United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL 346

1979

ABSTRACT

Bending tests of 28 untreated log

stringers from 12-year-old native

timber bridges in southeast Alaska

showed significant reductions in

strength due to decay. Compared to

results on fresh logs, strength

reduction was about 25 percent, and

could be predicted based on the loss

in section modulus due to decay. Log

stiffness was not significantly

affected.

Results will be helpful to engineers

with responsibility of rating the load

capacity of bridges having untreated

log stringers.

INTRODUCTION

Temporary log stringer bridges constructed of native logs with blast rock

surfacing are being extensively used on

logging roads throughout southeast

Alaska. Meeting the Bridge Inspection

and Load Rating requirements

Strength of

Log Bridge

Stringers

After Several

Year’s Use in

Southeast

Alaska1

By

R. C. MOODY, R. L. TUOMI,

Engineers

W. E. ESLYN,

Plant Pathologist

Forest Products Laboratory2

and

F. W. MUCHMORE,

Structural Engineer

Alaska Region

Forest Service,

U.S. Department of Agriculture

directed under the Federal-Aid

Highway Act of October 1968 presents

a problem—insufficient data have

been available to reliably predict the

load capacity of this type of bridge.

Thus, research was conducted to

determine the bending strength of log

bridge stringers.3

The data developed on bending

strength of log bridge stringers was

limited to green logs and did not

provide any measure of the strength of

untreated logs after several years of

service. To more accurately assess

service life of these temporary log

stringer bridges, strength data are

needed from bridges which have been

in service. Thus, this study was to

determine the relative strength and

stiffness of stringers after 12 years of

service in bridges in southeastern

Alaska.

RESEARCH MATERIAL

In the fall of 1975, three log stringer

bridges near Thorne Bay on Prince of

Wales Island in the Tongass National

Forest of southeastern Alaska were

scheduled for replacement. These

three bridges—Rio Roberts, Rio

Beaver, and Thorne River—are

described in table 1. Each bridge had

carried heavy logging traffic during

much of each year from the time they

were constructed.

Each bridge consisted of 10 Sitka

spruce logs. Eight logs were stringers

while the other two, placed on top of

the two outside stringers, were called

brow logs (fig. 1). During removal and

subsequent handling, two logs from

the Thorne River Bridge were broken.

Thus, 28 logs were available for

testing.

The eight stringers from each bridge

were numbered from 1 to 8, and the

1 Conducted in cooperation with the Alaska

Region of the USDA, Forest Service, Juneau,

Alaska and Louisiana Pacific-Ketchikan Division

(formerly Ketchikan Pulp Co.), Ketchikan, Alaska.

2 Maintained at Madison, Wis., in cooperation

with the University of Wisconsin.

3 Tuomi, R. L., R. W. Wolfe, R. C. Moody, and F.

W. Muchmore. 1979. Bending strength of large

Alaskan Sitka spruce and western hemlock log

bridge stringers. USDA For. Serv. Res. Pap. FPL

341 For. Prod. Lab., Madison, Wis.
Table 1.—Description of bridges

Figure 1.—Cross section of a typical

log stringer bridge. Bridge shown

consists of 10 logs (8 stringer and 2

brow logs).

upstream and downstream brow logs

referred to as BU and BD, respectively.

Log identification consisted of the

abbreviated bridge name (RR, RB, or

TR) and the stringer or brow log

designation.

RESEARCH METHODS

The same equipment described in a

previous report3 was used for the

stringer evaluations. The structural

testing was conducted between June

21 and July 2, 1976, and examination

to estimate the extent of decay

followed two weeks later.

Test Procedure

The logs, which had been stored in

an abandoned rock quarry used as the

test site, were all examined prior to

test. To the extent possible, knot

sizes and areas of decay were

recorded. Cables used to hold the

stringers together in the bridges had

cut several inches into the surfaces of

some logs. Also, some logs were

damaged by fork lifts during handling.

This damage appeared to be most

extensive in areas appearing to be

partially decayed, i.e., the outer few

inches of the diameter.

The circumference at each 10-foot

increment, beginning at the log tip,

was determined and converted to

2

M 148030

diameter readings. Nearly all of the

bark had either been removed during

handling or fell off during service;

thus diameter measurements were

actually inside bark. The use of insidebark diameters differs from the

previous study of fresh logs which

had the bark intact.3 Surface

weathering of the 12-year-old log

stringers indicated that most of the

bark had been off the logs for some

time. Therefore, the diameter without

the bark would be the value used for

rating the in-place capacity of the

bridges and is the appropriate

dimension to use in calculating the

residual unit strength of the logs. No

attempt was made at the time of

examination to judge the extent of

decay—rather it was an effort to

measure the outside diameter.

