United States

Department of

Agriculture

Forest Service

CaSQ Effect of Moisture Cycling On Mechanical

Response of Metal-Plate Connector Joints

With and Without an Adhesisve Interface

Southern Forest

Experiment Station

New Orleans,

Louisiana

L. H. Groom

Research Paoer

so-291 - September 1995

5,000

DlSPLACEMENi (INCH)
Wood trusses are frequently located in light-frame structures where they

are subjected to significant shifts in moisture conditions. However, little is

’ known about the effects of moisture cycling of the wood members on the mechanical behavior of metal-plate connector (MPC) joints. Thus, the primary objective of this study was to quantify the effect of wood moisture content fluctuations

on MPC joint behavior. A secondary objective was to investigate maintaining

interfacial integrity between the metal-plate teeth and wood by applying an

epoxy adhesive at the interface and noting the changes in mechanical performance of MPC joints.

Results of this study indicate that moisture cycling significantly affects the

mechanical performance of MPC joints. Some of the decline in mechanical performance could be attributable to pIate “backout.” It was shown that in addition to a decrease in mechanical performance, moisture-cycled specimens

probably have an altered stress transfer mechanism as exhibited by load-slip

traces. Initial stiffness of the load-slip traces is most drastically affected by

moisture cycling, but ultimate load and load-at-0.015~inch-slip were also affected. Addition of an adhesive interface between the teeth and wood appears

to increase the mechanical performance of MPC joints. However, the apparent

increase in mechanical performance could not be quantified due to the lack of

identically matched specimens. The degrade ofultimate load and load-at-O.OlSinch-slip of MPC joints with an adhesive interface was similar to that of standard MPC joints. Initial stiffness of MPC joints with an adhesive interface

degraded more rapidly than standard MPC joints, probably due to weakening

of the adhesion between the teeth and wood. Addition of an adhesive to the

tooth-wood interface resulted in less variability of mechanical properties when

compared to standard MPC joints, with this variability remaining constant

regardless of number and severity of moisture cycles.

.

.

N
Effect of Moisture Cycling on Mechanical Response of Metal-Plate

Connector Joints With and Without an Adhesive Interface

L.H. Groom

INTRODUCTION

The fundamental purpose of a truss system in construction is to transfer applied loads to the structure

framework and eventually the foundation, with some

of the load being absorbed in the truss system through

internal friction and damping. The trend in truss design in the last 15 years has been to reduce the amount

of material necessary to resist the design loads. This

increase in efficiency has led to higher stresses in the

truss members. However, restrictions placed on oldgrowth timber stands and an increase in plantationgrown timber will result in a timber base with

markedly lower mechanical properties. This paradox

of increased demand for superior lumber with a decreased availability of a quality-product lumber supply will lead to research aimed at changing current

design values and practices (Galligan 1990).

In most cases, truss performance is a reflection of

the properties of the joints (Wilkinson 1984). Failure

of truss systems occurs primarily at the joints due to

stress concentrations, whereas large deflections in

truss systems are generally the result of small joint

displacements (Reardon 1971). Thus, evaluation of

truss performance must begin by understanding the

mechanical behavior of metal-plate connector (MPC)

joints.

Predictive models have been developed for the static

structural behavior of MPC joints by various researchers (Crovella and Gebremedhin 1990, Foschi 1977,

Groom 1988, Triche and Suddarth 1988). However,

these models concentrate on the fundamental mechanisms of stress transfer and ignore secondary variables such as moisture cycling. Moisture cycling is

critical for truss systems because trusses are often

found in the most environmentally extreme areas:

attics and basements. Joist and rafter moisture content (MC) in these environments has been shown to

fluctuate seasonally by anywhere from 10 percent

(Cunningham 1984, Duff 1978) to as much as 18 percent (Cleary 1984, Cunningham 1983, Sherwood 1985).

This type of moisture cycling can affect truss performance. Unfortunately, the effect of moisture cycling

has not been quantified. As a result, design values have

been adopted that are too conservative and make for

inefficient use of lumber whose mechanical properties

are ever decreasing.

There are several studies that have evaluated the

effect of changing MC under a constant load for timber connectors (Keenan and others 1983, Kunesh and

Johnson 1968, Mayo 1980, Noren 1968, Wilkinson

1971). The results of these studies share a common

theme: The combined effect of changing environment

with loading significantly decreased truss stiffness.

In most cases, strength was also significantly reduced.

However, the research neither quantified the effect of

moisture cycling on stiffness or strength nor defined

the level at which moisture cycling proved significant.

McAlister (1990) found that MPC joints containing

laminated veneer lumber and pine and subjected to

an outdoor environment for a l-year period showed a

substantial decrease in stiffness and strength. The

decrease in mechanical properties was due in part to

“plate backout,” a phenomenon in which the metal

plate backs out of the wood members due to shrinking

and swelling stresses. Plate backout significantly

weakens the interfacial contact between the

metal-plate teeth and wood.

Recommended design values for truss members are

dictated primarily by the materials comprising the

truss, the type of truss, and the expected loading during the life of the truss (Truss Plate Institute 1985).

The design plan laid out by architects and, to some

degree, engineers, for a particular structure, generally dictates the type of trusses that will form the

framework of the roof and floor systems. The required

mechanical properties of the trusses are determined

by the loadings as prescribed by local building codes.

