Monotonic and Cyclic Tests to Determine Short-Term Load Duration ...

Monotonic and Cyclic Tests to Determine Short-Term Load Duration Performance of Nail and Bolt Connections Volume I: Summary Report

Virginia Polytechnic Institute and State University Timber Engineering Report No. TE-1994-001

by: J.D. Dolan Assistant Professor of Wood Engineering S.T. Gutshall Research Assistant Department of Wood Science and Forest Products Brooks Forest Products Center 1650 Ramble Road Blacksburg, Virginia 24061-0503 and T.E. McLain Department Head and Professor of Wood Engineering Department of Forest Products Oregon State University Corvallis, Oregon 97331-7402

October, 1995

Report No. TE-1994-001

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SUMMARY An experimental study of the monotonic and cyclic properties of nailed and bolted wood connections is described. The objectives of the investigation are 1) determine the effects of cyclic loading on the performance and safety of nail and bolt connections, 2) determine if the value of 1.6 used as the load duration factor for wind and seismic design in the 1991 National Design Specification for Wood Construction (NDS) is conservative. Three typical nail and three typical bolt connections were constructed to include three of the four possible yield modes for wood connections. Two matched samples of each connection type (15 specimens for nails and 10 specimens for bolts) were tested monotonically to either catastrophic failure or a connection slip of 1.0 inches. One matched sample was subjected to cyclic loads prior to being tested monotonically, while the other matched sample represented a control that was tested according to methods used to determine design values published in the 1991 NDS. An additional sample of lumber-to-lumber nail connections were fabricated without pilot holes and tested under monotonic loading. Results indicate that prior cyclic loading to magnitudes as high as 2.0 times the nominal design values published in the 1991 NDS do not have adverse effects on connection capacity, or ductility for nailed and bolted wood connections. This indicates that design seismic events will not significantly lower the connection’s ability to resist the loads. Factors-of-safety based on the allowable seismic design values ranged from 1.7 to 3.2 for nail connections, from 2.3 to 4.6 for bolted connections loaded parallel-to-grain, and from 3.3 to 8.1 for bolted connections loaded perpendicularto-grain. High ductilities associated with nail connections and their ability to dissipate large amounts of energy indicate that a factor-of-safety of 1.7 is sufficient to guarantee acceptable performance. The factors-of-safety associated with bolt capacities are sufficient to provide acceptable performance of wood connections. Results presented also quantify several cyclic properties such as hysteretic damping, cyclic stiffness, and equivalent viscous damping. Values of these cyclic properties illustrate the ability of wood connections to dissipate significant quantities of energy during cyclic or dynamic loadings expected during natural hazard events such as earthquakes. The ability to dissipate energy improves a structure’s performance and reliability.

INTRODUCTION An increase in the load duration factor, CD, for wood construction under seismic and wind loading from 1.33 to 1.6 was included in the 1991 edition of the National Design Specification® for Wood Construction (NDS®). This change is based on long-term rational that has been applied to wood construction. While this increase does not reflect any change in philosophy from previous design codes, it highlights an area of limited research support for connections. Lack of research data raises questions about the load duration increase for seismic loading from historic levels. The basis of the questioning relates to previous connection tests used to determine design values for the NDS which were performed with monotonic quasi-static and impact loading, while seismic events produce a reversing load effect on the connections.

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Equations presented in the 1991 NDS for determining allowable design load include two factors. The first factor adjusts short-term strength to allowable 10-year duration strength. This factor is equal to 1.6 and is based on engineering judgement and results of load duration tests of wood members. The second factor is a factor-of-safety for the particular yield mode considered. In Equations 8.2-1 to 8.2-6 in the 1991 NDS for bolted connections, constants in the denominator of each equation combine the duration-of-load (DOL) and factor-of-safety (FOS). The factor-of-safety includes adjustments that account for uncertainty, variability, and calibration to historic performance. Therefore, the constants of each equation can be broken into two components as follows: !4.0 for MODES Im and Is becomes 1.6 x 2.5 for DOL and FOS respectively !3.6 for MODE II becomes 1.6 x 2.25 for DOL and FOS respectively and !3.2 for MODES IIIm, IIIs, and IV becomes 1.6 x 2.0 for DOL and FOS respectively. The change from 1.33 to 1.6 for the DOL factor was made without changing the FOS for all NDS fastener equations. The previous value of 1.33 essentially increased the factor-of-safety for seismic over that used for normal duration loads. To understand why CD = 1.6 is proposed for short-term loads such as seismic actions, consider the method used to calculate the nominal design values shown in the 1991 NDS. Connection design values are based on experimental yield loads derived from monotonic tests with a rate of loading causing failure of specimens in 5-10 minutes. This short-term load is then indexed to a nominal design value based on a service duration of 10 years. (Normal or 10-year load duration assumes that wood structures experience a cumulative design load effect of 10 years duration over the useful life of the structure.) To adjust short-term values to the nominal 10-year loads, values are divided by 1.6. The 1991 NDS readjusts the nominal values back to a short-term (10-minute) design values for seismic loads by multiplying by the load duration factor of 1.6. There are no other factors implied in the use of CD = 1.6 other than indexing back to short-term design capacity. The 1.6 load duration factor is conservative since it adjusts the nominal design value to a 10-minute design value, whereas an earthquake is a much shorter load event. In fact, if factor-of-safety values remain unchanged, a more accurate load duration factor for seismic design most likely will be higher than 1.6 if the capacities continue to be based on monotonic test results. However, monotonic tests are not representative of cyclic loads such as earthquakes. Monotonic tests do not provide information on any effect of prior cyclic load history on connection capacity and/or ductility. There is concern that previous load history may affect connection reserve capacity and/or ductility in situations of cyclic loading. Long-term static performance information is needed since there currently are no experimental results on which to directly base the expected long-term performance of connections. As stated earlier, current procedure for adjusting the short-term experimental results to a 10-year nominal value are based on long-term material strength tests of wood and engineering judgement. Therefore, while long-term connection tests will not address the effect of reversing load such as the cyclic loading associated with seismic events, they will provide information that will be useful in correctly setting the 10-year nominal capacities. This research project was necessary to determine the effects of cyclic load history on nailed and bolted connections and determine effective factors-of-safety for cyclicly loaded connections.


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The objectives of this report are as follows: 1. Present the results of monotonic tests and load-controlled cyclic tests of nail and bolt connections. 2. Show that short-term cyclic load history on connections, at levels as high as 2.0 times nominal design loads, does not adversely affect on capacity or ductility of nail and bolt connections. 3. Show that the proposed load duration factor of 1.6 is conservative for laterally loaded nail and bolt connection in wood if fabricated according to the minimum requirements of the 1991 NDS. 4. Show that the apparent factors-of-safety for nail and bolt connections, subjected to prior cyclic loading, provide an acceptable level of performance. The specific information presented cannot determine directly whether the load duration factor presented in the 1991 NDS is correct or not since long-term load tests of connections have not been performed. The long-term connection tests are required for setting the nominal design values to which all of the other durations are calibrated. Detailed and summary information on the connections tests is presented in companion reports that are available from the Department of Wood Science and Forest Products, Timber Engineering Center, which is located in the Brooks Forest Products Research Center at Virginia Polytechnic Institute and State University. Detailed data for each specimen tested to determine monotonic and load-controlled cyclic performance tests are presented in Virginia Polytechnic Institute and State University (VPI) Timber Engineering Report No. TE-1994-002. The summary report and detailed data for each specimen tested under the Sequential Phased Displacement (SPD) procedure are presented in VPI Timber Engineering Reports No. TE-1994-003 and No. TE-1994-004 respectively.

TEST PROCEDURES Specimen Configurations Connection geometries were chosen to represent typical connection details found in construction in the United States, and to include three of four yield modes that can occur in timber connections. Connections were in single shear with one dowel fastener. Fifteen specimens of each nailed connection type and ten specimens of each bolted connection type were tested. Table 1 summarizes the types and numbers of replications used for nail connections in monotonic and loadcontrolled cyclic tests. Table 2 summarizes the same information for bolt connections. All lumber and plywood used for the tests was purchased at local lumber retailers. Specimens were constructed from southern pine, and were cut so as to avoid localized defects in the wood as much as possible. The wood was conditioned at a temperature of 20 ±30 C and relative humidity of 65±5% for a minimum of 14 days, or until the equilibrium moisture content was reached. The steel plate used in two connection geometries was also locally purchased and consisted of ¼-inch, ASTM A36 mild carbon steel (ASTM 1989-a) and 18-gauge A446 galvanized sheet steel (ASTM 1989-b). Nails used in two connection geometries were 10 penny (10d) common nails with a diameter of 0.148 inches and length of 3 inches. The 16 penny (16d) common nails used in the third nailed connection geometry were 0.162 inches in diameter and 3.5 inches long.

