A Performance Assessment of Flood-Damaged Shearwalls

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1 A Performance Assessment of Flood-Damaged Shearwalls RJ Leichti R Staehle & DV Rosowsky Department of Forest Products Oregon State University USA Summary: Flood events affect more people than any other form of natural disaster. Flooded structures are partially submerged and then re-dried, which leads to swelling and shrinking over an arbitrary wall height. One repair scheme is to remove the interior cladding (drywall), dry the wood system, and re-install new drywall. This investigation was conducted to evaluate the effects of long-term water exposure on the mechanical properties of oriented strand board (OSB) sheathing and the impact of those changes on the capacity of light-frame shearwalls. OSB was submerged for up to seven days, air-dried and then tested for embedment strength, shear strength through the thickness, and shear modulus. Nine walls were built for the test program. Three were tested at ambient conditions and monotonically loaded in shear to establish the displacement criteria for the quasi-static methods; three walls were tested at ambient conditions (control walls) using a quasistatic loading protocol; and three were submerged in 1 m of water for seven days, re-dried, and then tested using the same quasi-static protocol as the control walls. The results from the OSB material tests showed that most of the material property degradation occurred in the first 48 hr of submersion, for example, the embedment strength was reduced by almost 40 percent after 48 hr and showed no further reduction even with 120 hr of additional soaking. The shearwall test results did not follow the materials results -- the quasi-static shearwall tests showed that water submersion did not reduce shearwall capacity, the energy absorption of the wall, or change the yield mode. However, shearwall stiffness was reduced, apparently the result of reduced embedment stiffness of the OSB sheathing. Keywords: Shearwall; monotonic testing; quasi-static testing; timber; durability 1 INTRODUCTION Durability of structures has been the focus of study and design innovation in recent years. The design engineer implements design features and materials that, when used with appropriate construction methods, yields buildings that have long-term durability (TenWolde and Rose 1994). The presumptive design assumption is that the condition of the building system will remain unchanged throughout its service life. However, after a flooding event, there may be a loss in capacity to resist major loading events. Designing for durability requires quantification of the changes in the mechanical properties and resulting performance of the structure. Condition assessment combined with quantified data on the impact of the given material and connection condition provides a vital link to predicting residual structural capacity to resist future loading events given the existing level of damage that is revealed by physical inspection. At present, durability assessment has incorporated only the observed condition of the materials and essentially a go/no-go basis for replacement. It would be desirable to include the existing levels of damage in the framing and sheathing components in the fragility of residual life assessment of a structure after a natural disaster. Flooding is a natural disaster that affects more people world wide than any other form of natural disaster (Hausmann & Perils 1998). Geo-scientists have reported that floods account for 23 percent of global natural catastrophes but accounted for 67 percent of casualties and caused 53 percent of the economic losses in the US (Munich Re 2001). The insurance industry has many definitions of flooding because flooding can originate from many different events and takes many forms. In general, the insurance industry agrees that a flood is a general and temporary inundation of normally dry land from the overflow of inland or tidal water, rapid accumulation of runoff of surface waters, mud flows or mud slides, or waters due to the collapse of shores and retaining systems. Data reported by Hausmann and Perils (1998) showed that most flood events in buildings involve water depths less than 2 m. The number of affected properties can be significant, as in 1993, when approximately100,000 houses were damaged by flooding in the Mississippi River Basin. Thus, flooding represents a potential hazard to the durability and serviceability of residential and industrial buildings. 9DBMC-2002 Paper 050 Page 1