The truss manufacturer engineers the trusses based

on design values that are concerned primarily with

the lumber mechanical properties and the metal plate

connectors (Truss Plate Institute 1985).

The lack of quantifying data regarding environmental and loading effects presents a difficulty in truss

design. Design specifications for duration of loading

based on the Madison Curve are available from the

Truss Plate Institute located in Madison, WS. However, the effect of moisture cycling is compensated for

Leslie H. Groom is a research forest products technologist at the Alexandria Forestry Center, U.S. Department of Agriculture, Forest

Service, Southern Forest Experiment Station, Pineville, LA 71360.

1
by dividing the average load of (a) five moisture-controlled specimens (MC = 15 percent plus or minus 4

percent) by 3.0, or (b) five moisture-response specimens (MC = 15 percent assembly, then 7 percent, then

greater than 10 percent) by 2.5. An adequate understanding of environmental variables on MPC joint

behavior must be attained to knowledgeably assign

design values for truss manufacture.

These environmental variables clearly need to be

quantified for sensible design values to be assigned

for MPC joints. Thus, the primary objective of this

study was to quantify the effect of wood MC fluctuations on MPC joint behavior. Specifically, this study

compared the effect of severity and number of wood

moisture cycles with metal-plate backout, joint stiffness, critical slip, and strength. A secondary objective

was to investigate maintaining inter-facial integrity

between metal-plate teeth and wood by applying an

epoxy adhesive at the interface and noting the changes

in MPC joint mechanical behavior.

MATERLALS AND PROCEDURES

Experimental Design

A randomized block design, summarized in table 1,

was used to minimize the high variability customarily seen in lumber mechanical properties. Two separate blocks were constructed to evaluate the effect of

wood MC ranges and number of moisture cycles with

various physical and mechanical properties of MPC

joints. The two separate blocks were as follows: (1)

standard MPC joints and (2) MPC joints with an adhesive applied at the tooth-wood interface.

Each block consisted of two factors: the severity of

MC levels to which MPC joint specimens were subjected and the number of MC cycles. Ten replicates

per cell were subjected to varying MC conditions (i.e.,

[al MC held constant at 12 percent, [bl MC cycled between 9 and 15 percent, and [cl MC cycled between 5

and 19 percent and a varying number of MC cycles

from zero to eight-table 1). These replicates yielded

a total of 150 MPC joint specimens per block (3 MC

ranges times 5 MC cycles times 10 replicates = 150

specimens). Although the material used to construct

the replicates in the blocks containing standard MPC

joints and joints with an adhesive were from different

boards, the boards were chosen so as to have comparable specific gravity and stiffness values between

blocks, thus allowing for a relative comparison between blocks.

Lumber and Metal Plates

The MPC joints were constructed with No. 2 grade

KD15 southern pine lumber representing a narrow

2

Table l.- Summary of experimental design showing the number

of samples for each moisture cycle moisture range of no

adhesive vs. adhesive metal-plate connector (MPC) joints

for each number of moisture cycles

Number of moisture cycles

Moisture content

range 0 1 2 4 8

--- Percent --- _______________ Number of samples ---__-_________

No adhesive

1 2 1 0 1 0 1 0 1 0 1 0

9 to 15 1 0 1 0 1 0 1 0 1 0

5 to 19 1 0 1 0 1 0 1 0 1 0

Adhesive

1 2 1 0 1 0 1 0 1 0 1 0

9ta 15 1 0 1 0 1 0 1 0 1 0

5 to 19 1 0 1 0 1 0 1 0 1 0

range of specific gravities and moduli of elasticity. The

lumber used to construct the MPC joints was nominal

2- by 4-inch dimension and 1%ft length and was

sampled from a local lumber retailer located in central Louisiana. Modulus of elasticity (MOE) of each

board was estimated by applying a static center-point

load to each board oriented flatwise over a 16-ft span

and measuring centerpoint deflection. Specific gravity was based on volume at the time of the test, and

MC was determined on an approximately 1.5- by 1.5-

by 3.5~inch block of wood removed adjacent to the

metal plate after testing.

The die-punched metal plates used in this study

were standard truss-plate type plates made of 20-

gauge grade C sheet metal, 3 inches wide by 4 inches

long with an average tooth density of eight teeth per

square inch. Average tooth length of the metal plates

was 0.360 inch with teeth being pointed, slightly

crimped, and having no surface coating or visible

machining oil on the metal surface. The metal plates

were not washed to remove any residual machining

oil before joint assembly.

Joint Construction and Testing

Figure 1 shows a standard MPC joint used in this

study. In accordance with Truss Plate Institute (1985),

all teeth were removed within a lumber-end distance

of 0.5 inch. The teeth were removed with a milling bit

at the plate surface. Thirty-six pairs of teeth were

present in the upper member, and 24 pairs were

present in the lower member. This arrangement ensured failure in the lower member, which was equipped

with linear variable differential transformers (LVDT).

