RESPONSE OF TIMBER JOINTS WITH METAL FASTENERS

TO LATERAL-IMPACT LOADS

By

C. A. JORDAN,1Engineer

Forest Products Laboratory, 2 Forest Service

U.S. Department of Agriculture

----

Abstract

Recent studies at the Forest Products Laboratory were made of two-member

wood joints fastened with nails, bolts, or lag screws and subjected to lateralimpact loads. These studies established a relationship, which is independent

of the type of metal fastener, between magnitude of impact force, joint slip,

fastener diameter, and specific gravity of wood (Douglas -fir) members.

Electronic instrumentation was employed to measure and record the instantaneous forces that were applied to joints in both pendulum and drop tests.

Three types of fasteners were tested: Eight-, sixteen-, and thirty-penny

nails; 1/4-, 1/2-, and 3/4-inch bolts; and 5/16-, 1/2-, and 3/4-inch lag

screws. The work was done in cooperation with the U. S. Air Force, and will

be the basis for revised load ratings for metal fasteners in a new blocking and

bracing guide under preparation for Air Force use.

Method

Available design data applicable to wood-member joints that are secured with

metal fasteners are based, almost entirely, on results from tests in which

1Acknowledgment is made of the contributions of W. D. Godshall, Forest

Products Laboratory engineer, who helped to design and assemble the instrumentation for this study, and supervised its operation.

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

Report No. 2263
loads were slowly applied to test joints. When such fasteners are used to

secure or brace items in shipping containers, however, they are likely to experience severe dynamic forces from impacts that are applied to the container

during handling and shipment. At present the number, direction, and severity

of impacts that may be experienced by a given shipment can only be estimated,

but a preshipment performance test that involves specific impacts may be required for military packages. Because packaged aeronautical equipment, of a

size or weight likely to require wood blocking and bracing, may be required to

pass the pendulum-impact test of Military Specification MIL-P-7936, that test

is the design basis for the pendulum tests reported here. A free-fall drop

test was also used that was simpler in some respects than the pendulum test.

The tests employed mostly the 9-inch drop height specified in MIL-P-7936;

however, in a series of check tests with sixteenpenny nails, the drop height

was varied between 4 and 30 inches.

Test joints consisted of a main member of nominal 4- by 4-inch size and a

shorter cleat, 1 by 4 or 2 by 4 inches, which was fastened to the main member.

All wood used was Douglas -fir, except that white fir was used for cleats with

the eightpenny nails. The mean specific gravity of the Douglas-fir material

was 0.459 with a standard deviation of 0.055. The white fir mean was 0.361

with a standard deviation of 0.026. Moisture content ranged from 12 to 18 percent.

In one series, which involved all 12 kinds and sizes of fasteners, grain direction in the two wood members was parallel and coincident with the direction of

applied load. In a second series, which involved only the two smaller sizes

of each of the three kinds of fasteners, grain directions were mutually perpendicular, and the load was applied to the side grain of the cleat. Recommended

fastener spacing, member penetration, and installation techniques were adhered to in preparing all test joints.

The number of fasteners per joint varied from 19 for the pendulum test of

eightpenny nails, in which two joints were tested simultaneously, to one fastener per joint for several tests that involved the larger fasteners. In the

pendulum tests, all fasteners were tested with a 260-pound cast iron plate as

the impact head. Ten-, fifteen-, and twenty-pound impact heads were used

for the drop tests. Except for the drop tests with two sixteenpenny nails per

joint, the number of fasteners in each test joint was chosen on the basis of

exploratory tests to give an anticipated slip of approximately 0.20 inch on the

initial impact from a 9-inch fall.

For safety and ease in manual handling, the impact head that was used in the

free-fall drop test was limited to a maximum of 20 pounds. This limitation

Report No. 2263 -2-
did not permit loading the larger fasteners to the same extent as in the

pendulum-impact tests.

In all impact tests, both the test joint and the impact head were either swung

or dropped as a unit, so that a protruding end of the main wood member

struck squarely and solidly against either a massive concrete abutment or a

steel plate that was embedded level in the top horizontal surface of another

large mass of concrete. Sandwiched between the impact head and each test

cleat, and in intimate contact with both during each fall, was a resistance

strain-gage load cell.