2 1.1 Technical Background Flooding in residential structures has the potential to affect the sheathing, the wood frame and the nailed connection. The literature has many reports of accelerated aging tests and durability studies of wood-based panel products. The various accelerated aging protocols are generally correlated to a given number of years in service under a stated exposure condition. One of the most important factors causing deterioration or strength reduction of wood-based panels is the change in moisture content (Suchsland 1982). Changing moisture content leads to permanent thickness swelling that consists of two parts: swelling of the wood and release of compression stresses that were induced into the panel at the manufacturing process (McNatt 1982). The standard methods of test for water absorption and thickness swelling and moisture-related effects on wood-based panel materials are based on 24 and 48 hr tests. However, flood events do not correlate to accelerated aging protocols, and flood events can last much longer than the test protocol durations. Hence, a method of testing the effects of water exposure for longer periods than 48 hr may be needed. Shearwalls and nail connections between sheathing and the wood framing have been studied extensively. Neisel & Guerrera (1956) showed that racking strength and lateral nail resistance vary exponentially with moisture content and that racking strength is correlated to lateral nail resistance. Toumi & McCutcheon (1978) used energy method to develop a method of predicting racking strength of light-frame walls. Patton-Mallory & McCutcheon (1987) predicted racking displacement-load behavior of a shearwall from the known behavior of the fasteners and sheathings that gave accurate predictions for small-scale walls. Their models showed that wall displacement was the sum of fastener deformation and displacement due to shear deformation in the sheathing. Shear deformation in the sheathing contributed only 10 to 15 percent of the wall displacement. Hence, most of the deformation in the shearwalls can be attributed to the nail deformation. Others have written more sophisticated models for nonlinear dynamic analysis; the most recent computer model incorporates shear modulus, nail hysteresis, and shear strength to predict load-displacement characteristics and energy dissipation under general cyclic loading (Folz & Filiarault 2001). The computer models and earlier closed-form models can assess the performance of shearwalls with nail and stiffness properties that are constant over the shearwall. The effect of a flood exposure over some arbitrary height of the wall would have to be approximated by evaluating the limits, which could be a wall that has not been exposed to water and a wall that was fully submerged. Nailed connection capacity and yield mode depend on the embedment strength of the main and side members, bending strength of the nails, and geometry of the joint (NDS 1997). The yield mode of a nailed connection can be managed by using different materials or by changing the geometry. Chou & Polensek (1987) studied damping and stiffness of nailed joints as they were affected by the drying of the green lumber using cyclic load tests of single nailed joints. Damping ratios and slip moduli were significantly smaller for joints with gaps than for those without gaps. Variations of interlayer surface roughness did not appear to significantly affect either damping or stiffness. Mohammad & Smith (1996) reported the results of a study with OSBlumber nailed connections that were subjected to various moisture cycling regimes between 5 and 15 percent moisture for samples made with dry lumber and between 5 and 12 percent for samples made with green lumber. The mechanical testing used a cyclic test protocol and showed that the moisture cycling had an adverse effect on connection stiffness. Then, if a structure was flooded, re-dried, and a gap developed between the stud and the sheathing as a result of shrinkage, the stiffness and damping of the building would be adversely affected. The Consortium of Universities for Research in Earthquake Engineering (CUREE) grew from the observation that woodframe structures did not fair well in some recent seismic events. They have sponsored many studies on shearwalls for the purpose of improving hazard mitigation. Researchers expressed a need for full-scale tests of on wood structures that use a standard design in order to make results comparable (Zacher 1999). For this reason, a quasi-static test method was developed by CUREE to assess the lateral force resistance and energy related properties of shearwalls. The method was adapted for this study. 