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Table 1 shows that there were three types of nail connections tested, and each type of connection was tested in two configurations for the monotonic test (parallel-to-grain and perpendicular-to-grain) and one configuration for the cyclic test (parallel-to-grain). Two configurations used for monotonic tests were included to investigate effects of grain orientation with respect to load, while cyclic load history effect was investigated only for the parallel-to-grain direction. Since nailed connections are characterized by yielding of the nails as well as the wood, two yield modes that represent fastener yielding (modes IIIs and IV) were included. Three types of commonly used connections were included, 1) plywood to lumber which is used in shear walls and diaphragms, 2) lumber to lumber which is typical of light framing connections, and 3) light-gauge sheet steel to lumber which is typical of joist hangers and other light-gauge metal connectors. An additional sample of lumber-to-lumber nail connections without pilot holes was tested to investigate the effect of pre-drilling pilot holes on nail connection performance. A matched sample of the connections without pilot holes was also tested under the SPD protocol to investigate the cyclic performance, with the results being reported in VPI Timber Engineering Reports TE-1994-003 and TE-1994-004. Fifteen replicates were tested in each configuration to provide some information on the statistical variation in connection performance. Summary information on type and number of replicates for bolt connection tests is presented in Table 2. Bolted connections were subjected to monotonic and cyclic loading in the parallel-tograin orientation and monotonic loading only in the perpendicular-to-grain orientation. Similar to nail connections, monotonic tests were used with two configurations to investigate effects of grain orientation on the monotonic connection performance, and cyclic load history effects were investigated only for the parallel-to-grain direction. The stationary member for all bolted connections was defined as the member that was clamped in a fixed position in the testing fixture, and was located on the nut side of all bolted connections. For all nailed connections, the stationary member was the penetrated or main member of the connection. The active member for all connections was defined as the member that was moved by the MTS hydraulic actuator in either a monotonic or cyclic manner during the tests. All bolted connections tested with the load acting perpendicular-to-grain to the stationary member required that the stationary members be wider than the active members in order to meet edge distance requirements for the full 1991 NDS design value. Three types of bolted connection geometries were tested in an effort to include three of four yield modes possible for bolted connections. Connections were also chosen to simulate typical connections used in wood structures. The 2-inch-to-2-inch nominal lumber connection with a ¾-inch bolt represents typical diaphragm chord connections. The 4-inch-to-4-inch nominal lumber connections with a ¾-inch bolt represents the yield mode expected in a concrete to wood connection such as a ledger board for a roof attached to a tilt-up concrete wall. Finally, the 4-inch nominal-to-¼inch steel plate connection represents typical connection hardware used in glulam and post-frame connections.


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Table 1: Summary of type and number of nail connection specimens tested under monotonic and load-control cyclic loads. Fastener Type (inches)

Main / Side Member Materials

Load Direction

Load Type

Expected Yield Mode

Number of Replicates

10d (0.148 x 3.0)

Lumber / 15/32-in Ply

Parallelto-Grain

Monotonic

IIIs

15

10d (0.148 x 3.0)


Perpendicularto-Grain

Monotonic

IIIs

15

10d (0.148 x 3.0)


Parallelto-Grain

Cyclic / Monotonic

IIIs

15

16d (0.162 x 3.5)

Lumber / Lumber

Parallelto-Grain (pilot Holes

Monotonic

IV

15

16d (0.162 x 3.5)

Lumber / Lumber

Perpendicularto-Grain (Pilot Holes)

Monotonic

IV

15

16d (0.162 x 3.5)

Lumber / Lumber

Parallelto-Grain (Driven)

Monotonic

IV

15

16d (0.162 x 3.5)

Lumber / Lumber

Parallelto-Grain (Pilot Holes)

Cyclic / Monotonic

IV

15

10d (0.148 x 3.0)

Lumber / 18-ga. Steel

Parallelto-Grain

Monotonic

IIIs

15

10d (0.148 x 3.0)



Monotonic

IIIs

15

10d (0.148 x 3.0)


Parallelto-Grain

Cyclic / Monotonic

IIIs

15

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Table 2: Summary of type and number of bolt connection specimens tested under monotonic and load-control cyclic loads. Fastener Type

3/4-in Bolt

3/4-in Bolt

1/2-in Bolt

Main / Side Member Materials

2-in / 2-in Nominal Lumber

4-in / 4-in Nominal Lumber

1/4-in Steel Plate / 4-in Nominal Lumber

Load Direction

Load Type

Expected Yield Mode

Number of Replicates

Parallelto-Grain

Monotonic

II

10


Monotonic

II

10

Parallelto-Grain

Cyclic / Monotonic

II

10

Parallelto-Grain

Monotonic

IV

10


Monotonic

IIIM

10

Parallelto-Grain

Cyclic / Monotonic

IV

10

Parallelto-Grain

Monotonic

IIIS

11


Monotonic

IIIS

10

Parallelto-Grain

Cyclic / Monotonic

IIIS

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Specimen Fabrication The full test program required four samples of matched specimens. Matched samples were obtained by cutting four replicates of each component from adjacent locations in a single board of southern pine lumber. This matching technique provides specimens in each sample with as close to identical physical characteristics as possible. Obvious local variations such as knots or splits were avoided in choosing the locations of each set of four components. Matched components were then marked so that sets of four "identical" specimens produced four samples, each with 15 or 10 matched specimens depending on whether the fasteners were nails or bolts, respectively. Results of tests on three of the four matched samples are presented in this report. The fourth matched sample was tested using the SPD procedure, and the results are presented in subsequent reports. Nail Connections Nailed connections were fabricated and then placed in an environmental chamber (at 200 C 0 (68 F) and 65% relative humidity) for a minimum of 14 days to allow for relaxation of wood fibers around the nail and to achieve approximate equilibrium moisture content. This conditioning time


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provided a more accurate representation of a nailed connection that has been in service for a period of time. Nailed connections usually have a higher initial stiffness immediately after assembly because wood fibers in contact with the nail shank have not relaxed. Pre-drilled nail holes, meeting the guidelines established in the 1991 NDS, were used to guide the nails and prevent splitting of the wood members during driving with a hand-held hammer for all except one sample. The 1991 NDS allows a pre-drilled hole no more than 75% of nail diameter for wood with a specific gravity less than 0.60. Therefore, connections using 10d common nails were pre-drilled with a 1/16-inch hole, or 42% of nail diameter, and connections using 16d common nails were pre-drilled with a 3/32-inch hole, or 58% of nail diameter. For 18-gauge steel plate to 2x4 connections, a 9/64-inch hole (95% of nail diameter) was pre-drilled in the steel plate. Members used in the testing had actual average specific gravities ranging from 0.48 to 0.62 depending upon the grade and size of lumber used. One sample of lumber-to-lumber connections, using 16d common nails, was fabricated without drilling pilot holes. Bolted Connections Bolted connections were fabricated immediately prior to testing. A bolt hole that was 1/16inch larger than the bolt was used for both wood and steel members of all bolted connections. This was in accordance with assembly tolerances given in the 1991 NDS and 1989 Manual of Steel Construction, Allowable Stress Design published by the American Institute of Steel Construction. All bolt holes were centered between the edges of members and drilled with high speed steel drill bits to ensure smoothness and uniformity. In all connection configurations, end and edge distances exceeded minimum requirements given in the 1991 NDS for use of full design values, and standard A307 mild carbon steel bolts or their equivalent were used. Active members of the connections were held by a gripping fixture attached to the testing machine. For bolted connections a grade-eight bolt was inserted through the gripping fixture into a pre-drilled hole of the same diameter to prevent any slipping between the wood or steel member and the grip. Stationary members of the connections were blocked to prevent any movement. Test Equipment All tests were conducted in the Wood Engineering Laboratory of the Brooks Forest Products Research Center at Virginia Polytechnic Institute and State University. Two MTS servohydraulic test machines were used to conduct displacement-controlled monotonic tests and load-controlled cyclic tests. Displacements and loads were measured using two linear variable differential transformers (LVDT) that were attached to the sides of specimens to measure connection slip, and load cells attached to the MTS actuators. Data was acquired using commercial data acquisition software on a micro computer in the engineering laboratory. Acquired data was analyzed using commercial spreadsheet software. Test specimens were held in place and guided by a steel fixture to prevent rotation that would result in forces other than pure shear being applied to the specimen. Figure 1 shows a diagram of the test fixture. One important aspect of the fixture alignment is that the center of the load cell is in the connection shear plane. This minimizes any moment introduced into the specimen. Rollers are included in the fixture to minimize the effects of friction between the fixture and the moving side of the specimen.

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Load cell

Grip

Rollers

Pins to prevent slippage, if required

Rollers LVDT

Specimen

Figure 1: Test fixture used for connection lateral load tests.

Monotonic Loading Monotonic connection tests were conducted using a displacement-controlled rate of 0.1 in./min. A sample of specimens for each connection type was tested to ultimate capacity, or to approximately one inch displacement if catastrophic failure did not occur prior to reaching a limiting displacement. Monotonic test specimens were not subjected to any prior cyclic loading and provided a control group tested with traditional procedures. Key information obtained from these tests were yield load and displacement, defined by the five-percent of dowel diameter offset method described in the 1991 NDS. Also, initial stiffness, capacity, displacement at capacity, and ductility were determined. (Ductility is defined as the ratio of displacement at capacity to yield displacement.) Monotonic tests were conducted for both parallel-to-grain and perpendicular-to-grain orientations, with data acquired at five points per second.