Each test joint was subjected to repeated impacts from the same drop height.

The test was terminated (a) when complete separation of the members occurred or was imminent, (b) when several successive impacts produced no

residual increase in the accumulated slip, or, (c) as in most of the later tests,

after the first three impacts.

In most instances, a given size and type of fastener was tested with four different cleats, each cleat fastened to a different face of the same main member.

Where it was necessary to assure full-length bearing against solid wood for

all fasteners in the joint, succeeding cleats were shifted lengthwise of the

main member.

The residual accumulated slip in the joint was measured after each impact. A

force-time display, or in the pendulum tests two simultaneous displays, by a

dual-beam oscilloscope was photographed for each of at least the first three

impacts, except in infrequent instances of picture failure.

The force-time displays consisted of a fundamental wave, or pulse, of 5 to

20 milliseconds duration, and oscillations of considerably higher frequency,

which were superimposed on the pulse. Rough calculations of natural frequencies indicated that elastic stress waves set up by the impact in the various

members of the test assembly may have been responsible for the oscillations.

While these higher frequency components may have had some effect on the

overall behavior of the test joint, there is considerable precedent for ignoring

them. Therefore, the values of dynamic force that were used in the analysis

of data correspond to peak values of the main force-time pulse obtained by

visual readings that disregarded the higher frequency oscillations.

Making such readings of the pictures from the pendulum-impact tests (fig. 1)

was not difficult, but this was not true for some of the pictures from the drop

tests. In the pendulum tests, because the heavy (260-pound) impact head

generated fundamental pulse amplitudes at least several times greater than

the amplitude of the superimposed oscillations, the oscillations often resulted

in only a comparatively minor vertical widening of the trace. However,

Report No. 2263 -3-
because the drop test involved much lighter impact heads and fewer fasteners

per test joint, the amplitude of the main pulse that was generated in this test

sometimes was little or no greater than that of the superimposed oscillations

(see upper trace in each of the pictures in fig. 2). This made accurate

measurement of the main pulse extremely difficult. In the later drop tests,

both channels of the oscilloscope were utilized; one displayed the signal after

passing through a filter that eliminated all components above 2,000 cycles per

second and was easily read, and the other displayed the unfiltered signal

(fig. 2) and retained all of the force-time information for possible future reference.

Equipment and Instrumentation

The equipment used for the pendulum tests is shown in figure 3. A heavy,

rigid structural steel framework that was securely anchored to solid concrete

piers provided overhead anchorage for four aircraft cables 20 feet long.

These cables supported, at its four corners, a pendulum platform of welded

steel construction. The precise leveling and symmetrical alinement of cable

attachment points contributed to the steady, true swing of the platform.

The platform surface was galvanized sheet iron over plywood. The forward

edge of the platform, at rest, was parallel to and about 8 inches from the

adjacent vertical face of the massive concrete bumper. A half-round strip of

resilient polyurethane foam was glued to the forward edge to cushion impacts

between platform and bumper. On the longitudinal centerline of the platform,

at its rear edge, a ring was provided for engagement of the quick-release

clevis on the pulling cable.

The pulling cable passed through a pulley that was attached to an anchor post

on the extended centerline, and was connected to the hook of an overhead

electric hoist.

Two test members, each 8 feet long and cut from the same 16-foot nominal

4- by 4-inch timber, rested on the platform. These members were similarly

oriented on the platform with respect to direction of their growth rings.

Their inner edges were positioned parallel to and 10 inches from the platform

centerline, and their leading ends overhung the forward edge of the platform

to just contact the bumper face with the platform in the rest position. Provision was made for locking the platform in the rest position.

The test cleats, 1 by 4 or 2 by 4 inches, were attached to the top faces of the

4 by 4's, and 8 inches from the leading ends. Also attached, by wood screws,

to each top face near the leading end was a small reference block for the slip

Report No. 2263 -4-
measurements. A dial micrometer compression gage, together with precision

steel gage blocks when needed, was inserted between the reference block and

the end of the cleat to measure joint slip after each impact.

