LATERAL AND W ITHDRAWAL STRENGTH OF NAIL

CONNECTIONS FOR MANUFACTURED HOUSING

By Steve G. Winistorfer1 and Lawrence A. Soltis,2 Member, ASCE

ABSTRACT : Current methods used in the design of nailed connections do notdirectly relate to the types of joints found in manufactured housing. These methodsdo not account for the construction practices used today, such as power nailingequipment, the use of nails with coated shanks, or the fabrication of joints withfiller materials separating main wood members. A total of 640 joints were testedto investigate the effects of these five variables on lateral and withdrawal strengthof nails (1) Two wood species groups: (2) low- and high-humidity environments:(3) hand- and power-driven nails; (4) uncoated and coated nail shanks; and (5)presence or absence of filler material in a joint. Tests verified that lateral strengthincreases with increasing wood density and that as environments become less humidand the moisture content of wood decreases. lateral strength and stiffness of nailedconnections increase. In the dry environment, the withdrawal strength of coatednails was almost 90% higher than that of uncoated nails in joints with an orientedstrandboard (OSB) filler and almost 40% higher when no filler was used. In high-humidity conditions, no differences were observed between joints with coated nailsand those with uncoated nails. In most other cases, the OSB filler reduced with-drawal strength by an overall average of 25%. This reduction is proportional tothe amount of penetration into the solid wood member; therefore, nail length shouldbe increased when an OSB filler is present.

INTRODUCTION

Connections in the manufactured housing industry are designed usingfastener properties contained in the National Design Specification for WoodConstruction (NDS) (1991). However, because many connections. specifi-cally those with fillers, are not addressed in the NDS, designers must usejudgment in modifying NDS methods and values to provide adequate con-nection designs. The NDS values were established before manufacturingstandards and practices were developed for manufactured housing and be-fore the manufactured put-in-place environment became known.

Current NDS values for withdrawal resistance were established usingstatic, dry, hand-driven test specimens that determined wood-to-wood de-sign characteristics; current NDS lateral design values from yield equations,even though based in mechanics of materials theory, were calibrated tomatch empirical data based on test specimens similar to those used in with-drawal tests. However, current manufacturing practices in the manufacturedhousing industry take advantage of efficiencies gained using power-drivennails and staples. Manufacturing efficiencies are also realized by applyinga panel board subfloor to the floor joist system before setting wall panelsin place.

Although most nailed connections in housing are designed to carry lateralforces, nail joints are often subject to withdrawal forces from other loads,

1 Res. General Engr., USDA Forest Service, Forest Products Lab., Madison, WI53705-2398.

2 Supervisory Res. Engr., USDA Forest Service, Forest Products Lab., Madison,WI.

Note. Discussion open until May 1, 1995. To extend the closing date one month,a written request must be filed with the ASCE Manager of Journals. The manuscriptfor this paper was submitted for review and possible publication on July 9, 1993.This paper is part of the Journal of Structural Engineering, Vol. 120, No. 12, De-cember, 1994. ASCE, ISSN 0733-9445/94/0012-3577/$2.00 + $.25 per page. PaperNo. 6460.

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including wind uplift. Information is needed concerning fastener and con-nection characteristics in the manufactured housing environment.

ObjectivesThe present study investigated connection properties that involve various

combinations of connection characteristics used in the construction of man-ufactured housing. The variables studied were: (1) Adhesive coatings onnails; (2) presence of subfloor filler materials between wood members of aconnection; (3) hand- versus power-driven nails: (4) two species groups;and (5) two moisture content (MC) levels. The lateral stiffness/strength andwithdrawal properties were determined and used as measures in comparingthe connection variables.

BackgroundPrevious research conducted at the USDA Forest Service. Forest Products

Laboratory (in cooperation with the U.S. Department of Housing and Ur-ban Development) determined that joints modeling typical manufacturedhousing connections, specifically those in which gypsum board and carpetfiller material separated the wood members, can reduce the strength of theconnection significantly (Soltis et al. 1993).

