Long-Term Bending Failure Tests of Structural Insulated Panel

Transcription

1 Long-Term Bending Failure Tests of Structural Insulated Panel Okabe, M. Head, Tsukuba Building Test Laboratory Center for Better Living Japan Yasumura, M. Professor, Shizuoka University Department of Environment and Forest Resource Science Japan Summary As well as the structural performance, the Structural Insulated Panels (SIPs) housing system could be expected one of the most environmental friendly housing. SIPs are widely applied in the residential application for walls, floor panels, and roof panels. Duration of load (DOL) factors for SIPs is the significant factors that reduce allowable design stresses. Unfortunately limited data for DOL was existed on SIPs. This paper firstly describes the estimation of stress levels applied the short-term static bending test of SIPs. Three test specimens were cut off from the SIPs. Center specimen was carried out static bending for estimation of the stress level and the other was curried out creep failure test applied stress level by using the failure load from matched center specimen. Applied stress levels are 95%, 90%, 85%, 80%, 75%, 70% and 65% and creep bending test curried out two SIPs specimens on each stress level. This paper finally deals with the relations of stress level to log time-to-failure of SIPs and fitting regressions to stress level versus log time-to-failure plots give DOL factor for SIPs. 1. Introduction Increase of concentration of carbon dioxide due to human activity have raised issues would intensify the greenhouse effect, definitely leading drastic changes to global climate system. SPIs could be expected one of the most environmental building systems available. A SIP home requires much less energy for heating and cooling hence it emits Sheating (OSB) Form Core (EPS) less greenhouse causing carbon dioxide. SIPs are a very efficient use of resources. Photo 1 Construction site of SPIs house in Japan

2 OSB is made from small, plantation grown trees that can be sustainable harvested. Because engineered wood products use wood more efficient than sawn lumber, it requires less forest acreage to build a SIP home than a conventional wood frame house. The EPS insulation used in SIPs is lightweight foam composed of 98% air. The structural characteristics of SIPs are similar to that of a steel I-beam. The OSB skins act as the flange of the I-beam, while the rigid foam core provides the web. In Japan some company would make use of SPIs for high energy efficient on residential houses. Photo 1 shows the construction site of SPIs house in Japan. Duration of load factors for SPIs is the most significant factors that reduce allowable design stresses. Unfortunately limited data for duration of load are existed on SIPs. This report describes the long-term bending failure test results in SPIs and obtained the DOL factor of SPIs. Scope of the long-term bending test is shown in Figure Static Bending Test 2. Long-Term Bending Failure Test Pmax Load(kN) 3. Duration of Load factor Displacement (mm) 応力レベル (%) 10 分 1 時間 1 日 1 ヶ月 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 破壊までの時間 (min) Figure 1 Scope of the Long-term Bending Test 2. Specimen SIPs are made by sandwiching a core of expanded polystyrene (EPS), between two structural skins of oriented strand board (OSB).Thickness of EPS is 90mm and OSB is 12.5mm in both side. SIPs total thickness is 114mm. Preparative SPIs samples are 850mm width and 1500mm long. Surface strand direction of OSB is parallel with width direction. Three test specimens were cut off from the SIPs and one specimen at the center of the panel tested static bending and the other tested creep bending determined a stress level by using the failure load from matched specimen. SIPs test sample cut out the static and creep bending test is 250mm width and 1500mm long. Shape of SIPs and specification of SIPs is shown in Table 1 and test

3 specimen of static and creep bending test is shown in Figure 2. Table 1 Shape of SIPs and Specification of SIPs Thickness (mm) Width (mm) Length (mm) SIPs Test specimen OSB; 12mm thickness JAS EPS; 90mm thickness Adhesives; Water based polymer-isocyanate adhesives Direction of Strand A : Creep failure of bending test B : Static bending test A : Creep failure of bending test Figure 2 Static and creep bending test specimen 3. Static Bending Test 3.1 Test procedure of static bending Test specimen cut the center of the SIPs was tested under monotonic one-third point loading as shown in Figure 3 and Photo 2. General loading rate is approximately 30mm per minute. In addition, three group of two SIPS specimen was tested by 3mm per minute, 10mm per minute and 30mm per minute. Oij Jack Load cell Capacity 10kN Displacement instrument Figure 3 One- third point load bending set up Photo 2 SIPs third point load bending set up 3.2 Result of bending and estimation of stress levels Result of static bending test was shown in Table 2 and load-displacement curve was shown in Figure 4. Average maximum load of bending was shown 7.07kN (C.V 2.7%) and average failure displace was shown 60.4mm (C.V. 10.5%). Dominant failure mode of the SIPs test specimen was horizontal shear at the OSB and EPS on loading point. Effect of the loading rate was shown in Figure 5. The strength of SIPs increases with increased loading rate. Stress

