CREEP BEHAVIOR OF WOOD I-JOISTS WITH WEB OPENINGS EXPOSED TO NORMAL AND LOW RELATIVE HUMIDITY CONDITIONS

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1 CREEP BEHAVIOR OF WOOD I-JOISTS WITH WEB OPENINGS EXPOSED TO NORMAL AND LOW RELATIVE HUMIDITY CONDITIONS Richard W. Runyen 1, David W. Dinehart 2, Shawn P. Gross 3, and W. Gary Dunn 4 ABSTRACT: The purpose of this study was to investigate the creep behavior of wood I-joists with and without web modifications subjected to various environmental conditions. The experimental testing program consisted of twenty inch depth I-joists all with a length of The I-joists were divided into two environmental conditions based on relative humidity (RH): 50% and 75%. The web modifications consisted of holes deemed either circular acceptable, circular unacceptable, square acceptable, square unacceptable, or no modifications. All I-joists were simply supported and loaded with an equivalent 20 psf floor load with I-joist spacing at 16. Deflection at center span, temperature, and humidity readings were taken over a six month period throughout the testing program. It was concluded that I-joists exposed to 50% RH had an initial creep rate nearly half that of I-joists exposed to higher RH conditions. In an environment with a relative humidity within expected ranges, I-joists with web openings in unacceptable locations will experience higher creep rates than those with openings meeting code or manufacturer specifications. Conversely, there is no discernable difference between I-joists with acceptable and unacceptable openings a low relative humidity. KEYWORDS: Wood; I-joists; Creep; Openings; Constant Humidity, Constant Temperature 1 INTRODUCTION Compared to sawn lumber products, engineered wood materials are relatively new to the construction industry. Wood I-joists found their beginnings in 1959 when the Douglas Fir Plywood Association published a document entitled DFPA Specification BB-8, Design of Plywood Beams [1]. Using this specification which outlined the procedures for manufacturing the first wood I-joists, the Trus Joist Corporation (now a Weyerhaeuser Company) began commercializing I-joists for high end home construction. Ever since this introduction of the wood I- joist, most manufacturers have had their own proprietary standards for manufacturing and designing these members. Although a lack of consistent codes and specifications was the norm for many years in the I-joists 1 Richard W. Runyen, Graduate Student, Villanova University, Dept. of Civil and Environmental Engineering, 800 Lancaster Ave., Villanova, PA richard.runyen@villanova.edu 2 David W. Dinehart, Professor, Villanova University, Dept. of Civil and Environmental Engineering, 800 Lancaster Ave., Villanova, PA david.dinehart@villanova.edu 3 Shawn P. Gross, Associate Professor, Villanova University, Dept. of Civil and Environmental Engineering, 800 Lancaster Ave., Villanova, PA shawn.gross@villanova.edu 4 W. Gary Dunn, Senior Engineering Manager, Boise Cascade EWP, P.O. Box 2400, White City, OR w.dunn@boisebuilding.com industry, there has been a recent drive for consistency. One major move toward a universal code was the release of ASTM D-5055 in 1990 [2]. This was the first universally recognized standard for wood I-joists [3]. Such standards have contributed to the growth of wood I-joists in the housing sector in the past 10 to 20 years. Along with additional codes and standards being released, two other factors played significant parts in the growth of this industry. The first is the need for affordable family dwellings. Engineered wood, while costing more initially, can cut the overall cost of housing drastically due to fewer structural members being needed and lower installation expenses. The other principle cause for this increase relates to the environment. Engineered wood I-joists utilize less wood to achieve the same if not better properties of solid sawn members. Not only are the I-joists themselves fabricated from less wood, the overall construction of a home uses fewer I- joists than if solid sawn members were used. Together, these two factors lead to fewer trees being used by the housing industry. Wood I-joists consist of two components, a web and flange members, all held together by an adhesive. Flanges are typically fabricated from sawn lumber, laminated strand lumber, or laminated veneer lumber. Web sections are made from plywood or oriented strand board (OSB). Adhesives are used to secure all

