Performance of BOLTED JOINTS in DOUGLAS-FIR

Performance of BOLTED JOINTS IN DOUGLAS-FIR

U.S. FOREST SERVICE RESEARCH PAPER FPL 2 MAY 1963

FOREST PRODUCTS LABORATORY U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE - MADISON, WIS. This U.S. Forest Service Research Paper, FPL 2, is the second in a new series issued by the Forest Products Lab oratory describing research results and developments. SUMMARY

The strength and behavior of machine bolts in timber joints with wood and metal side plates were evaluated by subjecting three-member joints with one and four bolts to lateral loads bearing either parallel or per pendicular to the grain in the wood center member of the joint. Joints were made with 3/4-inch bolts in lumber at three stages of seasoning and with 1/2-, 3/4-, and 1-inch bolts in nominal 3-inch-thick Douglas- fir dry lumber.

The results of this study indicate that current design procedures for bolted joints are technically sound in sofar as this study applies. The bearing stresses on a per-bolt basis for the four-bolt joints were about 10 percent lower than those for single-bolt joints with bolts bearing parallel to the grain. CONTENTS

Page

INTRODUCTION...... 1

DESCRIPTION OF SPECIMENS ...... 2 METHODS OF TEST...... 3

PART I: EFFECTS OF SEASONING ON JOINT PERFORMANCE ...... 5

Description of Material ...... 5 Presentation of Data...... 7 Discussion of Results ...... 8

PART II: EFFECTS OF BOLT DIAMETER ON JOINT PERFORMANCE . 17 Description of Material...... 17 Presentation of Data...... 18 Discussion of Results ...... 21 CONCLUSIONS ...... 30

APPENDIX ...... 33 PERFORMANCE OF BOLTED JOINTS IN DOUGLAS-FIR

By DONALD V. DOYLE and JOHN A. SCHOLTEN, Engineer

Forest Products Laboratory,1 Forest Service U.S. Department of Agriculture

INTRODUCTION

This report presents the results of of bolt diameter was studied in a sec experimental research conducted at ond phase of the investigation with the Forest Products Laboratory to joints constructed of closely matched study the strength and behavior of Douglas-fir lumber at about 12 per bolts in Douglas-fir lumber. The cent moisture content. factors investigated included one-bolt and four-bolt joints, loading either Previous investigations conducted at parallel or perpendicular to the the Laboratory concerning bolted grain, side plates of wood and steel, joints, onwhichthe present published holes of bolt size and 1/16-inch over design information is primarily size, stage of seasoning, and bolt di based, 2, 3, 4 were for the most part ameters of 1/2, 3/4, and 1 inch. The conducted on joints with a single bolt effect of seasoning was studied in an in seasoned lumber. The research in initial phase of the investigation with this report was planned to supplement 3/4-inch bolts in joints constructed the information obtained in the previ of green and dry lumber. The effect ous studies.

1 Maintained at Madison, Wis., in cooperation 3 National Lumber Manufacturers Association. with the University of Wisconsin. National design specification for stress- grade lumber and its fastenings. 64 pp., illus. 1962 Edition. 2 Trayer, George W. The bearing strength of 4 U. S. Forest Products Laboratory. Wood wood under bolts. U.S. Dept. Agr. Tech. Handbook. U.S. Dept. Agr. Hdbk. NO. 72. Bull. 332. 40 pp., illus. Oct. 1932. 528 pp., illus. 1955. DESCRIPTION OF SPECIMENS The specimens evaluated in this in- All specimens were three-member vestigation were fabricated at two dif- joints consisting of a single wood cen ferent times and of material from two ter member bolted between two wood different sources. In the initial phase or steel side members with machine of the investigation (Part I), the effect bolts placed in double shear. The of lumber seasoning on joints with center member was a nominal 3 3/4-inch bolts in green and dry lum- inch-thickwood member and the side ber from one Douglas -fir log was members were nominal 2-inch-thick studied. The second phase of the in- wood members or 5116-inch-thick vestigation (Part II) concerned eval- steel plates. The members were ar uating the effect of bolt diameter ranged so that the direction of the ap (1/2-, 3/4-, and 1-inch bolts) on plied load was parallel (figs. 1 and 2) joints of Douglas-fir lumber at about or perpendicular to the grain of the 12 percent moisture content. Three center member (fig. 3). Wood side specimens were evaluated for each members had the grain direction par- variable investigated. allel to the direction of applied load

M 114 294 M 114 295 Figure 2. --Test setup showing a bolted assem Figure 1. -- Test setup showing a bolted assem bly being loaded parallel to the grain. The bly being loaded parallel to the grain. The 5/16-inch-thick steel side plates are bolted wood side plates are bolted to the center to the wood center member with four 3/4 member with four 3/4-inch bolts. inch bolts.

2 FPL 2 (figs. 1 and 3). The specimens were assembled with either one or four bolts, positioned as shown in sketches of figures 4 to 7. The boltholes in the wood and steel members were drilled 1/16 inch larger than the di ameter of the bolts except for a few specimens of Part I in which the wood members had holes the same size as the diameter of the bolt. The holes were carefully drilled and centered to produce a smooth, accurately posi tioned hole.

M 114 293 Figure 3. --Test setup showing a bolted assem bly being loaded perpendicular to the grain of the center member. The wood side plates are bolted to the center member with four 3/4-inch bolts.

