The significance of basic and applied research on mechanical fasteners for residential construction in Australia

Build. Sci. Vol. 1, pp, 33-44. Pergamon Press 1965. Printed in Great Britain

UDC 694.4

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The Significance of Basic and Applied Research on Mechanical Fasteners for Residential Construction in Australia J. D. BOYD*

Extensive investigations have been made with plain nails in wood-to-wood joints and others with metal, hardboard and plywood side plates using mainly hardwoods having a wide range of strength properties. Variables studied included the effect of direction, nature and duration of loading, moisture content, and single and double shear of nails. Few tests have been made on other connectors. The research indicates a number of gaps in knowledge and some incompatibility with overseas data. It leads to consideration of desirable changes throughout the worm in methods of estimating allowable loads of fasteners in structures.

1.

INTRODUCTION

it was necessary to pre-drill some of the dense timbers before driving the nails. Lateral shrinkage of our hardwoods affects building practice considerably. The shrinkage varies with the species, but generally it is two to three times as great as in the softwoods of the northern hemisphere. At mechanical fasteners, it causes increased splitting with seasoning subsequent to fabrication. The seasoning of hardwoods is comparatively slow. Substantial deformation in the joint tends to occur as a result of a moisture change during loading, and this is additional to the normal creep under dead loads. Also, growth stresses are of considerably higher intensity than those in softwoods[l], and can cause difficulties in fixing the timber due to any consequent bow or spring. These distortions tend to be greatest in material from relatively young, fast-grown trees[2]. A major problem of improving building practice in Australia arises from the range of properties of the very large number of species involved. Probably 30--40 species are used structurally in substantial quantities, and more than 100 are important in local areas. Natural conservatism of builders is an additional handicap in efforts to achieve efficient practices. With big distances separating the main

THE first settlers from Britain found that most of the readily available Australian timbers were hardwoods. These were very dense and difficult to work, and generally their woodworking tools and nails were inadequate, or their nails caused severe splitting. Usually it was impossible to nail the timber after it had seasoned. The first reaction to this situation was to import European and, later, North American softwoods. However, with time, techniques were gradually developed for building with the Australian species. It was found that nails with blunt points caused the minimum of damage in species particularly susceptible to splitting; thus the practice arose of placing the nail upside-down on the timber and giving it a sharp hit with the hammer to blunt the point before driving it. Subsequently, for making boxes and crates, nails were manufactured without a point. One of the most far-reaching practices was to fabricate all structural timber in the green condition to facilitate nailing and working, but even then * Timber Mechanics Section, Division of Forest Products, Commonwealth Scientific & Industrial Research Organization, P.O. Box 310, South Melbourne, S.C.5, Australia. "¢

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J. D. Boyd

centres of population, building practices have developed somewhat differently in the several states. Although climatic conditions justify some of this, much is due to irrational adherence to procedures which developed in the relative isolation of the last century. This position is aggravated by a fairly widespread lack of good technical leadership in most bodies responsible for housing finance. Government building regulations are not progressive, and much building is under the control of housing authorities that tend to be conservative. Further, Australian building timber is usually poorly presented, and strength-graded material is generally not available. Also, the timber industry is an essentially conservative and substantially non-technical agglomeration of small businesses which, up until recent years, have not found it necessary to be highly competitive. Over the last 35 years, research in timber technology in Australia has developed against this background. The few investigations made before then were of limited value, because of inadequate control of testing conditions. Our research has been done chiefly by the Division of Forest Products (DFP) of the Commonwealth Scientific and Industrial Research Organizati~,~. The DFP was originally staffed by young Australian wood scientists who received their basic training at the Forest Products Laboratory at Madison, Wis., U.S.A., and there is evidence of this in much of our early work. However, we have now developed methods and techniques closely related to our special problems. Particularly notable has been the formulation of a sampling method enabling more efficient testing of our many species. Also of note is the extent to which statistical methods are used in our experimental designs and analyses. Australian buildings do not have to withstand severe earthquake forces, and rarely are they subject to extremely high velocity winds. Further, the holding power of nails in our hardwoods is high, and the lateral strength of simple joints in conventional construction is generally adequate. Thus, until fairly recent years, there was no apparent need of research on the mechanical fasteners used in house construction. However, when the Division turned to timber engineering, it became apparent that reliable data on the strength of nailed joints was essential to improve the efficiency of light timber framing. Recently, mechanical fasteners, other than simple nails, have assumed importance. 2. DESIGN OF NAILED JOINTS IN AUSTRALIA

Nailed joints were not generally used in important structures in Australia before 1940, as it was considered they were not sufficiently reliable. This reservation was due to the splitting that tended to * These recommendations in the TEDH were unchanged in the second edition (1962).

occur with nailing near the ends of pieces ot" hardwood, and that so obviously affected the strength of joints. Under some conditions, this splitting was very severe, but through lack of study its extent was unpredictable. A supplement to the Handbook o/ structural timber design[3] was published by Langlands{4] to provide assistance in the design of war-time structures, including those depending on nails. As no experiments had been made by the DFP to determine the lateral strength of nailed joints, the design recommendations were based mainly on American and German figures, and the assumption that 'splitting of the timber did not occur to any extent ". Guidance was given for minimizing splitting. Apparently tile recommended loads on nails took no note of the higher values likely to be associated with the comparatively high density of the Australian timbers, and thus they included a conservative factor which could be said to allow for a significant amount of splitting. The recommendations were modified in the Timber engineering design handbook[5]* (TEDH). At that time also, research data were not available on the allowable lateral loads on nails in the side grain of our species, although some tests were then being undertaken. The new recommendations took account of recent overseas data and our experience with nailed joints in Australian species. Our recommendations were based on the formula published in the Wood Handbook of U.S.A. (1955): P = Kd3/2 where P is the allowable permanently applied lateral load, d is the diameter of the nail and K is a constant depending on density of the timber. The loads recommended in U.S.A. correspond approximately to the test loads at 0.015-in joint displacement, divided by a factor of 1.6 (which no doubt includes an allowance for long-duration loading), and are approximately ~ of the maximum loads for joints in softwoods, and ~,~ of the maximum loads for joints in hardwoods] 6]. For comparison, the German recommendation is the lesser of ] of the maximum load, or the load at 0.059-in displacement in a joint subject to short-duration loading. The Swedish Building Code, by contrast, bases recommendations on a 'flow l o a d ' involving the assumption that the supporting force in the wood and the bending moment in the nail ultimately reach constant values[7]. Values of K for seasoned Australian species were obtained by extrapolation of the U.S. curve of K vs. density. Values of P, calculated from the formula, were reduced by 33 per cent for the timber strength groups A, B and C and by 25 per cent for strength group D. It was considered these reduction factors would allow for the likelihood o f splitting in the hardwoods, and for the reduced strength of joints made in green timber (the U.S. values of K are for seasoned timber). The basic loads thus derived were appreciably higher than those recommended by Langlands [4].

