Fire Resistance of 

Structural Composite 

Lumber Products

Robert H. White

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–633
April 2006

White, Robert H. 2006. Fire resistance of structural composite lumber 

products. Research Paper FPL-RP-633. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 28 p.

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Abstract

Use of structural composite lumber products is increasing. 

In applications requiring a fire resistance rating, calculation procedures are used to obtain the fire resistance rating 

of exposed structural wood products. A critical factor in 

the calculation procedures is char rate for ASTM E 119 fire 

exposure. In this study, we tested 14 structural composite 

lumber products to determine char rate when subjected to 

the fire exposure of the standard fire resistance test. Char 

rate tests on 10 of the composite lumber products were also 

conducted in an intermediate-scale horizontal furnace. The 

National Design Specification/Technical Report 10 design 

procedure for calculating fire resistance ratings of exposed 

wood members can be used to predict failure times for 

members loaded in tension. Thirteen tests were conducted in 

which composite lumber products were loaded in tension as 

they were subjected to the standard fire exposure of ASTM 

E 119. Charring rates, observed failure times in tension 

tests, and deviations from predicted failure times of the 

structural composite lumber products were within expected 

range of results for sawn lumber and glued laminated 

timbers. 

Keywords: structural composite lumber, LVL, fire resistance, char rate

Acknowledgments

This study was suggested by Rodney McPhee of the Canadian Wood Council (CWC). Funds for this study were provided by the CWC. The following manufacturers provided 

the structural composite lumber (SCL) test materials: Boise 

Cascade, Georgia–Pacific, Louisiana–Pacific, Trus Joist 

MacMillan (now Trus Joist, a Weyerhaeuser Business), and 

Willamette Industries (now part of Weyerhaeuser Company). Advice and suggestions were provided by the members 

of the CWC/FPL SCL Charring Rate Study task group of 

the Wood I-Joist Manufacturers Association. This task group 

was chaired by Bruce Craig. Comazel Caldwell of the FPL 

fire research work unit made the specimens and conducted 

the tests. The author greatly appreciates the support, assistance, and patience from the parties involved.

Contents

Page 

Introduction...........................................................................1

Background...........................................................................1

 Fire Resistance Calculations.............................................1

 Char Rate of Wood............................................................2

 National Design Specification Technical 

 Report 10 Procedure .........................................................3

Char Rate Experiments.........................................................3

 Materials ...........................................................................3

 Small Vertical-Furnace Tests.............................................4

 Test method...................................................................4

 Test results.....................................................................5

 Horizontal-Furnace Tests................................................10

 Test methods ...............................................................10

 Test results...................................................................12

 Discussion.......................................................................13

Tension Tests.......................................................................15

 Methods and Materials....................................................18

 Failure Times ..................................................................20

 National Design Specification Model Predictions..........20

Conclusions.........................................................................26

Literature Cited ..................................................................26

Appendix—Individual Test Results for Each Small 

Vertical-Furnace Test ..........................................................28
Fire Resistance of Structural Composite 

Lumber Products

Robert H. White, Research Wood Scientist

Forest Products Laboratory, Madison, Wisconsin

Introduction

Production and use of structural composite lumber (SCL) 

products are increasing (McKeever 1997, Schuler and others 2001). Such products include laminated veneer lumber 

(LVL), parallel strand lumber (PSL), laminated strand 

lumber (LSL), and oriented strand lumber (OSL) (Green 

and Hernandez 1998, Moody and others 1999, ASTM International 2005) (Fig. 1). Introduction of composite lumber 

products started in the 1970s with LVL and continued with 

introduction of PSL in the 1980s and LSL in the 1990s 

(Yeh 2003). Oriented strand lumber is the latest of these 

products. For each of these products, adhesives are utilized to manufacture the composite product from veneers 

or strands of wood. Questions are sometimes raised about 

the performance of the adhesive when exposed to elevated 

temperature. The code and market acceptance of products 

depends on documentation of acceptable performance. 

Code acceptance of calculation procedures for determining 

fire resistance ratings of exposed wood beams and columns 

permitted their use in applications requiring structural members to have specified fire resistance ratings. As a result, the 

potential market for such wood products increased. This report provides test data on the charring rate of three types of 

composite lumber products (LVL, LSL, and PSL) and their 

performance when loaded in tension and subjected to the 

fire exposure specified in the standard fire resistance test. 

Several species of composite lumber products were included 

in testing. In the first two phases of this project, we documented charring rates of composite lumber products. Fire 

tests conducted in the first two phases of this project did not 

include any load being applied to the member. Results of 

Phase One were initially published in a conference proceedings article, “Charring Rate of Composite Timber Products” 

(White 2000). These one-dimensional charring experiments 

were conducted in the small vertical-furnace at the USDA 

Forest Products Laboratory (FPL), Madison, Wisconsin. In 

the second phase, we measured temperatures in the interior 

of a non-loaded beam subjected to the ASTM E 119 (ASTM 

International 2000) fire exposure in an intermediate-scale 

horizontal furnace. Charring rates determined in the two 

furnaces were compared. In the third and final phase of 

the study, a series of SCL test specimens were subjected to 

ASTM E 119 fire exposure while loaded in tension. 

Background

Fire Resistance Calculations

Fire resistance ratings of structural members are normally 

determined by conducting the full-scale test described in 

specifications of ASTM E 119 or similar standards. Calculation procedures for determining fire resistance rating of 

wood members have code acceptance (White 2002). Lie 

(1977) developed the first procedure to gain code acceptance in the United States and Canada for wood beams and 

columns (American Institute of Timber Construction 1984, 

Canadian Wood Council 1997, American Forest & Paper 

Association 2000, 2003). A more recent calculation procedure, the National Design Specification (NDS) method, was 

developed by the American Wood Council (AWC) of the 

American Forest & Paper Association. It was included in the 

NDS (American Forest & Paper Association 2001) starting 

with the 2001 edition. This new, more explicit procedure is 

applicable to other structural members besides beams and 

columns. Per NDS, the fire design procedure is applicable to 

all wood structural members and connections covered under 

NDS. These include wood members of solid-sawn lumber, 

structural glued-laminated timber, and structural composite 

Figure 1. Examples of types of composite lumber products tested in this study: (left to right) laminated veneer 

lumber, parallel strand lumber, and laminated strand 

lumber.
Research Paper FPL–RP–633

2

lumber. Methodology and supporting data for solid-sawn 

lumber and structural glued-laminated timber are fully discussed in “Calculating the fire resistance of exposed wood 

members,” Technical Report 10 (TR 10) of the American 

Forest & Paper Association (2003).

Tests of three glued-laminated specimens were conducted 

to verify calculation procedures for determining the fire 

resistance rating of axially loaded wood tension members 

contained in TR 10 (White 2004). Tests were conducted 

using the horizontal furnace and tension apparatus at the 

FPL. Glued-laminated specimens were exposed to an 

ASTM E 119 fire exposure while loaded in tension for 

the full duration of the test. 

Other options for analytical methods for determining fire 

resistance of timber members are reviewed by White (2002). 

Analytical methods in the updated Eurocode 5 document of 

Europe are discussed by König (2005).

A critical parameter in calculation procedures is charring 

rate of the member while directly subjected to fire exposure 

of the time–temperature curve specified in ASTM E 119. 

Char Rate of Wood

Parameters affecting charring rate for solid wood have 

been extensively studied at the FPL (Schaffer 1967, White 

and Nordheim 1992, White and Tran 1996) and elsewhere 

(White 1988, White 2002). For solid wood and gluedlaminated members, the value for charring rate generally 

used in the United States and Canada is 0.635 mm/min. The 

0.60 mm/min value was used in developing calculation procedures for large wood members (Lie 1977).

The following time–location models for the time needed to 

obtain a char depth (xc) were considered by White and Nordheim (1992) and White (1988):

(1)

(2)

or its linear form

(4)

and 

Equation (1) is the simple linear single parameter (m1) 

model (zero-intercept) that is generally reported for the 

charring rate of wood products. Equation (1) is also the oneparameter model assumed when charring rate is calculated 

from residual section and duration of fire exposure. The value of m1 for the conventional 0.635-mm/min charring rate 

is 1.575 min/mm. Equation (2) is a two-parameter model 

(m2 and b) that allows a fast initial char rate followed by a 

slower linear char rate. Equation (3) is a nonlinear charring 

model with two parameters (m3 and a). Equation (4) is the 

linear form of Equation (3). Results for Equation (3) can be 

obtained by linear regression of Equation (4). The model of 

Equation (5) was developed by White and Nordheim (1992). 

This nonlinear model was used in the methodology of TR 

10. The value of m5 for the conventional 38-mm char depth 

at 1 h is 0.682 min/mm1.23. 

Charring data on composite lumber products in the public 

domain have been limited. In a series of cone calorimeter 

tests of sawn lumber and wood composites, Mikkola (1990, 

1991) tested a 37-mm-thick laminated veneer lumber product with a density of 520 kg/m3. For an external exposure 

of 50 kW/m2, char rates were 1.05, 0.82, and 0.68 mm/min 

for moisture contents of 0%, 10%, and 20%, respectively. 

For the 38-mm-thick spruce sawn lumber with density of 

490 kg/m3, char rates were 1.06, 0.80, and 0.60 mm/min for 

moisture contents of 0%, 10%, and 20%, respectively. Ignition properties and char rates from cone calorimeter tests of 

radiata pine were reported by Lane and others (2004). Getto 

and Ishihara (1998) found that fire resistance of untreated 

wood or fire-retardant-treated wood was improved by compression of the board. Fire resistance of lathed-veneer laminated boards was greater than that of the solid-sawn board. 

In a study of LVL with different types of joints, Uesugi and 

others (1999) obtained charring rate of 0.6 to 0.7 mm/min 

in tests of Douglas-fir and larch LVL and concluded that 

these materials showed the same performance in fire as 

heavy timber. Subyakto and others (2001) investigated fire 

resistance of a LVL–metal plate connection and its improvement with graphite phenolic sphere sheeting. Experimental 

results for parameters of both Equations (1) and (5) for six 

wood composite rim board products were reported by White 

(2003). The six products included three oriented strandboards (OSB), a plywood, an LVL, and a com-ply product. 

In addition to data for the exposed rim boards, White (2003) 

also provided parameter estimates for rim boards protected 

with gypsum board. More recently, fire resistance of LVL 

was investigated at the University of Canterbury in New 

Zealand (Lane and others 2004). In a report on variability 

of wood charring rates, Hietaniemi (2005) included unpublished data for Kertopuu LVL specimens obtained from 

private communication with T. Oksansen. For LVL data obtained from T. Oksansen, Hietaniemi (2005) reported mean 

charring rates of 0.60 and 0.76 mm/min for charring perpendicular and parallel to the veneers, respectively. For each 

direction, coefficients of variation were 8% to 9%. 

Charring tests in the small vertical-furnace included in this 

report were previously discussed by White (2000). Results 

for the composite lumber products were comparable to those 

for solid-sawn lumber. As with solid-sawn lumber, charring 

(3)

(5)
Fire Resistance of Structural Composite Lumber Products

3

rate of a specific composite lumber product would depend 

on density, moisture content, and species. 

