EFFECT OF CONFINING PRESSURE ON THE COMPRESSION PARALLEL-TO-THE-GRAIN STRENGTH OF SMALL CLEAR WOOD SPECIMENS

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1 U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADlSON, WIS U.S. FOREST SERVICE RESEARCH NOTE FPL-057 November 1964 EFFECT OF CONFINING PRESSURE ON THE COMPRESSION PARALLEL-TO-THE-GRAIN STRENGTH OF SMALL CLEAR WOOD SPECIMENS

2 Summary Since wood foundation piles are normally used under conditions of confining lateral pressure from the soil in which they are imbedded, it has been suggested this pressure may have an effect upon the compression parallel-to-the-grain strength of the piles. Accordingly, a study was conducted to evaluate the effect of this pressure on the compression-parallel strength of small clear specimens of Douglas-fir and red oak. To simulate earth pressures, oil pressures of 50 and 100 pounds per square inch were used. No effect was found on the strength of the two species at either pressure.

3 EFFECT OF CONFINING PRESSURE ON THE COMPRESSION PARALLEL-TO-THE-GRAIN STRENGTH OF SMALL CLEAR WOOD SPECIMENS By THOMAS LEE WILKINSON, Engineer 1 Forest Products Laboratory, Forest Service U.S. Department of Agriculture Introduction This research was conducted as a supplement to a strength evaluation study of round timber piles at the U.S. Forest Products Laboratory. Since wood foundation piles are driven into the ground and normally used under conditions of confining lateral pressure from the surrounding soil, a realistic evaluation was desired to determine possible effects of confining pressure on the compression strength parallel to the grain. A test method similar to the tri-axial shear test for soils was suggested. In this procedure, a uniform pressure is applied to the sample being evaluated while an external compression load is applied. To develop this type of test for evaluating piles 4 feet in length would have been quite costly and, therefore, evaluation was based on small, clear wood specimens to determine whether confining pressure had any effect upon the compression strength parallel to the grain. The confining pressure on a pile is dependent upon the depth and specific weight of the surrounding soil. For cohesive soils, it is also dependent on the shear strength of the soil, and for granular materials it is dependent on the angle of friction. The confining pressure at a depth of 50 feet can vary from practically zero to 26,000 pounds per square inch in a granular material; however, the latter pressure is for an extremely compacted soil in which wood piles normally would not be used. Average pressures in cohesive soils would range from 15 to 70 pounds per square inch. To obtain comprehensive data, pressures of 50 and 100 pounds per square inch were selected for use in this study. 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. FPL-057

4 Specimens Douglas-fir and red oak were the two species selected for this study. Three groups of 10 specimens from each species were prepared, The specimens were cylindrical in shape and measured 2 inches in diameter and 4 inches in length One group was evaluated at atmospheric pressure to serve as controls; another group at a confining pressure of 50 pounds per square inch; and the third group at 100 pounds per square inch. All specimens had a moisture content above the fiber saturation point at the time of testing and each group of specimens was end matched, Experimental Procedure Figure 1 shows the equipment used to evaluate the effect of confining pressure on the properties of small, clear wood specimens in compression parallel to the grain. The test machine applied compressive loading through a pressure plate or piston bearing directly on the end of the specimen. Specimens were enclosed in an oil filled, cylindrical chamber mounted on the machine and a hand-operated hydraulic pump connected to the chamber applied the desired lateral confining pressure. Strains were measured using clip-on type, SR-4 electrical strain gages (fig. 2), mounted on bands of spring steel which were secured on knife edges fastened to the specimens. The distance between knife edges was 2-5/8 inches. Discussion of Results The data obtained in this study are shown in tables 1 and 2, and indicate the following effects of lateral confining pressure on the maximum crushing strength, modulus of elasticity, and fiber stress at the proportional limit for the two species at each pressure level: Maximum crushing strength.--the maximum crushing strength of the red oak specimens was virtually unaffected by confining pressure at either pressure level, while some increase was noted in the Douglas-fir specimens at 100 pounds per square inch confining pressure. A statistical analysis of the data, however, showed this increase to be insignificant except at levels of probability below 60 percent. A similar analysis of the red oak data showed no effect of confining pressure except at levels of probability below 50 percent. FPL