The Significance of Basic and Applied Research on Mechanical Fasteners The strength groups referred to above are defined in the TEDH. Strength group D is based on the properties of Douglas fir, and the average values of strength and density of timbers in the other groups increase progessively to approximately double in group A. This system of classifying a wide range of species greatly facilitates structural design and rational marketing. 3. OUTLINE OF AUSTRALIAN RESEARCH ON T H E S T R E N G T H OF NAILED J O I N T S SUBJECT T O LATERAL LOADING Practical considerations required that the investigation of the strength of nailed joints in Australian species be closely related to our strength grouping system. Certain species were selected so that for each of the four strength groups, the one or more species subjected to intensive study could be reregarded as reasonably representative of all others in the group. Also, the selected species were widely used structurally. The range of variables involved in the investigations of lateral strength of nailed joints included the direction of loading, growth-ring orientation, nail direction relative to grain, moisture condition at the time of driving and also when loading, the significance of the nails being in double or single shear, the number of nails in the joint, the penetration of the nails, moment on the joint, nail size and type, short-duration, cyclical and dead loading, thickness of timber, temperature, relationship between load, displacement, and time, wood-to-wood joints, and also metal, plywood, and hardboard-to-wood joints. Similarly, for investigations of the withdrawal resistance of nails, a series of variables was investigated. Tests made to date on other mechanical fasteners have been very much less extensive. Only very brief comment will be made on most of the experimental work indicated above. Somewhat more comment, though still brief in comparison with the extent of the experiments, will be made on the rheological studies. It is convenient to discuss all this work against the background recommendations of the TEDH. This is because the position in respect to design of nailed joints in Australia is similar to that in other countries, where it has been necessary to make some assumptions not fully supported by research data. It will be shown that recommendations based on those assumptions can provide a good basis of engineering design, until such time as modifications and refinements are fully justified. It is hoped to show also, that through the results of our research, no only Australia, but most countries should consider a review of their design recommendations at the earliest practicable date. 4. LATERAL S T R E N G T H OF NAILED J O I N T S IN SOLID TIMBER The basic allowable loads on nails as given in the T E D H are for one nail under dead loading, in side grain, and in single shear, and for joints made with

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green timber subsequently seasoned. In deriving design loads, the Handbook requires that the basic loads be modified for moisture condition, duration of loading, nails in double shear, nails in end grain, nail penetration, the use of metal side plates, and excessive splitting of the timber. The Handbook indicates that modifications are not required for direction of loading, direction of grain and multiplenail joints. Research has been done with short-duration loading on joints assembled with green timber and tested green, on others assembled with green timber and tested when the timber has fully seasoned, and on a third group assembled with seasoned timber and tested in the same condition. Species investigated were messmate stringybark (Euc. obliqua L'Herit.), jarrah (Euc. marginata Sm.), grey ironbark (Euc. drepanophylla F. Muell.), yellow stringybark (Euc. muelleriana Howitt), radiata pine (Pinus radiata D. Don), karri (Euc. diversicolor F. Muell.), spotted gum (Euc. maculata Hook.), and Douglas fir (Pseudotsuga menziesii Mirb.). Plain-shank nails of 12, 9, 7 and 4 standard wire gauge were used with various thicknesses of timber. The tests with messmate stringybark were the most detailed of the series as regards all levels and also the range of variables. Other species were tested, so that their results tied in with the messmate stringybark results. Brief comments will be made on the significance of all the variables referred to above, as deduced from this series of tests.

(a) Direction of loading Tests on hardwoods made by Mack[8] have indicated that if one or more members of a joint are loaded perpendicular to the grain, the strength at 0.015-in deformation is reduced by 20 to 30 per cent with both green and dry timber. Also with joints made with seasoned timber, there was a loadreducing effect apparent at maximum load. In view of the findings of overseas laboratories that there is no effect of grain direction and a general recommendation accordingly in the TEDH, it may be necessary to investigate this variable further; no tests of direction of loading have yet been made with radiata pine.

(b) Growth ring orientation Short-duration loading tests with messmate stringybark indicate that probably the recommendation of the TEDH, that no distinction be made between fiat-sawn and quarter-sawn timber in joints, is substantially correct for hardwoods[8]. However, no work has been done to check its applicability to Australian softwoods such as radiata pine; conceivably the effect could be different in very-fast-grown timber.

(c) Nails in end grain For nails in end grain, the T E D H recommends that the basic lateral load be reduced by one-third; no tests have been made in Australia to confirm the validity of this. However, nailing into end grain is discouraged where there is any tendency for the

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J. D. Boyd

nail to be withdrawn during handling of the structural unit, or as a result of forces applied during construction, or as a result of high lateral loads in service. This decision was made for plain nails, and may later be reviewed to take account of the higher withdrawal resistance to be expected with helically threaded nails in end grain. However, pending general availability of the threaded nails, our research studies of other factors are taking precedence.

