Performance of wood joints fastened with power-driven nailsChui, Ying H.1, Ni, Chun2

ABSTRACT

The design provisions in the Canadian timber design code and the National Building Code for nailed wood joints arebased on performance of hammer-driven nails. However, the use of power-driven nails has become the normal practice inwood frame construction because of their speed. Power-driven nails are available in sizes which differ from those forhammer-driven nails. Moreover, it is unclear if the properties of power-driven nails are significantly different from thoseof hammer-driven nails. The main purpose of this paper is to compare mechanical properties of power-driven andhammer-driven nails, and strengths of nail and toe-nail joints made with these two types of nails. Two nail lengths weretested, with samples obtained from four power nail manufacturers and one hammer-driven nail manufacturer. Nailbending, direct nail withdrawal, lateral strength of nail and toe-nail joint tests were conducted. It was found that there werelarge variations in mechanical properties of power-driven nails from different manufacturers. Available nail diameters aresimilar for both types for small diameter nails, but they deviate significantly for larger diameter nails. An overallconclusion is that, although power-driven nails appeared to have higher yield strength, the detected differences in lateralstrength capacity of nail joint were influenced primarily by the diameter. Power-driven nails also appeared to have ahigher direct withdrawal resistance, which may explain the higher capacities for toe-nail joints made with power-nails.

INTRODUCTION

Hammer-driven common bright wire nails have for decades been the prime mechanical fasteners for light wood-framebuildings. In Canada prescriptive construction details given in its model National Building Code (NRC, 1995) containfastening details which are based on decades of demonstrated satisfactory performance of bright wire nails. Engineeringdesign provisions in the Canadian timber design standard CSA O86 (CSA, 1994) are based on dimensions andspecifications for bright wire nails (CSA, 1974). Tabulated design strengths are given for standard nail sizes based on theEuropean Yield Model (EYM). As expected mechanical properties of nails such as yield strength and elastic modulus playan important role in performance of nailed joints. Mechanical properties of common bright wire nails have beencharacterized in studies by Smith and Whale (1985) in Europe and by Loferski and McLain (1991) in US, which provideddata for transition of the national design codes into limit states design format in these respective regions.

In recent years, the use of power tools has become standard practice in building construction in North America forfastening purposes. At the present time there is no generic product standard governing the manufacturing of power-drivennails. Consequently all power-driven nails are proprietary products and each nail manufacturer typically provides nail gunswhich only work with its own products. This has created a situation that, for any given standard length nail diameters maydiffer considerably between manufacturers. In most cases, they are generally smaller than standard diameters for brightwire nails. Moreover, it is unclear if the mechanical properties of power-driven nails are significantly different from thoseof hammer-driven nails. Despite these differences and uncertainties, power-driven nails are widely used interchangeablywith hammer-driven nails in building construction. In the USA, an evaluation report has been published for power-drivennails and staples which are used in wood-frame building construction (BOCA, 1997). That report provides designcapacities of nailed joints, shear walls and diaphragms fastened with power-driven nails and staples. These designcapacities were determined by using EYM and a minimum yield strength requirement for power-driven fasteners.

The main purpose of this test program is to provide experimental data by comparing mechanical properties of power- 1 Professor, Faculty of Forestry and Environmental Management, University of New Brunswick, Fredericton, NewBrunswick, Canada2 Wood Engineering Scientist, Forintek Canada Corp., 2665 East Mall, Vancouver, British Columbia, Canada

driven and hammer-driven nails, and strengths of nail and toe-nail joints made with these two types of nails. Because ofthe small number of replicates used here, this test program is not intended to provide general conclusions regarding thedesign capacities of joints made with power-driven nails.

MATERIALS AND TEST METHODS

The tests conducted were: 1. Nail bending, 2. Lateral strength of nail joints, 3. Direct nail withdrawal strength, 4.Withdrawal strength of toe-nail joints, and 5. Lateral strength of toe-nail joints. Table 1 provides details on test materialsand method for each test group. For tests 1 to 3, two nail lengths were selected: 51mm and 83mm. The 51mm (2 inch) nailis the smallest nail size used in the wood-frame building construction industry. The 83 mm length (3¼ inch) nail is acommonly required size for toe nailing and end nailing in wood frame construction. Test 1 was intended to provide someindication of the variability in moduli and yield strengths of power-driven nails from three or four manufacturers. The testalso provided a comparison between power-driven and hammer-driven nails. Tests 2 and 3 compared the lateral strengthof nail joints and the direct withdrawal strengths of the two types of nail. Tests 4 and 5 compared strengths of toe-nailjoints subjected to loading in various directions.

