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

Compiled by Leslie H. Groom

Presented by

USDA Forest Service and

Forest Products Research Society

June 24,199l

New Orleans, LA

CONTENTS

Introduction to Mechanical Fasteners Plenary Session

LeslieH.Groom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Wood Connection Design: 1991 National Design Specifications and Beyond

T. E. McLain and D. G. Pollock, Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Eccentricities in Metal-Plate Connected Joints

MichaelH.Triche................................:..............5

Determining Failure Mechanism in Bolted Joints Using Acoustic Emissions

Marcia Patton-Mallory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Strength and Stiffness of Shear Transfer Plate Connections

Ron Wolfe and Dave Bohnhoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Mechanical Connections: The Weak Link in Light-Frame Wood Construction

GaryJ.Klein..................................................13

..*

111

Introduction to Mechanical Fasteners Plenary Session

Leslie H. Groom

The title of this publication-“Current and Future

Applications of Mechanical Fasteners for LightFrame Wood Structures”-is the theme of this plenary session from the 1991 annual meeting of the

Forest Products Research Society (FPRS). This theme

was chosen to address current issues in the forest

products industry: changing design codes, fastener

performance, and legal responsibilities. It is hoped

that this session helped provide a forum for forest

products industries influenced by mechanical fasteners and also for research agencies, resulting in an

increased exchange of current knowledge and concerns for the future.

Mechanical fastener research has blossomed in

redent years as a result of the changing wood base and

of liability issues and foreign competition. Liability

has become a significant problem because of the increasing frequency and severity of litigation, resulting

in the financial collapse of some businesses and increased insurance premiums for all. A key defense

against litigation is proper assessment of product performance. This information is needed not only by

manufacturers of mechanical fasteners but also by

other potential lawsuit defendants, such as design

agencies and trade associations. Economical and innovative designs of engineered wood structures

(whose properties are largely governed by mechanical

fasteners) must be coupled with product performance

to be competitive in world markets.

The presentations given at this plenary session

were chosen because of their timeliness and importance. Individual articles in this publication are based

on the presentations. The first article, by Tom McLain, addresses the current revamping of the National Design Specifications (NDS) for mechanical fasteners, which are undergoing a fundamental change.

The new design specifications will be based on the

European concept of yield theory and will be incorporated in the 1991 NDS manual. Tom discusses the

NDS changes and how they will affect light-frame

construction practices.

Product performance is addressed in the next three

articles, with topics progressing from theoretical

modeling to mechanical behavior of mechanically fastened joints. The modeling article, by Mike Triche,

focuses on the analysis and design of truss-plate

joints. Marcia Patton-Mallory explores experimental

techniques for determining the integrity of mechanical fastened joints in her article. She discusses the use

of acoustic emissions to determine the failure modes

of bolted joints. In the product performance article,

Ron Wolfe addresses the mechanical behavior of shear

transfer plates. Traditionally, this mechanical fastener has been limited to laminated beam columns;

however, shear transfer plates have exceptional

mechanical properties and’may prove useful in other

engineered systems such as timber bridge decks. Ron

details the load capacity of stress transfer plates and

the factors that affect their performance.

The final article of this publication is by Gary Klein,

who discusses the consequences of a lack of product

performance understanding by examining the collapse of structures as a result of failure of the

mechanically fastened joints. Gary presents forensic

case histories and discusses the liability associated

with the failures.

The contributors selected were chosen on the basis

of their knowledge of the subject area and respect

among their colleagues. They represent industry,

academia, and U.S. Government research facilities. I

should note that there had not been an FPRS plenary

session on mechanical fasteners for several years, and

the enthusiasm of the contributors was very high. A

high level of interest was shown by both industry and

researchers. I hope the session was enjoyable and

educational and that discussions continue.

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

Southern Forest Experiment Station, Pineville, LA 71360.

