Educational Overview Revised 10/2/2017

Educational Overview Revised 10/2/2017
2-Hour Fire Endurance Assembly for MPCWT Structural Floor and Roof Framing Systems Educational Overview Revised 10/2/2017

Copyright © 2017 Structural Building Components Association.
SBCA has been the voice of the structural building components industry since 1983, providing educational programs and technical information, disseminating industry news, and facilitating networking opportunities for manufacturers of roof trusses, wall panels and floor trusses. SBCA endeavors to expand component manufacturers’ market share and enhance the professionalism of the component manufacturing industry. Copyright © 2017 Structural Building Components Association.

Introduction Fire resistance ratings of materials and assemblies are not always readily available from prescriptive tables and tests. Theoretical methods offer an alternative to full scale fire tests.

Introduction Several historical codes permit fire resistance ratings to be determined by analysis: Section of the 1996 BOCA National Building Code Section of the 1994 Uniform Building Code of the 1994 Standard Building Code

Introduction Current versions of the International Building Code also offer guidance regarding fire-resistance ratings. IBC Section 703.2 IBC Section 703.3 The analysis used in SRR is based on the method listed in IBC Section 703.3, method number 4, Engineering analysis.

Introduction Two items to note:
While calculations in accordance with Section 722 (#3) for wood are limited to a maximum of 1 hour (Section ), that limitation does NOT apply to method 4. Prescriptive design in accordance with Section 721 (#2) Table 721.1(3), item , includes a ceiling design using I-joists, 3 layers of 5/8 Type C gypsum and 7/8 furring channel with a 2 hour rating.

Introduction One theoretical method known as the “Ten Rules of Fire Endurance Ratings” was published by T.Z. Harmathy in the May, 1965 edition of Fire Technology. Harmathy’s Rules provided a foundation for extending fire endurance assembly data. These rules are still in use, including extensive use within this presentation.

Introduction Fire endurance assembly calculations are also delineated in: Fire Safety Design in Buildings Canadian Wood Council Component Additive Method for Calculating and Demonstrating Assembly Fire Endurance (DCA 4) American Wood Council (AWC) Chapter 7 of BOCA’s Guidelines for Determining Fire Resistance Ratings of Building Elements

Introduction Additionally, the UL Fire Resistance Directory, Design Number L538 assembly was used in conjunction with the component additive method (CAM) principles. The calculations in this presentation are based on CAM principles for metal plate connected wood truss construction on behalf of the Structural Building Components Association.

Two Hour Calculated Assembly
Fire Rating: 2 hours Finish Rating: More than 90 minutes

Specifications – (1) Subfloor/Finish Floor
Interior plywood or OSB in 4 x 8 sheets 5/8 or greater thickness Manufactured with exterior glue Have tongue-and-groove edges along the 8 side

Sheets shall be: Installed perpendicular to the trusses with end joints centered over the top chord of the truss Placed so the end joints are staggered Applied in compliance with specifications and recommendations provided by the American Plywood Association.

A lightweight concrete topping may be applied over the plywood or OSB. A topping is not required If applied, this topping should be minimum ¾” thick or thicker, following the architectural specifications and topping manufacturer’s guidelines.

Specifications – (2) Structural Members
A minimum 12-deep metal plate connected wood truss spaced at a maximum of 24 o.c. can be used in this assembly. The truss application should follow the installation recommendations developed by SBCA.

Specifications – (3) Ceiling Membrane
Three layers of Type C gypsum wallboard are used in this assembly. Each sheet used is assumed to be a minimum of 4 wide For the most current listing of acceptable Type C gypsum board see UL Design Number L538.

Specifications – (3) Ceiling Membrane
The gypsum wallboard attached directly to the trusses should be oriented perpendicular to the trusses. The gypsum wallboard attached to the furring channels should be oriented perpendicular to the furring channels.

Specifications – (3a) Ceiling Membrane Layer 1
The application of the Type C gypsum wallboard shall follow the manufacturer’s installation instructions. The wallboard end joints should be centered on the bottom chord of the trusses, and should be staggered. Ceiling Membrane Layer 1 Material 5/8 Type C gypsum wallboard Attached to Bottom chord of truss Fastener type 1-1/4 Type S bugle-head screw Spacing along edges/in field 6:12 Minimum end distance 3/8 Minimum edge distance 1-½

Specifications – (3b) Furring Channel
Resilient or inverted hat-type furring channels are placed over the first layer of gypsum The channels are made of 25-gauge galvanized steel, and installed perpendicular to the structural members. Channel spacing: max 24 o.c. Attached to each truss (through the gypsum) with one 1-7/8 Type S screw.