National Design Specification Technical 

Report 10 Procedure

The NDS method described in TR 10 (American Forest & 

Paper Association 2003) is a reduced-section method for 

calculating fire resistance of an individual structural wood 

member (White 2002). For each surface of the member subjected to fire exposure, charring reduces the cross-sectional 

area of the member. In TR 10, a form of Equation (5) is used 

to calculate char depth at time t. A nominal char rate, βn 

(mm/min or in/h), is initially assumed. This is a linear char 

rate based on char depth for 1-h exposure, and in most cases 

a value of 38 mm at 60 min is used. As noted in the NDS 

(American Forest & Paper Association 2001), this nominal 

char rate of 38 mm/h is commonly assumed for solid-sawn 

and structural glued-laminated softwood members. The char 

depth, xc, at time t is calculated from

 (6)

To account for loss of strength from elevated temperatures 

within the residual cross-sectional area of the member and 

rounding the corners of a rectangular member, the crosssectional area is further reduced to an effective cross-sectional area. In the TR 10 procedure, additional reduction in 

dimensions is 20% of char depth. Thus, the effective char 

rate used to calculate section properties of the charred member at time t is

 (7)

For the nominal char rate of 38 mm at 1 h, the effective 

char rates are 46 mm/h, 42 mm/h, and 40 mm/h for 60, 90, 

and 120 min, respectively. For a rectangular member with 

dimensions of B and D and all four sides exposed to the fire, 

the area of the cross section at time t, A(t), is

 (8)

With this effective reduced cross-sectional area, the ultimate 

load bearing capacity of the member at time t is calculated 

using normal room temperature assumptions. The failure 

criteria for tension members are

 (9)

where

D is design dead load,

L design live load,

K factor to adjust from nominal design capacity to 

 average ultimate capacity,

RASD nominal allowable design capacity or FtAf,

Ft tabulated tension parallel-to-grain design 

 value, and 

Af area of cross section using cross-section 

dimensions reduced from fire exposure 

that will result in failure of the tension 

member.

For a member in tension, the allowable design stress to average ultimate strength adjustment factor, K, is 2.85 (American Forest & Paper Association 2003). For the fire design 

procedure in the NDS, this value for design stress to member strength factor is considered valid for solid-sawn, gluedlaminated timber, and structural-composite lumber wood 

members (American Forest & Paper Association 2003). 

Calculated failure time or fire resistance rating is the time 

for which the effective cross-sectional area has been reduced 

to Af per the failure criteria of Equation (9).

Char Rate Experiments

This project included the determination of charring rate of 

composite lumber products in two fire resistance furnaces 

at the FPL. The small vertical-furnace was used to obtain 

the one-dimensional charring rates for the widest range of 

products and included replicates for statistical comparison. 

Tests in the intermediate-scale horizontal furnace provided 

verification that data obtained in the smaller furnace would 

likely be valid for the even larger test furnaces specified in 

ASTM E 119.

Materials

Fourteen different materials were tested in the small vertical-furnace experiments of Phase 1 (Table 1). Ten of the 

fourteen materials were tested in horizontal-furnace tests of 

Phase 2 (Table 2). Three general types of products were tested: LVL, PSL, and LSL. No OSL product was tested. Species included two aspen products, four Douglas-fir products, 

three Southern Pine products, four yellow-poplar products, 

and one eucalyptus product (Table 1). Except for Material 

Number 7 (Table 1), the products tested were those 

commercially available. The eucalyptus LVL (Material 

Number 7) was a prototype sample of the product. 

Laminated strand lumber is made of strands of wood that 

are glued together such that strands are parallel to the longitudinal or axial direction of the lumber product. Laminated 

veneer lumber is a composite made by laminating sheets of 

veneer with an adhesive so the longitudinal grain of the majority of veneers is parallel to the longitudinal or axial direction of the lumber product. The veneers are end-jointed with 

a lap, butt, or scarf joint. In typical applications, the glue 

lines are vertical. Parallel strand lumber is made of strands 

obtained by clipping sheets of veneer with strands aligned 

parallel to the longitudinal axis. The wide face of the strands 

is parallel to the tangential direction and perpendicular to 

the radial direction of the wood itself. Aligned strands are 

pressed together with adhesive.

Information on adhesives used in the products (Tables 1 

and 2) was provided by the manufacturers. Exterior-type 
4

adhesives typically were phenol–formaldehyde-based 

adhesives. Other adhesives included isocyanate-based adhesives (MDI, or methylene diphenyl diisocyanate). 

Individual pieces of test materials were glued together to 

make test specimens for the small vertical-furnace tests. For 

some of the horizontal-furnace charring tests of Phase 2, 

FPL researchers glued manufactured products together with 

a phenol–resorcinol adhesive to obtain a thicker specimen 

(Table 2). 

All specimens were conditioned at 23oC, 50% relative humidity prior to testing as specified in ASTM E 119. Moisture contents were 6% to 8%.

Small Vertical-Furnace Tests

Test method 

In the small vertical-furnace tests, the specimens of the 

structural composite lumber products were inserted in the 

510-mm-square opening of the small gas-fired verticalfurnace (Fig. 2) to obtain the one-dimensional charring 

rates. The procedures were similar to those used to determine char rate for eight species of solid lumber (White 

1988, White and Nordheim 1992). Natural gas supply was 

controlled so the temperature determined with a thermocouple in an iron-capped pipe near the exposed surface of 

the specimen followed the ASTM E 119 time–temperature 

curve (Fig. 2a). A single center thermocouple was used to 

 Table 1. Composite lumber products tested 

Material 

number Type Species Adhesivea

Densityb

(kg/m3)

 Material number 

 in White (2000) 

1 LSL Aspen MDI 674 1 

2 LSL Yellow-poplar MDI 678 9 

3 LVL Aspen PF 464 – 

4 LVL Douglas-fir PF 529 2 

5 LVL Douglas-fir PF 535 4 

6 LVL Douglas-fir PF 552 3 

7 LVL Eucalyptus PFc 586 – 

8 LVL Southern Pine PFc 652 6 

9 LVL Southern Pine PF 635 7 

10 LVL Yellow-poplar PF 536 11 

11 LVL Yellow-poplar PF 554 10 

12 PSL Douglas-fir PF 610 5 

13 PSL Southern Pine PF 728 8 

14 PSL Yellow-poplar PF 628 12 a

PF, phenol–formaldehyde. 

 MDI, methylene diphenyl diisocyanate. b

Density calculated from oven-dried mass and volume as tested. 

c

Hot setting phenol–resorcinol–formaldehyde adhesive is used to glue 1/10-in.-thick veneers into 

 7/8-in. panels in plywood press. These panels are finger-jointed with a cold setting PRF 

 adhesive and pressed into final product using a cold setting isocyanurate adhesive.

Table 2. Tests of unloaded specimens in horizontal furnace 

Test

number

Material 

numbera Species 

Composite

 type 

Width

(mm)

Height

 (mm) 

Range of char 

depthb

 (mm) 

Max. timec

 (min) 

FPL test 

number

1 1 Aspen LSL 175 d 356 14–57 95 2110 

2 4 Douglas-fir LVL 180 455 13–60 91 2112 

3 5 Douglas-fir LVL 44 352 3–13 16 2117 

4 5 Douglas-fir LVL 83 351 12–28 45 2108 

5 5 Douglas-fir LVL 167 d 352 12–60 85 2113 

6 6 Douglas-fir LVL 179 e 352 14–57 86 2118 

7 8 Southern Pine LVL 169 e 407 13–57 63 2120 

8 9 Southern Pine LVL 173 e 352 13–58 76 2115 

9 10 Yellow-poplar LVL 170 e 353 12–59 76 2111 

10 11 Yellow-poplar LVL 85 d 403 12–27 34 2109 

11 11 Yellow-poplar LVL 170 e 405 13–59 82 2119 

12 12 Douglas-fir PSL 177 357 12–57 86 2116 

13 13 Southern Pine PSL 176 355 12–60 92 2114 a

Numbers correspond to those listed in Table 1. 

b

Range of char depths used in the regression calculations of the charring rates (Tables 8 and 9). c

Maximum time for the data used in the regression calculations of the charring rates (Tables 8 and 9). d

Thickness obtained by FPL gluing two of the manufactured products together with phenol–resorcinol glue. e

Thickness obtained by FPL gluing four of the manufactured products together with phenol–resorcinol glue. 

Research Paper FPL–RP–633
5

control the furnace, but there are four additional thermocouples at the centers of the quadrants. 

Specimens were 510 mm wide and 89 mm deep. The 

250- or 264-mm-high specimens were constructed by gluing five 50-mm-thick pieces (or six 44-mm-thick pieces) 

together with phenol–resorcinol adhesive (Fig. 2b). Which 

of the two setups was used depended on available dimensions of the lumber product. Charring occurred in the 

89-mm direction.

Because of their construction, products are anisotropic. 

We obtained charring rates for both transverse directions 

(perpendicular to wood grain). No tests were conducted 

for charring in the longitudinal direction (parallel to wood 

grain). The LVL specimens were tested with the direction of 

charring either parallel or perpendicular to the plane of veneer laminates. We tested PSL and LSL specimens with the 

direction of charring either perpendicular or parallel to the 

wide face of the original beam that was cut to obtain pieces 

for the specimens. Two replicates of each specimen type and 

transverse direction were tested. For test materials that were 

only provided with a width of 44 mm, we glued two pieces 

together to get the 89-mm dimension in the charringperpendicular-to-veneer tests.

Temperature measurements were taken within the middle 

three pieces or layers of the test specimen, where two thermocouples were embedded at each of four distances from 

the exposed surface, 13, 25, 38, and 51 mm (Fig. 3). For 

each test, specimens 250 or 264 mm high were placed at 

mid-height in the 510-mm-square furnace opening so that 

they were opposite the center control furnace thermocouple 

(Fig. 2). Thermocouples were made from Type K (chromelalumel), 30-gauge (0.254-mm-diameter) insulated thermocouple wire. Thermocouple data were collected on a personal computer data acquisition system. Data acquisition scans 

in these small vertical-furnace tests occurred every 3 or 5 s. 

Tests were terminated when the last of the six thermocouples at 51-mm depth reached 300°C. The 300°C criterion 

for the base of the char layer was used to calculate 

charring rate. This criterion of 300°C or the criterion of 

288°C were successfully used in earlier charring studies 

(Schaffer 1967, White 1988). The 288°C is the exact conversion of 550°F used in the early studies. Because temperatures are rapidly increasing at that point of the test, we consider the difference in times for the 288°C and 300°C criteria not to be significant. In a study of composite-rim boards, 

White (2003) found the average times for the 300°C criteria 

to be less than 1% greater than for the 288°C criteria. 

Test results

As just explained, charring was assumed to have occurred 

when thermocouples embedded in the specimen recorded a 

temperature in excess of 300°C. Visual observations of char 

depths after the tests were consistent with depths calculated 

from the 300°C criterion.

For each specimen, 24 pairs of time t and char depth (xc) 

were used to calculate char rates for each test (Fig. 4). These 

24 pairs are for char depths of 13, 25, 38, and 51 mm from 

the fire-exposed surface. We obtained estimates for char rate 

parameters for Equations (1) and (5) discussed previously. 

Figure 2. Opening of small vertical furnace (a) without and (b) with a test specimen in place.

Fire Resistance of Structural Composite Lumber Products
6

These time–location models are

and

Results for linear regression of data for each small verticalfurnace test are given in the Appendix. A total of 58 small 

vertical-furnace tests were conducted. Individual thermocouples of a test specimen were in one of three different 

layers of the test specimen. A full model includes as a variable the height of the thermocouple in the specimen. In this 

model (White 1988), overall coefficients of variation of predicted values were 6.6% for m1 and 6.2 % for m5. Individual 

parameter estimates for each individual layer are included in 

the Appendix. Results for each layer provide an indictor of 

the variability of char rate determinations. For the reduced 

model (Eqs. (1) and (5)) that considers only distance 

from the exposed surface, xc, the overall coefficients of 

variation of predicted values were 7.1% for m1 and 6.7% 

for m5. Results presented in this paper are for the reduced 

model.