The National Building Code (NRC, 1995) specifies the nail length but not the nail diameter. While the 51mm hammer-driven and power-driven nails had almost identical diameter (2.75mm and 2.85mm), the diameters for the 83mm hammer-driven and power-driven nails differed significantly. The diameters for 83mm power-driven nails varied between 2.97mmto 3.25mm whereas the hammer-driven nail had a diameter of 3.8mm. None of the power-driven nail manufacturersproduces a nail diameter close to that of the hammer-driven nail.

All wood material used in this test program was eastern Canadian spruce-pine-fir lumber. Dry short (less than 6 ft) lumberpieces intended for a finger-joining process were purchased from a local sawmill. They were conditioned at 20ΕC and65% relative humidity before the gross density of each piece was measured. The mean and standard deviation of all piecesat this condition were 0.455 g/cc and 0.047 g/cc respectively. Only pieces within the range of mean ±1 standard deviation(0.408 and 0.502 g/cc) were retained for fabrication of joint specimens for tests 2 to 5. A stratified sampling approach wasadopted to segregate lumber pieces into various test groups to ensure that all groups had a similar spread of density. Thiseliminated the possibility of the influence of wood density masking any differences in test results from the test groups.With this sampling approach, a relatively small sample size of 6 replicates was adopted for each test group.

To allow the wood material to completely relax around the nail, all joint specimens were fabricated at least 24 hours priorto testing. All nails were driven with care so as to prevent over-penetration of the nail head. Nail guns supplied byrespective nail manufacturers were used and preliminary trials were required to prevent this over penetration. Thecommon nails were driven by a hammer. Different guides were used to ensure that the nail penetrated into the woodmember at the appropriate angle (i.e. normal to the surface for nail joint and 30 degree to normal axis for toe-nail joint).Nail joint specimen details for test 2 are given in Figure 1. All toe-nail joints were fabricated with an end distance of one-third of nail length and at an angle of 30Ε as illustrated in Figures 2 to 7, which provide details for the toe-nail joints intests 4 and 5. Test 4 was intended to determine the withdrawal strength of toe-nailed joints with arrangement as shown inFigure 2. Test 5a differed from 5b to 5e in the way loading was applied to the specimen. Tests 5b to 5e had the sameloading direction in relation to the member grain axes, but each differed from others by either the number of nails ormember restraint condition. These differences are illustrated in Figures 3 to 7.

ANALYSIS OF DATA AND DISCUSSION OF RESULTS

Test 1 - Nail bendingThe nail bending tests were conducted using a three-point bending arrangement and spans of 30mm and 40mmrespectively for the 51 mm and 83 mm nails. The test procedure was as described by Loferski and McLain (1991). Theobjective of the bending tests was to determine the elastic modulus and yield strength. The elastic modulus wasdetermined from the initial slope of the load-deflection response, as shown in Figure 8.

In comparison, the determination of yield strength, Φy, is less straight forward. Loferski and McLain (1991) used theproportional limit (transition from linear to non-linear behaviour) for calculating nail yield strength. However, as can beseen in Figure 8, there is no well-defined transition point. Hence other alternate definitions of yield strength were adopted

when analysing the test data. Three definitions are presented in Figure 8. One is based on the intersection of the initialslope and asymptote. The second and third definitions are based on offsets of 5% and 10% of nail diameter as illustratedin Figure 8. The yield strength, Φy, can be calculated from Equation (1).

323dLPy

y =σ (1)

where Py = Load at yield stressL = Spand = nail diameter

The test measurements and results are presented in Table 2. For the 51mm nails, it was found that, except for power-driven nail from manufacturer B which had lower modulus and yield strength, power-driven nails from manufactures Pand S had similar properties to the hammer-driven nail. Only power-driven nails from manufacturer S met the yieldstrength requirement of 689 MPa in the National Evaluation Report NER-272 (BOCA, 1997). All 51mm nails testedexhibited significantly lower mechanical properties compared with the hammer-driven common nails of similar sizestested by Loferski and McLain (1991), but the E results agreed with those reported by Smith and Whale (1985) andMohammad (1996) for nails with similar diameters.