An epoxy resin with a cure time of 15 minutes was

applied with a sponge to the tips of the metal-plate

teeth immediately before pressing into the wooden

members preconditioned to an MC of 12 percent. An
average of 0.0031 g of epoxy was applied to each tooth

with the resin spread along the tooth length during

pressing. Metal plates were pressed into the wooden

members so as to make intimate contact, but not so

much that the plate surfaces became embedded in the

wood. The joints were placed in a hygrothermally controlled room at 79 “F and 66-percent relative humidity with an MC of approximately 12 percent after

assembly for about 4 weeks to allow for relaxation of

stresses induced by pressing. After conditioning, the

joints were moisture cycled by placement in one of

three hygrothermally controlled units set to cycle between the following MC ranges: 5 to 19 percent, 9 to

15 percent, and 12 percent (constant). All moisture

was atmospheric in nature, and at no time were the

joints exposed to direct surface water. Moisture content was monitored during cycling with a hand-held

surface moisture meter. All joints were reconditioned

to an MC of 12 percent for 4 weeks prior to testing.

Loading was applied in tension by a 30,000-lb capacity, screw-driven crosshead testing machine (fig. 1).

A constant displacement rate of 0.015 inch/min was

applied so as to produce failure in 5 to 10 minutes.

Figure l.- fipical metal-plate connectorjoint specimen before tensile testing.

The specimen was attached to the testing machine

with universal joints to prevent potential moments

induced by misalignment. The upper member slip was

restrained by the attachment of two C-clamps. Four

simultaneous LVDT readings were taken every second, located in pairs at the two opposite sides of the

wood-wood juncture (fig. 1). These LVDT values were

then averaged to provide overall slip between the

metal plate and lower wood member. Load was monitored with a lO,OOO-lb capacity load cell. A computercontrolled data acquisition system acquired the load

and slip readings. Load-slip traces were standardized

to an MC of 12 percent by adjusting mechanical properties by 5 percent per l-percent MC change (Bodig

and Jayne l982).

Metal-plate backout is defined in this study as the

dimensional difference between the wooden member

thickness and the wood-plus-plate thickness. This

method was chosen so as to minimize errors due to

wood dimensional changes related to stress relaxation

or MC. The wood-plus-plate thickness was measured

preceding and after MC conditioning; the dimension

was based on the average of two readings located on

each side of the long axis center line of the metal plate

and midway between the rows of teeth. Wood-member dimensions were recorded at the same time. Initial stiffness was calculated as the slope of the load-slip

curve between 700 and 1,100 lb.

RESULTS AND DISCUSSION

The conditioning history of the moisture-cycled

specimens is shown in figure 2. The moisture history

for the constant 12-percent MC specimens is not shown

due to the lack of moisture fluctuations. The compressor motor needed maintenance during the sixth and

seventh cycles in the mild-cycle conditioning chamber, whereas compressor problems were encountered

during the fifth cycle in the more severe cycle conditioning chamber. The duration of each moisture cycle

varied with the operation of the conditioning chambers and was coordinated to be of about equal length

for both conditioning chambers.

Some of the physical properties of the MPC joints

at the time of the test are shown in table 2. The MC

and specific gravity of standard MPC joints and those

joints with an adhesive interface were very similar. The

average moduli of elasticity from static, flatwise bending tests of the lumber comprising standard and adhesive MPC joints were 1.76 (lo? psi and 1.86 (lo? psi.

Plate Backout

Plate backout was significantly affected by the severity and number of moisture cycles as seen in figure 3.

3
!A

Table 2.-Average value by moisture content mnge and number of cycles for selected physical and mechanical properties of metal-plate connector (MPC) joints tested in this study

Moisture content

range

Moisture content

No. of

Cycles No adh. Adhesive

9to15 0

1

5 to 19

-

Percent

12 (constant) 0

1

-------- percent _-______ Percent tooth length _________ Lb _________ ____ Kips /inch* ---- __________ Lb _________

12.1 12.1 0.49 0.49 0.002 0.000 0.6 0.1

11.4 11.6 0.48 0.48 0.002 0.005 0.6 0.1

2 11.3 11.3 0.49 0.49 0.004 0.004 1.1 1.1

4 10.3 10.4 0.47 0.49 0.004 0.007 1.2 1.8

8 11.2 11.1 0.49 0.47 0.004 0.003 1.2 0.8

12.1 12.1 0.49 0.49 0.002 0.000 0.6 0.1

12.0 12.0 0.49 0.48 0.006 0.008 1.7 2.2

2 10.8 10.8 0.49 0.48 0.007 0.010 2.1 2.8

4 10.1 0.49 0.49 0.013 0.014 3.5 4.0

8 11.3 0.49 0.49 0.012 0.015 3.3 4.2

0 12.1

10.2

11.3

12.1

12.1

10.3

11.0

11.4

0.49 0.49 0.002 0.000 0.6 0.1

1 11.6 0.48 0.47 0.013 0.011 3.6 3.0

2 0.49 0.49 0.023 0.016 6.4 4.4

4

10.3

10.5

11.4

0.48 0.49 0.037

0.061

0.027 10.1

17.0

7.6

8 0.51 0.48 0.045 12.5

Load-atSpecific gravity Plate backout Plate backout Ultimate load StiEness 0.015~inch-slip

No adh. Adhesive No adh. Adhesive No adh. Adhesive No adh. Adhesive No adh. Adhesive No adh. Adhesive

4,080 4,431 729

(11.2)+ (6.4 (14.6)

3,907 4,367 677

(8.3) (7.4) (12.8)

3,778 4,201 613

(7.0) (6.1) (9.5)

3,598 4,095 565

6.9) (7.3) (17.2)