METHODS OF TEST

Joint loads were applied in com pression in a hydraulic testing ma chine (figs. l, 2, and 3). In joints tested parallel to the grain, the load was applied parallel to the grain of the center and side members. For joints with loads applied in a direction perpendicular to the grain, the center member was supported on 5-inch-wide blocks as shown in figures 3, 5, and 7, and the load was applied parallel to the grain of the side members. In the fabricationof the joints, the bolts were drawn snugly and then loosened by one-half turn of the nut. In the Figure 4. --Sketch showing construction details joints that were permitted to season, for parallel-to-the-grain test assemblies the bolts were not retightened after with four bolts. This specimen was used in seasoning, the tests of 1/2-, 3/4-, and 1-inch bolts.

3 M 123 680 Figure 5.--Sketch showing construction details of perpendicular-to-the-grain test assemblies with four bolts. The slip in the joint was determined from relative movement between side members and center member as measured with dial gage micrometers graduated to 0.001 inch, shown in fig ures 1, 2, and 3. The load was ap plied continuously, and readings of the joint slip were taken atuniform increments ofincreasing load of such magnitude as to give a suitable load- slip curve. The joint slip was meas ured from the beginning of the appli Figure 7. --Sketch showing construction details for perpendicular-to-the-grain test assem cation of the load. During load appli blies with one bolt. cation, the movable head of the test ing machine was driven ata rate of of the general behavior of the joint 0.032 inch per minute. Observations under load, the first drop ofthe test load, the type of failure, seasoning Figure 6. --Sketch showing construction details for parallel-to-the-grain test assemblies checks, and other similar details with one bolt. were noted and recorded. Tests were stopped at joint slips of 0.6 inch, and the maximum loads listed in the tables are the highest test loads obtained to a slip of 0.6 inch. For those tests in which the slip reached 0.6 inch, the load was increasing very little with relatively large increases in slip so that the actual maximum was prob ably but little higher than the recorded value.

4 FPL 2 PART I:

EFFECTS OF SEASONING ON JOINT PERFORMANCE

The effects of seasoning on the lar to the grain and planks 7 to 9 (fig. strength and behavior of 3/4-inch 9) furnished the center members load bolts in timber joints with the wood ed parallel to the grain. The material at three conditions of seasoning were for the wood side plates was cut from studied in Part I. Some joints with adjoining planks and resawed to pro green and some withair-dry lumber vide a pair of' side plates. The bolts were tested soon after assembly. were 3/4-inch steel bolts with square Others were assembled with green heads and nuts. New bolts were used lumber and tested after seasoning to in each test. Plain steel washers with the air-dry condition. None of the holes 1/16 inch larger than bolt sizes joints were subjected to any load prior were used at each end of the bolts. to the tests. The moisture content of the unsea soned wood at time of test (values in Description of Material tables 10 to 13 of the Appendix) ranged from 44 to 74 percent. The moisture The material used for the specimens content of the seasoned wood in Part I in Part I of the investigation consisted ranged from 9.7 to 10.9 percent with of nine planks 4 by 10 inches by16 an average of 10.3 percent (tables 10 feet cut from one coast-type Douglas- to 14 of the Appendix). fir log and stored in the green condi tion. All planks were practically The specific gravity of the specimens clear and essentially flat sawn. that were obtained from one log, based Planks 1 to 6 (fig. 8) furnished the on weight when ovendry and volume at center members loaded perpendicu test, ranged from 0.33 to 0.38 with

Figure 8. --Sketch showing method of cutting test material from Douglas-fir planks for perpendic ular-to-the-grain specimens. Material for center member of specimens for each replication with wood or steel side plates was cut from different planks. Sections A and E were used in joints built and tested while wood was green. Sections B and D were used in joints built while wood was green and tested after wood was air dry. Sections C and F were used in joints built and tested after wood was air dry.

5 Table 1.--Bearing strength properties of 3/4-inch bolts in Douglas-fir at three conditions of seasoning1

STEEL SIDE PLATES5

PERPENDICULAR TO THE GRAIN (WOOD SIDE PLATES)3

STEEL SIDE PLATES5

1 Values are the average of three tests with loads and bearing stresses expressed on a per bolt basis. 2 Specific gravity based on ovendry weight and volume at test. 3 The center members were nominal 2-5/8 by 4-1/2 inches and 2-5/8 by 9-1/2 inches and the wood side plates were 1-5/8 by 4-1/2 inches and 1-5/8 by 9-1/2 inches for the single and 4-bolt joints, respectively. 4 The diameter of the boltholes in the wood was the same as the nominal diameter of the bolt. All other holes were 1/16 inch larger than the bolt diameter. 5 The center members were nominal 2-5/8 by 4-1/2 inches and 2-5/8 and 9-1/2 inches and the steel aide plates were 5/16 by 4 inches and 5/16 by 8 inches for the single and 4-bolt joints. respectively. Figure 9.--Sketch showing method of cutting test material from Douglas-fir planks for parallel to-the-grab specimens. Sections A, E, I, and J were used in joints built and tested while wood was green. Sections B, D, F, and H were used in joints built while wood was green but air dried before test. Sections C, G, K, and L were used in joints built after wood was air seasoned. an average of 0.36 for the green ma Presentation of Data terial, and from 0.35 to 0.42 with an average of 0.38 for the dry material The experimental data for series A, (values given in tables 10 to 14 of the B, and C of Part I of the investigation Appendix). The average specific are summarized and compared in gravities of the material used in the table 1 and in figures 10 to 15. Re tests of both Part I and Part II of the sults obtained for joints in series B investigation are somewhat lower than of Part I with 3/4-inch bolts in mate the average specific gravity of the rial assembled dry and tested dry are species, which is 0.48 for material also incorporated, for comparative at 12 percent moisture content. 4, 5 purposes, in tables 6 and 9, of Part II. The data listed in tables 1, 6, and 9 The specimens were grouped into se and plotted in the figures are average ries A, B, and C corresponding to values per bolt obtained from tests of three differentmoisture or seasoning three specimens of a kind. Loads for conditions. The specimens in series four-bolt joints were divided by four A were constructed of green material to obtain comparative values per bolt. and were tested without seasoning. Tables 2 to 5 and tables 10 to 14 of In series B, the green material was Appendix A also present effects of the cut to the approximate size of the three conditions of seasoning and oth specimens and the pieces stored in er variables on the strength of 3 an unheated building for about 1 year. member bolted joints. The specimens were then constructed from the seasoned pieces and tested. Figures 10 to 15 present composite load-slip curves (average values of In series C, the specimens were fab loads at chosen slip values) from tests ricated and assembled of green mate of three joints of a kind for 3/4-inch rial, stored along with the material bolts in lumber in three conditions of of series B for 1 year, and tested. seasoning. The loads plotted in the

