Moisture Cycling of Trussed Rafter Joints
U. S. FOREST SERVICE RESEARCH PAPER FPL 67 NOVEMBER 1966
MOISTURE CYCLING OF TRUSSED RAFTER JOINTS
U. S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON WISCONSIN FOREST PRODUCTS LABORATORY U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE ··· MADISON, WIS.
In Cooperation with the University of Wisconsin SUMMARY
Prefabricated wood trussed rafters are widely used in small building con struction, and various connector systems are used in preassembling the wood members, including nailed or glued wood and plywood plates and newly developed metal gusset plates of various designs. Trussed rafters installed in a building are subjected to many conditions that could affect the load-carrying capacity of the joints. To illustrate, changes in relative humidity and temperature could cause dimensional changes in truss members and this could cause a loosening of the connector; cyclic live-loading, resulting from changes in the amount of snow wind pressure on the roof, may cause the joints to “work” and thus alter their performance; or a constant load, such as a dead load, could cause creep. It was considered to determine actual effect of these con ditions, by means of accelerated tests, so that the information could be made readily available to aid in the design of wood trussed rafters. This Research Paper presents the results of the first phase of a longtime performance study of trussed rafter joints, namely, the effects of initial mois ture content and moisture cycling under load on strength and rigidity of trussed rafter joints. Included in the investigation were nailed wood and plywood joints; phenol-resorcinol and casein glued joints; and nailed. barbed, and toothed metal-plate joints. Joints tested in tension and bending were fabricated at 10, 17, and 25 percent moisture contents and half of the specimens were subjected to moisture cycling while under load. These specimens were then conditioned to 10 percent moisture content, destructively loaded, and the results compared with those of matched, uncycled control specimens. The results of this study showed that the moisture-cycled specimens had from I to 3-1/2 times more elongation or deflection than the control specimens. Losses in maximum load for the cycled specimens ranged from 0 to 30 percent greater than for the controls. The initial moisture content had a significant effect upon the elongation and deflection of the mechanically fastened joints but not on the maximum load, with the exception of the barbed metal-plate joints. Wet material had an adverse effect upon the maximum loads of the glued joints. The results of this research provide information and data essential to design of trussed rafter joints to insure their satisfactory longtime performance under varying conditions of loading, moisture content, and moisture cycling.
i CONTENTS
Page
INTRODUCTION ...... 1 REVIEW OF PAST RESEARCH ...... 2 OBJECTIVES ...... 2 SPECIMENS ...... 2 MOISTURE CONTENT OF MATERIAL . . . . . 5 EXPERIMENTAL PROCEDURES ...... 5 RESULTS AND DISCUSSION ...... 9 Moisture Cycling ...... 9 Destructive Loading ...... 16 Nailed-wood joints ...... 18 Nailed-plywood joints ...... 22 Nailed metal-plate joints ...... 24 Barbed metal-plate joints ...... 25 Toothed metal-plate joints ...... 27 Phenol-resorcinol glued joints ...... 29 Casein-glued joints ...... 31 STATISTICAL ANALYSIS ...... 33 “t” Values ...... 33 “F” Values ...... 35 SUMMARY OF FINDINGS ...... 37 LITERATURE CITED ...... 38
FPL 67 ii MOISTURE CYCLING OF TRUSSED RAFTER JOINTS
by THOMAS LEE WILKINSON, Engineer
Forest Products Laboratory1 Forest Service U. S. Department of Agriculture
INTRODUCTION
The preassembled wood trussed rafter, a Trussed rafters installed in a building may be a decade ago, has become widely accepted subjected to many conditions that could affect the and used in the small building industry. An esti load-carrying capacity of the joints. For example, mated one-quarter billion dollars is spent annually changes in relative humidity and temperature in the United States on trussed rafters. could cause dimensional changes in the truss Trussed rafters are made in many shapes and members or connectors resulting in a sizes, the most popular types being the king-post of the connectors. This same condition could and Fink or W-truss. Many different connection result from in moisture content of the systems are used to assemble the wood members, truss members as they come into equilibrium including nailed wood and plywood plates, glued with their surroundings. Also, cyclic live-loading, plates, and the newly developed metal gusset resulting from changes in the amount of snow or plates, available in various configurations. These wind pressure on the roof, may cause the joints connectors resist axial forces, shear forces, and to “work” and thus alter their performance; or moments in varying degrees. Newly developed a load, such as a dead load, could cause design procedures consider all of these forces to creep. of these could affect the secure a balanced design. Many full-size trussed longtime service performance of wood trussed rafters have been tested soon after fabrication rafters. and have been found to be adequately designed. Prior to this particular research study, little
1 Maintained at Madison, Wis., in with the University of was known concerning the permanence long nailed joints. In this study, small trussed rafters time service performance of the newly developed were subjected to high and low humidities in a metal gusset plates or glued joints under load. controlled atmosphere. Results showed that the Available information the longtime glued trussed rafters suffered some loss in stiff performance of nailed joints was obtained mainly ness and considerable loss in strength, while the from service records. Yet, a large number of nailed trusses were less affected. After exposure, trussed rafters being fabricated and used both types had ample stiffness and strength for without knowing how well they will continue to normal service requirements, but the glued perform during the expected life of the buildings trussed rafters were considerably stiffer and in which they are installed. If the trussed rafters stronger than the nailed ones. Other now in service give satisfactory per at the Forest Products Laboratory showed little formance, it will prove that their design was loss in strength and of some glued- adequate to meet in-service conditions. but this nailed king-post and W-type trusses after 3 years will take 20 to 30 years to determine. It was con of outdoor exposure. The trusses, however, were sidered important, therefore, that the effect of not loaded during exposure. these conditions be investigated by means of The foregoing studies have proved practica accelerated tests, so that the results could be bility of trussed rafters, but their longtime per made readily available to aid in the design of formance has not been determined. trussed rafters. This Research Paper presents the results of the first phase of a longtime performance study OBJECTIVES of trussed rafter joints, namely, the effects of initial moisture content and moisture cycling under load on the strength and rigidity of trussed Because of the broad scope of the longtime rafter joints. performance study of truss joints, the research to be conducted by the Forest Products Laboratory has been into six phases, namely (1) mois REVIEW OF PAST RESEARCH ture cycling, (2) temperature effects, (3) creep at constant load, (4) cyclic loading, (5) geometry studies have been conducted by other effects, and (6) exposure of full-size trusses. researchers on full-size trussed rafters. In Phase I of this study has been completed and practically all of the studies. however, the tests the results are presented in this Research Paper. were conducted shortly after fabrication of the The specific objectives of Phase I were to deter rafters. These included by: Angleton (1)2 mine the effect of moisture cycling under load on roof trusses using nailed-plywood gusset and the effect of initial moisture content of the plates; Luxford and Heyer (3) on glued and nailed truss material on the final strength rigidity roof trusses; Pneuman (5) on king-post type of nailed wood and plywood joints, three types of trussed rafters with plywood gussets on one aide; metal-plate connector joints, and phenol and Radcliffe, and others (6,7,8) on nail-glued resorcinol- and casein-glued gusset joints. type trussed rafters. In addition. various manu facturers have tested trussed rafters using the new metal-gusset plates of the more popular con SPECIMENS figurations and constructions. In all of these studies, trussed rafters were found to be adequately designed. Two types of specimens were chosen for this Luxford (2) studied the results of changes in study. One was a simple tension specimen and the relative humidity on strength and rigidity of other a simple bending specimen. These were trussed rafters with glued joints and also with selected mainly because of their simplicity and