1

Wood Connection Design: 1991 National Design

Specifications and Beyond

T. E. McLain, D. G. Pollock, Jr.

Design provisions for engineered wood connections

are changing substantially for the first time in over 50

years. These changes can be found in the upcoming

1991 edition of the National Design Specification

(NDS) for Wood Construction and in a proposed load

and resistance factor or design (LRFD) Specification

for Engineered Wood Structures. The purpose of this

presentation is to outline some of these changes.

BEHAVIORAL EQUATIONS

A major change in design basis is the adoption of a

well-verified theoretical base for lateral yield strength

criteria for connections using nails, wood screws,

bolts, and lag screws. Yield strength is defined as

shown in figure 1. The European Yield Models (EYM)

are used to calculate design strength based on the

bending resistance of the fastener, the crushing

strength of wood or member material, joint geometry,

and a set of assumed mechanical relationships. The

5 % OF FASTENER DIAMETER

0

DEFORMATION (INCHES)

Figure l.-Fastener yield strength as based on 5 percent of the

nominal shank diameter. Note: PL = Proportional Limit.

EYM describe a set of possible yield modes for a single

fastener under lateral load (see fig. 2) for double-shear

connections. Results of EYM for common connections

are calibrated to 1986 NDS criteria to minimize

change. However, because of the wholesale change in

design equations and calibration to only one point in

Yield Theory Behavioral

Failure Mode Equation

Ii2 D 2f Fe,,,

3.2 (2+R,) Ke

2D2

3.2 Ke

Figure 2.-Yield modes and corresponding behavioral equations for

double-shear connections with dowel-type fasteners.

T. E. McLain is professor, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061; D. G. Pollock, Jr. is director of

engineering, National Forest Products Association, Washington, DC 20036.

3
design space, there will be changes in future connection design. For lateral loading of connections, the

EYM require a dowel-bearing strength, Fe, and fastener yield Fy. The Fe is directly related to specific

gravity, which is used as an index variable for species

effects and replaces the traditional species connection

groups.

Specific Gravity

Eight species were tested in the National In-Grade

Test Program, and the average specific gravities (SC)

obtained have been adopted. For all other species, the

methods of ASTM D2555 with a lo-percent grouping

limit are proposed. These methods result in slight

changes from historic practice. The principal change

is not with the level of SG chosen for a species but

elimination of species groupings in favor of a more

continuous strength-SG relationship.

Dowel-Bearing Strength, Fe

Dowel-Bearing Strength, Fe, is determined through

a simple compression test of a dowel into a predrilled

half-hole. For solid wood, the USDA Forest Service

Products Laboratory (FPL) has obtained the relationship shown in table 1. For connections loaded at an

angle to the grain, Fe is found with Hankinson’s formula. This is more convenient than applying Hankinson’s formula when solving the connection yield

strength, and it gives essentially the same results.

Fastener Yield Strength, Fy

The yield strength of the fastener, Fy, on a &percent

nominal shank diameter (D) offset basis may be found

by standard tests or from published values. Historically, with allowable stress design, 45 ksi (310 MPa)

has been assumed as the F, of common steel bolts.

This is not contraindicated by recent research from

FPL. Research at Virginia Polytechnic Institute and

elsewhere indicates that for common wire nails, Fy =

130 ksi minus 2140 where D = nominal diameter. For

a 16-penny nail with D = 0.162 inch, Fy = 95 ksi.

MODIFICATION FACTORS

Most of the historic modification factors used to

design strength for end-use conditions remain unchanged. Exceptions in the NDS include minor adjustments to the penetration factor. The only substantive

change in the NDS is with the group action or multiple fastener factor, C,, which is now in an equation

format and also tabulated. This factor is changed from

MDS-86 to more accurately reflect the supporting research results, which show that C, is a function of connection stiffness. Consequently, C, for bolted connections and for split ring/shear plate connections is now

different.

CALIBRATION

Historically, engineered wood connections have

been safe. The NDS-91 design values were derived

from yield equations and calibrated to the safety

levels found in NDS-86. Calibration was done by a

yield mode that minimized change but did not

eliminate it. The yield equations were directly

calibrated to steel side plate connections with a lowto-intermediate range of bolt L/D ratios. The NDS-91

nail connection design values were generally increased over the NDS-86 values.

