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Khademibami, L., Shmulsky, R., Senalik, C. A., Seale, R. D., Ross, R. J., Mohammadabadi, M., Ward, K., and Williamson, T. (2023). “Flexural testing of structural insulated panels before and after creep testing,” BioResources 18(2), 3319-3327.

Abstract

The effect of duration of load testing on flexural properties of structural insulated panels was investigated herein. Structural insulated panels were manufactured by a member of the Structural Insulated Panel Association (SIPA) in accordance with International Code Council-Evaluation Service Report 4689. Two panel depths 16.5 cm and 31.1 cm (6.5 in. and 12.25 in.) were tested in short duration 1/3-point bending per American Society for Testing and Materials standards. All structural insulated panels had joints or discontinuities in the foam layer in a location that was subject to shear stress during the bending tests. Failure mode for all panels was horizontal shear within the foam layer. Within each panel depth, no statistically significant differences were detected between the maximum load values before and after creep testing. This finding indicates that the creep test loading was not detrimental to the strength of the structural insulated panels. While the results were not deemed to be statistically different for the Δymax (midspan deflection at Pmax) for the 31.1 cm depth class, they were statistically different from the 16.5 cm depth class. Overall, it appeared that there was minimal effect of the creep test loading on Δymax of the SIPs.


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Flexural Testing of Structural Insulated Panels Before and After Creep Testing

Laya Khademibami,a,* Rubin Shmulsky,a Christopher Adam Senalik,b Roy Daniel Seale,a Robert J. Ross,b Mostafa Mohammadabadi,a Keith Ward,a and Thomas Williamson c

The effect of duration of load testing on flexural properties of structural insulated panels was investigated herein. Structural insulated panels were manufactured by a member of the Structural Insulated Panel Association (SIPA) in accordance with International Code Council-Evaluation Service Report 4689. Two panel depths 16.5 cm and 31.1 cm (6.5 in. and 12.25 in.) were tested in short duration 1/3-point bending per American Society for Testing and Materials standards. All structural insulated panels had joints or discontinuities in the foam layer in a location that was subject to shear stress during the bending tests. Failure mode for all panels was horizontal shear within the foam layer. Within each panel depth, no statistically significant differences were detected between the maximum load values before and after creep testing. This finding indicates that the creep test loading was not detrimental to the strength of the structural insulated panels. While the results were not deemed to be statistically different for the Δymax (midspan deflection at Pmax) for the 31.1 cm depth class, they were statistically different from the 16.5 cm depth class. Overall, it appeared that there was minimal effect of the creep test loading on Δymax of the SIPs.

DOI: 10.15376/biores.18.2.3319-3327

Keywords: Structural insulated panels (SIPs); Duration of load testing; Bending test; Foam; Building products; Construction materials

Contact information: a: Department of Sustainable Bioproducts, Mississippi State University, Box 9820, Mississippi State, MS 39762 USA; b: Forest Products Laboratory, Forest Service, US Department of Agriculture, One Gifford Pinchot Drive Madison, WI 53726 USA; c: SIPA TAC Chair, Managing Partner, Timber Engineering LLC, Vancouver, WA 98683 USA;

* Corresponding author: Lk475@msstate.edu

INTRODUCTION

Structural insulated panels (SIPs) are sandwich-type products that are available commercially throughout North America. The SIPs contain an insulating foam core sandwiched between two structural facings, which typically are oriented strand board (OSB). They are widely used in both residential and nonresidential construction. They have highly reliable strength and stiffness properties that allow designers to use them in a wide range of structural applications. Moreover, they are lightweight, prefabricated for ease of installation, highly energy efficient, and meet the requirements of the 2021 International Energy Conservation Code (IECC). Additionally, they are compliant with International Residential Code (IRC) and the International Building Code (IBC), the latter through International Code Council Evaluation Service (ICC-ES) reports.

During typical manufacturing, OSB is applied as face layers over expanded polystyrene (EPS) foam panel cores using adhesives that must comply with ASTM D2559 and ICC AC05. Other facers or foam cores can be used. Discontinuities may occur in the foam where two pieces are butted together in the core layer. It has been previously shown (Shmulsky et al. 2022) that foam discontinuity influences overall mechanical properties of a SIP in bending when it is located within the shear zone (that is between the reaction support and the load head of the 1/3-point bending fixture). With this influence in mind, each specimen in this study had foam discontinuities located in the shear zone to ensure consistent, conservative property estimates.

