INVESTIGATING METHODS TO EVALUATE IMPACT BEHAVIOR OF SHEATHING MATERIALS

U.S.D.A. FOREST SERVICE RESEARCH PAPER FPL 260 1975

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U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WISCONSIN

ABSTRACT

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

INTRODUCTION

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.

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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

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RESEARCH METHODS

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)

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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 device, which con- consisted of two steel channels (C8 x 18.7) 4 sisted of wire mesh attached to the sandbag feet long (fig. 3). The clear span of 14-½ inches and to the test panel at the impact point.

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Figure 4.-Panel cut to show instrumentation. (M 141 898-5)

Immediately before impact, the two pieces of wire mesh made contact and completed the circuit. The completed circuit triggered the sweep which produced the deflection-time curves. These curves were then recorded with a Land process camera mounted on the os-cilloscope (fig. 5).

After each drop the residual deflection of each panel was measured relative to three fix-ed points on the top side of the frame (fig. 6). The measuring device was a dial gage mounted to a rigid frame.

Testing Procedure Prior to each test the sandbag was rolled

on the floor to loosen the sand. The sandbag was initially dropped from a height of 6 inches, and the height was successively increased in increments of 6 inches until failure occurred or the test was terminated. The dynamic deflec-tion and residual deflection were measured for each drop. The procedure for dropping the bag and measuring deflection was as follows:

(1) The sandbag was hoisted to the desired height and checked for alinement.

(2) The shutter of the Land process camera was opened.

(3) The bag was released via the remote controlled solenoid-activated jaws.

Figure 5. -Cathode ray oscilloscope with Land Failed particleboard panel in background.

(M 141 984-1)

process camera attached.

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(4) After impact the shutter was closed and the picture developed.

(5) After initial impact the sandbag was manually restrained with a rope and pulley to prevent additional impacts from the same drop.

(6) Residual deflection measurements were then taken.

All tests were conducted in this manner. For single-span tests, the panels were im-pacted at the center. One end span of the full 4- by 8-foot panel was impacted at the center, and the other end span of the same panel was impacted at the center of the span 6 inches from the unsupported edge, as in figure 5.

For each sheathing material, it was plan-ned to conduct six full-size panel tests, six single-span tests with long edges simply sup-Figure 6.-Residual deflection gage in place. ported, and six single-span tests with long

(M 141 984-4) edges clamped. Some of the tests were not performed because of damage to the sand-bag.

ANALYSIS OF RESULTS

Effect of Edge Conditions simply supported condition, four tests for the clamped condition, three tests for center drop

The curves for the height of drop versus on an end span of a full panel, and two tests for average peak dynamic deflection of par- an edge drop on an end span of a full panel. ticleboard panels for each edge condition are For this study a residual deflection of 1/8

presented in figure 7. Each point represents inch or greater is defined as failure. The the average of four tests for the clamped sup- average height of drop causing failure of the port condition and five tests for each of the small particleboard panel with clamped edges other three conditions. These curves illustrate was slightly less than that for a center drop on the relative stiffness provided by the various an end span of a 4- by 8-foot nailed panel support conditions; therefore, the curves have (table 2). The average height of drop at failure not been extended to include the height of drop for small panels with simply supported edges at failure. was approximately two-thirds the value for a

The curve for a center drop on the end center drop on the end span of a 4- by 8-foot span of a full 4- by 8-foot panel continuous panel. The average heights of drop causing over five joists lies between the two curves for failure were approximately equal for a center simply supported and clamped edge con- drop on the panel with edges simply sup-ditions. The curve for a drop 6 inches from the ported and for an edge drop on a 4- by 8-foot unsupported edge of the end span of the 4- by panel. This indicates that the edge conditions 8-foot panel is nearest to the curve for the do affect the deflection and ultimate load of the simply supported panel. The position of these particleboard sheathing subjected to impact curves is consistent with the relative stiffness loading, as expected, but the additional of each system. stiffness provided by clamped edge conditions

The relative position of the curves (fig. 8) did not increase the maximum height of drop for each test condition for plywood panels with above the level of that for the end span of a full face-ply grain parallel to span are the same as panel. This is probably due to the greater im-for particleboard. Each point in figure 8 pact force generated by the more rigid support represents the average of five tests for the conditions.

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Figure 7.-Particleboard panels, height of drop in relation to average peak dynamic deflection:

A- 17 ½ - x 48-in. panel, 16-in. span, longer edge simply supported and shorter edge unsupported, center drop.

B- 4- x 8-ft panel nailed to joists spaced 16 in., bag dropped 6 in. from unsupported edge of end span.