1.2 Objectives The duration of water events often exceeds 48 hr, which is the period used to characterize moisture effects on wood-based panels. Then, an important structural issue exists: Does long-term water exposure cause a reduction in capacity of shearwall systems? It is logical to presume that significant changes in wall performance would result for structures that have experienced one or more water events where the sheathing has been wet. The specific objectives of the study were: To investigate the effect of long-term water exposure on the material properties of the shearwall constituents; To investigate the effect of long-term water exposure on static and energy-related properties of shearwall assemblies. 2 MATERIALS AND METHODS 2.1 Wood-Based Materials Twenty-seven panels of OSB were purchased from a local lumber yard. The OSB was APA rated sheathing and rated for EXPOSURE 1 (APA 2000), which means that the adhesive was considered to be fully waterproof. The panels were 11.9 mm (15/32 in.) thick and 1220 by 2440 mm in plane dimension. The lumber for the study was No.2 Douglas fir-larch, 38 by 89 mm, which by visual inspection was determined to be Douglas-fir. 9DBMC-2002 Paper 050 Page 2

3 2.2 Materials Properties Tests Five OSB panels were randomly selected for the materials properties tests and the remaining panels were used for wall construction. The five panels were cut in half resulting in ten half-sheets, 1220 mm in length and width. The engineering properties to be tested were embedment strength, edgewise shear strength, and shear modulus. These were to be tested following the standard methods given in ASTM (2001a, 2001b, 2001c). The embedment strength of the OSB was assessed in only the principal direction of the panel because it has been shown that embedment strength of small diameter fasteners is not affected by angle to the grain. The diameter of the nail used for the embedment tests was 2.9 mm. The edgewise shear strength was tested in both principal directions of the panels. Shear modulus was tested using square plate. The test was conducted on both diagonals of the plate and the result was the average of the two tests on each plate. The specimens were located on the replicate panels as shown in Fig 1. Each test and water exposure period was assigned to each half-sheet of OSB, so in the end, n = 10 for each of the exposure periods for each test. The treatments to be applied were various periods of water exposure: 0, 24, 48, 96, and 168 hr of submersion in fresh water. The specimens were soaked in a single batch and designated specimens were removed from the water at the specified times. The specimens were air-dried after the water exposure. Figure 1. Cutting diagram for materials properties testing. 2.3 Shearwall Preparation Nine shearwalls, 2440 by 2440 mm, were built like those by Krawinkler et al. (2000) with the exception of nail spacing, which matched the nail spacing of a recent study by Langlois (in preparation). The 38 by 89 mm studs were placed at 406 mm centers. The OSB sheathing was oriented vertically on the stud frame and pneumatically nailed with 8d box nails (60.3 by 2.9 mm). The OSB panels were installed with a 3-mm gap between them. The end studs were doubled and nailed together with 16d box nails (82.6 by 3.3 mm). The top plate was a double member while the sill plate was a single member. Tie-down straps were nailed to the end studs and bolted to the foundation 109 mm from the ends of the sill plates. Two other anchor bolts were located at 610 mm from each end of the sill plate. Three of the shearwalls were partially submerged in fresh water by placing them in a specially constructed tank so that the sill plate and the lower 1 m of the wall were below the water line. These were kept in the water tank for 168 hr (seven days). At the end of the water-exposure period, the walls were lifted out of the tanks and stood upright in a covered outdoor area to dry. A handheld resistance-type moisture meter was used to check the moisture content of the studs during the drying period. One wall was used as an indicator wall, that is, it was weighed several times during the drying period and used to indicate the dryness of all three walls. 2.4 Shearwall Tests Monotonic Tests. Three shearwalls were subjected to monotonic testing, which is a ramp load to failure in one direction. The racking load was applied to the top of the wall through a steel C-channel that was bolted to the top plate of the wall. The wall was attached to a foundation fixture with four anchor bolts. Four LVDTs for displacement measurements were positioned as shown in Fig 2. The LVDTs had an error of less than 0.1 percent. Force was measured by an electronic load cell mounted on 9DBMC-2002 Paper 050 Page 3

