COMPARISON OF STATIC AND DYNAMIC ...

COMPARISON OF STATIC AND DYNAMIC RESPONSE OF TIMBER SHEAR WALLS By David W. Dinehare and Harry W. Shenton m z AB.STRACT: Static and. dynamic tests were conducted on wood frame shear walls to (1) detennine the wall resIstance to lateral loadm.g; (2) examin~ the wall performance under fully reversed cycles of dynamic loading; and (3) compare the statIc and dynamIc performance as measured using the same test facility. Tests were condu~ted on stand~d 2.44 by 2.44 m (8 by 8 ft) walls, constructed of plywood or oriented strand board sheathmg. ~our specIm~ns were t~sted s~tically u.sing the traditional ASTM E-564 test procedure: three halfcycles, loadmg monotomcally to fatlure. EIght specImens were tested dynamically, using a proposed test standard that wa~ recently dev.eloped by the Structural Engineers Association of Southern California. A comparison of the sta~c and dyna~c test results demonstrates several key differences. The ultimate loads measured in the dynamtc tests arc: shgh~ly less than those measured in the static tests; however, the ductility of the wall, when ~easured .dyna~cally, IS be~ween 34 and 4~% less than the corresponding static ductility. Furthermore, damage m ~he statIC t~s~s IS charactenzed by the pulhng away of the sheathing from the frame, extraction of the sheathing natls, and spl~ttmg ~f the ~ttom plate. Damage in the dynamic tests is generally confined to pullout and fatiguing o~ the sheathmg natls, which eventually break off. The dynamic tests show a reduction in load and stiffness and pmched hysteresis with continued cycli~g at a given displacement. The results of this study suggest that the act,:,al load factors for a shear wall subjected to an earthquake will be significantly lower than the intended deSIgn.

INTRODUCTION In most wood frame structures constructed today, timber shear walls resist lateral loads due to wind and earthquakes. Wood frame shear walls can be found in homes, schools, apartments, low-rise commercial buildings, and other light industrial buildings. These buildings constitute a major portion of the U.S. Building stock, and therefore, it is important that engineers understand how these structures and their primary lateral load resisting system perform under high winds or earthquakes. Wood frame structures have historically performed well under strong earthquakes, i.e., experience shows that properly built wood frame structures can withstand major earthquakes without collapsing. In that sense, timber structures have satisfied the spirit of the modern seismic building code. Costly nonstructural and secondary damage, however, is still a problem, and adds significantly to the total economic loss from an earthquake. Wood frame structures have historically not performed well under strong winds. The vulnerability of poorly constructed wood structures in high winds was again dramatically demonstrated by hurricanes Hugo and Andrew (Assessment 1993), which hit the east coast of the United States in 1989 and 1992, respectively. Maintaining the structural integrity of vertical shear walls and horizontal diaphragms is extremely important for adequate wind resistance; experience shows that once a shear wall or diaphragm loses a sheathing panel, the integrity of the entire structure may be compromised. Historically, the lateral load capacity of wood frame shear walls has been established using standards ASTM E-72 ("Standard" 1995) and ASTM E-564 ("Standard" 1994). The strength of the sheathing material is evaluated using ASTM E72 and a standard wood frame, while the racking strength of IRes. Asst., Dept. of Civ. and Envir. Engrg., Univ. of Delaware, Newark, DE 19716. 2 Asst. Prof., Dept. of Civ. and Envir. Engrg., Univ. of Delaware, Newark, DE. Note. Associate Editor: Dan L. Wheat. Discussion open until November I, 1998. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on March 14, 1997. This paper is part of the Journal of Structural Engineering, Vol. 124, No.6, June 1998. ©ASCE, ISSN 0733-9445/98/0006-0686-0695/ $8.00 + $.50 per page. Paper No. 15452.

the entire wall, which includes the frame, sheathing, fasteners, and any anchors or hold-down devices, is evaluated using ASTM E-564. The loading procedure is essentially the same in both standards: the wall is laterally loaded at the top with a static, monotonically increasing load. The specimen is subject to three (ASTM E-564) or four (ASTM E-72) half-cycles of loading. The target loads within each cycle are specified in ASTM E-72, and are a function of the estimated ultimate load in ASTM E-564. These test procedures were developed many years ago and numerous tests and studies have been conducted since, to determine the racking strength of wood frame walls (Tissell 1993). The effects of sheathing type, thickness and orientation; stud spacing, material and size; and fastener type and density have all been established using the static test procedure. Comprehensive bibliographies of work in this area can be found in Carney (1975) and Peterson (1983). Foliente and Zacher (1994) provided an excellent review of historical and more recent work. It should be noted that the code [e.g., Uniform (1994)] allowable shear for a wood frame shear wall, whether for wind or seismic design, was determined based on tests conducted in accordance with the ASTM static test procedures. In recent years there has been a move toward testing wood frame shear walls dynamically, using full reversed load cycles. This type of test is believed to be more representative of the load/deformation history that the wall will experience during an earthquake, than the static, monotonic procedures outlined in ASTM E-72 or E-564. This kind of test is more akin to tests that have been used for years to evaluate the seismic performance of steel, concrete, and masonry structures and subcomponents. Having recognized the need for a new test standard, an ad hoc committee of the Structural Engineers Association of Southern California (SEAOSC) recently developed a "draft" standard for testing wood frame shear walls (Shepherd 1996; "Standard" 1996). The test procedure, modeled after a standard developed by the Technical Coordinating Committee on Masonry Research (TCCMAR), is conducted under stroke (displacement) control and uses fully reversed cycles of increasing amplitude, conducted at a frequency of between 0.2 and 1 Hz. The test sequence is referred to as a Sequential Phased Displacement (SPD) test procedure. A few studies have been conducted in which walls have been tested under fully reversed cycles, but even fewer have been tested at realistic dynamic rates. Hanson (1990) tested

