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Effeet of UPPer Storeyfloor on the performance of wood shear walls


Effect of Upper Storey / Floor on the Performance of Wood Shear Walls
Yan Liu, Professor, Yangzhou University, Jiangsu Province, P.R. China Chun Ni, Wood Engineering Scientist, Forintek Canada Corp., Vancouver, B.C., Canada Hans Rainer, Consultant, Rainer Dynamics Inc., Vancouver, B.C., Canada Lu Wensheng, Associate Professor, Tongji University, Shanghai, P.R.China

Abstract
In shear wall tests, in-plane shear loads are usually applied to the test specimen by means of a continuous load spreader connected along the top of the shear wall. The stiffness of the load spreader has a direct impact on the lateral load capacity and failure mode of a shear wall, particularly if the wall does not have hold-downs at the ends and around openings. A flexible load spreader that permits the ends of the shear wall to freely lift from the foundation when a lateral in-plane load is applied will result in an estimate near the lower bound of the lateral load capacity of the wall. Conversely, a rigid load spreader will result in an estimate near the upper bound of the lateral load capacity. In this paper, results are presented from a testing program designed to study the restraining effect from an upper story on the performance of shear walls. Sixteen full-size single-storey shear walls 6.0 m in length and 2.44 m in height were tested: half with a flexible and half with a rigid load spreader. In addition, four full-size shear walls representing a typical two-storey configuration were tested. Comparisons of the initial stiffness, ultimate load and displacement, energy dissipation and axial force in anchor bolts of the test walls are presented. An increase of lateral load capacity due to rigid load spreader beam and from a second storey has been confirmed.

1. Introduction
Continuous load spreader beam along the top of shear walls is commonly used in shear wall tests. Although this approach seems reasonable for evaluating shear walls with hold-downs, it may have a direct impact on the performance of walls in conventional wood frame structures where hold-downs are usually omitted at the ends and around openings of the walls. For shear walls without hold-downs, the use of continuous load spreader beam could result in higher shear strength than the actual strength of the shear wall. In this paper, identical full-size shear walls with a flexible load spreader beam and with a continuous stiff beam were tested to study the influence of load spread beam. For each wall specimen, monotonic and reversed cyclic loadings were applied. The influence of boundary conditions is discussed in terms of shear strength, ultimate displacement, initial stiffness, energy dissipation and axial force in anchor bolts.

2. Test program
2.1. Wall Configurations Table 1 summarizes the wall configurations. For each wall configuration, monotonic and reversed cyclic tests were carried out. Except for Wall K in which commercially available hold-downs were used at both ends, no hold-downs are used in the rest of the walls. Walls A to D and F to I are 2.4 m in height and 6.0 m in length. To study the influence of upper storey on the performance of shear

walls, Walls A to D were tested with a flexible load spreader beam while a continuous rigid beam was added on identical walls to simulate the upper storey (Walls F to I). To verify the findings, shear walls in a two-storey building (Walls E and J) were also tested. A uniformly distributed vertical load of 3.5 kN/m was applied on the top of Wall J. The amount of vertical load represents the gravity load exerted on walls in a typical conventional wood frame building. All shear wall specimens were constructed using 1650f-1.5E machine stress rated (MSR) Spruce-Pine-Fir 38 mm × 89 mm lumber for the wall studs and the top and bottom plates. The top plate, end studs and studs around openings consisted of double members, while the bottom plate and the interior studs, spaced at 400 mm on center, consisted of single member. The 9.5 mm oriented strand board (OSB) was used for the sheathing panels, and was connected to the framing members with 3.0 mm diameter spiral nails (60 mm long) spaced 150 mm along the panel perimeter and 300 mm elsewhere. The framing members were assembled with 3.9 mm diameter (90 mm long) spiral nails. Nailing schedule of framing members followed the prescriptive requirements in Part 9 of the National Building of Canada (NBCC, 1995). Table 1 Test Matrix of Wood Frame Shear Wall
Wall No. Opening (m) Boundary condition at top of wall