4 the end of the actuator. The displacement was applied at a uniform rate of mm/min. Load and displacement at the actuator and the LVDTs were recorded at 10 Hz. Control and data acquisition were implemented via Labview software. The properties to be determined from the monotonic shearwall tests were stiffness (N/mm), racking strength (kn), energy absorption (kn*mm), and the reference displacement (mm). The stiffness was calculated as the secant modulus between the origin and a point 0.8 of the maximum load (F max ) in the ascending portion of the displacement load curve. The racking strength was taken to be F max. Energy absorption was calculated as the area under the displacement-load curve between the origin and 0.8 F max on the degrading slope of the displacement-load diagram. The point 0.8F max on the degrading slope was defined as failure (Ceccotti 1999). The reference displacement was calculated as 0.6 times the displacement at failure. Steel bar (Load distributor) Direction of applied force Hydraulic actuator Top plate bolt LVDT and Load cell Measurement direction LVDT LVDT Concrete foundation Anchor bolt Tie-down bolt Figure 2. The shearwall test apparatus. Quasi-Static Tests. The quasi-static tests were a fully reversed-cyclic test that followed the protocol for ordinary ground motions (Krawinkler et al. 2000). The apparatus and method of load application was the same as for the monotonic tests. Data were recorded at 25 Hz. The time-displacement function was based on the reference displacement of the monotonic test. The test protocol began with a series of small amplitude initiating cycles. Then, each amplitude change was led by a single leading cycle followed by two or three trailing cycles at 0.75 of the current leading cycle amplitude. The time-displacement diagram for the quasi-static tests is given in Fig 3. 9DBMC-2002 Paper 050 Page 4

5 Defl (Inch) Time (sec) Figure 3. Time-displacement diagram for the quasi-static shearwall tests. The properties to be assessed by using the quasi-static shearwall test data were stiffness (N/mm), backbone energy absorption (knαmm), total energy absorption (knαmm) and racking strength (kn). The initial stiffness was calculated as the secant modulus of the cycle that included the maximum load. The line was drawn from the origin to a point at 0.8 of the maximum load in the ascending portion of the designated cycle. The backbone energy absorption was calculated as the area under the backbone curve extending from the origin to the displacement at 0.8F max on the degrading slope of the backbone loaddisplacement curve. The total energy absorption was calculated as the sum of the areas in all cycles up to the first post-peak primary cycle where the highest value was less than or equal to 0.8F max. Finally, the racking strength was the maximum load measured during any cycle of the test. 3 RESULTS AND DISCUSSION 3.1 Materials Properties The results of tests with OSB that was submerged in water from 0 to 168 hr are summarized in Table 1. The embedment strength, edgewise shear strength, shear modulus, and specific gravity follow the same trend most of the loss in properties occurred in the first 48 hr of water soaking. After that time, the losses were minimal. Water absorption at 48 hr was 43 percent, but by 186 hr water absorption had reached 73 percent. The residual thickness swelling, that is the thickness swelling after being re-dried, remained at the same level for soaking periods of 48 hr and greater. It is noted that the moisture content after re-drying did not return to the initial moisture content. In fact, the moisture content in the desorption process returned to only 11.5 percent, which was 4 percent greater than the initial condition. Some of the properties loss is attributed to the somewhat elevated moisture content. Embedment stiffness, the slope of the linear portion of the embedment-force diagram, shifted from 95 N/mm to 60 N/mm after 24 hr of water soaking and did not change with further soaking. To summarize the results of the materials investigation, it appears that embedment strength and shear modulus are most severely affected by the long-term water exposure. 9DBMC-2002 Paper 050 Page 5