686/ JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1998

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test procedures. Presented in this paper are the results of an experimental investigation, the objective of which was to evaluate the strength of typical wood frame shear walls, using both the traditional static (ASTM) monotonic test procedure and the newer, dynamic (SEAOSC), fully reversed procedure. The performance of the wall, both in terms of load and displacement capacity, and the mode of failure were studied and compared. A total of 12 wood frame shear walls were tested, six of which were sheathed with plywood and six with OSB. For each type of sheathing, two specimens were tested statically and four were tested dynamically. In the following is presented a description of the test specimens, the test facility and procedure, the results and discussion of results, and conclusions.

four 1.22 by 1.22 m (4 by 4 ft) walls, and two 2.44 by 2.44 m (8 by 8 ft) walls, sheathed in plywood or oriented strand board (OSB). The specimens were tested under fully reversed cycles of increasing amplitude; however, the rates of loading were effectively static (particularly for the large specimens): 0.1 and 0.05 Hz, for the 1.22 mm (4 ft) and 2.44 m (8 ft) walls, respectively. Stewart et al. (1988) tested 11 walls, which included four under reversed cyclic quasi-static loading, and four walls on a shake table with a sinusoidal motion of 1.5 Hz. Dolan (1989) conducted a variety of different tests on 2.44 by 2.44 m (8 by 8 ft) panels sheathed in plywood and waferboard. The test specimens he described as being cyclic were tested under fully reversed cycles; however, the rates of loading were effectively static. Dolan also conducted tests on a shake table using recorded ground motions. Schmid et al. (1994) tested three 1.22 by 2.44 m (4 by 8 ft) walls under fully reversed cycles at increasing amplitudes of displacement (the rate of loading was not reported). Shepherd and Allred (1995) tested a total of five high-aspect-ratio walls of dimensions 0.71 by 2.44 m (2.33 by 8 ft), two statically and three dynamically, using fully reversed cycles at a frequency of 2 cyc/s. Karacabeyli and Ceccotti (1996) tested 4.88 by 2.44 m (16 by 8 ft) walls under a ramp load, and a cyclic displacement of increasing amplitude at a frequency of 0.5 Hz. Walls were sheathed with either plywood or OSB and gypsum. Johnson and Dolan (1996) tested ten 12.19 by 2.44 m (40 by 8 ft) walls to investigate the effect of openings on the wall performance. Five configurations of walls were tested, with one static and one dynamic test conducted on each configuration. The dynamic tests used the SPD test procedure at a rate of 0.5 Hz. Skaggs and Rose (1996) discuss the results of dynamic tests of eight 2.44 by 2.44 m (8 by 8 ft) shear walls. Walls were tested under fully reversed cycles in accordance with the SEAOSC procedure; however, no matching monotonic tests were conducted. To date, no studies have been conducted to evaluate and compare the static and dynamic performance of the "standard" 8 by 8 ft wall, using the same test facility, replicate specimens, and a standard dynamic test procedure, to determine the dynamic response of a standard specimen, and to investigate how the responses of the walls compare in the two

TEST SPECIMEN

Specimens were built in accordance with standard construction practice; a detailed diagram of the specimen is shown in Fig. 1. The 2.44 by 2.44 m (8 by 8 ft) walls were constructed of No.2 Spruce-Pine-Fir (SPF) 2 by 4 studs, 38 by 89 mm (1.5 by 3.5 in.) net. The walls consisted of double end studs, a double top plate, a single bottom plate, and interior studs spaced at 406 mm (16 in.) on center. The top and bottom plates were end-nailed to the studs using two 16d common nails; the double end studs were nailed together using 16d nails at 305 mm (12 in.) on center from both sides. American Plywood Association (APA) rated plywood, 11.9 mm (15/32 in.) thick, or 12.7 mm (1/2 in.) OSB were used as the sheathing materials. The 1.22 by 2.44 m (4 by 8 ft) sheets of sheathing were oriented vertically and fastened to the frame using 8d nails. The edge nailing density was 102 mm (4 in.) on center and the field nailing was 305 mm (12 in.) on center. The same construction was used for both the static and dynamic specimens. According to UBC Table 23-I-K-l (Uniform 1994), the allowable shear force for both the plywood and OSH walls is 5.54 kN/m (380 lb/ft). Thus, the design allowable load for the walls is 13.53 kN (3,040 lb). TEST SETUP

The test frame with a specimen installed is shown schematically in Fig. 2. The test fixture is a self-reacting braced

Double IDp plate nailing: 16d @ 406mm on center. staggered 38mm x 89nun

'"

X

2044m (2 typical)

Bottom plate to stud nailing: 2 - 16d end nail 1.2m x 204m, 11.9mm plywood

7

or 12.7rnm OSB sheathing

v-:

38mm x 89mm X 2. 32m - 9 total

Sheathing to Stud Nailing:

..-

~

Edges: 8d @ I02mm on cent... Field: 8d @ 30Srnm on center Min. edge distance: 9.Smm

Doubleend stud nailing: 16d@ 30Smm on center, both sides slaggered

3 8rnm x 89mm x 2.44m Bottom plate to stud nailing: 2 • 16d end nail (typical)

FIG. 1.