For wood-based products, the duration of load and creep effects tests follow American Society for Testing and Materials (ASTM) D6815 (2015). Therein, the dead load values (and associated bending stress values) for the long-term “creep-rupture” or duration of load testing are based on laboratory short-term static bending tests: The specimens selected for these tests shall be tested at a constant stress level, fb, as determined in accordance with Eq. 1 of D6815 where fb= 0.55 × (5% PE), defined as fb = minimum applied bending stress, and 5% PE = the lower 5% point estimate, as determined from the short-term bending tests. For the 30-specimen samples herein, the respective 5% non-parametric point estimates were determined in accordance with ASTM D2915 (2017). As referenced in ASTM D6815 (2015), the short-term bending (hereafter static bending) tests were conducted per ASTM D4761 (2013), with testing following the joist form materials and Test Methods.” This standard specifies full-scale flexural testing, that is testing in the structural size(s) in 1/3-point bending at a span: depth ratio ranging from 17:1 to 21:1. Per ASTM D4761 (2013), the rate of loading was maintained such that time to failure was approximately 1 min. Previous works (McDonald et al. 2014, 2018) investigated the creep behavior of SIPs with varying sample sizes. To date, no definitive information has become available regarding the full-scale flexural properties of SIPs after long term flexural stress at creep design bending loads. Therefore, the objective of this research is to compare the strength (Pmax) and midspan deflection at Pmaxymax) of SIP panels before and after load duration testing. Two depths, 31.1 and 16.5 cm (12.25 and 6.5 in.), were considered. Because the section properties of all specimens were the same, maximum load (Pmax) trends are equivalent to both MOR and maximum moment (Mmax) trends. Consistent with previous works, midspan deflection Δymax at Pmax are reported herein as an indication of panel stiffness.

EXPERIMENTAL

Specimens’ Characteristics

The SIPs were manufactured at a Structural Insulated Panel Association (SIPA) member’s commercial facility in accordance with ICC, Evaluation Service Report (ESR) 4689. Two depth classes, 16.5 and 31.1 cm (6.5 and12.25 in.) deep, were evaluated. All specimens were approximately 29.8 cm (11.8 in.) wide with 317 cm (125 in.) length for depth class 16.5 cm and 591.8 cm (233 in.) length for depth class 31.1 cm. Because they were manufactured at two different times, approximately half of the matched specimen pairs in each depth class had a foam density of approximately 0.016 g/cm3 (1.0 lb/ft3), while approximately half of the matched specimen pairs in each depth class had a foam density of approximately 0.02 g/cm3 (1.2 lb/ft3). Data from beams of differing foam densities were pooled because the SIPs were considered commodity products with non-differing design properties. Thus, potential differences related to foam densities are not considered herein.

The SIPs had EPS foam cores with 1.11 cm (7/16 in.) thick OSB facers. The OSB was APA-the Engineered Wood Association- Performance Rated Panels (PR-N610) that were produced by a North American-based commercial production facility. Test specimens had facers with a strong axis orientation with the OSB strength axis oriented parallel with the length of the SIP panel. The OSB was full length. That is, there were no visible end joints in either the top or bottom OSB facers.

Testing

The research detailed herein occurred in three phases. Each phase used similarly specified SIPs. Prior to testing, specimens were segregated into matched pairs. Each pair was from the same parent (or master) panel. From each side matched pair, one specimen was used in a short-term bending test (phase 1), while the other specimen was used in the duration of load testing (phase 2) followed by a short-term bending test (phase 3). In this manner, variation was minimized within the matched pairs in the effort to compare flexure properties before and after duration of load testing.

Phase 1: Initial full-scale short-term bending tests

Destructive static testing of two SIP’s depths, 16.5 and 31.1 cm (6.5 and 12.25 in.), was conducted in short duration 1/3-point bending per ASTM-D6815 (2015). Figure 1 illustrates an exemplar full-scale 16.5 cm (6.5 in.) deep SIP test specimen in the 1/3-point bending test fixture awaiting the short-term bending test.