5Markwardt, L. J., and Wilson, T. R. C. Strength and related properties of woods grown in the United States. U.S. Dept. Agr. Tech. Bull. 479. 99 pp., illus. Sept. 1935. 7 Figure 10. --Load-slip curves for bolted joints moisture content, the test results are with wood side plates and with load bearing compared without adjustments for parallel to grain of center member. The differences in specific gravity or curves are composite load-slip curves from tests of three joints of a kind. Curves A, moisture content. assembled and tested when green; curves B, assembled and tested when air dry; curve C, The initial slip, or mat part of the assembled when green and tested when air slip not associated with elastic dis dry. tortion that occurs when the bolt is coming into full bearing (tables 10 to 14 of the Appendix), averaged about 0.02 inch for all specimens loaded in a direction parallel to the grain and about 0.01 inch for all specimens loaded in a direction perpendicular to the grain. Starting at the origin of the curve, the load-slip curve fillets into a linear portion of the curve at about 0.03 inch for the parallel to the grain specimens and about 0.02 inch for the perpendicular to the grain specimens (tables 10 to 14 of the Appen dix). In general, the initial slip and the slip at the point of fillet to the linear portion of the curve were greater

figures were averaged at relatively large increments of slip. Thus, as a result, the composite curves are not smooth, and the proportional limit as indicated by the curves differs in some instances from the average pro portional limit values given in the Figure 11. --Load-slip curves for bolted joints tables. with wood side plates and with the load bear ing perpendicular to the grain of the center Discussion of Results member. The curves are composite curves from tests of three joints of a kind. Curves Because of the careful matching of A, assembled and tested when green; curves B, assembled and tested when air dry; curve material and resultant narrow range C, assembled when green and tested when air of values for specific gravity and dry.

8 FPL 2 Table 2.-- Relative effect of wood and steel side plates on bolt- bearing properties of Douglas-fir

PARALLEL TO GRAIN

PERPENDICULAR TO GRAIN

1 Diameter of boltholes in the wood was the same as the nominal diameter of the bolt. All other holes were 1/16 inch larger than bolt diameter.

9 for the four-bolt joints than for the showed greater initial slip than all single-bolt joints. The joints with other joints. The joints with bolts in bolts bearing parallel to the grain that snug-fitting holes showed less initial were constructed green and permitted joint slip than the corresponding to season before test showed greater joints with bolts in oversize holes, nonelastic slip than the one- and four- particularly when wood side plates bolt joints of drylumber and the four- were used. The joints with the bolts bolt joints of green lumber. The bearing perpendicular to the grain in joints with one bolt in green lumber snug-fitting and oversize holes that were constructed of green material and permitted to season before test did not exhibit any initial nonelastic slip. Presumably, the shrinkage of the members brought the bolts into firm bearing with the wood.

Joints with Bolts Bearing Parallel to the Grain Joints with wood and steel side plates . --In joints constructed of green lum ber, the joints with steel sideplates had bearing stresses at 0.04-inch and 0.08-inch slip that were from 40 per cent to nearly 5 times greater than the stresses with wood side plates, but the proportional limit stresses were about the same for the two types of side plates (table 2). Thebearing stresses obtained for the joints that were assembled green and subjected to a period of seasoning were somewhat erratic but, in gen eral, the joints with steel plates gave considerably lower values than sim ilar joints with wood side plates, ex cept for one value at proportional limit (table 2). Figure 12. --Load- slipcurves for bolted joints with steel side plates and with the load bear ing parallel to the grain of the center mem Joints with one and four bolts. --For ber. The curves are composite curves from four-bolt joints in green lumber, the tests of three joints of a kind. Curves A, proportionallimit load with wood side assembled and tested when green; curves B, plates was 72 percent and, with steel assembled and tested when air dry; curve C, assembled when green and tested when air side plates, 76 percent that of the dry. joints with one bolt (table 5).

10 FPL 2 Table 3.--Relative effect of hole size on the bolt-bearing properties of Douglas-fir

PARALLEL TO GRAIN

PERPENDICULAR TO GRAIN Table 4.--Relative effect of moisture and moisture change on the bolt-bearing properties of Douglas-fir

PARALLEL TO GRAIN

PERPENDICULAR TO GRAIN

1Diameter of boltholes in the wood members was the same as the nominal diameter of bolt. All other boltholes were 1/16 inch larger than the bolt diameter.