Nail diameter for 83 mm power-driven nails varied significantly between different manufacturers (from 2.97 mm to 3.25mm). There were significant variations in mechanical properties among the power-driven nails from the fourmanufacturers. All power-driven nails had lower elastic moduli but higher yield strength compared with the hammer nail.Only nails from manufacturers D and S met the yield strength requirement of 689 MPa in the National Evaluation ReportNER-272 (BOCA, 1997). Their yield strengths were also comparable with those reported by Loferski and McLain (1991)for hammer nails of similar size.

Test 2 - Lateral strength of nail jointsResults for the lateral strength of nail joints are presented in Table 3. Two manufacturers and 2 nail lengths were chosenfrom those which supplied the nails for test 1. Joints made with 51mm power-driven nails had greater strengths than thosemade with hammer-driven nails. Despite their lower yield strength, joints made with power-driven nails frommanufacturer P produced the highest lateral strengths. The joints made with 83 mm power-driven nails from the twomanufacturers had similar joint capacities, even though the yield strength and diameter of nails from manufacturer D weregreater than those from manufacturer P. This seemed to suggest that the strength of the nail joints are not sensitive to thevariation of nail mechanical properties within the range encountered here. The joints made with 83 mm hammer-drivennails had a higher strength than joints made with 83 mm power-driven nails. This was probably because the hammer-driven nail had a much larger nail diameter.

Test 3 - Direct withdrawal strength of nailsTest results presented in Table 4 show that power-driven nails had considerably higher withdrawal strengths than hammer-driven nails. The strengths differed by a factor of 2. This difference in behaviour may be due to the fast speed ofpenetration of the nail into the wood which leads to less splitting, a phenomenon observed by Lau and Tardiff (1987) onhammer-driven nails. However, tests similar to those conducted by Lau and Tardiff (1987) are required in order tosubstantiate this explanation.

Test 4 - Withdrawal strength of toe-nail jointsThe results of test 4 are presented in Table 5, which shows that despite the larger nail diameter, the hammer-driven nailshad significantly lower withdrawal strengths compared with joints made with power-driven nails from manufacturer D.This was probably due to the higher withdrawal strengths of power-driven nails as observed in test 3.

Test 5 - Lateral strength of toe-nail jointsTests 5a to 5e were designed to study the effect of loading direction on toe-nail joints. All nails in test 5a specimens wereloaded perpendicular to the nail axis, despite the fact that they were toe-nail joints, whereas in test 5b some of the nailswere subjected to a larger withdrawal effect. The authors were interested in comparing the strengths of 5a joints with thoseof the laterally loaded nail joints shown in Table 3 (83mm hammer and D nails only). It can be noted that the toe-nail

joints exhibited only slightly lower strengths than nail joints made with same nails, with the difference being about 2%.The Canadian timber design code C SA O86.1 (CSA, 1994) calls for a reduction of 17% for toe-nailed joints. Thisconservative factor may have been adopted to account for possible fabrication deviation in service.

The above provides an indication that the direction of loading in relation to the nail axis may influence strength of toe-nailjoints. To verify this, tests 5b and 5c were conducted. Joint 5b had four nails, with two nails placed on one side and othertwo nails placed on other side. In test 5c, only two nails were used and these were placed on one side of the jointspecimen. Due to the nailing arrangement, it was predicted that joint 5b would be significantly stronger than joint 5c.However as can be seen in Table 5, the test results are relatively close and inconclusive. The lack of clear differencesbetween the two arrangements was thought to be related to the fact that the jointing members were restrained againstrotation in all the toe-nailed joint tests reported thus far. This may ‘force’ a failure mode involving yielding of nail inpreference to withdrawal. To elucidate this further, additional tests 5d and 5e, which used the joint configurations of 5band 5c respectively, were conducted with point-side members free to rotate (Figures 6 and 7). As can be noted from Table5, the results still indicate that the influence of loading direction in relation to nail axis may not be significant.

CONCLUSIONS

Tests were conducted to study the mechanical properties of power-driven and hammer-driven nails, and strengths of nailand toe-nail joints made with these two types of nails. Based on test results the following conclusions can be drawn.

1. Yield strengths and elastic moduli of power-driven nails differ significantly between manufacturers. For 51 mmnails, most power-driven nails have higher elastic modulus and yield strength than hammer-driven nails. For 83mm nails, power-driven nails have a lower elastic modulus than hammer-driven nails. However, yield strengths ofpower-driven nails are higher than those of hammer-driven nails.