3,490 3,979 522

(9.4) (8.9) (14.2)

1,422

(18.5)

1,316

(13.2)

1,017

(15.2)

853

(20.9)

(1%

3,612 4,285

(9.9) (7.4)

3,513 4,261

(7.6) (8.8)

3,462 4,062

0.2) (7.6)

3,304 3,856

(6.8) (9.6)

3,185 3,758

(10.4) (9.2)

4,080 4,431

(11.2) (6.4)

3,821 4,035

(8.4) 6.6)

3,585 3,899

03.0) (5.6)

3,421 3,694

(8.7) 6.7)

3,312 3,596

(8.6) (8.1)

729

(14.6)

472

(14.4)

383

(10.7)

(:5:)

335

(14.1)

1,422 3,612 4,286

(18.5) (9.9) (7.4)

619 3,389 3,850

(8.4) (8.3) (7.3)

532 3,187 3,652

03.0) (7.4) (7.8)

470 3,020 3,428

(16.1) (9.7) (7.7)

471 2,878 3,375

(16.6) (7.7) (8.2)

4,080 4,431 729 1,422 3,612 4,285

(11.2) (6.4) (14.6) (18.5) (9.9) (7.4)

3,545 3,752 366 466 3,127 3,503

(11.8) (6.1) (13.7) (12.5) (10.1) (6.5)

3,199 3,438 271 358 2,725 3,174

(12.6) (9.9) (14.1) (11.1) (10.5) (9.4)

3,027 3,241 267 336 2,575 2,954

(18.8) (9.4) (20.2) (13.7) (14.3) (10.1)

2,812 2,980 249 295 2,457 2,678

(17.2) (12.8) (20.0) (14.2) (15.9) (11.6)

-

*Kips = kilopounds

+&efficient of variation in parenthesis
1 6

1 5

3

5 1 4

:!a2

e!z 1 3

!s8 1 2

w

i5 1 1

G

g 1 0

9

8

25

0

3 19.1 18.9 I 5 2

18.6 18.9

I

4

4.6 5.8 5.3 5.6 5.5 5.2 5.5 5.6

I I I 1

0 5 0 loo 150 200 250

TIME (days)

Figure 2.-Moisture history of metal-plate connector joints that were cycled between (a) 9- and 15.percent moisture content (MCI and (b)

5- and 19-percent MC.

5
Numerical values of plate backout are also shown in

table 2. The MPC joints held at a constant la-percent

MC showed plate backout values of approximately 1

percent of the total tooth length. The mild

moisture-cycled specimens seemed to stabilize at about

4 percent of total tooth length after eight moisture

cycles. In both the constant la-percent MC and mild

moisture-cycled specimens, addition of an adhesive

had no apparent effect on plate backout.

The more severe moisture-cycled standard and adhesive MPC joints backed out 17 percent and 12.5

percent of the total tooth length after eight moisture

cycles. Unlike the constant la-percent MC and mild

moisture-cycled specimens, the rate of plate backout

for the more severe moisture-cycled specimens showed

no signs of leveling off. Addition of an adhesive to the

tooth-wood interface retarded plate backout, but the

adhesive specimens did continue to back out after eight

moisture cycles. This suggests that plate backout continues for an extended period of time for MPC joints

subjected to wide variations in MC. A similar study,

but of an extended timeframe, may help to define the

upper limit of this variable.

Mechanical Behavior

Load-slip traces for standard MPC joints subjected

to a constant la-percent MC, mild moisture cycles,

and more severe moisture cycles are shown in figure

4, with the adhesive tooth-wood interface counterparts

summarized in figure 5. Average load-slip traces (measured in kips, or kilopounds) along with plus and minus one standard deviation for adhesive and standard

joints after zero, one, two, four, and eight moisture

cycles are shown in the appendix.

Figure 4 shows that successive moisture cycles do

appear to change the nature of the load-slip traces.

Metal-plate connector joints subjected to a constant

MC exhibited a slight degradative behavior with time.

This is to be expected to localize relaxation of the wood

surrounding the tooth. Although the MPC joints exhibited diminished load-slip traces with time, each

trace exhibited nonlinear behavior throughout the

entirety of the loading.

Moisture cycling not only accelerated the rheological degradation of the MPC joints but also changed

the shape of the load-slip trace (fig. 4b, 4~). Mild moisture cycling of the MPC joints reduced the amount of

nonlinearity in the initial portions of the load-slip

traces, a factor more evident with each subsequent

moisture cycle. This effect is even more apparent in

the more severe moisture-cycled MPC joints, with the

load-slip traces remaining almost linear up until approximately 50 percent of the ultimate load. This suggests that a different stress transfer mechanism is

being demonstrated. This altered mechanism is most

likely a result, either singularly or in concert, of less

surface area between the tooth and wood due to plate

backout, accelerated localized relaxation of the wood

surrounding the teeth, and/or an additional rotational

load being applied to the teeth due to eccentric loading of the backed-out plate (Chou 1987, Groom 1988).

This same phenomenon was also present in the MPC

joints with an adhesive tooth-wood interface (fig. 5).

The adhesive interface appeared to remain intact for

constant MC specimens. However, moisture cycling

degraded the adhesive interface rather rapidly, with

most of the degrade occurring in the first moisture

cycle.

