United StatesDepartment ofAgriculture

Forest Service

ForestProductsLaboratory

ResearchPaperFPL-RP-503

Bending Propertiesof Reinforced andUnreinforced SplicedNail-Laminated PostsDavid R. BohnhoffRussell C. MoodySteve P. VerrillLeo F. Shirek

Abstract

Efficient design of post-frame wood buildings requiresaccurate information on the properties of the nail-laminated posts. To characterize structural post prop-erties, 140 three-layer nail-laminated posts were testedto failure in bending. Of these assemblies, 28 containedno splices, 56 contained unreinforced splices, and theremaining 56 contained splices reinforced on the out-side butt joints. Spliced posts were fabricated withgun-driven or machine-driven nails. In addition to theposts, 56 individual pieces of lumber were loaded tofailure in bending. The average strength (modulus ofrupture) of the single members was found to be higherthan that of the unspliced posts. However, the 5th per-centile of strength for the unspliced posts, which is thebasis for design, was significantly higher than that forthe single members. Joint reinforcement significantlyincreased bending strength and stiffness of splicedposts. However, even with butt joint reinforcement,spliced posts had substantially lower bending strengthand stiffness than did unspliced posts.

August 1991

Bohnhoff, David R.; Moody, Russell C.; Verrill, Steve P.;Shirek, Leo F. 1991. Bending properties of reinforced andunreinforced spliced nail-laminated pests. Res. Pap. FPL-RP-503. Madison, WI: U.S. Department of Agriculture,Forest Service, Forest Products Laboratory. 24 p.

A limited number of free copies of this publication areavailable to the public from the Forest Products Labo-ratory, One Gifford Pinchot Drive, Madison, WI 53705-2398. Laboratory publications are sent to more than 1,000libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in coopera-tion with the University of Wisconsin.

Acknowledgments

This research was supported by the College of Agricul-tural and Life Sciences, University of Wisconsin, Madi-son; the U.S. Department of Agriculture, Forest Ser-vice, Forest Products Laboratory, Madison, Wisconsin;Wick Building Systems Inc., Mazomanie, Wisconsin;and Alpine Engineered Products, Inc., Pompano Beach,Florida.

Bending Properties of Reinforced andUnreinforced Spliced Nail-Laminated Posts

David R. Bohnhoff, Assistant ProfessorAgricultural Engineering Department, University of Wisconsin, Madison, Wisconsin

Russell C. Moody, Supervisory Research General EngineerSteve P. Verrill, Mathematical StatisticianForest Products Laboratory, Madison, Wisconsin

Leo F. Shirek, Product Engineer ManagerWick Building Systems, Inc., Mazomanie, Wisconsin

Introduction

The basic structural components of a post-frame woodbuilding include wood posts, trusses, girts, and purlins(Fig. 1). An individual truss and the posts to which itis connected are referred to as a post-frame. Each post-frame receives its lateral support and loads from girtsand purlins. Girts and purlins are laterally supportedby the exterior sheathing (generally metal panels), andwith the sheathing they form large wall and roof di-aphragms. These diaphragms add considerable rigidityto the building.

Today, builders of post-frame wood structures competewith builders of “pre-engineered” low-rise, steel-framestructures. In many respects, both building systemsare identical. The only essential difference is the ma-terial used for the frame, However, with respect to firecodes, this is a major difference, and it generally deter-mines under what conditions a wood post-frame build-ing will be economically competitive with a steel-framestructure.

The wood posts make the post-frame building unique.Although the posts may be bolted to the top of a con-crete frost wall or floating slab foundation, they aregenerally embedded in the soil. When embedded, theposts transfer load directly to the soil and thereby func-tion as the foundation for the building. When designingposts, engineers are most interested in their bendingcapacity: under the load combinations that typicallycontrol post design, usually at least 75 percent and of-ten more than 85 percent of the maximum fiber stressin the post is due to the applied bending moment.

A recent trend has moved the post-frame building in-dustry away from the use of solid-sawn wall posts to-ward the use of nail-laminated posts. This trend canbe attributed to the high cost and scarcity of long,stress-rated, solid-sawn, preservative-treated posts.This trend can be expected to continue as (1) a greaternumber of taller structures are built, (2) long timberbecomes increasingly more expensive and difficult toobtain, (3) the cost of preservative-treated lumber in-creases, and (4) an economic advantage exists for usinglaminated members when posts exceed a certain length(see Fig. 2 and App. A).

Nail-laminated posts fall into two main categories: un-spliced and spliced. Unspliced posts are those assem-blies in which each layer consists of only one piece ofdimension lumber. Spliced posts are those assemblies inwhich at least one layer contains two or more pieces ofdimension lumber butt-jointed together. Spliced postscan be classified further by whether or not the buttjoints are reinforced.

Both spliced and unspliced nail-laminated posts aredesigned to resist loads applied parallel to the interlayerplanes or wide faces of the layers. Posts designed toresist loads in this direction are referred to as verticallylaminated. Nail-laminated posts are seldom designedto resist loads applied normal to the interlayer planesbecause the strength in this direction is only a fractionof that for bending about the strong axis. Thus, postsmust be provided with adequate lateral support. Whenspliced nail-laminated posts are not laterally supported,loads applied normal to the interlayer planes tend tocause delamination of the assembly in the joint area.

Figure 1—Framing for a post-frame building.

A commonly used spliced-post design is a three-layer(trilaminated) assembly fabricated from six pieces ofnominal 2- by 6-in. (38- by 140-mm) lumber. Thethree butt joints are normally staggered and spacedeither 18 or 24 in. (0.5 or 0.8 m) apart for an overallsplice length of 3 or 4 ft (0.9 or 1.2 m), respectively(Fig. 3). The general objective of this report is toprovide information that will assist in developing de-sign bending properties for nail-laminated posts of thistype.

Figure 2—Relative cost of solid-sawn and nail-laminated Southern Pine posts.

Background

Unspliced Beams

Only a few studies have been conducted on the ef-fect of number of layers on strength properties ofvertically laminated beams without butt joints.Bonnickson and Suddarth (1966) showed that three-layer nail-laminated bending specimens had about thesame average strength as that of single members butsignificantly reduced variability for a Standard andbetter grade of green Douglas Fir. Wolfe and Moody(1979) found similar results for glued specimens of thehigher grades of lumber but also found a slight increasein strength of lower grades. Sexsmith and others (1979)studied 6-, 12-, and 18-layer specimens held togetherwith a transverse stressing technique. They found thatallowable properties for the multiple layer assemblieswere significantly higher than those for single members

Overall, research on unspliced beams indicates thatmultiple-layer members, compared to single members,have the following properties:

1. similar mean strength for high grades and somewhatincreased mean strength for lower grades of lumber;

2. similar mean stiffness regardless of grade;

3. reduced variability in both strength and stiffness,with coefficient of variation decreased inversely assquare root of number of layers; and

4. significantly higher increases in lower 5th percentilevalues than are presently recognized by the 15-percent “load sharing” factor from the National De-sign Specification (NFPA 1988).

2

Figure 3—Spliced post design. Nailing pattern taken from Bohnhoff (1989).

Spliced Beams

Test results for three-layer, nail-laminated assem-bly designs with staggered butt joints that havebeen tested and reported to date are compiled inTable 1. Although this table includes only 10 differentdesigns, the data gathered from these tests have beenquite valuable; they have significantly increased our un-derstanding of spliced nail-laminated assembly proper-ties. Specifically, the tests have shown that (1) strengthproperties are related to lumber grade, (2) 20d (4.5-by 102-mm) ring-shank nails can be used if properlylocated, and (3) maintenance of an adequate overallsplice length is extremely important.