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Cyclic Loading Load-controlled cyclic tests were performed at a rate of 1 Hz. Nailed connections were subjected to fully reversing cyclic loading at two load levels: the 1991 NDS nominal design load for 30 cycles, followed by loading at 1.75 times design load for 15 cycles. The 1.75 factor consists of the load duration factor, 1.6, multiplied by the diaphragm factor, Cdi= 1.1. One nailed connection geometry, with 15 replicates, was tested with an additional 8 cycles at 2.0 times the 1991 NDS nominal design load. Bolted connections were cycled at three load levels: the 1991 NDS nominal design load for 30 cycles, followed by cycling at 1.6 times nominal design load for 15 cycles, and then at 2.0 times nominal design load for 8 cycles. Upon completion of the cyclic load regime, all specimens were tested under monotonic loading, to either catastrophic failure or a displacement of 1.0 inches. Data was acquired at a rate of 100 points per second for all cyclic tests. This high rate of acquisition ensured an accurate representation of the complete hysteresis for analysis. The time lapse between changes in cyclic load levels, and between the final cycling level and the monotonic loading at the end of cycling, was only long enough to change settings on the MTS and reset the data acquisition program.

RESULTS AND DISCUSSION Property Definitions Six properties that define monotonic performance of the connections were determined for each specimen. These properties were 1) initial stiffness, 2) yield load, 3) yield displacement, 4) capacity, 5) displacement at capacity, and 6) ductility. Figure 2 illustrates five of the six properties. Initial stiffness is the slope of the initial section of the load-displacement curve. Yield load and displacement are defined by the point where a line drawn parallel to the initial stiffness, but offset along the displacement axis by 5% of fastener diameter, intersects the load-displacement curve. Yield displacement is the distance between the intersection of the initial stiffness line with the displacement axis and the displacement corresponding to the yield load. Capacity of the connection is defined as the ultimate load or the load resisted at a slip of 1.0 inches. The load at 1.0 inch slip is used to define capacity for two reasons. First, if a connection in a real structure were to slip as much as 1.0 inch, load would be most likely transferred to other locations in the structure due to load sharing. Second, the 1.0 inch displacement is close to the maximum displacement that the fixture could tolerate and continue to maintain the specimen's correct alignment. Ductility of a connection is a measure of how much displacement the connection can sustain after yielding and not fail. This property is one indicator of how a structure will perform during an earthquake. Higher ductility of structures provides a means to sustain deflections imposed during a seismic event. Ductility is defined as displacement at capacity (as defined in the previous paragraph) divided by yield displacement, and always has a value greater than 1.0.

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Figure 2. Typical monotonic load-deflection curve for a connection with properties defined.

Three properties were used to define cyclic performance of the connections. These properties were 1) hysteretic damping, 2) equivalent viscous damping, and 3) cyclic stiffness. Figure 3 shows a typical hysteresis for a nail connection along with information required to calculate the three properties. Figure 4 show a typical hysteresis for a bolt connection with similar information required for calculating the properties. In Figure 3, the area enclosed by the load-deflection curve during one cycle represents hysteretic damping. This is a measure of actual energy dissipated by the connection. The values for this property are determined by integrating the area inside the hysteresis loop using a Simpson's rule algorithm. Connection potential energy is defined as the area enclosed by triangles OMN and OQP and are used to calculate the equivalent viscous damping for the connection with the equation > '

D × 100 2BP

where: > = the equivalent viscous damping ratio in percent D = the hysteretic damping (dissipated energy during one cycle, in-lb) P = the potential energy for the same cycle (in-lb)

(1)


11 A

HYSTERESIS

E

C B POTENTIAL ENERGY CYCLIC SLOPE

D

DISPLACEMENT Figure 3. Typical hysteresis for a nail connection with properties defined.

Figure 4. Typical hysteresis for bolt connection with properties defined.

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The equivalent viscous damping is an indicator of how much energy an equivalent single degree-of-freedom, mass-dashpot system would dissipate, and is a useful property for numerical modeling structural systems or for making comparisons between connections manufactured of different materials. Finally, cyclic stiffness is defined as the slope of the line connecting points Q and M shown in Figure 3. This property indicates how the connection "softens" or degrades during the loading. The definitions of viscous damping and cyclic stiffness are different for bolt connections due to "slop" in the connection associated with holes being 1/16-inch larger than the bolts. For bolted connections, two potential energies were defined. The first includes the effect of oversized holes (areas MNO and OQP shown in Figure 4), and the second excludes the effect of oversized holes (areas LMN and RQP shown in Figure 4). This change results in two calculated values of equivalent viscous damping for bolt connections. Cyclic stiffness was also defined for the two cases, i.e. including the effect of oversized holes (slope of line QM shown in Figure 4) and excluding the effect of oversized holes (the average of the slopes of lines LM and RQ shown in Figure 4). The definition of hysteretic energy was not changed from that used for the nail connections and is equal to the area enclosed by the load-deflection curve for one complete cycle. Changes in the definition for equivalent viscous damping and cyclic stiffness were made to provide information on the bounds of expected cyclic response of connections used in real structures. The 1991 NDS provides for bolt holes to be a minimum of 1/32-inch and a maximum of 1/16-inch oversize in Section 8.1.2.1. Therefore, the two values calculated for each property provide information on the bounds of possible performance of connections manufactured within the allowance of the 1991 NDS provisions. While total exclusion of the effects of oversized bolt holes may not be a realistic assumption since the minimum size for the holes is 1/32-inch oversize, it is a conservative assumption for determining bounds of performance. Excluding the effect of oversized holes results in overestimated values for cyclic stiffness. After the cyclic load regime was completed, each specimen was tested monotonically until either catastrophic failure occurred or a slip of 1.0 inch was reached. This is the same failure definition that was used for monotonic only tests of specimens not subjected to cyclic loading. Two properties were determined from the post-cyclic monotonic tests; capacity and the corresponding displacement. These two properties were then compared to results of the tests where only monotonic loading was used, to determine if the prior cyclic load history affected the performance of the connections.

Nailed Connection Results Average values for properties of interest for the nail connection tests and the corresponding coefficients of variation (COV) are presented in Table 3. Both parallel- and perpendicular-to-grain properties are included in the table to allow for comparisons between performance in the two types of tests. The monotonic results listed in this table are for those specimens tested with and without any prior cyclic testing. Defining capacity with both strength and maximum displacement criteria affected the magnitude of the capacity as well as the displacement at capacity results. If capacity had been defined as only the catastrophic failure load, the average value for the test parameter would have been higher.


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The NDS yield loads, shown toward the bottom of Table 3, were determined by multiplying the nominal design values by the corresponding diameter coefficient, KD, given for Equations 12.3-1 through 12.3-4 in the 1991 NDS. Average yield loads obtained for parallel-to-grain nailed connections not subjected to prior cyclic loading ranged from 29 percent lower to 15 percent higher than the yield loads associated with nominal design values listed in the 1991 NDS. This discrepancy is partially due to the average specific gravities and moisture contents of the tested connection members being slightly higher than the values assumed in the 1991 NDS for southern pine. Friction in the test fixture and different end fixity conditions from those used in the tests on which the NDS values are based could also contribute to the discrepancies. Finally, yield strengths for nails and/or steel plates may have been different than values assumed in the 1991 NDS. The coefficient of variation (COV) in the 5 percent diameter offset bending yield strength of common nails is known to be as high as 12% COV (Loferski and McLain, 1991). Table 3: Averages and coefficients of variation (%) for monotonic tests of nailed connections fabricated with pilot holes.. 2x4/2x4 16d Common Nail Monotonic Parameters Yield Load: Yield Displacement: Initial Stiffness: Capacity: Displacement at Capacity: Ductility: Yield Mode: 1991 NDS Nominal Value: 1991 NDS Yield Load:

Parallelto-Grain 241 lbs. (16.4) 0.021 in. (33.3) 21800 lbs./in. (42.9) 410 lbs. (7) (14.6) 0.617 in. (60.5) 30.1 (66.8)

Perpendicularto-Grain 213 lbs. (8.8) 0.020 in. (14.1) 19200 lbs./in. (24.4) 455 lbs. (0) (9.7) 0.415 in. (23.9) 21.4 (25.0)

15/32" Plywood/2x4 10d Common Nail Parallelto-Grain 160 lbs. (22.4) 0.019 in. (18.9) 15700 lbs./in. (39.7) 369 lbs. (6) (12.2) 0.698 in. (42.6) 38.2 (45.5)


18 Gage steel Plate/2x4 10d Common Nail Parallelto-Grain 305 lbs. (28.0) 0.015 in. (12.7) 44000 lbs./in. (36.8) 527 lbs. (0) (11.5) 0.206 in. (30.8) 14.3 (30.8)


IV

IIIs

IIIs

154 lbs.

101 lbs.

121 lbs.

339 lbs.

222 lbs.

266 lbs.

Note: Numbers in parenthesis to the right of the capacities are the number of specimens in which capacity was controlled by the limiting displacement.

