BEARING STRENGTH OF WOOD UNDER

EMBEDMENT LOADING OF FASTENERS

U.S.D.A. FOREST SERVICE

RESEARCH PAPER

FPL 1 6 3 1971

U.S. Department of Agriculture

Forest Service

Forest Products Laboratory

Madison, Wis.

ABSTRACT

The bearing resistance of wood to embedment loading of mechanical fasteners was investigated. Results show that loads at 0.05-inch embedment are related to the bearing area of the fastener and that maximum embedment load is related to the perimeter of the fastener head. An engineered design of anchorage based on the data provided by this study will result in structures that can safely withstand heavy wind storms or earthquakes.

BEARING STRENGTH OF WOOD UNDER EMBEDMENT LOADING OF FASTENERS B y THOMAS LEE WILKINSON, Engineer

Forest Products Laboratory,1 Forest Service, U.S. Department of Agriculture

INTRODUCTION

Mechanical fastenera are often used to anchor a structure to a foundation. The fastener prevents the structure from being moved from its foundation by forces such as wind, earthquake, or wave action. In areas subject to hurricanes, adequate anchorage is particularly important.

Wind causes uplift or overturning forces on a structure, This generally results in a direct tensile load on the anchorage fastener, although in some situations the fastener may also be loaded laterally. In designing an anchorage fastener subject to tensile load, three factors are of importance: (1) Withdrawal resistance of the fastener from the foundation, (2) tensile strength of the fastener, and (3) embedment of the fastener head into the sill or other framing member. Because the bearing resistance of wood to embed­ment loading of a fastener into a framing member has not been known, the design for anchoring a structure could not be complete. In this study, the data are determined for the bearing resistance of wood to embedment loading,

In previous work at the Forest Products Laboratory, Scholten determined the bearing strength of wood and plywood under fastener heads so that designs could be established for ship bulkheads. No information has been found in the literature on embedment of fastener heads or on design for anchor bolts. Part of Scholten’s data is included in this report.

EXPERIMENTAL PROCEDURE

Specimens--Wood with Fastener

Here a “specimen” is a combination of a wood species and a mechanical fastener, The specimens were divided into two classifications based on method of support during test: Short specimens, those supported an a 4-inch span to prevent excessive bending (used in this study); and long specimens, those supported on a 20-inch span (used by Scholten).

Five Species, white fir (Abies concolor (Gord. and Glendl.) Lindl.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), southern pine (Pinus spp.), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and red oak (Quercus rubra L.), were used for the short specimens. Clear straight-grained specimens were obtained, but no attempt was made to select either edge- or flat-grain pieces.

The types and the dimensions of the fastenera included in both studies are listed in table 1. Used in the short-specimen study were 3/8-inch steel bolts with washers and with square nuts, 3/4-inch steel bolts with washers and square nuts, and tenpenny common nails with 1/8-inch steel washers and without washers. In the long specimen study, Scholten included Nos. 14, 18, and 24 flat-head wood screws, 3/8-inch lag bolts with and without washers, and 1/2-inch lag bolts with and without washers. Scholten used washers of silicon

1Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

Table 1.--Dimensions of fasteners used in short and long specimens1

bronze that conformed to Federal Specification QQ-C-591, composition B.

All combinations of fasteners and species were evaluated with the wood at 10 percent moisture content. Additional Douglas-fir specimens with 3/4-inch bolts with and without. washers, and ten­penny nails without washers were evaluated at 17 and 26 percent moisture content.

Groups of six short specimens for each species-fastener combination were evaluated. The groups were formed by randomly assigning one 14-inch­long piece from each of six 8-foot long sticks to a particular fastener. The specimens were planed to a uniform thickness of 1-1/2 inches, and were about 3-5/8 inches wide.

The long specimens were of Douglas-fir and of southern pine; both were about 5 to 5-1/2 inches wide. The Douglas-fir was obtained from nominal 3- by 6-inch construction-grade material; south­ern pine, from 2- by 6-inch tongue-and-grooved decking. To obtain a thickness of 2-1/2 inches, the wide faces of two 2- by 6-inch members were glued together and subsequently finished to a thickness of 2-1/2 inches.

Bolts and screws were installed in snug but free-fitting holes. The nails were driven into holes prebored to 90 percent of the nail diameter. For the short specimens, the nut, washer, or nailhead was flash with the wide surface. For the long specimens, counterbored bolt holes 1 inch in diameter and 0.38 and 0.44 inch deep were used with the 3/8-inch bolts with and without washers, respectively. With the 1/2-inch bolts, a counter-bore 1-1/4 inches in diameter and 0.45 inch deep without washers and a counterbore 0.54 inch deep with washers were used. Thus, the top of the bolt head was 1/8 inch below the top surface of the specimen in all tests. The holes for the wood screws were countersunk to a depth that permitted the top of the head to be flush with the surface of the specimen

Test Procedure

In the experimental arrangement for deter mining the resistance to fastener embedment of all short specimens with bolts (fig. 1), the speci­men was supported on 2-inch-diameter rollers

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spaced 4 inches on center. The span was made as short as possible to prevent excessive bending of the wood specimen. A special deformation gage employing a microformer was developed to con­tinuously measure the amount of fastener-head embedment of all short specimens. The legs of the gage were spaced 3-5/8 inches.

