Ken-ichi SUGIMOTO Team Leader Forestry and Forest Products Research Institute Tsukuba, JAPAN

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1 Strength Performance of Particleboard Subfloor Under Concentrated Static Loads Ken-ichi SUGIMOTO Team Leader Kenji AOKI Researcher, Ph.D. Hideki AOI Senior Researcher, Ph.D. Fumio KAMIYA Principal Research Coordinator, Ph.D. Yasuto CHIBA Japan Fiberboard and Particleboard Manufacturers Association (Retired) Tokyo, JAPAN Summary To evaluate the performance of subfloor joints experimentally, we conducted concentrated loading tests with the floor system nailed to particleboard subfloors mm thick. Four types of specimens with different subfloor joint shapes were used: ) a butt joint with blocking, ) a tongue-and-groove joint, ) a groove-and-spline (made of MDF) joint, and ) a butt joint without blocking. Specimens were supported with four pairs of channel steel bars and subjected to concentrated loads applied through a 7-mm diameter loading disk. We measured eight points of absolute deflection from one end to the other of a specimen, on the bottom surface of the subfloor sheathing, midway between the framing members under concentrated loads. To evaluate the stiffness, a slightly more than kn concentrated load was applied to each center and each point near the joint of the subfloor sheets continuously and reloaded. After this test, a concentrated load was applied to the point near the joint of the subfloor sheets or just on the joint until a failure occurred. Consequently, it was found that the relationship of load to deflection was linear under kn of concentrated load in all specimens. The deflection under kn of concentrated load stayed within the criteria of Span/, except the butt joint without blocking. If a concentrated load acted on the vicinity of the joint, the tongue-andgroove and the groove-and-spline joints had similar performance for maximum load and sitiffness. The butt joint with blocking appears to have a better performance than the tongue-and-groove and the groove-and-spline joints. The deflections corresponding to the maximum load were alike in the overall kinds of specimens. If a concentrated load acted on the joint, the maximum load, the corresponding deflection and the stiffness of the specimens, except the butt joint with blocking, were not very different.. Introduction Recently, in Japanese conventional wooden houses, thick subflooring and widely spaced joists have been used to simplify the construction of floor system. The use of thick particleboard sheets as subflooring promotes the effective utilization of wood resources. Usually butt joints with blocking are used in particleboard subfloor jointing, but if tongue-and-groove joints or groove-and-spline

2 joints are available in particleboard subfloor jointing, construction could be simplified. To evaluate the performance of subfloor joints experimentally, we conducted concentrated static loading tests with a floor system nailed to particleboard subfloors mm thick. Part of this report was presented at the 7 th Annual Meeting of the Japan Wood Research Society held in Hiroshima in August 7.. Materials and Method. Specimens A schematic of the specimens and the test setup in this study is shown in Fig.. The test was conducted based on the method used in the study Development of new construction method of small-scaled wood housing in the General Technology Development Project managed by the Ministry of Construction in Japan in 97. There are four types of specimens with different particleboard subfloor joint shapes: ) butt joint with blocking (BWB), ) tongue-and-groove joint (TG), ) groove-and-spline (made of MDF, medium-density fiberboard) joint (GS), ) butt joint without blocking (BWOB). The particleboard sheets used as subflooring were mm thick and type 8M (in accordance with JIS A 98:, melamine-resin adhesive, 8 N/mm or higher in bending strength, 9 N/mm or higher in wet bending strength). The actual measurement values of the particleboard sheet were 77 kg/m in density,. kn/mm in bending strength,. kn/mm in wet bending strength, and. kn/mm in modulus of elasticity. Each particleboard sheet (8 mm by 9 mm) was cut in half longitudinally. In the case of the tongue-and-groove joint specimens and the groove-and-spline joint specimens, each side edge of two pieces cut from the sheet was processed into tongues or grooves with a groove cut circular saw. Cut pieces were applied perpendicular to the span of the joists and nailed to each joist(species: Daglus-Fir) with the common nails N7 (JIS A 8, the diameter of the body and the head is. mm and 7.9 mm respectively and the length is 7mm in nominal) spaced at mm on centers.. Experimental On occasions when tests are conducted following the ASTM E66-[], Standard Test Method for Performance of Wood and Wood-Based Floor and Roof Sheathing Under Concentrated Static and N7@ 9 9 Joist(Daglus-Fir, 8) Bridging(Sugi, ) Tongue-and-Groove Upper View A B Blocking (only Blocking Type) Sugi 6 CD EF G H 9 Groove-and-Spline Spline: MDF Type P length mm thickness 9mm Butt Joint with Blocking 9.. Blocking Blocking Type Butt Joint without Blocking Loading Disc Φ7 Joint LVDT 8 C-Shaped Steel Bar Lateral View In the case of loading on B Unit:mm Fig. A schematic of the specimens and the test set-up