(d) Moisture condition The effect of various moisture conditions on lateral strength of nails has been examined in a series of experiments [8-10]. Short-duration testing of various hardwood species and radiata pine has shown that during seasoning the ultimate strength of joints increases by between 30 and 60 per cent. On the other hand, the change in load at 0"015-in displacement of the joint is negligible. Relevant recommendations of the T E D H are that there should be no increase in the basic load if seasoning occurs after the joint is made and put into service, and an increase of 25 per cent in the basic load is permissible for joints made with timber below 15 per cent moisture content and kept dry in service. Thus, if the load at 0.015-in displacement is adhered to as a basis of design, our recommendation is supported. However, neglect of the effect on ultimate load may not be justified. Tests on radiata pine showed that the ultimate strength of joints made with seasoned timber was only about 13 per cent higher than that of joints made with green timber, while the load at 0.015-in joint displacement was about 36 per cent higher than that of the corresponding green joints. Thus, for radiata pine, the recommendation of a 25 per cent increase of basic load for a joint made with seasoned timber is a reasonable compromise, whether one considers the 0.015-in displacement or the maximum load as the basis for design.

(e) Nails in double shear Mack[8, 9] has shown that generally the maximum loads on joints with nails in double shear, and also the loads at 0.015-in displacement are only 1 1 to l ~ times the corresponding values for nails in single shear. The U.S. National Design Specification[ 11] recommends an allowable load 1½-1 ~ times the single shear value, according to the thickness of the members. The Wood Handbook[6] and the B.S. Code of Practice112] make no reference to double shear in nailed joints, while the German Standard Specification[13] and the T E D H each allow twice the basic load for single shear. Clearly, if our research data were supported by results with other species, there would be justification for making amendments to some of these recommendations.

( f ) Multiple-nail joints Where a number of nails was used, Mack[8J showed that for those of 9 and 7 S.W.G. in initially green messmate stringybark, the average load per nail was affected, both at maximum load and at 0.015-in displacement, when there were more than

six nails in the joint. The strength per nail was reduced by approximately 20 per cent for 12 nails in a joint. In comparison, Deutscher NormenausschussJ 13 j specifies a reduction in allowable load per nail of 10 per cent for 10 nails, and 20 per cent for more than 20 nails in the joint. However, the Australian recommendation, based on that of U.S.A., did not advocate any reduction. To determine the significance of the number of nails in a joint, it is desirable to do additional testing with hardwoods and softwoods, such as radiata pine.

(g) Nail penetration For Australian strength groups A, B and C, the T E D H makes a recommendation, based on overseas practice, e.g. National Design Specification (1954, 1960), that at least half the length of the nail should penetrate the last piece of timber, and for group D at least two-thirds of the length should penetrate the last piece. No tests to check the validity of this recommendation have been made with Australian species. With the lengths of nails generally used in our framing timbers, usually sufficient penetration is achieved with the hardwoods. However, in some areas where hardwoods have been used invariably for many years, locally-grown exotic softwoods, particularly fast-grown radiata pine, are now being used. The nail length and gauge suited to the hardwoods tend to be adopted for the softwoods, and in such cases inadequate penetration can be expected. However, this problem is not one for urgent research, but for effective guidance, as in a code of good practice for nailing. However, the apparent sufficiency of data on penetration is related to design recommendations being based on the load at 0.015-in displacement under load. If more note is to be taken of maximum load, or even of an appreciably higher displacement, this matter should be reconsidered. Nail penetration could become of importance for short nails through side plates of metal, hardboard or plywood.

(h) Spfitting and nail spacing No precise information on nail spacings is given in the American literature and the recommendations of the T E D H are those given in the British Standard Code of Practice[ 12]. Observations of splitting have been made during nailing of test joints of the various species. Immediate effects and the aggravated effect of seasoning have been noted. Considerable variability occurs within and between species, but it appears that for free-splitting species, the spacings recommended are the minimum likely to lead to satisfactory joints. There are indications that spacings of the order of 50 per cent greater than the basic ones are required when nailing some green dense hardwoods, such as grey ironbark, without pre-drilling. However, the spacings with pre-drilling appear to be satisfactory. Where significant splitting occurs, the basic spacings should be increased.

The Significance of Basic and Applied Research on Mechanical Fasteners With hardwoods, small surface splits almost invariably occur at the nails, and these are inclined to become more pronounced during seasoning. The splits are not directly associated with nail spacings, but undoubtedly they affect the loads at small displacements of the joint. Further, when the splits are combined with seasoning checks, the effect on displacement under load can be severe. There is justification for more intensive study of splitting. Nail spacing and board thickness appear to be related; this needs investigation. Attention should be given also to the problems associated with the use of large diameter nails through thin boards.

(i) Special nails A few short-duration loading tests of hardwood plain nails in green messmate stringybark, and of annularly threaded hardened nails in dry radiata pine, gave loads at 0.015-in displacement that were little different from those for ordinary plain nails of the same gauge. On the other hand, helically threaded, low-carbon-steel nails in dry radiata pine showed an increased load-bearing capacity of about 33 per cent at the basic displacement. In all cases, the maximum loads were increased by 30-80 per cent. Because technically efficient, special structural nails, such as those helically threaded, have not generally been available in Australia, no mention of design loads for them is made in our Handbook. However, the indication of increased ultimate loadbearing capacity for the special nails, again calls attention to the desirability of reviewing the criterion of recommendations on joint design. As more special nails become available in Australia, additional testing will be done. (j) Mixed species The possibility of the use of mixed species in joints is more real in Australia than in most countries in the northern hemisphere, because many timbers that are difficult to identify in the sawn form may be marketed in groups. However, the accidental use in a joint, of timber from more than one strength is fairly unlikely, as timbers of different strength groups are usually fairly readily separated. It should be safe to assume that a joint made with mixed species is no weaker than one made with the weaker of the species included, but it could be intermediate in value. No tests of joints with mixed species have been made in Australia, but limited testing may be justified. Meanwhile, no recommendation on this matter has been given in our Handbook. (k) Skew- or slant-driven nails and clinched nails Limited tests of lateral load-bearing capacity have indicated that, for slant-driven nails, there should be no change in the basic design load. This is the result of an investigation of the load at 0.015-in displacement, rather than the maximum load. Similarly no effect at 0.015-in displacement was indicated for the clinched nails, and the effect