In the arrangement for determining embedment resistance of nails in the short specimens (fig. 2), the specimen was supported on a flat plate with a 2-inch-diameter hole through which the nail protruded and was gripped by a tension grip. A different arrangement was needed for these specimens because of the tension grip.

All specimens were loaded at a constant rate of head movement of the testing machine of 0.075 inch per minute.

The specimens evaluated by Scholten were simply supported on knife-edges and roller-bearing plates (fig. 3). The span length was 20 inches. The specimens were loaded at a rate of testing machine head movement of 0.048 inch per minute.

After testing, the moisture content and specific gravity of each specimen were determined.

RESULTS

Load-embedment curves typical of the different fasteners are shown in figure 4. Specimens with bolts were fairly elastic during initial loading whereas those with nails were loaded to 200 to 500 pounds without detecting any embedment of the fastener head. This may have resulted from driving the nails into holes smaller than the nail diameter; thus friction along the shank had to be overcome.

Maximum load and a load at a fairly small deformation, 0.05 inch, were selected for deter­mining empirical relationships with fastener dimensions. Maximum load is apparently related to the fastener perimeter, figure 5, and the load at 0.05-inch deformation is related to the fastener bearing area, figure 6. These relationships ap­parently are fairly linear with no appreciable difference between round and square fasteners. In the embedment tests of nails, the load deformation curves at 0.05-inch flattened somewhat possibly because of the friction along the shank that had to

The relationships between load at 0.05-inch deformation and fastener bearing: area, figure 6, can be described by the equation

(1)

where yo equals load at 0.05-inch deformation,

pounds; x o, fastener bearing area, inches2; and

A and B are parameters dependent on species. Parameter A can be related to average species specific gravity, figure 7, and parameter B can be related to the species proportional limit stress in compression perpendicular to the grain as given in Wood Handbook2 (fig. 8). With these relationships, it is possible to use the results of this study other species. For example, a species with specific gravity of 0.40 and proportional limit stress in compression perpendicular to the grain of 500 pounds per square inch would have a parameter A of 360 and parameter B of 860. Equation 1 for this species would then be

(2)

The relationship between maximum load and fastener perimeter, figure 5, can be described by

(3)

where Y equals maximum load; x, fastener max perimeter, inches; and C, a parameter dependent on species. Parameter C was found related to average species specific gravity, figure 7. Thus, equation 3 for the species in the preceding example would be

(4)

In figure 9, load increases at 0.05-inch defor­mation with decreasing moisture content; how­ever, the amount of increase is greatly dependent on fastener size. The figure shows no relationship

be overcome before obtaining any deformation. between maximum load and moisture content. 2Forest Products Laboratory. Wood Handbook. Handbook No. 72. forest Service, U.S. Department of

Agriculture. 1955.

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SUMMARY (2) Parameters describing the empirical rela­tionships between loads and fastener dimensions were in turn found to be related either to average species specific gravity or proportional limit stress in compression perpendicular to the

(1) The maximum resistance to fastener-head grain. embedment is apparently related to the perimeter (3) Moisture content apparently has no effect on of the fastener, whereas the resistance at small maximum embedment resistance, whereas at amounts of embedment, 0.05 inch, is apparently small amounts of embedment, 0.05 inch, the load related to the fastener bearing area. decreases with increasing moisture content.

Figure I.--Equipment to determine bearing strength of wood under nuts and washers for short specimens. Two-inch rollers are spaced 4 Inches on center; the gage allows continuous recording of load versus embedment. M 137 644

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Figure 2.--Equipment for determining embedment resistance of nails for short specimens. The plate on which specimens rests has a 2-inch-diameter hole through which the nail protrudes toward the lower tension grip.

M 137 645

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Figure 3.--Equipment to determine embedment resistance of long specimens of 2-1/2-inch-Thick southern pine. Fastener is a 3/8-inch bolt with a silicon-bronze washer.

M 129 695

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Figure 4.--Relationships of load to fastener embedment typical of different fasteners (Douglas-fir). M 139 243

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Figure 5.--Relationship of maximum load to fastener perimeter for five wood species. M 139 244

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Figure 6.--Relationship of load at 0.05 inch deformation to fastener bearing area for five species. M 139 242

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Figure 7.--Relationship of parameter A, equation I, or parameter C, equation 3, to average specific gravity of species. (Specific gravity based on ovendry weight and volume at test.) M 339 245

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Figure 8.--Relationship of parameter B to proportional limit stress (PLS) in compression perpendicular to grain (from Wood Handbook). M 139 241

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