Tests on Partially Anchored Wood-Framed Shear Walls

Transcription

1 Tests on Partially Anchored Wood-Framed Shear Walls Ulf Arne GIRHAMMAR Umeå University Sweden 1980 Dr. Eng.; 1981 Assoc. Prof.; 1987 Eng. Consultant, wood and building industry; 1989 Adjunct Prof Senior Specialist, mechanics industry. Presently at Umeå University. Bo KÄLLSNER Växjö University Sweden 1977 Dr in Civ Eng. Since 1975 at the Trätek - Swedish Institute for Wood Technology Research in Stockholm. Since 2001 also at Växjö University as Adjunct Prof. Summary The structural behaviour of wood-framed shear walls primarily depends on the sheathing-to-timber joints, the anchorage of the studs and bottom rail, the vertical loads on the studs, and the intercomponent connections between the shear wall and the surrounding structures. Tests have been conducted to study the racking resistance of partially anchored shear walls and to verify an analytical plastic model. In the paper test results of shear walls of different sheet materials, configurations, anchoring conditions and load applications are described. Fully and partially anchored shear walls were studied by varying number of segments and magnitude of vertical loads on studs. Test results show significant reduction of racking resistance due to partial anchorage of the walls and they are fairly well in agreement with results of the analytical model. Key words: Wood-framed shear wall, partially anchorage, racking resistance, racking test. 1 Introduction 1.1 Background For the horizontal stability of multi-storey buildings, diaphragm action in the walls is of vital importance. Shear walls are the main structural elements used to resist forces of wind and seismic loads. In wooden houses, such wall panels usually comprise of wood-based sheets nailed on a timber framework. The structural behaviour and capacity of wall diaphragms are primarily dependent on the sheathing-to-timber joints, the anchorage of the studs and bottom rail, the magnitude of the vertical loads on the studs, and the inter-component connections between the shear walls and the surrounding structures. The sheathing-to-timber joints are studied in a companion paper [1]. Analytical models that are capable of quantifying the effect of partial anchorage of shear walls on the racking capacity are presented in another companion paper [2]. Racking resistance of timber-framed shear walls are usually (in Sweden) determined either by calculation using a simplified elastic procedure or a lower limit value of the plastic capacity [3], or by testing of prototype structures [4]. However, these methods are not consistent between themselves with respect to the boundary conditions. The analytical procedure is based on the assumption of fully anchored shear walls, i.e. fully restrained studs and bottom rail, while the testing is carried out on partially anchored shear walls, where the degree of anchorage depends on the magnitude of the applied vertical loads on the studs. The highest racking capacity of a shear wall is attained when the leading stud is fully anchored. In case of partial anchorage, the studs on the tension side of the wall panel are subjected to substantial vertical uplift. Fasteners located along the bottom rail on the tension side of the test panel will then be subjected to vertical tension forces, which strive to draw the sheet apart from the bottom rail. These tensile forces will act almost perpendicular to the edge of the sheet (and bottom rail). The strength of the joint is often considerably lower when the fastener forces act perpendicular to the edge of the sheet and bottom rail than when they act parallel to the edge. This is especially true when the edge distances of the fasteners in the timber member and the sheet are small. 1001