Sheaf Wall Specimen JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1998/687


A Uullle

I~

B""", L I

~

~

r

-

".

r

..

L- ..- ,

Lateral Guides

89 kN Actuator

ecimeu

r

L

j

Section A-A

-IBottom

support channel

A

FIG. 2.

Test Frame with Specimen Installed

frame constructed of steel channels, angles, and wide flange sections bolted or welded together. The fixture consists of two lateral load resisting frames, spaced 762 mm (30 in.) apart, that are connected together by channels at the top and bottom. When installed, the specimen is positioned between the two load resisting frames and is anchored to the bottom support channel of the test frame. The lateral load is applied by an 89 kN (20 kip) hydraulic actuator, through a guide beam that is bolted to the top of the specimen. The wall anchorage at the bottom was designed to take the shear and moment reactions, and to allow the sheathing to rotate during testing. A 25 by 76 mm (1 by 3 in.) steel shim plate was placed between the support channel and specimen so that the sheathing could rotate as the specimen was loaded. The specimen was anchored to the bottom support channel using 19 mm (3/4 in.) diameter, A307 bolts that run through the steel shim and specimen bottom plate. The bolts are evenly spaced, with two bolts between each set of interior studs, for a total of 12 bolts. Instead of washers, a 254 by 76 by 13 mm (10 by 3 by 112 in.) steel plate with two 22 mm (7/8 in.) holes was placed over the bolts and on top of the specimen bottom plate. Although this shear anchorage is far more than is required by the code, or normally used in practice, the detail was adopted to ensure that there would be no relative movement at the base of the specimen during testing (previous studies that have used the minimum required shear anchorage have found only limited movement at the base, with negligible effect on the overall response of the wall (Schmid et al. 1994). Commercially available hold-down anchors were used at each end of the specimen to resist uplift forces. The anchors were attached to the double end studs using two 19 mm (3/4 in.) bolts. The anchor was positioned so that the bottom of the hold-down was approximately 25 mm (1 in.) above the bottom plate. The holes in the support channel and bottom plate, through which the anchor bolt for the hold-down was placed, were oversized so that the bolts were free to rotate as the wall deformed. A 2.74 m (9 ft) W16 X 67 wide flange beam placed on top of the specimen served two primary functions during the tests: (1) To distribute the lateral load evenly along the top of the specimen; (2) to prevent any transverse movement of the specimen during testing. Referring to Fig. 2, the beam was turned so that the web was horizontal and rested on top of the specimen. Again. a steel shim was placed between the specimen and beam to allow the sheathing to rotate. The beam was bolted through the steel shim and double top plate of the spec-

imen, using the same type of bolt and plate system that was used to anchor the bottom of the specimen. The hydraulic actuator was connected to one end of the W beam and reacted off of a crossbeam between the two load resisting frames. Four roller bearings, two at each end, were placed between the flanges of the W beam and the lateral load resisting frames to prevent any transverse movement of the specimen during testing. The vertical load on the specimen due to the wide flange beam and steel shim was approximately 1095 N/m (75 Ib/ft). During the dynamic tests the actuator, which has a stroke of ±76 mm (±3.0 in.), was placed at the center of its stroke and the guide beam was connected. This provided a working stroke of ±76 mm (±3.0 in.) during the fully reversed cyclic tests. During the static tests, the actuator was fully retracted before the guide beam was connected, thus providing a working stroke of 0 to 152 mm (6.0 in.). A PC-based data acquisition and control system provided simultaneous data acquisition and control for the test. The actuator load or displacement test sequences (described in the next section) were generated digitally and output through a digital-to-analog converter to the hydraulic controller. At the same time, data from several transducers placed on the specimen was recorded. Up to eight channels of data were recorded, this included load (from the actuator load cell), lateral displacement at the top of the wall via the actuator LVDT, lateral displacement at the top of the wall via an independent cable potentiometer at the opposite end of the specimen, uplift displacement at each corner, and other displacement measurements on the specimen that varied for the different tests. TEST PROCEDURE

The shear walls were tested statically in accordance with ASTM E-564, with the exception that higher test loads were used. The higher loads were necessary in order to exceed the design allowable load of the wall before the third half cycle. The use of higher test loads is consistent with the test procedure used by the APA (Tissel 1993). The load history used for the static tests is shown in Fig. 3(a). The specimen was loaded at a rate of 89 N/s (20 IbIs) to a peak load approximately equal to the design load of the wall, 13.35 kN (3,000 Ib). The specimen was unloaded to zero load at the same rate and then reloaded to approximately two times the design load 26.7 kN (6,000 Ib). Following a second unloading to zero, the specimen was loaded to failure, or until the limits of the test equipment were reached. Two plywood and two OSB walls were tested



so 40

30

20

10

o+--.--~~..---r---r-..>I----,-----,----,----''--I

o

400

200

600

800

1000

1200

1400

1600

Time (sec)

a. Static Test Load History 75

50

25

o -25

-50

-75

o

20

60

40

80

Time (sec)

b. Dynamic Test Displacement History FIG. 3.

