United States 

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Paper 

FPL-RP-488 

Duration of Load

on Bolted Joints

A Pilot Study 

Thomas Lee Wilkinson 
Abstract 

Design values for bolted joints are adjusted for 

the effect of duration of load by using an 

arbitrary factor based on bending tests of small 

clear wood beams. Designers of timber structures 

need a more realistic factor. This report describes 

an initial effort to determine the effect of time 

under load on bolted joints. This is the first such 

study in the United States. 

The study included three-member wood joints 

with 1/2-inch bolts subjected to three levels of 

constant load for a period of 1 year. The creep 

rate approached zero after 3 months for joints 

loaded at 30 and 60 percent of the ultimate 

short-term load. Joints loaded at 85 percent of 

the short-term load had a decreasing creep 

rate with time, but significant creep was still 

occurring at the end of 1 year. The results 

of this study were quite variable, which 

indicates that large sample sizes would be 

required to model creep behavior. There appears 

to be an insignificant duration-of-load effect for 

the first year of service under controlled 

environmental conditions. 

Future research efforts should be directed at 

short width-to-bolt diameter ratios (Mode I failure) 

and large width-to-bolt diameter ratios (Mode III 

failure), as these modes may produce different 

time-dependent effects. 

Keywords: Connection, bolt, duration of load. 

July 1988 

Wilkinson, Thomas Lee. 1988. Duration of load on bolted 

joints: A pilot study. Res. Pap. FPL-RP-488. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 9 p. 

A limited number of free copies of this publication are 

available to the public from the Forest Products Laboratory, 

One Gifford Pinchot Drive, Madison, WI 53705-2398. 

Laboratory publications are sent to over 1,000 libraries in 

the United States and elsewhere. 

The Forest Products Laboratory is maintained in cooperation 

with the University of Wisconsin. 
Duration of Load 

on Bolted Joints 

A Pilot Study 

Thomas Lee Wilkinson, Research General Engineer 

Forest Products Laboratory, Madison, WI 

Introduction 

Published design values for mechanical fasteners 

in wood are derived by modifying test results for 

such effects as variability of results, moisture 

content, and duration of load. The test value used 

as a basis for design depends upon the particular 

fastener and may be the load at a given 

deformation, the apparent proportional limit load, 

or the ultimate load. The design value may be 

further modified for various physical and 

geometrical properties of the joint. 

Arbitrary adjustment factors have been applied to 

fastener test values to account for 

duration-of-load effects. These factors are 

primarily based on results of bending tests of 

small clear wood specimens under long-term 

constant load. The factors have been applied to 

all fastener values, whether the controlling joint 

property is the wood property or a combination of 

wood and fastener properties. The factors are 

also applied whether the test value is an ultimate 

load or a load at a given deformation. Those 

arbitrary factors have been used because only 

limited data exist for time under load effects 

on joints. 

The trend in reliability-based design concepts has 

pointed out the lack of knowledge about 

mechanical fastener joints and, in particular, the 

lack of knowledge on the duration-of-load effects. 

With the new limit states-based codes, there is a 

need to understand time effects on ultimate 

strength and on deformation at given loads. 

This report describes an initial effort to determine 

the time effects of load on bolted joints. The 

study was limited to one species, one bolt 

diameter, and one joint geometry, as it was 

intended as an initial effort for determining the 

need for additional studies. 
Literature Review 

Palka (1981) has done a selective literature review 

on the effect of load duration upon timber 

fasteners. Palka’s review revealed that (1) the 

most frequently used fasteners in light-timber 

constructions are nails and truss plates; (2) the 

most common fasteners may be considered as 

variants of pins or dowels; (3) both the short- and 

long-term behavior of timber joints can be 

predicted from an understanding of the behavior 

of five major physical components; (4) the 

short-term deflection of wooden structures is 

dominated by the lumber elements, while their 

long-term deflection is dominated by joint 

behavior; (5) both short- and long-term joint 

strength and stiffness can be substantially 

increased by gluing mechanical fasteners to, or 

into, wooden structural elements; and (6) a 

damage accumulation model developed for 

describing the long-term behavior of lumber can 

be readily adopted for describing the long-term 

behavior of timber joints. 

Palka further states that complete theoretical and 

experimental evaluation of both short- and 

long-term behavior of timber joints would require 

analytical models and test data for each of the 

five major physical components of timber joints. 

These components are (1) the flexural strength 

and stiffness of single pins; (2) the force required 

for their withdrawal; (3) the bearing strength and 

stiffness of wood members under the pins; (4) the 

lateral strength and stiffness of single-pin joints; 

and (5) the lateral strength and stiffness of 

multiple-pin joints. 