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at maximum load depended on the direction of clinching. Other tests have been made of ceiling and roof members of Douglas fir secured by skewdriven nails and subject to long-duration load testing[14]. Although skew-driving slightly increases the withdrawal resistance of nails, the TEDH recommends against it, except in joints where there is no reversal of stress and where the direction of slant is such that the joint would tighten under load. Because of the limitations on this procedure, no additional research is being planned.

(l) Temperature Long-duration load studies have indicated, incidentally, that temperature has some effect on the behaviour of nailed joints under load. However, no direct investigations have yet been carried out, although some are at present being planned. Recommendations on this effect have not been available from overseas, and none are made in our Handbook. (m) Metal-to-wood joints Observation of the tendency of wood to split in a way related to nail spacing in an unrestrained member, has indicated that a closer spacing of nails than is recommended for wood-to-wood joints may be practicable with metal side plates used in conjunction with green messmate stringybark. Tests with 20-gauge steel plates, joined to green messmate stringybark with 11 S.W.G. nails, showed an effect of nail lengths up to ~ in on the load at 0.015-in displacement, and an effect on maximum load of nail length up to 1¼ in (the maximum tested). There was no apparent difference between joints made with 18- and 20-gauge plates. The test data for messmate stringybark indicated that the Wood Handbook, British Code of Practice, and TEDH recommendations of a 25 per cent increase in allowable load with metal side plates may not be justified for joints in green timber. It appears that the green centre member may be the critical part of the joint at a displacement of 0-015 in and also at maximum load. However, more extensive short-duration and long-duration studies are desirable. (n) Plywood-to-wood nailed joints Some experimental work involving plywood side plates has been reported by Mack[10]. In this, the short-duration loading tests were made with ¼-in plywood of mixed D group species nailed to green messmate stringybark main members. Other tests were made with plywood of spotted gum and karri (B group), in thicknesses of ¼, ½, ~ and ~ in, nailed alternatively to green messmate stringybark main members and to dry radiata pine main members. Research data from these tests are not completely analysed, but several conclusions have been reached. When comparing different plywood thicknesses, there is little effect on the load at 0"015-in displacement of thickness of plywood about .~ in, but with i-in plywood the load-bearing capacity is

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J. D. Boyd

reduced by approximately 10 per cent. Nail lengths above three times the plywood thickness have no apparent effect on the load at 0.015-in displacement, but they have a significant effect on the maximum load. Grain direction of the plywood has no effect, and the maximum load appears to be affected more by the solid timber than by the species of plywood. Insufficient evidence is available to determine the proper procedure in design of joints with plywood side plates. However, although matched specimens were not tested, it appeared that as with metal side plates, dry plywood in combination with a green main member had no appreciable effect on increasing the load-bearing capacity over that of an all-green timber joint. As plywood may frequently be used with green timber, additional research is necessary with other species to determine the mechanics of a joint with plywood plates, and to enable a review of current recommendations.

(o) Hardboard-to-wood joints Short-duration loading tests have been made with i% and k-in tempered hardboard nailed to green messmate stringybark. The results have not been reported, but they have been used extensively in the design of trusses using hardboard gussets, and similarly for other structures such as box-beams, I-beams and rigid frames. The information available from these tests has been supplemented by long-duration loading studies which will be referred to later. There is insufficient evidence to completely determine the mechanics of joints with hardboard side plates. Because the quality of the hardboard varies with manufacturers, and it may vary from time to time with the same manufacturer, it is not intended to publish recommended loads until the quality of material is standardized. Meanwhile, limited additional testing will be undertaken.

(p) Cyclic loading Modification factors for repetitive or cyclic loading have not been referred to in the T E D H or other references cited, although some joints in houses are subject to load reversals, or at least to cyclical loading above a base load. However, a modification factor of 25 per cent is given in our Handbook to increase the basic load for combinations of dead and live loads, and one of 50 per cent for combinations of dead and wind loads. These factors are in accordance with the normal allowances for duration of load on solid timber. Repetitive loading tests have been made by Mack[15] on radiata pine, jarrah and karri joints, with up to 10,000 cycles of loading based on shortduration-load displacements ranging from 0.010 to 0-060 in. The displacement of the joints increased considerably with the repetitive loading, being greater the higher the initial displacement, and relatively less with dry timber. There was some indication that the increase of displacement was less with joints made with thick nails. Fatigue failure of the nails in the joint did not generally occur, but some joints in jarrah, which

were subject to the initial high displacement of 0.060 in, failed in this way at about 9000 cycles. The strength of joints tested to failure under shortduration loading, after being subject to 10.000 cycles of load, was not significantly different from the strength of matched joints tested initially under short-duration loading. Rates of increase of displacement due to repetitive loading were shown to depend substantially on the maximum loads applied in the cycling, and on the moisture content of tile wood. Where the load was moderate, such as corresponding to an initial displacement of 0.015 in, the increase in displacement after 10,000 cycles was relatively small: it was 50 to 60 per cent for joints fabricated from seasoned wood. However, the hardwood joints that were kept green throughout the test increased in displacement by about 1 } times, and the green radiata pine by about 3.1, times. Even with joints having a high initial displacement of 0.060 in, 10,000 repetitions of load caused a displacement increase of only l~- to 3~ times the initial value, the higher ratio being associated with green timber. Another series of cyclic loading tests was made on joints of initially green messmate stringybark that had been under long-duration loading. Loads were varied daily for 2 weeks within the limits of 25-50, 50-75 and 75-100 per cent of the original long-duration loads. Although these tests were not extensive, they indicated that a recurring live load definitely increased the permanent displacement of the joint.