The program FEAST was developed to learn moreabout the strengths and weaknesses of various nail-laminated assembly designs (Bohnhoff 1988, Bohnhoffand others 1989). FEAST is a finite element methodof analysis program for analyzing vertically and me-chanically laminated assemblies. Bohnhoff (1989) mod-eled three-layer, nail-laminated assemblies with stag-gered joints and no reinforcement using this analyticalmethod. He concluded the following:

1. Without additional butt joint reinforcement, anoverall splice length of 2 ft (0.6 m) is too short toeffectively redistribute forces in the immediate vicin-ity of a butt joint. An overall splice length of atleast 3 ft (0.9 m) appears necessary for a three-layerassembly.

2. High localized stresses in the wood layers cannot beeffectively decreased by increasing nail density.

3. Regardless of splice length and individual woodmember modulus of elasticity (MOE) values, thelongest wood member in the center layer is almostalways the most highly stressed member.

4. Regardless of splice length and individual woodmember MOE values, the most highly stressed nailsare always located adjacent to a butt joint in anouter layer.

5. Increasing nail density decreases maximum nailshear forces. However, it is not known to what levelnail density can be increased without having an ad-verse effect on the ultimate strength of an assem-bly (that is, without increasing the number of nail-related or nail-induced assembly failures).

On the basis of conclusions 3 and 4, two rational al-ternatives for increasing the strength of the assemblieswould be (1) to increase the grade of the longest mem-ber in the center layer because this member-is the mosthighly stressed and (2) to reinforce the outside buttjoints. By reinforcing only the outside joints, less loadis channeled into the center layer and stresses in thecenter layer are reduced. Reinforcing the joints also re-duces the amount of interlayer slip at the joints, thuslowering nail forces in the vicinity of the joints. Assum-ing that reinforcing the outside butt joints increasesassembly strength, it is not surprising that of the 10designs in Table 1, design 7 was associated with thehighest mean ultimate strength. Design 7 not onlyfeatured an overall splice length of 4 ft (1.2 m) butalso was the only design tested that featured No. 1 orbetter lumber in combination with outside butt jointreinforcement.

3

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Although laboratory tests and modeling indicate thatreinforcing outside butt joints should increase assem-bly strength, the magnitude of such an increase in as-semblies fabricated with No. 1 lumber is not known.Woeste and others (1988) did not test unreinforced as-semblies of No. 1 or better lumber. Although Bohn-hoff (1988) did test unreinforced specimens (designs 8,9, and 10), the work of Bohnhoff and that of Woesteand others cannot be compared for the following rea-sons: (1) different nails and nailing patterns were used,(2) lumber came from different. “batches” and there isno good way to account for differences in lumber prop-erties, and (3) test conditions, such as loading rates,specimen conditioning, and lateral constraints, were notthe same.

As Figure 2 shows, the addition of splice plates to apost increases the material cost of the assembly byabout 10 percent (see App. A). Before this cost can bejustified, the structural benefit of additional reinforce-ment must be ascertained.

There is also a need to determine the “design strength”of assemblies fabricated using the nailing pattern shownin Figure 3. This pattern was recommended by Bohn-hoff (1989) for assemblies fabricated using nails equiva-lent in size to a 20d (4.5 by 102-mm) ring shank. Thepattern is a hybrid of the nailing patterns associatedwith designs 9 and 10 in Table 1. The relatively highultimate strengths associated with designs 9 and 10demonstrated that nails equivalent in size to 20d (4.5by 102-mm) ring shanks could be used effectively inlaminated assemblies. In the majority of the tests con-ducted to date, nails no larger than a 12d (3.8- by 76-mm) common nail have been used. There is a majordisadvantage to using the smaller nails: typically, twiceas many nails are needed per assembly.

Teat specimens used to determine allowable designproperties for spliced posts are generally fabricatedwith all untreated wood or with preservative-treatedwood on one end of the post and untreated wood onthe other end. Regardless of which arrangement is usedto establish design properties, a designer may want tointerchange treated and untreated wood for a partic-ular field application. Such interchanging is not likelyto affect post strength and stiffness if the strength andstiffness of the untreated and preservative-treated woodare not significantly different. When the design proper-ties of the treated and untreated lumber are different,additional laboratory testing may be required to de-termine the strength and stiffness of the new design.In teats with No. 1 Southern Pine lumber, Winandyand Boone (1988) found relatively little difference be-tween the strength of untreated and treated lumberwhen preservative retentions were limited to 0.6 lb/ft3

(9.6 kg/m3) and redrying temperature was limited to190°F (88°C).

Design Properties

A major difference between spliced and unspliced nail-laminated posts lies in the procedure used to determinethe allowable bending capacity of the assemblies. Thebending strength of unspliced posts is almost alwayscalculated using the “repetitive member use” bendingstress values listed in the National Design Specifica-tions (NDS) for Wood Construction (NFPA 1986). Therepetitive member use values are 15 percent greaterthan the single member design values established ac-cording to ASTM Standard D 245. This standard per-mits the use of repetitive member use values any time“three or more load-carrying members such as joists,rafters, studs, or decking are contiguous or are spacednot more than 24 in. (0.6 m) in frame construction andare joined by transverse floor, roof, or other load dis-tributing elements” (ASTM 1989a). Nail-laminatedposts fall into this category if the nails connecting thelayers are categorized as load-distributing elements. Be-cause relatively little interlayer slip occurs in unsplicedposts, the size, type, and location of nails have virtuallyno influence on the strength of the assemblies.

The strength and stiffness of spliced nail-laminatedposts, unlike those of unspliced posts, are highly depen-dent on nail type, size, and location as well as the rela-tive location of the butt joint or joints in each layer andthe type, amount, and location of butt joint reinforce-ment (when such reinforcement is used). Because ofthe complex interaction of these variables, the strengththeof spliced posts is currently determined by laboratorytests of a representative sample of actual assemblies.A two-point load, applied in accordance with ASTMStandard D 198 (ASTM 1989b) is commonly used toestablish the ultimate bending strength of each postTo arrive at an allowable bending moment for designthe 5th percentile of the distribution of the ultimatebending moment for all sample posts tested is dividedby 2.1, which is a product of a load duration factor of1.6 and a traditional safety factor of 1.3 (ASTM 1989b;Hoyle and Woeste 1989). The load duration factoris used to adjust the strength determined in a 5-mintest to that expected under a load with-a duration of10 years.

Because different procedures are used to arrive at theallowable bending moment values for spliced and un-spliced posts, a design value for a spliced post withbutt joint reinforcement can be found that is greaterthan that calculated for an unspliced post fabricatedusing the same size, grade, and species of lumberThis leads to confusion because the designer is led toconclude that the spliced region of the post is

5

stronger than the unspliced region. Because of the rel-atively light reinforcement currently used in splicednail-laminated posts, this is seldom, if ever, true. Thebottom line is that if the same procedure used to deter-mine the allowable bending moment for spliced postswere used for unspliced posts, the allowable bendingstress for unspliced posts would, in most instances, behigher than that based on published allowable stressvalues.

Research Needs

To date, laboratory tests comparing properties ofspliced and unspliced assemblies have not been con-ducted. Consequently, the true reduction in strengthassociated with the addition of butt joints has neverbeen accurately presented. In addition, only Winistor-fer and others (1987) have conducted side-by-side testson spliced posts with and without butt joint reinforce-ment, Inasmuch as these tests involved only five spec-imens of each design, more testing is needed on bothreinforced and unreinforced joints. Such test data areneeded to determine in what applications joint rein-forcement would be economical.

