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INVESTIGATING METHODS TO EVALUATE IMPACT BEHAVIOR OF SHEATHING MATERIALS
U.S.D.A. FOREST SERVICE RESEARCH PAPER FPL 260 1975
U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WISCONSIN
No current test methods evaluate how sheathing itself resists impacts. This study approaches the problem by investigating the effect of panel size and edge conditions on impact behavior and the feasibility of testing a smaller panel over a single span with the joists fully supported.
Results indicate that all of these factors affect the impact behavior, and that the characteristics of the impacting object are also important.
Since data from tests of this type are best analyzed by comparison with other test results or with a standard, a smaller scale, single-span test with the joists fully supported is recommended for evaluating the sheathing developed as part of the Forest Service Structural Particleboard Program. This test procedure will eliminate the variability in- troduced by nailing and by the stiffness of joists and provide a more meaningful method of evaluation.
The results of this study will be useful to code and standard groups, to sheathing manufacturers, and to other researchers.
INVESTIGATING METHODS TO EVALUATE IMPACT BEHAVIOR OF SHEATHING MATERIALS1
By MICHAEL J. SUPERFESKY, Engineer
Forest Products Laboratory, 2 Forest Service U.S. Department of Agriculture
Sheathing materials for walls, roofs, and floors are subjected to a variety of loads during construction and throughout service life. The loads that are the most difficult to determine and which may produce con- siderable damage are the impact loads-those caused by the impact of the sheathing material with a moving body. Humans can cause this type of loading by jumping or falling. The loads may also be the result of dropping an object such as a roll of roofing, a tool, or a piece of furniture.
To design a sheathing material to resist impact loads, the magnitude of the load and the response of the sheathing must be deter- mined. But no design or test methods are available to evaluate sheathing material for im- pact, and there are no performance criteria for sheathing subjected to impact loading (8).3
The lack of a suitable method of evalua- tion is probably due to the complexity of the problem. The dynamic load and the response of sheathing depend both on the characteristics of the impacting body and properties of the sheathing. Some of the other important factors are weight, rigidity, and velocity of the impacting body and the geometry and the edge conditions of the sheathing.
Impact tests have historically been per- formed using methods similar to those described in ASTM E 72-68 (2). This test method provides for the evaluation of the en- tire wall, roof, or floor assembly which includes the framing members and coverings. Usually the weakest link in the component fails first and the other members are not evaluated. For example, if a floor joist fails in bending, no knowledge of the sheathing behavior will be obtained other than the fact that it did not fail. Since the data from a drop bag impact test of this type are utilized mainly for comparative purposes, it may not be necessary to conduct tests of full-scale components to evaluate materials such as sheathing. Considerable economy and simplicity could be achieved if tests were conducted using a smaller, single- span panel. Also, full support of the joists and elimination of nailing would block out nuisance variables, such as stiffness of the joists and
1 Acknowledgment is made to W.D. Godshall, Engineer, for his assistance in the develop- ment of instrumentation and interpretation of data.
2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
3 Italicized numbers in parentheses refer to litera- ture cited at end of report.
stiffness introduced by nailing, and facilitate testing.
The sandbag impact test method was probably originally developed to demonstrate the impact resistance of certain structural components (e.g., a built-up floor assembly). This type of test only indicates the relative abilities of structural components to resist simulated impact loads. Such information about an individual component is of most value when compared with other components or with a standard.
The main purposes of this study were to evaluate the effect of panel size and edge con- dition upon impact behavior and to investigate the feasibility of testing a smaller panel over a single span. Other objectives were to obtain impact deflection data with the joists fully sup- ported and to determine the feasibility of utiliz- ing a linear variable differential transformer (LVDT) and cathode ray oscilloscope for recording the dynamic deflection of the pan- els.
