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PRE
-
DESIGNED TIMBER BRIDGES OF THREE TYPES FOR ARKANSAS
COUNTY ROADS

MBTC FR
-
1057
-
1

Dr. Larry G. Pleimann

The contents of this report reflect the views of the

authors, who are responsible for the facts and accuracy

of the information presented herein. This document is

disseminated under the sponsorship of the Department of

Transportation, University Transportation Centers

Program, in the interest of information exchange. The

U.S. Government assumes no liability f
or the contents or use
thereof.

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2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
June 2000
Technical report, 1/96
-
12/97

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
Pre
-
Designed Timber Bridges of Three Types for Arkansas

County Roads

1


AUTHOR(S)

Dr. Larry G. Pleimann


2

PERFORMING
ORGANIZATION
NAME(S) AND ADDRESS(ES) 8. PERFORMING


Mack
-
Blackwell Transportation Center
ORGANIZATION

REPORT NUMBER

4190 Bell Engineering Center University of .Arkansas
FR
-
1057_.1 .Fayetteville, AR 72701 (report
)

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/

U.S. Department of Transportation
MONITORING AGENCY'
Research and Special Programs
Administration
REPORT NUMBER
400 Seventh Street, SW Washington, DC 20590
-
0001

11. SUPPLEMENTARY
NOTES

Supported by a grant from the U.S. Department of Transportation

University Transportation Centers program

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

National Technical Information Service 5285 Port Royal Road
N/A
Springfield,

VA 22161

13. ABSTRACT (MAXIMUM 200 WORDS)

According to the annual National
Bridge Inventory, the highest percentage of US
bridges that are "structurally deficient" or "functionally obsolete" is in the
"city/county/township" category . In recent years the major causes of rapid
deterioration of bridges with typical steel or concret
e superstructures have been
deicer chemicals and lack of adequate maintenance monies .

Well
-
designed "modern" timber bridges could be a cost
-
effective alternative for the
replacement of substandard bridges on county and city roads. Three types of simple
s
pan timber bridges have been designed with accompanying specifications for ready
use : (1) solid sawn stringers with transverse solid sawn deck planks, (2) glulam
stringers with transverse glulam decks, and (3) stress
-
laminated full
-
span glulam
stringers,
all constructed of southern pine (SP). This report contains designs for
the first type using 6 different SP stringer sections, for the second type using
4 different standard widths of SP glulam stringers, and for the third type using
the same 4 separate wi
dths of SP glulam stringers . Designs focused on flexural
adequacy. Procedures are included for changing span length or depth of the primary
bending sections to fit with any desired allowable deflection limitation or smaller
"load duration factor ."

14. S
UBJECT TERMS 15. NUMBER OF PAGES

1
01 timber, bridges, county roads

16. PRICE CODE

N/A '

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF

OF REPORT OF THIS PAGE OF ABSTRACT ABSTRACT
none none none
-
N
/A.

1


1


1


Technical Report Documentation Pa

1 . Report
No.

2. Government Accession
No.
3. Recipient's Catalog
No.

4. Title and Subtitle Pre
-
Designed Timber Bridges of Three Types
for Arkansas County Roads

5. Report Da e June 000 6. Performing
Organization Code

-

7. Author(s)

8. Performing Organization Report No.


Dr. Larry G. Pleimann

FR
-
1057
-
1 (report)

9. Performing Organization Name and Address

10. Work Unit No. (TRAIS)

Mack
-
Blackwell Transportation Center


4190 Bell Engineering Center

11 . Contract or Grant No.

University of Arkansas


Fayetteville, AR

72701

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address


U.S .

-
Department of Transportation

Technical Report, 1/96
-
12/)

Research and Special Programs Administration


400 Seventh Street, SW

14. Sponsoring Agency Code

Washington, DC

20590
-
0001


15. Supplementary Notes



Supported by a grant from

the U.S. Department of Transportation University
Transportation Centers program

16. Abstract

According to the annual National Bridge Inventory, the highest percentage
of US bridges that are "structurally deficient" or "functionally obsolete"
is in the "
city/county/township" category . In recent years the major causes
of rapid deterioration of bridges with typical steel or concrete
superstructures have been deicer chemicals and lack of adequate maintenance
monies .

Well
-
designed "modem" timber bridges co
uld be a cost
-
effective alternative
for the replacement of substandard bridges on county and city roads. Three
types of simple span timber bridges have been designed with accompanying
specifications for ready use: (1) solid sawn stringers with transverse s
olid
sawn deck planks, (2) glulam stringers with transverse glulam decks, and (3)
stress
-
laminated full
-
span glulam stringers, all constructed of southern
pine (SP). This report contains designs for the first type using 6 different
SP stringer sections, fo
r the second type using 4 different standard widths
of SP glulam stringers, and for the third type using the same 4 separate widths
of SP glulam stringers . Designs focused on flexural adequacy. Procedures
are included for changing span length or depth of
the primary bending sections
to fit with any desired allowable deflection limitation or smaller "load
duration factor."