3 Impact Loads, floor sheathing specimens are subjected to concentrated loads of 89 N applied through a 76-mm diameter loading disk to evaluate the stiffness of subflooring. However, in this study, specimens were subjected to concentrated loads to each center (Points B and G) and each point near the joint (Points C and F) of the subfloor sheets to a little more than kn continuously applied through a 7-mm diameter loading disk and reloaded with a hydraulic material testing machine. Specimens were supported with four pairs of channel steel bars in such a way that points A, B, C and D were on the tongue side of the joint and subjected to concentrated loads We measured eight points, from points A, B, C, D, E, F, G and H, of absolute deflection from one end to the other of a specimen, on the bottom surface of the subfloor sheathing, midway between joist members under concentrated loads. After evaluating the stiffness, a concentrated load was subjected to the point near the joint of the subfloor sheets (Point C or F) or just on the joint until a failure occurred.. Result and Discussion. Deflections under kn of concentrated load... Deflection at point B Fig. Examples of load-deflection curves (loading at point B) 8 A B 8 CDEF 8 G 8 H Deflection (Loading Ponit B ) A B CDEF G H Deflecton (Loading Point F ) A B 8 CDEF 8 G 8H Deflection (Loading Point C ) 8 A B 8 CDEF 8 G 8H Deflection (Loading Point G ) 本実 Tongue-and-groove joint (TG) 雇い実 Groove-and-spline joint (GS) 突付受材あり Butt joint with blocking (BWB) 突付受材なし Butt joint without Blocking (BWOB) Fig. Deflections under kn of concentrated load There are examples of load-deflection (at point B) curves with six tongue-andgroove (TG) specimens loading up to a little more than kn concentrated load at point B in Fig.. The relationship of load to deflection is linear under kn of concentrated load in all the specimens. Fig. shows the deflections of specimens at points A to H, subjected to kn of concentrated load at points B, C, F, and G. Each deflection for loading points B and G is an average of values measured in six specimens, and each deflection for loading points C and F is an average of values measured in three specimens. In TG specimens, the deflections for loading at points B (tongue side) and G (groove side), and the ones for loading at points C (tongue side) and F(groove side) are virtually symmetric to the joint. The maximum deflection occurred at Point D or E for the TG, GS and BWOB specimens, while the maximum deflection occurred at Point B or G for BWB. The maximum deflections were approximately.mm,,7mm,.mm and.mm for the TG, GS, BWB and BWOB specimens, respectively. The difference of deflections on both sides of the joint (Points C and F) were approximately.mm,.7mm,.6mm and.8mm for the TG, GS, BWB and BWOB specimens, respectively.

4 Table Apparent stiffness Apparent stiffness (kn/maximum Deflection) (kn/mm) TG-C Average ( specimens).878 Coefficient of variation [%] 7.8 TG-F Average ( specimens).86 Coefficient of variation [%] 6.7 GS-C Average ( specimens).7 Coefficient of variation [%]. GS-F Average ( specimens).7 Coefficient of variation [%]. BWB-B Average (6 specimens). Coefficient of variation [%] 9.9 BWB-G Average (6 specimens).97 Coefficient of variation [%]. BWOB-C Average ( specimens).6 Coefficient of variation [%]. BWOB-F Average (6 specimens).7 Coefficient of variation [%]. Table Concentrated loads for deflection criteria Concentrated load per mm (Deflection criteria:span/) calculated from apparent stiffness (kn) TG-C.76 TG-F.7 GS-C. GS-F.8 BWB-B.8 BWB-G.9 BWOB-C.9 BWOB-F.9 Table shows apparent stiffness defined in this paper, calculated from the value of kn (applied concentrated load) divided by the corresponding maximum deflection at from points A to H. The last letter of the specimen name, such as -C, shows the position of loading. For the TG, GS and BWOB specimens, only the values for loading at points C and F, in the vicinity of the joint were shown and for the BWB specimen, only the values for loading at points B and G, at which maximum deflection occurred. Apparent stiffness was approximately.87kn/mm,.7kn/mm,.kn/mm and.6kn/mm for the TG, GS, BWB and BWOB specimens, respectively. Table shows concentrated loads for the deflection criteria provided by the Standard for Structural Design of Timber Structures []. Each concentrated load was calculated from apparent stiffness by Span (=9 mm)/. If kn of concentrated load acts on the specimens, except for BWOB, the deflection stays within deflection criteria.. Failure test loaded at point C or F TG Loaded at F 6 Deflection at Loading Point BWB 6 Deflection at Loading Point GS 6 Deflection at Loading Point BWOB 6 Deflection at Loading Point Load-deflection (at a loading point) curves loaded at point C or F up to failure are given in Fig.. Each curve shows a different sample. Only one specimen of TG was loaded at point F and the rest of the specimens were loaded at C. The maximum loads of the TG and GS specimens were alike. The maximum load of BWOB was approximately 6-7% of the ones of TG and GS. The maximum load of BWB was approximately -% higher than the ones of the TG and GS. The load-deflection curve of the TG specimen loaded at point C was similar in shape to the ones of the GS specimens. In the GS and BWOB specimens, the difference between the curves was small. In the BWB specimens, the difference between the curves appears to be greater than the ones in GS and BWOB due to the quality of blocking beneath the joint. Fig. Load-deflection curves loaded at point C or F