A test span approximately equal to

the bridge span was selected for each

of the three bridges and the logs were

placed on the test equipment with at

least 3 feet of overhang on the butt

end. Supports were so positioned that

the load could be applied near midIength. For each log tested, the exact

location of the supports and load

point were referenced to the tip end.

Logs were tested following the

general procedure previously

described. 3 The load was applied in

increments to produce about 0.1-foot

increments in log deflection.

Following each increment of

deflection, the loading was stopped

for up to 10 seconds to determine the

load by a strain indicator. Then the

load was increased to deflect the log

an additional 0.1 foot, which required

up to another 10 seconds. During

loading, the strain indicator was

adjusted to closely follow the load so

that when logs failed, the maximum

load could be determined. Using this

procedure, most logs could be tested

to failure in 1 1/2 to 5 minutes.

Determination of Extent

of Decay

After the log failed, the fractures

were examined and characteristics

believed to contribute to rupture

noted. Within the next 2 weeks, each

log was sawn at midlength and the

extent of decay determined in this

area. The latter determinations were

based upon surface appearance of

the log and probings into both the

peripheral and cross-sectional faces.

Peripheral decay was subtracted from

the gross area and an equivalent

cylindrical sound section was

estimated.

Calculations

Modulus of Rupture

Two methods were used to calculate

the modulus of rupture (MOR). For

both, the dead weight of the test

equipment (1,200 Ibs) was added to

the load determined from the strain

indicator. Also, the weight of the log

was used to determine the dead load

stress by assuming 40 pounds per

cubic foot for sound wood and 20

pounds per cubic foot for the decayed

portion. These dead load stresses

were added to the applied stresses to

derive an MOR value.

An MOR value, assuming a tapered

section, was calculated according to

equations developed for a previous

report. 3 The taper and diameter at the

load point used were based on a

regression analysis of the diameters at

10-foot intervals.4 This modulus of

rupture based on full log diameter was

referred to as MOR1

. Another value,

referred to as MOR2

, was calculated

based on a diameter that was reduced

by the amount of decay.

The location of the most highly

stressed section in center-loaded

4 For logs containing obvious butt swell, the diameter at the butt was not included in the regression analysis.
tapered members is not necessarily at

midspan. The degree of taper

determines the point of maximum

stress. However, in all these tests, the

maximum stress was at the load point.

Modulus of Elasticity

As with MOR, two methods were

used to calculate modulus of elasticity

values (MOE). MOE1 and MOE2 were

both calculated assuming a tapered

section and using procedures

previously developed.3 The difference

was that MOE1 assumed diameters

determined from a linear regression of

the diameter measured at 10-foot

intervals while MOE2 assumed a

diameter reduced by the amount of

decay.

Analysis of Variance

To determine if the results of log

tests from the three bridges were

different and, if they differed from

previous tests of fresh logs, an

analysis of variance was conducted.

Differences were tested to determine if

they were significant at the 95 percent

level.

RESULTS AND

DISCUSSION

The logs ranged in diameter at the

tip from about 24 to 40 inches. All but

two had taper of between 0.1 and 0.2

inches per foot, with one 8 percent

below the extreme values of this range

and the other 2 percent above.

Results of the tests are summarized

in table 2. There was no significant

difference between strength or

stiffness results from the three

different bridges; thus, results were

combined for subsequent comparisons. Neglecting the reduced section

due to decay, the modulus of rupture

(MOR1

) for the 28 logs averaged 3,480

lb/in. 2 and the modulus of elasticity

(MOE1

) averaged 1.28 million lb/in.2.

The lowest strength log had a MOR of

2,100 lb/in.2

.

Logs Broken Prior

To Test

There is a question of how to

properly consider the two logs from

the Thorne River Bridge that broke

during removal and subsequent

handling. It was difficult to judge

whether they were damaged during

service and, if not, the level of loading

they experienced during handling.

The one that was available for visual

inspection had extensive decay

pockets near midlength. If not

previously broken, stresses induced

during removal and handling may have

approached or exceeded those in

several of the tested logs. Without

this information, they could not be

included in the analysis. However, the

fact that they broke during handling

suggests that they were likely low in

strength and implies that the practical

limit for their use may have been

reached in the 12 years.