2 Underlined numbers in parentheses refer to Literature Cited at the end of this Paper.
67 2 Figure 1.--Method of fabricating specimens for cycling and control. A, general configuration; B, selection of joint members to obtain matched specimens.
M 127 609 the ease with which they could be tested. They and (3) toothed metal plates, which depend on also facilitated the application of forces to the teeth punched in the plate to carry the load with connectors during testing that were similar to out any nails. The plates selected for this study those found in an actual trussed rafter. from each of the three groups are shown in Both types of specimens had the same general figure 3, All plate connectors selected were of configuration (fig. 1A). They were made of galvanized sheet metal--the nailed and barbed straight-grained Douglas-fir 2 4’s were types being 20-gage material and the toothed free of defects in the area of the gusset plates. type gage. Joints with toothed barbed The specimens were 3 feet long with a joint at the metal-plate connectors were fabricated with the midlength. The halves butted together so that aid of a hydraulic testing machine, as shown in no gap was present, figure 4. Toothed plates were pressed one face Three general groups of fastenings were eval at a time and the barbed plates were first uated in this study, namely, nailed joints, metal- positioned on each face and then pressed. plate joints, and glued gusset joints. For the glued joints, a phenol-resorcinol and The nailed joints included plywood and solid- a casein glue were selected as being the more wood gusset plates. The gussets were 20 inches popular glues in the manufacture of trussed long and fastened sixpenny common nails. rafters. The gussets were made of 1/2-inch The nail pattern is shown in figure 2. Clear Exterior grade Douglas-fir plywood and were straight-grained Douglas-fir 1 by 4’s were used inches in length. Pressure was applied to the for the wood gussets, and 1/2-inch Exterior glued joints in a glue press until the glue had set, grade Douglas-fir plywood was used for the ply so that nails were not needed. Gluing was done at wood gussets. room temperature and a period of 2 to 3 weeks Although metal-plate connectors are available allowed for curing after the removal of pressure. in numerous configurations and constructions, A control specimen was matched with each they were divided into three basic groups and one cycled specimen. This was done by taking every representative type selected from each group for other piece as one-half of a specimen (fig. 1B). evaluation in this study. The three basic groups The term “side,” as shown in figure 1, refers to were: (1) nailed metal plates, which rely on the one of the wood members used in a nails alone to carry the loads transmitted through specimen. No attempt was made to match speci the joint; (2) barbed metal plates, which rely on mens of different groups. the barbs and also some nails to carry load; Specimens simulated a tension splice in the
3 2.--Face edge views of naiIed-wood (top) and nailed-plywood (bottom) joints. Mem- bers and wood gussets are of Douglas-fir. The plywood gussets are of 1/2-inch Exterior grade. Common sixpenny nails were used. M 126 001
Figure 3.--Three general types of metal truss plates selected for the moisture cycling study. Left, barbed metal plate (20-gage); center, toothed plate (18-gage); and right, nailed metaI pIate (20-gage). M 124 773
67 4 Figure 4.--Method of fabricating toothed and barbed metal-plate joints. Wood members were held in position by a jig while the hydraulic testing machine pressed in the plate. Toothed plates were pressed one face at a time. as shown. Barbed plates were first positioned on each face and then pressed. M 129 928 lower chord of a trussed rafter and were designed EXPERIMENTAL PROCEDURES for a 2,000-pound tensile load of 2-months’ duration. This value represented a typical design Twelve bending and tension specimens at stress and the duration was representative of the each initial moisture content (wet, airdry, and time the specimens were loaded. Since the lower kiln-dry) were evaluated for each of the seven chord splice usually carries bending moment as joint types. Six specimens of each sample were well as direct stress, the same joint. con subjected to moisture cycling and six were used figuration was used in bending as in the tension as uncycled controls. specimens, The moisture cycling consisted of three cycles of high and low relative humidity. The first phase of each cycle was at 90° F. and 95 percent rela MOISTURE CONTENT tive humidity. This gave the wood an equilibrium moisture content of about percent. The second OF MATERIAL phase of each cycle was at 160° F. and 45 per cent relative humidity, which gave an equilibrium Three general moisture content groups were moisture content of about 6 percent. Each phase established to simulate the range in moisture lasted for 1 week, At the end of the three cycles, content of material from which a trussed rafter the specimens were conditioned at 160° F. and may be made, namely. (1) wet (23 to 27 percent); 73 percent relative humidity for 3 to 4 days. (2) air-dry (15 to 19 percent); (3) kiln-dry which resulted in a final moisture content of (6 to percent). Specimens of the seven types about 10 percent., After conditioning they were of joints studied were made from each of these stored at 74° F. and 50 percent relative humidity. three initial moisture content groups. The mois During this entire time, the uncycled control ture content of the gusset material was approxi specimens were stored at 74° F. and 50 percent mately 11 percent for all specimens using wood relative humidity to attain an equilibrium mois or plywood. ture content of approximately 10 percent.