Wall configuration
2440

A
B C D

1.2 2.4 3.6

Flexible beam
Flexible beam Flexible beam Flexible beam

6000

2400

1200

2400

1200 1200 1200 1200 1200

1200

3600

1200

900 1200 2440

2100 2440

900 1200 2440

2100 2440

1200

3600

1200

2100 2540

E

3.6

Second storey

F G H I

1.2 2.4 3.6

Rigid beam Rigid beam Rigid beam Rigid beam

6000

2400

1200

2400

1200 1200 1200 1200 1200

1200

3600

1200

900 1200 2440

2100 2440

900 1200 2440

2100 2440

2440

1200

3600

1200

For Walls E and J, commercially K Flexible beam available wood I-joists, 240 mm in height, were used as floor joists. They were placed at right angle on the top plate of the shear wall and were spaced at 600 mm on center. The gaps between I-joist were blocked with the same I-joist placed at the center of the top plate. Connections between floor joists and the walls below and above the floor also followed the prescriptive requirements in Part 9 of the National Building of Canada (NBCC, 1995).
hold-down 6000

Conditioning and testing of the walls was performed at ambient laboratory conditions where average moisture contents of the lumber and OSB were approximately 12.2 and 6.0 percent,

2440

2100 2540

J

3.6

Second storey

respectively. The average relative density of the lumber was 0.47. Bottom plates were attached to the steel foundation with 12 mm diameter anchor bolts spaced at about 400 mm on center. Except for walls with a 3.6 m opening (Walls D, E, I and J) where two bolts at each end of wall segments were used, one bolt was used at each end of wall segments. The distance between the first anchor bolt and the outer edge of the wall was 175 mm. The same anchor bolt was used between the top plates and the load spreader beam. 2.2. Test Facilities and Instrumentation Figure 1 shows a diagram of the shear wall test set-up. A monotonic or cyclic displacement is applied in-plane along the top of the wall through a load spreader beam which is bolted to the top plate and attached to the hydraulic actuator. The load spreader beam consists of 80 × 80 × 5 mm 1.22 m long square steel tubes hinged at the ends to allow for the deflection of the top plates and is hereafter called the "flexible load spreader beam". Guides are provided along the load spreader to prevent lateral out-of-plane movement. For Walls E and J, a different load spreader beam was used in order not to disturb the connections between floor and walls. Instead of on the top of top plate, the load beam was connected to the sides of the top plates. To reduce the influence of beam stiffness the load beam was only extended into the top plate to a full panel width (1.2 m). For all the shear walls, displacement transducers were placed at the top and bottom plates to measure the overall horizontal displacements (U1), slip between bottom plate and foundation (U2), relative movements between end studs and top and bottom plates (V1, V2, V3 and V4), and relative movements between studs where panels were joined or studs around opening and top and bottom plates (V5 to V12), as shown in Figure 1. For walls fitted with the flexible load spreader beam, ring load cells (F1 – F6) were placed at anchor bolts around wall ends and at openings or studs where the panels joined to measure the change of axial force in the anchor bolts. For shear walls fitted with the additional continuous stiff beam, ring load cells were placed around the end of walls and at hinge joints of the flexible load spreader beam. At each location, two load cells, one on top of the stiff beam and the other between the load spreader beam and the stiff beam, were used to measure the compressive or tensile reaction force from the continuous stiff beam.
F15
F V4 V12 V11 V10 V9 V3 U1

F13 F8 V12 F7 V11 V10

F12 F6

F11 F5 V9

F10 F4 U1 V3

F

F9 V4

V2 F6

V8 F5 F4

V7

V6 F3

V5 F2

V1 F1

V2 F3

V8 F2

V7

V6

V5

V1 F1

(a) Instrumentation of Wall A

(b) Instrumentation of Wall F

Figure 1

Diagram of the shear wall test set-up

2.3. Load Protocol Monotonic and reversed cyclic displacement schedules were used in the test program. For monotonic tests, the displacement rate was 7.5 mm/min. The reversed cyclic displacement schedule followed the ISO 16670 Standard (ISO, 2003), in which the cyclic protocol consisted of the following reversed cycles: one cycle at each displacement level of 1.25%, 2.5%, 5%, 7.5% and 10% of the reference ultimate displacement, and three cycles at each displacement level of 20%, 40%, 60%, 80%, 100% and 120% of the reference ultimate. For reversed cyclic tests a displacement rate of 3 mm/s was used.