Log Failures

Twenty-two of the 28 logs failed by

fracturing on the tension side. Six

others did not completely fail. Most of

the 22 tension failures occurred near

mid length but two were within 10 feet

of the tip support. It was necessary to

stop the tests on another five logs

before complete rupture because the

deflection limit of the equipment had

been reached. For all of these, the

load did not appear to be increasing

and it was believed that the maximum

load had been reached. The remaining

log, RB05, was loaded to over 140,000

pounds before loading was stopped

because this significantly exceeded

the design load of the test equipment.

The curves of load versus deflection

plotted from the data for most of the

logs were Iinear to or beyond 50 percent of the ultimate load. Several

curves appeared nearly linear to

failure, but most departed from linear

behavior by 70 to 80 percent of the

ultimate load. The one log that

showed no indication of failure had

departed from linear load-versusdeflection behavior, and the load likely

would not have increased much above

the level reached. The resulting MOR1 of

4,090 lb/in.2 was one of the higher

values based on the full diameter.

When based on net section, its MOR2

was near the overall average. Although

the failure load may have been 5 to 10

percent higher, the actual calculated

stress resulting from the over 140,000

pound load was used in the analysis.

Decay Effects

Examination of the failed logs

revealed various amounts of decay.

The estimated effective reduction in

diameter varied between 2 and 8

inches with most logs reduced 4 to 6

inches. Based on the net diameter of

sound wood, the modulus of rupture

(MOR2

) averaged 4,820 lb/in.2 or

significantly (39 pct) higher than that

based on the full diameter. On this

same basis, the modulus of elasticity

(MOE1

) averaged 2.09 million lb/in.2.

Comparison With

Results Of Fresh

Log Tests

Results of previous tests3 on 25

Sitka spruce logs from southeast (SE)

Alaska are included in table 2. Eleven

of these 25 were from Prince of Wales

Island and their average results are

also listed separately.

Full Diameter (MOR1, MOE1)

The logs from the three bridges had

significantly lower modulus of rupture

values than found previously—averaging 23 percent less than all 25 logs

from southeast Alaska and 25 percent

less than the 11 logs from Prince of

Wales Island. The average modulus of

elasticity of 28 logs was not significantly different from that of either

grouping of previously tested logs.

Sound Diameter

(MOR2

, MOE2)

When strength calculations were

based on sound diameters, there was

no significant difference between the

average modulus of rupture of the 28

logs from the three bridges and the

previous results. However, the modulus of the elasticity calculated using

this approach resulted in values

significantly higher than found for

fresh logs.

Notable Points

The comparison with previous tests

of Sitka spruce indicated two

significant findings:

(1) Log stringer strength is significantly reduced by decay after 12 years

in a bridge in southeast Alaska, and

the amount of the loss can be accurately determined by measuring the

3
Table 2.—Strength properties of Sitka spruce log stringers from southeast Alaska1

reduction in the diameter and calculating the reduced section modulus.

(2) Log stiffness is not significantly

reduced by the presence of decay

under similar conditions.

The inconsistent effect of decay on

strength and stiffness might be

explained in that strength is controlled

by conditions at specific sections,

while stiffness is a function of the

entire log.

The actual rate of decay is obviously

a function of the geographic environment. The bridges on Prince of Wales

Island were located in an area of

substantial rainfall, 160 to 180 inches

per year. This, coupled with the rock

wearing surface, probably kept the

logs at an extremely high moisture

content throughout their service life.

Bridges of this design in a different

location would undoubtedly have a

different rate of deterioration. However, this study suggests that, if a

good estimate of the decayed zone

can be made, the residual strength can

be accurately determined.

SUMMARY

Tests of 28 Sitka spruce logs following over 12 years’ service in log

stringer bridges on Prince of Wales

Island in southeastern Alaska,

indicate that:

1. Bending strength was significantly lower than would be estimated

for fresh logs.

2. Log stiffness was about the

same as estimated for fresh logs.

3. Decay in the 24- to 50-inchdiameter logs reduced the effective

diameter between 2 and 8 inches.

A comparison of results with 25

previously tested Sitka spruce logs

from the same area suggested that the

amount of the bending strength reduction after 12 years was 25 percent, and

that this reduction could be accurately

predicted by the actual loss in section

modulus due to decay.

Considering the amount of strength

reduction and the fact that two

additional logs broke during handling

and could not be tested, it appears

that 12 years may be a practical limit

for Sitka spruce log stringers in southeast Alaska.

2.0-8-79

4 ★ U.S. GOVERNMENT PRINTING OFFICE 1979-650-028/14