THE INFLUENCE OF MGEWRE CGNFENT
HESI‘QRV 0N TE-{E QEFLECWCN EEHAV’EOR
AND ULTEMATE LOAD LEVEL OF
INDIVIDUAL WOGD YENSELE IOENFS
FASTENED WITH METAL PLATES

Thesis Em- i'ito Dogma eff M. 5.
MICHHGAN STATE UNIVERSITY

Donald; H. Baumgutuer
1967

 
   

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THE INFLUENCE OF MOISTURE CONTENT HISTORY
ON THE DEFLECTION BEHAVIOR AND
ULTIMATE LOAD LEVEL OF INDIVIDUAL WOOD TENSILE JOINTS

FASTENED WITH METAL PLATES

BY

DONALD H. BAUMGARTNER

a thesis
Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of
MASTER OF SCIENCE

Forest Products Department

ACKNOWLEDGMENTS

Appreciation is due Dr. Alan Sliker and Professor Byron Radcliffe
for suggesting the thesis tOpic and for their valuable advice concerning
the research and formulation of this thesis. I am also indebted to all
the faculty members of the Forest Products Department and to my fellow

graduate students for their encouragement and assistance.

In addition, I wish to thank my wife, Janet, for her tireless assis-

tance and patient understanding.

ii.

ACKNOWLEDGEMENTS . . .
LIST OF TABLES . .
LIST OF ILLUSTRATIONS .

INTRODUCTION . . . . . .

REVIEW OF LITERATURE . .

PURPOSE . . . . . . . .

TABLE OF

CONTENTS

DESCRIPTION OF MATERIALS, TEST APPARATUS, AND TEST

General . . . . . .

Geometry of Test Joints .

Selection of Lumber

Moisture Conditioning

Moisture Content Determination .

Member Specific Gravity

Joint Fasteners

Specimen Coding

Fabrication

Test Apparatus
Test Procedure ,

TEST RESULTS

Test Data

Cumulative Joint Gap
Incremental Load

Deflection Versus Time and

Levels

iii.

Page

ii

0\

\o \o -q —a —a

12
15
15
18
18
21
29
32

32

35

Magnitude of Deflection at "Creep Limit" Versus Load . . . .

Load at Failure . . . . . . . . . .
ANALYSIS AND DISCUSSION OF DATA . . .

General . . . . . . . . . . . . . .

Deflection Versus Moisture Content of Member Lumber . . . .

Deflection Versus Moisture Content History of Member Lumber

Statistical Significance of Deflection Versus Moisture

Content History of.Member Lumber .

Ultimate Load Versus Moisture Content of Member Lumber . . .

Ultimate Load Versus Moisture Content History of

Lumber O I O O O O O O O O 0 O 0
Type of Joint Failure . . . . . . .

Statistical Significance of Ultimate
Content History of.Member Lumber
CONCLUSIONS AND RECOMMENDATIONS . . . .

APPENDIX I - - Notation . . . . . . . .

APPENDIX II - - Table I . . . . . . . .

LITERATURE CITED . . . . . . . . . . . .

iv.

Load Versus

Member

Moisture

A6
A6
52
52
53
55

60
63

6h
6h

69
72

76
77

87

LIST OF TABLES

Kiln Schedule for Drying Wood Members . . . . .
Summary of Test Data . . . . . . . . . . .
Description of Plate Fasteners . . . . . . .

Summary of Creep Limit Deflection at the 1200, 1800,
and 2h00 Pound Load Levels . . . . . . . . .

Summary of Load at Failure and.Type of Failure for
Test JOintS O O O O O O O O O O O O O 0

Correlation and Regression of Deflection on Moisture
Content History . . . . . . . . . . . .

Deflection versus Moisture Content History . . .
Comparison of Moisture Content History . . . . .

Correlation and Regression of Ultimate Load on
Moisture Content History . . . . . . . . .

Ultimate Load versus Moisture Content History . .

Page

11
77
17

50

51

58
61
62

66
71

10.

ll.

l2.

13.

1h.

15.
l6.
l7.

18.

LIST OF ILLUSTRATIONS

Joint Test Specimen . . . . . . . . . . .

Location of Moisture Content Sample in One Member
of Test Specimen at Time of Fabrication . . . .

Moisture Content Sample from One of Two Symmetrical
Members of the Test Specimen Shown in Dotted Cross

Section and Labeled . . . . . . . . . . .
Plate Types A and B . . . . . . . . . . .
Clamping of Joints in Jig Prior to Pressing . . .

Pilot Hole Bit (A) and Shear Plate Boring Tool (B)

Wood Members with Pilot Hole and Shear Plate Hole
Bored O O O O O O O O O O O O O O O

Chord'with Shear Plates in Position and Steel Grips
Being Slid into Position (Note the Eye—Bolt and

Universal-Joint) . . . . . . . . . . . .

Bolts Being Passed Through Steel Grips and Shear
Plates and Fastened by Means of a Nut . . . . .

Universal-Joint Fastened to Reaction Which is
Bolted to Floor . . . . . . . . . . . .

Strain Gage Attached to Steel Rod and Wrapped in
Protective Covering . . . . . . . . . . .

Strain Indicator . . . . . . . . . . . .

Hydraulic Cylinder Fastened to Reaction Showing Yoke

Used to Apply Force . . . . . . . . . . .

Dial Gages and Jigs Used to Read Deflection at
JOj-nt Gaps O O O O O O O O O O O O O O

Positioning of Dial Gages on the Test Specimen . .
Complete Test Set-Up . . . . . . . . . .
Testing of Joint Specimen . . . . . . . . .

Application of Load with Control of Time . . . .

vi.

Page

13

1h
16
2O

22

22

23

23

2%

2h

25

27

27
28
28
31

31

Figure Page

19. Type of Joint Failure . . . . . . . . . . . . 3h
20. Deflection versus Continuous Time from Start of Test

for JOint A-MCl-g o o o o o o o o o o o o o 36
21. Deflection versus Continuous Time from Start of Test

fOr JOint A-MC2-9 o o o o o o o o o o o o o 37
22. Deflection versus Continuous Time from Start of Test

for JOint A—MC3-9 o o o o o o o o o o o o o 38
23. Deflection versus Continuous Time from Start of Test

for JOint A-AMCh-lo o o o o o o o o o o o o 39
2h. Deflection versus Continuous Time from Start of Test

for JOint A-AMC5-9 o o o o o o o o o o o o 0 1+0
25. Deflection versus Continuous Time from Start of Test

for JOint B-MCl-9 o o o o o o o o o o o o o ’41
26. Deflection versus Continuous Time from Start of Test

for JOint B-MC2-9 o o o o o o o o o o o o 0 I42
27. Deflection versus Continuous Time from Start of Test

for JOint B-MC3-lo o o o o o o o o o o o o 0 1+3
28. Deflection versus Continuous Time from Start of Test

for JOint B-AMCL9 o o o o o o o o o o o o 0 1+1}
29. Deflection versus Continuous Time from Start of Test

for JOint B-AMCS-g o o o o o o o o o o o o o ’45
30. Creep Limit Deflection versus Load for Test Joint

series B-MC2 o o o o o o o o o o o o o o o ’47
31. Creep Limit Deflection versus Load for Plate Type "A"

Subjected to Various Moisture Content Histories . . . A8
32. Creep Limit Deflection versus Load for Plate Type "B"

Subjected to Various Moisture Content Histories . . . A9
33. Joint Gap Deflection versus Moisture Content with

Fabrication and Test Moisture Content Levels Equal for

Plate Type "A" at the 1800 Lb. Load Level . . . . . 57
3h. Joint Gap Deflection versus Moisture Content with

Fabrication and Test Moisture Content Levels Equal for

Plate Type "B" at the 1800 Lb. Load Level . . . . . 57

vii.