Based on his literature review, Palka (1982) 

proposed a general approach for the development 

of design procedures concerning the long-term 

behavior of timber joints. He developed 

appropriate testing procedures and data 

acquisition systems and suggested a plan for 

initiating load-duration experiments on truss-plate 

joints. Palka (1983) reported work performed on 

testing the short-term behavior of truss-plate 

joints, on modeling the short-term behavior of 

truss-plate joints, and on testing the long-term 

behavior of truss-plate joints. Palka began creep 

testing of 24 truss-plate joints on a small test 

frame built for evaluating the most economical 

methods of collecting long-term joint test data. 

Soltis and Wilkinson (1987) have shown that the 

European yield theory predicts the short-term 

behavior of bolted connections. This yield theory 

assumes that the bearing capacity of a bolted 

connection is attained when either (a) the 

compressive strength of the wood beneath the 

bolt is exceeded (Mode I failure) or (b) one or 

more plastic hinges develop in the bolt (Mode II 

or III failure). These assumptions provide for 

several modes of failure, depending on 

connection member dimensions, member 

strength, and bolt strength. Because of the 

different failure modes, the effect of duration of 

load may be more important for Mode I than for 

Mode III failures. 

No research in the United States has been done 

on duration of load on bolted joints. 

2 
Experimental Methods 

Specimens 

The specimens were three-member wood joints 

(fig. 1) with a single bolt. The main members 

were 3 inches wide and the side members were 

1-1/2 inches wide. All members were 1-1/2 inches 

thick. All material was Douglas-fir with a moisture 

content of approximately 10 percent. 

Figure 1 —Specimen 

geometry. Member thickness 

was 1-1/2 inches. (All 

dimensions are inches.) 

(ML88 5387) 

Bolts were 1/2 inch in diameter and 7 inches in 

length. They conformed to ASTM Standard A 307 

(Low Carbon Steel Externally and Internally 

Threaded Standard Fasteners). Washers were 

used under the bolt head and the nut. Nuts 

were finger-tight. The bolt holes were 9/16 inch 

in diameter. 

The specimens had a ratio of main member 

width-to-bolt diameter of six (Mode II failure). This 

ratio was selected because it is commonly found 

in glulam construction. 

End distance was 4 inches in both main and 

side members. 

Load Conditions 

For this study, four load conditions were used: 

(a) Short-term ramp load (constant test machine 

head movement of 0.05 inch per minute). 

(b) Constant load of 4,080 pounds–85 percent of 

the short-term mean. 

(c) Constant load of 2,880 pounds–60 percent of 

the short-term mean. 

(d) Constant load of 1,440 pounds–30 percent of 

the short-term mean. 

All loads were applied in tension. Sixteen 

replications were evaluated at each 

load condition. 

Matching 

The specimen members were cut from 

12-foot-long No. 2 and Better grade 2 by 4’s. One 

piece from each 2 by 4 (fig. 2) was assigned to a 

joint within one of the four load conditions. No 

attempt was made to match side members within 

a joint or to match main and side members within 

a joint. 

The area near the bolt hole was clear of knots. 

Some members did have rather large knots away 

from the bolt. 

Testing Procedures 

The short-term specimens were loaded in tension 

in a universal testing machine. Load-deformation 

curves were recorded to failure load. Test 

machine head movement was used to 

measure deformation. 

The constant load test utilized tension test 

frames which were designed to load full-size 

structural lumber. Specimens were suspended in 

the frames (fig. 3) and the load applied through a 

system of pulleys and cables where weights were 

attached. The machines were located in 

controlled conditions which maintained the 

lumber at a constant moisture content of. 

10 percent. 

3 
Figure P-Scheme for matching specimens between the four load conditions (a, b, c, or d) for 

left, main, and right-side member. Each sequence was repeated 4 times to obtain 16 replications. 

(ML88 5386) 

Figure 3—Bolted joint located in tension test frame for constant load test. (M85 0330-7) 

4 
Joint deformation was measured with variable 

resistor-type extensometers (fig. 4). 

Time-deformation data was monitored with 

a data logger and recorded on cassette 

tape. During application of the load, 

deformation readings were taken every 

3 or 4 seconds. After the load had been on for 

approximately 1 minute, deformation readings 

were taken every 5 minutes. After 1 or 2 hours, 

this was extended to reading every hour. After 

1 day, readings were taken every 6 hours for the 

remainder of the time the specimens were under 

load. Monitoring continued for a period of about 

1 year. 

After the specimens were unloaded, they were 

loaded to maximum load in a universal testing 

machine. This loading took place approximately 

8 weeks after the specimens were unloaded. 

Specific gravity was determined for both side 

members and the main member. 

Figure 4—Method of measuring joint 

deformation during constant load test. 

(M86 0254-3) 

5 
Results and Discussion 

Strength Data 

Based on the specific gravity of the joint 

members (table 1), the matching between load 

conditions was good. 

