PREDICTING PERFORMANCE

OFHARDBOARD IN I-BEAMS

U.S.D.A., FOREST SERVICE RESEARCH PAPER

FPL 185 1972

U.S. DEPARTMENT OF AGRlCULTURE FOREST SERVICE

FOREST PRODUCTS LABORATORY MADISON. WISCONSIN

ABSTRACT

Hardboard has for many years been used in Struc­tural applications after an assembly was constructed and tests were made to determine performance under the anticipated service environment. A better method, particularly for structural purposes, is to consider hardhoard an engineering material like timber, steel, or concrete and we design stresses and other basic information for rational design

The study of the behavior of hardboard under stress at Forest Products Laboratory provided the basic information for developing design values. These values were used in designing three experimental I-beams that spanned 30 feet with a 2-foot overhang on each end. They were constructed with web material of hardboard and flanges of 2- by 4-inch lumber. The design stress values of the hardboard were based on a factor of 0.45 for load duration, a statistical consi­deration of variability, and a factor of safety equal to 1.5. The beams were tested under simulated roof loading. The test results verified the predictions of deflection and strength

The close agreement between the standard design theory and the test results confirm that the perform­ance of hadboard in a structure can be predicted. This should encourage further work in the use of this material for structural applications.

PREDICTlNG PERFORMANCE OF HARDBOARD

IN I-BEAMS 1

By

TERRY J. RAMAKER, Engineer and MICHAEL D. DAVISTER, Engineer

Forest Products Laboratory.2

Forest Service U.S. Department of Agriculture

INTRODUCTION

Both production and use of hardboard have more than tripled in the United States during the last 15 years. Use in construction, a major part of the market, has been limited to nonstructural or to limited-structural applications like wall paneling, floor underlayment, and siding. If hardboard was used structurally, the design was on an empirical basis; prototypes were tested to determine ade­quacy of design for deformation and strength.

The full potential of hardboard as an engineer­ing material can only be realized if design stress values and other basic information are available so that the same rational design procedures can be applied to hardboard like they are when de­signing with other engineering materials.

Research at the Forest Products Laboratory by Lewis and McNatt on representative high-density

standard and tempered hardboard products pro­vided the fundamental data on which future design could be baaed. The research reported here demonstrates how these fundamental design cri­teria can be applied in designing hardboard for structural use. Components in the form of three glued I-hems with lumber flanges and stiffeners and tempered hardboard webs were teated and strength and behavior under load determined. The beams were 34 feet long, 16 inches deep, and spanned 30 feet with a 2-foot overhang on each end.

All data, tables, and figures used in this report are from a thesis by Michael D. Davister.1 Cer­tain of his decisions, such as those for factors of duration of load and Statistical variability, have been used here.

1Condensation of a thesis, "Hardboard design stresses for farm buildings," by Michael D. Davister for the degree of Master of Science (Civil Engineering) at the University of Wisconsin, Madison, 1968. Research was conducted at the Forest Products Laboratory.

2Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

DESIGN STRESSES FOR HARDBOARD

For this work, a tempered 1/4-inch hardboard, Lewis and McNatt were drawn typical of the hardboards produced commercially, population. Therefore, values of Lewis and McNatt was selected. Sufficient material was obtained for were adopted far design of these beams because testing both the major structural units and a num- they were from a sample larger ber of minor specimens to determine properties Davister. The values are summarized in table 1, of the materials,, Tension and compression stress and are for the particular 1/4-inch parallel to the surface and the edgewise and the used for webs, and are not applicable interlaminar shear stress for the hardboard were additional teat information for determined. The mean ultimate stress values from thicknesses, or qualities of hardboard. these tests were essentially the same as the mean The calculations of design stresses values established by Lewis and McNatt for the ultimate stresses were based on same product. Davister, from a statiatical analy- three considerations: Statistical sis, concluded both his samples and those of tor of safety; and duration of load.

Table 1.--Basic strength and elastic properties of 1/4-inch tempered hardboard used for webs of I-beams1

from the same

than that of

hardboard without

other products,

from mean the following

variability: fac­

1ASTM standard procedures used in all tests w i t h material a t 6.4 percent moisture content (equilibrium at 75° F. and 64 percent relative humidity).

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Statistical Analysis

The ultimate stresses and the elastic proper­ties were analyzed statistically according to guidelines in Military Handbook No. 5 (4, Table 6.4.1) to obtain design stresses and elastic values. The ultimate stress data were analyzed by

the test, and the results indicated the data were normally distributed. Therefore, the design s t r e s s e s were calculated with the following equation

_ X = sample mean based on N observations; S = standard deviation; and K = one side tolerance limit factor corre­

y sponding to a proportion of at least 0.95 of a normal distribution and a confidence coefficient of 0.95. (See (4) for tables of K y.) K y was taken

as 2 for this study. Obtaining a design modulus of elasticity and a

modulus at rigidity involved an altered approach. Whereas the design stress values were set low enough to enable approximately 95 percent of the population to have a rupture stress in excess of the established value, the values for modulus of elasticity and modulus of rigidity should reflect the more average condition described by the following equation

If a representative sample with a normal dis­tribution i s assumed, 69 percent of the elastic property values should be greater than y.

