Effect of Openings on the Racking Strength of Structural Log Walls

Abstract

The racking performance of log home construction under lateral loads became a concern after the 1999 great earthquake in central Taiwan. Racking tests were performed on western redcedar 94.5- by 83.8-inch D-log walls assembled with lag screws. The purpose of this study was to evaluate the effects of wall thickness and window and door openings on the horizontal shear strength of log home walls. The results indicated that the horizontal shear strength of D-log walls assembled with nominal 6-by 6-inch D-log members was about twice that of walls assembled with nominal 4- by 6-inch members. The horizontal shear strength of walls with 28.1 percent window openings was 28 percent lower than a similar wall without openings, but 69 percent higher for walls with 14 percent openings due to reinforcement by additional lag screws around the openings. The horizontal shear strength of walls with door opening rates of 30.5 percent and 60.9 percent were 26 percent and 72 percent, respectively, lower than that of similar walls without any opening. Based on the failure mode of D-log walls, the performance of withdrawal and lateral resistance of lag screw joints was shown to play an important role for walls subjected to lateral load.

In recent years, wood-framed housing construction has become prevalent in Taiwan, especially in recreational areas and the countryside. The major motivation to replace the materials currently used in residential houses, such as brick and reinforced concrete, with wood is the desire for a healthier living environment. The majority of structural lumber products are imported from North America. According to a survey, consumers prefer log homes instead of light-frame construction for recreational purposes. Western redcedar is preferred for wall materials over hemlock, pine. Douglas- fir, and yellow cedar due to its overall durability, machining quality, and cost, based on recent construction experience. Ninety percent of people believe the living quality of wood-framed houses is superior based on its advantages of comfort, health, insulation, light, and ventilation (Yeh 1999).

An investigation of building damage performed after the 1999 Gi- Gi great earthquake, which was 7.3 on the Richter scale, found that poor connections between wood purlins and clay walls, large stiffness differences between wood beams and stone posts, and deterioration of wood members due to decay and termite attack were the major causes of failure in historic or traditional buildings (Yeh 2000a). Splitting always occurred at the corner of window and door openings on clay walls, due to stress concentration, even though they are quite small in traditional buildings. Although 90 percent of the 8,487 damaged buildins cases were classified as brick, reinforced concrete, or clay structures, a few newly constructed wood-framed houses near the faults were also damaged. Fiftyseven percent of the damaged wood structures had deformed or destroyed wall sections, including 35 percent with damage around wall openings (ABRl 1999. Yeh et al. 2001 ). Similar failure modes in windows, doors, and garage doors were also found in the 1994 Nortrmdge earthquake and contributed to weak resistance to seismic forces by wood diagonal bracing or steel strapping (Andreason and Rose 1994).

It is believed that log homes perform well during earthquakes, except for weak wall openings. Yeh investigated the racking strength of log walls fabricated with inside-out machined lumber elements, i.e.. half-sawn and quarter-sawn profile. They showed 197 percent and 90 percent higher racking strength, respectively, than wood- framed structural walls sheathed with 9-mm-thick plywood (Yeh 1998, 2000b). Chuang et al. (2002) reported that the racking strength of plywood sheathed walls with 63- and 78.7-inch-wide window openings was reduced by 17.8 percent and 14.7 percent, respectively, compared to wafls without any opening. Also, reductions of 10.5 percent, 46.8 percent, and 68.5 percent in racking strength of walls with 31.5-, 78.7-, and 110-inch-wide door openings, respectively, were found. No significant difference in racking strength was found for the wall with door openings located at various positions relative to the loaded comer. Many detailed results can also be found for various wood-framed wall systems with different species of wood (Kawai 1998. Ni et al. 1999. Jang 2000), but there has been little research on the racking performance of log home wall systems with openings. This study investigated the effect of window and door openings on the racking strength of log home wall systems made with horizontal machined western redcedar log members, which is the common construction approach used in Taiwan.