5 Figure 5.--General arrangement used for loading bending specimens during moisture cycling. Total load on each specimen was 500 pounds, which produced a bending moment on the joint similar to that caused by a 10-pound-per-lineal-footceiling load. M 125 941
A total dead load of pounds was applied to outer, intermediate, and inner sections, with the all moisture-cycled specimens during the entire outer and intermediate sections each 1/4 inch time of cycling and to all uncycled control speci thick, and the moisture content of each section mens during the time of storage. Bending speci was determined. This gave the gradient of mois mens were loaded at the quarter points as shown ture penetration as well as the overall moisture in figure 5, to produce a moment at the joint content. equivalent to that caused by a uniform ceiling At the end of each phase of cycling, deflection load of 10 pounds per lineal foot. Tension speci elongation readings were made on the cycled mens were loaded as shown in figure 6, which specimens. This was done to determine the effect resulted in a stress on the joint equivalent to of moisture cycling upon the rate and amount of that caused by a dead load of pounds per lineal creep. Figure 7 shows the method used to meas foot on a 28-foot-span trussed rafter. ure the deflection of the bending specimens and To obtain some idea of the moisture content figure 8 shows the method used. to measure elon the cycled specimens reached at the end of each gation of the tension specimens. For comparison, phase of cycling, a moisture determination was similar readings were taken of the uncycled con made on a piece of nominal 2 by 4 lumber which trol specimens at the same time intervals. had been placed in the cycled atmospheric con After the specimens were conditioned to 10 per ditions and was. representative of the material cent moisture content, elongation and deflection used for the specimens. It was end-coated to readings were made. The load was then removed prevent moisture penetration from the ends and, and the immediate recovery recorded. Next, all after each phase of cycling, a moisture sample specimens were loaded to destruction at room taken from it. The sample was divided into temperature. Figure 9 shows the general arrange-
FPL 67 6 Figure 6.--General arrangement used for tension specimens during moisture cycling. The load on the joint was 500 pounds, which is a force similar to that produced by a dead load of 20 pounds per lineal foot on a trussed rafter. M 124 776
Figure 7.--Method used to measure deflection of bending specimens at end of each phase of moisture cycling. M 126 003
7 Figure 8.--Method used to measure elongation Figure 9.--Destructive loading arrangement of tension specimens at the end of each for tension specimens. Shear plates were phase of moisture cycling. Also shown are used to apply load to the specimen and C- the round shear plates used in applying clamps were used to close splits and final destructive load to the specimen. increase the capacity of shear plates. Elongation was measured over a 6-Inch gage M 126 004 length.
M 124 777
FPL 67 8 Figure 10.--Destructive loading arrangement for bonding specimens. The span of the join? was 33 inches and loads were applied at the quarter-mints. M 124 775 ment used for destructive loading of the tension both halves of each specimen were determined specimens. Shear plates, 2-1/2 inches in diam immediately after the destructive loading of the eter, were used to apply load to the specimens joints. and were located 5 inches from the ends of the members. This gave a maximum capacity of approximately 12,000 pounds. When the shear RESULTS AND DISCUSSION plates were placed in wet material and the mate rial allowed to dry, there was a tendency to split Moisture Cycling members; therefore, to obtain loads large enough to break the joints, C-clamps The data obtained during cycling consisted of between the shear plates and ends of the members moisture determinations of sample material to close the splits. Load was applied at a con located with the cycled specimens and creep stant machine-head movement of 0.010 inch per measurements at the end of each phase of minute and elongations were measured over a cycling, 6-inch gage length on both sides of the joints. Table 1 gives values of moisture content Figure 10 shows the destructive loading attained at various sections of the specimens arrangement for the bending specimens. The during each phase of moisture cycling as deter specimens were loaded at the quarter-points and mined from sample material. The sample pieces deflections were measured at the load points. were divided into three sections, as described The deflection equipment permitted readings under “Experimental Procedures,” to obtain a beyond design-load levels, but was removed before moisture gradient. Values presented in the table maximum load was reached. Load was applied at are the averages for all seven types of joints at a constant machine-head movement of 0.010 inch each of the three initial moisture contents for per minute. both tension and bending specimens. The. values The moisture content and specific gravity of obtained fox the whole section compare favorably
9 Table 1. --Moisture contents attained at various sections of the specimens during each phase of moisture cycling as determined from sample material 1
with the range that could be encountered in This was probably caused by incomplete condi trussed rafters located in buildings. The largest tioning of the sample material, as it was taken range of moisture contents was noted in the directly from a saturated condition and used after outer 1/4 inch of the material, but a large vari a short period of drying. ation was also obtained between phases of cycling: Table 2 gives the average elongation and within the second 1/4 inch. This outer 1/2 inch deflection measurements of initially wet, air-dry, is the portion of the material in which fastener and kiln-dry cycled and control specimens of each was located and where the greatest variation in type of joint evaluated in tension and bending. moisture was desired. The initial moisture con These data show a substantial increase in the tent of some of the “wet” sample pieces selected amount of creep when the specimens were sub higher than that desired for the specimens. jected to cycling. The cycled tension specimens
FPL 67 10 Table 2 .--Elongation and deflection2 measurements of initially wet, air-dry, kiln-dry cycled and control specimens of each type of joint evaluated in tension and bending3
had from 2.0 to 37.0 times greater elongation Figures 11-14 show typical elongation and than the controls and the cycled bending speci deflection curves of various types of joints mens had 2.1 to 6.9 times greater deflection than during moisture cycling. At the end of condition the controls, The larger ratios were for the ing while under load, the control specimens had kiln-dry group in which the control specimens ceased creeping, while creep was still occurring had less creep than the controls of the wet or in the cycled specimens. The largest increases air-dry groups. For the glued joints. there was in deformation generally occurred during the dry less variation in the ratios of elongation phases of cycling. During this phase the members deflection of cycled and control specimens for shrank, leaving a slight gap between the members the different initial moisture contents than for and gussets, thus possibly allowing the speci the other joint types. For specimens with mechan mens to deform by removing friction. Another ical connectors, the ratios increased the initial contributing factor could be the expansion of the moisture content decreased, primarily because metal gussets because of the increase in temper- of the smaller deformation of the controls. ature from 90° 160° F.