The addition of an adhesive to the tooth-wood interface reduced the variation in load-slip traces of MPC

joints (appendix). Table 2 and figures Al, A3, A5, A7,

and A9 show that the variability of standard MPC

joints increased with the severity and number of moisture cycles. The tooth-wood interface of adhesive MPC

joints maintained more intimate contact during moisture cycling, resulting in mechanical performance variability that seems little affected by moisture cycling

(table 2; figs. A2, A4, A6, A8, and AlO). This suggests

that the most important attribute of the adhesive interface may not be the average increase in mechanical performance that degraded during subsequent

moisture cycling, but instead the reduced variability

associated with moisture cycling.

UZthute Load.-The effect of moisture cycling on

ultimate load is summarized in figure 6 and table 2.

The MPC joints with an adhesive interface had ultimate-load values about 9 percent higher than the standard joints. This difference may be partially due to

the different lumber samples used for the adhesive

and standard joints. Although the specific-gravity values of the two samples were identical, the MOE of the

adhesive joints was about 6 percent higher than that

of the standard joints.

The decrease in ultimate load for subsequent moisture cycles was consistent in both adhesive and standard MPC joints (table 3). The percentage decrease in

ultimate load after eight moisture cycles for constant

la-percent MC, mild moisture-cycled, and more severe

moisture-cycled specimens was approximately 10,20,

and 30 percent, respectively. The rate of decrease also

seemed to decline for all specimens with subsequent

moisture cycles. However, the number of moisture

cycles in this study was inadequate for determining a

lower limit for each moisture range.

Figure 7 shows that an inverse relationship existed

between plate backout and ultimate load. Kline (1993)

reported a similar relationship for MPC joints not

subjected to moisture cycling. This study showed that

the ultimate load of standard MPC joint specimens

held at a constant la-percent MC decreased at a much

faster rate than did the severely cycled specimens for

a given degree of plate backout, with the mild mois6
MC = moisture content (4

l

ìM%c - 15%

= 12% MC

0

I I I I I I I

0 1 2 3 4 5 6 7 8 9

MC = moisture content 04

l 9% - 15%

M C

n 12%MC

I I , I

0 1 2 3 4 5 6 I 8 9

NUMBER OF MOISTURE CYCLES

Figure 3.-Effect of number and severity of moisture cycles on plate backout of metal-plate connector joints (10 replicates per data point)

constructed (a) without an adhesive interface and (b) with an epoxy adhesive interface.

7
r (4

09

0.03 II.04

I

(c)

cm

DISPLACEMENT (inch)

Figure 4.-Load-slip traces in kips (kilopounds) showing the effect of sequential moisture cycling for standard metal-plate connector joint

specimens (10 replicates per moisture-content cycle) subjected to (a) constant 12-percent moisture content (MC), (b) moisture

cycling between 9- and lb-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

8

‘“,, :,_ ,,a I,>/
Table 3.- Percentage decrease, based on zero moisture cycles, by moisture content range and number of cycles of

selected mechanical properties

Moisture content No. of

range cycles

Ultimate load

No adh. Adhesive

Stiffness Load-at-O.OlB-

(Kips* /inch) inch-slip

No adh. Adhesive No adh. Adhesive

. . ..~...............____________________......... percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12 (constant) 0 0.0 0.0 0.0 0.0 0.0 0.0

1 4.2 1.5 7.2 7.5 2.7 0.6

2 7.4 5.2 15.9 28.5 4.2 5.2

4 11.8 7.6 22.5 40.0 8.5 10.0

8 14.5 10.2 28.4 51.7 11.8 12.0

9to 15 0 0.0 0.0 0.0 0.0 0.0 0.0

1 6.4 8.9 35.2 56.4 6.2 10.1

2 12.1 12.0 47.4 62.6 11.8 14.8

4 16.2 16.6 50.1 66.9 16.4 20.0

8 18.8 18.8 54.0 66.9 20.3 21.2

6 to 19 0 0.0 0.0 0.0 0.0 0.0 0.0

1 13.1 16.3 49.8 67.2 13.4 18.3

2 21.6 22.4 62.8 74.8 24.6 25.9

4 25.8 26.8 63.4 76.4 28.7 31.1

8 31.1 32.8 65.9 79.3 32.0 37.5

*Kips = kilopounds.

ture-cycled specimens falling between the two moisture ranges (fig. 7a). Because MPC joints in service

are subjected to varying moisture cycles as well as

in-service loads, it is difficult to develop a simple predictive equation for plate backout and ultimate load

due to other compounding factors.

Initial Stifie88.-The initial stiffness of MPC

joints is the mechanical property most affected by

moisture hysteresis, with results shown in figure 8

and tables 2 and 3. Metal-plate connector joints kept

at a constant la-percent MC showed a steady reduction in initial stiffness for the first several months,

with a reduced rate of degradation in the following

months. These constant la-percent MC specimens

showed about a 30-percent reduction in initial stiffness over the approximate g-month timeframe, with

a lower limit appearing to be about 35 percent of the

original initial stiffness.

Moisture cycling significantly increased the degradation of MPC joint initial stiffness, especially with

regard to the first moisture cycle (fig. 8a). Mild

moisture-cycled specimens showed a drop in initial

stiffness of 35 percent after the first moisture cycle.