Material and methods

Material

Western redcedar (Thuja plicaia) from North America with sizes of nominal 6 by 6 inches and nominal 4 by 6 inches were machined into D profile cross sections (D-tog), i.e., one side of the lumber is flat and other side has a round surface. Each log had two tongue and groove configurations on the top and bottom surfaces for sealing during wall stacking. The width and height of the tongue were 1/2 inch and 5/16 inch, respectively. All of the logs were air-dried before wall assembly.

Lag screw joint tests

The length of the lag screws was 7.875 inches with a 5-inch-long spiral shank and a diameter of 0.375 inch. The penetration depth for the withdrawal resistance test of the lag screwjoint was 2.75 inches, the same depth as used in the wall assembly, with a testing speed of 0.08 in. min. The schematic testing diagram for the lateral resistance of a lag screw joint is shown in Figure 1. The testing speed was 0.12 in/min. The moisture content (MC) of D-log lumber elements was measured using an electrical resistance-type device (Hydramatte H35, GANN, Germany).

Wall assembly

The D-log lumber elements were stacked horizontally into 94.5- inch-wide and 83.8-inch-high wall sections with lag screws. The end distances of lag screw joints at each end of the wall section were 3.5 inches and 4.7 inches alternately for each layer of D-log lumber elements, and a third lag screw was located at the center of the wall section as shown in Figure 2. This makes the maximum spacing 39.8 inches for lag screw joints at the same layer of lumber stacking. In the ease of window and door openings, lag screws are fastened at both sides of the opening for reinforcement. The lag screw was tightened by an air-driven wrench, and the head of the lag screw and washer were sunk beneath the surface of the lumber with the point penetrating 2.8 inches into the next D-!og member. Two width sizes of window and door opening were used in this study, i.e., 35.4 inches and 70.9 inches, respectively, as shown in Table 1 and Figure 3. The heights of window and door openings were 31.4 inches and 68.1 inches, respectively. A total of six configurations of wall section were assembled and each treatment had two replications.

Test procedure

The D-log wall specimens were mounted on the shear wall testing machine with bolts securing the bottom layer of the lumber element to the steel frame. Two sets of lateral supporting wood frames were installed near both ends of wall specimens to prevent lateral buckling deformation. A pressure roller was installed on top of the wall specimen at the load application end to prevent wall tilting. Six guiding rollers were against both sides of the upper wall section to prevent lateral twisting during load application.

The racking test procedure for D-log walls was performed according to ASTM E564 and E72 (ASTM 1995a, 1995b). The loads were applied at the top of wall specimens up to 790, 1,570, 2,360 poundf for the first, second, and third stages, respectively, at a uniform rate. The lateral residual deformation was recorded after the force was removed for each step. The lateral load was then applied to the wall until failure occurred at the fourth stage, and maximum load with corresponding lateral deformation and failure mode were recorded. The testing speed was set to complete four stages within 30 minutes. The maximum horizontal shear strength, Su, of the D-log walls was calculated as:

Su = Pu/b - max. load level/width of wall specimen [1]

and shear stiffness, G, is computed as:

G = (ΔP/ΔP) (a/b) [2]

where: ΔP is the differential lateral force between 550,115 poundf and 770,157 poundf estimated from each stage of load application and Δd is the corresponding lateral displacement of the wall specimen. The height and width of the D-log wall specimen are designated as a and b.

Results and discussion

The average MC of the western redcedar lumber elements of the wall section assemblies was 35.2 1.4 percent. The major fastener used for holding the D-log elements together was lag screws, since top-down bolting or long bolt application is limited to only a few builders in Taiwan. The evaluation of joint performance was based on the same holding depth of lag screws in the wall assemblies. The results indicated that the maximum lateral resistance of a lag screw was 419 42 poundf, and the lateral slip of the joint at maximum load is 1.06 inch\es. The withdrawal resistance of a lag screw penetrating to a depth of 2.75 inches in western redcedar lumber is 740 185 poundf/in.. or 2044 503 poundf in maximum load for a single lag screw joint.