The maximum load (table 1) of the short-term 

specimens appears to be about 1,000 pounds less 

than the residual maximum strength of the 

constant load specimens. Similar strength 

increases have also been reported by Palka (1987) 

for truss-plate joints subjected to constant loads 

of different duration. 

Only a few specimens (table 1) failed during the 

year of constant loading. These failures were 

usually associated with knots or brash grain 

pattern (perhaps pre-existing compression 

failures) in the wood members and were always 

away from the joint area. 

The published design value for the joints in this 

study is 1,270 pounds. The design value was 

derived by applying certain factors to the 

short-term proportional limit load. The average 

proportional limit load for the short-term 

specimens was 2,940 pounds. Therefore, the 30 

and 60 percent load conditions were at or below 

the average proportional limit load and above the 

design value. 

Creep Data 

Examination of a typical static load-deformation 

curve (fig. 5) can explain some of the results 

obtained. A typical curve has a nonlinear initial 

portion where the joint is being tightened and 

members start to bear on the bolt. This nonlinear 

portion depends on how well the joints are 

fabricated, how smoothly the holes are drilled, 

and it varies from joint to joint. This variation is 

included in the initial deformations (table 2) and 

consequent creep data. 

The creep data (table 3) are presented as a ratio 

of the total deformation divided by the initial 

deformation to eliminate some of the effects of 

variable data. Results indicate that the creep rate 

was nearly zero after 3 months for the two lower 

load conditions. For the high-load condition, the 

creep rate decreased with time, but significant 

creep was still occurring at the end of 1 year. The 

continued creep of the high-load condition is as 

expected since these specimens were loaded 

beyond the linear portion of the static 

load-deformation curve (fig. 5). 

The total deformation increased about 20 percent 

for the 30 percent load condition, about 

40 percent for the 60 percent load condition, and 

about 140 percent for the 85 percent load 

condition at the end of 1 year. 

Typical creep curves (fig. 6) indicate fluctuations 

in the data. These were due to breakdowns in the 

climatic conditions of the room. The massive size 

of the test frames also contributed to fluctuations 

in the data, as the load on the specimen was 

sensitive to the position of the weight attached to 

the cable. 

6 
Table 1–Specific gravity and maximum joint load of specimens 

Specimens loaded at Specimens loaded at Specimens 

Short-term specimens 85 percent of short term 60 percent of short term 30 percent of 

Member Specific gravitya 

Joint Specific gravitya Joint Specific gravitya 

Joint Specific gravity

number of members maximum of members maximum of members maximum of members 

Main Left Right loadb 

Main Left Right loadb Main Left Right loadb 

Main Left R

side side (lb) side side (lb) side side (lb) side s

C 

1 0.44 0.37 0.51 4,420 0.45 0.41 0.50 5,950 0.44 0.48 0.49 — 0.47 0.40 0

2 0.49 0.48 0.56 4,570 0.45 0.50 0.58 4,670 0.45 0.48 0.56 5,140 0.46 0.51 0

3 0.43 0.46 0.55 4,600 0.45 0.47 0.55 5,830 0.43 0.45 0.51 5,030 0.45 0.47 0

4 0.46 0.48 0.53 4,640 0.47 0.46 0.48 5,590 0.50 0.50 0.50 4,870 0.44 0.51 0

5 0.48 0.46 0.44 4,440 0.49 0.41 0.42 4,870 0.49 0.40 0.42 4,920 0.47 0.41 0

6 0.42 0.40 0.49 5,020 0.44 0.38 0.47 — C 0.44 0.38 0.46 5,110 0.45 0.43 0

— C

7 0.55 0.45 0.46 4,620 0.52 0.42 0.44 0.51 0.46 0.49 5,090 0.57 0.42 0

8 0.40 0.42 0.46 4,860 0.41 0.44 0.46 4,510 0.39 0.45 0.45 5,560 0.40 0.46 0

9 0.44 0.43 0.57 4,110 0.43 0.55 0.41 6,140 0.43 0.53 0.40 5,320 0.42 0.51 0

10 0.56 0.52 0.50 5,340 0.48 0.51 0.53 — C 

0.50 0.47 0.53 — C 

0.51 0.57 0

11 0.43 0.47 0.53 5,110 0.44 0.47 0.55 5,960 0.46 0.45 0.51 6,400 0.46 0.47 0

12 0.47 0.42 0.51 4,800 0.48 0.38 0.48 6,000 0.45 0.38 0.50 6,730 0.48 0.40 0

13 0.50 0.44 0.52 4,600 0.48 0.45 0.50 6,230 0.46 0.46 0.54 6,620 0.45 0.46 0

14 0.50 0 43 0.49 5,420 0.46 0.48 0.49 6,200 0.47 0.52 0.45 5,200 0.47 0.48 0

15 0.55 0.48 0.51 5,370 0.55 0.53 0.50 7,810 0.58 0.51 0.45 4,810 0.57 0.53 0

16 0.57 0.44 0.45 4,820 0.57 0.41 0.48 — C 

0.55 0.43 0.47 5,110 0.52 0.48 0

Average 0.48 0.45 0.50 4,800 0.47 0.45 0.49 5,810 0.47 0.46 0.48 5,420 0.47 0.47 0

a

Specific gravity based on volume at test and ovendry weight.