Specifications – (3c) Ceiling Membrane Layer 2
This middle layer of 5/8 Type C gypsum wallboard is attached to the furring or resilient channels. The end joints of each gypsum wallboard sheet shall be centered on the resilient or furring channel. Ceiling Membrane Layer 2 Material 5/8 Type C gypsum wallboard Attached to Furring/resilient channel Fastener type 1-1/4 Type S bugle-head screw Spacing 6 maximum Minimum end distance 5/8 Minimum edge distance 1-½

Specifications – (3d) Ceiling Membrane Layer 3
The end and edge joints of the finish layer of gypsum should be staggered a minimum of 24 from the joints in layer 2. The end joints of the face layer must be centered on the furring channels. If this is not the case, end joints shall be attached to Wallboard Layer 2 with 1-1/2 Type G screws spaced 6 o.c. with an end and edge distance of 1-1/2. All screws shall be set so that they are flush with the face of the wallboard and do not damage the core of the wallboard. Ceiling Membrane Layer 2 Material 5/8 Type C gypsum wallboard Attached to Furring/resilient channel through Layer 2 Fastener type 1-5/8 or 1-7/8 Type S bugle-head screw Spacing 6 maximum Minimum end distance 5/8 Minimum edge distance 1-½

Specifications – (4) Fasteners
Type S bugle head screws that are self-drilling and self-tapping shall be used. Where needed, Type G wallboard screws can also be used. Screws shall meet ASTM C1002 or ASTM C954 standards. Type G Type S

Specifications – Finishing Systems
The face layer joints shall be covered with tape and coated with joint compound. Screws shall also be covered with joint compound.

Analysis – Membrane Protection
The critical feature of a fire endurance assembly, particularly a horizontal assembly, is the performance of the gypsum membrane. For an assembly to obtain a given fire endurance resistance performance, the membrane must stay intact for the full duration of fire resistance performance before falling off.

Harmathy states: “The thermal fire endurance of a construction consisting of a number of parallel layers is greater than the sum of the thermal fire endurance characteristics of the individual layers when exposed separately to the fire.”

Wallboard Membrane Description Time (minutes) 3/8 Douglas fir plywood phenolic bonded 5 1/2 Douglas fir plywood phenolic bonded 10 5/8 Douglas fir plywood phenolic bonded 15 3/8 gypsum wallboard 1/2 gypsum wallboard 1/2 Type X gypsum wallboard 25 5/8 gypsum wallboard 20 5/8 Type X gypsum wallboard 40 Double 3/8 gypsum wallboard 1/2 and 3/8 gypsum wallboard 35 Double 1/2 gypsum wallboard For example, a single layer of 1/2 gypsum wallboard yields a membrane rating of 15 minutes. If two such layers are used, the rating is 40 minutes, instead of the expected 30.

Another rule states that the fire endurance of a construction does not decrease with the addition of additional layers. By adding layers of material, both the resistance to heat flow and heat capacity of the construction increase. This reduces the rate of temperature rise in the plenum, and therefore, at the unexposed surface.

However, the added layer: Must have similar thermal expansion and transmission properties to the adjacent layer Must be made of a thermally resistant material In other words, adding a layer of steel to the unexposed surface would not enhance the fire performance of the assembly, whereas adding a plaster layer would.

The table at right shows a protective membrane of 120 minutes using the CAM. Direct addition of membrane times is conservative, based on the rules and the 1/2 gypsum wallboard example given. Type C gypsum is specified and is conservative, since it has much better fire endurance performance than Type X. Wallboard Membrane Description Time (minutes) 5/8 Type X gypsum wallboard 40 Total 120

Analysis – Performance Enhancements
Harmathy states that the fire endurance of constructions with continuous air gaps or cavities is greater than the fire endurance of equivalent constructions without air gaps or cavities. This occurs because the insertion of voids provides additional resistance to heat flow, similar to a storm window

Inserting resilient channels between Wallboard Layer 1 of the assembly and the two layers below it serves several functions. First, it creates a continuous air space that enhances the fire performance of the membrane system and creates dead air space that is insulating.

Second, attaching the resilient channel over the first layer of gypsum provides additional support for, and therefore enhances the stability of, the first layer of gypsum wallboard. This will aid in keeping this layer of gypsum in place, resulting in better assembly fire performance.