Objectives

The specific objectives of the research were as follows:

1. to determine how bending strength and stiffness ofsingle members of dimension lumber are related tobending strength and stiffness of three-layer, un-spliced nail-laminated posts;

2. to compare bending strength and stiffness of threetypes of three-layer nail-laminated posts: (a) un-spliced, (b) spliced without butt joint reinforcement,and (c) spliced with butt joint reinforcement; and

3. to determine the effect of nail type on bendingstrength and stiffness of three-layer, spliced nail-laminated posts.

Methods

Experimental Design

The experimental design for the single-member testsand three-layer nail-laminated post tests is described inTable 2. As indicated in the table, five different postdesigns were tested: one unspliced design and fourspliced designs. In addition to the 140 posts, fifty-six 12-ft (3.76-m) nominal 2- by 6-in. (standard 38-by 140-mm) pieces of lumber were loaded to failurein bending. The single-member tests were conductedto characterize the properties (stiffness and ultimate

6

Table 2—Experimental design

Board andnail type

Numbertested

Butt jointreinforcement

Single member 56 NAUnspliced posts

Machine driven 28 NASpliced posts

Gun driven 28 No28 Yes

Machine driven 28 No28 Yes

strength) of the lumber used to fabricate the laminatedposts.

The sample size of 28 was suggested by Woeste (per-sonal communication) and based on ASTM D 2915 re-quirements for estimating the 5-percent nonparametrictolerance limit (75 percent confidence level)1989c). If the test data are normally distributed witha coefficient of variation of 20 percent and there is nointeraction between nail type and butt joint reinforce-ment, then this sample size is large enough to permitone to detect a 10-percent difference in means at a 0.05significance level with 0.75 probability.

Nail-Laminated Post Designs

Member designs for spliced and unspliced posts are il-lustrated in Figures 3 and 4, respectively. All postswere three-layer assemblies, 12 ft (3.76 m) in length,which were fabricated from nominal 2- by 6-in. (stan-dard 38- by 140-mm) No. 1 dense (hereafter calledNo. 1D) KD15 Southern Pine lumber. As shown inFigure 3, the three butt joints in the spliced posts werestaggered and spaced 24 in. (0.6 m) apart for an over-all splice length of 4 ft (1.2 m). All spliced posts werefabricated from three pieces of treated lumber andthree pieces of untreated lumber. We originally consid-ered fabricating the assemblies entirely from untreatedwood. The decision to use treated wood on one end ofeach spliced post was based on the premise that preser-vative treatment is more likely to decrease rather thanincrease lumber strength. Thus, using some treatedwood should produce a more conservative estimate ofpost strength than using all untreated wood.

The nailing pattern for the spliced posts was designedby Bohnhoff (1989) to be used in conjunction witha nail similar in type and length to a 20d (4.5- by102-mm) ring shank. As shown in Table 2, two dif-ferent nails were used in the assembly of the lami-nated posts. The first type was a gun-driven ring-shank nail, 0.145 in. (3.68 mm) in diameter and 4 in.

Figure 4—Unspliced post design.

(102 mm) in length. This nail was used in the fabrica-tion of one-half the spliced posts. The other half of thespliced posts and all the unspliced posts were fabricatedusing a autonailing machine. This machine drove andcut a spirally threaded wire (outside diameter 0.187 in.(4.8 mm)) to a 4.5-in. (115-mm) length.

High strength, 20-gauge (0.9-mm) toothed metal plateconnectors with a width of 5.25 in. (133 mm) andlength of 8.75 in. (222 mm) were applied to the outsidebutt joints of those spliced posts designated for rein-forcement. The plates had a tension yield stress ratingof 60,000 lb/in2 (417 MPa).

Lumber Allocation

A total of 440 pieces of lumber 16 ft (4.98 m) in lengthwere obtained for the study. Of these pieces, 146 wererandomly selected and treated with chromated cop-per arsenate (CCA) to a retention level of 0.6 lb/in2

(9.6 kg/m3). Both the treated and untreated pieceswere conditioned to an equilibrium moisture contentof 12 percent. Each piece was then numbered andweighed, and the MOE value was determined usinga flatwise vibration technique. Prom the 146 treatedpieces, 140 were selected for the experiment. Similarly,280 untreated pieces were selected for the experiment.

The lumber was divided into six groups: a group foreach of the five post designs (one unspliced and fourspliced) and a group for the single-member tests. Lum-ber allocation was designed so that each group con-sisted of wood with a similar MOE distribution. An-other goal of the allocation process was to apportionthe lumber so that 28 matched sets were created for thefour spliced post designs. The allocation process was asfollows:

The 280 untreated boards were first ranked by MOE.An adjacent pair of boards was selected from the 10boards with the lowest MOE values. These boards werecut in half to produce four 8-ft (2.4-m) boards. A two-step procedure was used to allocate the boards into theteat groups:

1. A set (or replicate) number between 1 and 28 wasselected.

2. The boards were randomly distributed to the fourspliced test groups within the selected set.

Another adjacent pair of untreated boards was thenrandomly selected from the eight remaining boards andcut into four 6-ft (1.8-m) lengths (plus two “excess”pieces 4 ft (1.2 m) in length). These boards were thenassigned to a randomly selected matched set of four as-semblies using the preceding two-step process. Anotherboard from the original group of 10 boards was thenrandomly selected, cut into four 4-ft (1.2-m) pieces, andrandomly assigned to one of the 28 matched sets.

The five remaining boards were cut to a length of 12 ft(3.7 m). Two of these boards were assigned to thesingle-member test group, and the remaining boardswere randomly assigned to the unspliced post testgroup (see Table 2 for experimental design).

This procedure for allocating lumber was repeated forthe remaining 27 groups of 10 boards.

The 140 treated boards were also ranked by their MOEvalues and then divided into 28 groups of five boardseach. The five boards in each group were assigned tothe spliced post test group by the same method usedfor assigning the untreated wood to test groups.

Specimen Fabrication

Posts manufactured with gun-driven nails were assem-bled in a conditioning room; those manufactured withmachine-driven nails were assembled at a manufactur-ing facility and then returned to the conditioning room.None of the assemblies was tested until 2 weeks afterfabrication.

Problems were encountered when driving both typesof nails. Because of the high density of some theSouthern Pine lumber, the nail gun did not completelydrive all the 4-in.- (102-mm-) long nails. This prob-lem was considerably reduced after the air pressure

7

Figure 5—Problem associated with machine-driven nails. Nails deflected (A) outward and(B) inward by denser latewood.

to the gun was increased to 140 lb/in2 (965 kPa),which is 20 lb/in2 (138 kPa) beyond the recommendedmaximum safe operating pressure. Where nails wereunderdriven, attempts were made to finish driving thenails by hand. However, because of the ductility ofthe gun-driven nails, these attempts were not alwayssuccessful.

The problem associated with driving the machine-driven nails is illustrated in Figure 5. The nails tendedto follow the path of least resistance as they were be-ing driven. In several cases, the nails were deflectedby the dense latewood. Thus, the nails followed an-nual rings, staying in the earlywood until they exitedfrom the assembly (Fig. 5A). This problem could bevirtually eliminated by turning boards so that an-nual rings formed arches instead of troughs (Fig. 5B).With this arrangement, nail points were deflectedtoward the center of the assembly. Because therewas no guarantee that the orientation of the lum-ber would be controlled during future productionof nail-driven posts, no attempt was made to con-trol lumber orientation during fabrication of the testspecimens.

Although reinforcing plates can be pressed into placebefore individual layers are nailed together, all platesused in this study were pressed into their respectiveassemblies after the layers had been nailed together.