Figure Page

35. Joint Gap Deflection versus Change in Moisture
Content to Base Moisture Content of 7% -- Fabrication
and Test Moisture Content Levels Not Equal -- for
Plate Type "A" at the 1800 Lb. Load Level . . . . . . 59

36. Joint Gap Deflection versus Change in Moisture
Content to Base Moisture Content of 7% -- Fabrication
and Test Moisture Content Levels Not Equal -- for
Plate Type "B" at the 1800 Lb. Load Level . . . . . . 59

37. Ultimate Load versus Moisture Content with
Fabrication and Test Moisture Content Levels Equal
for Plate Type "A" . . . . . . . . . . . . . . 65

38. Ultimate Load versus Moisture Content with
Fabrication and Test Moisture Content Levels Equal
for Plate Type "B" . . . . . . . . . . . . . . 65

39. Ultimate Load versus Change in Moisture Content to
Base Moisture Content of 7% -- Fabrication and.Test
Moisture Content Levels Not Equal -- for Plate Type "A" . 67

ho. Ultimate Load versus Change in Moisture Content to

Base Moisture Content of 7% -- Fabrication and.Test
Moisture Content Levels Not Equal —- For Plate Type "B" . 67

viii.

INTRODUCTION

 

Since their introduction in the l9h0's, lightweight wood trusses
have become an important and integral part of residential and other light

frame construction. Some of their advantages are the following:

1. The exterior walls and roof of a structure can be erected,

providing a sheltered, unobstructed work area.

2. The interior ceiling and walls can be erected and finished in

large sections for labor and material economy.

3. The interior layout of a structure is completely flexible

without the limiting aspect of bearing walls.

A. Wood trusses require less material and are more economical to

set in place than conventional framing.

Split rings were the first joint connectors for the trusses. Sub-
sequently, glued-plywood splice plates were developed and, more recently,
metal plates attached by nails or by teeth stamped from the plate surface

have been designed.

One of the reasons for the popularity of metal plates has come about
because of their adaptability to mass production techniques in truss manu—
facture. At present, production methods have neared peak efficiency and,
because of steep competition, manufacturers have striven toward better use of
materials and.improved designs for their market edge. The use of metal plates

has also introduced some unique problems, of which an important one is

creep. The creep found in a truss lies almost solely within the truss
joint connector. Previously, with the use of glued plywood connectors,
creep was negligible, but with the introduction of metal plate connectors,
the limiting of creep became important in order to eliminate sagging roofs,

shifting structures and cracking plaster.

Due to the complexity, variability and plastic-elastic nature of wood,
designers have had to make many assumptions as to the effect of moisture
content, moisture content history, long term loading and the modulus of
elasticity of the wood members on the ultimate truss design specifications.
Many such assumptions have not been clearly validated in preliminary stur
dies so that future research in these areas must be conducted to preserve
the integrity in design of wood trusses. Sizes for chord members, sizes
for splice plates, tooth geometry, number of teeth per unit area of plate,
gage of steel for plates and moisture contents suitable for lumber at time
of fabrication can only be arrived at if true engineering analyses for wood

joints are made through research.

REVIEW OF LITERATURE

Most of the research in the last 20 years on the behavior of light
wood trusses has been done in the area of full scale truss testing. This
work has been mainly associated with ascertaining the performance of
trusses related to acceptance criteria of building codes and quality control
specifications. A great percentage of this research with regard to metal
plates has been conducted by the truss plate manufacturers and the techni-
cal results have frequently not been published. In many cases, the results
do not prove valuable to others concerned with truss manufacture because of
the uniqueness of the gusset plate fastener, lack of moisture content con-
trol during testing and fabrication, varying test procedures, poorly designed
test programs and lack of ideal facilities. Published research with de-
tailed analyses have notably come from universities, the U. S. Forest Pro-
ducts Laboratory, and other independent and association research institutes.

Specifications have been written for light metal plate trusses (2).*

Many individual investigations have been made in the area of full
scale truss testing with respect to load-deflection and ultimate strength
characteristics (7, 2, lg, 12,.11). Usually this work was conducted with
little regard to moisture content history and chord member stiffness, ET,
(2, lg, 12, 11). In most cases, this research involved a comparison of

the performance of various fastening systems (1, l_5_, _l_7_, L8).

 

* Underlined numbers in parentheses refer to literature cited at the
end of this thesis.

h.

The effect of relative humidity variation upon the strength character-
istics of light wood trusses has been researched (g, 12, ;§,,2,.gg). Luxford
(g) showed a loss in stiffness and ultimate strength of glued plywood gusset
plates with cyclic high and low relative humidity. Nailed plywood gusset
plates were less affected. Stern and Stoneburner (18) found a loss in per—
formance of nailed, burrlock, bolted, and ring connectored trussed rafters
when fabricated green and dried for testing. Radcliffe and Sliker (3g)
found similar results for nail-on metal plates and stamped metal plate fas-
teners, but found that nailed-glued plywood gusset plates were little af—
fected by moisture content change prior to testing. Kawal (2) found little
effect of moisture content level or moisture content history on the deflec-
tion characteristics of nailedemetal plates, punchedrtooth metal plates,
and stamped metal plates. Wilkenson (29) showed from 1 to 3% times more
deflection and 0 to 30 percent loss in maximum load for moisture-cycled
joints over control specimens for nailedsplywood joints; phenol-resorcinal
and casein glued joints; and nailed, barbed and toothed metal-plate joints

tested in tension and bending.

Other than the work done by Wilkenson (29) on moisture content cycling
of individual truss joints, little has been investigated as to the effect
of duration of load, cyclic loading, moisture content level, moisture
content history and member ET on the performance of individual truss joints.
Previous work on isolated truss joints was concerned mainly with comparing
truss plate types to ultimate strength and load—deflection with little con-

cern for other variables (E,_16,.l8).

In the area of truss design and analysis, work has been done by
empirical and theoretical approaches as well as a combination of both.
Trusses made with metal plate fasteners usually are classified as being
semi-rigid in nature, which means they are intermediate in deflection
characteristics between a truss with pin connected joints and one with
rigid joints. Electrical resistance strain gages have been used to gain
information empirically on the moments and forces present in truss mem-
bers and joints to be used in the analysis of connectors (3,;pl, Struc-
tural analyses of trusses with rigid and semi-rigid joints have been made

by slope—deflection and energy methods (19, 3).

PURPOSE

The purpose of this investigation was to determine the effects
of moisture content and moisture content history on the deflection

characteristics of metal plate fastened wood joints subjected to tension.

The effect of moisture content was studied under the following

classifications:

1. Influence of a base moisture content (the joints were tested

at the same moisture content level as that of fabrication).

2. Influence of a change in moisture content (the joints were
tested after being dried from a higher moisture content of

fabrication).

Two commercial metal plate types were selected for comparison at
the various moisture content history levels. One plate type was of the
stampedptooth variety, the other was of the punchedrtooth variety. Al-
though the plates were of different size and tooth geometry, both were
designed for use in the lower chord tension joint of a 26' span W-type

truss.

DESCRIPTION OF MATERIALS,L

 

TEST APPARATUS

 

AND TEST METHOD

 

GENERAL

A total of one hundred and eighty individual tension joints were fab—
ricated and tested in this research. The variables included the type of
metal connector plate, moisture content level at manufacture, and the mois-
ture content at the time of test. There were eighteen replications for a
given combination of test variables. Two types of metal plate connectors
were used: A) plates with stamped triangular teeth, and B) plates with
punched rectangular prongs. Lumber used in the joints was nominal 2" x h"
western hemlock. The lumber was carefully conditioned before and after
fabrication of the truss joints to predetermined levels of moisture content
in a standard dry-kiln. Each plate type was subjected to two series of
moisture content conditions. Series I truss joints were fabricated and
tested at the same moisture content at three moisture content levels.
Series II truss joints were manufactured at two moisture content levels and

dried to a lower base moisture content for testing.