Some connection design strengths increased, and

some decreased as a result of the combined effect of all

changes. Overall, the design strength of most connections with bolts and lag screws is within plus or minus

15 percent of the NDS-86 levels. Little or no change is

seen for split ring/shear plate connections or in the

design strength of connections relying on shank

withdrawal.

Table l.-Dowel-bearing strength equations for various fastener types and grain angles

Fastener type Grain angle Dowel-bearing strength

Nails, spikes, wood screws All grain angles F = 16600 SG1.s4*

Bolts, lag screws, large dowel Parallel-to-grain F: = 11200 SG

Bolts, lag screws, large dowel Perpendicular-to-grain Fe = 6100 SG1.45D-0.5

*SG = specific gravity based on ovendry weight and volume; D = nominal shank diameter

(inches); and F,, = dowel-bearing strength (psi)

4
Eccentricities in Metal-Plate Connected Joints

Michael H. Triche

Design methods for metal-plate connected joints

have evolved rapidly over the past several years.

Specific requirements are published by the Truss

Plate Institute, Design Specifications for Metal Plate

Connected Wood Trusses, TPI-85, (1). A new specification, TPI-92, (21, will be published in the near future.

Although TPI-92 includes a number of advances in

connector plate designs and testing requirements, it

does not cover specific methodology for handling eccentricities in connections. Typically, joints are

designed such that eccentricities are very small, and

the resulting moment in the connection is negligible

compared with the direct shear. However, in certain

odd situations, connections may have to be designed

to resist moment.

The method presented here accounts for eccentricities in connections. The proposed method

determines forces on individual teeth of the metal

plate using the nonlinear load-displacement relationships of the teeth, which are determined by standard

test (2). Variations in stiffness because of varying

angles of grain and tooth orientation are considered.

Several linear models have been used in the past to

design plates for moment resistance. The most common is the polar moment of inertia approach, which

was used successfully for years in steel construction.

This method, however, is very conservative when

compared to actual experimental behavior; the

American Institute of Steel Construction currently

publishes tables for standard connections used in

steel buildings (3) that are based on a nonlinear “instantaneous center of rotation” method, not the linear

polar moment of inertia approach. The nonlinear

method predicts joint capacity with much greater accuracy, and the connections designed by this method

are considerably more economical. The key to the

method is the use of the nonlinear load-displacement

characteristics of the fasteners, which allows for the

ultimate strength of the connection to be determined

and a factor of safety to be applied to the ultimate

strength of the entire connection rather than to individual fasteners.

When a load is applied to metal-plate joints, the

load is transferred from the wood into the steel plate

by the lateral resistance of the teeth of the plate. For

a joint under axial load, each tooth of the plate is assumed to share the load equally. This assumption,

while not completely accurate, is consistent with the

design procedure used for nailed joints. Under pure

moment, however, the teeth revolve around a common

point, and the teeth farthest from the center of rotation undergo much greater movement than those near

the center.

The use of a single linear load-displacement

relationship for the teeth, which does not account for

wood grain direction or tooth orientation, results in

tooth forces directly proportional to the distance of the

tooth from the center of rotation, This is the basis of

the polar moment of inertia approach, which has often

been proposed. However, the results obtained by this

method are inaccurate. In reality, the lateral resistance of the teeth of a plate is distinctively nonlinear

and direction dependent. This nonlinearity is beneficial to joints under eccentric load because the force for

a given displacement is lower than that calculated if a

linear load-displacement relationship were assumed.

Connections must also be designed on the basis of

ultimate strength rather than allowable stresses.

Under the allowable stress approach, the design

capacity of the connection is based on the load at

which the first fastener of the connection reaches its

allowable load. In this procedure, a factor of safety is

taken on the individual fasteners and not on the

capacity of the entire joint. At these low load levels,

the fasteners are still in the linear range, and the

benefit of the nonlinear load-displacement behavior is

not realized. This approach is unnecessarily conservative. The preferred method is to determine the ultimate strength of the connection and to apply the factor of safety to the entire connection.