RESEARCH MATERIAL Two types of sheathing were used for this
study, a ½-inch commercial flake par- ticleboard and a ½-inch group 1 A-C grade plywood. The purpose of the selection was not to compare the different types of sheathing, but to investigate impact test methods using two materials with distinctively different characteristics. Four-by eight-foot sheets of each material were purchased from local suppliers and conditioned at 73 degrees F and
50 percent relative humidity. Smaller panels and specimens for determining mechanical properties were cut from the 4- by 8-foot sheets. Table 1 lists the mechanical properties of the materials obtained by small specimen tests in accordance with ASTM D 1037-72 (3) and ASTM D 805-63 (1). Two- by eight-inch joists for the panel test frames were Construc- tion grade and were purchased locally.
Table 1 . Propert ies of sheathing mater ia ls used in this s tudy
All impact tests were conducted using the 60-pound leather sandbag described by ASTM E 72-68. Two different test panel sizes were evaluated. Four- by eight-foot panels were nailed to joists spaced on 16-inch centers, the common spacing for floor joists and studs. Smaller panels were tested over a single clear span of 14-½ inches with the longer edges either simply supported or clamped. The joists for all panels were sup- ported on rigid members during the testing. It is probable that these procedures developed for a 16-inch span can also be utilized for a 24- inch span.
Test Frame The main support frame consisted of two
9-¼ by 36-inch glued-laminated girders 10 feet long spaced 5 feet on center. The girders were supported on a concrete floor and
laterally braced. Five- by nine-inch glued- laminated beams 5 feet long were spaced 16 inches on center and bolted to the girders as shown in figure 1. This system provided an es- sentially nondeformable support of the test frames and elevated the test area ap- proximately 3 feet. Elevation of the test frame aided instrumentation and allowed us to see the bottom face of the panel during testing. The frame for the 4- by 8-foot panel (fig. 1) consisted of 2- by 8-inch joists 5 feet long, spaced 16 inches on center with a 2- by 6-inch header nailed to each end. The 4- by 8-foot panel was nailed to the joists with eightpenny scaffold nails, which is the common size for this application, spaced 12 inches on interior joists and 6 inches on exterior joists. The ex- terior joists were bolted to the glued-laminated beams of the main support frame.
Figure 1.-Impact test frame with 4- by 8-foot panel in place for testing. (M 141 984-2)
The frame for the test with the longer edges of a 17-½ by 48-inch panel simply sup- ported consisted of two 2- by 8-inch joists 4 feet long and spaced 16 inches on center (fig. 2). The spacing was maintained with 2- by 4- inch struts and ½! inch-diameter bolts. The top edge of each joist was rounded to ¾ inch radius along the entire length. The joists were then bolted to the 5- by 9-inch glue-laminated girders which supported them. The panel was held in position during impact by 2- by 4-inch members which were fastened to the support joists. Each member was 4 feet long and the edge in contact with the panel was rounded to ¾-inch radius.
between the channels was maintained with 2- by 6-inch struts and ½ inch-diameter bolts. The channels were supported by the 5- by 9- inch glued-laminated girders. After a panel was in place on the support channel, a steel angle (A3 x 3 x ¼) was placed directly over the channel flange and clamped to it with three C- clamps torqued to 300 inch-pounds.
Figure 3.-Impact test frame for clamped edge condition.
(M 142 035-2)
Sandbag The 60-pound lather sandbag (ASTM E
72-68) was used as the projectile for falling body for all tests. The boag was released by a solenoid-activated pari of jaws remotely con- trolled by the operator of the recording equip- ment.
Instrumentation The deflection of the panels was
measured by an LVDT. One end of the LVDT was affixed to the panel and the other end rigidly supported by the concrete test floor (fig.Figure 2.-Impact test frame for simply sup- 4). The deflection signal from the LVDT wasported edge condtion.
(M 141 984-5) transmitted to a cathode ray oscilloscope which produced a deflection-time curve. A
The frame for the test with the longer single sweep of the oscilloscope was obtained edges of a 20-½ by 48-inch panel clamped with an electrical triggering