Testing Procedures

The MOR and MOE for each single member were de-termined according to ASTM D 198 (ASTM 1989b).The two-point load arrangement shown in Figure 6was used in combination with a loading rate of0.30 in/mm (7.62 mm/min). To measure midspandeflection, a spring-tensioned wire was drawn be-tween nails driven into the centroidal axis of the

8

member directly above the supports. The relativedisplacement between the wire and the member atmidspan was measured by clamping a linear variabledifferential transformer (LVDT) to the specimen atmidspan and hooking the core of the LVDT to thewire. To avoid damage to the LVDT, it was removedonce the total deflection reached 2 in. (51 mm). Acomputer-based data acquisition system was usedto record midspan deflection and load data at 2-sintervals.

Specimens from the five different nail-laminated assem-bly groups were randomly selected for test. Whereapplicable, ASTM D 198 (ASTM 1989b) was fol-lowed. The load-head rate was fixed at 0.40 in/min(10 mm/min) for all tests. The location of the loadpoints, support reactions, and points of lateral supportfor all laminated assembly teats are shown in Figure 6.Four single-turn potentiometers were used to measurethe deflection of the outside laminae at the load points.A computer-based data acquisition system was usedto record load-point deflection and load data at 2-sintervals.

Results

Lumber Properties

Lumber properties for the CCA-treated lumber, un-treated lumber, and both groups combined are com-piled in Table 3. The table lists mean values and cor-responding coefficients of variation for both dynamicMOE and specific gravity.

Single-Member and Post Properties

The MOR distribution characteristics for single mem-bers and unspliced posts are compared in Table 4(see App. B for individual data); the MOR distri-butions are shown in Figure 7. The mean MOE val-ues for single members and unspliced posts were2.48 × 106 lb/in2 (17.1 GPa) and 2.45 × 106 lb/in2

(16.9 GPa), respectively. Corresponding coefficientsof variation were 20.6 and 9.23 percent. The ra-tio of post to single-member values for MOE was0.99.

Distribution characteristics for ultimate midspan bend-ing moment and initial stiffness are shown in Table 5for unspliced and spliced posts (see App. B for individ-ual data). Values in Table 5 are presented in terms ofultimate moment resistance and stiffness rather thanMOR or MOE. For spliced posts, MOR and MOE haveno physical meaning because of the complex stressdistributions in the assemblies. However, moment

Figure 6—Location of bad points, support reactions, and points of lateral support for all laminated

assembly tests.

resistance and stiffness are realistic bases for comparingpost types.

Failure Types

Tables 4 and 5 both contain parametric and non-parametric estimates of the 5th percentiles for bend-ing strength as well as a lower 75-percent confi-dence bound on the 5th percentile (referred toas the 5-percent tolerance limit). The Shapiro-Wilk test only rejected the normal distribu-tion for the MOR of the single members. Forthis particular case, the lognormal distributionfit well.

Table 6 summarizes post properties that were sig-nificantly different at the 5-percent level. Lev-els of significance for various analyses are given inAppendix C. As Table 6 indicates, spliced postshad significantly lower strength and stiffness prop-erties than those of unspliced posts. The mag-nitude of the differences is shown in Table 7,which contains ratios of spliced to unspliced postproperties. Reinforced posts were also signifi-cantly stronger and stiffer than unreinforced posts(Table 6); ratios of these properties are shown inTable 7. Table 6 also indicates that posts made withgun- and machine-driven nails did not have significantly.

All specimens were examined after the bending teststo characterize the probable types of failure. For thesingle members, failure types were as follows:

Failure type Percentage of failure

Tension at grade-controllingknots or slope-of-grain 15

Splintering tension in clear 56wood or from small knots

Compression wrinkling at or 29between load points

Because the middle layers of the three-layer unsplicedposts could not be readily examined, their failures weremore difficult to characterise. However, the failures ap-peared to closely parallel the failure types of the singlemembers.

For spliced posts, wood and nail joint failures were cat-egorized into 13 types as shown in Figure 9. Although

different strength and stiffness properties. Thus, re-sults of the two nail types are combined in Table 7 (last

the diagrams do not provide information on the type ofwood failure, they help identify regions of high force or

column) and Figure 8. Figure 8 compares ultimate stress. In general, wood failures that occurred at mem-midspan bending moment of unspliced posts, spliced ber ends were parallel-to-grain splits, and those thatposts with reinforcement, and spliced posts without occurred away from member ends were parallel-to-grainreinforcement. tension failures or, in some instances, tension failures

resulting from knots and/or slope-of-grain. In rein-forced posts, the metal plate connectors failed beforemaximum load was reached, in almost all cases.

9

Table 3—Lumber properties

SpecificModulus of elasticity b

gravity c

NumberLumber of

Mean(×106 lb/in2 COV COV

type a pieces (GPa)) (percent) Mean (percent)

Treated 140 2.35 (16.2) 17.217.9

0.63 9.4Untreated 280 2.31 (15.9) 0.62 9.0Combined 420 2.32 (16.0) 17.7 0.62 9.2

a Chromated copper arsenate (CCA) treatment. Combined refersto data from both treated and untreated groups.

b Dynamic values. COV is coefficient of variation.c Based on ovendry weight and volume at a moisture contentof 12 percent.

Table 4—Distribution characteristics for modulus of ruptureof single members and three-layer unspliced posts a

Ratio ofModulus of rupture b post to

Distribution(lb/ins (MPa)) single-

membercharacteristic Single members Unspliced posts values

Mean 11,040 (76.1) 9,800 (67.6) 0.89[33.9%] [13.6%]

5-percent exclusion valueNormalLognormal

4,940 (34.1)c

6,060 (41.8)7,640 (52.7) — c

Weibulle7,760 (53.5) 1.28d

Nonparametric5,580 (38.5) 7,630 (52.6) 1.37 f

6,130 (42.3) 7,790 (53.7) 1.27

5-percent tolerance limitg

NormalLognormal

4,370 (30.1)c

5,760 (39.7)7,340 (50.6) — c

7,510 (51.8) 1.30Weibull e 5,320 (36.7) 7,380 (50.9) 1.39Nonparametric 4,800 (33.1) 6,730 (46.4) 1.40

a Modulus of rupture values for posts are average (“effective”) valuescalculated using post width of 4.5 in. (114.3 mm) and height of5.5 in. (139.7 mm). Posts were made with machine-driven nails.

b Values in brackets are coefficient of variation.c Normal fit of single members rejected by Shapiro–Wilk testd95-percent confidence bound = 1.10–1.49.eThree-parameter Weibull distribution.f 95-percent confidence bound = 1.14–1.59.g One-sided lower 75-percent confidence bound on 5-percentexclusion value.

10

Figure 7—Cumulative distributions for modulus of rupture for single members andthree-layer, unspliced assemblies.

More than half the plates were completely torn in halfby high bending forces. The only plates that did notfail were those connecting members 1 and 2 in postsexhibiting failure types 1, 2, and 3. The failure typesand number of failures associated with the four splicedpost designs are described in Tables 8 and 9. The fail-ure types identified with individual tests of the splicedposts are given in Table B3, Appendix B.

Discussion

Single Members

The strength and stiffness of the 56 single membersexceeded expected values based on published de-sign properties (NDS). The design stress in bendingis 1,850 lb/in2 (12.65 MPa) and the design MOE is1.9 × 106 lb/in2 (13.1 GPa) for No. 1D Southern Pinelumber used in dry conditions.

Using a lognormal distribution for bending strength(a normal distribution was rejected) and the reductionfactors in ASTM D 2915 for the 5th percentile fromTable 4, a design stress of 2,740 lb/in2 (18.9 MPa)

-would be applicable to the sample. This is 48 percenthigher than the published value. The mean MOE of

2.48 × 106 lb/in2 (17.1 GPa) would be applicable forthe designs using the sample material. This is 31 per-cent higher than the published value.