2 connections in the I-joist including flange-flange joints, web-web joints, and flange-web connections [3]. 2 MOTIVATION The rise in the amount of wood I-joists used throughout the construction industry points to an expanding trade; however, this also raises the potential for the misuse of these products. There are several key ways in which I- joists can be severely misused including improper design and cutting of openings through flanges and in allowable locations in webs. It has been shown that the effects of web openings can be significant on the load-deflection behavior of I-joists [4]. Additionally, wood is a natural material, and as such, its material properties vary with temperature and relative humidity. The combination of the effects of web openings and moisture on the long term deflection of I-joists is reported herein. Limited research exists concerning the effects on the creep behavior of I-joists with web openings and web openings combined with various moisture environments. Therefore, the goal of this research was to investigate the effect web openings have on the creep behavior of engineered wood I-joists. The research also focused on how lower than normal humidity conditions along with unacceptable web opening locations effects creep. 3 EXPERIMENTAL TESTING 3.1 TEST SPECIMENS The experimental portion of the study consisted of testing twenty engineered wood I-joists manufactured by Boise Engineered Wood Products. All specimens were inch deep, AJS 20 Series I-joists with cross sections shown in Figure 1. The clear span of the I-joists was held constant at Flanges consisted of machine stress rated lumber designated as MSR 2100F b - 1.8E. Webs were made from OSB panels. openings. For the I-joists with web openings an additional designation of A or U was used; A-acceptable web openings (meets manufacturer specifications), U- unacceptable web openings (violates manufacturer specifications). Opening size and location were determined to be acceptable or unacceptable based on the AllJoist Specifier Guide (2009) [5]. In the case where more than one specimen was used for a single design a designation of.1 or.2 was used. Renderings of the opening layouts are presented in Figure 2. Table 1: Details of inch depth I-joists Joist Number Joist Desciption Humidity Conditions Location of 6" Opening (in) L-B.1 Baseline Low L-B.2 Baseline Low L-C-A.1 Circular Acceptable Low L-C-A.2 Circular Acceptable Low L-C-U.1 Circular Unacceptable Low L-C-U.2 Circular Unacceptable Low L-S-A.1 Square Acceptable Low L-S-A.2 Square Acceptable Low L-S-U.1 Square Unacceptable Low L-S-U.2 Square Unacceptable Low N-B.1 Baseline Normal N-B.2 Baseline Normal N-C-A.1 Circular Acceptable Normal N-C-A.2 Circular Acceptable Normal N-C-U.1 Circular Unacceptable Normal N-C-U.2 Circular Unacceptable Normal N-S-A.1 Square Acceptable Normal N-S-A.2 Square Acceptable Normal N-S-U.1 Square Unacceptable Normal N-S-U.2 Square Unacceptable Normal 18 Figure 1: Dimensions of in. depth I-joists The location of two 6-inch web openings and the environmental conditions were varied during testing. Table 1 provides details of the inch depth test specimens using the following nomenclature. The refers to the depth of the I-joist. The letter following the I-joist depth corresponds to the humidity conditions the I-joist was tested in; L-low humidity, N-normal humidity. The next designation corresponds to the type of web opening in the I-joist; B-baseline specimens consisting of no openings, C-circular openings, S-square Figure 2: Test specimens I-Joist modifications Prior to testing, all I-joists with the exception of the baseline specimens were modified. For each specimen a total of two openings were drilled in the I-joist web. To simplify the modification process, jigs were constructed using plywood. Jigs measured 8 long and inch

3 wide in order to fit in the web area of the I-joists. Because the opening layouts were symmetric, the jigs were used for both sides of the I-joists. Circular holes were drilled using an electric drill and centered with an arbor drill bit. For square openings, the electric drill, equipped with a ¼ drill bit, was used to remove the material located in the corners of the square openings. An electric jig saw was then inserted into the corners and used to remove the remaining material. Once the jigs were completed they were clamped to the web of the appropriate I-joist. The openings were then created in the web using the same techniques as used for the jigs Environmental conditions I-joists were separated into two testing conditions based on the relative humidity (RH) of the environment. The first condition exposed the I-joists to fluctuating RH and temperatures. This environment replicated the conditions an I-joist may experience in the basement or attic of a home. These I-joists were kept indoors and were not exposed to the elements, but rather the changes in RH and temperature associated with exterior climate changes. These I-joists, called normal I-joists, were exposed primarily to RH values of 70% and temperatures around 20 C. In total, ten I-joists with inch depth were tested in the normal environment. A replicate set of ten specimens were tested in a 20 x 15 environmental chamber housed in the Villanova University Structural Engineering Teaching and Research Laboratory. The environmental condition for these I-joists, called low I-joists, were a constant RH of 50% and a constant temperature of 21 C. throughout the test. 3.2 EXPERIMENTAL SETUP All I-joists were supported by steel supports and secured to concrete masonry unit (CMU) block. The blocks were stacked in groups of three in order to ensure sufficient clearance for the recording of the dial gauges. The rollers, shown in Figure 3, were fabricated from a ¾ steel rod welded to a 10 x 2 x 3 / 4 plate. A liquid nail adhesive was used to secure the steel to the CMU block. All I-joists were subjected to a simulated 20 psf floor load