(q) Long-duration loading tests Long-duration loading tests have been made by Mack[ 10] with initially green messmate stringybark and dry radiata pine using 12 S.W.G. nails in single shear, and 9 S.W.G. nails in double shear. All timber was l-in thick, and loads on the messmate stringybarkjoints corresponded to 1, 1 ~, 2, 2½ and 3 times the basic loads proposed in the TEDH. The joints were sheltered from rain and the wind, but otherwise moisture conditions and temperature were not controlled. With the basic loads, displacements in messmate stringybark joints reached a more or less stable average of 0.04 in after 1 year. This was approximately 10 times the initial displacement. In the same period, displacements of radiata pine joints, under 1.1.,times the basic load, reached a stable value of 0.03 in, or 3 times the immediate displacement. Ninety per cent of the creep displacements, which occurred over the 2-3 years of testing, took place during the first year of loading. Data from initially green messmate, stringybark joints indicated that seasoning after fabrication, and rate of drying could have an effect on the initial displacement rate. Also, results of tests on joints of hardboard and dry messmate stringybark, at higher-than-ambient temperatures, indicated that displacement is affected by variations of atmospheric conditions that change the moisture content of the timber. For convenience in the following

The Significance of Basic and Applied Research on Mechanical Fasteners discussion, the total displacement which occurs under long-duration loading will be referred to as creep. However, in the case of green timber, a substantial part of the deformation is directly attributable to the extent of the moisture change during seasoning under load. A few tests were made with hardened annularly threaded nails in dry radiata pine. After 2 months there was little difference between the creep of joints with these nails, and that of other joints with plain nails of the same gauge. From these limited data, there is an indication that little or no increase in basic loads should be allowed for hardened and annularly threaded nails in solid timber. Overseas data on this factor appear to be somewhat limited, and clearly there is a need for more testing before formulating any firm recommendation, particularly if an allowance for the higher ultimate strength and possible higher stiffness of joints made with such annular and helically threaded and hardened nails is to be justified. From data on joints made with plywood side plates and messmate stringybark and radiata pine main members, and also for similar joints made with hardboard, it was concluded that the displacement of the joint at any given time was approximately proportional to the square of the load, and that for a given load the displacement could be expressed as an exponential function of time. If the creep is expressed as a fraction of the initial displacement of the joint, it appears that a reduction in that fraction occurs as the load is increased[16]. This is different from the behaviour of timber alone, as reported by Kingston and Clarke[17]. In tests made with plywood of Group D species, nailed to initially green messmate stringybark, and subjected to loads of 40 to 120 lb per nail, creep ceased to be significant after about 30 days. This compares with approximately 500 days for a similar reaction with joints of solid timber. Other tests were made with t-in tempered hardboard nailed to initially green messmate stringybark, and with loads from 40 to 120 lb per nail. These gave similar results to the joints with plywood side plates, except that after 1 year the creep displacement was about 50-70 per cent of that of the plywood joints. Only limited data are available for creep tests on joints made with messmate stringybark and metal side plates. Using 20-gauge plate and 11 S.W.G. nails, and loading for 1 year with 214 lb per nail, the creep was much less than in joints made with solid timber. In fact there was a considerable gradation in creep between the joints with metal side plates and those with hardboard, and also between the latter and the plywood and the solid timber joints. This could justify higher working loads based on stiffness for some of these joints, even though shortduration load data did not indicate this. Summarizing, it might be said that for several species of timber, different nail sizes, and different moisture conditions, there is a gradual increase in the relative displacement of the timber members with duration of loading. At the several load levels

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examined, the creep rate was at first relatively high, but diminished at an increasing rate. With joints under long-duration loads of up to half their shortduration load strength, the major portion of the creep occurred during the first year. With joints using plywood or hardboard gussets, most creep occurred in approximately 1 month. 5.

WITHDRAWAL RESISTANCE OF NAILS

The withdrawal resistance of nails is important with roofing and flooring timbers and in prefabricated components subject to severe transport and handling forces. Many withdrawal tests have now been made in Australia on a number of species. Langlands[18] reported studies of withdrawal resistance under static and impact loading on some special nails in air-dried western hemlock (Tsuga heterophylla Sarg.). Mack[19] discussed similar tests on special nails in radiata pine. Other withdrawal tests on special nails were reported by Mack[20], but some additional investigations of withdrawal resistance of plain and processed cementcoated nails in radiata pine, mountain ash and karri have been discussed only in ~an internal report of the DFP. Generally, results have shown that the basic loads given in the T E D H are in the range of ~ - ~ of the maximum withdrawal resistance in green timber, and therefore they appear to be at least quite safe. Our research has shown that withdrawal resistance is affected by the orientation of growth rings, though not to a practical extent. It is increased slightly by pre-boring, and in end grain, the resistance is approximately half that in side grain. Withdrawal resistance in seasoned timber is not much different from that in green timber. Moisture conditions have been shown to have a considerable effect. As a result of seasoning of the timber after driving, plain nails lose up to 60 per cent, and all types of so-called improved nails, except threaded nails, up to 70 per cent of their initial withdrawal resistance. Even in seasoned timber, the ' i m p r o v e d ' nails lose some withdrawal resistance with time; in many cases, they are not significantly better than plain nails. On the other hand, as a result of seasoning of the timber after driving, annularly and helically threaded nails increase their withdrawal resistance by up to 60 per cent. Also, it has been confirmed that nail ' popping' or ' backing out ', with changing moisture content, is reduced with annularly threaded nails. Using dry radiata pine, Mack[21] investigated the relationship between withdrawal resistance, nail diameter and penetration. He confirmed that the static resistance was proportional to the nail diameter and penetration, while the impact value was proportional to the diameter and the square of the penetration. However, a similar analysis of data on Australian hardwoods may be desirable. Some tests are under way to study the creep characteristics of nails in withdrawal.