Finally, there will be two connection points of the first layer into the truss system. The first will be attaching the gypsum layer to the truss. The second will be attaching the resilient channel through the first gypsum layer into the truss. This will enhance the stability of the membrane attachment, keeping layer 1 on longer, and improving the overall fire performance of the assembly

An analysis for the National Fire Protection Research Foundation (NFPRF), National Engineered Lightweight Construction Fire Research Project (Design FC-240 and Design FC-235) indicated an additional 6 minutes of performance due to the addition of resilient channels. For the assembly described above, the following calculation could then be made: Description Time (minutes) 3 layers of 5/8 Type X gypsum wallboard 120 Resilient channels spaced to a maximum of 16 o.c. 6 Total 126

Analysis Note that the multiple layer effect has not yet been taken into account. This membrane could be calculated as at least a 136 minute assembly by adding the 10 minute performance increase, using the 2 layers of 1/2 gypsum example shown earlier. Description Time (minutes) 3 layers of 5/8 Type X gypsum wallboard 120 Resilient channels spaced to a maximum of 16 o.c. Credit for Multiple Layers of Gypsum 6 10 Total 136

Analysis – Engineering Evaluation
To evaluate the reliability and reasonability of this assembly, ASTM E119 testing can be reviewed. There are three examples of assemblies that utilize 2 layers of 5/8 Type C gypsum wallboard: UL L505 – 75 minutes L511 – 71 minutes L538 – >90 minutes In these assemblies, the resilient channel was spaced 24 o.c., and the wallboard was attached with 8d nails spaced 7 o.c. These tests illustrate the endurance performance of the wallboard, and provide the time it took for this membrane and connection system to provide thermal protection of the assembly to 250° F plus ambient on average, or 325° F, plus ambient at a single point.

Finish rating range of performance
Analysis Two metal plate connected truss tests indicate single gypsum layer finish rating with trusses spaced at 24" o.c. Factory Mutual FC-235 shows 5/8" USG Type C board – 24 minutes PFS shows 5/8" USG Type C board – 23 minutes Performing a combined finish rating analysis yields: Description Time (minutes) Single layer of 5/8" Type C wallboard attached to Trusses Resilient channels Single layer of 5/8" Type C wallboard attached to resilient channels 71-75 Single layer of 5/8" Type X or Type C 23-24 Finish rating range of performance 94-99

Analysis This range is conservative given the additive rule.
This analysis shows the time it takes the back side of the wallboard to reach an average of 250°F above the ambient temperature (approximately = 320°), which is well below the temperature that wood begins to char (482°F).

Assembly Performance Range
Analysis The performance of the trusses in the fire test assemblies FC-235 and PFS after the finish rating was reached was 26 and 29 minutes, respectively. By adding these values to the finish rating range of performance above, we have: Description Time (minutes) Finish Rating Range of Performance 94-99 Truss Performance after Finish rating is achieved 26-29 Assembly Performance Range 120 – 128

Assembly Performance Range
Analysis The ASTM E119 testing justifies the fire endurance performance shown previously. Including the beneficial effect of multiple gypsum layers will add a minimum of 10 minutes to the performance, for a final range of minutes, which is in excess of the 2‑hour rating desired. Description Time (minutes) Finish Rating Range of Performance 94-99 Truss Performance after Finish rating is achieved Beneficial Effect of Adding Gypsum Layers Together 26-29 10 Assembly Performance Range 130 – 138

Analysis – Connection System
In order for the membrane system to protect the trusses from the impact of fire, the connection detailing becomes extremely important. The connections must hold the gypsum membrane in place during the exposure time and have enough withdrawal resistance to hold the dead load of the gypsum wallboard.

Loading (Layer 1) Spacing of trusses 24 o.c. Spacing of screws 6 o.c. Area of gypsum each screw must hold 1 ft2 Number of layers 1 Weight of Type C gypsum per layer 2.5 psf Weight of gypsum held by each screw 2.5 lb To determine whether the fastening of the assembly is adequate, we need to determine for each fastener in the system: The weight of gypsum that must be held by the fastener (dead load) The withdrawal resistance of the fastener from the substrate it penetrates

Withdrawal Resistance (Layer 1) Type of fastener 1‑1/4 Type S, 6 gauge Wood specific gravity (SPF or better) 0.42 Withdrawal value of screw from wood 69 lb/in. Thickness of gypsum 5/8 Penetration into wood Withdrawal resistance per screw lb Withdrawal calculations are based on the 2015 National Design Specification for Wood Construction. With a withdrawal resistance per screw of lb, the Fastener 1 connection is far stronger than the 2.5 lb required.

Loading (Channel) Spacing of resilient channel 24 o.c. Spacing of joists Area of gypsum each screw must hold 4 ft2 Number of layers 2 Weight of Type C gypsum per layer 2.5 psf Weight of gypsum held by each screw 20 lb For the channel attachment we can perform a similar calculation. For this connection, the fastener is attached through the resilient channel and gypsum to the truss.

Withdrawal Resistance (Channel) Type of fastener 1‑7/8 Type S Wood specific gravity (SPF or better) 0.42 Withdrawal value of screw from wood 69 lb/in. Thickness of gypsum 5/8 Penetration into wood 1-1/4 Withdrawal resistance per screw 86.25 lb Using a 1‑7/8 Type S screw that penetrates a 5/8 thick layer of gypsum and the resilient furring channel leaves approximately 1‑1/4 of penetration. Each screw holding the resilient channel to the joist can handle lb, which is far greater than the 20 lb required.