Factor of Safety

The design stress derived by statistical analy­

sis was divided by a factor of safety, of 1.5 to account for variations in panel thickness, occa­sional high temperature, overload, and other unpredictable strength adversities.

Duration of Load

The design stress was further reduced by a load factor to account for a 10-year duration of load based on the following formula by McNatt (3)

(1)

X = duration of load in seconds; and y = duration of load factor, percent. Solution of the formula for a 10-year duration

gave a factor of 0.45 for reducing the stress. (Additional analysis of test data by McNatt has resulted in further refinement of formula (1) to

that for a 10-year duration of load­

ing gives a factor of about 0.48.) Based on the foregoing, the design stresses

and elastic values used in the I-beam design are given in the following:

Design properties P.s.i.

Tension parallel to surface 1,160 Compression parallel to surface 1,180 Edgewise shear 725 Interlaminar shear 100 Modulus of elasticity* 746,000 Modulus of rigidity** 317,000

* Average of tension and compression. **Associated with shearing deformations in the

plane of the panel.

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BEAM DESIGN AND CONSTRUCTION

The different moduli of elasticity for the two materials in the builtup section require using the transformed area method to compute the proper­ties of the section. Ordinary flexure theory is based on the two assumptions that a beam is homogeneous and that plane transverse sections remain plane. The second causes strains to vary directly with their distance from the neutral axis. In investigating the bending of composite beam, only one assumption is retained: Plane sections remain plane, i.e., the strains vary directly with their distance from the neutral axis (5).

The most common method of dealing with a nonhomogeneous beam is to transform it to an equivalent homogeneous beam. The area of wood in the section was transformed to an equivalent area of hardboard, the less stiff of the two mate­rials. This was accomplished by multiplying the area of wood by the ratio of the average modulus of elasticity of wood to the average modulus of elasticity of hardboard

A Transformed

Following this transformation, the normal flexure formulas were applied in the design of the beams.

The bending deflections were computed by the conjugate beam theory: deflection due to shear was computed by the unit load energy method, The deflections calculated at design load (155 lb/l.f.) at the one-quarter points and center of the span are given in table 2. The elastic properties of the flange material used to calculate deflections were measured before the I-beams were constructed.

Construction details are shown in figure 1. The flanges were of clear Douglas-fir with a maximum slope of grain of 1 in 16. Material for the web was 1/4-inch tempered hardboard, The 26 stiff­eners and the 2- by 4-inch flanges that were glued with resorcinol adhesive to the hardboard web are shown in figure 1. Both lumber and hard­board were conditioned at 50 percent relative humidity and 73° F. for 2 months prior to testing. The average moisture content at test was 8.9 per­cent for the flanges and 4.6 percent for the hardboard web.

Table 2.--Comparison of deflections at design load1 measured and calculated at one-quarter points and at center of three 30-foot I-beams

1155 pounds per lineal foot.

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Figure 1.--Construction details of I-beamwith hardboard web and wood flanges.

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BEAM LOADING

The horizontal axis of the beam was inclined at a slope of 1 in 10 with toad points every 3 feet along the beam The load was applied to the en­tire width of the tap flange by a cable and pulley system (fig. 2).

Strains were measured with wire resistance-type rectangular rosettes. In beam 1 and 2 the rosettes were mounted on opposite sides of the web, and located in the center of each of the first three panels beginning at the support.

In beam 3, all three rosettes were mounted in the second panel, one at the center and one at each earner. One gage was mounted on each flange 1.5 feet off center.

The beam deflections were taken in two phases. In phase I, each beam was loaded to design load of 5,160 pounds (155 lb./l.f.) in increments of 500 pounds. The strain and the deflection were taken at each Load increment. Observed and cal­culated deflections at one-quarter points and mid-span are compared in table 2. The load was then removed, and the set recorded. The set in all three cases was negligible. In phase II, the beam was loaded to failure in increments of 1,000 pounds, and deflections were taken at each incre­ment. Figure 3 is an example of a set of typical load-deflection curves.

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Figure 2.--Loading method for testing I-beams to failure showing, at lower left, pointand magnitude of load (in pounds) over an approximate 30-foot span.

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Figure 3.--Set of load-deflection curves typical for quarter- and center-points of I-beam.