Static and Dynamic Test Protocol

statically. Load and displacement readings were taken at a rate of 10 samples per second during the static tests. Note that the static tests were conducted with the actuator under load control. The shear walls were tested dynamically using the sequential phased displacement (SPD) procedure recently proposed by the ad hoc committee on testing of SEAOSC (Shepherd 1996). The SPD used for the dynamic testing, shown in Fig. 3(b), consists of 72 cycles of varying amplitudes of displacement. All amplitudes in the test procedure are based on what is referred to as the "first major event" (FME) amplitude. The FME could be a yield displacement, drift limit, or some other limit state; for these tests the FME was defined to be 9 mm (0.75 in.), which corresponds to a drift of 0.75%. This choice of FME also utilized the full working stroke of the actuator, ±76 mm (±3.0 in.) at the end of the test sequence. Referring to Fig. 3(b), the SPD procedure starts with a series of three cycles at 0.25, 0.5, and 0.75 times the FME. Next follows a repeated pattern of seven cycles, the primary amplitude of which is some multiplier of the FME. The basic pattern starts with fOUf "degrading" cycles that decrease in amplitude (1.0, 0.75, 0.5, and 0.25 times the primary amplitude), followed by three cycles at the primary amplitude. The

primary amplitudes are 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0, 3.5, and 4.0 times the FME. The dynamic tests were conducted at a frequency of 1 Hz [per "Standard" (1996)], with the exception that the last series of seven cycles was conducted at a frequency of 0.5 Hz; because of test equipment limitations, the final cycles at 4.0 X FME, or ±76 mm (±3.0 in.), could not be run at a frequency of 1 Hz. Note that tests and analyses show that the fundamental frequency of most low-rise wood frame structures is initially in the range of 10 to 20 Hz. (Foliente and Zacher 1994); however, stiffness degradation quickly reduces the fundamental frequency. The rate of loading for these tests was still below the actual initial rate a wall is likely to experience during an actual earthquake; however, the practical limitations of most structural test systems in use today prohibit testing at those rates and any reasonable amplitude. The frequencies used in these tests are a reasonable upper limit of what can be achieved and still subject the specimen to displacements corresponding to the ultimate load. Four plywood and four OSB walls were tested dynamically. Load and displacement readings were taken at a rate of 200 samples per second during the dynamic tests. Note that the JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1998/689


TABLE 1. Static Results for Plywood and aSB Walls 1st Cycle

2nd Cycle

3rd Cycle

.....

Pun

~@Pun

p.....

~@P.....

KE

p.....

~@P.....

KE

p@~ .....

Specimen (1 )

(kN) (2)

(mm) (3)

(kN)

(4)

(mm) (5)

(kN/mm) (6)

(kN) (7)

(mm) (8)

(kN/mm) (9)

(kN) (10)

(mm) (11 )

(kN/mm) (12)

PLY I PLY2 Average

32.7 33.9 33.3

82.2 83.9 83.1

12.3 12.4 12.3

6.9 5.6 6.2

1.79 2.22 2.01

27.9 22.5 25.2

0.91 1.13 1.02

31.2 33.0 32.1

127.6 127.8 127.7

0.24 0.26 0.25

OSBI OSB2 Average

33.1 30.7 31.9

78.8 79.5 79.2

12.3 12.2 12.2

6.1 5.2 5.6

23.3 25.8 24.6

1.09 0.98 1.04

31.3 29.5 30.4

111.2 127.6 119.4

0.28 0.23 0.26

~

KE

(a) Plywood

25.4 25.5 25.4

(b) Oriented Strand Board (OSB)

Note: 25.4 rnm

2.01 2.35 2.18

= 1 m. and 4.45 kN = 1,000 lb.

dynamic tests were conducted with the actuator under stroke (displacement) control. RESULTS Presented in Table 1 is a summary of the results of the four static tests. In columns 2 and 3 are the ultimate load and displacement at ultimate. In columns 4 through 9 are the maximum load (PmIlA)' displacement at the maximum load (a), and effective stiffness (KE ) for the first and second cycles of the test. The effective stiffness in cycle i, for both the static and dynamic tests, was calculated as K E,

P:

25.5 25.4 25.5

P; - Pi' - AI

= A;

where = force corresponding to the maximum positive displacement, and P 1- = force corresponding to the maximum negative displacement, a/-. Presented in columns 10 through 12 are the same quantities when the test was stopped [the static tests were stopped either automatically by the control system at a maximum displacement of 127 mm (5.0 in.), or manually by the investigators in the case of specimen aSBl]. Typical load-displacement curves from the static tests are shown in Fig. 4. Presented in Table 2 is a summary of the results of the eight dynamic tests. Like the static results, presented in columns 2 and 3 are the ultimate load and displacement at the ultimate load. Column 4 contains the initial stiffness of the specimen, from the first cycle of loading. Presented in columns 5 through 12 are the maximum load, displacement at maximum load, and effective stiffness at displacements corresponding to approximately 25 mm (1 in.), 51 mm (2 in.), and 76 mm (3 in.), respectively. Typical hysteresis plots of load versus horizontal displacement at the top of the specimen are shown in Fig. 5 for the plywood and aSB walls.

at;

