A Study of Metal Truss Plate 

Connectors When Exposed to Fire

Harman, K. A. 

Lawson, J. R. 

NISTIR 7393

 
NISTIR 7393

A Study of Metal Truss Plate 

Connectors When Exposed to Fire

Harman, K. A. 

Lawson, J. R. 

Fire Research Division 

Building and Fire Research Laboratory 

January 2007

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

Technology Administration 

Robert Cresanti, Under Secretary of Commerce for Technology 

National Institute of Standards and Technology 

 William Jeffrey, Director 
i

TABLE OF CONTENTS 

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii 

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 
ii

LIST OF FIGURES 

Figure 1 Typical lightweight wood construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Figure 2 Typical set up of test members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Figure 3 Locations of thermocouples A, B, C, and D . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Figure 4 Close up of surface thermocouples C and E . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Figure 5 Test apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6 One minute after ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Figure 7 Thirty minutes after ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Figure 8 Example: thermocouple measurements from a normal test specimen . . . . . . 9 

Figure 9 Example: thermocouple measurements from a 1/32 in. wood/plate gap 

 specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Figure 10 Example: thermocouple measurements from a 1/8 in. joint gap specimen . . 10 

Figure 11 10 minutes into the test the plate is still shiny . . . . . . . . . . . . . . . . . . . . . . . . 11 

Figure 12 40 minutes into the test the plate has lost its shininess . . . . . . . . . . . . . . . . . 11 

Figure 13 Post burn – 1/8 in. joint gap specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

Figure 14 Post burn – 1/32 in. wood/plate gap specimen . . . . . . . . . . . . . . . . . . . . . . . . 12 

Figure 15 Post burn – normal specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

Figure 16 Char pattern on the left side of the plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

LIST OF TABLES 

Table 1 Char depth range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 
1

A Study of Metal Truss Plate Connectors 

When Exposed to Fire 

By 

Kelly A. Harman, E.I.T and James R. Lawson 

Abstract 

The popularity of lightweight, metal plate connected wood truss construction is increasing due to 

cost effectiveness, versatility, and ease of construction. This type of construction brings many 

concerns to the firefighting community, since structural collapse has caused numerous injuries 

and fatalities in the fire service. In an attempt to determine the performance of metal plate wood 

truss connections during fire exposures, NIST conducted a series of twelve instrumented tests 

exposing one side of the test specimen to the thermal exposure. Load carrying ability of the metal 

plate truss connections was not measured during these tests. The tests were purely an attempt to 

study the heat transfer between the metal plate and the wood. Results from these tests suggest 

that the metal plates help to protect the wood beneath the plates. However, additional work is 

required to produce more detailed information. 

KEY WORDS: Building collapse, fire test, lightweight construction, metal plate, wood truss 
INTRODUCTION 

The popularity of lightweight, metal plate connected wood truss construction is 

increasing due to cost effectiveness, versatility, and ease of construction. These 

assemblies, as seen in Figure 1, are typically constructed of two-by-four members 

connected with light gauge metal connectors. This construction brings many concerns to 

the firefighting community [1]. Many in the fire service believe that lightweight 

construction does not have the fire 

resistance that typical solid joist 

and rafter construction offer. 

Many firefighters have been 

injured and over 180 have lost 

their lives in the past twenty years 

due to structural collapse [2]1

. 

However, it is noted that these data 

do not identify the types of building construction where the injuries and deaths occurred. 

Figure 1 – Typical lightweight wood construction 

The use of metal connector plates is one of the most widely discussed aspects of 

the lightweight wood truss. Some believe that these connectors serve as a heat sink and 

are weakening the wood at a faster rate while others believe that the metal plate actually 

reflects heat and therefore protects the wood behind the plate [3, 4]. While it is known 

that polished metal objects reflect heat efficiently Manny [3] questioned the reflective 

properties of these metal plates if they become masked by heavy smoke and soot. It is 

also important to note that the teeth on most truss plates only penetrate the wood about 

0.38 in. (9.5 mm) and in the case of a long duration fire the wood may char beyond this 

 

2

1

 From all causes (not including the World Trade Center). 
3

depth. While the strength of the truss construction depends on the integrity of all its 

parts, considerable charring causing just one connector plate to fail severely weakens the 

entire assembly [1]. 

Information on this type of connection is very limited. While conducted under 

different objectives, tests performed by the USDA Forest Products Laboratory gave an 

initial insight to metal plate behavior [5]. These results are addressed later in the 

discussion section of this article. 