A limited amount of research has been conducted on the relative strengthof power-driven nails with respect to comparable hand-driven connections.Two such studies (Youngquist and Scholten 1951; Stern 1952) investigatedholding properties of proprietary fasteners specifically for use by the man-ufacturers. The researchers found strength increases for connections madeby power-driving nails.

Other researchers conducted tests to determine the effect of penetrationrate on withdrawal strength values of common and plain sinker nails (Deutschand Mechler 1952). Their general conclusion was that a slight increase inwithdrawal resistance is obtained with increased penetration rates.

The effects of MC on wood strength properties are well documented inthe Wood Handbook (1987). However, because of variations in nail type,wood species. and moisture conditions, MC effect on connection lateralstrength is not as completely defined. The Wood Handbook and otherstudies (Borkenhagen and Heyer 1950; Heck 1950; Lawniczak 1956) havereported that cyclic moisture conditioning can reduce lateral strength andwithdrawal resistance of certain types of nails and staples. In one case, a25% reduction from the seasoned wood lateral load value is recommendedto avoid excessive deformation when fasteners will be used in unseasonedwood or in conditions where the wood will remain wet. However, one shouldrecognize that the possibility of having dramatically different fabricationand end-use environments in manufactured housing construction is unlikely.

EXPERIMENTAL METHODS

A total of 640 joints were tested, 320 laterally and 320 in withdrawal. Awood/wood joint and a wood/oriented strandboard (OSB)/wood joint wereused for both lateral (Fig. 1) and withdrawal (Figs. 2 and 3) nail tests. Thewood/OSB/wood joint models those joints typically found in floor construc-tion in the manufactured building industry (Fig. 4). The OSB subfloor isattached to floor joists before wail sections are put in place, creating a jointwhere OSB is filler material between main wood members of joint. Thebottom piece of wood represents the rim joist of the floor system, and the

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FIG. 1. Joints with No Fiiler: (a) with OSB Filler; and (b) for Lateral Tests

OSB represents the subfloor deck applied with glue and nails to the floorjoists. The top piece of wood represents the bottom wall plate of a prefab-ricated wall panel that is placed on the floor system and attached to thejoist through the subfloor layer. The wood/wood joints provided a baselinecomparison.

MaterialsThe lateral and withdrawal test specimens were fabricated from either

Southern pine (SP) or spruce-pine-fir (SPF) standard 38 × 89 mm (nominal2 × 4 in. ) no. 2 grade dimension lumber.

Nails, obtained from the International Staple, Nail, and Tool Association(Chicago, Ill.), had a 3.3-mm (0.131-in.) diameter (smooth) shank and were83 mm (3.25 in. ) long. The HUD department identifies this nail size asrepresentative of those used in manufactured housing construction. Thenails were produced during a single production run; half the nails werecoated with a thermosetting plastic and half remained uncoated. The coated

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FIG. 2. Withdrawal Joint with No Filler

FIG. 3. Withdrawal Joint with OSB Filler

nails were coated on the point-end of the nail to approximately 50% of thenail length. The plastic coating was formulated to conform with FederalStandard FF-N-105B (Federal 1971). This plastic coating melts from the heatgenerated at the high speed of driving with pneumatic nailers, then reso-lidifies, effecting a bond between the nail shank and the wood fibers.

American Plywood Association rated, 17-mm (0.69-in. ) thick OSB, typ-ical of that used in the manufacturing housing industry, was the filler material

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FIG. 4. Constructlon in Typical Manufactured Housing

between wood members of the joint. A polyvinyl acetate (PVA) structuraladhesive was applied between the wood and OSB members of joints priorto fabrication, with care taken to have the adhesive contact only the woodand OSB surfaces (and not coat the fasteners during penetration).

ProceduresTwo separate 25 factorial designs with 10 replications of each type of joint

provided 320 lateral and 320 withdrawal tests, and made it possible toidentify all main effects and any interactions among variables. The exper-

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imental design matrix in Table 1 was duplicated for the lateral and with-drawal tests.