Single Members and Unspliced Posts

As shown in Table 4 and Figure 7, the mean MORof the unspliced posts was found to be only 89 per-cent that of the single members. This is not surpris-ing. In a similar study involving Douglas Fir lumber,Bonnickson and Suddarth (1966) found that the meanMOR of three-layer assemblies was only 94 percent thatof single members. The lower mean MOR for threelayer unspliced posts can be attributed to the fact thatone layer in an assembly must fail before the remaininglayers are stressed to their maximum capacity. Unfor-tunately, most three-layer assemblies cannot take addi-tional load after the first failure occurs. Consequently,the resulting MOR is the average of the extreme fiberstresses in one layer that is loaded to its maximum ca-pacity and two layers that are not loaded to their maxi-mum capacity. Thus, it is not surprising that the meanMOR of three-layer assemblies is slightly lower than themean MOR of single members. On the other hand, aslong as defects are spread out, a weak area in one layeris supported by strong areas in adjacent layers. In ourstudy, this resulted in a lowering of the MOR

11

Table 5—Distribution characteristics for ultimate midspan bending moment and initial stiffness ofunspliced and spliced posts

Distributioncharacteristic

Ultimate moment resistance and stiffness for various post types a

UM SG SG–R SM SM–R

Ultimate midspan bending moment (× 103 in-lb (kN-m))

Mean 222 (25.1) 109 (12.3) 126 (14.2) 105 (11.9)[13.4%] [18.8%] [12.9%] [18.1%]

5-percent exclusion valueNormal 173 (19.6) 75.1 (8.49) 99.0 (11.2) 73.6 (8.32)Lognormal 176 (19.9) 76.2 (8.61) b

Weibull c 173 (19.6) 73.4 (8.29)98.6 (11.1)b

96.5 (10.9)77.5 (8.76)79.4 (8.97)

Nonparametric 176 (20.0) 67.8 (7.66) 95.6 (10.8) 81.3 (9.19)

5-percent tolerance limitd

Normal 167 (18.8) 70.4 (7.96) 95.1 (10.8) 69.1 (7.81)Lognormal 170 (19.3) 72.6 (8.20)b 95.4 (10.8)b 74.5 (8.42)Weibullc 168 (18.9) 69.1 (7.80) 91.7 (10.3) 77.9 (8.79)Nonparametric 153 (17.2) 56.6 (6.40) 75.6 (8.54) 74.5 (8.42)

Initial stiffnesse (lb/in (kN/m))

118 (13.3)[13.7%]

91.4 (10.3)92.0 (10.4)b

88.6 (10.0)88.5 (10.0)

87.6 (9.90)88.9 (10.0)b

84.2 (9.51)77.4 (8.74)

Mean 5,200 (910) 3,080 (540) 3,930 (688) 3,140 (550) 3,850 (674)[9.2%] [10.7%] [10.3%] [10.8%] [12.2%]

a Values in brackets are coefficients of variation. Abbreviations for post types:U is unspliced, M machine-driven nails, S spliced, G gun-driven nails, and R reinforced.

b Lognormal fit rejected by Shapiro–Wilk test.cThree-parameter Weibull.d One-sided lower 75-percent confidence bound on 5-percent exclusion value.e Total load as opposed to average load point deflection.

Table 6—Significant differences in post properties

Significant difference b

Mean ultimate midspan Ultimate strength (5-percent Mean bendingComparison of post types a bending moment exclusion value) stiffness

Unspliced and splicedUM and SG Yes Yes YesUM and SG–R Yes Yes YesUM and SM Yes Yes YesUM and SM–R Yes Yes Yes

Unreinforced and reinforcedSG and SG–R Yes Yes YesSM and SM–R Yes Yes YesSG + SM and SG–R + SM–R Yes — Yes

Gun- and machine-driven nailsSG and SM No No NoSG–R and SM–R Noc No NoSG + SG–R and SM + SM–R No — No

a See Table 5 (footnote a) for definitions of post types.b 5-percent confidence level.cSignificant at a 5- to 10-percent level of confidence.

12

Table 7—Property ratios for spliced to unspliced and reinforced to unreinforced postsa

Property anddistribution

characteristic

Ratio for spliced to unspliced posts

SG SG–R SM SM–R

Ratio for reinforcedto unreinforced posts

SG–R SM–R SG–R + SM–Rto SG to SM to SG + SM

Ultimate midspanbending moment

Mean 0.49 0.56 0.47 0.53 1.16 1.12 1.145th percentileb 0.43 0.57 0.42 0.53 1.32 1.24 1.2895-percent CI on 0.35–9.51 0.49–0.65 0.35–0.50 0.45–0.60 1.08–1.55 1.02–1.46 1.11-1.43

5th percentilec

5-percent tolerance limitb 0.42 0.57 0.42 0.53 1.35 1.27 1.31Initial stiffness

Mean 0.59 0.76 0.60 0.74 1.27 1.23 1.25a See Table 5 (footnote a) for definitions of post types.b Normal distribution.cCI is confidence interval.

coefficient of variation for the three-layer unsplicedposts compared to that for the single members. Conse-quently, the estimated 5th percentile of strength for theunspliced posts was approximately 28 percent higherthan that for the single members. When design val-ues are based on the 5th percentile, the ratio of the 5thpercentiles is equal to the ratio of the design bendingstresses. On the basis of our results, three-layer un-spliced posts should be assigned an allowable bendingstress about 30 percent greater than that assigned tosingle members. Unfortunately, the repetitive use crite-ria of the NDS allow a designer to use a design bendingstress for a three-layer assembly that is only 15 percentgreater than the single member value.

To justly reward those designers who currently use un-spliced nail-laminated posts, design criteria could beestablished for the posts that would allow the engi-neer to take advantage of higher design values than thevalues currently allowed, These design criteria shouldaddress the number of laminations; spacing betweenlaminations; type, number, and location of fasteners;and method of loading. More specifically, the designcriteria should include provisions that limit the gap be-tween two adjacent layers and do not allow nail fas-teners to be located too close to member edges. In allcases, specifications should be written to ensure thatthe applied loads will either be destributed uniformly toall layers or applied through a load-distributing elementthat forces all layers to have similar displaced geome-tries. Once a design methodology using higher designvalues is implemented, manufacturers of unspliced posts

will need to establish a system to assure these designvalues are maintained.

In an unspliced post, the effective MOE should equalthe average MOE of the individual layers when allthree layers in the assembly are the same size and areforced to have the same displaced geometry. Thus, aswas expected, the mean MOE values of the three-layerunspliced posts and the single members were essentiallyequal.

Unspliced and Spliced Posts

The ratios in Table 7 demonstrate the significant reduc-tion in strength associated with splicing. The estimated5th percentiles of strength for the spliced posts withoutbutt joint reinforcement (which are used to calculatedesign values for the posts) were found to be less than45 percent of the 5th percentile values for the unsplicedpost design. This is much lower than some engineerswould predict. A common belief is that a three-layerpost should have a design bending strength that is ap-proximately two-thirds that for an unspliced post be-cause two of the layers are continuous at each joint.The problem with this assumption is that it fails toconsider’ the following three factors: (1) bending mo-ments in the two continuous layers adjacent to a jointcan be quite different because of the redistribution offorces in the vicinity of the joint, (2) nail forces aremuch higher in spliced posts and precipitate failuresin the posts that are not common to unspliced posts,and (3) design strengths are highly dependent on the

13

Figure 8—Cumulative distribution of ultimate midspan bending moment for three-layer assemblies.

variability in strength of individual specimens, and such.variability is generally higher for spliced posts than it infor unspliced posts.