RESULTS

did not reach the target loads of 13.35 kN (3,000 Ib) and 26.70 kN (6,000 Ib), respectively, because of a low control gain on the actuator. The low gain was only noticed after the first test; in order to maintain a consistent test procedure the gain was not adjusted for the subsequent tests. The third and final half-cycle is characterized by an overall loss in stiffness and load carrying capacity. All of the walls, when tested statically, reached their ultimate load during the third half-cycle. The average ultimate load for the plywood walls was approximately 33.38 kN (7,500 Ib), the average ultimate load for the aSB walls was approximately 32.04 kN (7,200 Ib). The ultimate loads were reached at average displacements of 84 mm (3.3 in.) and 79 mm (3.1 in.), for the plywood and aSB walls, respectively. Assuming design allowable shears of 13.52 kN (3,040 Ib) for each wall, this yields a load factor (the ultimate load divided by the design allowable) of 2.5 for the plywood and 2.4 aSB walls. It should be noted that the wall maintains considerable load carrying capacity beyond ultimate, which is not the case for the dynamic tests, which will be discussed shortly. Several characteristic damage patterns emerged from the static tests. First, sheathing tended to pull away from the frame, pulling the nails along with it. The nails were pulled out of the stud, as though a tool had been used to extract the nail. In only a few instances were nails pulled through the sheathing. This lifting of the sheathing away from the frame occurred only along the edges of the sheathing. Second, the bottom plate split parallel to the grain at the uplift corner (i.e., the corner in tension). The splitting occurred along the line of the sheathing nails in the bottom plate, and was obviously precipitated by the sheathing pulling up and away from the bottom plate. Finally, some interior studs were observed to twist along their length, and split at their top or bottom along the line of sheathing nails in the stud. There was no noticeable difference in the damage observed between the plywood and aSB walls. Little or no damage was evident, or indicated by cracking or other noises, until the third half-cycle of the test [above loads of approximately 25.37 kN (5,700 Ib»).

Static Results The load-deformation behaviors of the plywood and aSB walls were very similar for the static tests, as shown by Fig. 4. The first half-cycle, up to a maximum load of approximately 12.28 kN (2,760 Ib), was nearly elastic for all the specimens. Some hysteresis is evident in the first half-cycle, but it is relatively insignificant. Inelastic behavior in the wall is clearly evident during the loading and unloading phases of the second half-cycle, which had a maximum load of approximately 25.37 kN (5,700 Ib). There is pronounced hysteresis on unloading from the second cycle and a permanent set or displacement of the top of the wall of about 6 mm (1/4 in.). Note that the maximum loads in the first and second half-cycles of the test

Dynamic Results During the dynamic tests the stud frame was observed to deform in a pure shearing type mode, while the sheathing, after a dozen or more small amplitude cycles, would rotate as a rigid body. The two sheathing panels would rotate by the same amount, while sliding relative to each other along the interior joint where they meet. During the tests the sheathing nails were deformed, back and forth, in alternating directions with each cycle. The dynamic tests were for the most part repeatable, as shown by the results in Table 2. The hysteretic load-deformation behaviors of the plywood and aSB walls were similar, with a few exceptions. The av-

690 I JOURNAL OF STRUCTURAL ENGINEERING I JUNE 1998


35 - . - - - - - - - - - - - - - - - - - - ,

~

20

characterized by a degradation in stiffness with continued cycling. and a pinched hysteresis loop. This is true even at moderately small amplitudes of displacement. The stiffness degradation can be attributed to the loss of load capacity during cycling at constant displacements. Porter (1987) defined the load to be stabilized if the reduction in load capacity was 5% or less between successive cycles. All specimens tested are considered stabilized by this definition for loads below ultimate load. Shenton et al. (1998) showed that although these specimens meet the 5% stabilization criteria, the stiffness will continue to degrade with continued constant amplitude cycling. Most of the damage observed to the shear walls in the dynamic tests was confined to the sheathing fasteners. Nails either fatigued and broke off just below the surface of the stud, or were pulled out from the stud: broken nails were more common than pulled-out nails. Converse to the static tests, however. the sheathing in general did not pull away from the frame, even in those locations where nails pulled out (Le., the nails appeared to be simply backed out of the stud by the cyclic action). Nail damage was concentrated along the bottom sheathing edges, the top corners, and the interior edges where the sheathing met. There was little or no damage to the interior field nails. When there was significant damage to nails around the bottom or top comers, the damage would extend further up or down along the sides of the wall as those nails picked up additional load. It is important to note that this type of damage, although observed in every dynamic test, was not consistent in terms of the location, distribution, or extent of damage within the wall. between the tests. The only noticeable difference in damage between the plywood and aSB walls was degradation in the aSB sheathing near the comers, in the later stages of the test. More specifically. pieces of aSB would break off of the wall near the comer, usually after many cycles.

~ ...:l

15

Comparison of Static and Dynamic Results

30

25

~

20

]

15

10

5

O..,u'---r--,..----,---r--,--,----j

o

40

20

60

80

120

100

140

Displacement (rom) a. Plywood (PLY1) 35 30

25

10

5

0 0

20

40

60

80

100

120

140

Displacement (mm)

b. OSB (OSB2) FIG. 4. "TYpical Measured Load Displacement Plots from (a) Plywood Shear Wall; (b) OSB Shear Wall

erage ultimate load of the plywood walls was approximately 31.60 kN (7,100 lb) and occurred at a displacement of approximately 56 mm (2.2 in.). The average ultimate load for the aSB walls was approximately 28.04 kN (6.300 Ib) and occurred at an average displacement of 46 rom (1.8 in.) It is worth noting that the ultimate load for the plywood wall occurred at the 52nd cycle of loading. and the ultimate for the aSB walls at the 45th cycle. For both types of walls. there was a gradual increase in load-carrying capacity up to the ultimate; however, after the ultimate load was reached there was a fairly rapid decrease in load capacity and stiffness, with increasing displacement. The reduction in load capacity and stiffness was more pronounced in the aSB walls than the plywood walls, as illustrated in Fig. 5. The response of the shear wall at a given displacement is