Horizontal shear- strength for D-log wall

As a D-log wall specimen is subjected to monotonic test (Fig. 4), the lateral load applied at the top corner is transmitted in the form of horizontal shear through lag screws between each layer of horizontal wall elements. The load path is different from a typical panel sheathed wood-framed wall when subjected to wind or earthquake loads. The results shown in Figure 5 indicate that the maximum horizontal shear strength of a D-log wall specimen fabricated with nominal 6- by 6-inch western redcedar lumber is 785 poundf/ft, which is 59 percent higher than that of structural light-framing walls sheathed with 3/8-inch plywood panels with 6-inch nail spacing along studs and 2-inch spacing along the bottom plate (Yeh 1998). The study also indicates that the maximum horizontal shear strength of walls constructed with European red pine studs in 16-inch spacing was 15 percent higher than that in 24-inch spacing, and 12 percent higher for a 3.8-inch plywood sheathed wall than that of a 9/32- inch plywood sheathed wall. The maximum horizontal shear strength of a 6-inch-thick D-log wall section is about twice that of a 4-inch- thick log wall. Besides the better tongue and groove joint performance for 6-inch-thick wall, this might contribute to a thick log wall acting as a cantilever beam with a better slendemess ratio than that of a thin wall.

The major failures of D-log walls subjected to lateral loads are shown to occur at the lower portion of the wall on the same side as the load application and act as a deformed cantilever beam. Similar failure can be found in plywood and oriented strandboard sheathed stud walls but is not the major cause of wall failure (Yeh 1997, 1998, 2000a. 2000b). There is tensile force between horizontal elements of a D-log wall due to the bending moment as the wall acts like a vertically erected cantilever beam. The lag screw joints in the second and third lumber elements from the bottom wall on the side of the load application are withdrawn vertically. The resisting moment estimates based on the evaluated withdrawal values of lag screws are well matched to the overturning moment resulted from lateral load application on the top of walls with a 4 percent deviation. Therefore, the racking performance of a D-log wall assembly is expected to be attributed to the withdrawal strength of the lag screw and can be improved by increasing the penetration depth of the lag screw. Furthermore, there is larger horizontal displacement for the 13th and 14th lumber elements from the bottom due to shear deformation of the lag screws at the joints in the upper part of the wall.

Effect of window openings

The comparison of horizontal shear strength of D-log walls without openings and with various window opening sizes was observed. The results indicated that the horizontal shear strength of walls with 70.9-inch-wide opening or 28.1 percent of opening space was 28 percent lower than that of walls without any opening (Fig. 5). Walls with 35.4-inch-wide openings, however, were 69 percent higher in horizontal shear strength than walls without openings, which indicated that the effect of opening space for the wall was insignificant since the value is small, such as 14 percent in this case. Actually, the bracing of the wall was increased as additional lag screws were put at the end of lumber elements on both sides of window openings during wall assembly. The net horizontal shear strength of walls with window openings is based on the length of full-height segments of wall sections, i.e., the overall length of the wall minus the width of openings. The higher values of net horizontal shear strength for walls with window openings than for those without openings indicated that the wall sections above and below a window opening may contribute certain lateral resistance. It was noted that the average of 1.8 times horizontal shear strength can be obtained for walls with window openings by ignoring the window width. These results may be useful for shear wall design.

The shear stiffness of walls with window openings estimated at each stage of lateral load application varied significantly. The shear stiffness measured at the second, third, and fourth stages was 34 percent, 54 percent, and 98 percent higher, respectively, than that at the first stage of lateral load application (Table 2). The shear stiffness of walls showed better performance during the fourth stage of load application, while stiffer results were found during the third stage for plywood sheathed walls in previous studies. For walls without any opening, the average shear stiffness estimated from second, third, and fourth stages was 163 percent higher than that at the first stage of load application, which indicates that the performance of shear walls assembled with lag screws becomes significant when subjected to a series of higher lateral loads.

The failures of wall specimens subjected to lateral loads were always initiated at the corner of a window opening. The lag screw joints failed in withdraw resistance at the sixth and seventh layers of short lumber elements. The lag screws located above a window opening were also laterally deformed due to horizontal shear that was resisted through lag screw joints. It was determined that an improvement in the racking behavior of D-log walls might be achieved by using larger sizes or more lag screws. Similar to the full wall specimens, failures at the lower portion of the wall and at the same side as the load application also occurred.