To illustrate the differences in the polar moment of

inertia approach and the method proposed here, five

moment splices were considered. The specimens consisted of a pair of 2 by 4’s spliced together with metal

Michael H. Triche is assistant professor, Civil Engineering Department, University of Alabama, Tuscaloosa, AL 35487.
Figure I.-Diagram of a moment splice joint; the “+“shows the location taken for the center of rotation used in the analysis

of the left connection area.

plates as shown in figure 1. Each connection was

analyzed by the two methods; plate size, orientation,

and the resulting moment capacities are shown in

table 1. For both methods, the center of rotation was

taken to be at the location of the bottom corner due to

the wood-to-wood bearing that occurred at the bottom

edge of the 2 by 4 ends.

The joints analyzed were previously tested in

another study (4); the results are shown in table 2.

Also given in table 2 are the ultimate capacities as

predicted by the proposed method, along with percent

errors. Although the proposed method of analysis underestimated the actual capacity of the joint in all but

one case, it was in general agreement with the actual

capacity. Referring back to table 1, note the linear,

polar moment of inertia method predicts considerably

less moment capacity and therefore would result in

extremely conservative designs.

In conclusion, methodology is proposed for analyzing metal-plate connections subjected to moment. The

method is based on ultimate strength and requires

load-displacement information, which is available

from standard test procedures. The method predicts

actual joint behavior much more accurately than

other commonly used linear methods. Also, this

method can be used to aid in the development of

reliability-based design procedures for wood trusses.

Table l.- Comparison of proposed method with linear method for

moment capacity of metal-plate connected joints with

varyingplate size and orientation

Plate size

Plate Proposed

orientation method

Linear

method

Inches Degrees In-lb In-lb

2.5 by 5.25 0 6,091 4,871

3.5 by 3.0 9 0 5,511 4,796

3.0 by 3.5 0 5,254 4,395

5.0 by 3.0 9 0 8,211 7,110

3.0 by 5.25 0 8,533 6,902

Table 2.-Comparison of experimental and proposed model results

for moment capacity of metal-plate connected joints with

varying plate size and orientation

Plate Experimental Predicted Percent

Plate size orientation capacity capacity error

Inches Degrees In-lb In-lb

2.5 by 5.25 0 7,689 6,091 -20

3.5 by 3.0 9 0 6,435 5,511 -14

3.0 by 3.5 0 6,798 5,254 -22

5.0 by 3.0 9 0 7,920 8,211 +4

3.0 by 5.25 0 9.454 8,533 -10

1 .

2 .

3 .

4 .

REFERENCES CITED

American Institute of Steel Construction. 1986.

Manual of steel construction, load and resistance

factor design. 1st ed. Chicago, IL: American Institute of Steel Construction. 1192 p.

Triche, M.H. 1984. Analysis and design of metal

plate connections. West Lafayette, IN: Purdue

University. 119 p. M. S. thesis.

Truss Plate Institute. 1985. Design specification

for metal-plate connected wood trusses, TPI-85.

Madison, WI: Truss Plate Institute. 47 p.

Truss Plate Institute. 1991. National design

specification for metal plate connected wood truss

construction. TPI-92; draft 3. Madison, WI: Truss

Plate Institute. 103 p.

6
Determining Failure Mechanism in Bolted

Acoustic Emissions

Joints Using

Marcia Patton-Mallory

Understanding the mechanisms by which failure

occurs in wood connections can lead to better criteria

for specifying their design load and design details.

The research described considered failure mechanisms that occur in wood connections containing steel

bolts. Failure processes were evaluated during

laboratory testing of connections. The sounds generated during the failure process-termed acoustic

emissions @+-were recorded and compared with

observed differences in failure modes.

Connections with the shorter end distance failed in

a brittle splitting mode with only minor crushing of

the wood beneath the bolt. This was the primary

failure mode for connections that had a small enough

bearing length that the bolt did not yield during the

test. The pattern of AE during these tests was a sudden increase in event rate, high amplitude events, and

high energy content of events just prior to failure

fig. 1.