In Tables 3 and 4, data for individual tests were combined 

for linear regressions to obtain parameter estimates. For 

each material (Table 1), estimates of the parameter m1

(Table 3) are provided for charring parallel and perpendicular to the plane of veneers for LVL or the plane of the 

normal vertical surface of LSL or PSL. Equation (1) is the 

simple linear one-parameter model most often used to define charring rate of wood when subjected to standard fire 

exposure. There were two replicates for each direction for 

all but one case. For Material Number 8, three replicates for 

each direction were tested. Table 3 gives the number N of 

pairs of time for 300°C and corresponding depth of thermocouple used in linear regressions to obtain estimates. The 

conventional 0.635 mm/min charring rate corresponds to m1

of 1.575 min/mm. Also in Table 3 are estimates for m1 when 

all data for a material are combined. For each estimate of 

the mean for the parameter m1, the standard error of estimate 

is listed in Table 3. Likewise, the coefficient of variation of 

predicted times for the Equation (1) model is listed. These 

measurements of variability were obtained by using individual pairs of times for 300°C and char depth for the data 

set. In Table 4, data for similar materials were combined for 

linear regressions to obtain parameter estimates of a more 

generic nature. Format and information listed in Table 4 are 

similar to those in Table 3. If the 38 individual m1 results for 

LVL tests are used for the data set, standard error and coefficient of variation for the m1 mean of 1.50 min/mm are 

0.02 min/mm and 8%, respectively. Regression of all data 

for different composite lumber products produced m1 of 

1.54 min/mm (Table 4). If the 58 individual m1 results for 

all tests are used for the data set, standard error and coefficient of variation for the m1 mean of 1.54 min/mm are 

0.01 min/mm and 7.1%, respectively.

Tables 5 and 6 are similar to Tables 3 and 4 except the data 

are for parameter m5 of Equation (5). The one-parameter 

model of Equation (5) is used in the fire resistance calculation procedures of TR 10. If the 38 individual m5 results for 

LVL tests are used for the data set, standard error, and coefficient of variation for the m5 mean of 0.638 min/mm1.23 are 

0.008 min/mm1.23 and 8%, respectively. For a char depth of 

38 mm at 60 min, m5 is 0.682 min/mm1.23. Regression of all 

composite lumber data together produced a value for m5 of 

0.652. min/mm1.23 (Table 6). If the 58 individual m5 results 

for all tests are used for the data set, standard error, and coefficient of variation for the m5 mean of 0.653 min/mm1.23

are 0.006 min/mm1.23 and 7.1%, respectively. 

In Equation (5), the exponent of the nonlinear model has a 

value of 1.23. For Equations (3) and (4), in which the exponent a is a second variable, regression of all data together 

produced a mean estimate for a of 1.151 and a standard error of the estimate of 0.006.

Figure 3. Location of thermocouples in one of the laminates that was glued together to make the specimen for 

small vertical-furnace tests. With thermocouples in three 

laminates, a total of six thermocouples was at each of 

the four distances (13, 25, 38, and 51 mm) from the fireexposed surface.

Figure 4. Example of the time–location data for one 

small vertical-furnace test. The data were obtained from 

24 thermocouples located at different depths in a specimen. The curve is a power function. 

Research Paper FPL–RP–633

(1)

(5)
7

Table 3. Estimates for parameter m1 of Equation (1) for the 14 materials tested in the small vertical furnace 

m1 of Equation (1) m1 of Equation (1) for both directionsg

Material 

numbera

Species 

composite type Directionb Nc

Meand

(min/mm)

Standard errore

(min/mm)

Predicted COVf

(%)

Meand

(min/mm)

Standard errore

(min/mm) 

Predicted COVf

(%)

1 Aspen LSL Parallel 48 1.601 0.013 6.4 

Perpendicular 47 1.557 0.013 6.2 1.579 0.009 6.5 

2 Yellow-poplar LSL Parallel 47 1.586 0.016 7.7 

Perpendicular 48 1.593 0.017 8.2 1.589 0.011 7.9 

3 Aspen LVL Parallel 48 1.511 0.020 10.4 

Perpendicular 47 1.448 0.012 6.4 1.480 0.012 9.1 

4 Douglas-fir LVL Parallel 48 1.531 0.012 6.0 

Perpendicular 48 1.526 0.011 5.6 1.529 0.008 5.8 

5 Douglas-fir LVL Parallel 48 1.660 0.015 7.2 

Perpendicular 48 1.559 0.015 7.5 1.610 0.012 8.1 

6 Douglas-fir LVL Parallel 48 1.626 0.014 6.6 

Perpendicular 48 1.620 0.014 6.7 1.623 0.010 6.6 

7 Eucalyptus LVL Parallel 44 1.674 0.027 11.9 

Perpendicular 46 1.661 0.018 8.4 1.667 0.016 10.2 

8 Southern Pine LVL Parallel 72 1.446 0.010 6.4 

Perpendicular 71 1.401 0.014 9.2 1.422 0.008 8.0 

9 Southern Pine LVL Parallel 48 1.552 0.013 6.6 

Perpendicular 48 1.409 0.015 8.2 1.480 0.012 9.1 

10 Yellow-poplar LVL Parallel 48 1.452 0.011 5.7 

Perpendicular 48 1.323 0.014 8.5 1.387 0.011 8.8 

11 Yellow-poplar LVL Parallel 48 1.421 0.011 6.2 

Perpendicular 47 1.323 0.020 12.0 1.372 0.013 10.1 

12 Douglas-fir PSL Parallel 47 1.625 0.016 7.7 

Perpendicular 48 1.623 0.012 5.7 1.624 0.010 6.7 

13 Southern Pine PSL Parallel 48 1.623 0.017 8.3 

Perpendicular 48 1.630 0.014 6.4 1.627 0.011 7.4 

14 Yellow-poplar PSL Parallel 38 1.556 0.015 6.9 

Perpendicular 48 1.579 0.012 6.0 1.570 0.010 6.4 

aNumbers correspond to those listed in Table 1. bDirection of charring relative to the plane of veneers in the laminated veneer lumber (LVL) or normal vertical surface of the laminated strand lumber (LSL) or parallel strand lumber (PSL). cNumber of pairs of time for 300°C and corresponding depth of thermocouples used in linear regression to obtain estimate of parameter. dMean estimate of the parameter m1 in Equation (1). eStandard error of estimate for the parameter m1 in Equation (1). fCoefficient of variation of the predictions from Equation (1). gData for both charring directions relative to the plane of the veneers are combined for these results. 

Fire Resistance of Structural Composite Lumber Products
8 Table 4. Estimates for parameter 

m1 of Equation (1) for combinations of the 14 materials tested in small vertical furnace 

m1 of Equation (1) 

m1 of Equation (1) for both directionsg

 Meand Standard errore Predicted COVf Meand Standard errore Predicted COVf Material 

numbersa

Species 

composite type Directionb

Nc (min/mm) (min/mm) (%) (min/mm) (min/mm) (%) 

4, 5 ,6 Douglas-fir LVL Parallel 144 1.606 0.009 7.6 

Perpendicular 144 1.568 0.008 7.2 1.587 0.006 7.5 

8.9 Southern Pine LVL Parallel 120 1.487 0.009 7.6 

Perpendicular 119 1.404 0.010 8.8 1.446 0.007 8.7 

10, 11 Yellow-poplar LVL Parallel 96 1.436 0.008 6.0 

Perpendicular 95 1.323 0.012 10.3 1.380 0.008 9.4 

1, 3 Aspen Parallel 96 1.556 0.013 9.1 

Perpendicular 94 1.503 0.010 7.5 1.530 0.008 8.6 

4, 5 ,6 ,12 Douglas-fir Parallel 191 1.611 0.008 7.6 

Perpendicular 192 1.582 0.007 7.0 1.596 0.005 7.4 

8, 9, 13 Southern Pine Parallel 168 1.526 0.009 9.0 

Perpendicular 167 1.469 0.011 11.2 1.498 0.008 10.3 

2, 10, 11, 14 Yellow-poplar Parallel 181 1.497 0.008 8.5 

Perpendicular 191 1.455 0.012 13.3 1.475 0.008 11.3 

1, 2 LSL Parallel 95 1.594 0.010 7.1 

Perpendicular 95 1.574 0.010 7.3 1.584 0.007 7.2 

3–11 LVL Parallel 452 1.534 0.006 10.0 

Perpendicular 451 1.470 0.007 11.9 1.502 0.005 11.2 

12–14 PSL Parallel 133 1.608 0.010 8.0 

Perpendicular 144 1.610 0.007 6.2 1.609 0.006 7.1 

1–14 All data Parallel 680 1.556 0.005 9.5 

Perpendicular 690 1.514 0.006 11.1 1.535 0.004 10.4 

aNumbers correspond to those listed in Table 1. bDirection of charring relative to the plane of the veneers in the laminated veneer lumber (LVL) or normal vertical surface of the laminated strand lumber (LSL) or parallel strand 

 lumber (PSL). 

cNumber of pairs of time for 300°C and corresponding depth of thermocouples used in linear regression to obtain estimate of parameter. dMean estimate of the parameter 

m1 in Equation (1). eStandard error of estimate for the parameter 

m1 in Equation (1). fCoefficient of variation of the predictions from Equation (1). gData for both charring directions relative to the plane of the laminates are combined for these results. 

Research Paper FPL–RP–633
9

Table 5. Estimates for parameter m5 of Equation (5) for the 14 materials tested in small vertical furnace 

m5 of Equation (5) m5 of Equation (5) for both directionsf

Material 

numbera

Species 

composite type Directionb

Meanc

(min/mm1.23)

Standard errord

 (min/mm1.23)

Predicted COVe

(%)

Meanc

(min/mm1.23)

Standard errord

(min/mm1.23)

Predicted COVe

(%)

1 Aspen LSL Parallel 0.6796 0.0052 5.9 

Perpendicular 0.6599 0.0057 6.6 0.6698 0.0040 6.4 

2 Yellow-poplar LSL Parallel 0.6753 0.0051 5.8 

Perpendicular 0.6765 0.0059 6.8 0.6759 0.0039 6.3 

3 Aspen LVL Parallel 0.6411 0.0088 10.6 

Perpendicular 0.6147 0.0033 4.1 0.6281 0.0049 8.5 

4 Douglas-fir LVL Parallel 0.6499 0.0052 6.1 

Perpendicular 0.6473 0.0058 6.9 0.6486 0.0039 6.5 

5 Douglas-fir LVL Parallel 0.7046 0.0069 7.5 

Perpendicular 0.6607 0.0084 9.7 0.6827 0.0059 9.3 

6 Douglas-fir LVL Parallel 0.6894 0.0075 8.3 

Perpendicular 0.6866 0.0077 8.5 0.6880 0.0053 8.4 

7 Eucalyptus LVL Parallel 0.7106 0.0134 13.8 

Perpendicular 0.7043 0.0064 7.0 0.7073 0.0072 10.8 

8 Southern Pine LVL Parallel 0.6126 0.0045 6.9 

Perpendicular 0.5951 0.0046 7.3 0.6039 0.0033 7.2 

9 Southern Pine LVL Parallel 0.6588 0.0050 5.9 

Perpendicular 0.5985 0.0051 6.7 0.6286 0.0047 8.2 

10 Yellow-poplar LVL Parallel 0.6160 0.0049 6.1 

Perpendicular 0.5624 0.0038 5.3 0.5892 0.0041 7.7 

11 Yellow-poplar LVL Parallel 0.6032 0.0043 5.6 

Perpendicular 0.5626 0.0065 9.0 0.5831 0.0044 8.3 

12 Douglas-fir PSL Parallel 0.6920 0.0054 6.0 

Perpendicular 0.6881 0.0065 7.2 0.6900 0.0042 6.6 

13 Southern Pine PSL Parallel 0.6902 0.0041 4.7 

Perpendicular 0.6919 0.0053 6.0 0.6911 0.0034 5.3 

14 Yellow-poplar PSL Parallel 0.6727 0.0056 5.9 

Perpendicular 0.6703 0.0042 4.8 0.6712 0.0034 5.2 

aNumbers correspond to those listed in Table 1. bDirection of charring relative to the plane of the veneers in laminated veneer lumber (LVL) or normal vertical surface of laminated strand lumber (LSL) or parallel strand lumber (PSL). cMean estimate of the parameter m5 in Equation (5). dStandard error of estimate for the parameter m5 in Equation (5). eCoefficient of variation of the predictions from Equation (5). fData for both charring directions relative to the plane of the laminates are combined for these results. 