5 In Fig., there are examples of failure loaded at point C. In the TG specimen loaded at point C (tongue side), a crack occurred in the groove-side particleboard from the joint to the other end at mid-span parallel to the joists, and another crack occurred in the tongue-side particleboard between the joint and the loading disk (a) TG: Loading at point C (tongue side) perpendicular to the joists. Both cracks occurred on the bottom surface of the particleboard (Fig.(a)). In the GS specimen, a crack occurred on the bottom surface of the particleboard from the joint to the other end at midspan parallel to the joists, and another crack occurred on the top surface of the particleboard between the joint and the loading disk perpendicular to the joists. Both cracks occurred in the (b) GS loaded-side particleboard (Fig. (b)). A similar failure mode was observed in the TG specimen loaded at point F. In the BWOB specimen, a crack due to bending was observed on the bottom surface of the particleboard at mid-span parallel to the joists (Fig. (d)). In addition, the bending failure of blocking beneath the joint was observed in the BWB specimen (Fig. (c)). (c) BWB (d) BWOB Table represents the maximum load, Fig. Examples of failure loaded at C or F maximum deflection at point C or F corresponding to maximum load and stiffness calculated in accordance with the method proposed by Moarcas and co-workers ()[]. The stiffness is calculated from the ratio of the difference between. and. of the failure load (F and F) to the difference between the corresponding deflections for F and F. In this study, the foregoing apparent stiffness and the stiffness calculated here appear to be alike. Maximum deflection was approximately -6 mm, except for the BWOB specimen. Table Maximum load, corresponding deflection and stiffness (loaded at point C or F) Maximum load (kn) Maximum deflection (at point C or F) corresponding to maximum load Stiffness (kn/mm) TG-C Average (Number of specimen ) Coefficient of variation [%]... TG-F (Number of specimens ) GS-C Average (Number of specimens )...79 Coefficient of variation [%].9.6. BWB-C Average (Number of specimens ).7.. Coefficient of variation [%] BWOB-C Average (Number of specimens ) Coefficient of variation [%]...6

6 . Failure test loaded directly on the joint 6 Deflection TG BWB 6 Deflection 6 Deflection 6 Deflection Fig.6 Load-deflection curves loaded on the joint GS BWOB Fig.6 shows load-deflection (average at point C and F) curves loaded directly on the joint up to failure. Each curve shows a different sample with three specimens. Except for the BWB specimen, the shapes of the loaddeflection curves appear to be similar to one another. Fig.7 shows examples of failure with loading on the joint. Almost all the specimens expressed typical bending failure. Two specimens of which each load-deflection curve dropped down vertically at the end of testing expressed punching shear failure after bending failure. The maximum loads, corresponding deflections and stiffness are given in Table. The last letter -J in a specimen name in Table means that the loading position was on the joint. The maximum loads of the TG, GS and BWOB specimens were approximately.6 kn and the corresponding deflections were 8 mm. The maximum load of the BWB specimen was 7. KN, % higher than other specimens. The deflection corresponding to the maximum load varied widely. (a) Typical bending failure (b) Punching shear failure Fig.7 Examples of failure loaded on the joint. Table Maximum load, corresponding deflection and stiffness (loaded at point C or F) Maximum load (kn) Maximum deflection (mean value at points D and F) corresponding to maximum load Stiffness (kn/mm) TG-J Average ( specimens) Coefficient of variation [%]... GS-J Average ( specimens) Coefficient of variation [%] BWB-J Average ( specimens) Coefficient of variation [%] BWOB-J Average ( specimens) Coefficient of variation [%]

7 . Conclusions To evaluate the performance of subfloor joints experimentally, we conducted concentrated static loading tests with a floor system nailed to particleboard subfloors mm thick. Four types of specimens with different subfloor joint shapes were used: ) a butt joint with blocking, ) a tongueand-groove joint, ) a groove-and-spline (made of MDF) joint, and ) a butt joint without blocking. As a result, it was found that the relationship of load to deflection was linear under kn of concentrated load in all specimens. The deflection under kn of concentrated load stayed within the criteria of Span/, except the butt joint without blocking. If a concentrated load acted on the vicinity of the joint, the tongue-and-groove and the groove-and-spline joints had similar performance for maximum load and sitiffness. The butt joint with blocking appears to have a better performance than the tongue-and-groove and the groove-and-spline joints. The deflections corresponding to the maximum load were alike in the overall kinds of specimens. If a concentrated load acted on the joint, the maximum load, the corresponding deflection and the stiffness of the specimens, except the butt joint with blocking, were not very different.. References [] ASTM E66-, Standard Test Method for Performance of Wood and Wood-Based Floor and Roof Sheathing Under Concentrated Static and Impact Loads, Annual Book of ASTM Standards, Vol..,, pp.9- [] Architectural Institute of Japan, Standard for Structural Design of Timber Structures, 6, pp.8-88(in Japanese) [] Moarcas O., Nicholls T, Thomas W., Matthews B., The bending performance of T&G joints subjected to concentrated loads, Holz als Roh- und Werkstoff, Vol.8, No.6,, pp.8-