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J. D. Boyd 6. OTHER M E C H A N I C A L FASTENERS

Although tests of both short- and long-duration loading on individual joints and structures have been made in Australia, with a number of mechanical fasteners in addition to nails, the range of the experimental work on special fasteners is much more limited than that on nails.

(a) Framing anchors As extremely severe wind loadings are rare in most parts of Australia, and seismic forces are of a low order and rare, there has been little need in most areas to provide rigid metal fastenings throughout house frames. Thus, only limited tests have been made. However, in some coastal areas, strong winds must be seriously considered in relation to the security of the roof structure particularly, and especially with low pitch roofs such as used on many contemporary homes. Because of the tendency of the Australian hardwoods to split when nailed close to the end of the members, as is generally involved with use of these fasteners, it appears that they cannot be given working loads higher than those advocated for less-dense timbers in U.S.A. Unfortunately, an inadequate investigation of variables, and the consequent limitations of data from Australia, U.S.A. and Canada, on the strength of joints made with framing anchors, make it difficult to derive safe working loads. The position is further complicated by the fact that sound general principles applicable to the derivation of working loads for all mechanical fasteners in timber have not been established.

(b) Punched plates A variety of different types of metal plates has been used as gussets, and as combined fastenings and gussets. Relatively large gussets shaped for the particular trusts have been used in moderately high pitched trusses in Australia. Generally these have been punched by the nail as it is driven, and the nail has been clinched over the second plate, or the plates have been drilled and riveted to the timber. However, such gussets have not been extensively used in Australia, and the following discussion refers to plates pre-cut and pre-formed for use with a wide range of truss sizes and shapes. ( 1) Pre-punched metal gussets-- The term ' prepunched metal gusset' is intended to apply to the type of plate that is punched to provide nail holes only, and is used with nails and basically as a simple gusset. To reduce the cost of the plates to a minimum, and so make them competitive with hardboard, for example, the tendency is to make them small and the nail spacings close. With this type of gusset, the critical problem in Australia is the splitting which tends to occur, particularly towards the end of the piece of timber. With green timber seasoned after nailing, this problem is aggravated. So far, these plates have been little used here, and because of the competition with other types of plates and gussets, an extension of their use seems

to be dependent on control of splitting. Such a control may not be achieved except through an extensive investigation of the fundamental factors involved, and a significant change in the design of the plates. Some design information can be derived from the data referred to earlier, but this is inadequate. (2) Pronged plates--Here, the term "pronged p l a t e ' is meant to describe metal plates that are punched in a manner producing short triangular projections standing out at approximately right angles to the sheet. These prongs are subsequently forced into the timber by a powerful press, so that they act as mechanical fasteners. Such gussetfasteners, with approximately circular holes in the finished sheet, have been extensively used in Australia for the manufacture of light timber roof trusses or trussed rafters. One type of pronged plate has been designed with a view to the prongs turning over slightly and locking in the timber as they penetrate. The manufacturers claim that this characteristic overcomes the problems of loosening and ' backing out ' that are generally associated with shrinkage of our hardwoods. Unfortunately, without the provision of additional nails to provide against withdrawal, this type of fastener is not always satisfactory. This is because it must be designed for use in very dense and sometimes in seasoned timber, and the locking action of the prongs depends on the hardness of the timber. From time to time, the plates may be used with timber of substantially lower hardness as, for example, with species that are less dense or at higher moisture content than is ideal for the particular plate thickness and prong design. In such a case, satisfactory locking of the prongs in the wood is not achieved, and the plates may be sprung out of position as a result of rough handling. Only relatively limited testing in respect to species and conditions of use has been done with this type of plate. Some tests on individual joints and small model structures were made at one of the Australian universities, and a few short- and long-duration tests of trusses have been made by the DFP. Although the engineering needs of an approximate basis of design may be claimed to be reasonably satisfied, the data are not adequate for the more analytical and demanding wood technologist, because of the lack of a widely acceptable basis of design of fasteners. However, in Australia there is a strong trend for this type of fastener to be replaced by the toothed plate fastener. (3) Toothed plates-- Here, the term " toothed plate ' refers to the type of fastener in which short, narrow and pointed metal strips are punched from the plane of the plate, but each is left attached to it at one point and standing out approximately at right angles. Each such punching takes the general shape of a flattened nail, and an appreciable number of nails are punched from each gusset. Subsequently, the projecting teeth or ' nails ' are forced into the timber by a powerful press, to form a combined gusset and fastener.

The Significance of Basic and Applied Research on Mechanical Fasteners As in the ease of the pronged plates, a number of tests have been made of simple tension joints, and others have been made on model structures designed to indicate the reaction of the plate bending moments applied to the joints. Some shortduration tests of trusses have also been made, but no data on long-duration tests are available. The data are somewhat inadequate for the rational derivation of allowable unit loads by wood technologists, although in conjunction with sufficient performance tests of particular forms of structures, they may be used to indicate a moderately effective basis of design.

(c) Threaded nails Because efficient special nails, such as the helically threaded and annularly threaded, hardened, steel nails used in U.S.A., are not generally available in Australia, no significant amount of testing has been done with them. However, it is anticipated that the manufacture of nails of this type will be commenced in Australia shortly, and investigations of their characteristics in our timbers will then be undertaken.