Loading (Layer 2) Spacing of resilient channel 24 o.c. Spacing of screws 6 o.c. Area of gypsum each screw must hold 1 ft2 Number of layers 1 Weight of Type C gypsum per layer 2.5 psf Weight of gypsum held by each screw 2.5 lb Next, the middle layer of the wallboard should be attached to each furring or resilient channel.

The calculated withdrawal resistance of the specified screw penetrating the resilient channel is >60 lb, which far exceeds the 2.5 lb required. Withdrawal Resistance (Layer 2) Type of fastener 1 or 1‑1/4 Type S Resilient Channel 25 gauge, 33 ksi Withdrawal value of screw from channel 60 lb

Loading (Layer 3) Spacing of resilient channel 24 o.c. Spacing of screws 6 o.c. Area of gypsum each screw must hold 1 ft2 Number of layers 1 Weight of Type C gypsum per layer 2.5 psf Weight of gypsum held by each screw 2.5 lb Finally, the face layer of the wallboard is attached to each furring or resilient channel through the middle layer wallboard.

The withdrawal resistance of 60 lb per screw in resilient channels is more than sufficient to carry the load. Each fastener in the assembly is more than adequate to support the dead loads it must carry. Withdrawal Resistance (Layer 3) Type of fastener 1‑5/8 or 1‑7/8 Type S Resilient Channel 25 gauge, 33 ksi Withdrawal value of screw from channel 60 lb

Analysis – Assembly Modifications (Depth/Spacing)
The trusses in this design have a minimum depth of 12. Trusses with depths greater than 12 may be used. Per GA-600, dimensions included for depth of assemblies are minimums, spacings are maximums.

Analysis – Assembly Modifications (Insulation)
Both UL and GA include specific guidance regarding the use of insulation in assemblies. Many designs listed by UL BXUV allow addition of any depth of insulation at any location in an assembly that does not include insulation, but only as long as an additional layer of identical gypsum is installed at the ceiling. Any other method of adding insulation is prohibited in assemblies tested without insulation. GA-600 includes a similar provision.

Analysis – Assembly Modifications (Insulation)
However, IBC methods #4 or #5 do allow modifications to assemblies, which would include inclusion or placement of insulation, based on rational design provided to the building official. In deeper trusses, the space above the 12 plenum depth may contain insulation, provided the insulation is supported adequately so that the plenum depth is maintained at 12 from the backside of the gypsum due to the conservative design methodology.

Conclusion This assembly has been analyzed and shown to achieve a 2-hour fire endurance performance rating, based both on the component additive method (CAM) and verified by comparison to existing ASTM E119 tested assemblies. We have deliberately been conservative in the development of this assembly to ensure 2-hour performance in the following ways: Type C gypsum wallboard is specified instead of Type X. This wallboard has proven to be a more stable wallboard, and we feel it will more adequately protect the truss assembly. By placing the resilient channels over the top of wallboard layer 1, the channels will hold the wallboard in place and enhance the performance of the fire endurance assembly. This is due to the increased likelihood the wallboard will not fall away from the trusses prematurely.

Conclusion Additionally:
The Type S gypsum wallboard screws are spaced at 6 o.c. to provide good connection support of the wallboard membrane. We have also checked the loads carried by each fastener to ensure that there is a large margin of reserve capacity for each screw. The key to an assembly's performance is an intact membrane. This fastener schedule seeks to ensure the membrane will stay intact and perform as expected. Given the above, we are confident that the described assembly will afford a 2-hour fire endurance performance as required by the building code.

References SBCA Research Report 1509-01
National Design Specification® for Wood Construction (NDS®) 2012 or 2015, American Wood Council (AWC), Section 16, Fire Design of Wood Members. Analytical methods for determining fire resistance of timber members, Robert White, 2008, included in the SFPE handbook of fire protection engineering. Quincy, Mass.; National Fire Protection Association; Bethesda, Md.: Society of Fire Protection Engineers, c2008: pages Guidelines on Fire Ratings of Archaic Materials and Assemblies. International Code Council (ICC). National Building Code of Canada, 2010.

References International Building Code (IBC), 2012 and 2015, International Code Council (ICC). BXUV.GuideInfo, Underwriters Laboratory. Gypsum Association Handbook, GA-600, 2012 Fire Resistance Provided by Gypsum Board Membrane Protection, GA-610, 2013 ESR-1338, Gypsum Wall and Ceiling Assemblies and Gypsum Board Interior and Exterior Applications Intertek Directory of Certified Products PFS Corporation Factory Mutual

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