C- 4- x 8-ft panel nailed to joists spaced 16 in., bag dropped at center of end span.

D- 20-½ - x 48-in. panel, 16-in. span, longer edge clamped and shorter edge unsupported, center drop.

M 143 366

Examination of the panels with clamped edges revealed that the sandbag usually punc-tured the panel rather than producing a flexure failure along the length as was the case for the particleboard panel with simply supported edge conditions. It is believed that the inability of the panel with clamped edges to deflect and absorb the impact force, and the low shear strength of the material, may have caused the puncture shear type of failure.

Basic engineering analysis suggests that the vulnerability of the panel to puncture shear failure increases as the size of the impact area decreases. The sandbag does not adequately simulate some of the objects that would con-centrate the impact load over a small area.

Similar comparisons of height of drop at failure for plywood panels cannot be made because the plywood panels with clamped edge conditions had not failed when testing was discontinued at a height of 90 inches. Dur-ing the testing of the plywood panels with clamped edges and the plywood 4- by 8-foot

panels, the sandbag sustained considerable damage and had to be repaired; therefore, it was decided to terminate testing of the 4- by 8-foot panels at 54 inches for the center drop. The average height of drop at failure for the plywood panel with simply supported edges was 60 inches and height for the drop near the unsupported edge of the end span of a 4- by 8-foot plywood panel was 54 inches.

Comparisons of the average deflection for the drop immediately prior to failure listed in table 2 revealed that the particleboard pan-els with simply supported edges and the end span of the 4- by 8-foot panels deflected ap-proximately 25 percent more than the panels with clamped edges prior to failure. This sup-ports the previous observations concerning the ability of the less rigid panel to absorb im-pact and concerning the mode of failure. Average residual deflection for the drop im-mediately prior to failure was approximately the same for all test conditions.

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Figure 8.-Plywood panels, height of drop in relation to average peak dynamic deflection:

A- 17- ½ - x 48-in. panel, 16-in. span, longer edge simply supported and shorter edge unsupported, center drop.

B- 4- x 8-ft panel nailed to joists spaced 16 in., bag dropped 6 in. from unsupported edge of end span.

C- 4- x 8-ft panel nailed to joists spaced 16 in., bag dropped at center of end span.

D- 20- ½- x 48-in. panel, 16-in. span, longer edge clamped and shorter edge unsupported, center drop.

M 143 365

The last column of table 2 lists the average time in milliseconds (ms) to maximum deflection for each edge condition and material type. These values, which were measured from zero deflection to maximum deflection for each drop, were scaled from the deflection-time curves similar to those shown in figure 9. The magnitude of the deflections il-lustrate the relative stiffness of each system. Time to maximum deflection depended only upon the stiffness of the system and the properties of the impacting body and did not change appreciably as the drop height in-creased.

Effect and Variability of the Sandbag

The falling object or projectile used for all impact tests was the 60-pound leather sand-bag described by ASTM E 72-68. After several tests the seam in the sandbag failed. It was restitched and the bottom third of the bag was reinforced by wrapping it with filament adhesive tape to prevent additional splitting. Three specimens were tested with the bag taped and it was observed that there was a substantial change in the behavior of the specimens. Deflections were greater and drop

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Table 2.--Data ob ta ined from impact tests of p a r t i c l e b o a r d and plywood pane l s

heights for failure loads were smaller. The tested to failure. Figure 10 shows that the reinforcing tape was then removed for the deflections caused by the taped bag were con-remainder of the tests. siderably greater than those caused by the un-

Figure 10 is the height of drop versus taped bag. The three drops with the bag taped deflection curve for a plywood panel with did not affect the results of tests for the same clamped edges. Drops at heights of 6, 12, and panel with the tape removed because the 18 inches were made with the bag taped, and proportional limit of the panel material was not then the tape was removed and the panel exceeded when using the taped bag.

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Figure 9. -Typical deflection-time curves for particleboard panels (16-in. span) impacted with a 60-lb leather sandbag: Single span simply supported- 17- ½ - x 48-in. panel, longer edge simply

supported and shorter edge unsupported, center drop.

Single span with clamped edges- 20- ½ - x 48-in. panel with longer edge clamped and shorter edge unsupported, center drop.

Center drop end span- 4 - x 8-ft panel nailed to joists with bag dropped at center of end span.

M 143 367

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Figure 10.-Height of drop in relation to deflection for a plywood panel tested with standard ASTM E 72-68 sandbag and the sandbag reinforced with tape.