After lumber was cut, cross-matched, and labeled, all pieces were storedin environmentally controlled rooms to stabilize the equilibrium moisturecontent (EMC). Two environments were used: low humidity (6% EMC,27°C (80°F) at 30% relative humidity) and high humidity (19% EMC, 27°C(80°F) at 90% relative humidity), in accordance with the experimental de-sign. Specimens were assembled and then returned to their respective con-ditioning rooms for at least 2 weeks prior to testing. This allowed the ad-hesives to set before tests were conducted. Also, for lateral test specimens,a piece of waxed paper was placed between the main wood member andthe rest of the joint material to reduce initial friction in the joint. Thishelped to smooth out the initial portion of the load-deformation curve butdid not affect strength or stiffness. Of course, this friction-reducing effectis not present in actual construction. Quite the contrary. large friction forcescaused by normal forces of system dead load do exist in the field.

Power-driven specimens were assembled using a Duo-Fast (Franklin Park,Ill.) compressed-air tool. Air pressure was set to ensure that each nail wasdriven fully with one blow of the tool. Hand-driven specimens were assem-bled using a standard 0.45-kg (1-lb) construction hammer, using approxi-mately five blows to obtain full embedment. All specimens were assembledby the same technician to limit assembly variability.

Contrary to ASTM D1761 procedures, uncoated nails were not degreasedprior to embedment to provide “as-built” data. (In the process of makingnails, die lubricants and other materials can coat the finished nail. Nailsthat are coated with the types of thermosetting plastic coatings studied inthis test are degreased to assure adhesion of the coating to the nail shank. )

The nail coating used in the present study was produced specifically toprovide a coating typical of those used in construction. Fastener manufac-turers provide a broad range of coated fasteners to manufactured and mod-ular housing builders with a variety of proprietary coatings that range fromless than 50% shank coverage to full shank coverage. The percentage ofthe shank that is coated and the specific coating formulation are but two ofthe many factors that determine the effect of coatings on connection per-formance.

Pilot holes half the diameter of the nail were drilled into the solid woodside members to ensure straight penetration of the nail. The pilot holepenetrated all 38 mm (1.5 in. ) of the side member. The solid wood mainmember (member holding the nail point) had a pilot hole drilled approxi-mately 6.4 mm (0.25 in. ) into the nailed face. The OSB filler material didnot require a pilot hole. Also, joints where the nail was power driven didnot require pilot holes in the main member.

For lateral testing, deformation control was used in testing because loadcontrol was not possible with the equipment available. Load was applied atthe cross-head rate of 2.5 mm (0. 10 in.) per minute. Specimens were testedto failure following revised ASTM D1761 procedures for lateral nail tests.The revisions involved test apparatuses that minimize the eccentric loadinginherent in the standard ASTM procedures for lateral nail testing. Defor-mations were measured with a linear variable differential transducer (LVDT)with an accuracy of + 0.032 mm ( + 0.00125 in.). All load-deformation datawere recorded digitally until failure occurred. Fig. 5 shows the apparatusused in the lateral tests. The metal frame. attached to the loading head in

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FIG. 5. Lateral Test Setup

the base of the test machine, applied force vertically through wood sidemember while OSB and wood main member were held stationary.

For withdrawal tests, deformation control was also used, with a rate of0.25 mm (0.010 in.) per minute. Specimens were tested to failure followingrevised ASTM D 1761 procedures for nail withdrawal tests. The revisedprocedure allowed head embedment (head pull through) as well as shankwithdrawal. Deformations measured were those for wood-to-wood move-ment and for wood-to-nail-head movement. Fig. 6 shows the apparatus usedfor the withdrawal tests.

A section was cut from one specimen from each group of 10 replicates,and specific gravity (SG) and MC were determined inASTM D143.