Another reason that the ratios in Table 7 are lowerthan typically perceived is that design values basedon test results for spliced posts are often incorrectlycompared to NDS allowable design values for unsplicedpoets. Because these NDS values are applicable to alllumber from a broad range of sources, they are of-ten conservative for specific groups of lumber. Engi-neers who compare the design values for spliced poststo NDS values for unspliced posts are mistakenly ledto conclude that splicing is not nearly as critical asis actually the case. For this reason, evaluation ofspliced posts must include unspliced posts built us-ing lumber randomly selected from the same lot asthat used to fabricate the spliced posts. The actualstrength reduction associated with splicing can beascertained only when lumber from the same lot isused.

The ratios in Table 7 also demonstrate the significantreduction in stiffness associated with splicing. Splicedposts with and without butt joint reinforcement wererespectively about 75 and 60 percent as stiff as un-spliced posts. It is important to realize that these per-centages are based on average load point deflections

14

and are unique to the test setup shown in Figure 4–the actual bending stiffness of a spliced post is not con-stant but varies along the length of the assembly. Forthis reason spliced posts should be sectioned into el-ements for modeling purposes. Those elements thatdo not contain joints can be treated like unsplicedposts. Those elements that contain one or more jointsshould be assigned a different bending stiffness, theactual value of which is dependent on the length ofthe elements as well as several other design variables(Bohnhoff and Moody 1991).

When evaluating spliced posts by tests, the stiffnessof the individual pieces that make up a spliced postshould be measured before the post is fabricated. Av-eraging the MOE values of the pieces allows one toestimate the stiffness of an unspliced post fabricatedfrom the same lumber. This information along with thedisplacements measured during spliced post tests canbe used to calculate the reduction in stiffness resultingfrom splicing. For this type of comparison, MOE valuesfrom the NDS should not be used to estimate the stiff-ness of an unspliced post because lumber is quite vari-able, even within the same grade and species. Whenindividual lumber strength and stiffness are not mea-sured, it is impossible to determine where the batchor lot of lumber ranks with respect to other batches orlots of the same grade.

Figure 9—Failure types and locations in splicedposts. Members 1, 3, and 5 treated with CCA.

Members 2, 4, and 6 untreated.

Reinforced and Unreinforced Posts

The addition of reinforcement to the spliced postswas found to significantly increase the mean strength,5th percentile value, and initial stiffness of the as-semblies (Table 7 and Fig. 8). Specifically, themean ultimate bending moment increased ap-proximately 14 percent, the 5th percentile about28 percent, and the initial stiffness approximately25 percent.

The location of the reinforcement was based on earliercomputer modeling (Bohnhoff and others 1989),

which, as previously mentioned, showed that splicedposts with unreinforced staggered joints have an inher-ent weakness–the longest member in the center layer(member 3 in Fig. 9) is almost always the most highlystressed member. This can be attributed to two factors.First, member 3 is the major load-distributing elementin the assembly, and it therefore experiences high loadsregardless of its individual stiffness. If joints are notreinforced, all forces in members 1 and 5 must be trans-ferred through member 3 to reach members 2, 4, and6. Second, the lap between member 3 and member 2 istwice as long as any other lap in the assembly. Conse-quently, these two members make up 8 very stiff com-ponent in the assembly and attract much of the load.For this reason, member 2 in Figure 9 is usually thesecond most highly stressed member in the assembly(Bohnhoff 1969). By reinforcing only the outside joints,we hoped that less load would be channeled into thecenter layer, resulting in a more uniform distribution ofthe forces in the assembly.

The data in Table 9 support the theoretical predic-tion that members 2 and 3 are usually the most highlystressed members in an unreinforced spliced post.Of the 56 unreinforced spliced posts (SM+SG posts)tested, 32 were associated with a wood failure in mem-ber 3 and 28 with a wood failure in member 2 (severalposts had failures in both of these members). Whenthe outside butt joints were reinforced, the number ofwood failures in member 3 decreased slightly, and, asone would expect, the number of failures in member 2increased slightly. Overall, the distributions of failuresin the reinforced and unreinforced posts were not verydifferent. This can be attributed to the fact that abovea certain load level, plates in reinforced posts were notvery effective, and consequently the reinforced postsbehaved like unreinforced posts. The major advantageof butt joint reinforcement was that it prevented morefailures from occurring at lower loads. Because of this,the coefficient of variation associated with the ultimatebending moment of reinforced posts was somewhatlower than that of the unreinforced posts. The differ-ence in coefficient of variation explains why the differ-ence between the mean ultimate bending moment val-ues of the reinforced and unreinforced posts was smallerthan the difference between the 5th percentile valuesfor the two post types.

Nail Type

Bending strength and stiffness values of spliced postsfabricated with gun-driven nails were on the averageslightly higher than those of spliced posts fabricatedwith machine-driven nails. However, the differenceswere not significant.

15

Table 8—Distribution of failures in spliced posts

Location of failure Number of failures in various post typesc

(post members)b

Failure SG + SG–R +typea 1 2 3 4 5 6 SG SG–R SM SM–R SM SM–R Total

1 * * — 2 — — — 2 22 * * 2 1 — 2 2 3 53 * 10 10 7 11 17 21 384 * 1 — — — 1 — 15 * 2 — 1 3 3 3 66 * * 1 2 — 1 1 3 47 * 1 — 3 2 4 2 68 2 — — — 2 — 29 * 9 10 15 5 24 15 39

10 * * — 1 1 — 1 1 211 * * — 1 — 2 — 3 312 * * — — — 2 — 2 213 * * — 1 1 — 1 1 2

aSee Figure 9 for definitions of-failure types.b See Figure 9 for post member location.cSee Table 5 (footnote a) for definitions of post types.

Table 9—Frequency of post failures in highly stressed members

PostNumber of posts with failuresa

memberb SG SG–R SM SM–R SG + SM SG–R + SM–R Total

2 14 17 11 18 25 35 60

3 12 15 20 13 32 285 0 1 1 2 1 3

604

6 3 3 0 2 3 5 8

a See Table 5 (footnote a) for definitions of post types.b See Figure 9 for post member location.

Conclusions

Bending strength and stiffness of three-layer, unsplicednail-laminated posts were compared to that of sin-gle members of dimension lumber. The unsplicedposts were also compared to spliced posts with andwithout butt joint reinforcement, The effect of nailtype on strength and stiffness was evaluated by com-paring spliced posts fabricated with gun-driven ormachine-driven nails. Our results led to the followingconclusions:

1. The three-layer unspliced assemblies evaluated couldbe assigned an allowable bending stress about 30 per-cent greater than that assigned to the single members.The mean MOR of unspliced posts was slightly lowerthan that of single members, but the variation in MORof the posts was considerably lower than that of the

16

single members. Based on a lognormal distribution,the 5th percentile of MOR for the posts was 28 percenthigher than that for the single members. This valueis considerably higher than the 15 percent presentlyallowed.

2. If the individual layers in an unspliced post areforced to have the same displaced geometry, the ef-fective MOE can be obtained by averaging the MOEvalues of the individual layers.

3. Splicing, even with reinforcement, substantially re-duces the bending strength of a post. The estimated5th percentile values (which are used to calculate allow-able design values) for the spliced posts without buttjoint reinforcement were found to be <45 percent ofthe 5th percentile value for the unspliced post design.The 5th percentile values of strength for the reinforcedspliced posts were found to be <58 percent of the 5thpercentile for the unspliced post design.

4. Splicing can significantly reduce the stiffness of nail-laminated posts. Spliced posts with and without buttjoint reinforcement averaged only 75 and 60 percent, re-spectively, of the mean stiffness of the unspliced posts.