Matched connections with longer end distances

failed by wood crushing beneath the bolt. In contrast

to the short end distance failure mode AE pattern,

emissions were primarily of a lower amplitude, at a

fairly constant event rate, and with an increasing

event rate at the yield point (fig. 2). Higher amplitude

events occurred only after considerable deformation

in the joint and only when small shear cracks formed

adjacent to the crushed wood.

The connection yield point was determined graphically as the intersection of a line parallel to the major

linear portion of the load-deformation curve but offset

by 5 percent of the bolt diameter. The yield point

determined in this matter corresponded closely to the

increase in AE events when bolted connections failed

in a yielding (crushing) failure mode.

This study showed that AEl patterns correlated

closely with observed brittle and ductile failure modes

in bolted connections. However, care should be taken

to minimize small knots and drying cracks because

these defects will cause unwanted AE during the test.

4,000

I

1,000

(END D ISTAN C E = 4d, vd = 2)

I

P

2; 3*ooo - 750 l

2

2

4 Irr

kI

h 2,000 - - 500 T

2

--PL

k

i:

4 1,000 - - 250 ‘,

%

2

0 I * 0

0 200 400 600

TIME (SECONDS)

Figure L-Brittle failure mode: acoustic emission increase below yield in bolted connection with a

short end distance. Note: d = bolt diameter; 1 = bolt bearing length; PL = proportional

limit; and A = acoustic emission event rate change.

Marcia Patton-Mallory is assistant director, U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO 80526.

7
4,000

(END DISTANCE = 7d, I/d = 2)

;;; 3,DDD -

s

--YIELD

a

--PL

s 2,000 -

2

4 1,000 -

0

0 200 400 600

TIME (SECONDS)

Figure 2.-Ductile failure mode: acoustic emission increase near yield in bolted connection. Note:

d = bolt diameter; 1 = bolt bearing length; PL = proportional limit; and A = acoustic

emission event rate change.

8
Strength and Stiffness of Shear Transfer Plate Connections

Ron Wolfe and Dave Bohnhoff

A common problem faced by post-frame builders is

the difficulty of getting stress-rated solid sawn 8- by

&inch timber posts. One solution to this problem is to

use posts fabricated by laminating stress-graded 2- by

&inch material. Glue-laminating may be prohibitively expensive, but mechanically laminated posts can be

fabricated at a reasonable cost. Mechanically laminated columns have the additional advantage that

they allow the builder to limit the use of treated

material to the embedded portion of the post.

One approach to mechanically laminating wood

posts is to use a double-sided connector plate called a

shear transfer plate (STP) (fig. 1). Sandwiching STPs

between adjacent layers provides needed post stiffness by limiting interlaminar shear displacement and

limits weakening effects of butt joints by transferring

shear forces around them.

Although the concept of a double-sided nail plate for

shear transfer between layers in a laminated assembly is not new, its use has been limited to custom applications. As a result, there have been few published

reports dealing with STP design properties or the

structural capacity of STP connected assemblies. To

model the behavior of mechanically laminated assemblies, data are needed to characterize the load-slip

behavior of the connections.

The objective of this study was to add to the limited

available data base on strength and stiffness of STP

connections. Variables considered include fabrication

methods, plate size and type, joint confinement, load

orientation with respect to the grain, and wood

species/specific gravity.

EXPERIMENTAL METHODS

The study was conducted in three phases, summarized in figure 2. The first phase concentrated on

the effects of plate size and fabrication method. The

second phase dealt with the effects of joint boundary

conditions. The third phase focused on effects of plate

type, wood species, and load orientation with respect

to the wood grain.

Figure L-Two sizes of shear transferplates were used in the initialphase of the study. One was a

4- by 5-inch plate containing 10 plugs per side, and the other was a 2- by 5-inch plate

containing 5plugs per side.

Ron Wolfe is a research engineer, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI 53706; Dave

Bohnhoff is a professor, Department of’Agricultura1 Engineering, University of Wisconsin, Madison, WI 53706.