Fire Resistance of Structural Composite Lumber Products
10

In these small vertical-furnace tests, the residual char surface was a fairly consistent vertical plane. The exceptions 

were tests of the eucalyptus LVL specimens, in which specimen orientation was such that the intended direction of charring was parallel to the plane of the laminated veneers. The 

charring of these eucalyptus LVL specimens caused holes 

to appear on the back surface. In some cases, smaller holes 

merged to form a larger hole at the end of the test 

(Fig. 5a). In an LVL, gaps are formed by termination of a 

veneer within the product and the veneer above and below 

being pressed together (Fig. 5b). These gaps went completely through the test specimens. Unlike the other test 

specimens, this test material was a prototype sample of the 

product. We concluded that these gaps allowed three-dimensional charring at that point and the resulting rapid flame 

penetration through the specimen. On the fire-exposed 

surface, a physical hole was formed in the char layer. The 

magnitude of the gaps in these prototype samples would 

not occur in the commercial products for non-fire reasons. 

In commercial products, the ends of veneers were tapered 

or otherwise constructed to reduce the size of any physical 

gap being created. The early penetration of test specimen or 

holes in the charred surface was not observed in the tests of 

other products. In most applications of LVL, the potential 

fire exposure would be perpendicular to the plane of the 

veneers.

Horizontal-Furnace Tests

Test methods

The 13 intermediate-scale tests in the horizontal furnace 

included 10 different materials (Table 2). Width of the test 

specimens ranged from 44 to 180 mm (Table 2). Heights 

ranged from 351 to 455 mm. 

The intermediate-scale horizontal furnace was a metal box 

lined with ceramic/mineral fiber blankets and heated by 

eight diffusion-flame natural gas burners on the floor of the 

furnace (Fig. 6). The central 1.8 m of a specimen was exposed to furnace temperatures. The interior dimensions 

of the furnace were 1.83 m long, 0.99 m wide, and 

1.22 m high. On each end of the furnace was an opening 

229 mm wide and 508 mm deep for the test specimen. The 

ceramic fiber-lined metal cover for the furnace was removable from the top of the furnace. All air for combustion was 

provided by natural draft through vents at the bottom of the 

furnace.

Three capped furnace thermocouples were 152 mm from 

the exposed specimen surface down the length of each of 

the two sides of the test specimen. These six thermocouples 

were located 305 mm from the top of the furnace interior. 

The gas was controlled so temperatures of the capped thermocouples followed the time–temperature curve in ASTM 

E 119. The ASTM E 119 specifies a distance of 305 mm for 

the capped pipe from the exposed surface in tests of floors 

and columns and 152 mm for tests of walls and partitions.

Figure 5. Example of (a) the hole burned though the 

eucalyptus specimen after a small vertical-furnace test 

in which charring was parallel to the plane of laminated 

veneer and (b) gaps formed in the manufacture of laminated veneer lumber.

Figure 6. Unloaded specimen in horizontal-furnace test.

Research Paper FPL–RP–633
11

Table 6. Estimates for parameter m5 of Equation (5) for combinations of the 14 materials tested in small vertical furnace 

m5 of Equation (5) m5of Equation (5) for both directionsf

Material 

numbera

Species 

composite type Directionb

Meanc

(min/mm1.23)

Standard errord

(min/mm1.23)

Predicted COVe

(%)

Meanc

(min/mm1.23)

Standard errord

(min/mm1.23)

Predicted COVe

(%)

4, 5, 6 Douglas-fir LVL Parallel 0.6813 0.0042 8.3 

Perpendicular 0.6649 0.0044 8.9 0.6731 0.0031 8.7 

8, 9 Southern Pine LVL Parallel 0.6311 0.0039 7.6 

Perpendicular 0.5965 0.0034 7.0 0.6139 0.0028 8.0 

10, 11 Yellow-poplar LVL Parallel 0.6096 0.0033 5.9 

Perpendicular 0.5625 0.0037 7.3 0.5862 0.0030 8.0 

1, 3 Aspen Parallel 0.6604 0.0055 9.0 

Perpendicular 0.6375 0.0040 6.9 0.6490 0.0035 8.3 

4, 5, 6, 12 Douglas-fir Parallel 0.6838 0.0035 7.8 

Perpendicular 0.6707 0.0038 8.6 0.6772 0.0026 8.3 

8, 9, 13 Southern Pine Parallel 0.6480 0.0037 8.2 

Perpendicular 0.6239 0.0044 10.2 0.6360 0.0029 9.5 

2, 10, 11, 14 Yellow-poplar Parallel 0.6380 0.0034 8.1 

Perpendicular 0.6183 0.0048 12.0 0.6276 0.0030 10.4 

1, 2 LSL Parallel 0.6775 0.0036 5.9 

Perpendicular 0.6682 0.0042 6.8 0.6728 0.0028 6.4 

3–11 LVL Parallel 0.6511 0.0029 10.5 

Perpendicular 0.6242 0.0030 11.3 0.6376 0.0021 11.1 

12–14 PSL Parallel 0.6867 0.0030 5.6 

Perpendicular 0.6835 0.0032 6.3 0.6850 0.0022 6.0 

1–14 All data Parallel 0.6614 0.0022 9.5 

Perpendicular 0.6426 0.0024 10.7 0.6518 0.0016 10.2 

aNumbers correspond to those listed in Table 1. bDirection of charring relative to the plane of the veneers in laminated veneer lumber (LVL) or normal vertical surface of laminated strand lumber (LSL) or parallel strand 

 lumber (PSL). 

cMean estimate of the parameter m5 in Equation (5). dStandard error of estimate for the parameter m5 in Equation (5). eCoefficient of variation of the predictions from Equation (5). fData for both charring directions relative to the plane of the laminates are combined for these results. 

Fire Resistance of Structural Composite Lumber Products
12

Horizontal-furnace specimens were subjected to fire exposure on three sides. Specimens were oriented in the furnace 

with the wider side vertical. The top surface was covered 

with gypsum board and ceramic fiber (Fig. 6). A strip of 

gypsum board was also placed along the top edges of the 

vertical surfaces to ensure no charring of the top surface. 

The top surface was protected to allow holes for the 16 thermocouples to be drilled from the top surface. Four groups 

of thermocouples were inserted into a test specimen. Each 

group of thermocouples included thermocouples at different depths from the sides of the specimen. Four groups of 

thermocouples were located at different locations along the 

length of the specimens. Depths of the thermocouples from 

the top of the specimen also varied between the groups. In 

the furnace, the length of the specimen was in an east–west 

direction. The four groups of thermocouples were as 

follows:

a. Group one consisted of five thermocouples at 50-mm 

intervals from 350 to 500 mm east of mid-span of the 

test specimen. Four thermocouples were placed at 

different distances from the north surface of the test 

specimen and one was at mid-width of the specimen. 

Design depths from the vertical surface were 13, 25, 

38, and 51 mm, but actual depths varied depending on 

the width of the test specimen. The ends of the thermocouples were 175 mm from the top of the specimen. For most specimens, this represented approximately the mid-height of the test specimen (Table 2). 

b. Group two consisted of three thermocouples at a 

50-mm interval along the specimen length 75 to 

175 mm east of mid-span. Design depths were two 

thermocouples at 13 mm and 25 mm from the north 

surface and one at the mid-width of the test specimen. 

Thermocouples were 270 mm from the top of the 

specimen. 

c. Group three was likewise three thermocouples 75 to 

175 mm west of mid-span. Design depths from the 

vertical surface for two thermocouples were 13 mm 

and 25 mm from the north surface and one at the 

mid-width of the test specimen. Thermocouples were 

located 80 mm from the top of the specimen.

d. Group four was similar to group one but located 

350 mm to 550 mm west of mid-span and at depths 

from the south surface of the test specimen. 

To install the thermocouples, we drilled 3.2-mm-diameter 

holes from the top of the test specimen at thermocouple locations so we could insert wood dowels. A groove was made 

on the side of each wood dowel for thermocouple wire. 

Placement of the dowel in the hole was such that the groove 

with the thermocouple was on the side of the nearest fireexposed surface. Thermocouples were made from Type K 

(chromel-alumel), 30-gauge (0.254-mm-diameter) insulated 

thermocouple wire. This procedure was used because the 

depths of the thermocouple holes made it impractical to use 

a drill bit comparable to the diameter of the thermocouple 

wire. 

Thermocouple data were collected on a personal computer 

data acquisition system. The scan rates of data acquisition in 

these horizontal-furnace tests were a reading every 15 s. 

Test results

As with data from small vertical-furnace tests, pairs of time 

for 300°C and thermocouple depth from the horizontalfurnace tests (Table 2) were used to calculate char rate parameters of Equations (1) and (5). Models of Equations (1) 

and (5) are for a semi-infinite slab. Char rates reported for 

small vertical-furnace tests are for a semi-infinite slab, 

because the last times for 300°C were obtained prior to 

any significant temperature increase on the unexposed or 

back surface of test specimens. In horizontal-furnace tests 

(Fig. 7), data at or near the mid-width of a test specimen deviated from models of Equations (1) and (5). Data indicated 

a very rapid charring of the test specimen as temperature 

at the center of the specimen increased. This is consistent 

with a slab insulated on the back or an unexposed surface. 

The center of a specimen fire exposed to both sides is often 

represented as an insulated boundary condition that prevents 

heat loss out the back surface. In Figure 7, data from locations less than 13 mm from the center of the test specimen 

are represented by open triangles. Open circles represent 

locations between 13 and 30 mm from the horizontal center. 

Data from locations 13 mm or closer to the horizontal 

center of the test specimen from linear regressions were 

excluded from the data set to obtain parameter estimates 

Figure 7. Plot of experimental data (symbols) and predictions based on xc model (- - -) and xc

 model (—) 

of data. Solid circles (●) are for locations greater than 

13 mm from horizontal center of test specimen. Open 

circles (○) are data for locations between 13 and 30 mm 

from the horizontal center of the test specimen. Open triangles (∆) are locations less than 13 mm from the center 

of the test specimen.

Research Paper FPL–RP–633

1.23
13

(Tables 7 and 8). For Test Number 3, data closer than 9 mm 

were excluded since the width of the specimen was only 44 

mm. Widths of the 13 horizontal-furnace specimens ranged 

from 44 to 179 mm (Table 2). Range of char depth data and 

maximum time for the 300°C recorded varied depending on 

the width of the test specimen (Table 2).

Data for the four groups of thermocouples within a test 

specimen were combined to estimate char rate parameters 

(Figs. 8 to 17). Three horizontal-furnace tests were Douglas-fir LVL of Material Number 5 (Table 2, Fig. 10). Two 

horizontal-furnace tests were yellow-poplar LVL of Material Number 11 (Table 2, Fig. 15). As with the tables for the 

small vertical-furnace tests, tables for horizontal-furnace 

tests include estimates for mean and standard error for char 

rate parameter m1 (Table 7) and m5 (Table 8) and coefficients of variation for predicted times. Numbers of pairs of 

data included in the regressions are also listed. Estimates 

were calculated for each individual test and for combinations of common test material (Tables 7 and 8).

Discussion

Comparison of small vertical-furnace data for three composite lumber products with earlier data and predictive equations for charring of solid lumber was previously reported 

(White 2000). In Table 9, average estimates of m1 of 

Equation (1) for LSL, LVL, and PSL obtained in small 

vertical-furnace tests are compared with values for m1 from 

earlier FPL tests of different species of solid lumber. 