The results of the tests indicate several differences between the static and dynamic responses of the shear wall. First is the ultimate load and corresponding displacement at ultimate; second, the postultimate behavior; and finally, the damage and the mode of failure was significantly different for the two types of tests. The ultimate load measured in the dynamic tests is comparable to the static ultimate load; however, it occurred at a much smaller displacement: 56 mm (2.2 in.) dynamic versus 84 rom (3.3 in.) static for the plywood, and 46 mm (1.8 in.) dynamic versus 79 mm (3.1 in.) static for the aSH. Wall ductility is determined by dividing the displacement at ultimate load by the yield displacement. Assuming the same yield displacement of 6 mm (0.24 in.), this corresponds to a static ductility of 14.0 and dynamic ductility of 9.3 for the plywood walls and static ductility of 13.2 and dynamic ductility of 7.7 for the aSB walls. These values correspond to a 34% reduction in ductility for the plywood wall and a 42% reduction for the aSB wall, relative to the static results. It is not possible to say at this time whether this difference is due to the rate of loading (the static ultimate load is reached in approximately 20 minutes, while the dynamic ultimate load is reached in less than one minute), or the load history (Le., the number of cycles). Also, the average ultimate loads from the dynamic tests are comparable to, but less than, the average static ultimates: the dynamic ultimate is 5% less than the static ultimate for plywood, and the dynamic ultimate is 12% less than the static ultimate for the aSB. It is difficult to say if these minor differences in ultimate load are statistically significant given the low number of specimens tested statically. Direct comparisons were made of the load-deformation JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1998/891


TABLE 2.

Dynamic R..ulta for Plywood and aSB Walls

1st Cycle

24th Cycle

52nd Cycle

~

KE

(kN/mm) (4)

P@1" (kN) (5)

(mm) (6)

(kN/mm) (7)

2.23 2.13 NA 2.72 2.36

28 26 25 28 27

26.6 26.6 27.7 27.7 27.2

Pun

~@Pun

K;nnlal

Specimen (1 )

(kN) (2)

(mm) (3)

PLY I PLY2 PLY3 PLY4 Average

34 31 30 32 32

53.9 53.8 55.4 55.5 54.6

66th Cycle

P@2" (kN) (8)

~

KE

p@~ .....

~mu

KE

(mm) (9)

(kN/mm) (10)

(kN) (11 )

(mm) (12)

(kNlmm) (13)

34 31 30 32 32

53.9 53.8 55.4 55.5 54.6

0.59 0.53 0.49 0.51 0.53

28 28 28 27 28

71.6 71.5 74.5 74.3 73.0

0.38 0.37 0.35 0.35 0.36

55.0 56.0 54.5 55.9 55.3

0.40 0.44 0.44 0.40 0.42

15 18 21 19 18

74.5 75.2 73.9 75.0 74.6

0.19 0.22 0.27 0.25 0.23

(a) Plywood

1.05 0.96 0.89 1.00 0.97

(b) Oriented Strand Board (OSB)

OSBI OSB2 OSB3 OSB4 Average

2.53 2.38 2.00 2.41 2.33

46.1 47.3 45.8 47.2 46.6

27 29 28 28 28

27.1 27.9 26.7 27.8 27.4

25 27 26 26 26

0.91 0.95 0.93 0.90 0.92

26 29 28 27 27

Note: 25.4 mm = I m. and 4.45 kN = 1.000 lb.

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a. Plywood (PLYI) 4O--.--~-~---.----.----.---,--,--,

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b. OSB (OSB3) FIG. 5. Typical Measured Hysteresis Curves from (a) Plywood Shear Wall; (b) aSB Shear Wall

curves from the static and dynamic tests by plotting both curves on the same graph: representative comparisons are shown in Fig. 6 for plywood walls, and in Fig. 7 for OSB walls. In a very general sense, the static results track the peak dynamic results reasonably well; however, subtle differences do arise. For the most part the dynamic tests yield slightly larger loads for displacements less than 51 rom (2 in.), as illustrated in Figs. 6(a) and 7(a) [and to a lesser extent in 7(b)]. This was also noted by Karacabeyli and Ceccotti (1996). At larger displacements, however, the static results consistently bound the dynamic results. Also, the walls exhibit considerable reserve load-carrying capacity beyond ultimate in the static tests, whereas the strength decreases rapidly once ultimate is reached in the dynamic tests. In a few instances the static results bounded the dynamic, over the entire range of displacements [for example, Fig. 6(b)]. Note that the dynamic test load results exceed or are equal to the static test load results, up to the dynamic ultimate load, only during the first of the four cycles at a given displacement. Because of the degradation in stiffness at constant amplitude cycling, the load capacity in the dynamic tests is substantially reduced from the first cycle. Fig. 8 shows a plot of envelope curves of maximum load vs. displacement for the static tests and the 1st, 2nd, 3rd, and 4th excursion to a given displacement for the dynamic tests. The static loads were determined by taking the average of the two tests, and the dynamic loads were taken as the average of four tests. The static test predicts the cycle #1 dynamic test results reasonably well up to ultimate load. The load-displacement plots then diverge, as the load capacity of the dynamic test decreases while the load from the static test continues to increase. This trend is more prevalent in the OSB walls [Fig. 8(b)]. There is clearly a huge decrease in load between cycles #1 and #2 that cannot be predicted by the static test. The load degradation continues, to a lesser extent, between cycles #2, #3, and #4. The load degradation exhibited in the dynamic tests was nearly identical for the plywood and OSB walls. The final major difference between the static and dynamic tests is the characteristics of the damage observed. In the static tests, the damage was restricted to the sheathing pulling away from the frame, and the bottom plate and some studs splitting. The damage in the dynamic tests was confined to the sheathing nails, which fatigued and broke off or were pulled out from the frame. Skaggs and Rose (1996) noted that the majority of load cycles in the TCCMAR produce exceed the FME displacement. Obviously the high number of large displacement cycles contributes to the fatiguing of the nails. This fatigue failure is not consistent with post earthquake inspections; how-