Connector plates are made of galvanized sheet steel, typically 16, 18, or 20 gauge 

measuring [approximately 0.062 in. (1.6 mm), 0.05 in. (1.27 mm), and 0.038 in. (0.9 mm) 

thick, respectively], and have teeth punched into one side. During truss fabrication truss 

plates are pressed into the lumber with either a hydraulic press or a roller to ensure that 

the teeth are fully embedded. According to ANSI/TPI 1-2002 [7], the wood members do 

not have to be completely flush against one another. Chapter 3, Section 3.7.6.1 allows 

for a wood member-to-wood member gap of 1/8 in. (3.2 mm) in roof truss assemblies. 

Full embedment of the metal plate teeth is also desired but not required. These tolerances 

will be discussed later. 

In an attempt to determine whether or not the metal plate is protecting the wood 

beneath or further progressing its degradation and to possibly re-create the test results 

published by the USDA Forest Production Laboratory, twelve tests were conducted at the 

National Institute of Standards and Technology (NIST) Building and Fire Research 

Laboratory. During these tests, the strength capabilities of neither the truss plate nor the 

wood members were analyzed. These tests examined the heat transfer between the metal 

plate and the wood. 
METHOD 

Sets of three different test specimens were constructed for this investigation, all 

using the same grade of dimension lumber. Dimension lumber is rated by its stress 

grade. Two by four and wider dimension lumber can be found in grades of No. 1, No. 2, 

No. 3, and select structural. They often appear in combinations of No. 2 and better (No. 2 

& BTR) or No. 3 & BTR [6]. Spruce-Pine-Fir (S-P-F) lumber with a No. 2 & BTR 

rating, a cross-section of 1.5 in. (38 mm) by 3.5 in. (89 mm), from here on referred to as 

2 x 4, with a moisture content of 11.5 percent, and an average density of 490 kg/m3

 was 

used. Moisture content was measured using a commercially manufactured, automated, 

precision, thermo-gravimetric moisture analyzer. Each specimen consisted of two 2 x 4, 

one ft. (0.30 m) long members connected by two 3 in. (76 mm) by 6 in. (152 mm) 

galvanized steel truss plates of 20 gauge (0.95 mm) with punched teeth approximately 

0.31 in. (7.9 mm) long, one on each side. Figure 2 shows this arrangement. No load was 

applied to these specimens before, during, or after testing.

 

 

Figure 2 – Typical set up of test members 

The first six test specimens were assembled with the truss plate teeth completely 

embedded in the pieces of wood and with no gap between the pieces of wood. These test 

specimens will be referred to as “normal” in the future. 

4
5

The last six test specimens consisted of three members with a 1/8 in. (3.2 mm) 

joint gap between the butted ends of the wood members having full plate embedment and 

three members with a 1/32 in. (0.8 mm) gap between the face of the metal plate and the 

surface of the wood to comply with the ANSI/TPI 1-2002 [7] manufacturing tolerances. 

Chapter 3, Section 3.7.6.1 allows a wood member-to-member joint gap of 1/8 in. 

(3.2 mm) in roof truss assemblies. It should also be noted that only a 1/16 in. (1.6 mm) 

gap is permitted in floor truss assemblies; this case was not tested. Section 3.7.5 allows a 

1/32 in. (0.8 mm) gap between the wood and the metal plate. These specimens will be 

denoted in the future as “1/8 in. joint gap” and “1/32 in. wood/plate gap”, respectively. 

The exact effects on structural stability of wood charring beneath a gusset plate, where 

there is a gap between the metal and the wood and where plates may be improperly 

installed, has not been well documented [8]. A commercially manufactured flat plate 

hydraulic press was used for attaching the plates to ensure even embedment of the metal 

plate teeth. Temperatures were measured at various locations on the specimens using 

0.01 in. (0.25 mm) diameter type K thermocouples. 

The first three tests specimens were constructed with four thermocouples (A, B, 

C, and D). This was done in order to see if the metal plate was protecting wood adjacent 

to the plate on the surface. The first thermocouple, denoted “A”, was placed on the 

surface of the wood directly behind the metal plate and was centered on the 2 x 4’s width, 

approximately 1.75 in. (44 mm) from the top and bottom surface edges. Thermocouple 

“B” was placed in the center of the 2 x 4 test specimens directly behind “A”. This was 

accomplished by drilling a hole into the wood just larger than the thermocouple diameter. 