GEOMETRY OF TEST JOINTS

 

The test joint was typical of a lower chord tension joint of a W-type
truss. The tension joint was of the same geometry for both types of con-
nector plates. Figure 1 shows the overall specimen length as well as the
area allowed for the connector plate, the test grip equipment and a buffer

zone between the two.

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SELECTION OF LUMBER

 

All wood members were of western hemlock (Tsuga heterophylla (Raf)

 

Sarg.). The members were chosen from easily obtainable and nearly defect-
free nominal 2" x 6" lumber from.which clear nominal 2" x h" lumber was cut.
The nominal 2" x A" lumber was free of all detectable defects, including
compression wood. The nominal 2" x h"s (1-5/8" x 3—5/8") were cut to 32

inches in length for later joint fabrication.

MOISTURE CONDITIONING

 

The moisture content of the lumber was carefully controlled to prede-
termined levels both at the time of truss joint fabrication and at the time

of test. MOISTURE CONTENT DETERMINATION is discussed in the next section.

Within each plate type series, truss joints were fabricated and tested
at three moisture levels (28%,* 20% and 7%). Also within each plate type
series truss joints were fabricated at two moisture content levels (28% and
20%) and dried to test moisture content level (7%). At the two higher lev-
els of moisture content, truss joints were fabricated, some to be tested at
that moisture content level and others to be dried to the lower test mois-

ture content level.

The general conditioning procedure was as follows. After the lumber

had been selected and cut to desired size, it was placed in a standard dry-

 

* Fiber saturation point for western hemlock as stated in WOOD HANDBOOK (6).

10.
kiln where it was conditioned to the selected moisture content levels. For
each moisture content level, the lumber remained in the kiln until the mois-
ture gradient throughout the kiln had disappeared and the desired equilibri-
um moisture content level was attained. These moisture content measurements
were made regularly with the use of an electrical resistance moisture meter.
When the predetermined level had been reached and maintained for several
days, the truss joints were fabricated for immediate testing, or were dried

for later testing. Table 1 shows the kiln drying schedule.

In the case of those members to be fabricated at or above fiber sat-
uration point (28%), the members were completely smeerged in water for a
period of six weeks. Those to be tested at 28% were fabricated and tested.
Those joints to be fabricated at 28% and tested at a lower moisture content
were fabricated and placed in the kiln at the initial phase of the drying

schedule.

The fiber saturation point of wood is the stage in the drying of wood
when the cell walls are completely saturated.with water and no liquid'water
is present in the cell cavities. Under these conditions the moisture content
of western hemlock is approximately 28%, based on its oven—dry weight. Any
added moisture above this point will enter the cell cavities and have no
effect on the strength prOperties of the wood. Any moisture content below
this point will reduce the water content within the cell walls and increase
the strength properties of the wood according to the following equation (g):

LogS = LogSp + b(Mp - M)
where:

Mp = fiber saturation moisture content

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M = moisture content level lower than fiber saturation point
Sp = strength property at fiber saturation point Mp

S = strength property at moisture content level M

b = experimental constant

All moisture content values above 28% were referred to as being at 28%
since any moisture content above the fiber saturation point does not affect
the strength properties of wood. However, an effect on the withdrawal of

fastener teeth could be present as discussed on page 68.

MOISTURE CONTENT DETERMINATION

 

Two levels of moisture content determination were of consequence --
that of fabrication and that of test. The method of moisture content de-
termination was the oven-dried technique described in ASTM Standard D lh3—

52 section 122 (_1_).

The sample for the moisture content at fabrication level was a 1" cross
section taken from the end of the member as it was trimmed to the required
27" length (Figure 2). This sample was taken 6" :1" from the end. The

sample was immediately weighed and placed in an oven at 10300.

The sample for the moisture content at test level was a 1" cross sec—
tion taken from the buffer zone between the plate and grip region as shown in

Figure 3. The sample was immediately weighed and placed in an oven at 103°C.

Two samples for each truss joint were obtained for both fabrication
and test level of moisture content. The two values were averaged for each

level as shown in Table 2 (see appendix).

moiflun content sample

waste

 

 

 

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4 27
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liar, '30 I fix-:2 15‘ 3'. 51.4%. I. egg-.1}-
1 1 * m 1 . 1' t 1‘ H r

 

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Figure 3. Moisture Content Sample From One of Two Symmetrical Members
of the Test Specimen Shown in Dotted Cross Section and Labeled.

15.

MEMBER SPECIFIC GRAVITY

 

The specific gravity was determined for each of the test moisture
content samples according to ASTM Standard D lh3—52 section llh by the
water immersion method. The specific gravity was based on oven-dry volume

and oven-dry weight.

The two values for each truss joint were averaged and are shown in

Table 2 (see appendix).

JOINT FASTENERS

 

Two types of fasteners were tested in this study. They consisted of
a stampedrtooth type plate and a punchedetooth plate type, referred to as
type A and type B, respectively (Figure A). Both plate types were designed

for use in the lower chord tension joint of a 26' span W-type truss.

The dimensions of plate types A and B are shown in Table 3.

 

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Plate TYpes A and B.

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SPECIMEN CODING

 

In order to simplify the identification of the specific test Specimen
as to plate type and moisture content history, a coding system was used.
Plate types were assigned codes A and B as discussed in the section on

Joint Fasteners. The moisture content histories were coded as follows:

 

 

Fabrication Test Moisture Change in
Moisture Content Content Level Moisture

M2 70 % w
M01 28 28 O
MC2 20 2O 0
MC3 7 7 0
AMCA 28 7 21
[AMCS 2O 7 13

These moisture content values are approximate, for the moisture con-
tents within a group vary to a degree, but the tabular values show the
correct moisture content history relationship. A Specific example would

have the following meaning:

EXAMPLE: A—MC3—l7

A — plate type, stamped triangular teeth

MC3 - fabrication moisture content and test moisture content
both 7%, with no change in moisture content

17 - specimen number for moisture content group and plate
type Specified

FABRICATION

 

When the wood members had reached the correct moisture content for

l9.
fabrication, they were cut to the desired 27" test length. The members
were aligned eight wide on a metal caul. The joint gap between wood.mem~
bers was held constant by placing a 1/32" cardboard strip between the mem-
bers. The members were then clamped in a jig to hold their alignment
(Figure 5). The plates were placed in position on one side of the joint
and held by 3/A" wire brads driven through a tooth hole into both members.

One plate type was fabricated at a time.

A single Opening plywood press with 36" x 52" platens was used to
imbed the plates in the wood. Pressure was applied until the plate came in
contact with the wood member without crushing the wood fibers. After the
plate had been pressed into the wood members, the caul was removed and the
jig turned over. Plates were then placed into position on the opposite side

and the press cycle was repeated.

The joints were then ready to be either tested or returned to the kiln

for further moisture content reduction.

metal
caul
plate

wood
members

melol
plates

cardboard
separaler

clamping
liq

 

wing nut

”CNN" cardboard metal wood clamping

separater plate; mmnb/ lid 7
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/

   
 
 

 

 

21.

TEST APPARATUS

 

The tension joints were held at either end by shear plate connectors
and bolted to a steel grip. A plywood jig with two 7/8" diameter holes
exactly 7" apart was used to mark the wood members for drilling. A pilot
hole 7/8" in diameter was drilled and then the Teco Shear plate boring tool
was used to cut an opening for the shear plate (Figures 6 and 7). The
shear plate holes were drilled immediately before testing because any loss
of water from the wood members would cause shrinkage resulting in very dif—
ficult fitting of the metal shear plates into the holes. The shear plates
were placed in the holes and the steel grips were slid into position (Figure
8). Bolts were then passed through the steel grips and the Shear plates and

the nut tightened by hand on the Opposite side (Figure 9).