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50

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100

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a. Static Test PLY2 and Dynamic Test PLYI

a. Static Test OSB I and Dynamic Test OSB3

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b. Static Test OSB I and Dynamic Test OSB4 ever, nail pullout like that observed in the dynamic tests was evidenced in the Northridge earthquake (Holmes and Somers 1996).

Impact on Shearwall Design Most timber structures in seismic regions do not require a dynamic analysis for their design; for these structures the codes provide a simplified alternative known as the static lateral force procedure. This procedure allows the designer to replace the earthquake loading and all of its associated analytical difficulties with a set of "equivalent" static forces that are defined by the code. The equivalent static loads determined by this procedure must be transferred through the structure's lateral load resisting system, including the shear walls. From a table of allowable loads (traditionally based on the results of static tests), for wind and seismic forces, the designer can choose the sheathing grade, thickness,and application (Le., applied directly to the framing or over gypsum), and nail size and spacing for the wall. It is generally accepted that an equivalent static procedure

FIG. 7. Static and Dynamic Load Displacement Behavior for OSB Shear Walls

is necessary for the design of timber structures until a "design friendly" dynamic analytic model is developed. However, the process of going from the dynamic to the static should include as little guesswork as possible. This leads to the question of why static tests results are used in determining the allowable shear for wind and seismic forces in design tables. The static and dynamic behavior of a wall have been demonstrated herein to be substantially different. The degradation of load at constant amplitude cycling shows that the results of a static test will closely predict the maximum load of the first cycle of a dynamic test. However, further cycles at that same amplitude, which is characteristic of a seismic event, will drastically decrease the load capacity. Table 3 shows the load factors (the ultimate load divided by the design allowable) for the static tests and the four constant amplitude cycles (shown in Fig. 8) of the dynamic tests JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1998/693


3S , - - - - - - - - - - -

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CONCLUSIONS

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so

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a. Plywood Results 35,..------------

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b. OSB Results FIG. 8. Maximum Load versus Displacement Plots for Static Tests and Constant Amplitude Cycles of Dynamic Tests for (a) Plywood Shear Walls; (b) OSB Shear Walls TABLE 3.

poorly constructed wall the load factor may begin to approach 1.0 and a dangerous environment will result. The city of Los Angles/SEAOSC task force investigating the Northridge earthquake recommended that a cyclic test program be carried out to determine realistic loads for plywood shear walls and until that time, the allowable shear loads for plywood greater than 9.5 rom (3/8 in.) thick should be reduced by 25% (Findings 1994). The results of this study support those recommendations and demonstrate that they should be applied to OSB as well plywood.

Load Factors for Plywood and OSB Walls

Test type (1 )

Plywood (2)

OSB (3)

Static Dynamic-Cycle #1 Dynamic-Cycle #2 Dynamic-Cycle #3 Dynamic-Cycle #4

2.5 2.4 2.1 1.9 1.8

2.4 2.1 1.9 1.8 1.7

for the plywood and OSB. The largest decrease in load factor occurs between the first and second cycle. The load factor continues to decrease as the number of cycles increases and does not stabilize after four cycles. There is a decrease of nearly 30% for both the plywood and OSB walls between the static load factor (used in design) and the fourth cycle load factor. Clearly, there is a marked difference between the load factor expected from design and what is likely to occur in the field during an earthquake. After the fourth cycle a load factor slightly less than two is maintained, which is probably adequate to prevent the loss of life in the event of an earthquake provided that the wall is properly constructed. However, in a

Tests have been conducted on 2.4 by 2.4 m (8 by 8 ft) wood frame shear walls to determine and compare the wall performance under static monotonic loading, and dynamic, fully reversed cycles of loading. A total of 12 tests have been conducted: four static and eight dynamic tests. The static tests were conducted in accordance with the ASTM standard that has been in use for many years; the dynamic tests were conducted in accordance with a proposed standard that was recently endorsed by the Structural Engineers Association of Southern California. Based on the results of these tests, the following conclusions can be stated: 1. The ultimate load measured in the dynamic tests was slightly less than the ultimate load measured in the static tests. However, the wall ductility, as indicated by the displacement at which the ultimate load occurs, is between 34 and 42% less when measured in the dynamic tests, as compared to the static ductility. Therefore, the static test can reasonably predict the maximum load carrying capacity of the wall, when subject to dynamic loads, but not the ductility. 2. The static test, in general, may under predict the maximum load at smaller displacements, but is an upper bound for larger displacements. However, the stiffness and load capacity of the wall decreases significantly with increased cycling at the same displacement amplitude, as evidenced in the dynamic tests. This behavior is extremely important to the seismic performance of the wall, and is not captured by the simple, static monotonic test procedure. The load factors expected from design will not be sustained during a seismic event. Dynamic tests are necessary to determine realistic loads for shear walls. 3. The damage and failure mode of the wall is very different in the static and dynamic tests. The damage in the static tests is characterized by the pulling away of the sheathing from the frame, extraction of the sheathing nails, and splitting of the wall bottom plate at the uplift comer. The damage in the dynamic tests is characterized by fatiguing and failing of the sheathing nails, which is due to the many cycles at displacements exceeding the FME. Pullout of sheathing nails exhibited in dynamic tests is consistent with earthquake reconnaissance observations. 4. There is nearly a 30% decrease, for both the plywood and OSB walls, between the static load factor (used in design) and the fourth cycle load factor. This large difference between the load factor expected from design and what is likely to occur in the field during an earthquake supports the findings of the city of Los Angeles/ SEAOSC task force investigating the Northridge earthquake. A cyclic test program should be carried out to determine realistic loads for plywood and OSB shear walls, and until that time, the allowable shear loads for sheathing greater than 9.5 mm (3/8 in.) thick should be reduced by 25%.