A cast iron rectangular plate that weighed 260 pounds rested on the two 4 by

4’s, and was properly centered.3 Between the plate and the adjacent end of

each test cleat, in intimate contact with both, was one of the two strain-gage

load cells (fig. 5). Each of these transducers weighed 0.85 pound and consisted of a solid aluminum alloy block 3 by 3-5/8 by 3/4 inches to which four

SR-4, type A-1 strain gages were cemented. The two active gages on each

block were oriented parallel to the 3-inch dimension on opposite 3/4- by 3-

inch faces. The other two gages, oriented perpendicular to the first pair,

were cemented side by side to one of the 3- by 3-5/8-inch surfaces. The

gages formed the four legs of a resistance bridge circuit that was connected

by shielded cable, through a calibration box, to the input terminal of one of

the two channels of the dual-beam oscilloscope. The other load cell was connected similarly, through a second calibration box, to the input terminal of

the other channel. Each load cell was calibrated in a standard testing machine, before and after the tests, by determining the compressive load required for a trace deflection equivalent to that produced by the calibration

resistor. Two horizontal sweep traces, one with and one without the calibration resistor, are displayed on the oscilloscope screen along with each impact

record.

Sweep synchronization was achieved through an external 3-volt direct current

circuit that used a trigger switch composed of two short lengths of flexible

steel strapping. One strap was attached to and extended upward from the

leading edge of the pendulum platform. The other strap rested, in part, on

the backs top abutment, and was held there by a weight on one end. The other

end protruded toward the first strap and contacted it near its free upper end

when the pendulum was in rest position. Before each impact, the straps were

adjusted so they would make contact 3/4 inch in advance of the impact.

A Land process camera and attachment for the oscilloscope were used to

photograph the transient displays.

The setup for the free-fall drop tests (fig. 6) involved single, rather than

paired, joints. The main member was a nominal 4 by 4, 40 inches long. A

3

In the later pendulum tests with thirtypenny nails, 3/4-inch bolts, and 3/4-

inch lag screws, the iron plate rested on rollers (fig. 4). These minimized

any effect of friction, between plate and 4 by 4's and between the latter and

the platform, that may have been a factor in preceeding tests. This required

some restraint to hold the various components together and in position on the

platform during the swing.

Report No. 2263 -5-
thinner and much shorter member (cleat) was fastened to one side of the main

member. A small reference block for slip measurements was fastened to the

same side of the main member about 8 inches from the cleat. One of the two

load cells used in the pendulum tests was positioned against both the main

member and the opposite side or end of the cleat from the slip reference block.

The impact head, a 2- by 4-inch steel bar, was positioned against the load

cell, opposite the cleat. One or two steel straps, whose ends were fastened

to opposite sides of the main member, loosely encircled the impact head on

three sides to prevent undue lateral shifting of the bar during the drop test.

The test assembly was suspended by an overhead electric hoist at the desired

height, vertically above the dropping surface. The electric hoist cable was

equipped with a solenoid-operated release with opposing hinged jaws. This

release applied no lateral force to the suspended item during disengagement.

Vertical alinement of the test assembly was checked with a carpenter's level

and necessary adjustments were made in the suspension rigging clamped to

the upper end of the main member.

The dropping surface was a heavy steel plate embedded flush with the top surface of a large concrete mass that formed part of the ground-level concrete

floor, as well as extended well below it. This steel plate formed one terminal

of the trigger in the external sweep synchronization circuit. The other terminal was a short length of bare copper wire that was fastened to the side of the

main member, near its impact end, and was bent to contact the steel plate

1/4 inch in advance of impact.

Because of the much lower impact forces that were generated in the drop tests

than in the pendulum tests, additional amplification of the signal pulse, in the

drop test, was necessary. Therefore, a direct current preamplifier set at a

gain of 250 times was connected between the calibration box and the oscilloscope. The signal was fed directly to one channel through a low-pass filter

with a cut-off frequency of 2,000 cycles per second.

Slip measurement equipment and procedures were the same for the drop test

as for the pendulum test.

Moisture content (ovendry basis) and specific gravity were determined from

sections taken adjacent to one end of each test member, using customary

procedures.

Report No. 2263 -6-
Presentation, Analysis, and Discussion of Results

The method used to analyze the data developed from the following relationships.