Table 4 shows the monotonic test results for 2x4-to-2x4 connections fabricated with and without pilot holes. As shown, the yield load for driven nail connections is lower and the associated displacement is higher than those connections fabricated with pilot holes. The initial stiffness of driven nail connections is also lower. These three parameters indicate that the pilot holes improve the initial performance of the connections by reducing some of the localized spreading of the wood fibers surrounding the nail, and therefore provide stiffer resistance to lateral deformation. However, the ultimate capacity and ductility of connections with driven nails are higher than for connections with pilot holes. This indicates that the friction provided by the spread wood fibers surrounding the

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nail is higher when the connection is loaded to its ultimate capacity and pilot holes are not used. Table 4: Averages and coefficients of variation (%) for monotonic tests of 16d common nailed connections fabricated with and without pilot holes..

Monotonic Parameters Yield Load: Yield Displacement: Initial Stiffness: Capacity: Displacement at Capacity: Ductility: Yield Mode: 1991 NDS Nominal Value: 1991 NDS Yield Load:

2x4/2x4 16d Common Nail

2x4/2x4 16d Common Nail

Parallel-to-Grain With Pilot Holes 241 lbs. (16.4) 0.021 in. (33.3) 21800 lbs./in. (42.9) 410 lbs. (7) (14.6) 0.617 in. (60.5) 30.1 (66.8)

Parallel-to Grain Without Pilot Holes (Driven) 178 lbs. (39.0) 0.026 in. (44.8) 12174 lbs./in. (48.0) 427 lbs. (0) (13.0) 0.420 in. (45.1) 43.6 (41.7) IV 154 lbs. 339 lbs.


Table 5 shows the capacity and associated displacement for the monotonic tests on specimens with and without prior cyclic loading. Only specimens with the load applied parallel-to-grain were used to investigate the effects of cyclic loading on connection performance. Capacity for all connections was higher for the connections that were loaded cyclicly prior to being tested monotonically to failure. This may have been due to cold working of the nails during the cyclic loading, since there was no difference in average moisture content for the two samples and average specific gravity for the cyclic samples was either equal to or lower than the monotonic sample. Table 5: Capacity, associated displacement, and their associated COV (%) of nail connections with and without prior cyclic loading. 2x4/2x4 15/32" Plywood/2x4 18 Gage Steel Plate/2x4 16d Common Nail 10d Common Nail 10d Common Nail Parameter Not Cycled Cycled Not Cycled Cycled Not Cycled Cycled Capacity: 410 lbs. (7) 439 lbs. (5) 369 lbs. (6) 374 lbs. (6) 527 lbs. (0) 532 lbs. (0) (14.6) (10.9) (12.2) (11.2) (11.5) (9.5) Displacement 0.617 in. 0.506 in. 0.698 in. 0.732 in. 0.206 in. 0.259 in. at Capacity: (60.5) (70.5) (42.6) (35.8) (30.8) (23.6) Note: Numbers in parenthesis to the right of the capacities are the number of specimens in which capacity was controlled by the limiting displacement.


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Table 6 lists averages and coefficients of variation for specific gravity and moisture content of the nailed connections. Average specific gravities of the members used in 2x4-to-2x4 connections and plywood-to-2x4 connections were slightly higher than the 0.55 given in the 1991 NDS, and the average specific gravity of wood used for steel plate-to-2x4 connections was significantly higher. Dimension lumber used in these tests was primarily graded No.1 Dense or Dense Select Structural, which will have a higher than average specific gravity. Moisture content and specific gravity samples were taken from the immediate vicinity of the nail hole for approximately half of the tested specimens, and were determined using ASTM D-2016 and D-2395 standard procedures. Table 6: Averages and coefficients of variation (%) for percent moisture content and specific gravity in nailed connections. 2x4/2x4

Parallel-to-grain

16d nail

Monotonic Stationary

Moving

Perpendicular-to-grain Cyclic

Stationary

Moving


Moving

Member

Member

Combined

Member

Member

Combined

Member

Member

Combined

MC

14.5

15.5

15.0

14.0

15.5

14.8

12.8

13.8

13.3

(w / Pilots)

(9.1)

(3.8)

(7.4)

(9.7)

(7.5)

(9.8)

(7.0)

(6.8)

(7.6)

SG

0.55

0.60

0.58

0.55

0.58

0.56

0.57

0.55

0.56

(w / Pilots)

(9.8)

(10.6)

(10.9)

(9.2)

(10.3)

(10.7)

(13.6)

(10.2)

(11.8)

MC

14.0

14.0

14.0

(w/0 Pilots)

(7.0)

(6.0)

(7.0)

SG

0.56

0.59

0.58

(w/o Pilots)

(9.0)

(8.0)

(9.0)

15/32"

Parallel-to-grain

10d nail

Monotonic

MC

SG

Stationary

Moving

Member

Member

Cyclic

Combined

Stationary

Moving

Member

Member

Combined

Moving

Member

Member

13.4

14.0

(5.1)

(1.3)

(8.1)

0.57

0.53

0.54

(10.4)

(14.2)

(12.4)

Parallel-to-grain

10d nail

Monotonic

SG


13.1

18 ga. steel

MC

Perpendicular-to-grain

Stationary

Moving

Member

Member


Combined

Combined

Stationary

Moving

Member

Member

Monotonic

Combined

Stationary

Moving

Member

Member

11.3

12.1

12.3

(5.9)

(5.0)

(3.1)

0.62

0.61

0.63

(5.0)

(3.9)

(8.8)

Combined

16


Parallel-to-Grain Orientation - Nails Tested Monotonically Average yield displacements for the three types of nailed connections indicate a joint slip at yield ranging from 0.015 inches to 0.021 inches in the parallel-to-grain orientation. The reason for lower yield displacement in the steel plate-to-2x4 connection can be seen graphically in Figure 2. As the slope of the initial linear region of the load-displacement curve becomes steeper, indicating higher stiffness, the corresponding yield displacement must become smaller. Yield displacements, determined by the 5 percent of diameter offset method, were higher for wood-to-wood connections. Connection capacities for the parallel-to-grain specimens without and with prior cyclic loading were higher than 1991 NDS nominal design loads. Corresponding apparent factors-of-safety were determined by dividing the experimental capacities by the 1991 NDS nominal design loads. Resulting apparent factors-of-safety are shown in Table 7, and range from 2.7 to 5.1. Table 7 also shows the apparent factors-of-safety for seismic and wind design. To obtain seismic design apparent factors-ofsafety, seismic design values for the connection were determined by multiplying the nominal design values by the 1991 NDS load duration and diaphragm adjustment factors (CD=1.6 and Cdi=1.1). Connection capacities were then divided by the seismic design value. As shown in Table 7, apparent factors-of-safety for the proposed seismic design range from 1.5 - 2.9 for specimens loaded monotonically, and from 1.6 - 2.5 for cyclicly loaded specimens. These results indicate that factorsof-safety for nailed connections were not affected by the cyclic loading experienced prior to the monotonic loading to failure. Due to the high ductility and energy dissipation caracteristics of nailed connections, a factor-of-safety of 1.5 for nails used to attach sheathing in diaphragms should be sufficient. Average capacities for the 2x4-to-2x4 and plywood-to-2x4 connections would have been higher had the limiting displacement not been reached before catastrophic failure occurred. Capacity was determined by the limiting displacement criteria for more than one-third of the individual woodto-wood connections tested parallel-to-grain. Capacity was determined by catastrophic failure for all of the steel plate-to-2x4 connections; in this case, capacity is a good indicator of ultimate load capacity. Higher levels of variation in displacement at capacity occurred in 2x4-to-2x4 and plywoodto-2x4 connections because many specimens continued to increase in load until the limiting displacement was reached and the test was stopped, rather than a catastrophic failure occurring. Capacity for all steel plate-to-2x4 connections and for the perpendicular-to-grain orientation of woodto-wood connections was determined by ultimate load. The limiting displacement criteria also had an effect on variation in ductility. Connections that were affected by the limiting displacement would have had higher ductilities if the monotonic tests had been load controlled rather than displacement controled. Average ductility of 2x4-to-2x4 and plywood-to-2x4 nailed connections was 29.2 and 27.5, respectively, while steel-to-2x4 connections had a ductility of only 13.7. The difference in ductility is directly associated with the low displacement at capacity for the 18-gauge steel plate-to-2x4 connection. This low displacement at capacity is related to the mode of failure observed. The steel plate showed localized buckling around the nail head at low displacements. This buckling caused the capacity to be reached at lower displacement levels than in the tests of connections with wood or plywood side members. In contrast, connections with wood or plywood side members showed localized crushing of wood material around the fastener which resulted in a gradual failure and larger displacements at capacity being observed.


17

While these two modes of failure are different, calculated ductility would translate into similar response of full structures and therefore the values should directly compared if connection influence on structural performance is of interest. Table 7: Apparent factors-of-safety for nail connections. 2x4/2x4

15/32" Plywood/2x4

18 Gage Steel/2x4

16d Common Nail

10d Common Nail

10d Common Nail

Average Capacity (w / Pilots)

410 lbs.

369 lbs.

527 lbs.

Average Capacity (w/o Pilots)

427 lbs.

1991 NDS Design Value

154 lbs.

101 lbs.

121 lbs.

2.7

3.7

4.4

246 lbs.

162 lbs.

194 lbs.