Subsequent moisture cycling resulted in a 54-percent

reduction in initial stiffness, with a lower limit appearing to be about 60 percent of the original initial

stiffness. More severe moisture cycling resulted in a

50-percent reduction in initial stiffness after only one

moisture cycle. Subsequent moisture cycles had a minimal effect, with a lower limit of approximately 70 percent of the original initial stiffness.

Metal-plate connector joints with an adhesive interface showed a heightened response of initial stiffness to moisture cycling (fig. 8b). In the constant

la-percent MC specimens and the mild and more severe moisture-cycled specimens, the initial stiffness

of the adhesive interface MPC joints approached that

of the standard MPC joints with each successive moisture cycle. The difference between the adhesive interface and standard MPC joints diminished with the

severity of the moisture cycling. Although the initial

stiffness of the adhesive interface MPCjoints diminished more rapidly than that of standard MPC joints,

the adhesive interface joints were still stiffer than

standard joints in all cases.

Load-at-0.016-Inch-SZip.-Load-at-O.OlB-i,nchslip was recorded instead of the traditional load-at0.030~inch-slip as the top member of the joint was

restrained and only the load-slip trace of the bottom

member was recorded (fig. 1). Load-at-0.015~inch-slip

followed a trend similar to the ultimate load of the

MPC joints, with strength reductions of about 10 percent, 20 percent, and 30 percent for constant la-percent MC specimens, mild-cycled specimens, and more

severe cycled specimens, respectively (fig. 9). Adhesive interface specimens also showed an increase in

load-at-0.015-inch-slip for all moisture-cycled specimens, along with a similar pattern of load-at-O.Olb

inch-slip degradation. Metal-plate connector joints

with an adhesive interface did show an accelerated

degrade in load-at-0.015~inch-slip, with successive

moisture cycles when compared to standard MPC

joints, especially with regard to the first moisture

cycle. This is most likely due to weakening of the adhesive interface caused by dimensional changes in the

wood surrounding the teeth of the metal plate.

9
Figure 5.--Load-slip traces in kips (kilopounds) showing the effect of sequential moisture cycling for adhesive interface metal-plate connector joint specimens (10 replicates per moisture-content cycle) subjected to (a) constant 12-percent moisture content (MC), (b)

moisture cycling between 9- and 15-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

1 0
r4,500

(4

m 12% MC

+ 9%-15%

M C

5%-19%

M C

MC = Moisture content

I 1 I I

2,500 I I I I

0 1 2 3 4 5 6 7 8 9

(‘3

MC = Moisture content

1 , I

, f I

I

2,500 0 1I 2 3 4 5 6 I 8 9

NUMBER OF MOISTURE CYCLES

Figure 6.-Effect of number and severity of moisture cycles on ultimate load of metal-plate connector joints (10 replicates per data point)

constructed (a) without an adhesive interface and (b) with an epoxy adhesive interface.

1 1
loo

Initial Stiffness

9 0 MC = Moisture content

III = 12 percent MC

0

% 80 =9to15prcentMC

!z

A =5to19percentMC

d 7 0

8 60

5!

n

8 S O

ii 40

$2

3 0

2 0

0 5 10 15 2 0

PLATE BACKOUT (percent tooth length)

Figure 7.-Effect of plate backout on the normalized ultimate load and normalized initial stiffness of m&al-plate connector joints (10

replicates per data point) constructed (a) without an adhesive interface and (b) with an epoxy adhesive intert?ux.

12
L

MC = Moisture content

1,400 -

800 -

n lZ%MC

*xi8 - 15%

A - 19% 200 I I I I I I I 1p$& .

0 1 2 3 4 5 6 7 8 9

MC = Moisture content 04

800

600

12% MC

200 L

I I I I , I I I I

0 1 2 3 4 5 6 7 8 9

NUMBER OF MOISTURE CYCLES

Figure 8.- Efict of number and severity of moisture cycles on initial stiffkssof metal-plate connector joints (10 replicates per data point)

constructed (a) without an adhesive interface and (b) with an epoxy adhesive interfcrce.

1 3
2

9 3,000

(a)

-m 12%MC

+

iti% - 15%

2,500 -

M C

MC = Moisture content

2,000 I I 1 I I I I I -

0 1 2 3 4 5 6 7 8 9

4,500 r

04

m 12%MC

g

3,500

MC

2

2 3,000

MC I

2,500

MC = Moisture content

2,000 -1

I I J

0 1 2

NUMBER OF MOISTURE CYCLES

Figure 9.-Effect of number and severity of moisture cycles on load-at-0.015~inch-slip of metal-plate connector joints (10 replicatesper data

point) constructed (a) without an adhesive interface and (b) with an epoxy adhesive interface.

1 4

_, I
CONCLUSIONS LITERATURE CITED

Moisture cycling significantly affects the mechanical behavior of MPC joints. Some of the decline in

mechanical behavior could be attributable to plate

backout, which was as high as 17 percent of the total

tooth length for more severe moisture-cycled specimens. Specimens subjected to a constant la-percent

MC over the timeframe exhibited curvilinear load-slip

traces. However, more severe moisture cycling resulted

in an almost linear load-slip trace up to about 50 percent of the ultimate load, followed by a curvilinear

response to failure. Thus, it appears that there is a

change in the stress transfer mechanism for moisturecycled specimens. Although the average load-slip

traces of standard MPC joints are degraded by both

severity and number of moisture cycles, the real gain

in design values by addition of an adhesive interface

between the tooth and wood may be in variability of

these traces, which is much less affected by moisture

cycling.

