provisions in DIN (DIN 1977). The time-temperature curve used to test specimens in the DIN requirements is the same as that prescribed in ISO 834 (ISO 1999). A comparison of the ISO 834 and ASTM E119 time-temperature curves is given in Figure 6. It is evident from this figure that there are some differences in the curves. However, it has been determined that the differences in severity between the two tests are negligible (Harmathy 1987). Criteria to determine the fire-resistance rating are also basically d 2 3 = d 3 6 ts 2 d 3 6 s 2 =t BUBBLEDECK BD 230 d 3 6s 2 VOID FORMER ID COBIAX CBM-E-225 t d s Equivalent Thickness Figure 5 Sample calculations for equivalent thickness of voided slabs. 4 Fire Resistance of Flat Plate Voided Concrete Slab Systems [ETN-B-2-16]

5 Figure 6 Comparison of ISO 834 and ASTM E119 time-temperature curves. the same. Therefore, it follows that the results obtained from specimens tested in accordance with the DIN requirements would essentially be the same as those that would be obtained if the specimens were tested in accordance with ASTM E119 requirements. The fire tests have revealed that the concrete cover to the reinforcing bars on the fire side is the controlling parameter in the determination of the fire resistance in all of the tests. It was found that the voids act as a thermal isolator: the heat from the fire is dammed below the void. This leads to slightly higher temperatures in the reinforcing bars positioned below the voids. A cover of ¾ in. to the main flexural reinforcing bars resulted in a fire resistance rating of at least 2 hours. The fire ratings obtained from the Cobiax tests have been verified by finite element analyses. Summary It is evident from the findings of fire tests that concrete cover to the reinforcing bars on the side of the fire is the controlling parameter in the determination of the fire resistance for flat plate voided concrete slab systems. A ¾ in. cover to the bottom reinforcing bars will provide a minimum 2-hour fire-resistance rating, which meets minimum required fire-resistance ratings for floor assemblies in common occupancies. The void formers were found to be intact after the fire tests. The internal temperature remained below the melting temperature of the HDPE, which is approximately between 200 and 300 degrees Fahrenheit. CRSI Technical Note 5

6 References ACI (American Concrete Institute) Building Code Requirements for Structural Concrete and Commentary. ACI , Farmington Hills, Michigan. ASTM (American Society for Testing and Materials). 2016a. Standard Test Method for Fire Tests of Building Construction and Materials. ASTM E119 16, West Conshohocken, Pennsylvania. ASTM (American Society for Testing and Materials). 2016b. Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 C. ASTM E136 16, West Conshohocken, Pennsylvania. CRSI (Concrete Reinforcing Steel Institute) Design Guide for Voided Concrete Slabs, Schaumburg, Illinois. DIN (Deutsches Institut für Normung) Fire Behavior of Building Materials and Building Components; Building Components; Definitions, Requirements and Tests. DIN , German Institute for Standardization, Berlin, Germany. Harmathy, T.Z., Sultan, M.A., and MacLaurin, J.W Comparison of Severity of Exposure in ASTM E119 and ISO 834 Fire resistance Test, Journal of Testing and Evaluation, ASTM, 15(6), , West Conshohocken, Pennsylvania. ICC (International Code Council) International Building Code, ICC, Washington, D.C. ISO (International Organization for Standardization) (Amended 2012). Fire-resistance Tests - Elements of Building Construction - Part 1: General Requirements. ISO 834-1, Geneva, Switzerland. UL (Underwriters Laboratories) Standard for Fire Tests of Building Construction and Materials. UL 263, Northbrook, Illinois. Contributors: The primary contributors to this publication are: David A. Fanella, PE, SE, FACI, FASCE and Mike Mota, PhD, PE, FACI, FASCE Keywords: empirical design, fire rating, fire resistance, fire test, non-combustible, rational design, restrained, structural integrity, unrestrained. Reference: Concrete Reinforcing Steel Institute CRSI [2016], Fire Resistance of Flat Plate Voided Concrete Slab Systems, CRSI Technical Note ETN-M-9-16, Schaumburg, Illinois, 4 pp. Historical: None Note: This publication is intended for the use of professionals competent to evaluate the significance and limitations of its contents and who will accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel Institute reports the foregoing material as a matter of information and, therefore, disclaims any and all responsibility for application of the stated principles or for the accuracy of the sources other than material developed by the Institute. 933 North Plum Grove Rd. Schaumburg, IL p f Regional Offices Nationwide A Service of the Concrete Reinforcing Steel Institute 2016 This publication, or any part thereof, may not be reproduced without the expressed written consent of CRSI.