RESULTS AND COMMENTARY

Average MC and SG are shown in Table 2. Actual

accordance with

values. with SGadjusted-to 12% EMC using methods in ASTM D2395, were comparedwith those found in the NDS. Southern pine used in the lateral tests had amean SG approximately 10% lower than that listed in the NDS, while theSPF values agreed closely with those in the NDS. The OSB filler usedbetween wood members of the specimens had a SG of 0.57 (adjusted to12% EMC using ASTM D2395 procedures).

A statistical analysis using a 95% confidence level was conducted on thedata from both tests; an ANOVA of these results is presented in Table 3.The results and discussion are separated into sections covering the maineffects. Mean lateral stiffness and strength values. and associated coefficientsof variation of each replicate group are listed in Table 4, and similar values

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for withdrawal resistance are found in Table 5. Main variable effects forlateral and withdrawal tests are found in Tables 6 and 7, respectively.

Lateral Testsl-mad-deformation curves obtained during static lateral tests were fitted

to a hyperbolic tangent curve. This curve-fitting technique provided a con-sistent, repeatable method of analyzing the load-deformation data and pro-vided a computerized method of obtaining slope and strength data. Fig. 7illustrates a typical lateral load-deformation curve and the two responsevariables that were used to compare values, given the factorial main effects:the 5% diameter offset yield load (stiffness measure) and the load at 6.4-mm (0.25-in. ) deformation (strength). The 5% diameter offset yield loadis the convention used in the 1991 NDS to define yield in a connection.Because this offset yield load is derived from the initial slope of the load-deformation curve, it is used here as a measure of joint stiffness.

Most joints did not fail abruptly; after they lost their initial stiffness. theload-deformation curve flattened out, yet the joint continued to carry load.

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It was judged that after 6.4-mm (0.25-in.) deformation, the test loadingwent from lateral to general withdrawal resistance; therefore, a load at 6.4-mm (0.25 -in.) deformation was chosen as a measure of ultimate lateral load.In cases where a joint failed before reaching 6.4-mm (0.25-in.) deformation,the ultimate load was used.

Secondary interactions were significant at the 0.05 level in some cases,as Table 3 shows. Although several lower level interactions were statisticallysignificant, few were significant at the practical level. For instance, at 6%EMC with an OSB filler, hand-driven coated nails had statistically significanthigher strength than did hand-driven uncoated nails. However, because therespective values are 1,441 N (324 lb) and 1,352 N (304 lb), the 7% differencehas little practical value. Reducing these ultimate strength loads to lateraldesign values with factor of safety adjustments renders the difference neg-

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Species GroupsAs expected, the higher density of the SP produced higher values for

both the stiffness-measure and strength compared to SPF values. In all butone of the mean values listed in Table 4, the values for SP are higher thanthe corresponding values for SPF. However, the difference in values is oftennot as great as would be expected given the difference in densities. Thisapparent discrepancy can be explained partly because of the relatively lowSG of the SP used in these tests (Table 2).

Secondary interactions arose between species groups regarding the drivingmethod and humidity level. In high-humidity conditions, the differencebetween hand- and power-driven nails was only significant in SP, not inSPF. The mean value of the stiffness measure for hand-driven joints in SP

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was 534 N (120 lb), whereas for power-driven joints, the mean value was454 N (102 lb). No similar interaction was observed in low-humidity con-ditions or in the mean values for strength.

Driving MethodThe values in Table 6 show that the driving method had no practical effect

on either stiffness or strength of the joints, considering the variation in thedata. The only secondary interaction that arose was between the drivingmethod and the filler material.

Table 8 illustrates that when an OSB filler material was used, hand- andpower-driven joints had the same stiffness measure, whereas joints with nofiller material had higher stiffness values (approximately 20% higher) whenthe nails were hand driven compared with when they were power driven.A similar relationship was not observed for stiffness measure in high-hu-midity conditions. However. when nail shanks were coated, lateral strengthof power-driven joints in low-humidity conditions with OSB filler was ap-proximately 10% higher than that of similar hand-driven joints (see sectionon the effect of nail coating).