Compared with the range of averages for four species of 

aspen, Douglas-fir, Southern Pine, and yellow-poplar (m1 of 

1.38 to 1.59 for LVL and m1 of 1.33 to 1.64 for lumber), the 

range of average values for four types of wood products was 

less (m1 of 1.50 to 1.61). Species effect on char rate of 

LVL was comparable to lumber data. Species was less of 

a factor in char rate of PSL and LSL. On average, char 

Figure 8. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 1 of aspen laminated strand lumber 

(LSL, Material 1). Line is the linear m1 model using data 

for locations greater than 13 mm from the horizontal 

center of test specimen (●). Open circles (○) are data for 

locations less than 13 mm from the horizontal center 

of the test specimen. The left edge of the graph corresponds to the horizontal center of the test specimen.

Figure 9. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 2 of Douglas-fir laminated veneer 

lumber (LVL, Material 4). Line is the linear m1 model using data for locations greater than 13 mm from the horizontal center of test specimen (●). Open circles are data 

(○) for locations less than 13 mm from the horizontal 

center of the test specimen. The left edge of the graph 

corresponds to the horizontal center of the test 

specimen.

Figure 10. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Numbers 3, 4, and 5 of Douglas-fir laminated 

veneer lumber (LVL, Material 5). Line is the linear m1

model using data for locations greater than 13 mm from 

the horizontal center (9 mm for Test 3) of test specimen 

(solid symbol). Open symbols are data for locations less 

than 13 mm (9 mm for Test 3) from the horizontal center of the test specimen. Left edge of the graph corresponds to the horizontal center of the widest test specimen. Vertical lines correspond to the horizontal center 

of the two narrower test specimens. Symbols: ∆, Test 3; 

○, Test 4; □, Test 5.

Fire Resistance of Structural Composite Lumber Products
14

Figure 11. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 6 of Douglas-fir laminated veneer 

lumber (LVL, Material 6). Line is the linear m1 model using data for locations greater than 13 mm from the horizontal center of test specimen (●). Open circles (○) are 

data for locations less than 13 mm from the horizontal 

center of the test specimen. The left edge of the graph 

corresponds to the horizontal center of the test 

specimen.

Figure 12. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 7 of Southern Pine laminated veneer 

lumber (LVL, Material 8). Line is the linear m1 model using data for locations greater than 13 mm from the horizontal center of test specimen (●). Open circles (○) are 

data for locations less than 13 mm from the horizontal 

center of the test specimen. The left edge of the graph 

corresponds to the horizontal center of the test 

specimen.

Figure 13. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 8 of Southern Pine laminated veneer 

lumber (LVL, Material 9). Line is the linear m1 model using data for locations greater than 13 mm from the horizontal center of test specimen (●). Open circles (○) are 

data for locations less than 13 mm from the horizontal 

center of the test specimen. The left edge of the graph 

corresponds to the horizontal center of the test 

specimen.

Figure 14. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 9 of yellow-poplar laminated veneer 

lumber (LVL, Material 10). Line is the linear m1 model 

using data for locations greater than 13 mm from the 

horizontal center of test specimen (●). Open circles (○) 

are data for locations less than 13 mm from the horizontal center of the test specimen. The left edge of the 

graph corresponds to the horizontal center of the test 

specimen.

Research Paper FPL–RP–633
15

rates of composite products were comparable to previous 

data for lumber. Estimates for m1 and m5 for all small 

vertical-furnace composite lumber tests combined were 

1.535 and 0.652 min/mm1.23, respectively. For 38 mm of 

char at 1 h cited in TR 10 and generally assumed for solid 

lumber, corresponding values are 1.575 min/mm for m1 of 

Equation (1) and 0.682 min/mm1.23 for m5 of Equation (5).

There was considerable scatter in the data from horizontalfurnace tests. Coefficients of variation for predicted times 

were considerably greater for horizontal-furnace tests 

(Tables 7 and 8) than for small vertical-furnace tests 

(Tables 3 and 4). This is not surprising given greater difficulties in placing thermocouples within a horizontal-furnace 

specimen at a precise location. Two furnaces are considerably different in terms of furnace linings and type and placement of natural gas burners. Despite these differences, a 

zero-intercept linear regression of average m1 results from 

two furnaces for 10 materials resulted in a slope of 1.001 

(Fig. 18). This suggests that there was not a bias between 

results for the two furnaces. The 52% R2 of the regression 

likely reflects high variability in horizontal-furnace results. 

Tension Tests

Fire tests conducted in the first two phases of this project 

did not include any load applied to the member. The third 

phase provides data on tensile strength of composite lumber 

products while subjected to standard fire exposure defined 

by the time–temperature curve in ASTM E 119 (ASTM International 2000). The intermediate-scale horizontal-furnace 

used in Phase Two was also used for tension tests. The FPL 

horizontal furnace is considerably smaller than dimensions 

specified in ASTM E 119, but the furnace is unique in that 

Figure 15. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 10 and 11 of yellow-poplar laminated 

veneer lumber (LVL, Material 11). Line is the linear m1

model using data for locations greater than 13 mm from 

center of test specimen (solid symbol). Open symbols 

are data for locations less than 13 mm from the horizontal center of the test specimen. The left edge of the 

graph corresponds to the horizontal center of the widest 

test specimen. Vertical lines correspond to the horizontal center of the two narrower test specimens. Symbols: 

○, Test Number 4; □, Test Number 5.

Figure 16. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 12 of Douglas-fir parallel strand lumber (PSL, Material 12). Line is the linear m1 model using 

data for locations greater than 13 mm from the horizontal center of test specimen (●). Open circles (○) are data 

for locations less than 13 mm from the horizontal center 

of the test specimen. The left edge of the graph corresponds to the horizontal center of the test specimen.

Figure 17. Experimental times to 300°C plotted against 

distance from fire-exposed surface for horizontal-furnace Test Number 13 of Southern Pine parallel strand 

lumber (PSL, Material 13). Line is the linear m1 model 

using data for locations greater than 13 mm from the 

horizontal center of test specimen (●). Open circles (○) 

are data for locations less than 13 mm from the horizontal center of the test specimen. The left edge of the 

graph corresponds to the horizontal center of the test 

specimen.

Fire Resistance of Structural Composite Lumber Products
16Table 7. Estimates for parameter m1 of Equation (1) for tests of unloaded specimens in horizontal furnace 

 m1 for 300°C(min/mm) 

Test

 numbera

Material 

numbera Species 

Composite

type Width (mm) Meanb

Standard

errorc

COV of 

predictionsd

Number of data 

pointse

FPL test 

number

 1 1 Aspen LSL 175 1.660 0.058 12.9 12 2110 

 2 4 Douglas-fir LVL 180 1.593 0.037 8.3 11 2112  3 5 Douglas-fir LVL 44 1.429 0.110 21.1 7 2117  4 5 Douglas-fir LVL 83 1.813 0.106 15.7 7 2108  5 5 Douglas-fir LVL 167 1.533 0.064 15.4 12 2113  6 6 Douglas-fir LVL 179 1.497 0.069 17.4 12 2118  7 8 Southern Pine LVL 169 1.336 0.071 19.4 12 2120  8 9 Southern Pine LVL 173 1.404 0.037 9.9 12 2115  9 10 Yellow-poplar LVL 170 1.349 0.075 20.8 12 2111 10 11 Yellow-poplar LVL 85 1.348 0.047 10.1 8 2109 11 11 Yellow-poplar LVL 170 1.454 0.046 11.9 12 2119 12 12 Douglas-fir PSL 177 1.641 0.048 11.3 12 2116 13 13 Southern Pine PSL 176 1.527 0.044 11.0 12 2114 

10, 11 11 Yellow-poplar LVL 85, 170 1.433 0.035 12.1 20 – 

3–5 5 Douglas-fir LVL 44, 83, 167 1.570 0.049 18.1 26 – 2–6 4–6 Douglas-fir LVL 44–180 1.554 0.031 15.5 49 – 

7, 8 8, 9 Southern Pine LVL 169, 173 1.370 0.040 15.1 24 – 

9–11 10, 11 Yellow-poplar LVL 85, 170, 170 1.393 0.037 16.6 32 – 2–11 4–11 All LVL 44–180 1.455 0.022 16.9 105 – 

12, 13 12, 13 All PSL 176, 177 1.580 0.034 11.7 24 – 

All All All All 44–180 1.497 0.019 16.3 141 – 

aNumbers correspond to those listed in Table 2. bMean estimate for parameter m1 in Equation (1) for temperature criteria of 300oC. cStandard error for the mean estimate for parameter m1 in Equation (1) for temperature criteria of 300oC. dCoefficient of variation for the predicted times from Equation (1). eNumber of data points included in the regressions. Excluded data that was for locations 13 mm or closer to the horizontal center of the test specimen. For test 3, excluded data 

 was for locations closer than 9 mm from the horizontal center of the test specimen. The width of the specimen for test 3 was only 44 mm. 

Research Paper FPL–RP–633
17

Table 8. Estimates for parameter m5 of Equation (5) for tests of unloaded specimens in horizontal furnace 

 m5 for 300°C(min/mm)

Test

numbera

Material 

numbera Species 

Composite

type 

Width

(mm) Meanb

Standard

Errorc

COV of 

predictionsd

Number of data 

pointse

FPL test 

number

 1 1 Aspen LSL 175 0.706 0.028 14.5 12 2110  2 4 Douglas-fir LVL 180 0.667 0.027 14.5 11 2112  3 5 Douglas-fir LVL 44 0.802 0.063 21.5 7 2117  4 5 Douglas-fir LVL 83 0.878 0.067 20.6 7 2108  5 5 Douglas-fir LVL 167 0.645 0.033 18.9 12 2113  6 6 Douglas-fir LVL 179 0.637 0.029 17.2 12 2118  7 8 Southern Pine LVL 169 0.566 0.034 22.0 12 2120  8 9 Southern Pine LVL 173 0.595 0.022 13.7 12 2115  9 10 Yellow-poplar LVL 170 0.568 0.031 20.4 12 2111 

10 11 Yellow-poplar LVL 85 0.648 0.034 15.2 8 2109 11 11 Yellow-poplar LVL 170 0.615 0.023 13.7 12 2119 12 12 Douglas-fir PSL 177 0.698 0.026 14.1 12 2116 13 13 Southern Pine PSL 176 0.647 0.019 11.3 12 2114 

10, 11 11 Yellow-poplar LVL 85,170 0.620 0.018 14.3 20 – 

3–5 5 Douglas-fir LVL 44, 83, 167 0.677 0.029 24.1 26 – 

2-6 4–6 Douglas-fir LVL 44–180 0.660 0.018 19.0 49 – 

7, 8 8, 9 Southern Pine LVL 169, 173 0.581 0.020 18.0 24 – 

9–11 10, 11 Yellow-poplar LVL 85, 170, 170 0.595 0.017 17.6 32 – 2–11 4–11 All LVL 44–180 0.619 0.011 19.5 105 – 

12, 13 12, 13 All PSL 176, 177 0.670 0.016 13.2 24 – 

All All All All 44–180 0.637 0.009 22.5 141 – 

aNumbers correspond to those listed in Table 2. bMean estimate for parameter m5 in Equation (5) for temperature criteria of 300oC. cStandard error for the mean estimate for parameter m5 in Equation (5) for temperature criteria of 300oC. dCoefficient of variation for the predicted times from Equation (5). eNumber of data points included in the regressions. Data that were for locations 13 mm or closer to the horizontal center of the test specimen were excluded. For Test 3, data  for locations closer than 9 mm from the horizontal center of the test specimen were excluded. The width of the specimen for Test 3 was only 44 mm. 

Fire Resistance of Structural Composite Lumber Products
18

it is located in the middle of a tension apparatus. Testing of 

tensile members is not specifically addressed in the ASTM 

E 119 standard. The FPL’s furnace test apparatus was previously used to test dimension lumber in tension and metalplate connections (White and others 1993), glued-laminated 

specimens (White 2004), and dimension lumber (Lau and 

others 1998).