17
. Key Words

18. Distribution Statement

No restrictions . This document is

timber, bridges, county roads

available from the National Technical
Information Service, Springfield, VA
22161.

19
. Security Classif. (of this
report)
20. Security Ciassif. (of this page) 21. No. of Pages22.
Price
-
unclassified unclassified 101 N/A

Form DOT F 1700
.7 (8
-
72) ep u ono co
mp a page
-
a

FINAL REPORT MBTC
-
1057

PRE
-
DESIGNED TIMBER BRIDGES

OF THREE TYPES FOR

ARKANSAS COUNTY ROADS


Principal Investigator

L. G. Pleimann Associate Professor of
Civil Engineering

Graduate Assistant Gregory
R. Riley

Conducted by

Department of Civ
il Engineering
University of Arkansas
Fayetteville, Arkansas

in cooperation with

Mack
-
Blackwell Rural Transportation Study Center

and

Arkansas State Highway and Transportation Department

and


U.S. Department of Transportation
Federal Highway Administration

University of Arkansas
Fayetteville, Arkansas 72701

June, 2000

DISCLAIMER


The findings and opinions expressed with
in this report and plans are those
of the author who is responsible for the facts and the accuracy of the results
presented herein. The contents do not necessarily represent the official views
or policies of the sponsoring agencies, the Civil Engineering D
epartment of the
1
University of Arkansas, the Mack
-
Blackwell Rural Transportation Study Center,
the Arkansas State Highway and Transportation Department, or the Federal Highway
Administration. This report does not constitute a standard, specification, or
regulation.

1

ii

FINAL PROJECT SUMMARY REPORT

The percentage of bridges in the National Bridge Inventory that are
"structurally deficient" or "functionally obsolete" is still largest for
"city/county/township" bridges, i .e ., for bridges that are outs
ide the
Interstate, U.S. or particular state highway jurisdictions. In recent years
the two major causes for the rapid deterioration of those bridges with typical
steel or concrete superstructures have been the increased use of deicer
chemicals and the lac
k of adequate maintenance monies .

An alternative that could be of competitive cost and that also could
contribute to the economy of the state of Arkansas by further developing its
timber industry would be the use of well
-
designed, well
-
constructed, and
p
roperly pressure treated "modern" timber bridges for the replacement of
county and city jurisdiction bridges . Simplicity of construction would be
an advantage of major importance for county bridge departments .

A computer program, PCBRIDGE, written in 19
91 was used to design a range
of adequate simple span timber bridges of three different types and with
accompanying plans and guide specifications for ready use by Arkansas county
road and bridge departments . The three types include the use of 1) solid
sa
wn stringers with transverse solid sawn deck planks, 2) glulam stringers
with transverse glulam decks, and 3) stress
-
laminated full
-
span glulam
stringers, all constructed of Southern Pine (SP) . All would have an asphaltic
concrete wear course underlain by

an adequate geotextile.

This report contains tables of designs for the first type using six
different SP stringer sections, for the second type using four different
standard widths of SP glulam stringers, and for the third type using the same
four specif
ic widths of SP glulam stringers. The designs were done
concerntrating on flexural adequacy . The reader is guided through simple
procedures so that the span length and/or the depth of the primary bending
sections may be changed so that designs may harmoni
ze with any desired
allowable deflection limitation, or desired smaller "load duration factor."

iii

TABLE OF CONTENTS


Item page DISCLAIMER


ii FINAL PROJECT SUMMARY REPORT


LIST OF FIGURES


vi LIST OF TABLES


vii ACKNOWLEDGEMENTS


viii 1 Introduction


1

1 .1 Deterioration of the American Infrastructure

1

1 .2 Timber as an Alternative


11

1 .3 Timber Bridges in Arkansas


15

2. Purpose and Scope


25

2.1 Project Purpose


25

2.2 Scope of the Project


27

3. Literature Review


30

4. Background on Typ
es of Timber Bridges


35

4.1 Historical Sequence of Bridge Type Development

36

4.2 Computer Program Scope and Bridge Types Used

46

4.3 Meetings with Industry Representatives

47

5. Bridge Design Computer Program


52

5.1 History of the Development of th
e Computer Program

52

5.2 General Structure of the Computer Program

53

5.2 .1 Screen Sequence in Designing Type 1, Solid Sawn Stringers
and Transverse Plank Deck


54

5.2 .2 Screen Sequence in Designing Type 4, Glulam Stringers and
Transverse Glulam Deck


55

5.2.3 Screen Sequence in Designing Type 9, Full Span Glulam
Stringers Stress
-
Laminated to an Orthotropic Deck

55

iv

TABLE OF CONTENTS (continued)