b

Maximum load determined after completion of constant load test.

c

Specimen failed during the constant load test at locations away from the joint area.

d

Specimen failed away from the joint area.
Table 2—Average and range of initial deformations for 

constant load specimens 

Constant Initial deformation (in.) 

load (lb) Average Range 

1,440 0.120 0.027 - 0.234 

2,880 0.183 0.099 - 0.342 

4,080 0.207 0.147 - 0.285 

Table 3—Ratio of total deformation to initial deformations for various lengths of loading 

Total deformation/initial deformation 

Specimens loaded at Specimens loaded at Specimens loaded at 

30 percent of 60 percent of 85 percent of 

Elapsed short term short term short term 

time Average Range Average Range Average Range 

1 day 1.073 1.016 - 1.160 1.230 1.144 - 1.564 1.587 a 1.341 - 1.944 

1 week 1.091 1.013 - 1.210 1.275 1.157 - 1.627 1.781 a 1.490 - 2.231 

1 month 1.126 1.024 - 1.281 1.349 b 1.166 - 1.802 2.035 a 1.656 - 2.444 

3 months 1.156 b 0.980 - 1.367 1.409 a 1.190 - 1.916 2.267 c 1.819 - 2.684 

6 months 1.182 b 0.991 - 1.325 1.438 a 1.214 - 1.983 2.330 d 1.881 - 2.700 

1 year 1.149 b 1.000 - 1.303 1.444 a 1.200 - 1.989 2.434 d 1.717 - 2.894 

a

Value based on 14 specimens. 

b

Value based on 15 specimens. 

c

Value based on 13 specimens. 

d

Value based on 12 specimens. 

Figure 6—Typical creep curves. (ML88 5388) 

Figure 5—Typical static load-deformation curve. 

(ML88 5385) 

8 
Concluding Remarks Literature Cited 

A few general remarks can be made as a result of 

this study. 

1. The maximum strength of the specimens 

appears to have increased after 1 year of 

constant load. A gain of strength after 1 year has 

also been observed for truss plate joints; however, 

it is not understood why this occurs. Failures 

during the constant load test were associated 

with wood failures at growth characteristics away 

from the joint. 

2. The creep rate approached zero after 3 months 

for specimens loaded at 30 and 60 percent of the 

ultimate short-term load. For specimens loaded at 

85 percent, the creep rate decreased with time, 

but considerable creep was occurring at the end 

of 1 year. 

3. The results of this study were quite variable. 

This would indicate that large sample sizes would 

be required to model the creep behavior of bolted 

joints reliably. 

4. There appears to be an insignificant 

duration-of-load effect for the first year of service 

under controlled environmental conditions for 

bolted-wood joints with a width-to-bolt diameter 

ratio of six (Mode II failure), as long as load levels 

are at or below those on which the design values 

are based. 

This study was very limited in scope. Future 

research efforts should be directed at short 

width-to-bolt diameter ratios (Mode I failure) and 

large width-to-bolt diameter ratios (Mode III 

failure), as these failure modes may produce 

different time-dependent effects. It would also be 

beneficial to produce a large enough data base to 

arrive at mathematical models for predicting 

time-dependent behavior. 

Palka, L. C. 1981. Effect of load duration upon 

timber fasteners—a selective literature review. 

Canadian Forest Serv. Contract Research 

50-57-009. Vancouver, BC: Forintek Canada 

Corp., Western Lab. December. 

Palka, L. C. 1982. Effect of load duration upon 

timber fasteners: Assessment of the problem. 

Identification of research needs and a proposal 

for initial experimental work. Contract No. 

02-50-57-009. Vancouver, BC: Forintek Canada 

Corp., Western Lab. June. 

Palka, L. C. 1983. Effect of load duration upon 

timber fasteners. Contract No. 02-50-57-009. 

Vancouver, BC: Forintek Canada Corp., Western 

Lab. June. 

Palka, L. C. 1987. Effect of load duration on timber 

fasteners: Short- and long-term models. Report 

to Canadian Forestry Serv., Rep. No. 

04-54-10-009. Vancouver, BC: Forintek Canada 

Corp. March. 

Soltis, L. A.; Wilkinson, T. L. 1987. Boltedconnection design. Gen. Tech. Rep. FPL-GTR-54. 

Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 

21 p. 

2.0-7/88