12 FPL 2 The maximum bearing stress of the Figure 14. --Load-slipcurves for bolted joints with snug-fitting and nominal size bolt holes four-bolt joints in green lumber with and with the load bearing parallel to the either wood or steel side plates was grain. The curves are composite curves about 85 to 88 percent of that for one- from tests of three joints of a kind which were bolt joints. assembled when green and tested when air dry. Curves W, assembled with wood side plates; curves S, assembled with steel side Effect of seasoning. --For joints with plates. wood side plates (table 4), the one- M 123 694 bolt joints in green lumber (series A) carried a proportionallimit load of 83 percent and the four-bolt joints car ried 63 percent that of similar joints withdrylumber (series B). The four- bolt joints with wood side plates that were constructed of green lumber and left to season (series C) carried a proportional limit load of 95 percent that of similar joints with dry lumber (series B). For joints with steel side plates (table 4), the one-bolt joints in green lumber (series A) carried a proportional lim it load of 50 percent, and the four-bolt joints carried a load of 41 percent

that of similar joints in dry lumber (series B). The joints with four bolts, which were constructed of green lum ber and left to season (series G), carried a proportional limit load of 111 percent that of similar joints with dry lumber (series B).

At maximum load, the joints of green lumber with wood side plates (series Figure 13. --Load-slipcurves for bolted joints with steel side plates and with the load bear A) and with one and four bolts carried ing perpendicular to the grain of the center 60 percent, and the joints made of member. The curves are composite curves green lumber and left to season (se from tests of three joints of a kind. Curves ries C) carried 95 percent that of A, assembled and tested when green; curves similar joints of dry lumber (series B, assembled and tested when air dry; curve C, assembled when green and tested when air B). With steel side plates, the joints dry. of green lumber (series A) with one

13 Figure 15. --Load-slipcurves for bolted joints higher loads at a given slip than the with snug-fitting and nominal size bolt holes joints with 1/16-inch oversize holes and with load bearing perpendicular to the grain of the center member. The curves are when wood side plates were used. composite curves from tests of three joints They also gave higher proportional of a kind which were assembled when green limit and maximum loads. With steel and tested when air dry. Curves W, assem side plates, the joints with bolts in bled with wood side plates; curves S, assem bolt-size holes in the wood member bled with steel side plates. M 123686 (table 3 and fig. 14) sustained higher loads at 0.04- and 0.08-inch slip, but lower proportional limit and maxi mum loads than the joints with 1/16 inch oversize holes. Joints with Bolts Bearing Perpen dicular to the Grain Joints with wood and steel side plates . --The results for the joints with bolts bearing perpendicular to the grain (table 2) show that the bearing strength throughout the tests of the joints as sembled of either green or air-dry lumber and tested soon after assem and four bolts carried 55 percent that bly was generally a little higher for of similar joints with dry lumber (se joints with steel side plates than for ries B). The joints made of green joints with wood side plates. After lumber and left to season (series C) undergoing seasoning, the joints with carried 92 percent that of the series steel side plates had much lower val B joints. ues of bearing stress at 0.04- and 0.08-inch joint slip than the joints Joint behavior with respect to strength with wood side plates. In general, and slip, when seasoning of the wood the average proportional limitbearing members takes place under load, may stress for all joints with steel side not be comparable to the joint behav plates ranged from 95 to 134 percent, ior in these tests where the load was and the maximum bearing stress not applied until after seasoning. ranged from 89 to 117 percent of the In joints made of green material and joints with wood side plates. left to season, the slip at the pro Joints with one and four bolts.--In portional limit was about twice that green lumber, the proportional limit of similar joints made of dry lumber, bearing stress of the four-bolt joints and at maximum loads was about a was 108 percent that of the one-bolt third greater. joints with wood side plates and 102 Effect of hole size.-- Joints con percent for joints with steel side structed of green lumber and left to plates. The maximum load per bolt season before test with bolts in bolt- of the four -bolt joints was 87 percent size holes (table 3 and fig. 14) gave that of the one-bolt joints for joints

14 FPL 2 Table 5.--Relative effect of one bolt and four-bolts per joint on the bolt- bearing properties of Douglas-fir

PARALLEL TO GRAIN

PERPENDICULAR TO GRAIN

1 All boltholes were 1/16 inch larger than bolt diameter.

15 with wood side plates and 93 percent cent that of similar joints of air-dry that for joints with steel side plates. lumber. Effect of hole size. --The joints con Effect of seasoning. --For joints with structed of green lumber and left to wood side plates (table 4), the one- season with bolts in bolt-size holes bolt joints of green lumber (series A) gave higher loads for a given slip, carried a proportional limit load of as well as somewhat higher propor 76 percent that of similar joints of tional limit and maximum loads (table air-drylumber (series B). The four- 3), than the joints with 1/16-inch bolt joints of green lumber carried oversize holes when wood side plates a proportional limit load of 74 percent were used. Similar joints with steel that of the four-bolt joints of air-dry side plates gave proportional limit lumber. The four-bolt joints con loads that were higher and maximum structed of green lumber and left to loads that were slightly lower than the season (series C) carried a propor joints with 1/16-inch oversize holes. tional limit load of 40 percent that of In general, the joints with bolt-size joints of air-dry lumber. holes showed less joint slip for a given load throughout the tests than the joints with oversize holes (fig. For joints with steel side plates (table 15). 4), the one- and four-bolt joints of green lumber carried a proportional Joint Failures limit load of 68 percent that of sim ilar joints of air-dry lumber, and the The failures that resulted in the joints four-bolt joints constructed of green are classified as bending of the bolts, lumber and left to season carried 36 crushingunder the bolt, and splitting percent that of similar joints of air ofthewood (tables 10 to 14 of the Ap drylumber. pendix).

