Creep response of engineered wood i-joist and oriented strand board (OSB) floor systems

Creep response of a full-scale floor system fabricated with composite wood I-joists and oriented strand board (OSB sheathing) was measured for 280 days at sustained uniform loads of 20 psf (0.96 kPa). Creep responses of bare I-joists and 3-joist T-beams with OSB sheathing (glued and unglued tongue and groove sheathing joints) were measured under similar load levels and duration. The creep deflections of each system nearly stabilized after 168 days of loading. Average ratios of creep to initial deflection were 1.70, 1.75 and 1.76 for the bare I-joist, T-beams (with unglued joints), and floor system, respectively. The observed percentage increase in creep deflection over initial deflection of the Ijoist/OSB floor system approximately 60 days after loading was greater (32% vs. 12%) than those reported by Fridley et al. (1997) for floor systems with solid-sawn dimensional lumber joists and plywood sheathing. INTRODUCTION AND OBJECTIVES This article summarizes the results of an ongoing research project to measure and predict the creep response of composite wood floor systems fabricated with engineered wood I-joists and oriented strand board (OSB) sheathing. In this study creep responses of bare I-joists, three-joist T-beams, and a full-scale floor system were measured at stress levels corresponding to those experienced in a typical residential floor system under dead plus sustained live load conditions. The focus of this article is on the experimental phase of the study. Total and relative (∆relative = ∆(t) – ∆o) deflection versus time responses of the I-joists, the three-joist T-beams, and the full scale floor system are presented in tabular and graphical formats for load duration periods up to 280 days as are the ratios of time-dependent to initial deflections. Finally, the ratios of time-dependent to initial deflections at several load duration periods between the I-joist, the threejoist T-beam, the full scale floor system, and solid-sawn dimensional lumber/plywood sheathed floor systems are compared and trends interpreted. The research was undertaken because there is little documentation of the creep response of floor systems fabricated with wood I-joists and OSB sheathing. Hereinafter this type floor system will be referred to as the I-joist/OSB floor system. Considering that engineered wood I-joists now represent approximately 33% of all residential floor systems being constructed in the United States (Industry Briefs, 1998) and that OSB sheathing is rapidly replacing plywood in floor applications in both stick-framed and manufactured housing, the need to document the long-term performance of composite floor systems is apparent. Current U. S. wood/timber design standards and codes (e. g, National Design Specifications for Wood Construction (NDS), 1997) take a simple and somewhat conservative approach to quantify creep effects. The effects of creep in deflection calculations for dimensional lumber wood systems are estimated by multiplying the instantaneous deflection due to the applied long-term load by a modification factor. The modification factor is 1.5 for seasoned lumber and 2.0 for unseasoned lumber. One can now only assume the same modification factors apply for composite wood systems. The overall objective of the ongoing research project is to characterize the time-dependent deflection response of a composite wood I-joist (laminated veneer lumber (LVL) flange and OSB web) and OSB sheathing floor system (Ijoist/OSB floor system). The scope of the research reported herein is limited to experimental measurement of the creep deflection of the bare I-joists, the three-joist T-beams fabricated with composite wood I-joists and OSB sheathing and a ten-joist I-joist/OSB floor system. The specific objectives of the research reported herein are to: (1) Measure the creep 1 Graduate Assistant, Agricultural and Biological Engineering, Penn State University, University Park, PA, USA. 2 Distinguished Professor, Agricultural and Biological Engineering, Penn State University, University Park, PA, USA. 3 Associate Professor, School of Forest Resources, Penn State University, University Park, PA, USA. deflection of the bare I-joist used in the full scale floor system; (2) Measure the creep deflection of three-joist T-beam sections of the I-joist/OSB floor system (one-way flexure); (3) Measure the long-term creep deflection of a 10-joist Ijoist/OSB floor system (two-way flexure); (4) Compare the creep deflections of the bare joists and the T-beams to those of the I-joist/OSB floor system and compare the creep deflections of the I-joist/OSB floor system to those reported for floors with dimensional lumber joists and plywood sheathing.