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Frictional Coefficients between Timber and Other Structural Materials Quin-jung MENG, Takuro HIRAI and Akio KOIZUMI Laboratory of Timber Engineering Hokkaido University, Japan

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Background In most of the design standards of timber constructions, frictional resistance is not counted as a mechanical element of structural resistance. This is because of the conservative considerations: (1)Reduction of initial friction due to stress relaxation, (2)Difference in mechanical characteristic between friction and other mechanical elements, and/or (3)Uncertain effective vertical loads reduced by vertical components of earthquake forces.

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However, in wooden light frame constructions; Earthquake Wind Floor Wall Shear force Shear forces are transmitted from the bottoms of walls to the floors by both frictional resistance due to vertical loads and lateral resistance of nailed joints.

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If we consider these actual mechanical behavior linked with the frictional resistance between structural members, it seems more reasonable to count properly the effect of frictional resistance in structural design of timber constructions. However, we have few information about frictional resistance between structural materials commonly used in timber constructions and their effects on mechanical behavior of timber constructions.

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What should we conduct for taking the frictional resistance into consideration? The first step: Practical evaluation of frictional coefficients The next step: Analyses of effects of frictional resistance on mechanical behavior of timber constructions The final step: Proposal of structural design considering the effects of frictional resistance

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Target of this study On the first step, we conducted experimental evaluation of frictional coefficients between timber and several kinds of structural materials commonly used in timber constructions in this study.

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Test materials We selected: Structural softwood timber (Mixture of Larix, Abies and Cryptomeria) Softwood plywood Hardwood plywood Oriented strand board (OSB) Medium density fiberboard (MDF) Volcanic silicate ｓ fiber reinforced multi-layer board (VS) Steel (SS400: Ra 3.6-6.3)

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Material Specific gravityMoisture content Timber0.30-0.5612.0-13.4 % Softwood plywood 0.54-0.5910.0-10.4 % Hardwood plywood 0.55-0.6210.7-11.7 % OSB0.69-0.748.7-9.5 % MDF0.78-0.807.5-8.2 % VS0.72-0.77 Basic properties of test materials.

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Mechanical characteristics of test materials The test materials are categorized as: Orthotropic: Timber Hardwood plywood Softwood plywood Oriented strand board (OSB) Isotropic: Medium density fiberboard (MDF) Volcanic silicate ｓ fiber reinforced multi-layer board (VS) Steel (SS400: Ra 3.6-6.3)

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Combinations of test materials We tested every combination of slip directions: For example, friction between timber and plywood were measured for four combinations of slip directions.

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Measurement of Frictional Coefficients Dead load Displacement transducer Load cell Hydraulic cylinder Structural sheet material Timber

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0 20 40 60 80 100 120 05101520 Displacement (mm) Frictional force (N) Determination of Frictional Coefficients Static frictional coefficient Dynamic frictional coefficient

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Friction between timber and SS400 steel Test results Timber: Parallel Static Dynami c Static Dynami c Timber: Perpendicular

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Results of frictional tests between timber and SS400 steel were summarized as: (1) Frictional coefficients had negative correlations with specific gravity of timber. (2) Frictional coefficients perpendicular to the timber grain were larger than those parallel to the grain. ＞ ＜

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Friction between timber and VS board Timber: Parallel Timber: Perpendicular Static Dynamic Static Dynamic

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Results of frictional tests between timber and VS board showed the similar tendencies: (1) Frictional coefficients had negative correlations with specific gravity of timber. (2) Frictional coefficients perpendicular to the timber grain were larger than those parallel to the grain. ＜ ＞

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Friction between timber and MDF Timber: Parallel Timber: Perpendicular Static Dynami c Static Dynami c

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Results of frictional tests between timber and MDF also showed the similar tendencies : (1) Frictional coefficients had negative correlations with specific gravity of timber. (2) Frictional coefficients perpendicular to the timber grain were larger than those parallel to the grain. ＞ ＜

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Friction between timber and OSB Timber: Parallel OSB: Parallel Timber: Perpendicular OSB: Parallel Static Dynamic Static Dynamic

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Friction between timber and OSB Timber: Parallel OSB: Perpendicular Timber: Perpendicular OSB: Perpendicular Static Dynamic Static Dynamic

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Results of frictional tests between timber and OSB were summarized as: (1)Frictional coefficients had negative correlations with specific gravity of timber. (2) On the other hand, we found little difference among all combinations of slip directions. ＞ ～ ～ ～ ～ ～ ～ ＜