At maximum load, the one-bolt joints In general, the bolts bearing parallel with wood side plates and constructed to the grain showed crushing of the of green lumber carried 71 percent, wood beneath the bolts. They also and the four-bolt joints carried 77 showed splitting of the wood through percent that of similar joints of air- the boltholes of all joints except the dry lumber. The joints constructed single-bolt joints with wood side of green lumber and left to season plates. Some bolts were bent, par carried 94 percent that of similar ticularly in the joints with wood side joints of air-dry lumber. With steel plates. side plates, the one- and four-bolt joints of green lumber carried a max All joints bearing perpendicular to imum load of about 67 percent. The the grain showed crushing under the joints constructed of green lumber bolts and splitting of the wood. Some and left to season carried 103 per bolts were bent.

16 FPL 2 PART II.

EFFECTS OF BOLT DIAMETER ON JOINT PERFORMANCE

The effects of bolt diameter in the center members of the specimens strength and behavior of 1/2-, 3/4-, with loads applied perpendicular to and 1-inch bolts in timber joints were the grain. The planks used for the studied in Part II. The joints were specimens loaded perpendicular to constructed of air -dry Douglas -fir the grain were not long enough to and were tested soon after assembly. provide matched specimens for three The results include some of the data sizes of bolts. Thus specimens with for 3/4-inch bolts in air-dry material 3/4- and 1-inch bolts were matched in from Part I. planks 13 to 15 (fig. 17) and speci mens with the 1/2-inch bolts were Description of Material matched with duplicate specimens with 3/4-inch bolts in planks 16 to 18 The material used for the specimens (fig. 18). The wood side plates used in Part II of the investigation con in conjunction with the center mem sisted of nine planks 3 by 10 inches bers were cut from the 2- by 12-inch by 18 feet and six planks 2 by 12 planks. The individual center and inches by 20 feet of S4S Construc side members of all specimens were tion grade lumber of dry Coast-type positioned along the planks so that the Douglas-fir. The nominal 3-inch areas occupied by the bolts were rea thickplanks were numbered 10 to 18. sonably free of knots. The planks, in Planks numbered 10 to 12 (fig. 16) general, showed a moderate amount were used for the center members of of seasoning degrade in the form of the specimens with loads applied par checking, twisting, and cupping. allel to the grain, and planks 13 to 18 They were not resurfaced prior to (figs. 17 and 18) were used for the the fabrication of the specimens. M 123682

Figure 16.--Sketch showing method of cutting test material from Douglas-fir planks (Nos. 10, 11, and 12) for parallel-to-the-grain specimens in Part II of the investigation.

17 Figure 17.--Sketch showing method of cutting test material from Douglas-fir planks (Nos. 13, 14, and 15) for perpendicular-to-the-grain specimens in Part II of the investigation. M 123 689

Material for the specimens was stored an average yield point stress of about prior to test under a controlled at 48,000 pounds per square inch and mospheric condition of 75° F. and 65 a tensile strength of about 66,000 percent relative humidity until peri pounds per square inch. odic weighing of representative pieces showed that the pieces had reached Presentation of Data constant weight. The experimental data for Part II of the investigation are summarized in The specific gravity of the specimens tables 2, 5, 6, 7, and 9 and in figures that were obtained from 15 planks, 19 to 28. Results obtained for joints based on weight when ovendry and in series B of Part I with 3/4-inch volume at test, ranged from 0.38 to bolts in material assembled dry and 0.55 with an average of 0.43 (values tested dry are also incorporated for given in tables 15 to 20 of the Appen comparative purposes in tables 6 and dix). The moisture content of the sea 9. sioned wood in Part II ranged from 11.0 to 14.5 percent, with an average As in Part I, data listed in tables 6 of 12.6 percent. As in Part I, the test and 9 and plotted in the figures are results are compared without adjust average values per bolt obtained from ments for differences in specific tests of three specimens of a kind. gravity or moisture content. Loads for four bolt joints were divided by four to obtain comparative values The bolts were 1/2-, 3/4-, and 1 per bolt. inch steel machine bolts with square heads and nuts. New bolts were used The effects of the variables on air-dry in each test. Plain steel washers lumber are presented in tables 2, 5, with holes 1/16 inch larger than the and 7. Table 8 presents a compar bolt sizes were used at each end of ison of proportional limit bolt-bearing the bolts. Tension tests on necked- stresses parallel to the grain of joints down shanks of four 1/2-inch, four having wood side plates with joints 3/4-inch, and two 1-inch bolts gave having steel side plates. The results

18 FPL 2 Table 6.--Bearing strength properties of 1/2-. 3/4-. and 1-inch bolts bearing parallel to the grain of seasoned Douglas-fir1

WOOD SIDE PLATES

STEEL SIDE PLATES

1Values are the average of 3 tests of a kind with loads and bearing stresses expressed on a per bolt basis. 2Specific gravity based on ovendry weight and volume at test. 3The diameter of the boltholes was 1/16 inch larger than the nominal bolt diameters. 4Values obtained in Part I (table 1). All other values from Part II. Table 7. --Relative effect of bolt diameter on the bolt- bearing properties of seasoned Douglas- fir