Resulting Factor of Safety (w / Pilot Holes)

1.7

2.3

2.7

Resulting Factor of Safety (w / o Pilot Holes)

1.7

Parallel-to-grain

Resulting Factor of Safety Wind/Seismic Design Value

Perpendicular-to-grain Average Capacity

455 lbs.

391 lbs.

614 lbs.


154 lbs.

101 lbs.

121 lbs.

3.0

3.9

5.1

246 lbs.

162 lbs.

194 lbs.

1.8

2.4

3.2

Average Capacity

439 lbs.

374 lbs.

532 lbs.


154 lbs.

101 lbs.

121 lbs.

2.9

3.7

4.4

246 lbs.

162 lbs.

194 lbs.

1.8

2.3

2.7

Resulting Factor of Safety Wind/Seismic Design Value Resulting Factor of Safety Post-Cyclic

Resulting Factor of Safety Wind/Seismic Design Value Resulting Factor of Safety

For nailed connections under monotonic loading, both initial stiffness and capacity increased as the strength and local stiffness of the side and/or active member increase. As shown in Table 3,

18


the plywood -to-2x4 connection has a lower initial stiffness and capacity than did the 18-gauge steel plate-to-2x4 connection. Increased stiffness of the steel plate and increased penetration of the nail in the main member are why this connection was stiffer and stronger; this behavior adds credibility to the yield model assumed for connection design in the 1991 NDS. Perpendicular-to-Grain Orientation - Nails Tested Monotonically A second set of matched specimens for each nailed connection type was tested in a perpendicular-to-grain orientation. These results are presented in the second column for each connection type in Table 3. Statistical t-tests ("=0.025, two-tailed test) were used to test a hypothesis that average values for each parameter for parallel- and perpendicular-to-grain orientations were equal. The t-tests indicated no significant differences between parallel- and perpendicular-tograin orientations for yield load, yield displacement, and initial stiffness for the nailed connections tested. One exception was the difference in average yield load in 2x4-to-2x4 connections for the parallel- versus perpendicular-to-grain orientation. The 2x4-to-2x4 connections had a 12 percent lower average yield load in the perpendicular-to-grain orientation, while yield displacement and initial stiffness were not significantly different. This is due to the degree of curvature in the loaddisplacement curve after the initial linear region. The perpendicular-to-grain orientation for this connection exhibited a greater curvature in the non-linear range than did the parallel-to-grain orientation, resulting in a lower yield load while having similar initial stiffness. T-tests on average capacities indicated no significant difference in mean capacity between parallel-to-grain and perpendicular-to-grain orientations for plywood-to-2x4 connections. There was sufficient evidence to reject the hypothesis of equivalency for the 2x4-to-2x4 and steel plate-to-2x4 connections. However, all connections loaded perpendicular-to-grain reached a capacity caused by catastrophic failure, whereas some wood-to-wood parallel-to-grain connections were governed by the limiting displacement criteria. Thus, catastrophic capacities of these specimens loaded parallel-tograin would be higher. The displacement criteria imposed on this test must be considered when evaluating the t-test on the capacity parameter for the two orientations in wood-to-wood connections. Both parallel-to-grain and perpendicular-to-grain orientations of steel plate-to-2x4 connections reached a catastrophic failure-controlled capacity before the limiting displacement was reached, therefore, the t-test for this connection can be considered on its face value and indicates that this connection may be stronger in the perpendicular-to-grain orientation. Since more individual perpendicular-to-grain wood-to-wood specimens reached catastrophic failure-controlled displacement at capacity than did the parallel-to-grain specimens, there was a corresponding decrease in average displacement at capacity for wood-to-wood connections oriented perpendicular-to-grain. However, steel plate-to-2x4 connections had an increase in displacement at capacity for the perpendicular-to-grain orientation. The higher average displacement at capacity for the perpendicular-to-grain orientation of steel plate-to-2x4 nailed connections was due to the way wood crushed around the nail in the main member for the two orientations. In the parallel-to-grain orientation, the nail caused crushing parallel-tograin. This resulted in wood crushing in early wood for a greater distance in the main member. In the perpendicular-to-grain orientation, there was less yielding of wood, due to high density latewood bands, and more nail bending as it was forced to withdraw (or caused the nail head to inbed in the side member). Therefore, it is reasonable to conclude that the perpendicular-to-grain orientation has


19

a higher displacement at capacity due to higher capacity and withdrawal resistance associated with reduced of wood damage along the grain. Post-Cyclic Monotonic Tests of Nailed Connections Monotonic tests were conducted on previously cycled specimens, following the end of the last series of cycles. Results of these post-cyclic tests are presented in Table 5. Displacement at capacity was determined by first adjusting the post-cyclic monotonic load-displacement curves. Loaddisplacement curves for the post-cyclic, monotonic tests were shifted so that the displacement at the maximum load, experienced during the last cycle of a cyclic test, equaled the displacement at the same load for the post-cyclic monotonic load-displacement curves. This was necessary because the LVDT's, used to measure connection slip, had to be repositioned after cyclic testing to allow the full range of motion of the instrument to be used for the monotonic tests. Also, as the cyclic tests were load-controlled, the MTS machine stopped a 0 load at the end of the cycling. Due to the effect of residual load, the displacement at which 0 load is reached at the end of the last cycle was a negative displacement in relation to the displacement at the beginning of the test. The three types of nailed connections seemed to exhibit a slight increase in average capacity after cycling. One contributing factor was the number of specimens in each orientation that reached a limiting displacement controlled capacity. Statistical t-tests were used to test the hypothesis of equivalence between average monotonic capacities of specimens with and without prior cyclic loading. Results of t-tests indicated that there was no significant difference in capacities of any nailed connection type tested. In other words, the prior cyclic loading did not affect the connection capacity. Statistical t-tests also indicated that there was not sufficient evidence to reject the hypothesis of equivalency for 2x4-to-2x4 and plywood-to-2x4 connections, but there was sufficient evidence to reject the hypothesis of equivalency for the average displacements at capacity for steel plate-to-2x4 connections. This would indicate that prior cyclic loading increased the displacement at which catastrophic failure occurred for the steel-to-wood connection. Cyclic Properties of Nailed Connections The initial hysteresis for all cyclic tests was defined as the second cycle beginning as the hysteretic loop crosses the load axis on the tension side of the cycle and ending as the hysteresis returns to the same position. The second cycle was used instead of the first because it provides the first complete hysteretic loop for analysis rather than including part of the virgin loading curve. The final cycle was defined as the last cycle of a set of cycles at a given load level, beginning as the displacement axis is crossed at a negative displacement on the tension side of the cycle. The final hysteresis loop ends at the same negative displacement on the displacement axis as it began. Since all cyclic tests were performed in load control condition, load returned to zero at the end of the test rather than zero displacement. In order to allow a complete hysteresis loop to be analyzed, the final cycle must begin at zero load and a negative displacement and end at the same point. Table 8 presents the results of cyclic tests performed on nailed connections. As shown in the table, the cyclic properties for all three connection types followed similar trends. Increases or decreases in values for the cyclic properties investigated were observed between the initial and final cycles at a given load level. Hysteretic energy dissipation increased between the initial and final

20


cycles at at a given load level for all connection types. The cyclic stiffness decreased for all connection types with increased cycling. Most of this decrease in stiffness ocurred during the first three to four cycles at each load level. Equivalent viscous damping remained fairly constant for all load levels, and ranged from 21.5 - 31.3 percent, with an average of 26.6 percent. Table 8: Average values and coefficients of variation (%) of cyclic properties for nailed connections. 2x4/2x4 16d Common Nail Cyclic Parameters Hysteretic Initial: Energy Final: Stiffness

Initial: Final:

Damping Ratio

Initial: Final:

1.0 NDS 30 Cycles 1.39 in.-lbs. (36.5) 1.61 in.-lbs. (34.1) 37600 lbs./in. (66.9) 32100 lbs./in. (64.1) 28.4 % (23.8) 28.2 % (19.8)


15/32" Plywood/2x4 10d Common Nail 1.0 NDS 30 Cycles 1.40 in.-lbs. (30.3) 1.48 in.-lbs. (29.3) 13400 lbs./in. (40.7) 12000 lbs./in. (41.0) 25.6 % (15.1) 23.8 % (12.2)


18 Gage Steel Plate/2x4 10d Common Nail 1.0 NDS 30 Cycles 0.31 in.-lbs. (112.3) 0.32 in.-lbs. (118.4) 117000 lbs./in. (37.8) 110000 lbs./in. (38.3) 27.6 % (27.1) 26.3 % (33.8)



These trends are not suprising since some damage does occur around the nail during the repeated cycling. Hysteretic damping increased within a given load level from the initial cycle to the final cycle as a result of incremental increases in joint slip. Figure 5 shows a typical load-displacement curve for a nail connection test, and illustrates the increase in joint slip with cycling for the nailed connections. Increases in hysteretic energy dissipation ranged from 3 to 16 percent at 1.0 times design load, and from 11 to 30 percent at 1.75 times design load. The connections tested at 2.0 times design load had a 9 percent increase in hysteretic energy dissipation. At lower load levels, incremental increases in slip occurred for only the first few cycles. As shown in Figure 5, slip increases at a decreasing rate with continued cycling at a given load level, with the greatest increases in slip occurring within the first three cycles. The decreasing rate of slip increase is important as it indicates that the system stabilizes with repeated cycling. Cyclic stiffness decreased from the initial cycle to the final cycle within a given load level for nailed connections due to increases in joint slip with cycling. Decreases in stiffness occurred at a decreasing rate as the number of cycles increased because incremental increases in displacement were smaller with successive cycling. Decreases in cyclic stiffness from the initial cycle to the final cycle ranged from 6 to 15 percent at the 1.0 times 1991 NDS design load level, and from 13 to 24 percent at the 1.75 times design load level. Connections tested at 2.0 times the NDS nominal design load had a 6 percent reduction in cyclic stiffness from the initial to final cycle. The effect of cycling on equivalent viscous damping ratio was small and inconsistent, with some individual specimens having an increase while others decreased. A decline in equivalent viscous damping is due to the hysteretic damping increasing, as would be expected, due to an increased displacement associated with the higher load. However, potential energy also increased due to higher joint slip. A decrease in damping ratio indicates a decrease in the numerator of Equation 1, the hysteretic damping, relative to the denominator (2 B times the available potential energy) of a


21

connection. However, this does not mean that an increase in potential energy of the connection was more than the associated increase in hysteretic damping. In fact, the actual hysteretic damping consistently increased more than potential energy at the higher load levels.