5. Reinforcing outside butt joints can significantly in-crease mean strength, 5th percentile of strength, andinitial stiffness of posts. The mean ultimate bendingmoment increased approximately 14 percent, the 5thpercentile about 28 percent, and initial stiffness approx-imately 25 percent when metal splice plates were addedto outside joints of spliced posts.

6. The two nail types used in this study performed sim-ilarly in the spliced posts. There was no significant dif-ference between the bending strength and stiffness ofspliced posts fabricated with gun-driven nails and thosefabricated with machine-driven nails.

17

Appendix ACost Comparison of Sawnand Laminated Posts

Table A2—Cost of typical laminated members andsolid-sawn treated postsa

Table A1 provides information on wholesale pricesof various sizes and types of Southern Pine lumber.Table A2 shows costs of typical laminated members andsolid-sawn treated posts.

Table A1—Wholesale Southern Pine lumber pricesa

Length ($/Type Grade Sizeb (m(ft)) board ft)

Untreated No. 1 KD 2 by 6 ≤4.3 (≤14) 0.280

CCA-treatedd No. 1 2 by 6 ≤4.3 (≤14) 0.375No. 2e 6 by 6 ≤4.8 (≤16)

6.1 (20)0.4550.505

7.3 (24) 0.565

a Approximate wholesale price during March 1989. Priceincludes cost of shipping to southern Wisconsin

b Nominal dimensions: 2 by 6 in. = standard 38 by140 mm, 6 by 6 in. = standard 140 by 140 mm.

c1 board ft = 0.0024 m3.d CCA is chromated copper arsenate. For lumber kilndried after treatment, AWPA Standard C16 (1985)requires a minimum retention level of 0.60 lb/ft3

(9.6 kg/m3) for any CCA-treated structural post.e Not stress-rated.

16-ft (4.8-m) nail-laminated memberb

24 BF untreated dimension lumber @ 0.280/BFc $6.7224 BP treated dimension lumber @ 0.375/BF 9.0042 20d pneumatically driven nails @ 0.028/nail 1.18Labor & equipment for fabrication–handling @ 20/hTotal

4.30$21.20

Solid-sawn treated 6 by 6 lumber (actual size =5.5 by 5.5 in.)

16-ft post - 48 BF @ 0.455/BF $21.8420-ft post - 60 BF @ 0.505/BF $30.3024-ft post - 72 BF @ 0.565/BF $40.68

aBased on prices in Table A1.b Actual member size is 114 by 140 mm (4.5 by 5.5 in.).cBF is board foot (1 board ft = 0.0024 m3).

20-ft (6.1-m) nail-laminated memberb

36 BF untreated dimension lumber @ 0.280/BF $10.0824 BF treated dimension lumber @ 0.375/BF 9.0050 20d pneumatically driven nails @ 0.028/nail 1.40Labor & equipment for fabrication–handling @ 20/h 4.65Total $25.13

24-ft (7.3-m) nail-laminated memberb

48 BF untreated dimension lumber @ 0.280/BF $13.4424 BF treated dimension lumber @ 0.375/BF 9.0058 20d pneumatically driven nails @ 0.028/nail 1.62Labor & equipment for fabrication–handling @ 20/h 5.00Total $29.06

Addition of splice plates to outside butt jointsof assembly2 Splice plates (18 gauge) @ 0.75/plate $1.50Labor & equipment for fabrication–handling @ 20/h 1.20Total $2.70

18

Appendix BResults of Bending Tests

Tables B1 through B3 present data from bending testson single members, unspliced posts, and spliced posts.The tables provide data on specific gravity, modulus ofelasticity (MOE), initial-stiffness, MOE based on initial

stiffness, modulus of rupture, and ultimate midspanbending moment. (1 lb/in = 175 N/m; 1 lb/in2 =6.89 kPa; 106 lb/in2 = 6.89 GPa; 1,000 in-lb =112.9 J.)

Table B1—Test data for single members

MOE based ModulusDynamic Initial on initial of

Replicate Specific MOE stiffnessa stiffnessb rupturenumber gravity (×106 lb/in2) (lb/in) (×106 lb+in2) (lb/in2)

1 0.6332 0.5933 0.7284 0.6185 0.7226 0.5717 0.6548 0.6599 0.661

10 0.68711 0.63212 0.70413 0.58314 0.55715 0.58616 0.60217 0.76118 0.61019 0.58520 0.59821 0.62222 0.67923 0.63724 0.46025 0.65526 0.53127 0.58128 0.53729 0.62130 0.70331 0.55032 0.53933 0.52334 0.65735 0.64836 0.63037 0.70338 0.556

2.6432.0833.6171.5343.6961.8402.3612.6452.1832.9611.8672.4722.1162.4381.9821.7892.6802.0092.2072.1902.4242.5262.2761.6561.9491.8622.5831.5512.8002.5302.2821.7461.8561.9562.1412.6342.9672.193

1,546 2.7361,324 2.3442,306 4.081

919 1.6262,089 3.6981,098 1.9431,315 2.3281,558 2.7581,386 2.4531,774 3.1411,071 1.8961,510 2.6721,210 2.1411,447 2.5611,112 1.9691,099 1.9461,556 2.7541,1061,330

1.9582.354

1,257 2.2261,380 2.4421,381 2.4441,231 2.1781,069 1.8921,153 2.0411,117 1.9771,604 2.839

952 1.6851,600 2.8321,578 2.7941,463 2.5891,073 1.8991,003 1.7751,278 2.2621,314 2.3261,725 3.0541,980 3.5041,393 2.466

10,75010,08613,4067,822

12,9397,5277,798

12,89011,90611,73322,40911,95512,2017,970

11,7839,544

16,08716,08714,88111,19211,0459,2499,9624,797

10,1848,905

12,2266,887

10,52816,82610,0128,1676,0278,929

12,96313,62510,7749,938

19

Table B1—Test data for single members—concluded

MOE based ModulusDynamic Initial on initial of

Replicate Specific MOE stiffness a stiffnessb rupturenumber gravity (×106 lb/in2) (lb/in) (×106 lb/in2) (lb/in2)

6,8508,029

10,57718,42811,65818,896

9,6258,0046,7767,982

10,4609,8399,8893,953

12,04721,48410,150

39 0.625 2.234 1,484 2.627 11.36540 0.501 2.021 1,27441 0.706 2.607 1,397

2.2552.4732.7913.4262.5162.7332.3492.4672.6642.4622.7002.7461.7062.1153.2292.1192.691

1,5771,936.1,422

0.725 2.8080.650 2.8070.693 2.3250.654 2.7330.735 2.4120.593 2.2850.684 2.6350.597 2.3130.654 2.4470.632 2.4700.541 1.6530.623 2.0200.681 2.8130.573 2.0780.629 2.380

0.626 2.3270.064 0.449

10.18 19.30

4243

44454647484950515253545556

(Mean)(Standard

deviation)(Coefficient

of variation)

1,399288.6

20.63

2.476 11,0360.511 3,740

20.63 33.89

1,5441,3271,3941,5051,3911,5251,551964

1,1951,8241,1971,520

a Ratio of total load to average midspan deflection.b Calculated by multiplying initial stiffness by 1,769.9/in.