PARALLEL TO GRAIN

PERPENDICULAR TO GRAIN

1 A11 boltholes were 1/16 inch larger than bolt diameter.

20 FPL 2 Figure 18. --Sketch showing method of cutting test material from Douglas-fir planks (NOS. 16, 17, and 18) for perpendicular-to-the-grain specimens in Part II of the investigation. M 123 688

of individual tests are presented in between the bolt-bearing stress at tables 15 to 20 of the Appendix. the proportional limit and L/d for parallel and perpendicular to the grain Figures 19 to 22 present load-slip loading of one- and four-bolt joints curves for 1/2-, 3/4-, and 1-inch in air-dry Douglas -fir. bolts in dry Douglas-fir. The loads plotted in the figures were averaged Figure 28 presents the relationship at relatively large increments of slip of the ratio of the bolt-bearing stress and, as in the preceding figures, the perpendicular to the grain at the pro- curves are not smooth and the proportional limit to the average pro portional limit differs in some in- portional limit stress perpendicular stances from the average proportional to the grain of air-dry Douglas-fir limit values given in the tables. and the bolt diameter. A comparison of these data and the data taken from Figures 23 to 25 present in chart form figure 13 of Technical Bulletin 332 2 a comparison of the bolt -bearing for maple and spruce is also pre stress for the various bolted joints sented in the figure. in air-dry Douglas-fir at 0.04- and 0.08-inch slip, proportional limit Discussion of Results and maximum load. Figure 26 presents a relationship be- Similarly, as in Part I, the initial 'tweentwo ratios: (1) the length of the slip, orthatpart of the slip not asso bolt bearing in the center member to ciated with elastic distortion that oc the diameter of the bolt (L/d) and (2) curs when the bolt is coming into full the bolt-bearing stress parallel to the bearing (tables 15 to 20 of the Appen grain at the proportional limit to the dix), averaged about 0.02 inch for all average crushing strength parallel to specimens loaded in a direction par- the grain of air-dry Douglas-fir. allel to the grain and about 0.01 inch for all specimens loaded in a di- Figure 27 presents the relationship rection perpendicular to the grain.

21 Starting at the origin of the curve, with wood side plates (table 2). At the load-slip curve fillets into a lin- the proportional limit, they had bolt- ear portion of the curve at about 0.03 bearing stresses 16 to 78 percent inch for the parallel to the grain spec- greater. imens and about 0.02 inch for the perpendicular to the grain specimens Atmaximumload, the bearing stress (tables 15 to 20 of the Appendix). In per bolt for the joints with wood side general, the initial slip and the slip plates was in general comparable to at the point of fillet to the linear por- that of the joints with steel side plates tion of the curve were greater for the except for the joints with 1/2-inch four-bolt joints than for the single- bolts in which the joints with steel bolt joints. side plates gave a bearing stress of 143 to 157 percent that of the joints with wood side plates (table 2). Joints with Bolts Bearing Parallel to the Grain A comparison of joints with bolts bearing parallel to the grain in air- Joints with wood and steel side plates. dry lumber with wood side plates and --In joints constructed of air-dry similar joints with steel side plates lumber, the joints with steel side is also given in column 7 of table 8 plates for the most part had bolt- for both one-bolt and four-bolt joints bearing stresses at 0.04-inch slip and with 1/2-, 3/4-, and 1-inch-diameter 0.08-inch slip from 7 percent less bolts. The average bearing stress to over 3 times greater than joints values at the proportional limit for

M 123 687 Figure 19. --Load-slipcurves for bolted joints with wood side plates and with load bearing parallel to the grain of air-dry Douglas-fir members. The curves are composite load-slip curves from tests of three joints of a kind.

22 FPL 2 Figure 20.--Load-slipcurves for bolted joints withsteel side plates and with load bearing parallel to the grain of air-dry Douglas-fir center member. The curves are composite load-slip curves from tests of three joints of a kind. M 123 692

the three bolt sizes are expressed crease with an increase in bolt diam as ratios of the average crushing eter. Previous tests2 with 1/2-inch strength of Douglas-fir (6,460 pounds bolts in five different thicknesses of per square foot) in columns 4 and lumber of four different species gave 6, table 8. The average crushing a ratio of values for wood to values strength of Douglas-fir5 was adjusted for steel side plates of 81 percent. for the difference in the average spec ific gravity of Douglas-fir and the av The data in figure 26 show that for erage specific gravity of the material corresponding values of L/d, the ra usedinthis investigation. These ra tios of bearing stress under the bolts tios are 31 percent for the one - and at proportional limit to crushing four-bolt joints with wood side plates strength of Douglas-fir are larger and average 45 percent for the one- for joints with steel side plates than and four-bolt joints with steel side for joints with wood side plates. This plates. A comparison of these ratios may be reflecting the possibility that shows that the joints with wood side failure can begin in the wood side plates with one bolt are 66 percent, plates. The data also show that the and those with four bolts are 72 per ratios of bearing stress to crushing cent of the values for corresponding strength tend to decrease generally joints with metal side plates or an with an increase in the ratios of L/d. average for all joints of 69 percent. A comparison of these data with those In general, the ratios tended to in obtained in previous tests2 of Doug