Each grip at either end of the tension Specimen had an eye-bolt uni-
versal joint attached. One end of the tension specimen was bolted by means
of the universal joint to a steel reaction fixture fastened to the concrete
floor (Figure 10). The other end of the test Specimen was attached to a
steel rod by means of an eye-bolt (Figure 11). This steel rod had two
bonded electrical resistance strain gages attached to it and was wrapped in
protective layers of Sponge rubber and tape. Wires led from this strain
gage to a strain indicator (Figure 12) where the load was read. The strain
gage on the rod had been previously calibrated in a universal testing may
chine. Load versus strain indicator calibration curves were plotted. A
like steel rod with two bonded electrical resistance strain gages attached
to it was hooked to the opposite Side of the strain indicator to compensate

for temperature changes.

Figure 6.

Figure 7.

 

 

 

Pilot Hole Bit (A) and Shear Plate Boring Tool (B).

 

Wood Members With Pilot Hole and Shear Plate Hole Bored.

22.

Figure 8.

Figure 9.

 

23.

 

 

 

Chord With Shear Plates in Position and Steel Grips Being Slid
into Position (Note the Eye-Bolt Universal—Joint).

 

 

Bolts Being Passed Through Steel Grips and Shear Plates and
Fastened by Means of a Nut.

 

 

Figure 10. Universal-Joint Fastened to Reaction Which is Bolted to Floor.

 

 

Figure 11. Strain Gage Attached to Steel Rod and Wrapped in Protective
Covering.

2h.

 

Figure 12.

 

Strain Indicator.

25.

26.

The steel rod in turn was attached to a hydraulic cylinder which was
fastened to the concrete floor by a steel reaction Shoe (Figure 13). The
hydraulic cylinder applied a force through a yoke to utilize the better

hydraulic characteristics of a piston in a pushing position rather than in

a pulling position.

Two ten-thousandths dial gages were used to measure the deflection at
either side of the joint gap. (Figure 1h). They were positioned at the

joint gap Shown in Figure 15.

The load was applied by a hydraulic constant speed gear pump while a
two—way bypass valve was used to control the pressure. The system allowed
the piston to follow creep deflection with a given valve setting. Figure

16 shows the entire test apparatus including the hydraulic pump.

 

Figure 13.

 

 

27.

 

Hydraulic Cylinder Fastened to Reaction Showing Yoke Used to
Apply Force.

 

Figure lh.

Dial Gages and Jigs Used to Read Deflection at Joint Gap.

28.

 

 

Figure 15. Positioning of Dial Gages on the Test Specimen.

 

 

Figure 16. Complete Test Set—Up.

29.

TEST PROCEDURE

 

The order of Specimen testing was from the green to the dry state.
This was to minimize the use of the dry kiln. When a moisture content level
had been reached in the dry kiln, the samples were removed, fabricated if
necessary, prepared for adaptation to shear plates, and immediately wrapped

in polyethelene to limit the loss of moisture.

The tension specimen was first placed in the test apparatus and the
holding grips were bolted in position. During testing the Specimen was
allowed to slide along the concrete floor riding on the bolt heads with a

minimum of friction.

The deflection measuring dial gages were adjusted to zero as soon as

the slack had been removed from the system.

Two men were required to run the test. One was stationed at the dial
gages and recorded the gap deflection. The other man controlled the hy-
draulic line pressure to Specific load levels as well as timing the deflec-

tion readings. The test positions are shown in Figure 17.

The truss was initially loaded to 600 lbs. force in tension where the
load was held and creep deflection readings were taken at the joint gap.
When the creep at the joint gap had settled the load was increased.by 600
lbs. to 1200 lbs. where the deflection readings were taken. This procedure
was continued with the 600 lb. load increments until A200 lbs. had been
reached or the dial gages had reached full extension. At this point the

dial gages were removed and the load increased to joint failure. The rate

30.

of load was 1200 lbs./min. between load increments and load to failure.

The deflection at joint gap was recorded with both dial gages on
either Side of the joint to the nearest ten thousandth of an inch. When the
initial 600 lb. load level had been reached the deflection was recorded on
command from the man loading the Specimen. The next reading was taken at
1/10 of a minute after time zero. Subsequent readings were taken at 2/10
of a minute intervals. Time was kept by means of a stop watch calibrated
in hundredths of a minute. Readings were taken until the creep at the joint
gap had approached equilibrium: the creep limit was determined by three
readings remaining the same on both dials. When the creep had settled the
load was increased to the next 600 lb. increment and the same procedure was
followed. Figure 18 shows the man applying the load by means of the hy-
draulic pump while reading the load on the calibrated strain indicator and

watching the stop watch.

A form data sheet was used to record the test information. The heading
of the data Sheet contained the sample number, both chord numbers, fabrica—
tion and test moisture content, change in moisture content, Specific grav-
ity, and zero readings for each dial. A Space was reserved for both dial
readings at each load level and for the incremental deflection as well as
the average incremental deflection. The final data page recorded the load

at failure and the type of failure.

 

3l.

 

 

Figure 17. Testing of Joint Specimen.

 

_ ___,l,

Figure 18. Application of Load With Control of Time.

32.

TEST RESULTS

 

TEST DATA

All truss joints of plate types A and B were tested in the same manner

described in TEST PROCEDURE. The physical characteristics measured were

load, deflection and time. These values are shown in Table 2 (see appendix)

and are

characterised as follows:

Plate Type - The plate types consisted of a stamped tooth plate

(A) and a punched tooth plate (B) (Figure A).

Moisture Content - Moisture content values given for the fabrica-
tion level and the test level consist respectively of the average
of the upper and lower moisture content for the two wood members
of each Specimen. The change in moisture content is the differ-

ence between these two average moisture contents.

Specific Gravity - The Specific gravity consists of the average
specific gravity of the two moisture content samples at the test

level, taken at oven—dry volume and weight.

Accumulated Creep - The accumulated creep consists of the average
accumulated creep for both dial gages for the 1200, 1800 and 2AOO

lb. load levels.

Ultimate Load - The ultimate load is the maximum load applied to

the joint before failure.

Type of Failure - Type of failure was classified as teeth-pull

diagonal (A), 2 plates failed in tension (B), teeth—pull parallel

(C), and 1 plate failed in tension (D). These are shown in

Figure 19.

33.

 

 

 

Figure 19.

Type of Joint Failure.

3A.

35-

CUMULATIVE JOINT GAP DEFLECTION VERSUS TIME AND INCREMENTAL LOAD LEVELS

A single joint for each of the two plate types and five moisture con-
tent histories was selected at random as being representative of its group.
The time deflection plots for these truss joints are shown in Figures 20
through 29. They are grouped as to plate type and moisture content history

for ease of reading, making ten plots in total.

The abscissa was time, measured in minutes. The ordinate was joint
gap deflection measured in inches. Individual deflection data points are

shown at the incremental load levels as functions of time.

The dotted lines represent the creep for the load level indicated.
Each dot represents a creep deflection reading. The solid lines represent
the deflection during the loading of the joint to the required level. The
time allotted for loading the joint between creep levels was set at 0.5
minutes although the actual load time was 0.5 minutes $0.03 minutes. This

error was introduced through the hand manipulation of the loading valve.

The time deflection plots for all joints were Similar, differing only
in magnitude of deflection, rate of change of slope and total time to reach

creep limit.

Joint Gap Deflection in Inches

 

Elapsed Time in Minutes

 

 

 

 

 

 

 

 

 

 

 

00 2.0 4.0 6.0 8.0 IQO l2.0
4. —- -— ~—~—-——+—— S
msoof}
.oo7y\
0.0: l +
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a I i
———‘_-_- «FIE ‘__—--.t.."ooooy WT i

..o.+oo o....}

0.03 - -* -L_ ..__.__.__i

___. ..__. l ___. __._._._.._.._.___.__l

OJI4

 

 

 

 

 

 

 

 

Figure 20. Deflection vs Continuous Time From Start of Test

for Joint A-MCl-Q.

38.