7. DESIGN OF JOINTS USING MECHANICAL FASTENERS Even if consideration is limited to joints made with plain nails, the comparison of experimental and theoretical studies, that have been made in different countries, leads to considerable confusion. For example, according to the data obtained in U.S.A.[6], the load-bearing capacity of a nail varies as d a/2. On the other hand, Mack[8, 9] concluded, within limitations, that for nailed joints in both messmate stringybark and radiata pine, the strength at 0.015-in displacement and at maximum load was proportional to d 2. He also showed that certain formulae developed in Europe could predict the maximum strength of joints, if certain basic data were known, but with our species they could not satisfactorily predict the load at 0.015-in displacement. Other observations were that the thickness of members could have an influence on strength, and that nail withdrawal resistance and also friction between members should have a considerable effect on the ultimate strength of joints subject to shortduration loading. If the present practical application of the research data is considered, much wider and more critical differences may be observed. For example, in U.S.A. and Australia, the design value is based on the load corresponding to a displacement in the joints of 0.015 in. American data indicate that this basis of design leads to a reduction factor of -~ in the maximum strength of the joint in softwoods, and r~T-in hardwoods. This is very different from the basis of design in Germany, where the basic load is at least k of the maximum load, and the load corresponding to a joint displacement of

41

0.059 in. Both methods of deriving design loads are different to that of Sweden. At least the German and the American recommendations have been current for a considerable time, and a very large number of structures must have been built using the widely different basic design criteria, yet in the technical press there is little evidence that the big differences have been given critical examination by engineers or wood technologists. This situation surely presents a challenge! The U.S. recommendation implies a primary emphasis on the stiffness of the joint, whereas the German approach to design places much greater emphasis on a more effective use of the ultimate load-bearing capacity of the joint. The Swedish method also takes account of the stiffness of the joint, but in a different way. It is recognized that the acceptable standards of performance of buildings, having particular regard to stiffness or deflexion, are likely to vary in different countries. On the other hand, economy requires the most efficient use of material and labour, so that expenditure on joints cannot be justified beyond where it is assured that there is reasonable stiffness associated with a reasonable probability that failure will not occur. Too much emphasis cannot be placed on the relevant probability factors. Many wood technologists recognize this, and use statistical methods to define the most probable value and the variability of each factor studied. However, the concept now referred to is broader than this. It is recognized by modern structural engineers that no joint or structure can be guaranteed absolutely against failure, or even against less than ideal performance, as in stiffness. On the other hand, given sufficient data and a suitable statistical analysis, it is possible to determine a basis of design such that the probability of adequate performance is acceptable, without being unrealistic or uneconomic. When a considerable number of factors are involved simultaneously, Wood[22] and Boyd[23] have shown that probability methods can be very useful in determining design recommendations for timber units, even when some of the statistical data are not exact or complete. The extreme difference between various current national recommendations indicate that the application of these methods to the design of joints made with mechanical fasteners is overdue. In considering a timber beam or a structure such as a truss, it is usual to regard strength and stiffness as criteria of performance that should both be satisfied, although considered separately. It is logical to treat the design of joints in a similar way. Experience in timber design has shown that either criterion taken separately is not likely to lead to an economic structure, even if the design value were set at such a level as to lead to a reasonably safe structure. Therefore, consideration should be given to the standards of stiffness that are required in different types of joints, in different types of structures, and in locations involving substantially different performance requirements. Separate but

42

J. D. Boyd

similar consideration should be given to a basis of design for checking the adequacy of strength. Conceivably strength reduction factors used in design might be varied for different classes of structures. Consideration allied to the foregoing have been referred to by workers in a number of countries. For example, Granholm124] analysed the effect of joint stiffness on the stability and strength of the timber scaffolding for a large concrete arch; Niskanen[25] analysed the significance of joint stiffness in composite wood columns; and Nor6nL 7] referred in detail to the Swedish method of basing the design of nailed joints on a ' flow ' load defined as a yield at constant rate at constant stress or load. It is conceivable that such studies may lead to an acceptable combined strength and stiffness criterion, but such a proposal would require very critical study. The need of a review of design methods, which appears to be a logical deduction from the review of research herein, is emphasized when consideration is given to special nails that assure high ultimate loads, and to other mechanical fasteners, such as toothed plates, where the relatively rigid combination of fastener and plate is used to make the joint. Design methods variously applied to nailed joints are not entirely satisfactory, but it is irrational to design timber joints using other mechanical fasteners, on principles that will inevitably lead to different standards of performance or efficiency, and that cannot be applied to the design of nailed joints. In deriving a new basis of design of joints made with mechanical fasteners, it is necessary to give careful consideration to the effect of long-duration loading as, under some circumstances, stiffness under continued loading can be more critical than under short-duration loading, and also different for various classes of joints. All direct experimental data on extended loading that are now available should be taken into account, and they should be supplemented by critical observations of a wide variety of structures in service. The results of observations in different countries should be collated, due care being taken to differentiate between the different methods of construction, exposure conditions, and standards of acceptance.

8. AVAILABILITY OF RESEARCH DATA TO USERS OF M E C H A N I C A L FASTENERS

Scientific papers dealing with research on mechanical fasteners are not consulted by the general user; they attract few people other than wood technologists concerned with timber engineering. Nevertheless they should be readily understood, and in this connexion it would be helpful if American wood technologists always gave the standard wire gauge and/or the diameter of nails as an alternative to the very confusing ' p e n n y ' sizes. In Australia, in contrast with the lack of

general reading of scientific papers, the T E D H is widely and frequently referred to by engineers and others, so that persons capable of making a structural design can obtain guidance on proper procedures in the use of fasteners for which reliable data are given. Those concerned chiefly with light timber framing are unlikely to refer to the TEDH. For the average builder, it is necessary to specify and illustrate the details of construction rather than the basis of design. Accordingly, a code of practice is being prepared by the Standards Association of Australia, with assistance from the DFP. It will include details of efficient nailing and possibly some additional mechanical fastenings for all important features of house construction, and will serve as a basis of specifications in conjunction with building by-laws. Somewhat related to the above project is one being sponsored by the DFP, to develop a specification for the performance of light timber trusses such as used in house construction. After formulation of short-duration tests to check the probable performance of the truss under service conditions, and possibly also its ability to withstand forces likely to be developed during rough handling and transport, the draft proposals will be submitted to truss manufacturers, lending authorities and others for comment, and later developed as an Australian standard. There is a complex problem of estimating the performance, under long-duration loading, of the members, joints, and the complete structure. Also, decisions must be taken on the relative importance of stiffness, and a satisfactory but not excessive margin of strength above the design value. Thus, all the factors discussed for the fasteners are involved, as well as the shape of the design, the reaction of the timber, and the general control o f quality and uniformity of production.