9
Phase Species Plate Pressing Testa Load to Joint code

grain

Size Type

SYP

SYP

SYP

2 by 5 ONE 25-l

STAGE

4by5 GALV 1 45-1

2by5 TWO 25-2

STAGE

4by5 45-2

I

1 WOA

4by5 GALV TWO 2 AH0

STAGE 3 AL0

4 AHI

ss SYP-S-E

GALV I

SYP-G-E

ss

GALV

ss

III SPF

TWO

4by5 GALV STAGE

ss

GALV

ss

GALV

DF

ss

GALV

’ Confinement conditions:

’ No test apparatus used.

‘Apparatus used with no restraint.

3 Apparatus used with 20 psi restraining pressure.

4 Apparatus used with 40 psi restraining pressure.

SYP-S-A

II

SYP-G-A

SPF-S-E

1

I

SPF-G-E

II SPF-S-A

II SPF-G-A

I DF-S-E

I

DF-G-E

II DF-S-A

I I

DF-G-A

Figure 2.-Experimental design: Species considered included southern yellow pine (SYP), sprucepine-fir (SPF), and Douglas-fir (DF); plates include stainless steel (SS) and galvanized

steel (GALV) 20-gauge shear transfer plates with one four-tooth roqette (plug) per square

inch. Twenty-two joints were tested for each configuration. Joint code: S=stainless steel,

G=galvanized steel, E=perpendicular, A=parallel.

The control joints referenced in each phase were

fabricated using the two-stage pressing method,

southern pine lumber, and 20-gauge, 4- by &inch galvanized metal STPs. Each joint was composed of three

parallel laminations, connected using two STPs.

Load, applied perpendicular to the grain of the middle

member, was transferred through the STPs to the

outer members, which were supported on the bed of

the test machine. In two-stage pressing, plates were

completely pressed into both sides of the center member, using special steel pressing plates that fit over

the STP. The outside pieces of wood were then positioned over the STP and pressed into place. In onestage pressing, STPs were positioned between the

wood members, and the four wood-plate interfaces

were pressed simultaneously.

SUMMARY OF TEST RESULTS

The control joint had an average maximum load

capacity of 455 pounds per plug with a coefficient of

variation of only 8 percent. For design purposes, the

average load at 0.015 inches slip was 390 pounds per

plug with a coefficient of variation of 9 percent. Test

10
results suggest that these values are significantly affected by fabrication method, wood specific gravity,

plate size, and joint boundary conditions. No significant effect was noted for load-to-grain orientation

or for galvanized versus stainless steel STPs.

Joints fabricated using single-stage pressing

methods required greater total fabrication energy and

had less stiffness and strength than those fabricated

using the two-stage process. For obtaining joints of

comparable appearance, fabrication pressure requirements for single-stage pressing were on the order of

25 percent greater for single-stage pressing than for

two-stage pressing. The single-stage 4- by 5-inch plate

joints were only 75 percent as stiff and 85 percent as

strong as the two-stage joints. For the 2- by 5-inch

plates, this effect on joint stiffness was slightly less

(89 percent); but strength effect was the same.

Joint code Stiffhess Strength

lb/in Rel lb/plug ReI

II-WOA 50790 1.05 449 0.99

Control III-HP-G-E 50641 1.05 470 1.08

146-2 42628 .90 447 .96

Total 46241 1.00 465 1.00

Species III DF-G-E 41022 .55 448 .98

III SPF-G-E 28582 .59 359 .79

Pressing I 46-l 35975 .75 381 .84

Orientation III SYP-G-A 34708 .72 472 1.04

Plate Type III SYP-G-E 31449 .55 511 1.12

Plate Size I 26-2 29776 .62 558 1.28

IIANO 25122 52 266 58

Confinement IIALO 25364 53 440 .97

IIAMI 24601 51 484 1.06

Species/Plate Type III DF6-E 82667 58 452 .99

III SPF-8-E 18536 .59 360 .79

Species/Orientation III DFG-A 32241 .57 300 1.10

III SPF-G-A 13352 58 3so .88

Orientation/Plate. III SYP-G-A 25965 .54 488 1.07

Species-Orient-Plate III DF-G-A 25537 .53 501 1.10

III SPF-O-A 11467 .24 337 .74

Pressing and Plate Size I 26-1 26682 .55 402 1.08

Figure 3.-Relative values of average joint load capacity. The stiffness values given were derived as

the stiffness (K) parameter derived on a per-plug basis to fit the Foschi nail-plate model.