Elapsed Time in Minutes

K10

ELO

 

 

 

 

'- .,Izoo#

 

 

 

 

 

 

.°°0.oi

 

 

 

 

 

 

O
0
05!
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0134

 

 

 

Figure 21. Deflection vs Continuous Time From Start of Test

for Joint A-MC2-9.

Joint Gap Deflection in Inches

Elapsed Time in Minutes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0° 2.0 40 6.0 0.0 10.0 12.0
600%?
..‘ 2
l
.1 00b“
0.0. \ ——
1
i
?
\
.‘0JOOCHl
0.02 - ”'m-A
1
.,_._..____._ M1? - ._ l _. __.i_.- -....._._..J
0.0: - a MW.“
'- ”24007?
° ‘r ..
0.04

 

 

 

 

Figure 220 Deflection vs Continuous Time From Start of Test
for Joint A-HCB-Q.

Joint Gap Deflection in Inches

39.

Elapsed Time in Minutes

 

 

 

 

 

 

 

 

 

 

 

 

0° 2.0 4.0 6.0 e0 :00 I20
1; .
4500*?
I
g
0.01 ——-
T .
212.00” 7 ':
° 0 O I
H. ...”- t I _ _._ l—
i l '
T l '
z a
0.02%.”- i \ i — ~ ..___..H .—-——«
' 2:900?
L -—-— - i .... H .—
o,oa_————~e~~—- — ~ ‘. -7 - . ._,____h ,___-__A___J
..___.__.__.._._. .-.--- Mes—..--..» 4% -—-- w - ---_.,_ 4-. ____,__.__ __-.- l ,. _.- ..,._"__ Jr --___._.__.-...4
(104

 

 

 

 

 

 

 

Figure 23. Deflection vs Continuous Time From Start of Test
for Joint A-AMC4-10.

Joint Gap Deflection in Inches

Elapsed Time in Minutes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0 0 2.0 4.0 6.0 8.0 l0.0 l2.0
[ i
I
I
l ,
, a
‘-.699\ f w
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0.0: €99?” .
g
‘- ”poi ;
0.02 H i
0.03~— — , —-—
enm— -4-“
I :
<104~ L

Figure 24. Deflection vs Continuous Time From Start of Test
for Joint A-AHCS-Q.

40°

 

Elapsed Time in Minutes

2;) 41) GA) 60* N10 l21)

 

0°
Raw”

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.0I ' A - . . — 4_-_.____-
gi 2 0014*

g; o cor-....
5 .
0 3
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' 0.02 s , -
g ‘ F0....3ma#
'+, c ... ......
U ......
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7‘
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+9
£1
..-1
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O£M4

 

 

 

Figure 25. Deflection vs Continuous Time From Start of Test
for Joint B-MCl-Q.

Joint Gap Deflection in Inches

‘lapsed Time in Minutes

21) 4L0 6£)

ex)

N10

”L0

 

 

GIN

 

 

 

 

 

 

 

 

 

 

 

0.02 a 7 a _- - ... - “HINT”
i... _ .4; _ i
0.03 - -— «Me—......Acm -.
~——~ e —— - —~+ --------- ——-.4
0.04

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 26. Deflection vs Continuous Time From Start of Test

for Joint B-MCZ—So

42o

.Joint Gap Deflection in Inches

Flapsed Tine in Minutes

.0 40 $0 e0 IQO IZO

 

0° 2
K59?"
\ 1200 #

V0.\ “.4

\IB 0#

.\,.,m\

a???“

F—_—'——"_' -_ . >. --.—.-. .7 -- t -_ r», . - -. ___.__--..___i._<,. ,,

 

 

 

 

 

 

°-.4200#
0.02 . ---. i-.. __ ..--..

 

 

 

 

 

 

0.03 ----— ~----<~s~~—- - 4 e- .- , - ---._ -_._.

 

 

 

 

 

 

 

 

 

 

0.04

Figure 27. Deflection vs Continuous Time From Start of Test
for Joint B-HCB—lO.

Flapsed Time in Minutes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

00 2.0 4.0 6.0 8.0 I00 I20
\5590 #
,IZIDO # i
O I ‘
V®°#l I A
, . , 2400’? I
am '4 ‘ ‘
i i
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I __ : V '- 4999'? *
m I
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U .
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,3 0.03 f l ,1
I I i i
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1 i I
I . I
I :
I
0.04

Figure 28. Deflection vs Continuous Time From Start of Test
for Joint B-AHC4p9.

Joint Gep Deflection in Inches

Elapsed Time in Minutes

20 40 60 e0 IQO IZO

 

 

 

GIN

(102

(103

0134

 

 

 

 

 

 

 

Oboe...

\

° - 420W

 

 

 

 

 

 

 

 

 

 

 

Figure 29. Deflection vs Continuous Time From Start of Test
for Joint BuSMCS-Q.

h6.

MAGNITUDE OF DEFLECTION AT "CREE? LIMIT" VERSUS LOAD

 

The "creep limit" deflection was determined from the data sheet of each
test joint for the various load levels. The values for the 1200, 1800 and
2h00 lb. load levels are shown in Table 2 (see appendix). A typical average
creep limit deflection-load plot is shown in Figure 30 for the eighteen rep-
lications for one moisture content and plate group B-MC2. Similar graphs
were made for each moisture content history and plate group. These deflec—
tion—load plots are shown in Figure 31 and Figure 32 by plate type. All
curves exhibited comparable curvilinear forms. A summary table of the aver-
age creep limit deflection for the 1200, 1800 and 2h00 lb. load levels is

given in Table h.

LOAD AT FAILURE

 

The ultimate load as well as the type of failure was determined for

each test joint. The summary of these values is found in Table 5.

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52.

ANALYSIS AND DISCUSSION OF DATA

GENERAL

Essentially, two variables were introduced into the study; moisture
content history and plate type. Measurements of the effect of these vari-
ables were made on Joint gap deflection at incremental load levels and on

the ultimate load of the test joint.

Although member specific gravity was determined, it was not considered
a variable due to the small range of values obtained. The selective nature
of choosing lumber for the wood members purposely eliminated specific grave
ity as a variable. Specific gravity values are given in Table 2 (see ap-

pendix).

The analyses are confined to the range of essentially linear behavior
of "creep limit" deflection versus moisture content history and ultimate
load versus moisture content history. Investigation may be categorized as
follows: 1) deflection versus moisture content of member lumber, 2) deflec-
tion versus moisture content history of member lumber, 3) ultimate load
versus moisture content of member lumber, and h) ultimate load versus mois-

ture content history of member lumber.

Two dimensional comparisons were made of the above four categories by
the use of plotted scatter diagrams for both plate types. A simple regres-
sion equation was calculated for each diagram (§, lg). Duncan's Range Test
(g, iii) was applied to both deflection and ultimate load values between all

possible moisture content groups to judge possible significant differences.

53.
The "creep limit" deflection values used in these plots and comparisons
were taken from the 1800 lb. load level. This level was chosen for compar-
ison basis as it was a median of the deflection levels within practical use
limits, and it lay within the most linear portions of the deflection versus

load curves.

Linear regression for each category was used to statistically investi-
gate the degree of trend and reliability. Linear regression was used for
comparison because three levels of moisture content were compared within
each moisture content series and moisture content history series. The re-
gression equations only apply for those moisture content values below that
of fiber saturation point for western hemlock due to the moisture-strength

relationship of wood discussed in the section MOISTURE CONTENT DETERMINATION.

Average values were plotted and joined by smooth curves which probably
fit the data better than the regression line. The analyses of the catego-

ries of behavior follow.

DEFLECTION VERSUS MOISTURE CONTENT OF MEMBER LUMBER

Scatter diagrams for each plate type (A and B) were made for gap de-
flection at the 1800 lb. level versus moisture content levels (28%, 20% and

7%) of member lumber. These are shown in Figures 33 and 3h.

