2-Hour Fire Endurance Assembly forMPCWT Structural Floor and Roof Framing Systems

Educational Overview

Revised 10/2/2017

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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 704.1.1 of the 1996 BOCA National Building Code

• Section 703.3 of the 1994 Uniform Building Code

• 701.2.1 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 1509-02 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 722.6.1.1), that limitation does NOT apply to method 4.

• Prescriptive design in accordance with Section 721 (#2) Table 721.1(3), item 28-1.1, 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 4x 8 sheets

5/8 or greater thickness

Manufactured with exterior glue

Have tongue-and-groove edges along the 8 side

Specifications – (1) Subfloor/Finish Floor

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.

Specifications – (1) Subfloor/Finish Floor

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 24o.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 5/8Type 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

Material5/8 Type C gypsum

wallboard

Attached to Furring/resilient channel

Fastener type1-1/4 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

Material5/8 Type C gypsum

wallboard

Attached toFurring/resilient channel

through Layer 2

Fastener type1-5/8 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 S

Type G

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.

Analysis – Membrane Protection

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.”

Analysis – Membrane Protection

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.

Wallboard Membrane Description Time (minutes)3/8 Douglas fir plywood phenolic

bonded5

1/2 Douglas fir plywood phenolic bonded

10

5/8 Douglas fir plywood phenolic bonded

15

3/8 gypsum wallboard 101/2 gypsum wallboard 15

1/2 Type X gypsum wallboard 255/8 gypsum wallboard 20

5/8 Type X gypsum wallboard 40

Double 3/8 gypsum wallboard 251/2 and 3/8 gypsum wallboard 35

Double 1/2 gypsum wallboard 40

Analysis – Membrane Protection

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.

Analysis – Membrane Protection

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.

Analysis – Membrane Protection

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 405/8 Type X gypsum wallboard 405/8 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

Analysis – Performance Enhancements

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.

Analysis – Performance Enhancements

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.

Analysis – Performance Enhancements

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

Analysis – Performance Enhancements

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

wallboard120

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.

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 88-03 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 250 + 70 = 320°), which is well below the temperature that wood begins to char (482°F).

Analysis

The performance of the trusses in the fire test assemblies FC-235 and PFS 88-03 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

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 130 -138 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.

Analysis – Connection System

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

Loading (Layer 1)

Spacing of trusses 24 o.c.

Spacing of screws 6 o.c.

Area of gypsum eachscrew 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

Analysis – Connection System

Withdrawal calculations are based on the 2015 National Design Specification for Wood Construction.

With a withdrawal resistance per screw of 43.125 lb, the Fastener 1 connection is far stronger than the 2.5 lb required.

Withdrawal Resistance (Layer 1)

Type of fastener1-1/4 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 5/8

Withdrawal resistance perscrew

43.125 lb

Analysis – Connection System

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.

Loading (Channel)

Spacing of resilient channel

24 o.c.

Spacing of joists 24 o.c.

Area of gypsum eachscrew 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

Analysis – Connection System

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 86.25 lb, which is far greater than the 20 lb required.

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

Analysis – Connection System

Next, the middle layer of the wallboard should be attached to each furring or resilient channel.

Loading (Layer 2)

Spacing of resilient channel

24 o.c.

Spacing of screws 6 o.c.

Area of gypsum eachscrew 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

Analysis – Connection System

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

Analysis – Connection System

Finally, the face layer of the wallboard is attached to each furring or resilient channel through the middle layer wallboard.

Loading (Layer 3)

Spacing of resilient channel

24 o.c.

Spacing of screws 6 o.c.

Area of gypsum eachscrew 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

Analysis – Connection System

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 fastener1-5/8 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 703.3 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 12from 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 4.346-4.366.

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