with I-joist spacing of 16 on center and corresponds to a service level load of 27 lb/ft distributed load on each I-joist. I-joists were loaded in pairs to ensure lateral stability. I-joists pairs were created by nailing a 4-inch piece of 2 x 4 lumber at the ends of each pair, as shown in Figure 3. The setup allowed for the individual I-joists to freely rotate independently of one another. Testing the I-joists in pairs created a 54 lb/ft load on each pair. Additional CMU blocks filled with gravel were used to simulate a service load. An 8-inch wide by 8-foot long piece of plywood was first placed on each I-joist pair. A level of ten CMU blocks was then rested on the plywood. Gravel was used to fill the cells of these blocks. An additional level of eight CMU blocks was then placed on the first row. All blocks were placed symmetrically on the I-joists. The loading configuration is shown Figure 4. Figure 4: Loading configuration A dial gauge was used to measure deflection at the midpoint of each I-joist. In order to obtain the height needed to reach the bottom flanges of the I-joists, stands fabricated from 7 x 7 x ¾ plate and four ½ threaded rods were used to support tracks made from 2 1-inch angles. The dial gauges were attached to magnetic bases which were secured to the steel tracks. The entire dial gauge setup can be seen in Figure 5. Figure 3: Support conditions Figure 5: Dial gauge setup

4 I-joists were loaded in pairs; consequently, the average deflection of the two I-joists in that pair was used in creep calculations. Moisture content readings were collected using a Delmhorst Instrument Co. model BD moisture content gauge. Moisture content readings were collected from the top flange, the web, and the bottom flange of every I-joist periodically before and throughout the testing program. 11-N-C-A 11-N-C-U 11-N-S-A 11-N-S-U Prior to loading, all I-joists were visually inspected to locate any irregularities in the wood. Finger joints, webweb joints, as well as knots, deformations and warping were also noted and recorded. Similar inspections were also conducted to note any changes in the I-joists during the testing period. 4 EXPERIMENTAL RESULTS 4.1 NORMAL HUMIDITY The I-joists exposed to normal environmental conditions sustained load for a period of 183 days (July-January). During this time, the I-joists were subjected to fluctuating temperatures and relative humidity. Temperatures ranged from 22 C to 15 C (71.6 to 59 F) while the relative humidity ranged from 90% to 40%. These changes in temperature and humidity usually occurred over a period of time and were related to the changing of the seasons. Abnormal weather fluctuations had a more rapid effect on the conditions on a few occasions, but overall were relatively constant as shown in Figures 6 and 7. Figure 8 shows the ratio of total deflection to intitial deflection over the entire test period for the 5 sets of I-joists in the normal environmental conditions. Relative Humidity (%) Time (Days) Figure 6: RH readings for normal conditions Temperature ( C) Figure 8: Creep ratio vs. time for normal humidity I-joists 4.2 LOW HUMIDITY The I-joists exposed to low humidity conditions sustained load for a period of 133 days (September- January). Unlike the I-joists in the normal conditions, these I-joists were not subjected to fluctuating temperatures and humidity. Temperature was held constant at 21.1 C (70 F) while the relative humidity was held at 50%. Figure 9 shows the ratio of total deflection to intitial deflection over the entire test period for the 5 sets of I-joists in the low environmental conditions. 11-N-C-A 11-N-C-U 11-N-S-A 11-N-S-U Figure 9: Creep ratio vs. time for low humidity I-joists 4.3 COMPARISON OF NORMAL AND LOW RH Figure 10 presents the data for both the normal and low conditions for the I-joists that meet the manufacturer s design specification, while Figure 11 highlights the creep behavior for the I-joists with unacceptable openings subjected to both environmental conditions. Baseline results are included in both Figures 10 and 11 for comparison Time (Days) Figure 7: Temperature readings for normal conditions

5 11-L-B 11-L-C-A 11-L-S-A 11-N-C-A 11-N-S-A Figure 10: Creep ratio vs. time for acceptable I-joists 11-L-B 11-L-C-U 11-L-S-U 11-N-C-U 11-N-S-U Figure 11: Creep ratio vs. time for unacceptable I-joists 5 ANALYSIS AND DISCUSSION 5.1 ENVIRONMENTAL EFFECTS Inspection of Figures 10 and 11 clearly highlights the difference in the creep behavior of I-joists when subjected to normal and low relative humidity. While the creep deflections of some I-joists in the normal conditions reached 1.73 times their initial deflections at 2500 hours, the largest creep deflections any I-joists experienced in the low conditions were only 1.43 times their initial deflections at the same hour mark. Figure 9 shows the creep rates in the low conditioned I-joists appear to have flattened completely at the 500 hour mark, with little evidence that any increase would be seen at a further time. The initial deflections also differ, as the normal I-joists display a much steeper creep rate in their first 500 hours compared to the low I-joists. This behavior is most likely due to the difference in relative humidity between the two environments at the time of loading. The low I-joists were loaded at 50% RH while the normal I-joists were loaded at 70% RH. This increase in RH is most likely the cause of this increase in initial creep rates. 