Plate types A and B showed similar characteristics. In both cases,
the regression coefficients were positive, suggesting a trend where an in-
crease in moisture content would result in an increase in deflection. The
magnitude of the coefficients was relatively low, but a consistent trend was

present. By using individual test joints many variables were easily elimi-

5h.
nated, such as lumber defects, member specific gravity, warping and twist.
The increase in test precision was shown by the increase in the regression
coefficients over the relatively low regression coefficients found in the
work of Kawal (I) experimenting on full scale trusses. Table 6 shows the
summary of correlation and regression of deflection on moisture content and

moisture content history.

From Table A the following values for gap deflection can be seen:

Average creepglimit deflection in inches for the 1800 lb. load level
Plate

Type MCI g28%2 MC2 {20%) MC3 g1%2
A 0.0153* 0.0098 0.0089
B 0.0068 0.0039 0.0036

* Each value is the average of 18 test joints

The reduction in gap deflection from MCl to MC2 and MC3 was approxi-
mately 30% to 50%. A practical limit to joint slip between the truss members
is given by the Truss Plate Institute (g) in conjunction with the Federal
Housing Authority as 0.0150 inches. This limit is thought to be the point
of plaster cracking in ceilings as related to the ceiling structure, however,
this point has been in question by many authorities for many years. Even if
a practical deflection limit was set there is no present means for comparing

a single joint tested in tension to the over-all full scale truss performance.

At all load levels, the averages of the differences in creep limit

deflection between the Specimens tested at 20% moisture content and those

55.

tested at 7% moisture content were less than or equal to the averages of
the differences in deflection between the specimens tested at 28% moisture
content and those tested at 20% moisture content. This was true for both

plate types. See Table A.

At the 1200 lb. load level, there appeared to be very little, if any,

correlation between creep limit deflection and moisture content.

At the 2hOO lb. load level, the general trend between creep limit
deflection and moisture content was similar to that at the 1800 lb. load

level.

Plate type B out-performed plate type A at all moisture content levels
in that it attained gap deflection values of 1/2 or less than those attained
by plate type A. Analysis was not made to see if the factor contributing

to the differences in gap deflection was plate design or plate size.
DEFLECTION VERSUS MOISTURE CONTENT HISTORY OF MEMBER LUMBER

Scatter diagrams for each plate type (A and B) were made for gap de-
flection at the 1800 lb. level versus moisture content history (change in
moisture content of 20%, 1h% and 0% from fabrication moisture content to
base moisture content of member lumber of 7%). These are shown in Figures

35 and 36.

Plate types A and B showed similar characteristics. In both cases the
regression coefficients were negative, indicating an increase in the change
of moisture content would result in a decrease in the deflection. The mag-

nitudes of the coefficients were very low, but a consistent trend was pres-

56.
ent. The degree of statistical significance of the linear regression equa-
tions are so low that a definite conclusion as to their being true indicators
of trends is in doubt. Table 6 shows the summary of correlation and regres—

sion of deflection on moisture content and moisture content history.

From Table A the following values for gap deflection can be seen:

Average creep limit deflection in inches for the 1800 lb. load level

 

 

 

Plate

Type MC3 $076) AMCS (11%) AMCII (20%;
A 0.0089* 0.0161 0.0106
B 0.0036 0.0060 0.0039

* Each value is the average of 18 test joints

It can be seen that the change in moisture content has little effect
on gap deflection. The difference in deflection between MC3 and.DMCh was
very small for both plate types A and B, indicating that if a joint is con-
structed green and dried to 7% it will perform.with the same deflection

characteristics as one built at 7%.

In each moisture content history group plate B out-performed plate A

by having 1/2 or less the deflection found in plate A.

 

0.03 - j

-- 8 = 0.00536 + 0.00032(MC)
+ AVERAGE VALUES

 

 

 

 

 

F’
O
n:

 

 

 

 

GIN

Joint Gap Deflection in Inches

 

 

 

 

I !
i i
' I
I
I
I

I I

 

 

1
O 5 IO IS 20 25

Moisture Content in Percent

Figure 33. Joint pr Deflection vs Moisture Content wit“ F brication and
Test Moisture Content Levels E ual for Plate Type "A" it the
1800 lb. Load Level.

 

C102 r 1'

-— 8 = 0.00l93+0.000|5(MC)
4- AVERAGE VALUES

 

 

01M

,4”

 

 

I
I

 

 

 

 

Joint Gap Deflection in Inches

 

 

 

 

0 5 l0 IS 20 25
Moisture Content in Percent

Figure 34. Joint Gap Deflection vs Moisture Content with Fabrication and
Test Moisture Content Levels Equal for Plate Type "B" at the
1800 lb. Load Level.

58.

 

 

 

 

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+4 I
8 L / / \ O 3
:3C)0l I .V_
0 't I
"I If I
9‘ I .
0 . °
4; D
-I-4
O
P” I

o ,

0 4 8 l2 IS 20

Change in Moisture Content in Percent

Figure 35. Joint Gap Deflection vs Change in Moisture Content to
Base Moisture Content of T7 a Fabrication .nd Test

Moisture Content Levels not Equal - for Plate Type "A"
at the 1800 lb. Lo.d Level.

0132

L

 

I I
-- 6 = 0.00537 -0.00006(AMC)

+ AVERAGE VALUES

 

O1N

 

 

 

 

Joint Gap Deflection in Inches

 

 

 

 

 

 

 

 

 

”......I-fl-fl’i— —'o_""-'~p\ II
# o x I
an" : \ ...
‘.,. I . . .\UF‘
0.
O 4 8 l2 I6 20

Change in Moisture Content in Percent

Figure 36. Joint Gap Deflection vs Change in Moisture Content t0
Base Moisture Content of 7% - Fabrication and Test
Moisture Contents not Equal - for Plate Type "B"

at the 1800 lb. Load Level.

60.

STATISTICAL SIGNIFICANCE OF DEFLECTION VERSUS MOISTURE CONTENT HISTORY ‘

 

OF MEMBER LUMBER

 

The mean value differences for "creep limit" deflection at the 1800
lb. load level were statistically compared at the various moisture content
history levels by Duncan's Range Test Cg, gg) and are shown in Table 7.
Comparisons were made on both the 0.0l and 0.05 levels of confidence. It
must be noted that although a comparison of mean value differences might
Show that a statistically significant difference exists, this has no bearing
on the practicality of that difference and is only useful in locating areas
for further investigation. Plate and moisture content level are contained

within the specimen code.

Looking at differences between plate types, we find that plate type A
is significantly different from plate type B at the 0.01 percent confidence
level for all moisture content history groups. This indicates that plate
type A had significantly higher gap deflection values than plate type B for

the same moisture content history.

Looking at both plate types A and B in the summary Table 8 of signifi—
cance differences, we see that both MCI and MC2 were significantly different
from MC3, thus indicating that the higher moisture content levels result in

a significantly greater deflection than the lower moisture content levels.

Looking at (IMCI.t and AMCS for both plate types, we see no significant
difference, indicating that the degree of change in these moisture content

levels resulted in a like deflection.

61.

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Plate

MOCI
Comgarison

 

MCl x MC2
MCl x MC3

MC2 x MC3

AMch x AMCS

A MCh x MC3

AMCS x MC3

M01 x MC2
M01 x MC3

MC2 X MC3

AMCu x AMOS

AMCLL x MC3

AMOS x MC3

COMPARISON OF MOISTURE CONTENT HISTORY

62.

Significance of Variables

Ultimate Load

9696

9696

9696

none

none

9696

9696

9696

*96

none

none

none

* Significant at 0.05 level of confidence.