11 Figure It.--Typical elongation curves for kiln-dry, toothed metal-plate joints in tension during moisture cycling. M 131 598
Figure 12.--Typical elongation curves for wet, nailed-wood joints in tension during moisture cycling. M 131 599
FPL 67 12 Figure 13.--Typical deflection curves for kiIn-dry, nailed metal-plate joints in bending during moisture cycling. was measured at the quarter-points. M 131 600
Figure 14.--Typical deflection curves for wet, barbed metal-plate joints in bending during moisture cycling. Deflection was measured at the quarter-points. M 601
13 Table 3.--Percent elongation and deflection recovery of initially wet, air-dry, and kiln-dry cycled and control specimens of each type of joint evaluated in tension and bending1
The percentage of elongation and deflection of metal fasteners was common, with rust appear recovery at the end of moisture cycling is shown ing on the nailed joints and oxidation of the zinc in table 3. The bending control specimens recov coating occurring on the metal gusset plates. The ered approximately 60 percent of their total glued joints had a small amount of glue-bond deflection, and the tension controls recovered failure, especially with those made of “wet” approximately 35 percent of their total elongation, material. Partial failure of the plywood gussets, with values ranging from 0 to 100 percent, The of unequal shrinkage of the gussets and cycled specimens recovered approximately members, was noted. This occurred in both the 20 percent of their total deflection elongation. cycled and control specimens. Gaps were often The larger amount of creep of the cycled speci present between gussets and members of nailed mens and the smaller percentage recovery joints. Shrinkage caused the barbed metal plates resulted in a large residual set in the cycled to buckle out of the wood, and were held only by specimens. the positioning nails along edge of the plate. Deterioration of the cycled joints was noted at A comparison of the general appearance of the end of moisture cycling. Members quite often various cycled specimens and their controls is were split or checked. Wood and plywood gussets shown in had checks and were often discolored. Corrosion
FPL 67 14 Figure 15.--Comparison of cycled specimens with controls for nailed-wood joints (A, B) and for nailed-plywood joints (C, D). Joints A and C were cycled and B and D were controls. M 126 002
Figure 16.--Comparison of cycled and control specimens for nailed metal-plate joints. Lower spec i men was cycled . M 128
15 Figure 17.--Comparison of cycled and control specimens for barbed metal-plate joints (A, B) and toothed metal-plate joints (C, D). Specimens A and C were cycled and B and D were controls. M 124 774
Figure 18.--Comparison of cycled specimen (top) with control (bottom) for glued joints M 129 984
Destructive Loading of the mechanical fasteners. For the glued joints and barbed metal plates, the maximum loads were General.--After cycling was completed, the of primary interest. specimens were destructively loaded to obtain Tables 4 and 5 summarize the results of the values for and maximum load. The destructive loading in tension and bending. The rigidity of mechanical fasteners is important in values of specific gravity shown in these tables determining design loads, and thus the rigidity indicate that the method of matching control and values obtained were of primary interest for most cycled specimens was very good. Specific gravity
FPL 16 Table 4.--Summary of moisture content arid specific gravity values, elongation measurements, and maximum loads far the various moisture cycled and control specmens of each type of subjected to destructive loading in tension1
values were determined for both halves of each values of deflection for the bending specimens specimen. The final moisture contents of the are given at a load of 1,000 pounds. All values cycled and control specimens were so close that of elongation and deflection due to destructive correction for the difference was unnecessary. loading are in addition to the permanent set that The joints were designed for tensile force and resulted from dead loading and moisture cycling thus the specimens loaded in tension can be com of the cycled specimens and the dead loading of pared to each other in determining their ability the control specimens. The total distortion of any to meet design requirements. In bending, however, particular joint specimen is equal to the sum of the different connector types should not he corn- the permanent set and elongation or deflection pared unless they have comparable gusset lengths. values shown for that specimen. These values are The values of elongation for the tension speci given in table 6. In general, the moisture cycling mens are given at a load of 2,000 pounds, which caused permanent sets that were much larger than was the assumed design load for the joints. The the deformations resulting from destructive load
17 Table 5.--Summary of moisture content and specific gravity values, deflection measurements, and maximum loads for the various moisture cycled and control specimens of each type of joint to destructive loading in bending1
ing. The permanent set in the control specimens losses in both rigidity and maximum load. was generally less than the deformation resulting A detailed analysis of the results of destructive from destructive loading. loading on the individual types of joints studied The results of destructive loading in tension is presented in the following portion of this indicate that the greatest effect of moisture Research Paper. cycling was on the rigidity of most of the mechan Nailed-wood joints.--Design values for ical fastener joints. For the glued joints and the mechanical connectors are based on the load at a barbed metal-plate joints, however. the maximum given elongation, usually 0.015 inch, or a portion tensile loads were affected. For the bending of the maximum load. The lower of these two specimens, moisture cycling caused similar values determines the allowable load. For nailed
FPL 67 18 Table 6.--Permanent set resulting from dead loading and moisture cycling, and additional elongation and deflection due to destructive loading of the various types of joints evaluated tension and bending
joints, the load at a given elongation generally to be two to three times less rigid than the kiln- governs the allowable load. The effect of moisture dry control specimens. Typical load versus cycling upon the elongation and deflection of the elongation curves for kiln-dry, nailed-wood joints nailed wood joints, therefore, was of primary in tension are shown in figure 19. concern. The cycled bending specimens had 1.09 to 1.42 The cycled tension joints had 1.06 to 2.00 times times greater deflection than the controls. Again, greater elongation than the controls, the smaller the largest ratio was for the kiln-dry specimens ratio being for the “wet” initial moisture content because of the smaller deflection of the controls. group and the larger ratio for the “kiln-dry” The effect of the gap left between the gussets group. The kiln-dry specimens, however, were and the members of the cycled specimens can be still more rigid than the higher moisture content seen in figure 20. The curve for the cycled groups. This greater percentage increase in specimen starts at a flatter slope than the curve elongation can be accounted for partially by the for the control. When the gap becomes closed, gap left between the gussets and members in the the two curves have nearly the same slope. cycled specimens. No gap was present in the kiln- Losses in maximum load due to moisture dry control specimens, while for the other con cycling were not as great as the losses in trols a gap was generally present due to rigidity. They ranged from 0 to 15 percent loss shrinkage as the specimens dried to 10 percent for the tension specimens and 11 to 19 percent moisture content. These gaps caused the control for the specimens. The greater losses specimens in the higher moisture content groups were for the initially wet specimens.