3. Test Results
3.1. General Table 2 summarizes the shear strength, ultimate displacement, initial stiffness and energy dissipation of the tested shear walls. To compare the performance of shear walls with different openings, the shear strength, initial stiffness and energy dissipation were divided by the total lengths of the full-height sheathed segments in the wall. Table 2 Summary of test results
Wall No. A B C D E F G H I J K Notes: Load protocol Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic Monotonic cyclic fvd 1 (KN/m) T5 6.4 6.1 5.8 5.1 10.5 10.4 7.1 6.0 9.3 9.6 8.1 7.3 6.7 7.7 11.4 11.5 7.9 6.5 12 9.1 9.7 9.2 C5 6.7 6.8 10.7 6.7 4.6 7.9 7.5 12.0 8.3 9.5 8.5
6

Δu 2 (mm) Avg. 6.4 6.4 5.8 6.0 10.5 10.6 7.1 6.4 9.3 9.6 8.1 7.6 6.7 7.6 11.4 11.7 7.9 7.4 12 9.3 9.7 8.9 T5 64 52 51 85 112 85
8

K 3 (KN/m/mm) Avg. 64 54 51 46 85 71 112 107 85 84 54 47 75 63 72 63 106 T5 0.38 0.30 0.46 0.41 0.61 0.63 0.43 0.38 0.55 0.38 0.49 0.45 0.46 0.42 0.65 0.67 0.43 0.42 0.7 0.58 0.46 0.24
7

C5 55 50 75
8

C5 0.43 0.54 0.69 0.39 0.54 0.54 0.53 0.77 0.53 0.52 0.38
7

Avg. 0.38 0.37 0.46 0.48 0.61 0.66 0.43 0.39 0.55 0.46 0.49 0.50 0.46 0.48 0.65 0.72 0.43 0.48 0.70 0.55 0.46 0.31

E 4 (J) 15385 6939 14801 13318 15657 11898 13337 13749 11440 10385 14334

42
8

67 104
8

110 83 51 65 63 104 70 64
8

85 54 42 75
8

60 72 63 106 104 68 98 63
8

104 114 69 98 64

114

1. 2. 3. 4. 5. 6. 7. 8.

fvd – unit shear strength, kN/m, which is the shear strength divided by the total lengths of the full-height sheathed wall segments in the wall. Δu – ultimate displacement of the shear wall, which is the displacement at 80% of ultimate load in the descending portion of the load-displacement curve (ISO, 2003). K – unit initial stiffness, kN/mm/m, which is defined as the secant stiffness between starting point and the point where displacement equals to 9.6mm, equivalent to 1/250 of the wall height. E – Energy dissipation, which is the sum of dissipated energy of each reversed cycle from the beginning to the end of last cycle step where the specimen reaches the ultimate displacement. T – specimen is loaded with actuator in tension; C – specimen is loaded with actuator in compression. Specimen didn’t reach ultimate shear strength due to local buckling of steel loading beam. The initial stiffness is obtained from the load-displacement response of the wall which was pre-loaded to 10 mm. Test is stopped before the specimen reaches the ultimate displacement.

3.2. Shear Strength It is found that the shear strengths for walls loaded with a continuous stiff beam are consistently higher than those with the flexible spreader beam on top of the top plate. For walls loaded with the stiff beam (Walls F, G, H and I), the increase of shear strength is in the range of 8 – 22% compared to their counterparts with the flexible spreader beam (Walls A, B, C and D). Similarly, the shear strength of the wall with an upper storey (Wall E) increased noticeably compared to an identical wall with the flexible load spreader beam (Wall D). For both walls with an upper storey (Walls E and J), their shear strengths are comparable to those of the wall with hold-downs, indicating that they have reached the full shear strength of the wall. This demonstrates that the upper storey can increase the shear strength of shear walls without hold-downs by effectively limiting the uplifts at the ends and around the openings of the shear wall. For Wall J, the vertical load on top of the second storey doesn’t seem to have any impact on the shear strength of the wall. This is similar to a wall with hold-downs where vertical load doesn’t increase the shear strength of the wall. It was observed that the shear strength of Walls C and H were higher than the wall with hold-downs. This indicates that wall elements above and below openings also contribute to the overall lateral resistance of the wall. 3.3. Ultimate Displacement Except for Wall B, the ultimate displacements of walls with the flexible load spreader beam (Walls A, C and D) are slightly larger than those of their counterparts with the continuous stiff beam (Walls F, H and I). Large displacement was observed for walls with high aspect ratio of wall segments (Walls D and I). This behavior was also observed in Chen et al. (2006). With higher aspect ratio, deflections due to bending and rotation of wall segments are greatly increased. The ultimate displacements for shear walls with an upper storey (Walls E and J) decreased compared to the ones with the flexible load spread beam (Wall D). This can be attributed to the upper storey limiting the rotation of wall segments, therefore reducing the overall ultimate displacement. Compared to Wall J the ultimate displacements of Wall I are comparable in the