2 1.2 Objectives The purpose of this paper is to present the results of tests that have been conducted to study the racking resistance and structural behaviour of partially anchored shear walls and to evaluate the proposed analytical model [2], which is based on the assumption of plastic behaviour of the sheathing-to-timber joints [1]. The focus of the paper is to address the behaviour and capacity of sheathed wood-framed shear walls of different sheet materials, configurations, anchoring conditions and load applications. Specifically, fully and partially anchored shear walls were studied by varying the number of segments and the magnitude of vertical loads on studs. 2 Testing Program 2.1 Specification of Shear Walls Tested Racking tests of shear walls with different sheathing materials, anchorage conditions, and vertical load configurations have been conducted. One series refers to three segment shear walls of different sheathing materials and with fully or partially anchored leading stud, see Table 1, Series 1. Other series refer to shear walls made of hardboard, where the influence of partial anchorage versus the number of segments (Series 2) and the degree of partial anchorage versus the magnitude of the vertical loads, Series 3 and 4, respectively, are investigated. Table 1. Specifications of shear walls tested. Bottom rail was fully anchored to the substrate. Series Configuration Load application Sheathing material Diagonal loading = D Fully anchored leading stud 1 H Horizontal loading = Partially anchored wall Particleboard Plywood 2 Each type loaded diagonally and horizontally 3 V V = 3.23 kn V = 6.46 kn 4 V V = 1.29 kn V = 3.23 kn V = 6.46 kn 2.2 Test Specimens and Testing Procedure All walls tested were made of sheathing fastened to a timber frame. Only one side of the frame was sheathed. The test specimens were designed as follows: Frame members: Pine (Pinus Silvestris), C24, mm, stud spacing 600 mm. Sheathing: mm. (a), C40, 8 mm (wet process fibre board, HB.HLA2, Masonite AB). (b) Particleboard, V100, 12 mm (Byggelit AB). (c) Plywood, P30, 9 mm ( ) (Spruce, Picea Abies, Schauman Wood Oy). 1002

3 Nails: Annular ringed shank nails, mm, f y MPa (Duofast, Nordisk Kartro AB). The joints were hand-nailed and the holes were pre-drilled, 1.7 mm, in case of hardboard. Nail spacing was 100 mm along the perimeter and 200 mm along the vertical centre lines of the sheets. Edge distance was mm along the vertical studs and 22.5 mm along the bottom and top rails. Framing joints: Two annular ringed shank nails of dimension mm were applied in the grain direction of the vertical studs. All tests were performed under deformation control with a constant rate of 8 mm/min corresponding to a time to failure of about 5 minutes. The diagonal load was applied as a tension force on each side of the frame. The horizontal load was applied as a compression force by a hydraulic jack in line with the top rail. The vertical loads were applied as compressive forces at the top of the studs by manually controlled hydraulic jacks. The bottom rail was prevented from moving horizontally by a stop at the trailing stud. For each test, the density and moisture content have been determined and a choice of fasteners has been tested. However, in this paper no attempt is made to adjust the test results with respect to these parameters. 3 Presentation and Evaluation of Test Results 3.1 Fully and Partially Anchored Shear Walls with Different Sheathing Materials (Series 1) General The test results for the various sheathing materials are summarized in Table 2 (4 tests with hardboard and 3 tests with particleboard and plywood). The different wall configurations have been loaded diagonally (F = fully anchored leading stud) and horizontally (P = partially anchored wall). The experimental (1) and theoretical (2) mean values for the maximum load are presented in the table. For theoretical results, which are based on a plastic analytical model, see the companion paper [2]. The comparison between experimental and theoretical values is given as the quotient (1)/(2). The mean values of the displacement at maximum load (3) and at final failure (4) are also shown in Table 2. The mean values of the density and moisture content of the frame members (5) and sheets (6) are given in the table. The plastic capacity per unit length of the sheathing-to-timber joints (7) is based on test results for maximum load as given in reference [1] and on a centre distance of nails of 100 mm. In the last column of Table 2, the overall type of failure mode of the wall during the unloading part of the load-displacement curve after the maximum load has been reached is given. Table 2. Mean values of test results for shear walls with different sheathing materials. Series 1 Maximum horizontal load H [kn] ment [mm] [kg/m 3 ] Content [%] capacity overall Displace- Dry density Moisture Plastic Character of (1) (2) (1)/(2) (3) (4) (5) (6) (5) (6) (7)* failure mode F > Ductile P Semi-ductile Particleboard F > Semi-brittle P Semi-brittle Plywood F Brittle P Semi-brittle * Plastic capacity of the sheathing-to-timber joints, [kn/m], cf. [1]. As is evident from Table 2, for shear walls with hardboard the experimental values are on the safe side; the experimental values are 12% higher than the theoretical ones in case of fully anchored leading stud and 29% in case of partially anchored wall, respectively. For particleboard and plywood, the experimental values are on the unsafe side. Due to the brittle type of failure of the shear wall, the analytical model is not fully applicable in those cases. As presented in the companion paper [1], the joint characteristics are not of a ductile type in case of particleboard and plywood. With other kinds of fasteners, shear walls with particleboards and plywood, respectively, 1003