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ANALYSIS OF RESULTS

Wood Flanges

Ultimate load of 18,300, 20,200, and 20,4000 pounds was achieved in beams 1, 2, and 3. All three failures occurred suddenly in the tension flange without any previous sign of distress. The average calculated stress at failure in the wood tension flange was 6.640 pounds per square inch. which was 55 percent of the average ultimate stress in tension from tests of minor specimens cut with the grain parallel to the axis of the spec­imen. Earlier work by Lewis, Heebink, and Cottingham (2) for a 1 in 15 slope of grain indi­cated this ratio to be about 68 percent. The failure loads were multiplied by 0.62 (1) to account for normal loading. This results in the load that should produce failure if it remains on the beam fox a period of 10 years. Dividing this adjusted failure load by the design load of 5,160 pounds (155 1b./l.f.) results in an average factor of safety against failure in the wood flange of 2.36.

Hardboard Web

The failure in tension in the wood flanges of the three beams rather than in the hardboard webs necessitated further investigation to analyze the performance of the hardboard panel in shear. The two end panels were cut from the three beams and tested to failure as shown in figure 4. The mean calculated shearing strength value was 2,870 pounds per square inch for the six panels compared to a mean value of 2,860 pounds per square inch for the minor specimens (table 1). The failure pattern of the end panel is shown in figure 5. Because shear strengths for minor and major specimens were almost the same, it is reasonable to conclude effects of buckling were negligible and the values for edgewise shear from minor specimens can be used a3 the basis of web strength.

The horizontal shear stress in the beams at failure was computed from VQ/It where

V = Maximum vertical shear; Q = Moment of area about the neutral axis: I = Moment of inertia of cross section about

the neutral axis; and t = Web thickness.

The resulting values for beams 1, 2, and 3 were 82, 96, and 98 percent of the ultimate shear strength of the material, respectively. Dividing the failure loads by these respective percentages results in the loads that would have produced a shear failure in the beam webs if a failure of the wood in tension had not occurred first. Multiplying these loads by 0.45 to account for normal loading and dividing by the design load of 5,160 pounds produced an average factor of safety against fail­ure of 1.87 of the hardboard web.

Beam Deflection

The actual deflection of beams 2 and 3 was close to the design deflection in table 2. The measured deflections resulted in deflection-span ratios from 1/176 to 1/221 at design load of 5,160 pounds (155 lb./l.f.), No explanation is offered for the excessive deflection of beam 1 other than the possibility that there was inferior material in the flange at the center position

An analysis of the curves for load vs. strain for a rosette in the end panel of the beam re­vealed a slight buckling at a load of about 12,000 pounds. The close agreement between the exper­imentally determined shear stress and the calcu­lated shear stress (table 3) indicates the values for modulus of elasticity and rigidity and for Poisson's ratio were valid. Poisson’s ratio of 0.25 was determined experimentally with a ten­sion specimen,

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Figure 4.--Loading method to produce failure in shear in end panels from I-beams.

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Figure 5.--Pattern o f failure typical of end panels.

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CONCLUSION

The results from the tests of three I-beams of 1/4-inch hardboard for web material and wood for flanges indicate that the mechanical behavior of a structural component using hardboard can be predicted.

The six following general conclusions resulted from the tests:

1. Flanges of all three beam failed intension; therefore an estimated factor of safety against hardboard web failure of 1.87 was obtained by testing in shear end panels taken from the beams.

2. The moduli of elasticity and rigidity for hardboard equal to the mean minus one-half the standard deviation and a Poisson’s ratio of 0.25 used in rosette analysis produced shear stresses that were within ±5 percent of the theoretical results.

3. Buckling of the 1/4-inch tempered hard­board web was negligible with stiffeners spaced 3 feet on center.

4. No appreciable set was measured when the beam was unloaded after loading to the design load of 5,160 pounds.

5. Below 6,000 pounds, load-deflection and load-strain plots were generally straight lines.

6. Ordinary beam theory and the transformed area method produced values that agreed closely with measured values when the derived mechan­ical properties were used for hardboard design.

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LITERATURE CITED

1. American Society for Testing and Materials 1971. Standard methods far establishing

structural grades and related allowable properties for visually graded lumber. ASTM standard D 245, fig. 5. In 1911 Annual Book of ASTM Standards, Part 16, Philadelphia, Pa. 19103.

2. Lewis, W. C., Heebink, T. B., and Cottingham, W. S. 1953. Effects of certain defects and

stress-concentrating factors on the strength of tension flanges of box beams. Forest Prod. Lab. Rep. No. 1513. Information re­viewed and reaffirmed 1965.

3. McNatt, J. Dobbin 1970. Design stresses for hardboard-

Effect of rate, duration, and repeated loading. Forest Prod. J. 20(1): 53.

4. Moon, Donald D., and Hyler, Walter 3. 1966. Military-Handbook 5, AFML TR No.

66-368. Guidelines for the pre­sentation of data. Battelle Memo­rial Institute, Columbus Labora­tories, 505 King Ave., Columbus, Ohio 43201.

5. Singer, F. L. 1962. Strength of materials, 2nd ed.

Harper & Row, 49 E. 33rd St., New York, N.Y. 10016.

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