6 Table 1. Summary of test results from OSB water exposure tests when the OSB was tested after re-drying. Parenthetical values are standard deviations. Property Water exposure period (hr) Embedment strength (MPa) 35.3 (13.06) 29.1 (4.92) 21.2 (6.66) 21.4 (7.62) 25.0 (6.25) Shear strength parallel (MPa) 11.1 (0.89) ---- a 9.7 (1.13) 7.9 (0.25) 8.6 (0.73) Shear strength perpendicular (MPa) 10.6 (1.73) 8.8 (0.53) 8.1 (0.61) ---- a 7.7 (0.76) Shear modulus (GPa) 1.8 (0.35) 1.3 (0.15) 1.2 (0.17) 1.2 (0.16) 1.1 (0.15) Moisture content b (%) 7.6 (0.42) 9.9 (0.36) 10.9 (0.41) 11.4 (0.67) 11.5 (1.24) Specific gravity (0.02) 0.55 (0.02) 0.53 (0.02) 0.55 (0.04) Water absorption (%) 22.2 (1.22) 43.2 (4.54) 61.8 (4.39) 73.1 (9.14) Thickness swell (%) 6.5 a problems with test fixture, data censored from data set. b moisture content of the embedment specimens when tested. (1.31) 12.1 (2.47) 15.0 (2.88) 14.1 (3.37) 3.2 Shearwall Tests The monotonic and quasi-static tests were conducted over a two-week period. The mean monotonic test results (n = 3) showed that the capacity of the walls was 29.1 kn, the energy absorption was 2759 kn*mm, initial stiffness was 870 N/mm, and the reference displacement was 71 mm. The monotonic test data were overlaid and are shown in Fig 4. After the submersion period, the shearwalls were weighed and reweighed again prior to testing. On the average, each wall absorbed 22.7 kg of water. The walls were considered to be air-dry when the weight of each wall returned to within 3 kg of the initial weight, which took about 14 days. Moisture content of the studs, as determined by using a resistance moisture meter, was found to be approximately 19 percent at the time of testing. Materials tests suggest that the moisture content of the OSB sheathing was probably near 12 percent, but it was not measured. Prior to testing the water-exposed shearwalls, they were visually inspected. It was found that the OSB surface was much rougher below the waterline than above it as a result of surface strand swelling. Also, surrounding each nail below the waterline, there was a circular dimple in the surface of the OSB where the nails had apparently restrained the thickness swelling. The nail heads did not pull through the surface of the OSB. This indicated that the pull-through capacity of the OSB exceeded the forces attendant to thickness swelling. It also showed that the forces related to thickness swelling did not exceed the withdrawal capacity of the nails. On the average, the dimples were 0.8 mm deep relative to the surface of the panel. A result of the residual thickness swelling was that the sheathing below the water line fit tighter to the wood studs after being submerged for 168 hr and re-dried than did the sheathing above the water line or on the control walls. 9DBMC-2002 Paper 050 Page 6

7 Load (lb) Figure 4. Displacement-load results for the monotonic tests. The results of the quasi-static testing of the control and water-exposed walls are summarized in Table 2. A typical hysteresis diagram for control and water-exposed shearwalls is given in Fig 5. The data indicated that the mean capacity of the shearwalls was not reduced by the water exposure. In fact, the backbone energy and total energy absorption were not reduced by the water exposure. However, the stiffness of the shearwalls was reduced by 27 percent. Table 2. Summary of the quasi-static tests of control and water-exposed shearwalls, n=3, where the parenthetical values are standard deviation. Property control Condition water-exposed F max (kn) 26.8 (3.2) 30.6 (1.7) F min (kn) (2.1) (1.0) Stiffness (N/mm) 954 (111) 696 (57) Backbone energy (kn*mm) 2405 (335) 2653 (124) Total energy absorbed (kn*mm) (1971) (779) 9DBMC-2002 Paper 050 Page 7