23 las-fir joints with steel side plates Effect of bolt size. --The results show shows good correlation in the L/d (table 7 and figs. 19 and 20) that while range of 4 to 5 but rather poor cor considerable variation in bearing relation at the smaller ratios of L/d. strength exists among the bolt sizes The data in figure 27 also show that and types of side plates, the 3/4-inch the joints with steel side plates gave bolts for the most part gave higher higher bearing stresses than the bearing strength than the 1/2- and 1 joints with wood side plates for bear inch bolts except for the 1/2-inchbolts ing parallel to grain, but not for bear with steel side plates. The average ing perpendicular to grain. bearing strength at the proportional limit for all joints with 1/2-inch bolts Joints with one and four bolts.--In was from 75 to 90 percent. All joints air-dry lumber, the bearing stress with 1-inch bolts had a bearing strength per bolt at the proportional limit for at the proportional limit of from 96 to the four-bolt joints was about the 136 percent that of the joints with 3/4 same as that of the one-bolt joints inch bolts. At maximum load, the when wood side plates were used and average bearing stress for the joints about 92 percent that of the one-bolt with 1/2-inch bolts was 76 to 125 per joints when steel side plates were cent, and with the 1-inch bolts was 66 used (table 5). The maximum bear to 74 percent that of the joints with ing stress of the four-bolt joints in 3/4-inch bolts. dry lumber with either wood or steel side plates was about 89 percent of Joints with Bolts Bearing Per that of the one-bolt joints (table 5). pendicular to the Grain

M 123 685 Figure 21. --Load-slipcurves for bolted joints with wood side plates and with load bearing perpen dicular to the grain of the air-dry Douglas-fir center member. The curves are composite curves from tests of three joints of a kind. The curves for the 3/4-inch bolts with the subscript "a" are for joints whose material is matched to that of the joints with 1/2-inch bolts, and the curves for the 3/4-inch bolts with the subscript "b" are for joints whose material is matched to that of the joints with 1-inch bolts.

24 FPL 2 Table 8.--Comparative proportional limit bearing strength parallel to the grain in seasoned Douglas-fir of bolted joints with wood side plates and those with metal plates

1Bearing values from table 2. 2Average crushing strength of Douglas-fir at 12 percent moisture content is given as 7,420 pounds per square inch in Tech. Bull. No. 479, "Strength and Related Properties of Woods Grown in the United States." This value was adjusted for the specific

gravity difference of the material Joints with wood and steel side plates. that for both types of side plates, the --As in Part I, the results for the bearing stress generally increased joints with bolts bearing perpendic with an increase in L/d. ular to the grain (table 4) show that the bearing strength throughout the Joints with one and four bolts.--In tests of the joints assembled of air- dry lumber (table 5), the bearing dry lumber and tested soon after as stress per bolt at the proportional sembly was generally a little higher limit of the four-bolt joints with 1/2-, for joints with steel side plates than 3/4-, and 1-inch bolts ranged from for joints with wood side plates. In 80 to 112 percent that of the one-bolt general, the average proportional joints when wood side plates were limit bearing stress for all joints with used and from 87 to 108 percent when steel side plates ranged from 95 to steel side plates were used. 118 percent and the maximum bearing stress ranged from 98 to 115 percent At maximum load, the bearing stress that of the joints with wood side plates. per bolt in dry lumber (table 5) of the four-bolt joints ranged from 72 to 81 The data in figure 27 show that the percent that of the one-bolt joints joints with steel side plates gave only when wood side plates were used and a slightly higher bearing stress than from 63 to 89 percent when steel side the joints with wood side plates and plates were used. M 123 691

Figure 22. --Load-slipcurves for bolted joints with steel side plates and with load bearing perpen dicular to the grainofthe air-dry Douglas-fir center member. The curves are composite curves from tests of three joints of a kind. The curves for the 3/4-inch bolts with the subscript "a" are for joints whose material is matched to that of the joints with 1/2-inch bolts, and the curves for the 3/4-inch bolts with the subscript "b" are for joints whose material is matched to that of the joints with 1-inch bolts.

26 FPL 2 Effect of bolt size. --The bearing portional limit compressive stress stress at the proportional limit for perpendicular to the grain of Douglas- all joints with 1/2-inch bolts in both fir (690 pounds per square inch) with wood and steel side plates ranged bolt diameter is presented in figure from 116 to 135 percent, and for all 28 for one- and four-bolt joints. The joints with 1-inch bolts from 86 to average proportional limit compres 102 percent that of similar joints with sive stress perpendicular to the grain 4 3/4-inch bolts (table 7). At maximum of Douglas-fir was adjusted for the load, the bearing stress for the joints difference in the average specific with 1/2-inch bolts ranged from 136 gravity of the species and the average to 165 percent, and with 1-inch bolts, specific gravity of the material used ranged from 76 to 91 percent that of in Part II. The results show that the joints with 3/4-inch bolts. most of the ratios obtained in these tests are greater than those obtained The relationship of the ratio of the in the previous tests with maple and proportional limit bearing stress per spruce; however, the trend of the data 2 pendicular to the grain and the pro is similar. M 123 690

Figure 23. --Chart showing the bearing stress parallel to the grain at 0.04 inch, 0.08 inch, pro portional limit, and maximum load for one- and four-bolt joints with 1/2-, 3/4-, and 1-inch bolts bearing in nominal 3-inch air-dry Douglas -fir with wood and steel side plates.

27 Table 9.--Bearing strength properties of 1/2-, 3/4-, and 1-inch bolts bearing perpendicular to the grain of seasoned Douglas-fir 1

1 Values are the average of 3 tests of a kind with loads and bearing stresses expressed on a per bolt basis. 2 Specific gravity based on ovendry weight and volume at test. 3 The diameter of the boltholes was 1/16 inch larger than the nominal bolt diameters. 4 Material matched to that of specimens with 1/2-inch bolts. 5 Values obtained from tests in Part I (table 1). All other valves from Part II. 6 Material matched to that of specimens with 1-inch bolts.