monotonic tests but are greater in the reversed cyclic tests. This is probably because the bolts connecting the stiff beam and the flexible load spreader beam were only finger tight and less resistant to the rotation of wall segments. For Walls A and B, the ultimate displacements are smaller to those of the wall with hold-downs (Wall K). In the study by Chen et al. (2006), ultimate displacements are compatible for walls with hold-downs and walls with transverse walls. As both series of the shear walls used the same flexible load spreader beam, it seems to indicate that transverse walls could also increase the ultimate displacement of a shear wall without hold-downs. 3.4. Initial Stiffness In this study, the initial stiffness is defined as the secant stiffness between the starting point and the point where the displacement is equal to 9.6 mm, which is equivalent to 1/250 of the wall height. The load-displacement response is more or less linearly elastic within this range. It was found that the initial stiffness for the walls with an upper storey (Walls E and J) was greater than their counterpart with the flexible load spreader beam (Wall D). This is expected as the upper storey can increase the shear strength of shear walls while reducing the overall displacement by limiting the uplifts at the ends and around the openings of the shear wall. Except for Walls A and F, initial stiffness are, however, not significantly different between walls with the flexible load spreader beam and their counterparts with the continuous stiff beam. This is not expected as the continuous stiff beam should act in a similar way as an upper storey, therefore the walls with continuous stiff beam should have higher initial stiffness. As the bolts that connected the flexible load beam and the continuous stiff beam were finger tight, the restraint to the uplift is limited when displacement is small. This may explain why the initial stiffness is similar for the walls with different boundary conditions.
Uplift (mm) Uplift (mm) Uplift (mm)

3.5. Stud Uplift For monotonic tests Figure 2 shows the uplift displacement of end studs, studs around openings or studs where panels are joined. For shear walls under reversed cyclic tests, a similar trend was observed when the specimen was loaded in opposite directions.

WALL A

Figure2 The uplift of studs For walls with an upper storey (Walls E and J) it was found that the uplift displacements at the same stud locations were smaller compared to the wall with the flexible load spreader beam (Wall D). This indicates that the upper storey is stiff enough to limit the uplifts at the ends and around the openings of the shear wall. As a result, the upper story acts as a “hold-down” for the wall below. This explains why very little uplift was observed in the shake table tests of a two-storey conventional wood frame house (Ni et al. 2006). 3.6. Axial Force of Anchor Bolts Ring load cells with washers on both sides were placed on the top of bottom plate and were

Uplift (mm)

tightened with the nut of the anchor bolt. All the nuts on anchor bolts were tightened to create an initial axial tensile force of approximate 10 kN in the anchor bolts before the tests. Although it is desirable not to tighten the anchor bolts in order to accurately measure the axial force (Chen et al, 2006), proper axial tensile force in the anchor bolts is needed to prevent slipping of the bottom plate during the test. For the shear wall with hold-downs (Wall K) the maximum axial forces at the hold-downs were around 27 kN and the maximum axial forces in anchor bolts at other locations were about 2.8 kN. For shear walls without hold-downs and with the flexible load spreader beam (Walls A, B, C and D), the maximum axial forces were about 6 kN in the anchor bolts around end studs and openings. Compared to walls with a flexible load spreader beam the maximum axial forces were greatly reduced for those with the continuous stiff beam (Walls F, G, H and I). Similar observations were made for Walls E and J. Figure 3 shows the axial force responses of anchor bolts in time history. The figure clearly indicates that anchor bolts reached their maximum axial force at different times. This is probably because after nail joints along the bottom plate around the end stud reached their capacities, the adjacent nail joints started to resist the uplift force until the wall failed. 3.7. Reaction Axial Force from Stiff Beam Figure 3 Response of anchor bolts in time history