8 a) b) Figure 5. Typical quasi-static displacement-load response of shearwalls; (a) control, (b) water-exposed. The walls failed in much the same manner whether they were the monotonically tested shearwalls or the control or waterexposed shearwalls tested by quasi-static protocol. In general, the wall performance was governed by nail bending and withdrawal (from side grain) capacity. Although, in the water exposed shearwalls, some of the nails below the waterline were partially pulled through the OSB. This demonstrates that there was a deterioration of the OSB below the waterline. Freitag (2001) conducted a study at Oregon State University with the same OSB and wood to evaluate the effects of wetting and assembly procedures. She reported an increase in the strength of joints when the wood had been soaked. Hence, it is thought that unrecovered swelling may have contributed to nail withdrawal capacity. The design capacity of the shearwall was based on the design guide (APA 1998), where the design capacity was reduced for the use of 8d box instead of 8d common nails. The diameter ratio for 8d box to 8d common nails was Thus, when the wall was new, the design capacity for normal duration was 11.6 kn. The actual capacity of control shearwalls was 29.1 kn. The ratio of design to actual capacity was DBMC-2002 Paper 050 Page 8

9 Given the embedment strength of the OSB as measured, the yield mode for the sheathing nails was expected to be a Mode IIIs as described the NDS (1997). Observation indicated that the nails in the test walls yielded as described by Mode IIIs. As the embedment strength as reduced by 40 percent after water soaking, the yield mode was expected to remain a Mode IIIs, and observation indicated that this was the case. The 40 percent embedment-strength loss translated to an expected reduction in wall capacity of 18 percent based on the yield mode equations, which do not include interlay friction. Because some of the nails in the quasi-static tests of water-exposed walls showed evidence of pull-through, it appears that the shift from a Mode IIIs failure to a pull-through yield mode would occur before any other yield mode can become critical. A method to calculate the design stiffness of shearwalls is not available because shearwall design is always governed by capacity. In this test, the stiffness of the shearwall was reduced by 27 percent by the water soaking. It is valuable to note that the nail embedment stiffness of the OSB was reduced by 37 percent as a result of the water soaking. Thus, it seems likely that the loss in wall stiffness can be linked to the loss of nail embedment stiffness. The results of the test program might be different with other sheathing materials or nails; wall performance will depend on the thickness swelling characteristics of the panel and the change in properties with water exposure. For example if the sheathing panel did not swell, then the loss of embedment strength might translate to a real reduction in wall strength. The 1-m depth of submersion was an arbitrary choice of water depth. Wall height submersion was expected to have only a minor effect on the results of wall submersion tests because the nails at the top and bottom of the shearwall are the most highly stressed. Hence, the test walls could have been submerged only enough to cover the nail connections at the bottom edge of the wall and still produce the same effect as a 2-m submersion. Since most floods involve less than 2 m of water in the structure (Hausmann & Perils 1998), it was felt that a 1-m depth was sufficient to identify the effects. The walls and sheathing materials tested in this study were air dry. The capacity of the system when wet (saturated) might be much lower than observed in this moisture condition. However, the probability of concommitent major lateral force generating event with the water exposure (other than the force of water flow around or through the structure) is considered to be small. 4 CONCLUSIONS One approach to rational post-flood analysis of building capacity would be to discount the allowable properties of the OSB embedment strength by 40 percent and then discount the wall capacity by a similar percentage. However, the reduction in embedment capacity results in a calculated reduction of only 18 percent. In actuality, the wall capacity and energy properties were not reduced as a result of water exposure. Strength and energy capacity were affected by the connection geometry as well as the interlayer friction, which in this case was increased by the thickness swelling of the OSB. The loss of stiffness was most likely a result of softening in the OSB sheathing, hence the reduction of embedment strength accompanied by a loss of stiffness in the nail bearing resulted in the loss of wall stiffness. The monotonic tests and quasi-static tests failed by nail bending and withdrawal, which demonstrated that the wall capacity was limited by nail bending and withdrawal and not embedment or pull-through characteristics of the sheathing. It appears that structures with OSB sheathing that are temporarily inundated with fresh water and dried expeditiously will have a loss of stiffness but should not experience a reduction in lateral resistance capacity or energy absorption capacity. 