Fig. 1. Exemplar full-scale 16.5 cm (6.5 in) deep SIP test specimen in the 1/3-point test fixture

Specimens were tested in 1/3-point flexure at an approximate 18:1 span to depth ratio. Table 1 lists the various test combinations along with the number of specimens within each category. The failure mode in each case was horizontal shear within the foam core layer. Consistent with ASTM D6815, time to failure was targeted at 1 minute and the rate of loading was set accordingly.

Table 1. Sample Sizes of Short-Term Bending Tests Before and After Creep Tests

Phase 2: 90-day full-scale duration of load tests with matched specimens, followed by 30 days with no load for creep recovery (creep test)

This phase was conducted according to the ASTM-D6815 (2015). Figure 2 illustrates the battery of matched specimens undergoing the 90-day full-scale duration of load testing using sandbags for the dead loads. The results of Phase 2 duration of load testing are not reported herein and will be published in a subsequent report. Creep load levels were at the 5%-point estimate for bending stress as per ASTM-D6815.

Fig. 2. Battery of full-scale matched SIP specimens undergoing the 90-day duration of load testing

Phase 3: Final full-scale short-term bending tests on the specimens subjected to the 90-day duration of load test followed by a 30-day recovery cycle

Phase 3 testing followed the same protocol as Phase 1, wherein destructive static testing of two SIP’s depths, 16.5 and 31.1 cm (6.5 and 12.25 in.), was conducted in short duration 1/3-point bending per ASTM-D6815 (2015). Specimens were tested in 1/3-point flexure at an approximate 18:1 span to depth ratio. However, the specimens in Phase 3 were those that had been subjected to the long-term duration of load testing (i.e., Phase 2). Consistent with ASTM D6815, time to failure was targeted at 1 minute and the rate of loading was set accordingly.

The results of the initial (Phase 1) and final (Phase 3) full-scale bending tests are reported herein.

STATISTICAL ANALYSIS

All data that belong to the maximum load and deflection were analyzed by both one-way analysis of variance (ANOVA) and paired t-tests using the procedure for linear mixed models (PROC GLIMMIX) and PROC TTEST of SAS 9.4© (SAS Institute, Cary, NC, USA). Differences were deemed significant at P ≤ 0.05. The following model was used for analysis of the data that belong to one-way ANOVA (Eq. 1),

Yi = μ + Li + Ei (1)

where μ is the population mean; Li is the effect of duration of load (creep test) (L = 1 to 2); and Ei is the residual error.

RESULTS AND DISCUSSION

Summary statistics for Pmax and ymax were computed within each depth class; 16.5 cm (6.5 in.) and 31.1 cm (12.25 in.), respectively. Summary statistic results of Pmax and ymax before and after creep test for both depth classes are presented in Table 2.

The results indicate that there was little to no difference in the average Pmax before versus after creep testing for both depth classes and for the Δymax for the 31.1 cm (12.25 in) depth class with the differences being less than 2%. For the 16.5 cm (6.5 in.) depth class, the data show that the after creep testing average Δymax was approximately 7% greater than the before creep testing value. These results indicate there was minimal effect of the creep test loading on the strength (Pmax) and Δymax for the test panels. To further analyze these results on a more rigorous statistical basis, an ANOVA and paired t-test analysis were completed as follows.

Both ANOVA and paired t-tests were used to test for differences in Pmax, before and after duration of load testing. T-test pairing is based on the matched specimens that originate from the same parent panel. The results indicated that any difference in Pmax before versus after duration of load testing was not statistically significant. The statistical ANOVA show that there was no difference in Pmax of SIP specimens before versus after creep tests in both depth class (Table 2). The same result was observed in the paired T-test results for two depth classes, 16.5 cm (6.5 in.) and 31.1 cm (12.25 in.) (Table 3).

Table 2. Summary Statistics and ANOVA Results for Both Depth Classes; Before Versus After Creep Test

Table 3. Paired T-test Results for Both Depth Classes; Before versus After Creep Test

Because the load configuration and section properties were the same, Pmax trends directly relate to MOR and maximum bending moment trends.