Figure 5. Typical load-displacement time history for a segment of a load-controlled cyclic test of a nail connection.

Bolted Connection Results Average values and COV for the properties of interest obtained from monotonic tests of bolted connections with no prior cyclic loading are presented in Table 9. Yield loads obtained for these sets of specimens were from 2 percent lower to 54 percent higher than yield loads corresponding to 1991 NDS nominal design loads. NDS nominal design loads were converted to their corresponding yield loads by multiplying the nominal values by the constant in the denominator of Equations 8.2-1 through 8.2-4 of the 1991 NDS. Table 10 presents averages and coefficients of variation for moisture content and specific gravity of the bolted connections tested. Moisture content and specific gravity samples were taken from every specimen in the immediate vicinity of the bolt hole. Lower yield loads determined for 4x4-to-4x4 connection can be partially attributed to the specimens having lower specific gravities than the 0.55 assumed for the 1991 NDS. The higher yield loads observed for 2-inch-to-2-inch nominal lumber, and steel plate-to-4-inch nominal lumber bolted connections could be due to the bolts having a higher bending yield strength than assumed for the 1991 NDS. Another factor that may have led to the higher yield value in the tested connections is the yield strength of the steel plate material may have had a higher yield strength than the values assumed by the 1991 NDS. Determination of the yield strength of bolts and steel plates was beyond the scope of this study.

22


Table 9: Average values and coefficients of variation (%) for monotonic properties of bolted connections without prior cyclic load history. 2x4/2x4 (2x8) 3/4" bolt Monotonic Parameters Yield Load: Yield Displacement: Initial Stiffness: Capacity:

1/4" Steel/4x4 (4x6) 1/2" bolt

4x4/4x4 (4x8) 3/4" bolt







1991 NDS Nominal Design Value:

800 lbs.

460 lbs.

780 lbs.

500 lbs.

1690 lbs.

960 lbs.

1991 NDS Yield Load:

2880 lbs

1656 lbs.

2496 lbs

1600 lbs.

5408 lbs.

3072 lbs.

II

II

IV

IIIM

IV

IIIS

Displacement at Capacity: Ductility:

Yield Mode:


Parallel-to-Grain - Bolts Tested Monotonically Average yield displacements for bolted connections ranged from 0.102 inches to 0.144 inches for the parallel-to-grain orientation. Yield displacement is inversely related to the initial stiffness. As initial stiffness of a given connection decreased, the corresponding yield displacement increased. The steel plate used as the active member in one of the connection geometries contributed to that connection type having the highest average initial stiffness due to the higher bearing strength and stiffness of steel plate. The lowest average initial stiffness occurred in the 4x4 to 4x4 connection, with a contributing factor being the lower average specific gravity for those specimens. Apparent factors-of-safety for bolted connections are shown in Table 11. As shown in Table 11, apparent factors-of-safety for nominal design loads in the parallel-to-grain orientation were 6.5 for 2x4-to-2x4 connections, 6.9 for steel plate-to-4x4 connections, and 3.7 for 4x4-to-4x4 connections. When the 1991 NDS design values are multiplied by the load duration factor (CD= 1.6) for wind and seismic loading, the associated apparent factors-of-safety for the tested connections were 4.1, 4.3, and 2.3 for the 2x4-to-2x4, steel plate-to-4x4, and 4x4-to-4x4 connections, respectively. Table 12 shows the apparent factors-of-safety for bolted connections loaded in the perpendicular-to-grain direction. As expected, the perpendicular-to-grain orientation specimens had apparent factors-of-safety that were significantly higher than those calculated for the parallel-to-grain orientation.

Report No. TE-1994-001 Table 10:

23

Averages and coefficients of variation (%) for percent moisture content and specific gravity for bolted connections.

2x4/2x x8)

Parallel-to-grain

¾” Bolt

Monotonic

MC

SG

Cyclic

Moving

Stationary

Moving

Stationary

Moving

Member

Member

Combined

Member

Member

Combined

Member

Member

Combined

12.6

12.3

12.5

13.0

12.5

12.8

10.6

13.3

12.0

(9.8)

(9.8)

(9.8)

(8.1)

(10.5)

(9.2)

(6.9)

(5.9)

(13.1)

0.54

0.59

0.57

0.56

0.61

0.59

0.50

0.58

0.54

(12.5)

(10.0)

(11.6)

(13.4)

(10.1)

(12.2)

(5.0)

(14.0)

(13.3)

Parallel-to-grain

½@ Bolt

Monotonic

SG

Monotonic

Stationary

¼@ Steel/4x

MC


Stationary

Moving

Member

Member


Combined

Stationary

Moving

Member

Member

Monotonic

Combined

Stationary

Moving

Member

Member

10.6

11.0

13.5

(6.2)

(3.2)

(4.8)

0.54

0.54

0.51

(15.9)

(16.5)

(2.9)

4x4/4x

Parallel-to-grain

¾@ Bolt

Monotonic

MC

SG

Stationary

Moving

Member

Member

19.6

15.8

(16.2) 0.48 (8.8)

Combined


Stationary

Moving

Combined

Member

Member

17.7

19.4

15.7

(4.4)

(16.8)

(12.1)

0.52

0.50

0.49

(11.5)

(10.7)

(10.0)


Moving

Combined

Member

Member

Combined

17.5

16.2

20.0

18.1

(4.6)

(14.2)

(1.8)

(17.5)

(17.1)

0.52

0.50

0.48

0.50

0.49

(14.0)

(12.1)

(9.5)

(5.9)

(8.1)

24


Table 11:

Apparent factors-of-safety for bolt connections loaded monotonically in the parallel-tograin direction. 2x4/2x4 (2x8)

1/4" Steel/4x4 (4x6)

4x4/4x4 (4x8)

3/4" Bolt

1/2" Bolt

3/4" Bolt

Average Capacity

5220 lbs.

5360 lbs.

6280 lbs.


800 lbs.

780 lbs.

1690 lbs.

6.5

6.9

3.7

1280 lbs.

1248 lbs.

2704 lbs.

4.1

4.3

2.3

Average Capacity

5938 lbs.

5626 lbs.

6244 lbs.


800 lbs.

780 lbs.

1690 lbs.

7.4

7.2

3.7

1280 lbs.

1248 lbs.

2704 lbs.

4.6

4.5

2.3

Parallel-to-grain

Resulting Factor of Safety Wind/Seismic Design Value Resulting Factor of Safety Post-Cyclic


Table 12:

Apparent factors-of-safety for bolted connections loaded monotonically in the perpendicular-to-grain direction. 2x4/2x4 (2x8)

1/4" Steel/4x4 (4x6)

4x4/4x4 (4x8)

3/4" Bolt

1/2" Bolt

3/4" Bolt

Average Capacity

5140 lbs.

6510 lbs.

5000 lbs.


460 lbs.

500 lbs.

960 lbs.

11.2

13.0

5.2

736 lbs.

800 lbs.

1536 lbs.