4 can be made to behave in a ductile way. Another reason for the values on the unsafe side for particleboard and plywood is that the plastic capacity of the sheathing-to-timber joints, as presented in the companion paper [1], is determined for test specimens with higher density of the frame members than of those used in the shear wall tests The test results for fully and partially anchored shear walls with hardboard are presented in Table 2 [5]. A typical failure mode around the bottom corner at the leading stud in the case of a fully anchored leading stud (diagonal load) is shown in Figure 1a. A ductile type of joint failure by yielding and withdrawal of nails took place, where the main direction of the nail forces was parallel to the grain direction of the frame members. The final failure mode of the wall is ductile. For a partially anchored shear wall (horizontal load), the failure mode is shown in Figure 1b. The joint failure was ductile by nail yielding and withdrawal. The direction of the nail forces was essentially perpendicular to the bottom rail. No failure of nails took place along the leading stud. The final failure of the wall was ductile, but the load decreased more rapidly than in the case of fully anchored shear walls (semi-ductile). (a) (b) Figure 1. Failure modes: (a) Fully anchored shear wall; and (b) Partially anchored shear wall Particleboard The test results for fully and partially anchored shear walls with particleboards are presented in Table 2 [6]. Fully anchored shear walls: The failure of the walls at maximum load was of a semibrittle character, meaning that after maximum load is reached the softening part of the curve is fairly steep. Particleboard is a much softer material than hardboard. Therefore, most joint failures occurred as punching of nail heads through the sheet (and occasionally tearing failure of the corner). Partially anchored shear walls: The mode was the same as for fully anchored walls, but with much less deformability. Again, joint failures occurred as punching of the nail head through the sheet (together with tearing failure of the corner) Plywood The test results for fully and partially anchored shear walls with plywood are presented in Table 2 [6]. Fully anchored shear walls: The failure of the walls at maximum load was brittle. Most joint failures occurred as punching of head through the sheet. The type of nail head used in this case was not optimal. Partially anchored shear walls: The failure mode of the walls in this case became more ductile, going from brittle to semi-brittle. 3.2 Fully and Partially Anchored Shear Walls with Different Number of Segments (Series 2) Shear walls with hardboards show ductile load-displacement characteristics. Therefore, the theoretical model presented in the companion paper [2] can be evaluated. This is the main objective of sections 3.2 and 3.3. Mean values of the test results for fully (diagonal load) and partially (V = 0) anchored shear walls with hardboards and with one, two, three, and four segments are summarized in Table 3 [5, 7]. The failure modes are the same as those discussed in section (however, in these tests there were 1004

5 more nail failures in the case of four segments). The plastic capacity of the sheathing-to-timber joints, f p, used to compare experimental and theoretical values, are evaluated by using the test results for the fully anchored two segment wall as a reference wall, i.e. f p = 29.9/2.4 = 12.5 kn/m. Table 3. Mean values of test results for shear walls with hardboards and with different number of segments and vertical loads. Vertical loads according to Table 1. Series 2, 3, and 4 Maximum load Dry density [kg/m 3 ] Moisture Content [%] Number of tests [kn] Timber Sheet Timber Sheet One Diagonal load segment V = Two Diagonal load segments V = Three Diagonal load segments V = Four Diagonal load segments V = Two V = 3.23 kn segments V = 6.46 kn Four V = 1.29 kn segments V = 3.23 kn V = 6.46 kn The relative horizontal capacity (H/mf p b = l eff /l) versus the number of segments (m) is shown in Figure 2a, where b is the width of the sheet, l = m b is the total length of the wall, and l eff is the so called effective length. This effective length is defined as that portion of the length of the wall that is active in resisting the horizontal load. For a wall with a fully anchored leading stud (diagonal load) the effective length equals the total length. Relative horizontal capacity H /mf pb = l eff/l Full anchorage Partial anchorage (a) Number of segments m Relative horizontal capacity H /mf pb = l eff/l Series 2 Series 4 Full anchorage (b) Relative vertical load V /f p h Figure 2. (a) Effect of partial anchorage on horizontal load-bearing capacity with respect to the number of segments (Series 2); and (b) Anchoring effect of vertical loads on horizontal load-bearing capacity (Series 3 and 4). Solid lines denote theoretical values, see [2], and solid points experimental mean values. As is evident from Figure 2a, there is good agreement between the theoretical and experimental (dots) values for fully anchored shear walls. Also, in case of partial anchorage, the experimental values (squares) are closely related to the theoretical ones (thick solid line). If the effect of the framing joints (maximum tension load, 1 kn) is taken into account (thin solid line), the agreement is improved. 1005