This hole was located on the center of the wood’s top surface 0.75 in (19 mm) from the 

edge. The hole was drilled 1.75 in (44 mm) deep, and the thermocouple was inserted into 

the hole until it stopped at the bottom, directly behind thermocouple “A”. 
Thermocouples “C” and “D” were placed in a similar fashion 5 mm from the edge of the 

metal plate. Thermocouple “C” was located on the wood’s surface and thermocouple 

“D” was inserted into the wood like “B”, as described above. This arrangement can be 

seen in Figure 3. 

6

 Figure 4 – Close up of surface thermocouples C and E 

Figure 3 – Locations of thermocouples A, B, C and D 

 A fifth thermocouple, “E” which was used in tests four through twelve, was 

placed 20 mm from the edge of the metal plate. A close up of this configuration can be 

seen in Figure 4. 

 

The tests were conducted using a gas fired thermal radiant panel that was 

calibrated in order to subject the wood to a nominal constant exposure of 20 kW/m2

. 

This situation simulates initial flashover conditions during fire growth. At flashover, all 
 Figure 5 – Testing apparatus 

combustible items in the room begin to 

burn almost simultaneously, causing a 

rapid increase in both heat release rates 

and temperatures. After flashover, the fire 

is often referred to as a “fully developed 

fire” [9]. The testing apparatus used is 

shown in Figure 5. The wood/metal plate 

joint was exposed to a piloted ignition 

using a gas fired torch in order to introduce flame at various times during the test. The 

testing method at NIST applied heat and flame to one side only, the testing done by the 

USDA heated the sample with a furnace at a maximum temperature of 325 °C, allowing 

heat from all directions; a flame was never introduced [10]. The NIST testing method is 

illustrated in Figures 6 and 7. Tests were conducted for 60 minutes with a temperature 

reading taken each second using a computer controlled electronic data logger. 

 

7

 Figure 6 – One minute after ignition Figure 7 – Thirty minutes after ignition 

RESULTS AND DISCUSSION 

Results of the twelve tests were similar, indicating that the parameters that were 

varied plus the randomness of the tests were not significant. Figure 8 shows the 
8

temperatures measured by the thermocouples averaged over the six tests referred to as 

“normal.” The sudden jump in temperatures at around 200 s is due to ignition of the 

wood. The thermocouple directly behind the metal plate, thermocouple “A”, recorded 

lower temperatures than thermocouple “C” which was located on the surface beside the 

plate. This indicates that the metal plate provides some protection for the wood directly 

beneath it. A lower temperature was recorded in thermocouple “B” than at the adjacent 

location, thermocouple “D”. This differed from the tests conducted by the USDA. The 

tests conducted at NIST suggest that the metal plate is protecting the wood beneath it, 

whereas the USDA test results suggest that the teeth from the metal truss plate are 

conducting heat deeper into the wood [5]. The differences in the tests could be due to the 

testing method (recall that the USDA test was conducted in a furnace with the specimen 

exposed on all sides) as well as the temperature at which the tests were conducted. It was 

also found in the NIST tests that the temperature on the surface was higher at a distance 

of 0.79 in. (20 mm) from the edge of the plate than it was at 0.20 in. (5 mm) from the 

plate’s edge. This was something not looked into in the USDA tests and is probably 

related to the burning behavior of the wood. 
Normal 

0

100

200

300

400

500

600

700

800

0 400 800 1200 1600 2000 2400 2800 3200 3600

TIME (seconds)

TEMPERATURE (degrees C)

A

B

C

D

E

Figure 8 – Example: thermocouple measurements from a normal test specimen 

1/32 Inch Gap

0

100

200

300

400

500

600

700

800

0 400 800 1200 1600 2000 2400 2800 3200 3600

TIME (seconds)

TEMPERATURE (degrees C)

A

B

C

D

E

Figure 9 – Example: thermocouple measurements from a 1/32 in. wood/plate gap 

specimen 

9
1/8 Inch Gap

0

100

200

300

400

500

600

700

800

0 400 800 1200 1600 2000 2400 2800 3200 3600

TIME (seconds)

TEMPERATURE (degrees C)

A

B

C

D

E

Figure 10 – Example: thermocouple measurements from a 1/8 in. joint gap specimen 

(Note that thermocouples “C” and “E” lost contact with the surface at around 

1700 seconds.) The highest temperature reached for an individual test during ignition 

and flaming, was 1007 °C while the lowest temperature was 605 °C. These differences 

have to do with the variability of fire development, variations in thermocouple placement, 

and the uniformity of wood. Despite the fact that the wood specimens came from the 

same member and the testing apparatus was calibrated, each piece burned slightly 

different. Figures 9 and 10 show the temperatures measured (averaged over three tests 

each) during the 1/32 in. (0.8 mm) wood/plate gap tests and the 1/8 in. (3.2 mm) joint gap 

tests, respectively. The gap between the plate and the wood leads to considerably higher 

temperatures measured behind the plate with thermocouple “A” (See Fig. 9). The joint 

gap also increases the temperature of thermocouple “A” when compared to the normal 

specimen. Qualitatively speaking, the trends for the remaining thermocouples are all 

similar. 