Methods and Materials

In a series of 13 tension tests, 10 of the materials tested for 

char rate (Table 1) were also tested for fire resistance 

under load (Table 10). Specimens (Fig. 19a) were loaded in 

a specially made tension apparatus. In the middle of the tension apparatus is the ceramic-fiber-lined horizontal furnace 

(Fig. 19d) discussed previously. Test specimens were 

2.97 m long. The tension apparatus, Model 403FPL, was 

a modification of the Model 401 and 402 Tension Proof 

Testers of Metriguard (Pullman, Washington). It was 

5.18 m long, 2.29 m wide, and 1.83 m high. The apparatus 

was capable of a tensile load of 445 kN. The apparatus used 

an electric-powered hydraulic loading system (Fig. 19b). 

The system consisted of two hydraulic cylinders with oil 

provided by a hydraulic power unit. The manual valve and 

relief valve limited flow of hydraulic fluid. The load was 

measured with an electronic load-cell force-measuring 

system. 

The wedge-gripping system of the tension apparatus only 

allowed for specimen thicknesses of 19, 38, and 64 mm. 

Special grips were made to test these wider specimens 

(Fig. 19c). Steel plates were used to grip timbers. Fixed 

and limited distance between wedge grips of the tension 

Table 9. Comparison of average estimates of m1 in Equation (1) for the different types 

of wood productsa

m1 for LSL m1 for LVL m1 for PSL m1 for lumber 

Species (min/mm) (min/mm) (min/mm) (min/mm) 

Aspen 1.579 1.480 – – 

Douglas-fir – 1.587 1.624 1.64b

Southern Pine – 1.446 1.627 1.33c

Yellow-poplar 1.589 1.380 1.570 1.36d

All 1.584 1.502 1.609 1.575e

a

All results are for tests in the FPL small vertical furnace and for specimens conditioned at 23°C, 50% relative 

 humidity

b

Ten tests from Schaffer (1967), White and Schaffer (1981), and White (1988). Average dry density is 457 kg/m3.

 Standard error for the mean of the 10 tests is 0.051 min/mm. 

c

Five tests from Schaffer (1967), White and Schaffer (1981), and White (1988). Average dry density is 504 kg/m3.

 Standard error for the mean of the five tests is 0.06 min/mm. 

d

Two tests from White (1988). Average dry density is 484 kg/m3. Standard error for the mean of the two tests is 

 0.04 min/mm. 

e

The 1.575 min/mm is the generally accepted value for wood charring (1.5 in/h; 0.635 mm/min). 

 Table 10. Tension tests conducted in horizontal furnace 

Test

number

Material 

numbera

Species 

composite type 

Width

 (mm) 

Height

 (mm) 

Densityb

(kg/m3

)

Percentage of full 

design loadc

FPL Test 

number

 1 2 Yellow-poplar LSL 89 233 668 36 2138 

 2 3 Aspen LVL 42 242 464 64 2134 

 3 4 Douglas-fir LVL 135 241 523 25 2140 

 4 4 Douglas-fir LVL 178 239 513 30 2142 

 5 5 Douglas-fir LVL 43 240 575 8d 2132

 6 5 Douglas-fir LVL 90 240 540 33 2137 

 7 5 Douglas-fir LVL 128 235 544 24 2139 

 8 6 Douglas-fir LVL 178 234 494 33 2141 

 9 7 Eucalyptus LVL 41 239 586 28 2135 

10 8 Southern Pine LVL 45 239 603 26 2136 

11 9 Southern Pine LVL 169 233 589 33 2143 

12 11 Yellow-poplar LVL 44 230 577 46 2133 

13 12 Douglas-fir PSL 176 251 601 26 2144 

a

Material numbers are consistent with those listed in Table 1 for the materials used in the small vertical-furnace tests. Except for 

 Material Numbers 3 and 7, the manufacturer supplied the tension test samples at a later date than the samples used in the small vertical- 

 furnace tests and horizontal-furnace charring tests. 

b

Density calculated using oven-dry mass and volume as tested. 

c

Applied load as a percentage of the full allowable design load. Applied load reported is the average load during last 15 s (5 readings) 

 before rapid drop in load. Allowable load design values obtained from the manufacturer’s literature or correspondence. d

In this initial test, the member failed in the bolt connection outside the furnace when the load was applied prior to initiation of the fire 

 exposure. Plywood was added to bolted connection and test conducted using the low load reported. 

Research Paper FPL–RP–633
19

apparatus and the edge of the furnace (Fig. 19c) limited the 

design of the bolted connection and resulting load capacity. 

Eight 25-mm bolts were used at each end. Holes in the 

13-mm-thick steel plates were oversized by 1.6 mm. Nuts 

on the bolts were not fully tightened to prevent a bending 

moment from being applied to the specimen. A 64-mm pin 

was used to connect the two grip plates to another plate that 

was inserted into the wedge-gripping system of the tension 

apparatus. Two surfaces of that plate were lined with oak to 

help ensure a secured gripping system. Prior to application 

of the load, rollers on support stands provided vertical support to the ends of the member. Application of tensile load 

lifted the specimen off the vertical support rollers. This was 

also done to limit any moment load being applied to the 

specimen during the test.

Selection of the size of test specimens (Table 10) was dictated by limitations of the equipment, desires to obtain certain fire endurance times, and desires to evaluate the validity 

of fire-endurance models for these materials. Because of 

Figure 18. For the different materials, comparison of 

charring rates (m1) obtained in the horizontal-furnace 

tests with those obtained in the small vertical-furnace 

tests. Labels are the material numbers listed in Table 1.

Figure 19. Test of loaded tension specimen in the intermediate-scale horizontal furnace. (a) Specimens tested in tension apparatus/horizontal furnace in this study were similar to this glued-laminated specimen. (b) Hydraulic loading 

system of tension apparatus. (c) Steel plates and bolted connections used to apply the tension load. (d) Burning 

specimen inside the ceramic-fiber-lined furnace after cover was removed at conclusion of a test.

Fire Resistance of Structural Composite Lumber Products
20

how the furnace is constructed, specimen size is limited in 

two ways. Mid-height of the test specimen is 305 mm from 

the ceiling of the furnace. A 250-mm-high specimen left 

180 mm from the top of the specimen to the furnace ceiling. Metal plates used to grip the ends were 190 mm high 

(Fig. 19). To use all eight bolts to apply the load, the test 

specimen needed to be at least 235 mm high. The opening 

at the furnace ends for test specimens was 229 mm wide. 

To evaluate the range of performance of the model, specimens were approximately 44, 89, 133, and 178 mm thick 

(Table 10). 

Specimens were stored in a 23°C, 50% relative humidity 

room prior to testing. Test load (Table 11) was applied to the 

specimen for 10 min prior to initiation of the ASTM E 119 

time–temperature exposure. Applied load was limited by 

load capacity of the bolted connections used to attach specimens to the grips of the tension apparatus (Fig. 19c). Load 

was a percentage of the full allowable design load 

(Table 10). Except for Test Number 5, percentage of full 

design load ranged from 24% to 64% (Table 10). In Test 

Number 5, initial application of the load resulted in failure 

of the bolted connections prior to the initiation of fire exposure. Plywood was attached to the specimen in the area 

of bolted connections and the test specimen was tested 

with a very low load of 9 kN (8% of the full design load) 

(Table 11). 

Calculation of full allowable tensile design load used allowable design values for tensile load obtained directly from the 

manufacturer or their literature. If available data required 

an adjustment for length, values were adjusted for a span of 

1.8 m. The furnace was controlled to follow the ASTM E 

119 time–temperature curve (Figs. 20 to 32). Furnace thermocouples were 152 mm from sides of the test specimen. 

Observed failure was recorded when the specimen could no 

longer support the load. Thermocouples data and load cell 

data were collected on a personal computer data acquisition 

system. Data-acquisition scan rates in these horizontalfurnace/tension apparatus tests were every 3 s. 

Failure Times

Observed failure times of the 13 tests listed in Table 10 

ranged from 13 to 100 min (Table 11, Figs. 20 to 32). Failure times for the five 44-mm-wide specimens were between 

13 and 21 min. Measured widths of the specimens are listed 

in Table 10. Failure times for the two 89-mm-wide specimens were 35 and 46 min. For the two 127-mm-wide specimens, failure times were 68 and 73 min. Failure times for 

the four 178-mm-wide specimens ranged from 77 to 

100 min. Recorded applied load with time is illustrated 

in Figures 20 to 32. A problem with the hydraulic loading 

system in a few tests caused a gradual drop in applied load 

during fire exposure (Fig. 26). Reported load (Table 11) was 

the average load recorded during the last 15 s (5 readings) 

before the rapid drop in load at failure (Figs. 20 to 32). 

Recorded furnace temperatures were in compliance with 

requirements of ASTM E 119 with regard to the area 

under the time–temperature curve (Figs. 20 to 32). The 

ASTM E 119 requires deviation from the standard curve 

calculated using the area under the curve to be within 10%, 

7.5%, and 5 % for tests of 1 h or less, 1 to 2 h, and greater 

than 2 h, respectively. In Test Number 1 of 35-min duration, 

the recorded curve was 5% greater than the standard curve. 

For all other tests, deviation in area was less than 5%. In 

Test Number 1 (Fig. 20), recorded furnace temperature exceeded the standard curve starting at 5 min and ending at 

30 min. Because of the construction and size of the horizontal furnace, it can be difficult to adjust the furnace temperature to lower temperatures once higher temperatures have 

been achieved and a large specimen is burning. 

The observed failure times for the five 44-mm-wide specimens were a function of the percentage of design load 

(R2 = 0.982). Estimated failure times from the regression for 

these LVL products of five different species were 21.8 min 

for an unloaded member to 7.1 min for a member loaded to 

full design load. The variations in percentage design load 

were not suitable for similar regressions of data for the 

thicker specimens. 

Of the two 89-mm-wide specimens, the yellow-poplar 

LSL failed in less time (35 min) than the Douglas-fir LVL 

(46 min). The two 127-mm-thick specimens were both 

Douglas-fir LVL from different manufacturers. Failure 

times were similar (68 and 73 min). Load on the four 

178-mm-wide specimens was very similar. Of the four, the 

observed failure time for the Douglas-fir PSL was greater 

(100 min) than the failure times of the three LVL products 

(77 to 86 min), which was consistent with its slightly slower 

char rate in the small vertical tests and the higher allowable 

design stress (i.e., lower percentage allowable load). 

National Design Specification Model 

Predictions

National Design Specification methodology (Eqs. (6) to (9)) 

was used to predict failure times of test specimens (Table 

11). To convert the m5 (min/mm1.23) char rate values reported in this paper to the nominal char rate βn (in/h) of TR 10, 

one needs to calculate char depth at 1 h:

(10)

For the m5 values in Table 11, the corresponding βn (in/h) 

range from 1.46 in/h (0.7073 min/mm1.23) to 1.70 in/h 

(0.5831 min/mm1.23). Except for the yellow-poplar LSL, the 

differences between observed and predicted failure times of 

the tension specimens ranged from 1% to 19% (Table 11). 

Difference for the yellow-poplar LSL was 27%. 

Estimated degradation of load capacity of the specimen with 

time is illustrated for each test (Figs. 20 to 32). Estimated 

failure time is the time at which predicted load capacity no 

longer exceeds applied load (Figs. 20 to 32).