About halfway through the testing program a strong relationship became apparent between dynamic load, P, which is applied to a joint, and the square root

of the resulting slip (S1/2 ). Previous studies at the U.S. Forest Products

Laboratory that involved lateral displacement of nails under slowly applied

load have indicated a direct relationship between nail diameter raised to the

4

three -halves power (D3/2 ) and load, and also between specific gravity to the

three-halves power (G3/2) and load.5

When values of dynamic load versus values of the product S1/2 D 3/2G

3/2 for

corresponding impacts were plotted to logarithmic scales for all impacts and

all fasteners, a reasonably close approximation to a straight line resulted.

When the data originating from initial impacts was plotted as one set, and

data from the second and third impacts as separate additional sets, the

straight-line fit improved.

The data from all the impact tests are summarized in table 1. The first three

columns of this table are self-explanatory. Column 4 lists the number of

fasteners of a given kind and size that were impacted simultaneously. Each

of these figures is the same as the number of fasteners per test cleat in the

drop tests, but is twice the number of fasteners per cleat in the pendulum

tests, where two cleats were tested simultaneously.

The figures in column 5 are the result of dividing the total restrained weight

(impact head plus load cell) by the corresponding figure in column 4.

Column 7 gives values obtained by averaging the dynamic force measurements

obtained from the initial impacts against the several similar test joints of a

series, and dividing by the number of fasteners per joint. The figures in

column 8 are similarly obtained from dynamic force measurements made on

second impacts, and the figures in column 9 are from measurements on

third impacts. Because either two or four joints that involve a given kind

and size of fastener were tested, and because of infrequent failure to obtain a

4U.S. Forest Products Laboratory. Wood Handbook. U.S. Department of

Agriculture Handbook No. 72, 528 pp. 1955.

5Scholten, J. A. Nail-Holding Properties of Southern Hardwoods. Southern

Lumberman 181(2273):208-210, Dec. 15, 1950.

Report No. 2263 -7-
force-time record for a particular impact, the number of impacts represented

by each average value in these columns varied from 1 to 4. This number is

shown immediately above the corresponding average value, in each instance. l

Values of slip are given in columns 10, 11, and 12, and each value is the

average of slip increments developed in the same impacts that are represented

in the corresponding average dynamic force value (columns 7, 8, or 9).

The specific gravity of each cleat or main member was assumed to be that of

a small transverse section taken adjacent to one end of the member. This is

a common procedure, but it does introduce the possibility of some error because of variation of specific gravity along the length of the stock timber. The

specific gravity associated with each test joint is the mean of the values for the

respective cleat and main member. In some of the later pendulum tests

(those for thirtypenny nails, 3/4-inch bolts, and 3/4-inch lag screws), both

test members were cut from the same piece, and matching for specific gravity was good. In other tests, the specific gravity matching varied from good

to fair, and probably accounted for some variability in test results. The

average specific gravity shown in column 13 is the average of the values for

the appropriate individual test joints.

Columns 15, 16, and 17 give values of S1/2( DG)

3/2 that were calculated from

the appropriate value of S from columns 10, 11, or 12, plus the corresponding

values of G and D from columns 13 and 14.

Column 18 gives values that are products of average dynamic force per fastener (F) and of average slip (S) both for initial impacts. These values represent the average energy expenditure, or work done, per fastener in the initial

impacts on the test joints of each particular series. Column 19 gives corresponding values for second impacts, and column 20, third impacts.

Column 21 gives the potential energy that is available for doing work on the

test joint during one impact, divided by the number of fasteners in the test

joint. Each value is the product of the corresponding entries in columns 5

and 6.

In figures 7 through 10, values of dynamic load per fastener from columns 7,

8, and 9 of table 1 are plotted against corresponding values of the term,

S

1/2(DG)3/2, from columns 15, 16, and 17. Data plotted in figure 7 represents initial impacts only, against all the joints tested in the dynamic tests.

Similarly, figure 8 represents only second impacts, and figure 9, third impacts, of all joints tested. Figure 10 combines figures 7, 8, and 9, thus

giving an overall representation of the first three impacts on all test joints.