6 3.3 Fully and Partially Anchored Shear Walls Subject to Different Vertical Loads (Ser. 3, 4) Mean values of the test results for shear walls with hardboards and subjected to vertical load are also presented in Table 3 [5, 7]. Series 3 refers to two segment walls and series 4 to four segment walls. The relative horizontal capacity (H/mf p b = l eff /l) versus the relative vertical load (V/f p h) is shown in Figure 2b, where h is the height of the sheet. The reference value f p h = 30 kn is the maximum vertical load that can be transferred in the sheathing-to-timber joints along a stud. From Figure 2b it is evident that the experimental values (squares) in the case of two segment walls with vertical loads on the leading stud (Series 3) are on the safe side of the theoretical ones (thick solid line). The same goes for four segment walls with vertical loads on all studs (diamonds). Again, if the effect of the framing joints is taken into account (thin solid lines), the agreement is improved somewhat. The fact that experimental values are higher than the theoretical values according to Figure 2b, may be due to the shearing capacity of the framing joints (magnified on studs loaded in compression). 4 Comments and Conclusions There is a fairly good agreement between theoretical and experimental results for fully and partially anchored shear walls with hardboard where the sheathing-to-timber joints show plastic characteristics. All values are on the safe side. The plastic model is a lower bound method. The effect of partial anchorage of shear walls with hardboard versus the number of segments is clearly demonstrated and the experimental values show close agreement with theoretical results. The anchoring effect of the vertical loads is also demonstrated. In this case the experimental values are much on the safe side compared with theoretical values. 5 Acknowledgement We gratefully acknowledge the assistance of Jonas Eltoft, B.Sc. and Samuel Palm, B.Sc., who performed the tests at Umeå University, Department of Applied Physics & Electronics, Civil Engineering. This work is part of a Nordic Wood project on panel structures. 6 References [1] Girhammar, U.A., Bovim, N.I., and Källsner B., Characteristics of sheathing-to-timber joints in wood shear walls, 8 th World Conference on Timber Engineering, Lahti, Finland, [2] Källsner, B. and Girhammar, U.A., A plastic lower bound method for design of wood-framed shear walls, 8 th World Conference on Timber Engineering, Lahti, Finland, [3] Källsner, B. and Lam, F., Diaphragms and Shear Walls, Holzbauwerke nach Eurocode 5 Step 3, Arbeitsgemeinschaft Holz e.v., Düsseldorf, Germany, 1995, pp. 15/1-15/19. [4] European Standard EN 594 Timber structures Test methods Racking strength and stiffness of timber frame wall panels, December 1995 [5] Eltoft, J. and Palm, S., Tests of Wood-Framed Shear Walls at Partial Anchorage With and Without Openings (in Swedish), Umeå University, Department of Applied Physics & Electronics, Civil Engineering, Report 2003:1, Umeå, Sweden, [6] Eltoft, J. and Palm, S., Tests of Partially Anchored Wood-Framed Shear Walls with Different Sheet Materials (in Swedish), Umeå University, Department of Applied Physics & Electronics, Civil Engineering, Report 2002:2, Umeå, Sweden, [7] Eltoft, J., Tests of Wood-Framed Shear Walls at Partial Anchorage (in Swedish), Umeå University, Department of Applied Physics & Electronics, Civil Engineering, Examination Thesis, BY0216, Umeå, Sweden [8] Eltoft, J., Tests of Anchoring Fittings (in Swedish), Umeå University, Department of Applied Physics & Electronics, Civil Engineering, Report 2003:3, Umeå, Sweden,