2. For 51 mm nails, the diameters of both hammer and power-driven nails are similar. For 83 mm nails however,the discrepancies between nail diameters are large, with the power-driven nails typically having significantlysmaller diameters than hammer nails.

3. Lateral strength of the nail joints is not sensitive to the variation of nail mechanical properties. Difference instrengths between laterally loaded nail joints appears to be more related to nail diameter than the yield strength ofthe nail.

4. Direct withdrawal strength of power-driven nails is significantly higher than that of hammer-driven nails. Thissuggests that the nailing methods have a large impact on direct withdrawal strength.

5. Withdrawal strength of toe-nail joints made with power-driven nails is significantly better than that of hammer-driven nails, when tested with the configuration shown in Figure 2.

6. For the toe-nail joints tested using the configurations shown in Figures 3 to 7, strengths of the two types of nailsare similar.

REFERENCES

ASTM. 1998. Standard test methods for mechanical fasteners in wood. Designation D1761, America Society of Testingand Materials, West Conshohocken, PA.

BOCA. 1997. Power-driven staples and nails for use in all types of building construction. National Evaluation ReportNER-272. BOCA, Country Club Hills, Illinois.

CSA. 1974. Wire nails, spikes and staples. CSA B111-1974. Canadian Standards, Association, Rexdale, ON.

CSA. 1994. Engineering Design in Wood (Limit states design). CSA O86.1-94. Canadian Standards, Association,Rexdale, ON.

Lau, P.W.C. and Tardiff, Y. 1987. Cracks produced by driving nails into wood effects - Effects of wood and nailvariables. Unpublished report, Forintek Canada Corp, Ottawa, ON.

Loferski, J.R. and McLain, T. E. 1991. Static and impact flexural properties of common wire nails. Journal of Tesing andEvaluation, JTEVA, 19(4):297-304.

Mohammad, M. A. H.1996. Stiffness responses of nailed OSB-to-lumber connections: influence of moisture conditioningand load cycling. PhD thesis, University of New Brunswick, Fredericton, N.B.

NRC. 1995. National Building Code of Canada. National Research Council, Ottawa, ON.

Smith, I. and Whale, L.R.J. 1985. Mechanical properties of nails and their influence on mechanical properties of nailedtimber joints subjected to lateral load. Part 1 - Background and tests on nails of UK origin. Research Report 4/85, TimberResearch and Development Association, Hughenden Valley, U.K.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the funding support of the Canadian Wood Council and Forintek Canada Corp tothis study.

Table 1 - Details of test program.Nail sizeTest

No. Type of test Manufacturer L (mm) d (mm) Test procedure

H (Hammer) 83 3.80P (Power) 83 2.97D (Power) 83 3.25B (Power) 83 3.05S (Power) 83 3.00

H (Hammer) 51 2.75P (Power) 51 2.85B (Power) 51 2.85

1 Nail bending

S (Power) 51 2.85

Three-point bending test for measurement of elastic modulusand yield strength. Span = 30mm for 51mm and 40mm for

83mm nails, and cross-head movement = d/min.

H (Hammer) 83 3.80P (Power) 83 2.97D (Power) 83 3.25

H (Hammer) 51 2.75P (Power) 51 2.85

2 Lateral strength ofnail joints

S (Power) 51 2.85

Lateral strength of shear joint using apparatus developed byMohammad (1996). Specimen details are given in Figure 1.

Each joint has four nails. Cross-head movement = d/min. Sidemember thickness = 19mm for 51mm and 38mm for 83mm

nails.

H (Hammer) 83 3.80D (Power) 83 3.25

H (Hammer) 51 2.753 Direct withdrawalstrength of nail

P (Power) 51 2.85

ASTM D1761 (ASTM, 1997). Nail penetration is 70% of naillength.

H (Hammer) 83 3.804 Withdrawal strengthsof toe-nail joints D (Power) 83 3.25

Specimen details and loading direction are illustrated in Figure2. Cross-head movement =d/min.

H (Hammer) 83 3.805 Lateral strengths oftoe-nail joints D (Power) 83 3.25

Five combinations of specimen details and loading direction asillustrated in Figures 3 to 7. Cross-head movement =d/min.