ACKNOWLEDGMENTS The writers would like to thank Timothy Elliott and Robert Schimmel, civil engineering undergraduates at the University of Delaware, for assisting in the testing of the shear walls. The writers would also like to thank Michael O'Hailoran and Thomas Skaggs of the American Plywood Association for their assistance in obtaining the sheathing for the study. The writers would like to thank the Louisiana-Pacific Corporation for donating the sheathing for this research. The U.S. Department of Agriculture (USDA) provided financial support for this project under grant No. 95-37103-2100; the writers would like to thank the USDA for their support.

APPENDIX.

REFERENCES

Assessment ofdamage to single-family homes by Hurricanes Andrew and lniki. (1993). U.S. Department of Housing and Development, Washington, D.C. Carney, J. M. (1975). "Bibliography on wood and plywood diaphragms." J. Struct. Engrg., 101(11), 2423-2436. Dolan, J. D. (1989). "The dynamic response of timber shear walls," PhD thesis, University of British Columbia, Vancouver, British Columbia, Canada. Findings & recommendations of the City of Los AngeleslSEAOSC Task Foree on the Northridge earthquake. (1994). Structural Engineers Association of Southern California, Whittier, Calif. Foliente, G. C., and Zacher, E. G. (1994). "Performance tests of timber structural systems under seismic loads." Analysis, design and testing of timber structures under seismic loads, G. C. Foliente, ed., University of California, Forest Products Laboratory, Richmond, Calif. Hanson, D. (1990). Shear wall and diaphragm cyclic load testing, cyclic shear fastener testing and panel durability performance testing ofWeyerhaeuser Sturdi- Wood oriented strand board. Weyerhaeuser Company, Federal Way, Wash. Holmes, W. T., and Somers, P. (1996). "Northridge earthquake of January 17, 1994 reconnaissance report vol. 2." Earthquake Spectra, 11, Supplement C. Johnson, A. C., and Dolan, J. D. (1996). "Performance of long shear

walls with openings." Int. Wood Engrg. Conf., V. K. Gopu, ed., Ornni Press, Madison, Wis. Karacabeyli, E., and Ceccotti, A. (1996). "Test results on the lateral resistance of nailed shear walls." Int. Wood Engrg. Conf., V. K. Gopu, ed., Ornni Press, Madison, Wis. Peterson, J. (1983). "Bibliography on lumber and wood panel diaphragms." J. Struet. Engrg., ASCE, 109(12),2838-2852. Porter, M. L. (1987). "Sequential phased displacement (SPD) procedure for TCCMAR testing." Proc., 3rd Meeting of the Joint Tech. Coordinating Committee on Masonry Res., U.S.-Japan Coordinated Research Program, Tomanu, Japan. Schmid, B. L., Nielsen, R. J., and Linderman, R. R. (1994). "Narrow plywood shear panels." Earthquake Spectra, 10(3),569-588. Shenton Ill, H. w., Dinehart, D. w., and Elliott, T. E. (1998). "Stiffness and energy dissipation degradation of wood frame shear walls." Can. J. Civ. Engrg., 25(3). Shepherd, R. (1996). "Standardized experimental testing procedures for low-rise structures." Earthquake Spectra, 12(1), 111-127. Shepherd, R., and Allred, B. (1995). "Cyclic testing of narrow plywood shear walls." ATC R-l, Applied Technology Council, Redwood City, Calif. Skaggs, T. D., and Rose, J. D. (1996). "Cyclic load testing of wood structural panel shear walls." Int. Wood Engrg. Conf., V. K. Gopu, ed., Omni Press, Madison, Wis. Standard method of cyclic (reversed) load test for shear resistance of framed walls for buildings. (1996). Structural Engineers Association of Southern California, Whittier, Calif. "Standard method of static load test for shear resistance of framed walls for buildings." (1994). E 564-76, ASTM, West Conshohocken, Pa. "Standard methods of conducting strength tests of panels for building construction." (1995). E 72-95, ASTM, West Conshohocken, Pa. Stewart, W. G., Dean, 1. A., and Carr, A. J. (1988). "The earthquake behaviour of plywood sheathed shearwalls." Proc.• 1988 Int. Conf. on Timber Engrg., Vol. 2, Seattle, Wash. Tissell, J. R. (1993). "Structural panel shear walls." Res. Rep. 154, American Plywood Association, Technical Services Division, Tacoma, Wash. Uniform building code. (1994). International Conference of Building Officials, Whittier, Calif.

JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1998/695