All the load cells on top of the stiff beam and between the load spreader beam and the stiff beam were finger tightened, so that no initial stress was applied to these load cells. By doing this, the actual reaction force from stiff beam can be measured (Chen et al. 2006). Table 3 summarizes the maximum reaction axial force from the stiff beam. As can be noticed, the largest of the axial forces occurred around the ends and openings of the wall. The axial forces within the uniform full-height wall segments were much smaller, as shown in Wall F. The results indicate that the stiffness of the load spreader beam has a direct influence on the load-displacement response of the shear wall. Since the continuous stiff beam can also be compared to an upper storey, it means that for a typical conventional wood frame building the upper storey could provide significant restraint to counteract the uplift force. This may partly explain why the superior lateral resistance of conventional buildings cannot be demonstrated by engineering calculations when the existing design codes for wood construction are used. Table 3
Wall No. F

Forces on the rigid load spreaders
Load protocol Monotonic Cyclic Internal force of the bolts (kN) along length of specimen 0 2.1 4.6/ -3.4 1.2 m -1.1 * / -11.2 2.4 m * 7.7/ * 3.6 m 2.9 2.8/ * 4.8 m 4.3 5.7 / * 6.0 m -3.5 -4.4/ 0.6

G H I Notes: 1. 2.

Monotonic Cyclic Monotonic Cyclic Monotonic Cyclic

1.7 2.0/- 3.0 * 1.5/ -4.4 11 10.9/ -9.1

* */* -7.8 -7.0/ 3.4 - 20 -18.7/ 11.7

- 13.8 -14/ 15.2 11.2 * / 6.7 -

7.9 8.3/ -10.3 -4.5 -3.3/ 4.2 -

10.8 8.6/ * 9.5 11.6/ -7.7 15 11.8/ -15.0

-10.1 -7.8/ 2.5 - 5.5 -7.9/ 3.4 -11.1 -10.8/ 9.2

The signs of + and - denote that the axial force in the load cell between the flexible load spreader beam and the stiff beam is in tension and compression, respectively. The values before and after / denote, respectively, the maximum recorded axial forces under the same and opposite loading directions to the monotonic test. Data were corrupted at the location.

*

3. Conclusions
A total of eleven full-size shear wall tests were carried out to study the effect of the load spreader beam on the performance of shear walls in conventional wood frame buildings. The tests demonstrated that with a stiff load spreader beam the shear strength of the wall can be significantly increased compared to that of a flexible spreader beam. A flexible load spread beam would result in an estimate near the lower bound of the shear strength. The improved performance due to a stiff loading beam has demonstrated that for a typical conventional wood frame building, a stiff upper storey could provide significant restraint to counteract the uplift force and thus provide increased lateral resistance. This may partly explain why the superior lateral resistance of conventional buildings cannot be demonstrated by engineering calculations when existing design codes for wood construction are utilized. Further testing and modeling are needed to calibrate the contribution of the upper storey, so that the performance of conventional wood frame buildings can be evaluated on a more rational technical basis.

4. References
[1] Chen, H.J., Ni, C., Lu, X.L., Karacabeyli, E. 2006. Effect of Transverse Wall and Vertical Load on the Performance of Shear Walls. Proceedings of World Conference on Timber Engineering, Portland, Oregon. [2] ISO. 2003. Timber structures-Joints made with mechanical fasteners-Quasi-static reversed cyclic test method. ISO 16670, International Organization for Standardization, Geneva, Switzerlad. [3] NBCC. 1995. National building code of Canada. Canadian Commission on Building and Fire Codes, National Council of Canada, Ottawa, Ont.

[4] Ni, C., Rainer, H., Chen. H.J., Lu, X.L., Karacabeyli, E., Follesa, M. 2006. Assessment of seismic resistance of conventional wood frame houses. Proceedings of 2006 Structural Congress, St. Louis, Missouri.


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