Initial stiffness of the load-slip traces was most drastically affected by moisture cycling, with reductions

of approximately 30 percent, 55 percent, and 65 percent for constant la-percent MC, mild moisture-cycled,

and more severe moisture-cycled specimens, respectively. Ultimate load was less severely affected, with

constant la-percent MC, mild moisture-cycled, and

more severe moisture-cycled specimens reduced approximately 15 percent, 20 percent, and 30 percent,

respectively. However, a study of longer duration is

necessary to determine the lower limit for each moisture range. Load-at-0.015-inch-slip showed reductions

similar to ultimate load.

It appears that addition of an adhesive interface

between the teeth and wood increases the mechanical

properties of MPC joints. However, the apparent increase in mechanical properties could not be quantified due to the lack of identically matched specimens.

The degrade of ultimate load and load-at-0.015-inchslip of adhesive interface MPC joints was similar to

that of standard MPC joints. Initial stiffness of adhesive interface MPC joints degraded more rapidly than

standard MPC joints, probably due to weakening of the

adhesion between the teeth and wood. Increased adhesion between the metal plates and wood, either through

plate cleaning or improved adhesive system, would most

likely further enhance structural performance.

It should be noted that in-service MPC joints most

commonly are subjected to MC somewhere between

the la-percent constant and mildly cycled specimens.

Caution should be exercised when applying the results

of this study to different metal-plate types and to

in-service structures because of factors that have been

ignored in this study, such as surface moisture and

in-service loading of the truss structures.

Bodig, J.; Jayne, B.A. 1982. Mechanics of wood and

wood composites. New York Van Nostrand Reinhold

Company, Inc. 712 p.

Chou, C. 1987. Modeling of nonlinear stiffness and

nonviscous damping in nailed joints between wood

and plywood. Corvallis, OR: Oregon State University. 181 p. Ph.D. dissertation.

Cleary, P 1984. Humidity in attics-sources and control methods. Lawrence Berkeley Lab. Rep. LVL17590, July 1984. Berkeley, CA: University of

California. 15 p.

Crovella, P.L.; Gebremedhin, K.G. 1990. Analyses of

light frame wood truss tension joint stiffness. Forest Products Journal. 40(4): 41-47.

Cunningham, M.J. 1983. A new analytical approach

to the long term behaviour of moisture concentrations in building cavities. 2: Condensing cavity.

Building and Environment. 18(3): 117-124.

Cunningham, M.J. 1984. Further analytical studies

of building cavity moisture concentrations. Building and Environment. 19( 1): 21-29.

Duff, J.E. 1978. Moisture content of a joist floor over

a crawl space. Res. Pap. SE-189. Asheville, NC: U.S.

Department of Agriculture, Forest Service, Southeastern Forest Experiment Station. 12 p.

Foschi, R.O. 1977. Analysis of wood diaphragms and

trusses. 2: Truss-plate connections. Canadian Journal of Civil Engineering. 4: 353-362.

Galligan, W.L. 1990. The impact of raw material on

lumber quality. In: Biographies and abstracts of the

44th annual meeting of the Forest Products Research Society; 1990 June 24-27; Salt Lake City,

UT. Madison, WI: Forest Products Research Society: 26.

Groom, L.H. 1988. Experimental verification and nonlinear modeling of truss-plate joints by Runge-Kutta

numerical technique. Corvallis, OR: Oregon State

University. 202 p. Ph.D. dissertation.

Keenan, F. J.; Suddarth, S.K.; Nelson, S.A. 1983. Wood

trusses and other manufactured structural components. In: Proceedings of the workshop on structural

wood research; 1983 October 5-6; Milwaukee, WI.

New York: American Society of Civil Engineering:

175-184.

Kline, G.J. 1993. Mechanical connections: the weak

link in light-frame wood construction. In: Groom,

L.H., ed. Current and future applications of mechanical fasteners for light-frame wood structures:

Proceedings of mechanical fasteners plenary session

at the Forest Products Research Society annual

meeting; 1991 June 23-26; New Orleans, LA. Gen.

Tech. Rep. SO-92. New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station: 13-17.

15
Kunesh, R.H.; Johnson, J.W. 1968. Strength of multiple bolt joints: influence of spacing and other variables. Rep. T-24. Corvallis, OR: Forest Research

Laboratory, Oregon State University. 19 p.

Mayo, A.P. 1980. Long-term performance tests on

trussed rafters. Rep. CPlk30. Aylesburg, UK: Building Research Establishment, Princes Risborough

Laboratory. 10 p.

McAlister, R.H. 1990. Tensile loading characteristics

of truss plate joints after weathering and accelerated aging. Forest Products Journal. 40(2): 9-15.

Noren, B. 1968. Nailed joints-their strength and rigidity under short-term and long-term loading. Rep.

158B. Stockholm: The National Swedish Institute

for Building Research. 80 p.

Reardon, G.F. 1971. A structural analysis of frames

with semi-rigid joints. Tech. Pap. 69. Melbourne,

Australiaz Division of Forest Products, Commonwealth

Scientific and Industrial Research Organization.

Sherwood, G.E. 1986. Condensation potential in high

thermal performance walls-hot, humid summer

climate. Res. Pap. FPL-445. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest

Products Laboratory, 29 p.