Research Paper FPL–RP–633
21

Table 11. Observed and predicted failure times for tension tests 

Test

number

Material 

numbera

Species and 

composite type 

Allowable stressb

(MPa)

Applied load 

(kN)

Char ratec m5

(min/mm1.23)

Observed failure 

time (min) 

Predicted failure 

timed using m5

(min)

Predicted time 

minus observed 

time (min) 

Difference between 

observed and 

predictede (%)

1 2 Yellow-poplar LSL 12.11 89.6 0.6759 35.3 44.8 9.6 27 2 3 Aspen LVL 10.21 66.8 0.6281 13.0 14.6 1.6 12 3 4 Douglas-fir LVL 13.76 113.8 0.6486 72.7 73.6 0.9 1 4 4 Douglas-fir LVL 13.76 176.7 0.6486 77.3 92.0 14.7 19 5 5 Douglas-fir LVL 11.14 9.0 0.6827 20.9 22.7 1.8 9 6 5 Douglas-fir LVL 12.45 88.7 0.6827 46.4 46.9 0.4 1 7 5 Douglas-fir LVL 12.45 90.3 0.6827 68.5 73.7 5.2 8 8 6 Douglas-fir LVL 12.65 176.2 0.6880 86.2 94.0 7.7 9 9 7 Eucalyptus LVL 16.55 44.5 0.7073 17.6 19.9 2.3 13 

10 8 Southern Pine LVL 16.08 44.4 0.6039 18.2 19.2 1.0 6 11 9 Southern Pine LVL 13.75 179.2 0.6286 79.6 82.1 2.5 3 12 11 Yellow-poplar LVL 14.65 67.5 0.5831 14.4 16.0 1.6 11 13 12 Douglas-fir PSL 15.50 176.7 0.6900 100.5 102.5 2.0 2 

aMaterial numbers are consistent with those listed in Table 1 for the materials used in the small vertical-furnace tests.

bAllowable load design values obtained from the manufacturer’s literature or correspondence. For design values that depended on the length of the member, values were adjusted for a length of 1.8 m. 

cChar rates obtained in the small vertical-furnace tests for both charring directions relative to the plane of the veneers (Table 5). dPredicted failure times obtained using calculation procedure in Technical Report 10 (American Forest & Paper Association 2003). Assumptions were (1) Ultimate to allowable stress ratio is 2.85, 

 (2) cross-sectional area from measured dimensions (Table 10), (3) char rate m5 from small vertical-furnace tests (Table 5 and 11), and (4) allowable stresses from manufacturer or their literature 

 (Table 11). 

ePercentage calculated as (Predicted – Observed)/Observed × 100. 

Fire Resistance of Structural Composite Lumber Products
22

Figure 20. Furnace temperature, applied load, and estimated residual load capacity in tension test of yellowpoplar LSL (Test Number 1).

Figure 21. Furnace temperature, applied load, and estimated residual load capacity in tension test of aspen 

laminated veneer lumber (Test Number 2).

Figure 22. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir laminated veneer lumber (Test Number 3).

Figure 24. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir laminated veneer lumber (Test Number 5).

Figure 25. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir laminated veneer lumber (Test Number 6).

Research Paper FPL–RP–633

Figure 23. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir laminated veneer lumber (Test Number 4).
23

Figure 26. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir laminated veneer lumber (Test Number 7).

Figure 27. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir laminated veneer lumber (Test Number 8).

Figure 28. Furnace temperature, applied load, and estimated residual load capacity in tension test of eucalyptus laminated veneer lumber (Test Number 9).

Figure 30. Furnace temperature, applied load, and estimated residual load capacity in tension test of Southern 

Pine laminated veneer lumber (Test Number 11).

Figure 31. Furnace temperature, applied load, and estimated residual load capacity in tension test of yellowpoplar laminated veneer lumber (Test Number 12).

Fire Resistance of Structural Composite Lumber Products

Figure 29. Furnace temperature, applied load, and estimated residual load capacity in tension test of Southern 

Pine laminated veneer lumber (Test Number 10).
24

Overall, predicted failures times were slightly greater than 

observed failure times. Linear regression with zero-intercept 

indicated that predicted failure times were 7% greater than 

observed failure times when m5 data from the small verticalfurnace tests are used in calculations (Fig. 33). For four 

tests (Numbers 3, 6, 11, and 13), differences between observed and predicted failure times were 3% or less. These 

included tests for Materials Number 4 (Douglas-fir LVL), 

5 (Douglas-fir LVL), 9 (Southern Pine LVL), and 13 (Douglas-fir PSL). The test of the wider specimen of Material 4 

(Douglas-fir LVL) resulted in a 19% difference between 

observed and predicted failure times. If 0.6817 min/mm1.23 

(βn of 1.5 in/h) is used for m5 in calculation of predicted 

failure times, the result of the linear regression with 

zero-intercept indicated that predicted failure times were 

10% greater than observed failure times (Fig. 33).

Predicted failure times for the 44-mm-wide specimens 

were also a linear function of percentage of design load 

(R2 = 0.975). Using the NDS predicted failure times for 

linear regression, estimated failure times from the regression for these LVL products of five different species were 

23.6 min for an unloaded member to 8.5 min for a member 

loaded to full design load (compared with 21.8 and 

7.1 min for observed failure times). Predicted times for 

two 89-mm-wide specimens were nearly identical 

(45 and 47 min) and consistent with observed failure time 

of the Douglas-fir LVL (46 min). Yellow-poplar LSL failed 

in less time (35 min). Predicted failure times for the two 

127-mm-wide Douglas-fir LVL specimens were both 73 min 

and consistent with observed 73-min failure time for the one 

specimen. The other specimen failed at a slightly shorter 

time of 68 min. 

As with observed failure times, Douglas-fir PSL had 

the longest predicted failure times among the four 

178-mm-wide specimens. Its predicted failure time also 

agreed with the observed failure time (102 and 100 min, 

respectively). 

The NDS model predicts loss in ultimate load capacity of 

the element over duration of fire exposure. Predicted failure 

time is the time at which applied load exceeds estimated 

ultimate load capacity. Ultimate load capacity used in the 

calculations is an estimate of the average value for the population. Comparison of this predicted time is with observed 

performance of an individual member of the population. The 

2.85 value for K, the nominal design capacity to average 

ultimate capacity adjustment factor, assumes the coefficient 

of variation in tensile strength of individual pieces is 16% 

(American Forest & Paper Association 2003). Char rates 

used in calculations are also average results for test data. As 

noted earlier, coefficients of variation of the char rate data 

(m1 and m5) for the LVL tests were 8%. Coefficients 

of variation of the predicted times in the small verticalfurnace tests ranged from 5.2 to 10.8 (Table 5). As discussed 

previously, the ASTM E 119 standard allows some deviation from the standard time–temperature curve. Besides fire 

exposure, factors that can affect char rate include density, 

moisture content, chemical composition, and permeability 

(White 1988, White and Nordheim 1992). All of these properties vary somewhat within the population of a particular 

material. Given this expected variability in materials and 

fire exposure, reported differences between “average” predicted failure times and actual observed failure times for 

individual test specimens (Table 11 and Fig. 33) were within 

expectation. 

Data reported in Table 11 are times for failure. Using observed failure times, decreases in estimated ultimate load 

capacity were calculated for the last minute of the tests. This 

provides some insight as to what the corresponding changes 

Figure 32. Furnace temperature, applied load, and estimated residual load capacity in tension test of Douglasfir PSL (Test Number 13). 

Figure 33. Comparison of the experimental observed 

failure times for the tension specimens with the predicted failure times from the TR 10 methodology.

Research Paper FPL–RP–633
25

in ultimate load capacity were in the tests. For the five 

specimens with widths of 41 to 45 mm, average decrease 

in estimated load capacity at the end of the test was 14 kN 

per min (standard deviation of 3 kN/min). Applied loads for 

these tests were 9 to 68 kN. For the eight specimens with 

widths of 89 to 178 mm, average decrease in estimated load 

capacity at the end of the test was 8 kN per min (standard 

deviation of 1 kN/min). Applied loads for these tests ranged 

from 89 to 179 kN. 

The 27% difference for the yellow-poplar LSL test 

(Number 1) was the highest for the 13 tests. As a result, 

two additional tests of the yellow-poplar LSL were conducted. In the first additional test, applied load was slightly 

greater (106 kN or 42% of design load). Observed failure 

time was 33 min or a 29% difference between the observed 

time and predicted time of 43 min. A second additional test 

of yellow-poplar LSL was conducted to obtain char rate 

data. Thermocouples were inserted from the top of the specimen at a distance of 25 mm from the fire-exposed surfaces 

of the test specimen, and the top of the specimen was protected with ceramic fiber. In the three tests of yellow-poplar 

LSL, recorded furnace temperatures were initially below 

the curve and then exceeded the standard time–temperature 

curve specified in ASTM E 119. While within tolerances of 

ASTM E 119 for area under the standard curve, deviations 

from the standard time–temperature curve were greater 

in yellow-poplar tests compared with the rest of the tests. 

Compared with the small vertical-furnace charring rates, 

char rate obtained in yellow-poplar LSL tension test by using the average time for eight thermocouples inserted at 

25 mm from the exposed surface to reach 300°C was considerably faster (0.475 min/mm1.23 vs. 0.676 min/mm1.23). 

If this 0.475 min/mm1.23 char rate is used to predict failure 

times, predicted failure times for the two yellow-poplar LSL 

tension tests are 11% and 9% greater than observed failure 

times in the two tests. Density gradient of yellow-poplar 

LSL was also examined. Density affects both char rate and 

ultimate load capacity of the element. These are the two 

main inputs to the NDS model. Densities of samples cut 

from the center of the 89-mm-wide yellow-poplar LSL specimens were 2% to 7% less than densities of samples from 

the outer third of the cross section. With an applied load 

representing a relatively low percentage of full allowable 

load (36% to 42%), the remaining section at the time of 

structural failure primarily would be the center third of the 

original cross section. At predicted time of failure for 

Test 1 (45 min), the corresponding predicted “effective” 

char depth (from βeff of Equation (7)) was 40 mm, and effective residual cross section was 9 mm wide. Without the 

1.2 factor of Equation (7), corresponding predicted char 

depth was 33 mm, and residual cross-section width was 

23 mm. With only the lower density core remaining, actual 

ultimate load capacity of the remaining section was likely 

lower than that calculated from allowable stress values for 

an intact product. It appears likely that differences between 

observed failure times for yellow-poplar LSL and predicted 

times were largely due to faster charring of specimens compared with results of the small vertical-furnace tests. 

Data for two tests were used to examine the effect of variations in input data on predictions from the NDS model 

(Table 12). Tests were of a 128-mm-wide Douglas-fir LVL 

(Test Number 7) and of a 44-mm-wide yellow-poplar LVL 

(Test Number 12). Variations in dimensions along the length 

of the specimen or measurement errors can affect the appropriate value for dimensions. A 2-mm reduction in the width 

of the specimen used as input to the model caused a 1.5-min 

reduction and a 1-min reduction in predicted failure times 

of the 128-mm- and 44-mm-wide specimens, respectively 

(Table 12). With the experimental nature of fire exposure 

and material variability, there is a degree of uncertainty in 

Table 12. Results for three tests of the yellow-poplar LSL (Material Number 2) and effect of input values on the 

predictions of the failure times 

Test

number

Model

lettera

Initial 

widthb

(mm)

Char rate

m5

(min/mm1.23)

Initial ultimate load 

capacityb

(kN)

Observed failure 

time 

(min)

Predicted failure 

timec

(min)

Difference

between observed 

and predictedd (%)

7 A 128 0.6827 1067 68.5 73.7 8 

7 B 126 0.6827 1051 68.5 72.2 6 

7 C 128 0.6144 1067 68.5 66.3 –3 

7 D 128 0.6827 961 68.5 72.1 5 

 

12 A 44 0.5831 422 14.4 16.0 11 

12 B 42 0.5831 403 14.4 15.0 4 

12 C 44 0.5248 422 14.4 14.4 0 

12 D 44 0.5831 380 14.4 15.6 8 

a Model A: Best estimates used for the required input to the model (same as Table 11). 

 Model B: The input for the initial width of the specimen was decreased by 2 mm. 

Model C: The input for the char rate parameter m5 was reduced by 10%. 