7.0

8.1

3.3




25

The limiting displacement criteria, used as part of the definition of failure, affected values for capacity, displacement at capacity, and ductility of bolted connections tested. About one-third of the 2x4-to-2x4 and steel plate-to-2x4 connections tested parallel-to-grain with no prior cyclic loading were considered as failed when the limiting displacement was reached. All of the 4x4-to-4x4 connections were considered to have failed when the limiting displacement was reached. Evidence of the effect of the displacement criteria is shown in Table 9 by the lower variation in capacity for this connection. The limiting displacement criteria had the greatest effect on capacity for the 4x4-to-4x4 connections, and higher capacities would have been seen if this criteria had not been used. Average displacements at capacity were relatively high. The lowest variation in average displacement at capacity was observed in 4x4-to-4x4 connections because all of these specimens were governed by the limiting displacement. Actual displacements at capacity for bolted connections that were governed by the limiting displacement were less than 1.0 inch due to the bolt hole oversize being subtracted. The total stroke of the test machine crosshead was 1.0 inch or greater. High average displacements at capacity for the other two bolted connection types reflect the observation that catastrophic failure occurred in the form of splitting, at relatively high displacements. Bolted connection end distances were between 1/2" and 3/4" greater than the minimum end distances required for use of the full NDS design value. Even with this requirement being adhered to, catastrophic failure occurred in the form of splitting of the wood members in the 2x4-to-2x4 and steel plate-to-2x4 connections. Greater end distances in the tested connections would have reduced the occurrence of splitting of the member, and thereby resulted in even higher capacities and associated displacements. Ductility, defined as the ratio of displacement at capacity to displacement at yield, was lower for bolted connections than nailed connections due to the higher average yield displacements for bolted connections. This property was also affected by the limiting displacement criteria and the presence of the oversize bolt holes. Perpendicular-to-Grain - Bolts Tested Monotonically A sample for each bolted connection type was tested with the stationary member oriented so that the load acted perpendicular-to-grain. Results of these tests are presented in Table 9. Lower yield loads for the perpendicular-to-grain orientations are due to the effect of grain orientation on larger diameter dowel connections, such as bolts. Connections tested perpendicular-to-grain had average yield loads between 18 percent lower and 54 percent higher than the corresponding 1991 NDS yield loads. As in the parallel-to-grain orientation, yield loads observed in these tests were not identical to the values used in the 1991 NDS due to possible differences in bolt bending yield strengths and differences in specific gravity. Higher bolt bending yield strengths may explain the higher observed yield loads even with lower average specific gravities for the tested connections. Steel plate-to-4x6 connections had an average main member specific gravity of 0.51 (the 1/4" steel plate was the side member), yet the tested average yield load was 50 percent higher than the corresponding NDS value. The 2x4-to-2x8 connections were yield mode II connections and therefore bolt bending yield strength would not affect the results if they were higher than the assumed value. Higher values determined from these tests may also be due to the somewhat subjective procedure used for determining the initial stiffness line on load-displacement curves. Determination of the initial stiffness line directly affects yield load and displacement.

26


Average yield displacements for perpendicular-to-grain bolted connections were higher than for parallel-to-grain orientations due to lower initial stiffness seen in the perpendicular-to-grain orientation. Lower initial stiffness was due to the effect of grain direction on the initial response of larger diameter dowels such as bolts. In the perpendicular-to-grain orientation, the bolt is primarily compressing wood fibers in their weakest direction. Compression parallel-to-grain is a stronger wood mechanical property than compression perpendicular-to-grain, therefore dowel bearing strength of wood for bolts is higher in the parallel-to-grain orientation. As shown in Table 11, average capacities of bolted connections loaded perpendicular-to-grain resulted in apparent factors-of-safety of 11.2, 13.0, and 5.2 for the 2x4-to-2x8, steel plate-to-4x6, and 4x4-to-4x8 connections, respectively. If NDS nominal design loads are multiplied by the wind and seismic load duration factor, CD= 1.6, the apparent factors-of-safety were 7.0 for 2x4-to-2x8 connections, 8.1 for 1/4" steel plate-to-4x6 connections, and 3.3 for 4x4-to-4x8 connections. Capacities of specimens loaded perpendicular-to-grain were governed by the limiting displacement criteria for all specimens in each of the three connection geometries. This is reflected by the low variation for this parameter seen in Table 9. Statistical t-tests indicated a significantly higher average capacity for steel plate-to-4x6 connections (perpendicular-to-grain) versus steel plateto-4x4 connections (parallel-to-grain). The 4x4-to-4x8 connection had an average capacity that was significantly lower in the perpendicular-to-grain orientation than parallel-to-grain. A factor that may have affected this was low specific gravity of 4x8 stationary members. Statistical hypothesis testing indicated no significant difference in average capacity for 2x4-to-2x8 connections loaded perpendicular-to-grain versus their parallel-to-grain counterparts. Average displacement at capacity for perpendicular-to-grain orientation was higher than average displacement at capacity for parallel-to-grain orientations for 2x4-to-2x8 and steel plate-to4x6 connections due to the fact that all of perpendicular-to-grain specimens had capacity determined by the limiting displacement, while only about one-third of parallel-to-grain specimens did. Coefficients of variation for the displacements at capacity were also significantly lower than the values for parallel-to-grain orientations for all but 4-by-to-4-by connections. Both orientations of 4-by-to-4by connections experienced a capacity determined by the limiting displacement, as shown by the similarity of the values shown in Table 9. Ductility values were lower for the perpendicular-to-grain orientations than for the parallel-tograin orientations since they were more affected by the increase in yield displacement than by the increase in displacement at capacity. Yield displacement properties are governed by the initial stiffness, and since bolts connections have lower stiffness in the perpendicular-to-grain direction, yield displacement is significantly higher in this orientation than when the load is parallel-to-grain. Post Cyclic - Bolts Tested Monotonically Tables 11 and 13 show results from the post-cyclic monotonic test conducted on bolt connections. Table 11 shows the apparent factors-of-safety for bolted connections loaded monotonically after being loaded cyclicly to three magnitudes of load, 1.0, 1.6, and 2.0 times the 1991 NDS nominal design value. Apparent factors-of-safety associated with the post-cyclic specimens were equal to or higher than those calculated for parallel-to-grain specimens with no prior loading, indicating that prior cyclic load history had no effects on capacity of the connections.

Report No. TE-1994-001 Table 13:

27

Average capacity, associated displacement, and COV (%) for bolted connections loaded monotonically in the parallel-to-grain direction, with and without prior cyclic loading.

2x4/2x4 1/4" Steel/4x4 4x4/4x4 3/4" bolt 1/2" bolt 3/4" bolt Parameter Not Cycled Cycled Not Cycled Cycled Not Cycled Cycled Capacity: 5220 lbs. (3) 5938 lbs. (1) 5360 lbs. (4) 5626 lbs. (6) 6280 lbs. (10) 6244 lbs. (10) (18.4) (8.6) (16.6) (14.0) (9.3) (7.9) Displacement 0.735 in. 0.828 in. 0.726 in. 0.826 in. 0.893 in. 0.936 in. at Capacity: (20.9) (12.1) (29.2) (19.7) (4.3) (3.2) Note: Numbers in parenthesis to the right of the capacites are the number of specimens in which capacity was controlled by the limiting displacement.

Table 13 Shows the capacity and associated displacement for monotonic tests on specimens with and without prior cyclic loading. Post-cyclic test specimens had been subjected to 30 cycles at 1.0 times the 1991 NDS nominal design load, 15 cycles at 1.6 times the nominal design load, and 8 cycles at 2.0 times the nominal design load. T-tests conducted on average capacities of post-cyclic specimens indicated no significant difference between average capacities of those specimens that had or had not been subjected to prior loading. The number of specimens that reached a limiting displacement versus a catastrophic failure also affected average capacities and corresponding displacements. Limiting displacements for bolted connections were less than 1.0 inch because slip due to bolt hole oversize was subtracted from the overall displacement, which was greater than 1.0 inch. Fewer 2x4-to-2x4 connection specimens reached the limiting displacement during monotonic testing after cycling. However, more of steel plate-to-4x4 connections loaded cyclicly reached the limiting displacement than specimens not loaded prior to monotonic testing. The 4x4-to-4x4 bolted connection capacities for all specimens were determined by the limiting displacement criteria. Displacements at capacity for the post-cyclic tests were determined by first adjusting the displacements of post-cyclic monotonic load-displacement curves. The curves were shifted along the displacement axis so that the point of maximum load obtained during the last cycle of the cyclic tests coinsided with the monotonic curve. This allowed the virgin loading curve to be accurately determined in the post cyclic tests. An initial stiffness line was fit tangent to the initial cycle at the 1.0 times the nominal design load level. Where the initial stiffness line intersected the displacement axis was the reference point, or zero displacement location, for determining the displacement at capacity values. T-tests on displacement at capacity for 2x4-to-2x4 and steel-to-2x4 connection types indicated no significant difference between specimens with and without prior cyclic loading. The 4x4to-4x4 connection type did indicate a statistical difference in the displacement property. However, this property was affected by themaximum displacement criteria as none of the 4x4-to-4x4 connections experienced a catastrophic failure, and the difference in the average values is small and has no practical significance. Cyclic Properties of Bolted Connections Average values and the coefficients of variation for cyclic properties of bolted connections are shown in Table 14. As defined in the Property Definitions section, cyclic stiffness and equivalent viscous damping were calculated for two conditions, including and excluding the effects of the oversized bolt hole. These two values can be considered as bounds on the expected performance of

Table 14:

Averages and coefficients of variation (%) for bolted connections subjected to load-controlled cyclic loading. 2x4/2x4 3/4" bolt

Cyclic Parameters Hysteretic Energy

Initial Final:

Stiffness Initial (Based on inclusion I of oversize hole) Final: Stiffness (Based on exclusion of oversize hole)

Initial

Damping Ratio (Based on inclusion of oversize hole) Damping Ratio (Based on exclusion of oversize hole)

Initial

Final:

Final: Initial Final:

1.0 NDS 30 Cycles 32.3 in.-lbs. (8.3) 28.3 in.-lbs. (8.9) 8900 lbs./in. (5.2) 8400 lbs./in. (6.4) 27400 lbs./in. (12.2) 23000 lbs./in. (14.1) 6.7 % (8.1) 5.6 % (9.4) 20.8 % (11.4) 15.3 % (14.7)


1/4" Steel/4x4 1/2" bolt 2.0 NDS 8 Cycles 110.1 in.-lbs. (12.2) 107.8 in.-lbs. (13.6) 13000 lbs./in. (5.9) 12500 lbs./in. (6.5) 26300 lbs./in. (12.0) 24300 lbs./in. (12.5) 8.8 % (14.7) 8.2 % (17.2) 17.8 % (19.0) 16.0 % (22.0)



4x4/4x4 3/4" bolt 2.0 NDS 8 Cycles 85.8 in.-lbs. (12.1) 83.8 in.-lbs. (12.0) 13400 lbs./in. (9.7) 13000 lbs./in. (10.3) 29300 lbs./in. (23.5) 27600 lbs./in. (24.0) 7.4 % (10.2) 7.0 % (11.1) 16.1 % (16.9) 14.7 % (18.5)





29

connections constructed according to the tolerances in the 1991 NDS and 1989 Manual of Steel Construction. Hysteretic damping values are not affected by the two methods of calculating the properties due to the amount of energy dissipated being a function of actual construction and simply placing constraints on calculations after the tests are complete does not change actual performance. These calculations are simply provided as estimates of the range of possible values for the properties. Unlike nailed connections a consistant decrease in hysteretic energy dissipation was observed with cycling of bolted connections between initial and final cycles at each load level. With successive cycling, the displacement at which loading began, increased at a declining rate. An example of this is shown in Figure 6. In effect, there was a "pinching" or narrowing of the bolted connection hysteresis with successive cycling, thereby reducing the area inside. The percent decrease was greater at the lower load levels.

Figure 6: Typical load-displacement time history for a load-controlled cyclic test of a bolt connection. Cyclic stiffness, as defined in the Property Definitions section, decreased from the initial to final cycles due to the incremental increase in displacements at maximum and minimum load for each cycle. As shown in Figure 6, this increase in displacement occurred at a decreasing rate. Therefore, cyclic stiffness of bolted connections, like that of nailed connections, decreased at a decreasing rate. The percent decrease between initial and final cycles remained fairly constant for a given load level, regardless of whether the effects of over-sized bolt holes were included or not. If the values of cyclic stiffness are compared for when the effects of the over-sized bolt holes are included and when they are excluded, it becomes evident that the over-sized holes reduce the effective stiffness of the connections. Connections with tightly fitting bolt holes will be significantly stiffer than connection with loose fitting bolts.

30


Equivalent viscous damping decreased from the initial to final cycles at all load levels for all bolted connections. This was due to the decrease in hysteretic damping with cycling due to narrowing of the hysteresis, and an increase in potential energy associated with increases in displacement at maximum and minimum load with successive cycling. Using Equation 1, these two factors combined to result in a lower equivalent viscous damping ratio for the final cycle. In the nailed connections, the increase in hysteretic damping was always greater than the increase in potential energy. The opposite was true for the bolted connections, due to the oversize bolt hole. The decrease in equivalent viscous damping ratio was less at higher load levels because the change in hysteretic energy with cycling was less at the higher load levels. These changes are insignificant from a practical point when the effects of oversized bolt holes are included, but become significant when the effects of the oversized bolt holes are excluded. This indicates that while tight fitting bolts will increase the effective damping of the connections, the magnitude will decrease significantly as the connection is cycled. However, the lowest average value of equivalent viscous damping calculated for connections excluding the effects of the oversized bolt holes was 12.3%. This amount of damping is still significant, and provides an excellent source of structural damping for wood buildings.

CONCLUSIONS Results of monotonic and load-controlled cyclic tests of nail and bolt connections have been presented. Results show that there is no significant effect of prior cyclic loading on capacity or ductility of nail and bolt connections when prior cyclic load levels as high as 2.0 times the 1991 NDS nominal design load. This indicates that design seismic events will not significantly lower the connection’s ability to resist the loads. Nail connection capacities showed no statistical change due to prior cyclic loading. The capacities of nail connections without prior cyclic loading resulted in factors-of-safety of 2.7 - 5.1 for normal duration loads and 1.7 - 3.2 for seismic and wind loads. Connections that were loaded cyclicly prior to being tested monotonically had factors-of-safety of 2.9 - 4.4 for normal duration loads and 1.8 - 2.7 for seismic and wind loads. High ductilities associated with nail connections and their ability to dissipate large amounts of energy indicate that a factor-of-safety of 1.7 is sufficient to guarantee acceptable performance. With the exception of light-gauge steel-to-wood connections, prior cyclic loading did not affect the value of displacement at capacity. Light-gauge steel-to-wood connections experienced a slight increase in displacement at capacity due to local yielding of the steel plate, but the magnitude was not of practical significance. Ductility of the connections loaded monotonically with no prior loading ranged from 14.3 for light-gauge steel-to-wood connections to 38.2 for plywood-to-wood connections. These results indicate that ductility of nailed connections is not adversely affected by prior cyclic loading, and ductility is slightly improved for light-gauge steel-to-wood connections. Pilot holes increased the initial stiffness and yield load of nail connections when compared to connections fabricated without pilot holes. However, the nail connections without pilot holes had higher average ultimate capacity and ductility that nail connections with pilot holes. These effects can be attributed to the reduced localized spreading of wood fibers surrounding the nail.


31

Test results indicate that prior cyclic loading, as high as 2.0 times the 1991 NDS nominal design values had no adverse effect on capacities of bolted connections for yield modes II, IIIS, IIIM, and IV. Capacities for monotonic tests with no prior loading resulted in factors-of-safety of 3.7 - 6.9 for parallel-to-grain orientations and 5.2 - 13.0 for perpendicular-to-grain orientations when compared to the 1991 NDS nominal design values for normal duration loads. The same test results provide factors-of-safety for seismic and wind loads of 2.3 - 4.1 for the parallel-to-grain orientations and 3.3 - 8.1 for the perpendicular-to-grain orientations. Post-cyclic test capacities resulted in factors-of-safety of 3.7 - 7.4 for normal duration loads and 2.3 - 4.6 for seismic and wind loads. Post-cyclic connections were oriented in the parallel-to-grain direction. These factors-of-safety associated with bolt capacities are sufficient to provide acceptable performance of wood connections. Ductilities of bolted connections ranged from 5.4 to 7.5. Comparisons of monotonic capacity for the connections tested with and without prior cyclic loading indicated that prior load history did not adversely effect ductility of the connections. The only connection that showed a statistical difference in the displacement property was the 4x4-to-4x4 connection. Results showed that displacement at capacity increased slightly, and would indicate that ductility of the connection increased slightly due to prior cyclic loading. Several additional properties were presented to completely describe the monotonic and cyclic performance of nail and bolt connections. Results presented quantify several cyclic properties such as hysteretic damping, cyclic stiffness, and equivalent viscous damping. Values of these cyclic properties illustrate the ability of wood connections to dissipate significant quantities of energy during cyclic or dynamic loadings expected during natural hazard events such as earthquakes. The ability to dissipate energy improves a structure’s performance and reliability.

References Manual of Steel Construction., 1989. American Institute of Steel Construction, Inc. (AISC), New York, NY. American Society for Testing and Materials (ASTM), 1989. "ASTM A-36/A-36m-88c, Standard specification for structural steel." Annual Book of ASTM Standards, Vol 01.06, pp. 106-108. ASTM, Philadelphia, PA. American Society for Testing and Materials (ASTM), 1992. "ASTM A-446/A-446M-89, Standard specification for steel sheet, zinc-coated (galvanized) by the hot-dip process, structural (physical) quality." Annual Book of ASTM Standards, Vol 01.09, pp. 37-78. ASTM, Philadelphia, PA. American Society for Testing and Materials (ASTM), 1990. "ASTM D-143-83, Methods of Testing Small Clear Specimens of Timber." Annual Book of ASTM Standards, Vol. 04.09. ASTM, Philadelphia, PA. Dolan, J.D. and S.T. Gutshall, 1994. Detailed Data for Monotonic and Load-Controlled Cyclic Tests of Nail and Bolt Connections. Virginia Polytechnic Institute and State University, Timber Engineering Report No. TE-1994-002.

32


Dolan, J.D., S.T. Gutshall, and T.E. McLain, 1994. Sequential Phased Displacement Tests of Nail and Bolt Connections. Virginia Polytechnic Institute and State University, Timber Engineering Report No. TE-1994-003. Dolan, J.D. and S.T. Gutshall, 1994. Detailed Data for Sequential Phased Displacement Tests of Nail and Bolt Connections. Virginia Polytechnic Institute and State University, Timber Engineering Report No. TE-1994-004. Loferski, J.R. and T.E. McLain, 1991. "Static and Impact Flexural Properties of Common Wire Nails." ASTM Journal of Testing and Evaluation, 19(4):297-304. National Design Specification for Wood Construction, 1991. National Forest Products Association., Washington, D.C.

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