19 Figure 19.--Typical load versus elongation curves for kiln-dry, nailed-wood joints in tension. M 131 602
Figure 20.--Typical load versus deflection curves for kiln-dry, nailed-wood joints in bending. M 131 603
67 20 Corrosion of the nails had an effect upon the corrode to the point of being loose and, if maximum loads. At small deformations, the extremely corroded, may even break. This will corrosion had practically no effect, since the cause a greater loss in and, of more lateral of the nails was dependent on importance, a greater loss in rigidity. Since the bending of the shank, but at maximum load design loads are based on rigidity, the initially there was some withdrawal of the nail and resist wet group is not as satisfactory as the drier ance to withdrawal was increased by the rust. groups, even though its maximum strength is All of the cycled specimens, as well as the con greater because of the initial corrosion of the trol specimens from the initially wet moisture nails. content group, had nails that were rusted. Nails Figures and 22 show typical failures. The from the controls of the other groups had joints usually failed by splitting of the members shiny bright shanks. The rusting resulted in or gussets with nearly simultaneous withdrawal higher maximum loads for the control specimens of the nails. of the initially wet moisture content group and, The results for the tension specimens com thus, a greater percentage loss in maximum load pared quite well with the usually design due to moisture cycling. This does not mean, values. There was still a ratio of about 4 to 1 in however, that joints made of wet material will be the maximum loads based on a 2,000-pound design more satisfactory in over a long period load. The elongation values for the tension con of time. Over a period of years, the nails will trol specimens nearly equalled or were less than
Figure 21.--Typical failures in tension for the nailed-wood and nailed-plywood joints. The plywood gusset (bottom) broke in tension. The nailed-wood specimen (upper) split in the gussets and members as nails were withdrawn. M 126 099
21 Figure 22.--Typical failures in bending for the nailed-wood and nailed-plywood joints. The plywood joint failed in the gusset, while the nailed-wood joint (bottom) failed in the member and gusset. M 126 098
the value of 0.015 inch used to establish design wet material. The greater percentage increase loads. The wet specimens showed the greater in elongation can be accounted for, in part, by elongation but, in design, the allowable load the much greater stiffness of the kiln-dry con would be multiplied by a factor of three-quarters trols that had no gap between the gussets and the to compensate for the moisture content (4). The members, while the other specimens did. The cycled specimens, both in tension and bending, effect of this gap can be seen from the load showed greater deformation than the uncycled versus elongation curves shown in figure 23. The controls. curve for the cycled specimens had a much flatter Nailed-plywood joints.--As with the nailed- slope under initial loading than the curve for the wood joints. the effect of moisture cycling upon controls. the elongation and deflection of the joints was of The cycled bending specimens had 1.44 to 1.61 primary concern, since deformation governs the times greater deflection than the controls. Again, allowable design load. the larger ratio for the specimens made of The cycled tension specimens had to kiln-dry material. This can be accounted for by 3.50 times greater elongation than the controls, the gap between the gussets and members of all larger ratio being for the kiln-dry group and specimens except the kiln-dry controls, which the smaller ratios for the wet specimens. The had a greater stiffness. This effect can be seen cycled specimens made of kiln-dry material, in figure 24, which shows typical load versus however, were stiffer than the controls made of deflection curves.
FPL 22 Figure 23.--Typical load versus elongation curves for kiln-dry, nailed-plywood joints in tension. M 131 604
Figure 24.--Typical load versus deflection curves for kiln-dry, nailed-plywood joints in bending. 131 605
23 Losses in maximum load due to moisture and bending, showed greater deformation than the cycling ranged from 0 to 16 percent in tension uncycled controls. and from 10 to 30 percent in bending. As with the The nailed-plywood joints had nearly same nailed-wood joints, specimens with rusty nails maximum loads in tension as the nailed-wood had higher maximum loads. As corrosion pro- joints. The maximum loads in bending, however, ceeds, however, these joints will become weaker were about 500 pounds less. This can be accounted than those made of dry material. for the fact that the plywood gussets were not Figures 21 and 22 show typical failures. The as thick as those made of wood and only three- joints usually failed in the plywood gusset with fifths of the plywood thickness was effective in very slight withdrawal of the nails. resisting bending moment. This is also true for The results for the tension specimens compared the tension specimens may account for the quite well with the usually accepted design values. greater effect of moisture cycling on the plywood There was still a ratio of about 4 to 1 in the joints than on solid-wood joints. The plywood ultimate loads on the basis of a 2,000-pound gussets developed checks in the surface veneers design lead. The elongation values for the tension from moisture cycling. This caused a greater control specimens nearly equalled or were less increase in deformation for the plywood joints than the value of 0.015 inch used to establish than for the solid-wood joints. design loads, except for the initially wet speci- Nailed metal-plate joints.--The allowable loads mens. The wet showed greater elonga- for this type of joint are usually based on load tion but, in design, the allowable load would be at a given elongation. The effect of moisture reduced one-quarter to compensate for moisture cycling upon the elongation and deflection of the content (4). The cycled specimens, both intension joints. therefore, was of primary concern.