20

Table B2—Test data for machine-driven unspliced postsa

UltimateAverage MOE based midspanMOE of Initial on initial bending

Replicate members stiffnessb stiffness c momentnumber (×106 lb/in2) (lb/in) (×106 lb/in2) (×103 in-lb)d

123456789

10111213141516171819202122232425262728

(Mean)(Coefficient

of variation)

2.4292.0352.5502.2522.3612.2471.9992.1981.9842.6362.3452.3192.2462.3632.4452.5882.3872.3292.0402.3562.3472.1502.1772.2062.3802.3392.6941.905

2.2978.67

5,219 2.4564,873 2.2936,006 2.8264,976 2.3425,509 2.5934,778 2.2494,482 2.1095,084 2.3934,615 2.1725,726 2.6955,007 2.3564,961 2.3355,018 2.3615,484 2.5815,636 2.6526,264 2.9485,102 2.4014,806e 2.2625,213 2.4535,619 2.6445,142 2.4205,473e 2.5765,226 2.4595,213 2.4535,321 2.5045,193 2.4445,615 2.6423,922 1.846

5,196 2.4459.23 9.23

189.4189.9235.1214.4199.6212.8227.1192.8185.5289.1195.7231.1249.8237.5234.1272.2243.9204.7238.5213.8187.1229.9228.6267.7233.4243.0226.5152.6

222.313.6

a Wood properties (MOE, specific gravity) for individual posts can befound in Bohnhoff and others (1091).

b Ratio of total load to average load point deflection.c Calculated by multiplying initial stiffness by 470.6/in.d To convert to modulus of rupture, divide by section modulus of

22.69 in3.e Based on three rather than four measurements.

21

Table B3—Test data for spliced postsa

SG posts SG-R posts SM posts SM-R posts

Ultimate Ultimate Ultimate UltimateRepli- midspan Fail- midspan Fail- midspan Fail- midspan Fail-cate Initial bending ure Initial bending ure Initial bending ure Initial bending are

number stiffnessb moment typec stiffnessb moment typec stiffnessb moment typec stiffnessb moment typec

(lb/in) (×103 (lb/in) (×103( lb/in) (×103 (lb/in) (×103

in-lb) in-lb) in-lb) in-lb)

123456789

10111213141516171819202122232425262728

3,4273,5632,3233,0023,0333,0933,4043,1662,8243,6113,2833,0072,5412,8762,5623,5413,3062,6213,1032,8353,0622,8613,389—

3,3213,0433,1753,329

123.0 8107.4 656.7 9

100.2 9105.3 2123.5 9104.0 5143.8 3119.2 2123.0 9128.1 3

99.0 9121.4 9117.1 3156.0 8

71.8 579.1 389.6 3

103.2 996.3 3

121.7 3103.1 7108.2 3121.7 9102.7 392.5 3

114.5 4112.4 9

3,717 91.3 9 4,773 118.4 93,599 129.4 3 3,982 133.2 92,793 86.1 9 3,433 99.8 32,984 134.4 9 3,419 135.3 92,932 103.5 3 2,880 97.2 22,788 101.0 9 3,429 77.3 123,230 129.7 3 3,919 103.6 53,078 125.1 9 3,775 118.4 113,260 93.4 9 4,105 125.6 93,519 103.1 9 4,838 137.0 33,038 151.4 3 3,519 112.0 33,174 92.6 7 3,626 98.9 122,463 101.5 3 3,298 130.6 33,089 97.7 9 3,404 118.3 32,695 83.7 9 3,524 126.4 33,562 136.5 9 4,221 125.5 23,469 125.6 9 4,154 114.5 32,356 87.9 9 3,491 92.6 92,946 96.8 13 3,851 105.2 72,990 96.4 10 4,490 134.8 73,346 98.1 7 4,158 119.7 33,238 98.5 3 3,726 128.0 32,920 89.6 5 3,365 99.7 113,393 83.7 3 4,207 140.3 33,332 101.9 9 4,565 129.3 63,053 109.9 9 3,852 113.7 33,530 74.4 7 3,913 131.9 53,489 117.6 9 3,922 139.1 5

3,08510.7

(Mean)(Coeffi-cient ofvariation)

108.718.8

4,616 143.4 94,510 135.7 33,670 114.6 33,370 111.6 93,581 75.7 14,133 137.0 93,921 128.1 34,077 139.1 134,332 139.0 34,662 140.3 93,924 133.5 34,021 117.9 93,336 104.8 93,429 105.6 103,908 126.8 114,444 125.1 63,458, 137.4 33,630 102.7 33,956 131.9 94,145 155.3 33,967 133.1 33,294 119.6 23,426 136.5 93,604 110.3 94,449 122.6 64,183 123.5 34,027 131.9 13,921 134.9 9

3,928 125.710.3 12.91

3,14210.8

105.0 3,851 118.118.14 12.2 13.7

a Wood properties (MOE, specific gravity) for individual posts can be found inBohnhoff and others (1991). Post types: SG, gun-driven nails, unreinforced joints;SG-R, gun-driven nails, reinforced joints; SM, machine-driven nails, unreinforced joints;SM-R, machine-driven nails, reinforced joints.

b Ratio of total load to average load point deflection.cSee Figure 9 for description of failure apes.

22

Appendix CSignificance Levels forAnalyses of Post Properties

Tables C1 through C3 provide significance levels forstatistical analyses of mean ultimate bending moment,mean initial bending stiffness, and ultimate midspanbending moment of spliced and unspliced posts. Thefollowing abbreviations are used for post types:

Abbreviation P o s t d e s c r i p t i o n

SG Spliced posts, gun-driven nailsSG-R Spliced posts, gun-driven nails,

reinforced butt jointsSM Spliced posts, machine-driven nailsSM-R Spliced posts, machine-driven nails,

reinforced butt jointsUM Unspliced posts, machine-driven

nails

Table C1—Significance levels for comparison of mean ultimate bendingmoment of various post types

Comparison of Student’s Paired Wilcoxon Signedpost types t-test ANOVA t-test test rank test

Unspliced and spliced posts

UM and SG 0.0001 — — 0.0001 —UM and SG-R 0.0001 — — 0.0001 —UM and SM 0.0001 — — 0.0001 —UM and SM-R 0.0001 — — 0.0001 —

Unreinforced and reinforced butt joints

SG and SG-R 0.0011 — 0.0007 0.0005 0.0008SM and SM-R 0.0076 — 0.0072 0.0046 0.0088SG + SM — 0.0001 0.0001 — —

and SG-R + SM-R

Gun-and machine-driven nails

SG and SMSG-R and SM-RSG + SG-R

and SM + SM-R

0.4799 — 0.5015 0.2446 0.38420.0858 — 0.0586 0.0629 0.0712

— 0.0986 0.0950 — —

23

Table C2—Significance levels for comparison of mean initial bendingstiffness of various post types

Comparison of Student’s Paired Wilcoxon Signedpost types t-test A N O V A t -test test rank test

Unspliced and spliced posts

UM and SG 0.0001 — — 0.0001 —UM and SG-R 0.0000 — — 0.0001 —UM and SM 0.0001 — — 0.0001 —UM and SM-R 0.0000 — — 0.0001 —

Unreinforced and reinforced butt joints

SG and SG-R 0.0000 — 0.0001 0.0001 0.0001SM and SM-R 0.0000 — 0.0001 0.0001 0.0001S G + S M — 0.0001 0.0001 — —

and SG-R + SM-R

Gun- and machine-driven nails

SG and SMSG-R and SM-RSG + SG-R

and SM + SM-R

0.5316 — 0.3076 0.6194 0.31620.5340 — 0.2505 0.4759 0.1838

— 0.8877 0.7026 — —

Table C3—Significance levels for comparisonof 5-percent exclusion values of ultimatemidspan bending moment of various post types

Comparison of Parametricpost types test

Unspliced and spliced posts

UM and SG 0.000UM and SG-R 0.000UM and SM 0.000UM and SM-R 0.000

Unreinforced and reinforced butt joints

SG and SG-R 0.002SM and SM-R 0.014

Gun- and machine-driven nails

SG and SM 0.849SG-R and SM-R 0.256

24