Report No. 2263 -8-
A straight line, of slope 1.0 and positioned visually to approximate the mean

of the data points, is shown on each of figures 7, 8, and 9. It should be

pointed out that the number of individual test fasteners involved in each data

point shown in these figures is the product of the appropriate small secondary

number given in columns 7, 8, or 9, of table 1, and the corresponding number

from column 4. Thus, for instance from the pendulum test, each data point

for sixteenpenny-nailed joints loaded parallel to the grain of the cleat represents tests of 4 times 28, or 112 nails; while each data point for sixteenpennynailed joints loaded perpendicular to the grain of the cleat represents tests of

only eight nails.

The line shown in figure 10 gives considerable weight to the data points for

sixteen- and thirtypenny nails, 3/4-inch bolts, and 3/4-inch lag screws, all

loaded parallel to the grain of the cleat. Confidence in these data is enhanced

by the close grouping of the points that represent each of the three impacts for

each of these fasteners. There probably are three main reasons for this.

One is that, except for sixteenpenny nails, these were the last fasteners tested

in the pendulum tests, after the techniques had been perfected and early “bugs”

were corrected. A second reason is improved matching of specific gravity of

cleats with that of the main member for tests involving thirtypenny nails,

3/4-inch bolts, and 3/4-inch lag screws. A third reason, which applies particularly to the nails, is that these data points represent tests of comparatively

large numbers of individual fasteners, and thus give better estimates of the

true mean values.

The equation for the line in figure 10 is as follows:

(1)

in which F equals dynamic force per fastener (pounds); S equals slip produced

by force F in one impact (inches); D equals diameter of fastener (inches);

and G equals mean specific gravity of the two wood members (grams per cubic

centimeter). The value, 90,000, is the value of F when S 1/2(DG)3/2 is equal

to unity.

In figure 11, values of the product of dynamic load per fastener (pounds) and

slip (inches) are plotted against corresponding values of the product of weight

of impact head per fastener (pounds) and drop height (inches). A line with a

slope of 1.0, and which if extended would pass through the origin is shown.

Although considerable scatter above and below this line exists, it does indicate an approximate mean. Further confidence in the relationship indicated

by this line is gained by noting its location with respect to the data points

representing pendulum-impact tests of joints that involve sixteen- and

Report No. 2263 -9-
thirtypenny nails, 3/4-inch bolts, and 3/4-inch lag screws, all loaded in the

grain direction of the cleat. Reasons for the increased confidence in the data

from these particular tests were given previously. The equation of this line

is as follows:

(2)

where F equals dynamic force per fastener (pounds); S equals slip produced

by force F in one impact (inches); W equals weight of impact head plus load

cell(s) on a "per fastener" basis (pounds); h equals vertical drop height

(inches).

Substituting in equation (1) the value obtained from equation (2) for F,

and simplifying gives:

(3)

Thus, we have an equation for two-member joints relating weight restrained

per fastener, drop height, slip, diameter of fastener, and specific gravity of

wood members.

It should be remembered that for the most part, h in the impact tests was

9 inches, but the relationship expressed by equation (1) was not noticeably

affected by drop heights as low as 4 inches and as high as 30 inches (see data

points for drop tests of sixteenpenny nails, parallel-to-grain loading, in

figs. 7, 8, and 9). However, because the test data for the 30-inch drop height

involved only two drops (first and third) of a single two-nail joint, the validity

of equation (1) for drops in excess of 18 inches has not been adequately verified.

Because the impact tests involved only two species and involved material of

limited range in specific gravity, extrapolation of the data to other species

and to other densities may involve some risk. However, well-established

procedures and data for determining allowable lateral-resistance loads for

metal fasteners under static conditions indicate that allowable load and specific gravity are quite closely related, regardless of species, throughout the

wood density range. Similar general relationships can be expected to hold

under conditions of dynamic loading.

It may be of interest to note that values of allowable lateral-resistance load

(static) that were calculated, for nails of various sizes in wood of various

species, according to the Wood Handbooks are about double the values of W

that were obtained for the same species and nail sizes with equation (3), with

h = 9 inches and assuming a slip of 0.30 inch. Data from this study further

Report No. 2263 -10-
indicate that two-member nailed joints loaded according to allowable static

values could be expected to slip about 0.50 inch under a single 9-inch impact

as applied herein.