Another significant secondary interaction was observed in stiffness-mea-

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sure values of both species groups for low-humidity joints. Table 9 illustratesthat stiffness-measure values for hand-driven joints were higher than thosefor power-driven joints in both SPF and SP, although the effect was greaterin SPF joints. Again, even though both differences are statistically signifi-cant, only that for SPF appears to be practically significant.

Nail CoatingTable 6 shows no apparent difference in stiffness-measure and strength

values between joints and coated nails and joints with uncoated nails. How-ever, at least one significant secondary interaction was revealed by thestatistical analysis.

The use of a thermosetting plastic coating on nail shanks appeared tomake a difference only in low-humidity environments when OSB was usedas a filler material. Table 10 shows that coated nails produced higher meanstrength values than did uncoated nails and that the effect was greater whennails were power driven.

Moisture ContentAs expected, there was a significant difference in the lateral strength and

stiffness of joints in different moisture conditions. Table 6 shows that thestiffness measure was approximately 50% greater in joints at 6% EMC thanin 19% EMC and that strength was approximately 33% higher in joints at6% EMC than in 19% EMC.

The general data trends show that in the majority of cases, when a sec-ondary interaction existed between two or more of the main effects, thatinteraction was only significant at the low-humidity level and not at thehigh-humidity level. That is, the high-humidity environment appears tonegate any difference between other factors tested in the present study,namely, driving method and the presence (or absence) of a coating on thenail shank.

The joints tested here were fabricated and tested in identical environ-

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mental conditions. That is, the 6% EMC joints were both fabricated andtested at 6% EMC, and the 19% EMC joints were both tested and fabricatedat 19% EMC. The NDS provides for no reduction in lateral design loadswhen these conditions exist. However, reductions are required in the NDSwhen connections are fabricated when wet or partially seasoned and areused in dry conditions. Holes formed during fabrication tend to shrink awayfrom the fasteners in these cases, and reductions in load are necessary.

The MCs of the high-humidity joints were in the 23-28% EMC range,higher than would be expected in the most humid regions of the UnitedStates. Any differences between high- and low-humidity environments inthese tests would probably not be as pronounced in the actual use environ-ment.

Filler MaterialThe use of an OSB filler did not affect joint stiffness and caused only a

slight increase in strength (Table 6). The most significant increase in stiff-ness-measure values occurred in joints with OSB fillers and power-drivencoated fasteners in low-humidity conditions. Averaging similar mean strengthvalues from Table 4 gives a value of 1,357 N (305 lb) for joints with noOSB filler and 1,579 N (355 lb) for joints with an OSB filler, an increaseof more than 16%.

At high humidity, the OSB filler swelled about 3.2 mm (0.125 in.), Al-though this thickness swell did not appear to reduce stiffness-measure orstrength values below those for joints with no filler, it did reduce bothproperties to values below those for low-humidity joints with OSB fillers.The increased thickness not only reduced the relative density of the OSBmaterial but also allowed less penetration of the nail point into the mainwood member.

Withdrawal TestsMaximum ultimate load observed during withdrawal tests was used as a

means of comparison. As illustrated in Fig. 8, the typical load-deformationcurve shows an abrupt failure, with the joint continuing to carry a reducedload after initial failure. Table 7 lists information for a group of 160 spec-imens that had specific main effects.

In addition to wood-to-wood deformation (Fig. 8), the displacement ofthe nail head relative to the wood (head embedment) was also measuredand was found to be negligible. In most cases, no more than 0.76 mm (0.03in.) of displacement was measured.

Species GroupsAlthough the SP withdrawal load was expected to be higher than that for

SPF. the statistical analysis revealed no difference between SP and SPFwithdrawal load. Possible explanations lie in the variability of the tests.Coefficients of variation for a replicate group of 10 specimens were some-times as high as 0.65 but more generally were in the 0.20-0.35 range. Theorientation of the nail shank to the grain direction was a significant factorin maximum withdrawal load. If the nail penetrated the wood tangent tothe ring orientation, withdrawal load was relatively low. Conversely, if thenail shank penetrated the main wood member perpendicular to the rings,withdrawal load was relatively high. Because SP has much larger and moredistinctly defined rings (that is, well-defined earlywood and latewood bands),this phenomenon affected withdrawal load more for SP than for SPF. Also,

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because the density of the SP used in these tests was lower than expected,the relative difference between SP and SPF withdrawal loads was dimin-ished.