‘I&he, M.H.; Suddarth, SK 1988. Advanced design

of metal plate connector joints. Forest Products Journal. 38(g): 7-12.

Truss Plate Institute. 1985. Design specification for

metal plate connected wood trusses. TPI-85. Madison, WI: Truss Plate Institute. 47 p.

Wilkinson, T.L. 1971. Theoretical lateral resistance of

nailed joints. Journal of the Structural Division,

American Society of Civil Engineers. 97(STS): 1407-

1422.

Wilkinson, T.L. 1984. Longtime performance of trussed

rafters with different connection systems. Res. Pap.

FPL-204. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Product Laboratory. 19 p.

16

,,,I,. I
APPENDIX
5

---- -- -- I

l --- --- -

,

0.01 0.02 0.03 0.04

DISPLACEMENT (inch)

Figure Al.-Average load-slip traces (solid lines), includingplus and minus one standard deviation (broken lines), for standard metal-plate

connector joints subjected to zero moisture cycles and (a) constant 12-percent moisture content (MC), (b) moisture cycling

between 9- and 15-percent MC, and Cc) moisture cycling between 5- and I9-percent MC.

1 8
0 0.01 0.02 0.03 0.04

_---- -

I---- w

--- - - -

00 0.01 0.02 0.03 0.04

DISPLACEMENT (inch)

0.03 0.04

Figure AZ- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for adhesive interface

metal-plate connector joints subjected to zero moisture cycles and (a) constant 12.percent moisture content (MC), (b) moisture

cycling between 9- and 15-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

1 9
(4

_----

-

------

0.01 0.02 0.03 0.04

03

- - ---

----

- -

--- - - - -

0.02

DISPLACEMENT (inch)

Figure A3.- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for standard metal-plate

connector joints subjected to one moisture cycle and (a) constant l&percent moisture content (MC), (b) moisture cycling between

9- and 15-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

20
5

4

?7

g 3

2

9 2

1

0

5

4

72

.n 3

&

2

9 2

1

0

/.--- -- -- (4

_--------- -_

0 0.01 0.02

L

0.01 0.02 0.03 0.04

0 0.01 0.02

DISPLACEMENT (inch)

Figure A4.- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for adhesive interface

metal-plate connector joints subjected to one moisture cycle and (a) constant 12-percent moisture cohtent (MC), (b) moisture

cycling between 9- and 15-percent MC, and (c) moisture cycling between 5 - and Is-percent MC.

,
(4

0.02 0.03 0.04

/----_---

---

---------

I

0

1

03

0 0.01 0.02

DISPLACEMENT (inch)

0.03

Figure AS.- Average load-slip traces (solid hes), includingplus and minus one standard deviation (broken lines), for standard metal-plate

connector joints subjected to two moisture cycles and (a) constant 12-percent moisture content (MC), (b) moisture cycling between 9- and 15-percent MC, and (c) moisture cycling between 5- and Is-percent MC.

22
5

_---- - (4

4 - --- - -

0.02 0.03 0.04

(b)

0 0.02 0.03 0.04

5

03

DISPLACEMENT (inch)

Figure A6.- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for adhesive interface

metal-plate connector joints subjected to two moisture cycles and (a) constant 12-percent moisture content (MC), (b) moisture

cycling between 9- and Z-percent MC, and (c) moisture cycling between 5 and IO-percent MC.
0.02 0.03 0.04

04

_---

_---

----

- --

-----

--

0.02

DISPLACEMENT (inch)

0.03 0.04

Figure AT.- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for standard metal-plate I

connector joints subjected to four moisture cycles and (a) constant 12-percent moisture content (MC!, (b) moisture cycling I

between 9- and 15-percent MC, and (c) moisture cycling between 5- and 19-percent MC. I

24
5

(4

-

---

- - - -

4 -

_--- -

- c

0 0.01 0.02 0.03 0.04

5

4

1

0

0 0.01 0.02 0.03 0.04

0 0.01 0.02

DISPLACEMENT (inch)

0.03 0.04

Figure A&- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for adhesive interface

metal-plate connector joints subjected to four moisture cycles and (a) constant 12-percent moisture content (MC), (a) moisture

cycling between 9- and IS-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

25
5

4 -

(4

- - ---

/-----

- - ---

0 0.01 0.02 0.03 0.04

(‘4

_---

------

--

/.---

--------

0 0.01 0.02 0.03 0.04

0.02

DISPLACEMENT (inch)

Figure A9.- Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for standard metal-plate

connector joints subjected to eight moisture cycles and (a) constant la-percent moisture content (MC), (b) moisture cycling

between 9- and 15-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

26
(4

- - - - - 4 - /----

/

---

---

0 0.01 0.02 0.03 0.04

0 0.01 0.02 0.03 0.04

5

w

4 -

3

. a 3

_--- -- --

-

25 _--- ---

/---

0.02 0.03

DISPLACEMENT (inch)

O.d4

Figure AlO.-Average load-slip traces (solid lines), including plus and minus one standard deviation (broken lines), for adhesive interface

metal-plate connector joints subjected to eight moisture cycles and (a) constant 12-percent moisture content (MC), (b) moisture

cycling between 9- and 15-percent MC, and (c) moisture cycling between 5- and 19-percent MC.

27
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