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Friction between timber and hardwood plywood Timber: Parallel Plywood: Parallel Timber: Perpendicular Plywood: Parallel Static Dynamic Static Dynamic

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Timber: Parallel Plywood: Perpendicular Timber: Perpendicular Plywood: Perpendicular Friction between timber and hardwood plywood Static Dynamic Static Dynamic

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Results of frictional tests between timber and hardwood plywood were summarized as: (1) Frictional coefficients had negative correlations with specific gravity of timber. (2) The observed order of frictional coefficients among four combinations of slip directions was: ＞ ＜ ～ ～ ＜ ～ ～ ＜

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Friction between timber and softwood plywood Timber: Parallel Plywood: Parallel Timber: Perpendicular Plywood: Parallel Static Dynami c Static Dynami c

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Timber: Parallel Plywood: Perpendicular Friction between timber and softwood plywood Timber: Perpendicular Plywood: Perpendicular Static Dynamic Static Dynamic

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Results of frictional tests between timber and softwood plywood showed the similar tendencies: (1) Frictional coefficients had negative correlations with specific gravity of timber. (2) The observed order of frictional coefficients among four combinations of slip directions was: ＞ ＜ ～ ～ ＜ ～ ～ ＜

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(1) Frictional coefficients had negative correlations with specific gravity of timber for every combination of materials and slip directions. (2) Frictional coefficients were qualitatively affected by the combination of slip directions. The differences among the combinations of slip directions, however, was quantitatively a little for the friction between timber and plywood and was not clear for the friction between timber and OSB. Summing-up of test results

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Evaluation of frictional coefficients for practical design Architectural Institute Japan classifies typical softwood species for structural use into the following three groups:

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Timber direction J1(0.47)J2(0.42)J3(0.37) Parallel Static Dynami c 0.256 0.212 0.272 0.223 0.288 0.235 Perpendicular Static Dynami c 0.287 0.225 0.306 0.240 0.325 0.255 From the negative correlations with specific gravity of timber, we can roughly estimate the frictional coefficients from the average specific gravity of each group of structural softwood timber. For example, frictional coefficients between timber and SS400 steel are estimated as:

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TimberPlywoo d J1(0.47 ) J2(0.42 ) J3(0.37 ) Par. Static Dynami c 0.296 0.200 0.315 0.222 0.335 0.243 Par.Per. Static Dynami c 0.293 0.205 0.312 0.230 0.330 0.250 Per.Par. Static Dynami c 0.314 0.231 0.332 0.255 0.350 0.279 Per. Static Dynami c 0.364 0.272 0.375 0.292 0.386 0.312 Frictional coefficients between timber and softwood plywood are estimated for four combinations of slip directions as:

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An example of structural calculation considering frictional resistance The shear force is transmitted from the bottoms of walls to the floor sheathings by friction and lateral resistance of nailed joints. Consider a wooden light frame construction subjected to an earthquake force.

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Here we assume: Shear force beard by the shear walls = base shear factor (α)×mass (m)×g = base shear factor (α)×vertical load (W ) Floor W α×W Wall Earthquake force Shear walls that resist the earthquake force

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We also assume: Vertical load distributed to the shear walls that causes frictional force = 0.5×W Earthquake force Shear walls parallel to the earthquake force Shear walls perpendicular to the earthquake force W = 0.5W + 0.5W

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VeVe HeHe V e =0.5H e There is the risk at the earthquake that the vertical component of the earthquake force may reduce the effective vertical load, which causes frictional force. Considering this risk, we assume: Ratio of vertical component Ve to horizontal component He of the earthquake force = 0.5

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The ratio r n of required lateral resistance of nailed joints to the total shear force results in: r n = 1-0.5μ(1-0.5α)/α, where μ= frictional coefficient. We temporarily adopt both static and dynamic frictional coefficients because of poor information about detailed dynamic response of timber constructions. If we use spruce (J3) as the bottom plates of walls and softwood plywood as the floor sheathing panels, we can conservatively estimate: Static frictional coefficient: 0.3 Dynamic frictional coefficient: 0.2

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Ratio of required lateral resistance of nailed joints Base shear factor Dynamic Static Estimated result

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Remained Problems What characteristics affect the frictional coefficients? Surface roughness? Surface hardness? Is effect of moisture content of timber practically negligible or not? How to estimate the effective vertical load or stress relaxation?

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