The results showed that ymax was from 2% to 7% higher depending on depth in SIP specimens after creep test compared to before. Here again, both ANOVA and paired t-tests were used to test for differences in ymax, before and after duration of load testing. T-test pairing is based on the matched specimens that originate from the same parent panel. The results indicate that the difference in ymax before versus after duration of load testing is statistically significant for the 16.5 cm (6.5 in.) but not for the 31.1 cm (12.25 in) depth class. This finding is consistent for both ANOVA and the paired t-test (Tables 2 and 3).

CONCLUSIONS

  1. Results of both the ANOVA and the paired T-tests indicated that no statistically significant difference was detected in the matched static bending specimen Pmax values before versus after creep testing in either SIP depth class. Because the specimens are of like section properties and span, the Pmax comparison is indicative of both MOR and maximum moment (Mmax) capacity.
  2. Results of both the ANOVA and the paired t-tests indicated that no statistically significant difference was detected in the matched static bending specimen Δymax values before versus after creep testing for the 31.1 cm depth class, but it was statistically different for the 16.5 cm depth class although the difference in average values was less than 7%.
  3. The finding of no statistically significant differences between the Pmax before and after creep testing indicates that the creep test loading is not detrimental to the strength of the SIPs. While the results were not deemed to be statistically different for the Δymax for the 31.1 cm depth class, they were statistically different for the 16.5 cm depth class. Overall it appeared that there was minimal effect of the creep test loading on the midspan deflection (Δymax) of the SIPs.
  4. SIPs would be applicable for any roof applications. The National Design Specification for Wood Construction (NDS) provides load duration and creep factors (Kcr) for a wide range of wood products but no values are provided for SIPs.

ACKNOWLEDGEMENTS

This publication is a contribution of the Forest and Wildlife Research Center, Mississippi State University. The authors acknowledge the support from USDA Forest Service Forest Products Laboratory (FPL) in Madison, Wisconsin, as a major contributor of technical assistance, advice, and guidance to this research. The authors also acknowledge USDA Forest Products Laboratory, Madison, WI and the Structural Insulated Panel Association (SIPA) for providing funding and materials, respectively.

Additionally, the authors would like to express appreciation to Dr. Jane Parish and the staff at Mississippi Agriculture and Forestry Experiment Station’s North Mississippi Research and Extension Center, Mississippi State University, Verona, MS.

REFERENCES CITED

ASTM D2559-12a (2018). “Standard specification for adhesives for bonded structural wood products for use under exterior exposure conditions,” ASTM International, West Conshohocken, PA, USA.

ASTM D2915-17 (2017). “Standard practice for sampling and data-analysis for structural wood and wood-based products,” ASTM International, West Conshohocken, PA, USA.

ASTM D4761-13 (2013). “Standard test methods for mechanical properties of lumber and wood-base structural material,” ASTM International, West Conshohocken, PA, USA.

ASTM D6815-15 (2015). “Standard specification for evaluation of duration of load and creep effects of wood and wood-based products,” ASTM International, West Conshohocken, PA, USA.

McDonald, D., Begal, M., Senalik, C. A., Ross, R., Skaggs, T. D., Yeh, B., and Williamson, T. (2014). Creep Behavior of Structural Insulated Panels (SIPs): Results from a Pilot Study (Research Note FPL-RN-0332), U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, USA. DOI: 10.2737/FPL-RN-332

McDonald, D. E., Begel, M., Senalik, C. A., and Williamson, T. (2018). Evaluation of Creep Performance of Structural Insulated Panels (SIPs): Phase 2 (Research Paper FPL-RP-697), U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, USA. DOI: 10.2737/FPL-RP-697

Shmulsky, R., KhademiBami, L., Senalik, C. A., Seale, R. D., and Ross, R. J. (2022). “The influence of foam discontinuity in the shear zone of structural insulated panel beams,” Wood and Fiber Science 54(3), 187-195. DOI: 10.22382/wfs-2022-18

Article submitted: January 25, 2023; Peer review completed: February 18, 2023; Revised version received and accepted: February 28, 2023; Published: March 21, 2023.

DOI: 10.15376/biores.18.2.3319-3327