** Significant at 0.01 level of confidence.

GagiDeflection

9696

9696

none

none

9696

9696

9696

9696

none

none

none

 

63.
Since AMCh and AMCS and MC3 had been tested at the same moisture con-
tent (7%), a comparison was made between these groups. For plate type A,
AMCH x MC3 and AMOS x MC3 proved significantly different, indicating a sig-
nificant difference in the gap deflection between those joints fabricated
and tested at 7% and those joints fabricated at a higher moisture content
(28% and 20%) and tested at 7%. Plate type B showed no significant differ-
ence between the groups except for AMCS x MC3 at the 0.05 percent confidence

level.

ULTIMATE LOAD VERSUS MOISTURE CONTENT OF MEMBER LUMBER

Scatter diagrams for each plate type (A and B) were made for ultimate
load versus moisture content levels (28%, 20% and 7%) and are shown in

Figures 37 and 38.

Plate types A and B showed similar characteristics in that their re-
gression coefficients were both negative, indicating a trend where an in-
crease in moisture content would result in a decrease in ultimate load.

The magnitude of the coefficients were relatively low, but a consistent
trend was present. Table 9 shows the summary of correlation and regression

of ultimate load on moisture content history.

From Table 5 the following values for ultimate load can be seen:

Plate Ultimate load at failure in lbs.

Type MCl( 28%) MC2( 20%) MC3( 7%)
A 3783.33* h572.22 h900.00
B 5533.33 6156.50 6&55.55

* Each value is the average of 18 test joints.

6h.

Both plates A and B increased substantially in ultimate load as the
Inoisture content level decreased. Plate A.increased from 3783.33 lbs. at
1mc1 to h9oo.oo lbs. at MC3, or an increase of 1116.67 lbs. Plate B increased

from 5533.33 lbs. at MCl to 6h55.55 lbs. at MC3, or an increase of 922.22 lbs.

Plate B attained.higher ultimate load levels than plate A at each of
the moisture content levels. Plate B ranged from 1555.55 lbs. to 1750.00
lbs. higher in ultimate load than plate A. Analysis was not made to see if
the factor contributing to the greater load carrying capacity of the one

plate type was its large size or its tooth geometry.
ULTIMATE LOAD VERSUS MOISTURE CONTENT HISTORY OF MEMBER LUMBER

Scatter diagrams for each plate type (A.and B) were made for ultimate
load versus moisture content history (change of moisture content of 20%,
1h% and 0% to base moisture content of member lumber of 7%) and are shown

in Figures 39 and hO.

Plate types A and B showed similar characteristics in that both re-
gression coefficients were very near zero, indicating that no relationship
between change in moisture content and resulting ultimate load exists.
Table 9 shows the summary of correlation and regression of ultimate load on

moisture content history.

From Table 5 the following values for ultimate load can be seen:

Plate
Type Ultimate load at failure in lbs.
MC3§ 0%) AMC§§ 111%) among 20%)
A h9oo.oo* . h283.33 h76h.hh
B 6h55.55 6283.33 6h88.33

* Each value is the average of 18 test joints.

in 1000 Pounds

Ultimgte Loud

Ultim:te Load in 1000 Pounds

 

f
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'6 AVERAGE VALUES :
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o 5 i0 '5 2° 25 30

Moi ture Content in Percent

Fifiure 37. Ultimuie Lo d vs Moisture Confevt with F brjua‘ on 'Ld
Tent Moistu 9 Content Levels Egu l for Pl'te Type "A".

 

 

 

 

 

 

 

 

 

 

 

 

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0 5 l0 IS 20 25 3O

Moisture Content in Percent

Figure 38. Ultimate Load vs Moisture Content with szricution .nd
Test Moisture Content Levels Equal for P1 te Type "B".

66.

 

 

 

 

 

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01

 

 

 

 

 

 

 

 

 

 

 

 

 

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Ctan:e in Moisture Content in Bereent

Figure 39. Ultimate Loxd vs Ch fie in MoisTurc Cortent to B 5e
Moisture Content of 7% — F brie tion and Test Moisture
Conten' Leveis not E7u:l for P] to Tfpe "A".

 

 

 

 

 

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.b

— U=6299.27+8.l8tAMC)
+ AVERAGE VALUES

 

 

 

 

l

L
O 4 8 l2 I6 20
Change in Moisture Content in Percent

 

 

Figure 40. Ultimate Lead vs Change in Moisture Content to Base
Moisture Content of 7% - FabriCation and Test Moisture
iContent Levels not E ual for-Plate Type "B".

68.

TYPE OF JOINT FAILURE

Type of failure was classified as teeth-pull diagonal (A), both side
plates failed in tension (B), teeth-pull parallel (c), and one plate failed
in tension (D). The types of failure are shown in Figure 19 and a summary

of the types of failure is given in Table 5.

In comparing plate types A.and.B it was found that plate type A failed
predominantly by the teeth pulling out of the wood fibers (failure types A
and C) while plate B failed predominantly by the metal plates failing in
tension (failure types B and D) at the joint gap. The failure category of
plate type A indicates that the teeth either lacked the holding power or
that the number of teeth was not sufficient to reach the tension strength
limit of the metal plate. The failure category of plate type B indicates
that the teeth had more than adequate holding power or that the number of
teeth exceeded that which was sufficient to reach the tension strength limit
of the metal plate in tension. In both cases it should.have been the db-
jective of the designer to reach a balance between teeth-pull and.plate
failure in tension to gain the highest ultimate load without over designing
the metal connecting plate. The factors of joint creep and the relation-
ship of a single joint versus a complete truss play an important role in

deciding whether a joint is over or under designed.

Looking at plate type B specifically, it can be seen that at the higher
moisture content level (MCl) the failure type was predominantly of the tooth-
pull variety where as at the lower moisture content levels the failure was

of the metal plate failing in tension. When the moisture content level rises

69.
above the fiber saturation point, as it did for MCl, the added water is held
within the wood cell cavities. This water acting as a lubricant, could have
allowed the plate teeth to slip free of the wood member. No specific research
has been done in this area, but such an investigation of the effects of
moisture content above fiber saturation point would prove valuable, not only
for future truss studies, but for much of the research comparing green and

air seasoned withdrawal values in the screw and nail design field.

STATISTICAL SIGNIFICANCE OF ULTIMATE LOAD VERSUS MOISTURE CONTENT

HISTORY OF MEMBER LUMBER

 

The mean value differences for ultimate load values were statistically
compared at the various moisture content history levels by Duncan's Range
Test (an IE) and are shown in Table 10. It must again be noted that al—
though a comparison of mean value differences might show that a statisti-
cally significant difference exists, this has no bearing on the practicality
of that difference and is only useful in locating areas for further investi-
gation. Plate and moisture content level are contained within the specimen

code.

Looking at the differences in plate types, we find plate type A is
significantly different from plate type B at the 0.01 percent confidence
level for all moisture content history groups. This indicates that plate
type B reached significantly higher ultimate load levels than plate type A

for the same moisture content history.

Looking at plate types A and B in Table 8 of significant differences

70.
of mean ultimate loads, we see that MCl, MC2 and MC3 were significantly
different for all paired comparisons. This indicates that as a test joint
was fabricated and tested at decreasing moisture content levels, the ulti—

mate load increased significantly.

Comparing AMCM x;AMC5 for both plate types, no significant difference
was found between ultimate load values, thus indicating that the degree of

change in moisture content does not alter the ultimate load value.

Since AMCh and.AMC5 and MC3 had been tested at the same moisture content
(7%), a comparison was made between the groups. No significant difference
occurred, indicating that a joint fabricated at 28% or 20% and tested at

7% attained the same ultimate load level as a joint fabricated and tested

at 7%.

Tl.

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' 72.
CONCLUSIONS AND RECOMMENDATIONS

Without regard for moisture content history, it was found that the test
Joints fabricated with plate type B had less gap deflection than plate
type A, as shown below. No analyses were made to see if differences

in deflections were related to relative plate sizes or to plate design.