5.2 EFFECTS OF WEB MODIFICATIONS The modifications made to the webs of the I-joists exposed to normal environmental conditions had a clear effect on those I-joists creep behavior. Figure 8 clearly shows distinct patterns in the creep rates of each type of I-joist. Firstly, both the unacceptable square (S-U) and unacceptable circular (C-U) I-joists had increasing creep rates throughout the entire test. Both types of I-joists had creep rates of x 10-4 in/hr from 400 hours on. Of these two types, the unacceptable square I-joists had a higher total creep ratio of compared to the unacceptable circular I-joists total ratio of The creep rates of the baseline (B) and acceptable square (S-A) I-joists both leveled off at 400 hours, with slight increases toward the end of the test period reported. The final creep ratios for the baseline and acceptable square I-joists were and 1.420, respectively. Finally, the acceptable circular (C-A) I-joists had changing patterns in their creep rates. After 400 hours, the creep rates of these I-joists began to slow, similar to the baseline and acceptable square I-joists; however, from the 1700 hour mark on these I-joists began to creep at a rate of x 10-4 in/hr. These results show that as the web holes move toward the supports, the creep rate of the I-joists increase. Also, the square holes at the supports appeared to have a larger effect on the creep rate than the circular holes at the same location. The I-joists exposed to low environmental conditions did not show the same results as the I-joists in the normal conditions. Figure 9 shows that square holes had the least effect on creep rate while circular holes had the most effect. The baseline I-joists fell in between these; however, because the difference between the highest creep ratio and lowest creep ratio is only compared to a difference of 0.6 in the normal conditioned I-joists, it is hard to determine with certainty whether or not the web openings had a significant effect on the creep behavior in this environment. 5.3 COMBINATION OF CONDITIONS As previously discussed, lowering RH conditions decreases the creep rates of the I-joists. It was also discussed that web openings placed near supports increases the creep; however, the creep rates of the unacceptable I-joists in low humidity conditions, shown in Figure 9, do not demonstrate an increased creep rate. All I-joists in this environment have a stable creep rate, implying that a certain RH value must be reached for the web holes to have an effect on the creep behavior of the I-joists. 6 CONCLUSIONS Based on the creep testing of twenty inch deep 15.5-foot span I-joists in both normal and low environmental conditions, the following initial conclusions can be made: 1. Low humidity conditions affect the initial creep rates of wood I-joists. I-joists exposed to 50% RH had an initial creep rate nearly half that of I-joists exposed to higher RH conditions. 2. In an environment with a relative humidity within expected ranges, I-joists with web openings in unacceptable locations will experience higher creep rates than those with openings meeting code or manufacturer specifications.

6 3. At a low relative humidity, I-joists with openings drilled in unacceptable locations will not demonstrate an increased creep rate. Rather, at a low enough RH these I-joists will have creep rates similar to those I-joists with acceptable or no holes. Due to wood s natural variation from member to member, future testing of I-joists should be conducted to create a larger sample size. Currently, a study similar to the one discussed is ongoing with 9.5-inch depth I-joists subjected to identical environmental conditions. Different changes in the environmental conditions, such as high and low temperatures and the effects of high RH, are also currently being investigated. Future research should include investigation of the effects of span length and other opening sizes and arrangements before these results can be used to aid in codes or specification modification. ACKNOWLEDGEMENT The writers acknowledge Boise Cascade for their donation of the I-joists used in this study and Villanova University and Michael Baker Corporation for financial support for the project. REFERENCES [1] Faherty K.: Wood Engineering and Construction Handbook. 3 rd Ed. McGraw-Hill, Inc. New York, [2] ASTM International. Standard specification for establishing and monitoring structural capacities of prefabricated wood I-joists. ASTM D , West Conshohocken, Pa, [3] Yeh, B., ASTM Standards for Engineered Wood Products, Focus on Glulam, I-Joists, and Structural Composite Lumber. ASTM Standardization News, June, [4] Morrissey, G. C., Dinehart, D. W., and Dunn, W. G., Wood I-Joists with Excessive Web Openings: An Experimental and Analytical Investigation, ASCE Journal of Structural Engineering, 135:6, [5] Boise AllJoist Limited. AllJoist specifier guide USA, 11 th Ed., Boise Cascade Corporation, St. Jacques, New Brunswick, Canada, 2009.