10
Over time the metal plate, as seen in Figures 11 and 12, began to collect soot and 

loose its shininess. The temperature at the thermocouples behind the plate increased at a 

Figure 11 - 10 minutes into the test the 

plate is still shiny 

Figure 12 - 40 minutes into the test the 

plate has lost its shininess 

constant rate. While the discoloration of the plate caused the plate to change its heat 

reflective properties, it is not clear how much this affected the subsequent heat transfer. 

One is also able see in Figure 11 how the wood beneath the plate is still its original color. 

Thirty minutes later in Figure 12 the plate has become quite discolored and the wood 

beneath the plate has also begun to char. 

In each of the tests conducted thermocouple “A” recorded lower temperatures 

than thermocouple “C”, and average of 215 °C lower. This generally occurred during the 

first half of the test. Similar results were seen in thermocouples “B” and “D” with an 

average difference of 110 °C. 

11
Figure 15 - Post burn - normal specimen 

Figure 14 - Post burn - 1/32 in. wood/plate gap 

specimen 

Figure 13 - Post burn - 1/8 in. joint gap specimen 

There were two occurrences when thermocouple “B” reached a higher 

temperature than thermocouple “D”. 

This was in the case of the 1/8 in. 

(3.2 mm) joint gap. In this arrangement 

fire was able to make its way between 

the wood members and char a large area 

behind the metal plate. This can be seen 

in Figure 13. 

During the early part of the test 

after ignition, thermocouple “C” 

recorded lower temperatures, by an 

average of 45 °C, than thermocouple 

“E”. 

There were a few obvious differences in 

the specimens with manufacturing 

tolerances. The 1/8 in. (3.2 mm) joint 

gap specimens recorded on average 

40 °C higher temperatures on 

thermocouple “B” which is located in 

the center of the wood behind the plate, 

than in the normal specimens. This was 

due to fire easily making its way 

between the two pieces of wood behind 

the plate. These also caused a large area 

of wood to burn away and char behind the plate exposing a number of the metal teeth. 

12
This can be seen when comparing Figure 13 to Figure 15. On average, charring that took 

place in the space between butt-joint ends for the 1/8 in. (3.2 mm) joint gap specimens 

grew from the starting point gap of 0.15 in. (3.8 mm) to a width of 0.57 in. (14.5 mm), 

whereas little or no area burned away or charred between the normal spaced wood joint 

specimens. The 1/8 in. (3.2 mm) joint gap specimens recorded the greatest char depths; 

the 1/32 in. (3.2 mm ) wood/plate gap specimen, as seen in Figure 14, and only one of the 

normal specimens, Figure 15, recorded char depths great enough to completely expose 

the bottom row of metal teeth. The range of char depth measurements for each type of 

specimen is presented in Table 1. The char depth measurements were made using 

precision drafting tool calipers with measurements transferred to and read from a 

machinist scale. All char depth measurements reported (between the wood members and 

behind the plate) were made at locations along the edge of the metal plate. The char 

depth measured between the wood members was measured at the surface of the 

wood/metal interface and perpendicular to the wood gap or across the gap. The char 

depth measured behind the metal plate was measured at the surface of the wood/metal 

interface and perpendicular to the metal plate or in the direction of the teeth. 

Test Specimen 

Range, char depth measured 

between wood members 

(mm) 

Range, char depth measured 

behind plate 

(mm) 

Normal 0 - 2 2 – 26 

1/32 in. wood/plate gap 3 - 6 3 – 23 

1/8 in. joint gap 0 – 15 7 – 27 

Table 1 – Char depth range 

13
Figure 16 – Char pattern on the left side of the plate.

While the bottom side of each member burned in a similar fashion in the normal 

and 1/32 in. (3.2 mm) wood/plate specimens, fire was able to penetrate into the wood 

under the plate charring directly behind the plate in the case of the 1/32 in. (3.2 mm) 

wood/plate gap. Because of this more than just the bottom rows of teeth were exposed. 

Similar char patterns on the surface were seen in each specimen, each having the 

maximum surface char depths at a distance of 0.79 in. (20 mm) or greater from the edge 

of the metal plate on the left side and a distance of 0.55 in. (14 mm) or greater from the 

edge of the metal plate on the right side. 