M 143 364

The effect of the reinforcing tape upon the impact behavior of the sandbag and panel can be analyzed with the following energy relationship:

where W is weight of bag, H is height of drop, Us is work done on the sandbag, Uc is work done on the sandbag contents,

and U is work done on the panel.

Chern and Kuenzi (4) have shown that for a paper shipping bag dropped on a rigid steel plate the following energy relation is valid:

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where K1 depends on the properties of the bag

material, K2 depends on the properties of the bag’s

contents and the bag material, t is thickness of bag material, and Ex is elastic modulus for bag material. If the bag is dropped on a panel that

deflects rather than on a rigid body, then the work done on the panel must be included in the above expression. This suggests postulat-ing the following energy relationship for a sandbag dropped on a panel.

where K3 depends on the panel geometry and

edge conditions, and

D is the flexural rigidity of the panel. Examination of this expression reveals

why reinforcing the sandbag affected the panel behavior. If the elastic modulus of the bag is increased by reinforcing with another material such that the bag and reinforcing act com-positely, then the work done on the bag and the bag’s contents is decreased. Since the total work done, WH, is constant, then the ad-ditional work is done on the panel. This would cause increased deflection and failure at lower drop heights. Also as the stiffness of the panel increases, the work done on the panel decreases and the energy absorbed by the sandbag and its contents increases. Therefore, it is very important that the specifications for the sandbag and its contents be strictly adhered to for valid qualitative com-parisons. Unfortunately, strict adherence to the specification may not preclude variability caused by the age and condition of the bag’s material and contents, and the effect of temperature and humidity upon the properties of the bag and contents.

Approximation of Impact Force The preceding discussion shows that

when an object such as a sandbag is dropped upon another deformable body, the total energy is absorbed by the bag, its contents, and the deformable body. During the impact, the acceleration of the sandbag varies and the forces acting vary from zero to a peak value and return to zero during a very short period of time.

The following impulse-momentum relationship was investigated for ap-proximating the impact force (6):

where F is impact force, M is mass of the sandbag, v is velocity at impact, and t is time to maximum deflection.

This approach assumes that the impact force can be approximated by a constant force applied during the contact time interval. The mass was known, the velocity was calculated, and the time was scaled from the time-deflection curves. The force calculated in this manner was then used to predict the peak dynamic deflection by treating a particleboard panel as a semi-infinite isotropic plate simply supported on two edges with the impact force uniformly distributed over an area at the center equal to the area of the bottom of the sandbag and using a known static analysis (7).

The calculated deflections did not com-pare well with the measured deflections because the primary assumption for the impulse-momentum relationship is that the force must be constant. This is not true for the sandbag impact because the impact force varies from zero to a peak value and then back to zero. Also, the magnitude of the peak dynamic deflection for each height of drop above 6 inches exceeded the deflection limita-tion for the thin plate theory used.

The ideal condition would be to be able to design for impact loads using engineering design theory and basic material properties. In order to accomplish this the impact forces and the structural response must be accurately determined. It is believed that a more realistic approach to this problem would be to study the actual inservice dynamic loads and attempt to duplicate the loading with programable electrohydraulic loading device. The response of the structure to the inservice loads could then be studied and an attempt made to establish realistic performance criteria and design methods.

CONCLUSIONS

(1) As expected, edge conditions affect end span of a 4- by 8-foot panel. Stiffness for a the behavior of sheathing material subjected drop 6 inches from the unsupported edge of to impact loading. The stiffness and height of the end span of a 4- by 8-foot panel was drop at failure for a center drop on a 17-%-by greater than the values for a center drop on a 48-inch simply supported panel is con- simply supported panel, but height of drop at siderably less than for a center drop on the failure was about the same.

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Failure of the particleboard panels with two edges clamped was usually limited to the area immediately beneath the point of impact rather than along the length as was the case for the particleboard panel with two simply supported edges.

(2) Modification of the leather sandbag by reinforcing it or altering its contents can sub-stantially affect impact test results. The age and condition of the sandbag can also affect results.

The sandbag does not adequately simulate a falling object that would concen-trate the impact load over a smaller area and produce a puncture-type failure of the sheathing. The falling ball impact test describ-

ed by ASTM D 1037-72 or the panel impact test described by FAO (5) should be used in conjunction with the sandbag impact test in order to evalute the puncture resistance of the sheathing material.

(3) The use of an LVDT and cathode ray oscilloscope to measure and record the data improves the quality and quantity of the data. This procedure provides a time-deflection curve for each impact.

(4) For sheathing materials there is a need to develop an impact test method that ac-curately simulates inservice loading con-ditions, and to ultimately develop a design procedure for impact loads using engineering design theory and basic material properties.