4 level baseline of 100% for duration of load assumed 10 minutes. Relationship between the time of loading to failure for each loading rate and maximum load was shown in Figure 6. Relationship between the time of failure and factor normalized by 10 minutes was shown in Figure 7. Stress level is the ratio of the load at which it would fail in a short-time static strength test. Stress levels are always approximate because strength of any specimen is unknown under a loading scheme. SIPs determined a stress level by using the failure load from a matched the specimen cut the center of the panel. And the stress level corrected by using stress level factor normalized by 10 minutes. Load value of 100% stress level using the equation (1). P max(kn) SL100%(kN) = ( Ln(t) ) Table 2 Result of static bending test specimen density sheathing Times of Pmax(kN) δmax(mm) (g/cm 3 ) M.C.%) failure(min) Failure mode side :44 horizontal shear at the OSB and EPS A center :28 side :07 B center :42 horizontal shear at the OSB and EPS C center :32 compression failure of loading point D center :40 horizontal shear at the OSB and EPS E center :45 horizontal shear at the OSB and EPS F center :15 horizontal shear at the OSB and EPS G center :26 horizontal shear at the OSB and EPS H center :21 horizontal shear at the OSB and EPS I center :01 horizontal shear at the OSB and EPS J center :41 horizontal shear at the OSB and EPS average :59 std C.V.(%) (1) Load(kN) 4 Load(kN) Pmax ave.=7.07kn C.V. 2.7% Dmax ave=60.4mm C.V. 10.5% Average: Ave+-std: Displacement(mm) 2 0 3mm/min 10mm/min 30mm/min Displacement(mm) Figure 4 Load - displacement curve of SIPs Figure 5 Effect of the loading rate

5 Pmax(kN) y = Ln(x) R 2 = mm/min 10mm/min 30mm/min Stress level factor nornalized by 10 minutes y = Ln(x) R 2 = Time of Loading to Failure (min) Time of Loading to Failure (min) Figure 6 Relation of strength to time of loading to failure of SIPs Figure 7 Effect of the loading rate 4. Long-term Bending Failure Tests 4.1 Design of the apparatus for long-term bending failure test and Test Environment Creep failure test apparatus designed and specified using the lever as shown in the Table 3 and Photo 3. Detail of apparatus is shown in Figure 8. When creep failure test started, first steel arm was supported using jack, second weight was set at the end of the steel arm, then oil jack was took off quietly. Table 3 specification of creep failure apparatus Span 1350mm Maximum load 10kN Minimun load 2kN Load/Weight Vertical movement 6 times Specimen:100mm Weight:600mm Number of specimen 2 Photo 3 creep failure apparatus Creep failure test was carried out in the room using air conditioner. Condition of test room is controlled only temperature. Measuring temperature and relative humidity with the test term was shown in Figure 9.

6 Steel Arm 6 times 300 sensor specimen weight Figure 8 Detail of creep failure test apparatus 80 Figure 9 Measuring temperature and relative humidity with the test term Temperature (deg.) Humidity(%) 70 Humidity Ave 48.9 % 30 Ave 17.1 deg Temperature 0 2/13 2/28 3/15 3/30 4/14 4/29 Time (days) 4.2 Results of creep failure Relationship between time and the deflection was shown in Figure 10 and relationship between relative deflection and log-time was shown in Figure 11. Relative deflection was calculated using the equation (2). Def (t) R.Def. = Def ( 1min ute ) (2) Where: R.Def: relative deflection, Def(t):deflection to measuring time t, Def(1 minute):deflection to 1 minute. Target stress level was shown in the figure of percentage value. Failure mode of long-term bending

7 failure test was shown in Photo 4. Decrease of dead load for the bending failure test of SIPs becomes increase time to failure of SIPs % Diflection(mm) % 85% 75% 80% 70% 65% Relative Diflection (dt(mm)/d1min) % 90% 85% 80% 75% 70% 95% 90% 85% 80% 75% 75% 70% 65% Time to failure(hours) E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 Log time(hours) Figure 10 Relationship between time and the deflection (65% data was not shown to the failure) Figure 11 Relationship between relative deflection and log-time Photo 4 Failure mode of the Long-term Bending failure Tests Relationship between the log time to failure and dead load was shown in Figure 12. Fitting regressions to stress level versus log time-to-failure plots was shown. Relationship between the log time to failure and stress level was shown in Figure 13. Regression line using original data of time to failure was indicated below 1.0 stress level on 10 minutes. Baseline of duration of load has often used 10 minutes based on the static bending test. Regression line shifted was shown the stress level 1.0 on 10 minutes and 50years stress level was shown almost same DOL factor of wood. Decrease of dead load for the bending failure test of SIPs becomes increase time to failure of SIPs.

8 y = x month Dead load(kn) 4 Stress level y = x R 2 = years 10 years 1hour minute 1day 1month 10 minutes 50 years 2 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 Log time to failure(hours) Figure 12 Relationship between the log time to failure and dead load Log time to failure (hours) Figure 13 Relationship between the log time to failure and stress level 5. Conclusions On the basis of the results obtained from both static bending test and long-term bending failure test, the followings could be concluded. 1. Estimation of stress level of SIPS applied not only maximum failure load of matched specimen but also the time to failure using factor normalized by 10 minutes in the static bending test. 2. DOL factor of SIPs was obtained by results of long-term bending failure test corrected 10 minutes stress level 6. Reference [1] ASTM. Standard Specification for Evaluation of Duration of Load and Creep effects of Wood and Wood-Based Products ASTM D a [2] Karacabeyli, E., Long-Term Structural Performance of Wood Products, Forintek Canada Corp. Report, Project No.1036,1998. [3] Carol L. Link, Statistical Consideration in Duration of Load Research, Forest Products Laboratory, Research Paper FPL-PR-486, 1988 [4] The Structural Design Guide for Wood-Framed Construction in Japan 2002, Japan 2 4 Home Builder Association