Figure 25.--Typical load versus elongation curves for kiln-dry, nailed metal-plate joints tension. M 131 606
FPL 67 24 The cycled tension specimens had from 1.00 to either by popping of the nailheads with a slight 2.67 times greater elongation than the controls. withdrawal or tearing of the plate, as shown in The greatest difference between cycled and con figure 27. Thus, there was no effect of moisture trol specimens was for kiln-dry group, in cycling upon the maximum loads. The wetter which the controls had approximately one-half moisture content groups in tension, however, did the elongation of any other group. All values of have slightly higher loads, This again was due to elongation nearly equalled or were less than the the rusting of the nails, as previously explained. value of 0.015 inch used to establish allowable Barbed metal-plate joints.--These joints were loads. relatively stiff, as will be noted from the typical The cycled bending specimens had 0.98 to load versus elongation or deflection curves in 1.49 times greater deflection than the controls. figures 28 and 29. The tensile load at 0.015-inch Here again, the values of deflection were practi elongation was about 4,200 pounds, which is cally the same for all groups. The reason there greater than one-half the maximum load. The was practically no difference in rigidity between Truss Plate Institute, Incorporated (9) requires the different groups, either in bending or tension, that the allowable load shall not be greater than is probably because the nail shanks were one-third of the maximum load; thus, the allow entirely in the wood members. This would cause able design load for the barbed metal-plate joints the shanks to bend less than with the nailed-wood is governed by maximum load and, therefore, the and nailed-plywood joints. Typical load versus effects of moisture cycling and initial moisture elongation or deflection curves are shown in content upon maximum load were of primary figures 25 and 26. concern. Typical failures of these joints were caused The cycled tension specimens had from 0 to
Figure 26.--Typical load versus deflection curves for kiln-dry, nailed metal-plate joints in bending. M 131 607
25 Figure 27.--Typical failures for the nailed metal-plate joints. The tension specimen (left) generally failed by popping of the nailheads with slight withdrawal, and splitting of the wood members. The bending specimen (right) generally failed by tearing of the plate. M 129 986
Figure 28.--Typical load versus elongation curves tor kiln-dry, barbed metal-plate joints in tension. M 131 608
FPL 67 26 Figure 29.--Typical load versus deflection curves for kiln-dry, barbed metal-plate joints in bending. M 131 609
18 percent loss in maximum load due to moisture initial moisture content, the initially wet control cycling. The greatest loss was in the kiln-dry joints having about 4 times greater elongation group, where the controls had a maximumload than the kiln-dry specimens in tension and about of about pounds more than any other group. 1.24 times greater deflection in bending. The wet specimens had no loss in maximumload; Toothed metal-plate joints.--For this type of however, the loads for the control and cycled connector, the deformation and maximum load specimens were below the 6,000 pounds needed to values were such that the allowable design load establish an allowable design load of pounds. could be governed by either value, Thus, the This was due to shrinkage of the members caus effects of moisture cycling initial moisture ing the plates to bow out of the wood, leaving the content en elongation, deflection. and maximum joint held together by only a few of the barbs plus load were of equal concern. the positioning nails along the outside edges. This The cycled tension specimens had from 1.00 to same. effect also occurred with the air-dry speci 2.00 times the elongation of the controls, the mens, but to a smaller extent. greatest ratio being for the kiln-dry specimens, The cycled bending specimens had from 2 to in which the controls had one-half to two-thirds 9 percent loss in maximum load due to moisture the elongation of the other moisture content cycling. The smaller effect on the bending speci groups. All values of elongation were below the mens, as compared to the tension specimens, is value of 0.015 inch used to establish allowable explained by the type of failures that occurred loads. (fig. 30). The bending specimens usually failed The cycled bending specimens had 1.12 to by tearing of the plates, while the tension speci 1.30 times greater deflection than the: controls. mens failed by withdrawal of the barbs from the Again, the greatest percentage increase in deflec wood members. tion was for the kiln-dry specimens, which had The barbed metal-plate joints were relatively the stiffer controls. stiff and moisture cycling had no effect on elonga Typical load versus elongation or deflection tion or deflection. There was some effect due to curves are shown in figures 31 and 32.