This can be seen in Figure 16. It was 

noted earlier that higher temperatures 

were recorded at a location further from 

the plate than closer to the metal plate 

which would result in this pattern. 

Char depth measurement locations varied based on where flames burned the most on the 

wood’s surface. The average surface char depth 0.75 in. (19 mm) was measured on 

average 1.54 in. (39 mm) from the left edge of the metal plate, and on the right edge of 

the metal plate an average surface char depth of 0.47 in. (12 mm) was measured on 

average 1.22 mm (31 mm) from the right edge. 

CONCLUSION 

From the results of these tests, it is shown that the metal plate appears to provide 

some level of protection for the wood beneath it as well as the wood in close proximity to 

the plate’s edge when there is little or no gap between the plate and the wood. When 

burning occurred on the wood’s surface behind a metal plate with a gap, teeth depth in 

14
the char/wood interface ranged from approximately 80 percent embedded to fully 

exposed. 

It is important to note that these experiments were conducted with heating and fire 

applied to only one side of the test specimen and with no external loads applied to 

examine the load carrying capability of the construction system. To better understand the 

significance of burning around metal plates, these experiments should be repeated under 

various loading and fire conditions. Variations in the number and placement of 

thermocouples should be considered in future tests as well as variations in the orientation 

of the wood. The wood is expected to burn differently if aligned vertically to the radiant 

panel as opposed to horizontally. 

 With what little is known about the behavior of metal truss plate connectors when 

exposed to fire and with the increasing numbers of lightweight wood trusses in 

production, there is a need to further quantify the response of lightweight wood trusses to 

the impact of fire exposure. 

Acknowledgements: 

Dr. Steve Cramer 

The Wood Truss Council of 

America: 

Kirk Grundahl 

Ryan Dexter 

Will Warlick 

The Truss Plate Institute: 

Mike Cassidy 

Dr. William Davis 

Mr. Mike Selepak 

Mr. Jay McElroy 

Mr. Laurean DeLauter 

84 Lumber 

 

Harman’s research was performed while on appointment as a U.S. Department of 

Homeland Security (DHS) Fellow under the DHS Scholarship and Fellowship Program, a 

program administered by the Oak Ridge Institute for Science and Education (ORISE) for 

DHS through an interagency agreement with the U.S Department of Energy (DOE). 

ORISE is managed by Oak Ridge Associated Universities under DOE contract number 

15
16

DE-AC05-00OR22750. All opinions expressed in this paper are the author's and do not 

necessarily reflect the policies and views of DHS, DOE, or ORISE. 

Note: NIST policy is to report numerically measured values in metric; however, the 

audience for this paper uses standard units. 

REFERENCES 

1. Mittendorf, John W.(1988). “Joint rafters versus lightweight wood trusses.” Fire 

Engineering, 141(7), 48-50. 

2. Brassell, L. and Evans, D. (2003). Trends in firefighter fatalities due to structural 

collapse, 1979-2002. NISTIR 7069, National Institute of Standards and 

Technology, U.S. Department of Commerce, Gaithersburg, MD. 

3. Manny, William F. (1991). “Lightweight wood trusses: More to consider.” Fire 

Engineering, 144(5), 62-63. 

4. Meeks, John E. P.E., (2001). “Truss plate performance in fire: One person’s 

observations,” Structural Building Components Magazine, [online] 1 

http://www.sbcmag.info/past/2001/01jun_jul/plateperformance.html. 

(Accessed: 23 June 2005). 

5. Shrestha, D., Cramer, S., and White, R. (1995). “Simplified models for the 

properties of dimension lumber and metal-plate connections at elevated 

temperatures.” Forest Products Journal, 45(7/8), 35-42. 

6. Forest Products Laboratory, Forest Service, U.S. Department of Agriculture 

(1999). Wood handbook: Wood as an engineering material. Forest Products 

Laboratory, Forest Service, USDA, Madison, WI. 

7. ANSI/TPI (2002). Quality criteria for the manufacture of metal plate connected 

wood trusses. in: National Design Standard for Metal Plate Connected Wood 

Truss Construction. Truss Plate Institute, Chapter 3. 

8. Malanga, Robert (1995). “Fire endurance of lightweight wood trusses in building 

construction.” Fire Technology, 31(1), 44-61. 

9. Buchanan, Andrew H. (2001). Structural Design for Fire Safety. John Wiley & 

Sons, LTD., New York. 

10. White, R., Cramer, S. (1994). “Improving the Fire Endurance of Wood Truss 

Systems.” Pacific Timber Engineering Conference Gold Coast Australia, 

July 11-15. 582-589.