5 ACKNOWLEDGEMENTS This paper is based on the Vertieferarbeit written by the second author while in residence at Oregon State University. The thesis was in partial fulfillment of the degree requirements at the University of Karlsruhe. Financial support for this work was provided by the Forest Research Laboratory, Oregon State University, Corvallis, Oregon. The technical contributions of Milo Clauson with the testing and data management are gratefully acknowledged. 6 REFERENCES 1. American Forest & Paper Association (NDS) National Design Specification for Wood Construction, American Forest & Paper Association, Washington, D.C. 2. APA The Engineered Wood Association. 2000, Oriented Strand Board, APA, Tacoma, WA. 3. APA The Engineered Wood Association. 2001, Residential and Commercial Design and Construction Guide, APA Tacoma, WA. 4. American Society for Testing and Materials (ASTM). 2001a, Standard test methods for evaluating properties of woodbase fiber and particle panel materials, D 1037, in 2001 Annual Book of Standards, vol , ASTM, West Conshohoken, PA. 5. American Society for Testing and Materials (ASTM). 2001b, Standard test methods for evaluating dowel-bearing strength of wood and wood-based materials, D 5764, in 2001 Annual Book of Standards, vol , ASTM, West Conshohoken, PA. 6. American Society for Testing and Materials (ASTM). 2001c, Standard test methods for shear modulus of wood-based structural panels, D 3044, in 2001 Annual Book of Standards, vol , ASTM, West Conshohoken, PA. 7. Chou, C. & Polensek, A. 1987, Damping and stiffness of nailed joints: response to drying, Wood and Fiber Science, 19(1), DBMC-2002 Paper 050 Page 9

10 8. Ceccotti, A. 1999, Analysis and Design of Woodframe Construction According to Eurocode 8, Proc. of the Invitational Workshop on Seismic Testing, Analysis and Design of Woodframe Construction, 5-6 March 1999, Los Angeles, CA. pp Folz, B. & Filiatrault, A. 2001, Cyclic analysis of wood shear walls, Journal of Structural Engineering, 127(4), Forest Products Laboratory Wood Handbook: Wood as an Engineering Material, U.S. Dept. of Agriculture. Forest Service, Madison, WI. 11. Freitag, C unpublished data, Oregon State University, Corvallis, OR. 12. Hausmann, P. & Perils, C. 1998, Floods An Insurable Risk, Swiss Reinsurance Company, Zurich, Switzerland. 13. Krawinkler, H., Parisi, F., Ibarra, L., Ayoub, A. & Medina, R Development of a Testing Protocol for Wood Frame Structures, Department of Civil & Environmental Engineering, Stanford University, Stanford, CA. 14. Langlois, J. in preparation, Seismic Performance of Wood Shearwalls Subjected to the CUREE Test Protocol, M.S. thesis, Department of Forest Products and Department of Civil, Construction, and Environmental Engineering, Oregon State University, Corvallis, Oregon. 15. McNatt, J.D How cyclic humidity affects static bending and dimensional properties of some wood-base panel products, Proc. of the Workshop on the durability of Structural Panels, 5-7 October U.S. Forest Service, Pineville, LA. pp Mohammad, M.A.H. & Smith, I. 1996, Effects of multi-phase moisture conditioning on stiffness of nailed OSB-tolumber connections, Forest Products Journal, 46(4), Munich Re. 2001, Naturkatastrophen 2000 (Natural Catastrophes 2000), MRN at Cat Poster #8, Munich Re, Munich, Germany (in German). 18. Neisel, R.H. & Guerrera, J.F. 1956, Racking strength of fiberboard siding, TAPPI, 39(9), Patton-Mallory, M. & McCutcheon, W.J. 1987, Predicting racking performance of walls sheathed on both sides, Forest Products Journal, 37(9), Suchsland, O. 1982, Durability, Proc. of the Workshop on the durability of Structural Panels, 5-7 October U.S. Forest Service, Pineville, LA. pp TenWolde, A. & Rose, W.B. 1994, Moisture control strategies for the building envelope, Wood Design Focus, 5(4), Toumi, R.L. & McCutcheon, W.J. 1978, Racking strength of light-framed nailed walls, American Society of Civil Engineers, 104(ST7), Zacher, E.G. 1999, Gaps in information for determination of performance capabilities of light woodframe construction, Proc. of the Invitational Workshop on Seismic Testing, Analysis and Design of Woodframe Construction, 5-6 March 1999, Los Angeles, CA. pp DBMC-2002 Paper 050 Page 10