9. CONCLUSION There is a need for more data on the short- and long-duration loading characteristics of metal fasteners, ranging from simple nails to the more complex pronged and toothed plates. Some manufacturers could give greater assistance to wood technologists and engineers, by publishing all strength data obtained for their products, and in many cases they would benefit by discussing their test proposals and the derivation of design stresses or loads with experienced timber technologists, such as are available in the national laboratories. It is of considerable importance and urgency to define the correct principles to be followed in the derivation of design stresses. A definition of the significance of deflexions and the acceptable levels in various parts and classes of structures is necessary. Also, consideration should be given to the fact that the comparative stiffness of different joints is not necessarily the same in short- and long-duration loading. At the same time, attention should be

The Significance of Basic and Applied Research on Mechanical Fasteners given to deciding suitable ratios between the maximum load-bearing capacity as indicated by shortand long-duration tests, and working loads or stresses. In defining criteria in design for stiffness and strength, it is necessary to give attention to the

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variability of the timber, the characteristics of joints made with various fasteners, the conditions of use of the structural units and the possibility that several classes of use might justify different standards.

REFERENCES 1. J.D. BOYD, Tree growth stresses I. Growth stress evaluation, Aust. J. Sci. Res. B 3, 270-93 (1950). 2. J. D. Bovo, Tree growth stresses II. The development of shakes and other visual failures in timber, Aust. J. Appl. Sci. 1, 296-312 (1950). 3.

I. LANGLANDSand A. J. THOMAS,Handbook of structural timber design (2nd edn.). Coun. Sci. Ind. Res. (Aust.), D.F.P. Tech. Pap. No. 32 (1941).

4.

I. LANGLANDS,ibid. Supplement No. 1. Large timber structures. Notes on design specification and inspection (1942).

5.

R . G . PEARSON,N. H. KLOOT and J. D. BOYD, Timber engineering design handbook (2nd edn.). C.S.I.R.O. and the Jacaranda Press, Melbourne (1962).

6.

U.S. Dept. Agric. Wood Handbook, Handb. No. 72. Supt. Documents, Washington, D.C. (1955).

7.

B. NOR~N, Nailed j o i n t s - a contribution to the theoretical analysis of yield and strength. Proc. 1st lnternat. Conf. Timber Engineering, Southampton, pp. 66-76. Southampton Univ. and Timber Res. and Develop. Assoc., London (1961).

8. J.J. MACK,Thestrength of nailed timber joints. I. In messmate stringybark. C.S.I.R.O. Aust. Div. For. Prod. Technol. Pap. No. 9 (1960). 9. J.J. MACK,The strength of nailed timber joints. II. Radiata pine. C.S.I.R.O. Aust. Div. For. Prod. Technol. Pap. No. 21 (1962). 10. J.J. MACK,A study of creep in nailed joints. C.S.I.R.O. Aust. Div. For. Prod. Technol. Pap. No. 27 (1963). 11. National design specification for stress-grade lumber and fastenings. National Lumber Manufacturers Association, Washington, D.C. (1960). 12. Council for Codes of Practice for Buildings, Great Britain; The structural use of timber in buildings. B.S. Code of Practice CP 112 (1952). 13. Deutscher Normenausschuss; Holzbauwerke, DIN 1052 (4th edn.) (1947). 14. K . J . WYATT, Movement of traditional timber eaves joints under long term loading, Build. Mater. 3, 28-29 (1961). 15. J. J. MACK, Repetitive loading of nailed timber joints. Prod. Technol. Pap. No. 10 (1960).

C.S.I.R.O. Aust. Div. For.

16. J.J. MACK, Creep in nailed joints, Nature, Lond. 193, 1313 (1962). 17. R . S . T . KINGSTONand L. N. CLARKE, Some aspects of the rheological behaviour of wood I. The effect of stress with particular reference to creep, Aust. J. Appl. Sci. 12, 211-226 (1961). 18. I. LANGLANDS,The holding power of special nails. C.S.I.R. Aust. Pamphlet No. 46, Div. For. Prod. Technical Paper No. 11 (1933). 19. J.J. MACK, Tests on the holding power of special nails in radiata pine, Aust. J. Appl. Sci. 2, 454-463 (1951). 20. J.J. MACK, Grooved nails, Aust. Timb. J. 26, 43, 44, 46, 50, 131 (1960). 21. J. J. MACK, The relation between nail withdrawal resistance and nail diameter and penetration. C.S.I.R.O. Aust. Div. For. Prod. Technol. Pap. No. 11 (1961). 22.

L.W. WOOD, The safety factor in design of timber structures, Proc. Amer. Soc. Civil Engng. 84, No. ST7 (1958).

23.

J.D. BOYD, The strength of Australian pole timbers II. Principles in the derivation of working stresses for poles. C.S.I.R.O. Aust. Div. For. Prod. Technol. Pap. No. 22 (1962).

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24.

H. GRANHOLM,Scaffolding for the arch of Sandoe bridge (Sandoedrons bagstaeUning), Transl. Trans. Chalmers Univ. Technol., Gothenburg, No. 239, D.S.I.R. (Gt. Britain) (1961).

25. E. NISKANEN,Investigations on the buckling of compressed columns assembled by nailing. Valtion Teknillinen Tutkimuslaitos (State Inst. Tech. Res., Finland) Julkaisu (Publication) No. 63 (1961).