28 FPL 2 Figure 24. --Chart showing the bearing stress perpendicular to the grain at 0.04 inch, 0.08 inch, proportional limit, and maximum load for one- and four-bolt joints with 1/2- and 3/4-inch bolts bearing in nominal 3-inch air-dry Douglas-fir with wood and steel side plates. M 123 693

Joint Failures inch bolts and the single-bolt joints with 3/4-inch bolts with wood side The failures that resulted in the joints plates. All 1/2-inch bolts were bent. are classified as bending of the bolts, Some 3/4-inch bolts were bent, par- crushing under the bolt, and splitting ticularly in the joints with wood side of the wood (tables 15 to 20 of the Ap- plates. None of the 1-inch bolts were pendix). bent.

In general, the bolts bearing parallel All joints bearing perpendicular to to the grain showed crushing of the the grain showed crushing under the wood beneath all sizes of bolts. They bolts and splitting of the wood. All also showed splitting of the wood 1/2-inch bolts were bent; some 3/4 through the boltholes of all joints inch bolts were bent; but none of the except the single-bolt joints with 12- 1-inch bolts were bent.

29 CONCLUSIONS

The bearing strength data obtained for a single-bolt joint. the three-member bolted joints in nominal 3-inch Douglas-fir with one The following specific conclusions are and four bolts bearing parallel or per based on the average proportional pendicular to the grain exhibited large limitbearing stress of groups of one- variations among various groups of and four-bolt specimens of similar specimens as well as among individ construction, except that only joints ual specimens of a kind. The results, with four bolts were used in speci in general, show that for the items mens that were made of green mate studied with one species and one rial and tested after seasoning. In thickness of member, the design data addition to these data, other factors currently in use for bolted joints are such as maximum load, load at a specific joint deformation, and quality technicallysound. In joints with four of craftsmanship should also be con bolts bearing parallel to the grain, sidered in the evaluation of joint be however, the bolt-bearing strength havior and are specifically mentioned per bolt should be 90 percent that of in some of the conclusions. M 123 679

Figure 25. --Chart showing the bearing stress perpendicular to the grain at 0.04 inch, 0.08 inch, proportional limit, and maximum load for one- and four-bolt joints with 3/4- and 1-inch bolts bearing in nominal 3-inch air-dry Douglas-fir with wood and steel side plates.

30 FPL 2 Figure 26. --Relationship between ratio of bolt bearing stress at proportional limit to crushing strength and ratio of bearing length to bolt diameter (L/d) for joints with bolts bearing parallel to the grain of air-dry Douglas-fir. The reverse curve is taken from the data for Douglas-fir in figure 5, Tech. Bull. 332, and has been inserted for comparative purposes. M 123 683

Bearing Parallel to the Grain size but the loads at specific slips below the proportional limit are con 1. The bolt-bearing proportional siderably less. limit stress of joints of dry material with steel side plates is about 150 4. The bolt-bearing stress of joints percent that of joints with wood side with 1/2-inch bolts in nominal 3-inch plates, but the maximum load of the lumber is about 80 percent, and with joints with steel side plates is only 1-inch bolts it is about 115 percent about 110 percent that of joints with that of joints with 3/4-inch bolts. wood side plates. 5. The bolt-bearing stress of joints 2. The bolt-bearing stress of joints with bolt-size holes constructed of of green lumber is about 60 percent green lumber and loaded after sea that of joints of dry lumber. soning is about 120 percent that of 3. The bolt-bearing stress of joints similar joints with 1/16-inch over constructed of greenlumber and load size holes when wood side plates are ed after seasoning is about the same used but only 60 percent that of joints as that of joints constructed of dry with oversize holes when steel side lumber when all boltholes are over plates are used.

31 Figure 27. --Relationship of the bolt-bear 5. The bolt-bearing stress of joints ing stress at the proportional limit in with 1/2-inch bolts in nominal 3-inch air-dry Douglas-fir and the ratio of lumber is about 130 percent and with bearing length to bolt diameter (L/d). 1-inch bolts it is about 95 percent that M 123 684 of joints with 3/4-inch bolts.

6. The bolt-bearing stress of joints with bolts in bolt-size holes is slightly higher than joints with bolts in 1/16 inch oversize holes. M 123 685

Bearing Perpendicular to the Grain

1. The bolt-bearing stress of joints of dry material with steel side plates is about the same as that of joints with wood side plates.

2. The bolt-bearing strength per bolt of joints with four bolts is about the same as that of single-bolt joints.

3. The bolt-bearing stress of joints constructed of green lumber is about Figure 28.--Relationship between ratio of 70 percent that of joints of dry lum the perpendicular to the grain bolt-bear ber. ing stress at the proportional limit of air-dry Douglas-fir to the average pro portional limit stress perpendicular to 4. The bolt-bearing stress of joints the grain of air-dry Douglas-fir and bolt constructed of greenlumber and load diameter. The dashed curve is taken ed after seasoning is about 40 percent from figure 13, Tech. Bull. 332, and that of joints constructed of dry lum has been inserted for comparative pur ber. poses.

32 FPL 2 APPENDIX

33 Table 11.--Strength of 3-member bolted joints with one 3/4-inch bolt bearing parallel to grain of Douglas-fir1

Table 12.--Strength of 3-member bolted joints with one 3/4-inch bolt bearing perpendicular to the grain of Douglas-fir 1

34 FPL 2