5 Modulus of elasticity and fiber stress.--the results for modulus of elasticity in compression parallel and fiber stress at the proportional limit were not significantly effected by confining pressure. The red oak data showed this quite clearly at both pressure Bevels. There appeared to be some effect, however, upon the modulus of elasticity for Douglas-fir, but the increase noted was statistically insignificant. The small apparent effects of confining pressure indicated by these results would be even smaller had full-sized pile sections been utilized. Since it is impossible to distribute the confining pressure equally throughout the cross section of a specimen, the 2-inch-diameter specimens used would have a larger percentage of their cross section affected than a pile 6 to 18 inches in diameter. Failures.--Crushing of the fibers near the end of the specimens generally was the cause of Douglas-fir failures. In some instances, values of fiber stress at the proportional limit could not be obtained for Douglas-fir specimens, as failure occurred outside of the gage points for measurement of strain. This accounts for the omission of certain values in table 2. Failures in red oak were generally shear failures with some fiber buckling. The inability of the oil to prevent buckling was considered unimportant, since the actual conditions found in soils would similarly permit local buckling of the fibers during compression failures. Conclusions This study shows that lateral confining pressure has no significant effect upon the maximum crushing strength, proportional limit fiber stress, or modulus of elasticity in compression parallel to the grain for either Douglas-fir or red oak. FPL

6 Table 1.--Comparison of properties of red oak in compression parallel to grain at various confining pressures FPL

7 Table 1.--Comparison of properties of red oak in compression parallel to grain at various confining pressures--con. FPL

8 Table 2. --Comparison of properties of Douglas-fir in compression parallel to grain at various confining pressures FPL

9 Table 2. --Comparison of properties of Douglas- fir in compression parallel to grain at various confining pressures--con. FPL

10 Figure 1.--Equipment used inevaluating the effect of confining pressure on the properties of small, clear wood specimens in compression parallel to the grain: A, oil-filled chamber in which the specimens were enclosed: B, hand-operated hydraulic pump connected to the pressure chamber: C, strain indicator; and D, electrical switching and balancing unit. M

11 M Figure 1.--Typioal specimen showing mounting of clip-on type, SR-4 electrical strain gages. Strain was. measured over a length of 2-5/8 inches. Specimens were 2 inches in diameter and 4 inches in length.

12 PUBLICATION LISTS ISSUED BY THE FOREST PRODUCTS LABORATORY The following lists of publications deal with investigative projects of the Forest Products Laboratory or relate to special interest groups and are available upon request: Box, Crate, and Packaging Data Chemistry of Wood Drying of Wood Fire Protection Fungus and Insect Defects in Forest Products Glue and Plywood Growth, Structure, and Identification of Wood Furniture Manufacturers, Woodworkers, and Teachers of Woodshop Practice Logging, Milling, and Utilization of Timber Products Mechanical Properties of Timber Pulp and Paper Structural Sandwich, Plastic Laminates, and Wood-Base Components Thermal Properties of Wood Wood Finishing Subjects Wood Preservation Architects, Builders, Engineers, and Retail Lumbermen Note: Since Forest Products Laboratory publications are so varied in subject matter, no single catalog of titles is issued. Instead, a listing is made for each area of Laboratory research. Twice a year, December 31 and dune 30, a list is compiled showing new reports for the previous 6 months. This is the only item sent regularly to the Laboratory s mailing roster, and it serves to keep current the various subject matter listings. Names may be added to the mailing roster upon request.

13 FOREST SERVICE REGIONAL EXPERIMENT STATIONS AND FOREST PRODUCTS LABORATORY PROGRAM LOCATIONS HEADQUARTERS OF REGIONAL FOREST EXPERIMENT STATIONS FOREST PRODUCTS LABORATORY WASHINGTON - BELTSVILLE Forest Service regional experiment stations and Forest Products laboratory