Driving MethodAs Table 7 shows, no difference in withdrawal strength was observed

between specimens that were hand driven and those that were power driven.

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Nail CoatingThe plastic coating on the nail shank was found to interact with the MC

of the wood and the presence of an OSB filler. Table 11 shows that althoughcoated nails had much higher withdrawal loads in low-humidity conditionsthan did uncoated nails (almost 40% higher), high-humidity tests revealedno practical effect. In both cases, the OSB filler accentuated the effect.

Moisture ContentGenerally, no difference was observed in withdrawal strength between

the two levels of moisture tested here (in light of the variation seen). How-ever, as discussed in the nail coating section, interaction existed. For ex-ample, plastic coatings increased joint withdrawal resistance in low-humidityconditions but provided no practical difference from joints with uncoatednails when used in high-humidity conditions.

Filler MaterialAn OSB filler, in most cases, reduced withdrawal strength by about 25%.

This reduction was proportional to the reduction in depth of nail penetrationinto the main solid wood member. Dividing penetration depth by the maineffect means from Table 7 shows that the unit length withdrawal load carriedby the solid wood member was almost identical between joints with andwithout the OSB filler, 20.14 × 103 N/m (115 lb/in. ) of nail penetrationwith no filler, 20.49 × l03 N/m (117 lb/in. ) of nail penetration with a 17-mm (0.69-in.) thick OSB filler. This verifies that the observed reduction inwithdrawal strength was caused by the OSB filler. Therefore, nail lengthshould be increased in connections where composite lumber filler materialslike OSB are used in nailed connections. The increased penetration depthof the nail restores the withdrawal capacity of a joint when this type of fillermaterial is present.

CONCLUSIONS

A total of 640 joints were tested (320 laterally, 320 in withdrawal) todetermine the effect on joint strength of five main factors: (1) Wood speciesgroup; (2) low- and high-humidity environments; (3) hand- and power-driven nails; (4) uncoated and coated nail shanks; and (5) presence orabsence of a filler material separating wood members of the joint.

Many of the results observed in the present study reinforced conclusionsfound by other researchers, namely, that lateral stiffness and strength in-crease with increasing wood density and decrease as moisture content in-creases.

The method of driving, hand or power, has no practical effect on eitherlateral or withdrawal strength of a connection. However, there are certaincircumstances when power-driven nails either increase lateral strength (inlow-humidity environments with coated nails) or reduce lateral strength(when no fillers are used). Coated nails in dry environments (6% EMC)increase both the lateral and withdrawal strength of connections comparedto joints with uncoated nails. Lateral strength is increased as much as 15%,and withdrawal strength is increased as much as 90%. However, no increaseoccurs when coated nails are used in areas with a high moisture level. Thepresence of an OSB tiller material does not appear to reduce the lateralstrength of connections. Withdrawal strength, however, should be reducedup to 25% (or more for thicker OSB fillers) when a 17-mm (0.69-in. ) thick

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OSB filler is present, except in cases where a coated nail is used in low-humidity conditions. lt is recommended that in joints with OSB fillers, thelength of the nail be increased to restore the full withdrawal capacity of theconnection.

ACKNOWLEDGMENTS

This test program was funded cooperatively by the Forest Products Lab-oratory and the United States Department of Housing and Urban Devel-opment (HUD) under Interagency Agreement Number DU-100191-0000029,and by support from member companies of the International Staple, Nail,and Tool Association (ISANTA). The guidance and advice of David Kirk-man and William Freeborne, both of HUD, and of John Kurtz, ISANTA,are appreciated.

on recycled paper

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