Model D: The input for the initial load capacity was decreased by 10%. b

Initial width and ultimate load capacity refer to values for the uncharred cross section prior to any fire exposure. c

Predicted failure times were calculated at 0.05-min intervals. Reported time is the last calculated residual load capacity of the specimen that 

 exceeded the applied load. 

d

Percentage calculated as (Predicted – Observed)/Observed × 100. 

Fire Resistance of Structural Composite Lumber Products
26

char rate. A 10% reduction in the char rate parameter 

(min/mm1.23, i.e., faster charring) reduced predicted failure 

times by 7.4 and 1.6 min for the 128-mm- and 44-mm-wide 

specimens, respectively (Table 12). Besides char rate, the 

other important parameter of the NDS model is reduced area 

for failure of the specimen Af, which depends on calculation 

of the ultimate load capacity. No tension testing of specimens at room temperature was done as part of this study. 

Values for allowable design stress were either published 

values or obtained from the manufacturer. To evaluate impact of variations in ultimate load capacity, initial ultimate 

load capacity prior to any fire exposure was reduced by 

10%. Resulting change in predicted times for the 

128-mm- and 44-mm-wide specimens (Test Numbers 7 

and 12) were 1.6 and 0.4 min, respectively (Table 12). These 

results are consistent with a conclusion that deviations in 

char rate had a greater influence on failure times than deviations in the ultimate load capacity of the specimen in tests 

conducted for this study. 

Relative impact of any deviations in char rate and ultimate 

load capacity would be different if applied loads are at or 

near full allowable design load. Using input values of 

Table 11, predicted failure times for a member loaded to 

100% of the allowable design load were 40.0 and 11.1 for 

the specimens of Tests 7 and 12, respectively. With these 

shorter failure times compared with that for the 24/46% 

loaded test specimens, there is less charring of the element 

before failure. In the case of these two test specimens, impacts of the 10% reductions in the inputs for char rate and 

initial ultimate load capacity on predicted failure times of a 

fully loaded member were very similar for the fully loaded 

scenario. For Test 7, reductions in predicted times were 

3.6 min for the 10% reduction in load capacity and 

4.0 min for the 10% reduction in char rate (m5). For Test 

Number 12, reductions in predicted times were 0.9 min 

for the 10% reduction in load capacity and 1.1 min for the 

10% reduction in char rate (m5). Conversely, charring rate 

will likely increase as the remaining cross section becomes 

very small for elements with little or no applied load as evidenced in the horizontal-furnace charring experiments.

Results of this study support the application of the TR 10 

method to structural composite lumber. In terms of potential 

structural composite lumber products that may be different 

than those tested in the study, the TR 10 method assumes 

a cross section with fairly uniform properties. In Section 

16.2.4 of the NDS, additional tension lamination(s) are required for fire-rated structural glued-laminated timbers that 

have an outer tension lamination. The assumption is that 

existing outer high-strength tension lamination for room 

temperature structural design will be charred away in the 

1- or 2-h fire test. Similar provisions or adjustments to calculations should be provided for any structural composite 

products that are designed with higher load capacity within 

the outer exposed portions of the cross section and lower 

load capacity within the inner core of the element.

Conclusions

In this study, we tested 14 structural composite lumber 

products to determine the char rate when exposed to the 

fire exposure of the standard fire resistance test. Products 

tested included LSL, LVL, and PSL. Products of five different species were tested. Based on the small vertical-furnace 

tests, we concluded that the char rates for composite lumber 

products were comparable to those of solid-sawn lumber 

and within the range previously found for different species 

of solid-sawn lumber. Additional char rate tests on 10 of the 

composite lumber products were conducted in an intermediate-scale horizontal furnace. There was greater variability 

in the data for the horizontal-furnace tests compared with 

the results obtained in the small vertical-furnace. Despite 

the differences in the construction and operation of the two 

furnaces, the data did not suggest any bias between the char 

rate results for the two furnaces. Thus, it is reasonable to 

expect the char rate data from the small vertical-furnace to 

be applicable to the large furnaces specified in ASTM E 

119. Initially, 13 tests were conducted in which the composite lumber products were loaded in tension as they were 

subjected to the standard fire exposure of ASTM E 119. 

These tension tests included 10 different materials. For most 

of the tests, the applied load was about 30% of the full allowable design load. The TR 10 design procedure for calculating the fire resistance ratings of exposed wood members 

was used to predict the failure times in the tension tests. 

The char rate data of the small vertical-furnace were used 

in the calculations. The differences between the “average” 

predicted failure times and the actual observed failure times 

for individual test specimens of structural composite lumber were within expectations for sawn lumber or structural 

glued-laminated timbers. In 11 of the 13 tests, the predicted 

failure times exceeded the observed times by less than 14%. 

In four tests, the differences were 3% or less. In the tests of 

this study, variability in the char rates was likely the primary 

factor responsible for differences between the predicted and 

observed failure times. If the specimens had been loaded 

to full design load, the potential impact of variability in the 

actual room temperature ultimate load capacity of the individual test specimens would have been greater than was the 

case in these tests. 

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Fire Resistance of Structural Composite Lumber Products
28

Research Paper FPL–RP–633

Appendix—Individual Test Results for Each Small Vertical-Furnace Test 

These estimates of parameters for two time–location models (Eqs. (1) and (5)) are for three individual layers of test specimens 

with inserted thermocouples and for the combined data set.

Material m1 of Equation (1) m5 of Equation (5) 

number

Species 

composite type Direction 

FPL test 

number Layer 1 Layer 2 Layer 3 All Layer 1 Layer 2 Layer 3 All 

1 Aspen LSL Parallel 1673 1.620 1.650 1.575 1.615 0.6879 0.7004 0.6686 0.6856

1676 1.638 1.570 1.552 1.586 0.6956 0.6662 0.6590 0.6736

Perpendicular 1663 1.548 1.535 1.595 1.560 0.6564 0.6470 0.6755 0.6598

1668 1.580 1.575 1.508 1.554 0.6707 0.6688 0.6404 0.6600

2 Yellow-poplar LSL Parallel 1662 1.649 1.602 1.550 1.600 0.7004 0.6794 0.6592 0.6797

1674 1.603 1.578 1.516 1.571 0.6810 0.6700 0.6562 0.6704

Perpendicular 1665 1.578 1.573 1.491 1.548 0.6705 0.6680 0.6347 0.6578

1670 1.649 1.649 1.615 1.638 0.7004 0.6995 0.6857 0.6952

3 Aspen LVL Parallel 1741 1.693 1.552 1.603 1.616 0.7181 0.6579 0.6798 0.6853

1739 1.460 1.423 1.334 1.406 0.6200 0.6050 0.5658 0.5969

Perpendicular 1742 1.419 1.447 1.485 1.450 0.6039 0.6147 0.6318 0.6168

1744 1.458 1.433 1.446 1.445 0.6170 0.6079 0.6135 0.6126

4 Douglas-fir LVL Parallel 1669 1.496 1.570 1.502 1.523 0.6349 0.6666 0.6362 0.6459

1672 1.561 1.533 1.526 1.540 0.6634 0.6508 0.6472 0.6538

Perpendicular 1664 1.515 1.562 1.490 1.523 0.6437 0.6626 0.6318 0.6461

1667 1.529 1.534 1.527 1.530 0.6476 0.6499 0.6481 0.6485

5 Douglas-fir LVL Parallel 1652 1.704 1.764 1.708 1.725 0.7225 0.7490 0.7249 0.7321

1659 1.584 1.633 1.571 1.596 0.6727 0.6920 0.6666 0.6771

Perpendicular 1646 1.529 1.536 1.458 1.508 0.6488 0.6502 0.6185 0.6392

1655 1.672 1.621 1.539 1.611 0.7077 0.6872 0.6519 0.6823

6 Douglas-fir LVL Parallel 1647 1.608 1.567 1.553 1.576 0.6823 0.6653 0.6588 0.6688

1656 1.776 1.606 1.645 1.676 0.7525 0.6809 0.6765 0.7100

Perpendicular 1651 1.694 1.702 1.635 1.677 0.7196 0.7208 0.6938 0.7114

1660 1.576 1.575 1.536 1.562 0.6659 0.6676 0.6520 0.6618

7 Eucalyptus LVL Parallel 1743 1.930 1.782 1.551 1.780 0.8170 0.7537 0.6624 0.7551

1740 1.648 1.473 1.587 1.567 0.6985 0.6243 0.6803 0.6661

Perpendicular 1745 1.708 1.566 1.597 1.622 0.7206 0.6648 0.6775 0.6870

1738 1.643 1.667 1.784 1.701 0.6960 0.7075 0.7590 0.7221

8 Southern Pine LVL Parallel 1677 1.505 1.404 1.536 1.482 0.6386 0.5952 0.6521 0.6286

1680 1.460 1.410 1.418 1.430 0.6203 0.5989 0.6013 0.6068

1683 1.484 1.369 1.406 1.420 0.6289 0.5806 0.5972 0.6022

Perpendicular 1678 1.368 1.326 1.394 1.364 0.5812 0.5606 0.5922 0.5789

1681 1.542 1.405 1.418 1.455 0.6559 0.5983 0.6026 0.6189

1682 1.420 1.384 1.340 1.382 0.6042 0.5876 0.5684 0.5867

9 Southern Pine LVL Parallel 1645 1.498 1.559 1.508 1.521 0.6364 0.6629 0.6396 0.6463

1654 1.578 1.573 1.595 1.582 0.6699 0.6667 0.6774 0.6713

Perpendicular 1650 1.424 1.427 1.404 1.418 0.6052 0.6083 0.5954 0.6030

1658 1.443 1.376 1.380 1.400 0.6118 0.5841 0.5861 0.5940

10 Yellow-poplar LVL Parallel 1644 1.472 1.435 1.482 1.463 0.6254 0.6091 0.6292 0.6212

1653 1.436 1.502 1.382 1.440 0.6090 0.6373 0.5858 0.6107

Perpendicular 1649 1.405 1.267 1.346 1.346 0.5978 0.5468 0.5723 0.5723

1657 1.308 1.255 1.336 1.300 0.5562 0.5330 0.5683 0.5525

11 Yellow-poplar LVL Parallel 1671 1.428 1.363 1.416 1.402 0.6065 0.5780 0.6007 0.5951

1675 1.429 1.418 1.472 1.440 0.6068 0.6016 0.6257 0.6113

Perpendicular 1661 1.439 1.386 1.270 1.365 0.6138 0.5891 0.5406 0.5812

1666 1.391 1.246 1.215 1.278 0.5910 0.5295 0.5154 0.5430

12 Douglas-fir PSL Parallel 1634 1.639 1.576 1.693 1.636 0.6956 0.6698 0.7201 0.6951

1640 1.649 1.626 1.574 1.613 0.7142 0.6904 0.6685 0.6885

Perpendicular 1637 1.582 1.678 1.590 1.617 0.6708 0.7119 0.6731 0.6853

1643 1.635 1.588 1.662 1.628 0.6946 0.6734 0.7048 0.6910

13 Southern Pine PSL Parallel 1633 1.569 1.614 1.690 1.624 0.6679 0.6865 0.7177 0.6907

1639 1.565 1.641 1.662 1.623 0.6654 0.6970 0.7070 0.6898

Perpendicular 1636 1.664 1.687 1.639 1.664 0.7060 0.7152 0.6946 0.7053

1642 1.574 1.606 1.610 1.596 0.6690 0.6824 0.6844 0.6786

14 Yellow-poplar PSL Parallel 1631 1.541 1.482 1.587 1.536 0.7560 0.6829 0.7312 0.7162

1638 1.548 1.576 1.559 1.561 0.6577 0.6691 0.6629 0.6632

Perpendicular 1635 1.595 1.543 1.561 1.566 0.6765 0.6547 0.6628 0.6647

1641 1.577 1.585 1.611 1.591 0.6707 0.6730 0.6843 0.6760