PROCEDURES ADOPTED These procedures are intended for

evaluating the impact behavior of sheathing materials that will be developed as part of the Forest Service Structural Particleboard Program. 4 The 60-pound leather sandbag described by ASTM E 72-68 was selected for use as the impacting body because of its historical acceptance. It is recognized that any test that consists of dropping an object onto sheathing provides only a qualitative indication of the impact resistance and pro-vides very little quantitative information. This type of test best serves for comparing different materials and components with others or with established performance standards.

(1) In order to achieve standard test con-ditions and provide test data that can be com-pared with data from other material or a stand-ard, and to minimize the effect of nuisance variables such as nailing and joist stiffness, the following testing procedures and conditions were selected:

(a) Use 60-pound leather sandbag described' by ASTM E 72-68. Begin tests with a 6-inch height of drop and increase the height in 6-inch in-crements. Prior to each test, roll the sandbag on the floor to loosen the sand. Specimen size-Span+ 1-½ inches by 48 inches.

(c) Edge conditions-Longer sides-simply supported. Shorter sides-unsupported.

(d) Test frame-Described in this report. (e) Location of drop-Center of panel. (f) Conditioning-Condition panels at a

relative humidity of 65 ± 1 percent and 68° ± 6° F to equilibrium moisture content.

(2) Performance criteria for impact behavior of structural components such as en-tire floor assemblies are not realistic for sheathing that is evaluated independent of other structural members. Based on a limited number of tests, static load-deflection re-quirements, and consideration of the panel edge conditions, the following criteria were selected: A 0.02-inch maximum residual deflection after an 18-inch drop and a 0.125-inch maximum residual deflection after a 36-inch drop. The performance criteria are in-tended for floor sheathing simply supported on a 16-inch span and tested in accordance with the procedures described in this report. Additional tests would be required to establish performance criteria for different spans and other uses.

4 A Forest Service program initiated in 1973 to use forest residues for making structural particleboards suitable for roof, wall, and floor sheathing.

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RECOMMENDATIONS BY NORDIC COMMITTEE

We recently learned that test methods and performance criteria for evaluating sheathing materials are also being considered by the Nordic Committee on Building Reg-ulations. After we completed testing and analysis of data for our study, we received a copy of Nordic Committee Recommenda-tions. 5 Even though these studies were con-ducted independently, several common rec-ommendations resulted. For example, the Nordic Committee recommends use of a 60-pound sandbag, rigidly supported joists, no panel continuity over joists, and a panel width equal to at least two times the span. The Nordic Committee test procedures that differ from the procedures selected as a result of this study are: (1) Tests are performed near the unsupported edge of panels that are not blocked, and (2) specimens are mounted with the same fastenings as for the intended application in practice.

The performance criteria recommen-dations of this study are also similar to the Nor-dic Committee recommendations. The main difference is that the Nordic recommendations allow for one panel in five to be damaged at the 18-inch height of drop and one panel in five to break through when the bag is subsequently dropped from a 36-inch height.

5 Nordic Committee on Building Regulations, Rec-ommendations for Strength and Rigidity of Flooring and Roof Sheathing. (Preliminary report for proposed publication.) 1972.

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LITERATURE CITED

1. American Society for Testing and Materials. 1963. Standard methods of testing veneer, plywood and other glued

veneer construction. ASTM Des. D 805-63. Philadelphia, Pa.

2. 1968. Conducting strength tests of panels for building construction.

ASTM Des. E 72-68. Philadelphia, Pa.

3. 1972. Standard methods of evaluating the properties of wood-base

fiber and particle panel materials. ASTM Des. D 1037-72. Philadelphia, Pa.

4. Chern, Joseph, and E.W. Kuenzi. 1972. Development of basic information for the design of paper shipping

sacks. Tappi 55(10).

5. Food and Agricultural Organization of the United Nations. 1959. Developments in test methods for fiberboard and particleboard.

FAO/59/4/2321. Rome, Italy.

6. Sears, F.W., and M.W. Zemansky. 1964. University Physics. Addison-Wesley Publ. Co.

7. Timoshenko, S., and S. Woinowsky-Krieger. 1959. Theory of plates and shells. 2d ed. McGraw-Hill Book Co.

8. Yancey, C.W.C., and L.E. Cattaneo. 1974. State-of-the-art of structural test methods for walls, floors, roofs,

and complete buildings. Natl. Bur. Stand. (U.S.) Bldg. Sci. Ser. 58.

U.S. GOVERNMENT PRINTING OFFICE 1975-650-257/81 15