Since either a permissible or an assumed value of slip, S, is required before

use can be made of equation (3), some idea of what constitutes a safe value is

necessary. Some of the pendulum-impact tests were continued until failure

of one of the two cleats, or until terminated arbitrarily after a comparatively

large number of repeated impacts. The following tabulation gives the number

of impacts to failure and indicates the corresponding total slip:

Fasteners

Number of impacts

to failure Total slip

(inches)

Sixteenpenny nails

1/4-inch bolts

5/16-inch lags

1/2-inch bolts

1/2-inch lags

The greatest average initial impact slip in table 1, except for the one 30-inch

drop test, was roughly 0.30 inch. A slip value of 0.30 appears to be a reasonable starting point for use of equation (3). Lateral-impact load ratings

for nails published in Air Force Technical Order T.O. 00-85-8, "Interior

Blocking, Bracing, and Cushioning," agree closely with values obtainable by

using 0.30 for S and 9.0 for h in equation (3). As evidence that permits better

correlation of package testing with actual shipping experience is developed, it

may become desirable to increase the existing lateral-impact load ratings for

wood fasteners. A convenient means of doing this in an equitable manner

for the various kinds and sizes of fasteners is provided by equation (3).

Conclusions

A useful and practical method was developed for testing and evaluating the

lateral resistance to impact loads for two-member wood lap joints that are

secured with metal fasteners.

Report No. 2263 -11-
A comparatively heavy impact head and a correspondingly large number of the

fasteners in a test joint were advantageous, particularly when joints with the

smaller fasteners were tested. Thus, the magnitude of the total impact force

that is to be measured may be increased with respect to the magnitude of the

"hash" that usually is present in unfiltered force-time records of impacts on

elastic systems.

Two-member wood lap joints secured with nails, bolts, or screws and subjected to impact loads that are applied parallel to the plane of contact between

the wood members should, for optimum utilization of fasteners, be designed

on the basis of permissible slip.

On the basis of their performance when they join two wood (Douglas-fir) members that are subjected to not more than three impacts, none of which can

slide one member with respect to the other more than 0.3 inch, no practical

difference exists between nails, bolts, and lag screws of the same diameter.

Report No. 2263 -12- 2.-26


Figure 2. - - Drawings of oscilloscope screen showing force-time pulses

for drop tests. Pulses in channel A show superimposed high-frequency

oscillations that have been filtered out in channel B. In the upper

drawing, pulses are shown from the third 9-inch drop test impact

against the side grain of a cleat secured with one 1/4-inch bolt. In the

lower drawing, pulses are shown from the initial 9-inch drop test impact against the side grain of a cleat secured with two thirtypenny

nails.

Report No. 2263
Figure 3. --Pendulum-impact test setup, with pendulum platfo

for 9-inch (vertical equivalent) impact. Hooked in the ring 

form is the quick-release clevis attached to the pulling cab

through a pulley attached to an anchor post (not shown) and 

hoist (not shown). The dual-beam oscilloscope with Land c

left foreground.

ZM 119 725
Figure 4.- - Test setup used for thirtypenny nails, 3/4-inch bolts, and 3/4-inch 

In all other pendulum-impact tests, the impact head plate rested directly on 

4-inch main members. Here, the plate rests on rollers to eliminate possib

friction between plate and main members during impact. Lightly nailed cle

and brace (B) were necessary to hold components in contact and alinement d

swing. Slip measurement reference blocks (C) are shown at the left near th

synchronization trigger (D).

ZM 120 284
Figure 5. --Closeup view of the two load cells in place between the impact

head plate and the ends of the two test cleats. Separate shielded cables

connect the load cells through the calibration boxes to the oscilloscope.

ZM 119 728
Figure 6.- - Test setup and equipment used in drop tests of two-member

joints. A, main member; B, cleat; C, impact head; D, load cell;

E, slip measurement reference block; F, dial gage and precision

gage block; G, solenoid-operated release mechanism; H, steel plate

impact surface; I, sweep trigger circuit; J, D-C preamplifier;

K, calibration and junction box; L, filter; M, oscilloscope with viewing and camera attachment.

ZM 120 777
Report No. 2263