Effect of door openings

The maximum shear strength of walls with a 35.4-inch-wide door opening or 30.5 percent of opening space was 26 percent lower than that without any opening. The maximum shear strength of walls with a 70.9-inch-wide door opening or 60.9 percent of opening space was 72 percent lower than that without any opening and 24 percent lower than that with a 35.4-inch-wide door opening. The net horizontal shear strength of walls, however, with door openings still can compete with those walls without any opening. The walls with door openings all failed before finishing the fourth stage of lateral load application (Fig. 6), and shear moduli were considerably lower than the walls with window openings measured at the first stage, i.e., 37 percent and 31 percent lower, respectively, than that of walls with 35.4-inch- and 70.9-inch-wide window openings (Table 2). Larger lateral displacement was found for walls with door openings during each stage of monotonic testing compared to walls with window openings or without any opening.

Failure occurred at the lower corner of a door opening on the same side as the lateral load application, accompanied with failure at the diagonal corner. The lag screws were pulled out vertically at the joints between the first and second lumber elements from the bottom. The lag screw joints along the narrow wall section near the door opening also showed large deformation and each layer of lumber element was displaced horizontally, becoming step stacked. Larger horizontal displacement resulted for the lumber elements above a door opening.

Conclusions

Racking tests on western redcedar D-log walls assembled with lag screws showed better horizontal shear strength than conventional plywood sheathed light wood-frame walls and also related to the thickness of the wall sections. The withdrawal and lateral resistance of lag screw joints played an important role for D-log walls subjected to lateral loads. The improvement in the racking performance of a D-log wall might be achieved through the size, number, and penetration depth of lag screws. The D-log wall allowed for small openings without weakening the racking performance, due to the reinforcement by additional mechanical fasteners around the openings; however, the horizontal shear strength of walls with large openings was reduced significantly.

Literature cited

American Society for Testing Materials (ASTM). 1995a. Standard test methods of conducting strength tests of panels for buildine construction. ASTM E 72-95. ASTM, West Conshohocken, PA.

_____. 1995b. Standard practice for static load test of shear resistance of frame walls for buildings. ASTM E 564-95. ASTM, West Conshohocken, PA.

Andreason, K.R. and J.D. Rose. 1994. Northridge, California earthquake: Structural performance of buildings in San Fernando Valley, California (Jan. 17, 1994). Rept. T94-5. American Plywood Assoc. (APA). APA, Tacoma, WA, 34 pp.

Architecture and Building Research Institute (ABRI). 1999. Initial report: Survey on the damages of buildings for 921 Great Gi- Gi Earthquake. ABRI, Ministry of Interior, ROC. 177 pp.

Chuang, C.C., M.C. Yeh, C.P. Lin, S.H. Hung, and Y.M. Chen. 2002. Effects of openings on horizontal shear resistance of structural light framing shear wall. Quarterly J. of Chinese Forestry 35(3):309- 320.

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Ni, C., E. Karacabeyli, and A. Ceceotti. 1999. Design of shear walls with openings under lateral and vertical loads. In: Proc. of the Pacific Timber Engineering Conf. Rotorua, New Zealand, pp. 360- 367.

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Min-chyuan Yeh*

Chi-lung Chiang

De-tsai Lin

The authors are, respectively, Professor, Associate Professor, and Graduate Student, Dept. of Wood Science and Design, National Ping-tung Univ. of Science & Technology, Nei Pu, Ping Tung, Taiwan, R. O. C. (yehmc@mail.npust.edu.tw; clchiangt@mail.npust. edu.tw; mm9135007@yahoo.com.tw). This project was funded by the National Science Council. This paper was received for publication in April 2004. Article No. 9877.

*Forest Products Society Member.

Forest Products Society 2006.

Forest Prod. J. 56(11/12):137-141.

Copyright Forest Products Society Nov/Dec 2006

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