Strength values represent the maximum load for the joint uniformly distributed to the

shear transfer plate plugs.

11
3 600

HIGH LATERAL

6s 500 - PRESSURE (dtil)

WITHOUT TESTING

B3 APPARATUS (WOA)

400-

PRESSURE (ANO)

0.02 0.03

SLIP (INCHES)

Figure 4.-Confinement effect on joint performance. Joints tested in

the confinement apparatus with no lateral pressure had

no external forces acting to resist lateral movement.

Results of tests conducted to evaluate plate size effect are inconclusive. The 2- by &inch plate size appeared to give a per-plug load capacity 25 percent

greater than that of the 4- by &inch plate. This difference was significant at the go-percent confidence

level. Differing modes of failure, however, suggest

that this difference in strength may have been partially influenced by the size of the joint wood members

relative to the size of the STPs used. For the 4- by 5

inch plate joints, wood deformation caused by bearing

stresses influenced joint strength.

Joint strength and stiffness are directly related to

specific gravity. Relative values of average joint load

capacity (fig. 3) measured at incremental displacements were fairly constant over the range from 0.005

to 0.05 inch and roughly equal to average relative

specific gravity values. Differences between the

southern pine and Douglas-fir joint strengths were

not significant at the go-percent confidence level, but

the southern pine joints were significantly stronger

than the spruce-pine-fir joints.

Test boundary conditions have a major effect on

failure mode and joint strength (fig.4). Joints tested

with no lateral restraint had a failure load 55 percent

less than those tested with some form of restraint.

Joint strength was not improved significantly by substituting lateral pressure for bearing friction or by increasing lateral pressure beyond 20 psi.

CONCLUSIONS

Our research on strength and stiffness of STP connections led to the following conclusions:

l The two-stage method of joint fabrication is

preferable to single-stage pressing.

l Further study is needed to verify/quantify a plate

size effect. Additional studies should consider

wood member size relative to STP size as a variable.

l Results affirm a strong and direct relationship

between joint strength and member specific

gravity.

l The effect of load orientation with respect to

wood grain direction appears to be species dependent. Although it was not significant at the

go-percent confidence level, the effect of load

orientation was most noticeable for Douglas-fir

joints.

l Further study is needed to relate test boundary

conditions to in-use conditions.

12
Mechanical Connections: The Weak Link in Light-Frame

Wood Construction

Gary J. Klein

Metal plate-connected trusses are one of the most

widely used structural systems in residential and

light commercial construction. When properly designed and built, plate-connected trusses can be expected to perform very well under most conditions.

In spite of advances in quality control, poorly installed connector plates often result in trusses that

are much weaker than intended. Among the most

common problems are mislocation and partial embedment of connector plates. These and other problems

with truss connections are described in this paper.

Methods for repairing deficient joints are also

presented.

CONNECTOR PLATE MISLOCATION

Connector plates are sometimes mislocated during

fabrication, resulting in a significant reduction in the

capacity of the connection. Often the contact area of

the connector plate on the diagonal web member is

less than required to develop the computed diagonal

force at service loads (fig. 1).

Sometimes individual plates are mislocated. This

problem is particularly likely where very small plates

are used. On the other hand, plate mislocation can be

systematic. Generally, this condition is not apparent

until the shear stresses between the connector plates

and wood members are computed.

Related deficiencies include connector plates that

are missing, improperly oriented (fig. 21, or undersized. Also, wane or knots in the joint region can significantly diminish the capacity of the connection

(figs. 3,4X These problems can result in joint failures

and sometimes collapse, although it has been my experience that wood truss systems have a remarkable

ability to hang together in spite of numerous connection failures.