27 Figure 30.--Typical failures for the barbed metal-plate joints. The tension specimen (left) generally failed by withdrawal of the barbs, while the bending specimen failed by tearing of the plates. M 129 985
Figure 31.--Typical load versus elongation curves for kiln-dry, toothed metal-plate joints in tens ion. M 131 610
FPL 67 28 Figure 32.--Typical load versus deflection curves for kiln-dry, toothed metal-plate joints in bending. M 131 611
Tension specimens had from 0 to 9 percent initial moisture content upon the bonding of the loss in maximum load, the larger loss being for plywood to the members was of first concern the kiln-dry specimens. Both the cycled and con and then, the effect of moisture cycling upon trol specimens of this group, however, had higher both the plywood and the bond. maximum loads than those specimens made of No difficulty was encountered in bonding the wet material. Shrinkage of the wet material gussets to the members with the phenol-resorcinol caused the members to split slightly, thus reduc glue at any of the initial moisture contents. AS ing their lead capacity. Values of maximum load the wetter material dried to 10 percent moisture still had a ratio of about 4 to 1 after cycling, content, however. partial failures occurred in based on a design load of 2,000 pounds. the plywood because of unequal shrinkage between Bending specimens had from 2 to 20 percent the gussets and wood members. Partial glue- loss in maximum load. The larger loss was for bond failure was also observed in some instances. specimens made with wet material. Values were These partial failures caused about a 200-pound nearly the same except for the wet cycled speci loss in maximum load for the tension specimens mens, which were about 500 pounds less. and about a 600-pound loss for the bending Typical failures are shown in figure 33. Tension specimens. specimens generally failed by withdrawal of The moisture cycling accentuated the partial teeth from the members and splitting failures due to shrinkage. This, coupled with of the members. Bending specimens usually checking of the plywood veneers, caused losses in failed by splitting of the members along the bot-. maximum load of 5 to 21 percent for the tension row of teeth. specimens and 7 to 20 percent for the bending Phenol-resorcinol glued joints.--Allowable specimens. The greater losses were for speci loads for glued plywood joints depend primarily mens made of wet material, on the strength of the plywood if an adequate bond Elongation values were small (0.001 or 0.002 can be made between the wood members and inch) and appeared to be unaffected by moisture gussets. For glued joints, therefore, the effect of cycling. Similarly, there was hardly effect
29 Figure 33.--Typical failures for toothed metal-plate joints. Tension specimen (left) gener ally failed by withdrawal of teeth, while bending specimen (right) generally failed by splitting of the members along the bottom row of teeth. M 129 988
Figure 34.--Typical load versus elongation curves for kiln-dry, phenol-resorcinol glued joints In tension. M 131 612
FPL 67 30 Figure 35.--Typical load versus deflection curves for kiln-dry, phenol-resorcinol glued joints in bending. M 131 613 upon deflection. Typical load versus elongation maximum load in bending than those joints made or deflection curves are shown in figures 34 of drier material. and 35. The cycled tension specimens had from 11 to Typical failures are shown in figure 36. Both 23 percent loss in maximum load. The smallest the tension and bending specimens generally failed loss was for the wet specimen group. The con in rolling shear in the plywood. Occasionally trol specimens for this group, had there would be partial glue-bond failure in the maximum loads approximately 2,000 pounds leas wet specimen group. than the maximum loads of the other controls. Casein-glued joints.--As with the phenol Some complete glue failure was noted for the wet resorcinol glued joints, the effect of initial mois specimens due to moisture cycling, and partial ture content upon the bonding of the plywood to glue failures were noted for the rest of the speci the members was of first concern and then, the mens. There were also some partial failures of effect of moisture cycling upon both the plywood the plywood gussets due to unequal shrinkage, and the bond. Cycled bending specimens had from 1 to 14 No difficulty was encountered in bonding the percent loss in maximum load due to moisture gussets to the members for the kiln-dry and air- cycling. The smaller loss was for the wet speci dry material. For the wet material, the bond mens. the wet control specimens were appeared to be adequate at the end of fabrica about 700 pounds weaker than the other controls tion, but during the dead loading for moisture and were even less than the other cycled speci cycling, three of the bending specimens failed. mens. Again, some complete glue failures were Examination showed complete glue failure. The noted for the wet specimens during moisture glue was still damp, even though 3 weeks had cycling and partial failures in the other speci been allowed for curing. Since casein glue is a mens. There were also some partial plywood water-base adhesive, the excess moisture in the failures due to shrinkage. wood tended to keep the glue soft. The joints The elongation values were relatively made of wet material had about 2,000 pounds less and showed no effect from moisture cycling. maximum load in tension and 700 pounds less deflection was unaffected by moisture
31 Figure 36.--Typical failures of glued-plywood joints. Both tension (left) and bending (right) specimens generally failed in rolling shear in the plywood. M 129
Figure 37.--Typical load versus elongation curves for kiln-dry, casein-glued joints in tension. M 131 614
FPL 67 32 Figure 38.--Typical load versus deflection curves for kiln-dry, casein-glued joints in bending. 131 615
cycling. Typical load versus elongation or deflec mean (in this instance it was assumed to be zero); tion are shown in figures 37 and 38. s = standard deviation of the differences; and Typical failures are shown in figure 36. Those N = sample size. The calculated t was then com specimens that held together until destructive pared to a t value associated with a 95 percent loading generally failed in rolling shear in the confidence level. This value comes from the t plywood. Some partial glue failures also occurred, distribution and was equal to 2.015 for all of the especially in the wet specimen group. sample groups except those that had only five specimens. For groups having five specimens, the t value was 2.132. STATISTICAL ANALYSIS The calculated t values for tension speci mens are shown in table 7. For all values larger than the 2.015 or 2.132 reference: values, the “t” Values hypothesis that there is no difference would be rejected and a significant difference shown, while A statistical analysis was made to determine for values less than the selected values, the if the differences between cycled and control hypothesis would be accepted. It is evident from specimens were significant. the calculated t values that moisture cycling had To do this, the hypothesis was made that there significant effects upon the elongation of all the was no difference between the cycled and control mechanical-connector joints made of kiln-dry specimens. The statistical relation material. This was expected, since the kiln-dry controls were much stiffer than the other controls. For the Specimens made of air-dry material, the elongation of the nailed-wood, nailed-plywood, toothed metal-plate, and casein-glued joints was calculated for the difference between control showed a significant difference between cycled and matched cycled specimens. In this equation, and control specimens. The fact that the difference X = mean of the differences; X' = the population in elongation for the air-dry casein-glued joints