Average Creep Limit Deflection in Inches

 

 

 

Plate at the 1800 lb. load level

we MClL28% ) * MC2 90%) MC3§ 1%) A MCM20% ) ** AMC§§ 111%)
A 0.0153*** 0.0098 0.0089 0.0106 0.0161
B 0.0068 0.0039 0.0036 0.0039 0.0060

* Fabrication and test moisture content equal.

.** . indicates change in moisture content at fabrication to base
test level of 7%.

*** Each value is the average of 18 test joints.

Without regard for moisture content history, it was found that the test
Joints fabricated with plate type B had higher ultimate load values
than plate type A, as shown below. No analyses were made to see if
differences in deflections were related to relative plate sizes or to

plate designs.

 

 

 

Plate Average Load at Failure in Lbs.

gm MCl(28%)*~ MC2(20%1 MC3§1%) nMch(2073)** AMC§§ll+%2
A 3783***~ M572 h900 h76h A283
B 5533 6156 6h55 6h88 6283

* Fabrication and test moisture contents equal.

** indicates change in moisture content to base level of 7%.

*** Each value is the average of 18 test Joints.

Plate type A failed predominantly by the teeth pulling out of the wood

73.

while plate type B failed predominantly by the metal plate failing in
tension at the joint gap. Coupled with lower deflection values and
higher ultimate load values of plate type B, it is clearLy indicated

that plate type B was the better of the two plates.

Although plate type B failed predominantly by plate tension failure over
all moisture content histories, it failed by the plate teeth pulling out
of the wood fibers at the moisture content level over fiber saturation
point. A recommended area for future study would.be the lubricating
effect of water on the plate teeth withdrawal at a moisture content

above the fiber saturation point of the wood members.

With fabrication and test moisture content levels equal, both plate
types A and B increased in ultimate load level and decreased in gap de-
flection as the moisture content decreased. Thus, as a Joint is lowered
in moisture content it gains in desirable design characteristics much

as wood itself does.

With fabrication and test moisture content levels equal:

a. Both plate types A and B increased in ultimate load carrying ca-
pacity as the moisture content variable decreased.

b. Both plate types A and B showed decreases in gap deflection at the
1800 lb. load level as moisture content decreased with the reserve
ation that there was not a statistically significant decrease at
the 1% confidence level between the 20% moisture content specimens
and the 7% moisture content specimens for either plate type.

Thus, it appears that as a joint becomes drier below the fiber satue

ration point it gains in desirable design characteristics much as wood

.71;

itself does.

With fabrication moisture contents (28%, 20% and 7%) lowered to test
moisture content of 7% for both plate types A and B, no difference in
creep limit deflection or ultimate load values occurred.with the degree
of moisture content change except for the following:

a. Specimens for plate type A fabricated at 20% moisture content and
tested at 7% moisture content showed a statistical difference in
ultimate load at the 1% level of confidence from those specimens
fabricated and tested at 7% moisture content.

b. At the 1800 lb. load level, specimens fabricated with plate type A
at 28% and 20% moisture contents and tested at 7% moisture content
showed a statistical difference in creep limit deflection at the 1%
level of confidence when compared to specimens fabricated.and
tested at 7% moisture content.

c. Specimens for plate type B fabricated at 20% moisture content and
tested at 7% moisture content showed a statistical difference in
creep limit deflection at the .05% level of confidence when compared
with specimens fabricated and tested at 7% moisture content.

Thus, indications are that deflection and ultimate load for a joint are

more dependent on moisture content of use than moisture content of

fabrication.

By limiting the tests to individual joints the precision of the regres-
sion coefficients was greatly increased over those found.by Kawal C1)
experimenting on full scale trusses. This increase in precision was due

mainly to the practicality of increased sample size and the ability to

75.

remove defect variables when dealing with individual test joints.

Additional test data are needed to determine the correlation of indivi-
dual joint performance with the performance of a full scale truss. Once
this is accomplished, the much needed performance standards can be formed

relating to the practical testing system of individual joints.

 

APPENDIX

MC .

ISMC .

APPENDIX I

 

NOTATION

. Y intercept
. Regression coefficient

. Base moisture content in %

Decrease in joint member moisture content
from fabrication to test in %

Correlation coefficient

Coefficient of determination

. Ultimate load of test Specimen in pounds

. Deflection in inches at joint gap

76.

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LITERATURE CITED

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ANON. 1962. "Design Specifications for Light Metal Plate Connected
Wood Trusses." Truss Plate Institute, TPI—62.. Miami, Florida.

BROWN, H.P., A.J. PANSHIN, C.C. FORSAITH. 1952. "Textbook of Wood
Technology," Volume II. McGraw-Hill Book Company. New York.

FELTON, K.E. and H.D. BARTLETT. 1963. "Effectiveness of Punched Truss
Plates." Maryland Agricultural Experiment Station Scientific Article
No. A1090, Contribution Number 3519.

FOREST PRODUCTS LABORATORY. 1961. "Dry Kiln Operator's Manual." U.S.
Department of Agriculture Handbook No. 188.

FOREST PRODUCTS LABORATORY.V 1955. "Wood Handbook." U.S. Department
of Agriculture Handbook No. 72.

KAWAL, D.E. 1965. "The Influence of Member Stiffness and Moisture
Content History on the Deflection Behavior of a Truss Fastened with
Metal Plates." Thesis. Department of Forest Products, Michigan State
University. East Lansing, Michigan.

LI, C.C. l96h. "Introduction to Experimental Statistics." McGraw—
Hill Book Company. New York.

LUXFORD, R.F. 1958. "Light Wood Trusses." U.S. Forest Products
Laboratory Report No. 2113. Madison, Wisconsin.

RADCLIFFE, B.M. and GRANUM. 1955. "A New LOW-Pitched Roof Truss with
NailedeGlued Connections." Purdue University Agricultural Experiment
Station Bulletin 617. Lafayette, Indiana.

RADCLIFFE, B.M. and S.K. SUDDARTH. 1955. "The Purdue-Illinois Nail-
Glued Roof Truss with a Pitch of 3:12 and #:12 for Spans of 2h Feet 8
Inches and 28Feet 8 Inches." Purdue University Agricultural Experiment
Station Bulletin 629. Lafayette, Indiana.

RADCLIFFE, B.M. and A. SLIKER. l96h.. "Effect of Variables on Perfor-
mance of Trussed Rafters." Michigan State University Agricultural
Experiment Station Research Report No. 21. East Lansing, Michigan.

RODDA, E.D. 1965. "The Analyses of Trusses with Semi-Rigid Joints."
Thesis.

1h.

15.

16.

17.

18.

19.

20.

88.

STEEL, R.G. and TORRIE, J.H. 1960. "Principles and Procedures of
Statistics." McGraw—Hill Book Company. New York.

STERN, E.G. 195h. "Nailed Trussed Rafters of 2h Ft. Span and 3 in 12
Pitch." Virginia Polytechnic Institute Wood Research Laboratory Bulleetin
No. 1h. Blacksburg, Virginia.

STERN, E.G. 1957. "Wood, Plywood or Steel Gusset Plates for Nailed
Trussed Rafters." Virginia Polytechnic Institute Wood Research Labor-
atory Bulletin No. 29. Blacksburg, Virginia.

I

STERN, E.G. 1959. "King-Post Nailed Trussed Rafters." Virginia Poly-
technic Institute Wood Research Laboratory Bulletin No. 36. Blacksburg,
Virginia.

STERN, E.G. and P.W. STONEBURNER. 1952. "Design of Nailed Structures."
Virginia Polytechnic Institute, Engineering Experiment Station Series
No. 81. Blacksburg, Virginia.

SUDDARTH, S.K. 1961. "Determination of Member Stresses in Wood Trusses
with Rigid Joints." Purdue University Agricultural Experiment Station
Bulletin 71h. Lafayette, Indiana.

WILKENSON, T.L. 1966. "Moisture Cycling of Trussed Rafter Joints."
Forest Products Laboratory Research Paper FPL 67. U.S. Department of
Agriculture. Madison, Wisconsin.

 

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