Figure L-Pull-out of overstressed connectorplate.

Gary J. Klein is a consultant at Wiss, Janney, Elstner Associates, Inc., Northbrook, IL 60062.

13
Figure 2.-Pull-out of misoriented plate.

Figure 3.-Wane in joint region.

14
Figure 4.-Diagonal failure at knott below connector plate. Note that plate does not cover the entire

joint, concentrating the diagonal force at the knot.

PARTIAL PLATE EMBEDMENT

Often connector plates are not properly pressed

when trusses are fabricated, or the plates tend to

“walk out” as wood dries during service (fig. 5). Partial

embedment can also occur because of a mismatch in

member thickness or width (fig. 6). The latter problem

is particularly common on 2- by 4-inch parallel chord

trusses.

Partial embedment is characterized by a uniform

gap between the connector plate and the wood member. Joints with partially embedded plates are much

weaker than joints with fully embedded plates. The

gap causes weak axis bending of the teeth. The problem is particularly severe for 20-gauge plates loaded

along the length of the plate. A limited research program has shown that a Vl6-inch gap below a 20-gauge

plate reduced the joint strength by roughly 50 percent

(fig. 7).

REPAIRS

Joint deficiencies can be repaired with plywood gussets attached to one or both sides of the truss using

thru-bolts or lag bolts. A limited testing program was

conducted to verify the effectiveness of %-inch

plywood gussets (APA B-B Plyform, class 1) connected

to southern pine 2 by 4’s using %-inch bolts.

Recommended design values are as follows:

Suggested

Connection Type design value

%-inch plywood each side (double shear) 900 lb

and %-inch diameter thru-bolts per bolt

%-inch plywood one side (single shear) 350 lb

and !&inch diameter lag bolts per bolt

These values are based on a proportional limit load

and allow for time effects and a safety factor. A reduction per the National Design Specification (ND@ for

multiple fasteners in a row should be considered, although the test results did not show an apparent

strength reduction for up to three thru-bolts and six

lag bolts in a row.

A hydraulic C-clamp can also be used for a variety

of repairs. The most frequently used C-clamp repair is

to repress partially embedded connector plates (fig. 8).

The C-clamp can also be used to install new connector

plates at truss joints following shoring (if required)

and removal of the existing plates.

15
Figure L-Partially embedded connector plate.

Figure &-Pull-out ofpartially embedded connectorplate due to mismatch of 2 x 4 width.

1 6
\

\

\

0

0

20 GA. PLATE

316 IN. TEETH

0 - “A” SPECIMENS

0 - “8” SPECIMENS

s

y 60-

k

\

\

0

l

\

0 0.02 0.04 0.06 0.06 0.10 0.12 0.14

PLAT E GAP ( INCHES )

Figure ‘7.-Relative joint strength versusplategap for a 20-gauge plate with s/s-inch teeth, where 0

and l represent specimen data from separate sample sets.

Figure C-Field repressing ofpartially embedded connector plates.

1 7
CLOSING REMARKS

The kinds of problems described are becoming less

frequent. In recent years, the plate-connected truss

industry has made very significant advances in establishing and enforcing quality. It is hoped that these

advances continue and that ongoing research and development will strengthen the reliability of mechanical connections in light-frame wood structures.

18
Groom, Leslie H. 1993. 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. Gen. Tech. Rep. SO-92. New Orleans, LA. U.S. Department of Agriculture, Forest Service,

Southern Forest Experiment Station. 18 p.

Articles based on the five presentations at the mechanical fasteners plenary session of the 1991 Forest Products Research

Society annual meeting are given. The information presented is

aimed at designers of light-frame wood structures and researchers

of mechanical properties of fasteners.
Persons of any race, color, national origin, sex, age, religion, or with any

handicapping condition are welcome to use and enjoy all facilities, programs,

and services of the USDA. Discrimination in any form is strictly against agency

policy, and should be reported to the Secretary of Agriculture, Washington, DC

20250.

"U.S. GOVERNMENT PRINTING OFFICE: 1993-766-017/80009