33 Table 7.--Calculated t values for differences between cycled and control specimens in elongation and maximum load for various types of joints evaluated in tension1
was significant should not be given too much than the maximum loads of the other controls. importance, since the values of elongation were The effect of moisture cycling on the wet nailed- very small. This is also true of the wet casein- wood joints would probably have been significant, glued joints, which also showed moisture cycling as it was for the air-dry specimens, except for to have a significant effect upon elongation. The the temporary effects of corrosion. only other joint showing a significant effect on The calculated t values for the bending speci elongation from moisture cycling was the nailed- mens are shown in table 8. Deflections of all the plywood type. With the wet specimens, the con mechanical-connector joints made of kiln-dry trols had larger values of elongation than the material were significantly affected by moisture drier controls, and thus their calculated t values cycling, except the barbed metal plate. Other were smaller. groups with significant effects of moisture cycling The calculated t values for maximum load in on deflection are the air-dry nailed-wood joints, tension showed less significant effects of mois wet and air-dry nailed-plywood joints, and wet ture cycling than on elongation. The specimens and air-dry toothed metal-plate joints. The rea most affected were wet and air-dry phenol son for the kiln-dry specimens having a more resorcinol joints and the air-dry casein joints. significant difference between cycled and control The reason the wet casein joints did not show a specimens was the smaller deflection of the kiln- significant difference was because of the low dry controls than the deflections of the other con value of maximum load for the controls. Of the trol specimens. mechanical connectors, only the air-dry nailed- The calculated t values for maximum load in wood, kiln-dry barbed metal plate, and kiln-dry bending showed a significant difference between toothed metal plates showed a significant differ cycled and control specimens for the glued joints ence between cycled and control specimens. In made of wet and air-dry material. Of the the case of the two metal plates. the kiln-dry mechanical-connector joints, the nailed-wood and controls had maximum loads that were higher nailed-plywood joints made of wet and air-dry
FPL 34 Table 8.--Calculated t values for differences between cycled and control specimens in deflection and maximum load for the various types of joints evaluated in bending1
material had significant effects on maximumload or elongation, an analysis of variance was made from moisture cycling. Others with significant on the control specimens for the three initial differences in maximum load were the kiln-dry moisture content groups. F values were calcu nailed-plywood joints, air-dry barbed metal- lated (table 9) and compared with a value of 3.68, plate joints, and wet toothed metal-plate joints. which is associated with a 95 percent confidence The fact that the more significant effects were level. This analysis showed a significant differ fox joints made of drier material does not mean ence in maximum load in tension due to initial that they are inferior to the joints made of wet moisture content for the three metal-plate joints material. In practically all instances, the kiln- [the calculated values were greater than dry controls were much stiffer and stronger than The only joints showing significant differences in the controls of the other moisture content groups; maximum load in bending were the toothed metal- thus, they had a greater percentage increase in plate and glued joints. Maximum loads of all deflection or elongation and a greater loss in other joints were not significantly affected by strength. even though the cycled kiln-dry speci initial moisture content. The differences in deflec mens were stronger than the other cycled tion or elongation were quite significant, except specimens. fox the glued joints in tension and the nailed metal plates in bending. Since design loads for mechanical joints are generally on deforma “F” Values tion, there is obviously an advantage in using dry lumber. To determine if initial moisture content had a significant effect upon the strength, deflection,
35 Table 9.--Calculated F values from an analysis of variance between initial moisture-content groups in deflection, elongation, and maximum load for control specimens of the various types of ,joints evaluated in bending and tension1
FPL 67 36 SUMMARY OF FINDINGS
1. Moisture cycling caused deflection of bending specimens to be from 2.1 to 6.9 times that of uncycled specimens and elongation of tension specimens to be from 2.0 to times that of uncycled specimens at the end of moisture cycling. 2. Moisture cycling caused residual deforma tion that was approximately two to three times that of uncycled specimens. 3. Results of destructive loading showed that cycled tension specimens had from 1.00 to 3.50 greater elongation than the control specimens at a design load of 2,000 pounds. 4. Results of destructive loading showed that cycled bending specimens had from to 1.61 times greater deflection than the control speci mens at a load of 1,000 pounds. 5. Losses in maximum load ranged from 0 to 23 percent for the cycled tension specimens. 6. Losses in maximum load ranged from 0 to 30 percent for the cycled bending specimens. 7. The maximum loads of cycled specimens were adequately larger than the design loads, except for the glued joints and barbed metal- plate joints made of wet material. 8. Elongation values for control specimens of the mechanical-connector type joints compared favorably with the value of 0.015 inch used to establish design loads. 9. Wet material had an adverse effect upon the glued joints, causing partial glue failures and partial plywood failures due to unequal shrinkage. 10. Wet material had an adverse effect upon the barbed metal-plate joints where shrinkage caused the plates to bow, thus removing the barbs from the wood.
37 LITERATURE CITED
1. Angelton, H.D. Nailed-plywood gusset roof trusses, 4/12 and greater slopes. maximum span-28feet 8 inches. Mimeo. F-40. Purdue Univ., Agr. Exp. Sta., Lafayette, Ind. 2. Luxford, R.F. 1958. Light wood trusses. U.S. Forest Serv., Forest Prod. Lab. Rep. 2113. 3. , and Heyer, O.C. 1954. Glued and nailed roof trusses for house construction, U.S. Forest Serv., For est Prod. Lab. Rep. 1992. 4. National Lumber Association, 1962. National design specification for stress-grade lumber and its fasten ings. Washington, D. C. 5. Pneuman, F.C. 1960, King-post trusses with plywood gussets one side. Tech. Bull. Douglas Fir Ass., Tech. Dep., Tacoma, Wash. 6. Radcliffe, B.M., and Granum, H. 1955. A new low-pitched roof truss with nail- glued connections. Agr. Exp. Sta. Bull. 617, Wood Res. Lab., Purdue Univ., Lafayette, Ind, 7. , Granum, H., and Suddarth, S.K. 1955. The Purdue-Illionis nail-glued roof truss with pitch of 3:12 and 4:12 for spans of feet 8 inches and 28 feet 8 inches. Agr. Exp. Sta. Bull 629. Wood Res. Lab., Purdue Univ., Lafayette, Ind. 8. , and Suddarth, S.K. 1955. The Purdue-Illinois nail-glued truss with a pitch of 2:12 for spans of 24 feet 8 inches and 28 feet 8 Agr. Exp. Sta. Bull. Wood Res. Lab., Purdue Univ., Lafayette, Ind. 9. Truss Plate Institute, Inc. 1965. Design specifications far light metal plate connected wood trusses TPI-65. Miami, Fla.
FPL 38 1.2-45