Table 2 - Nail bending properties.Nail sizeManufacturer L (mm) d (mm) E 1 (GPa) Φy 1 (MPa) Φ5% 1 (MPa) Φ10% 1 (MPa)

H (Hammer) 51 2.75 141 (12) 670 (118) 604 (115) 659 (117)P (Power) 51 2.85 153 (25) 653 (60) 636 (52) 682 (50)B (Power) 51 2.85 126 (15) 451 (48) 403 (52) 445 (48)S (Power) 51 2.85 143 (12) 694 (21) 653 (27) 702 (22)

H (Hammer) 83 3.80 210 (39) 541 (118) 490 (120) 542 (119)D (Power) 83 3.25 164 (11) 790 (79) 772 (72) 830 (64)P (Power) 83 2.97 168 (20) 580 (43) 535 (47) 580 (42)B (Power) 83 3.05 150 (12) 552 (48) 503 (53) 563 (51)S (Power) 83 3.00 197 (7) 821 (19) 754 (31) 808 (21)

1 Values are mean and standard deviation (in bracket), respectively.

Table 3 - Lateral strengths of nail joints (per nail).Nail sizeManufacturer L (mm) d (mm) Joint Strength 1 (kN) Wood Density 1 (g/cc)

H (Hammer) 51 2.75 0.637 (0.095) 0.455 (0.026)S (Power) 51 2.85 0.660 (0.051) 0.458 (0.024)P (Power) 51 2.85 0.744 (0.076) 0.455 (0.026)

H (Hammer) 83 3.80 1.234 (0.206) 0.458 (0.024)P (Power) 83 2.97 0.987 (0.193) 0.460 (0.023)D (Power) 83 3.25 1.141 (0.048) 0.460 (0.023)

1 Values are mean and standard deviation (in bracket), respectively.

Table 4 - Direct withdrawal strengths (N/mm) of nails.Nail sizeManufacturer L (mm) d (mm) Withdrawal Strength 1 (N/mm) Wood Density 1 (g/cc)

H (Hammer) 51 2.75 31.88 (11.28) 0.463 (0.023)P (Power) 51 2.85 80.05 (11.51) 0.463 (0.023)

H (Hammer) 83 3.80 47.16 (12.38) 0.463 (0.023)D (Power) 83 3.25 81.53 (20.09) 0.463 (0.023)

1 Values are mean and standard deviation (in bracket), respectively.

Table 5 - Strengths (per nail) of toe-nail jointsH (Hammer) D (Power)

Test d (mm) Strength 1 (kN) Wood Density 1 (g/cc) d (mm) Strength 1 (kN) Wood Density 1 (g/cc)4 3.80 0.69 (0.08) 0.464 (0.023) 3.25 0.87 (0.14) 0.464 (0.023)5a 3.80 1.22 (0.04) 0.464 (0.023) 3.25 1.11 (0.11) 0.464 (0.023)5b 3.80 1.01 (0.10) 0.468 (0.023) 3.25 1.05 (0.06) 0.468 (0.023)5c 3.80 1.11 (0.06) 0.468 (0.024) 3.25 0.91 (0.05) 0.464 (0.023)5d 3.80 1.00 (0.11) 0.456 (0.030) 3.25 0.83 (0.05) 0.456 (0.030)5e 3.80 0.85 (0.27) 0.467 (0.026) 3.25 0.95 (0.07) 0.467 (0.026)

1 Values are mean and standard deviation (in bracket), respectively.

50

50

63

38tt = 19 for 2" nail = 38 for 3¼" nail

89

Glued indummyend

Figure 1 - Lateral strength nail joint specimen - Test 2.

30 degree

1/3 Length

Fixed

Figure 2 - Toe-nail joint withdrawal test specimen - Test 4.

30 degree

1/3 Length

Fixed

Rotation restrained

Figure 3 - Toe-nail joint lateral test specimen - Test 5a.

30 degree

1/3 Length

Fixed

Rotation restrained

Figure 4 - Toe-nail joint lateral test specimen - Test 5b.

30 degree

1/3 Length

Fixed

Rotation restrained

Figure 5 - Toe-nail joint lateral test specimen - Test 5c.

30 degree

1/3 Length

Fixed

Free rotation

Figure 6 - Toe-nail joint lateral test specimen - Test 5d.

30 degree

1/3 Length

Fixed

Free rotation

Figure 7 - Toe-nail joint lateral test specimen - Test 5e.

0

0.1

0.2

0.3

0 0.5 1 1.5 2Deflection (mm)

3 1/4" Nail

5%d

10%d

Py Py10%

Py5%

Figure 8 - A typical load-deflection response of nail bending and various definitions of yield point.