Item page

6. Timber Bridge Designs of Three Types


65

6.1 Design of Solid Sawn Deck with Solid Sawn Stringers

67

6.1 .1 Initial Assumptions and Design Procedure

67

6.1 .2 Final Designs


72

6.2 Design of Glulam Transverse Deck with Glulam Stringers

79

6.2.1 Initial Assumptions and Design Procedure

79

6.2.
2 Final Designs


85

6.3 Design of Full
-
Span Glulam Stringers Stress Laminated to an
Orthotropic Deck


90

6.3 .1 Initial Assumptions and Design Procedure

91

6.3 .2 Final Designs


93

6.4 Guard Rail Design


98

7. Conclusions and Recommendations


99

7.1
Conclusions


99

7.2 Recommendations


99 List of References


100

v

LIST OF FIGURES


Figure

page

National Interstate and State Bridge Data



6

National City/County/Township Bridge Data



7

Arkansas Interstate and State Bridge Data



8

Arkansas City/County Bridge Data



9

Isometric Section of Solid Sawn Stringer Bridge


36


Isometric Section of Solid Sawn Stringers with Dowel Laminated Deck

37
Isometric Section with Glulam Stringers and Transverse Deck

38 Isometric
Section with Glulam Stringers and Doweled Transverse Deck

40 Isometric
Section of Longitudinal Glulam or Dowel Laminated Deck

41 Isometric
Section of Longitudinal Stress
-
laminated Deck

42 Isometric Section of
Stress
-
laminated Deck Using Glulam

Stringers

44 Cross
-
sections Used for
Stress
-
laminated Box Girder Bridges

45 Cross
-
section of T
-
Beams With
FRP Tension Reinforcement

46

vi

LIST OF TABLES

Table

page

1

Data published in "Better Roads" magazine from the National Bridge
Inventory for the
fifty United States and the District of Columbia

4

2 Data published in "Better Roads" magazine from the National Bridge
Inventory for the State of Arkansas


5 3 Responses to Questionnaire


18 4 Type 1 Designs with 6 X 12 Stringers


73 5 Type 1 Designs
wit
h 8 X 12 Stringers


74 6 Type 1 Designs with 10 X 12 Stringers


75 7 Type 1 Designs with 6 X 14 Stringers


76 8 Type 1 Designs
with 8 X 14 Stringers


77 9 Type 1 Designs with 10 X 14 Stringers


78 10 Type 2 Designs with 5 inch wide Glulam Stringers

86 11
T
ype 2 Designs with 6.75 inch wide Glulam Stringers

87 12 Type 2
Designs with 8 .5 inch wide Glulam Stringers

88 13 Type 2 Designs
with 10.5 inch wide Glulam Stringers

89 14 Type 3 Designs with 5
inch wide Glulam Stringers

94 15 Type 3 Designs with 6.75 inc
h wide
Glulam Stringers

95

16 Type 3 Designs with 8 .5 inch wide Glulam Stringers

96 17 Type 3
Designs with 10 .5 inch wide Glulam Stringers

97

vii

ACKNOWLEDGMENTS

The author takes this opportunity to thank many people who have been
of great help in ac
complishing the work of this project and in producing this
final report. These include three former graduate students, Mr. S. Grant
Jordan who wrote the original version of PCBRIDGE, Mr. Lee R. Shaw who made
some important improvements in the program, and
Mr. Gregory R. Riley who used
PCBRIDGE to do the designs listed herein and who drew all the pages of drawings
and specifications.

Many long
-
suffering colleagues have awaited the arrival of this report
. These include, in historical sequence, all the directors of the
Mack
-
Blackwell National Rural Transportation Study Center, Dr . E. Walter
LeFevre, Admiral Jack E. Buffington, and Dr.
Melissa S. Tooley. And at lower
eschelons but with no less suffering have been the staff
-
of Mack
-
Blackwell,
Ms. Shantu Roychudarhi, Ms. Lyn Gattis, and most especially Ms. Sandra
("Sandy")

Hancock, who took this "Dummy's" use of WordPerfect 5 .1 and rela
ted software,
and converted it to WordPerfect Suite 9 thereby saving the author a "learning
curve" for which he simply didn't have the time . Thank you all .

viii

I


I


1. INTRODUCTION

1

1.1 Deterioration of the American Infrastructure
1
It is a commonplace
currently to speak of the deterioration of the American transportation
infrastructure. The national media periodically have reports on the
crumbling of pavements that should have lasted much longer, or of bridge
failures caused by the c
ombination of such
1
deterioration and the lack
of adequate funding for maintenance within the separate jurisdictions


responsible for particular road and bridge systems . Some of the
se failures
have led to loss of life which demonstrated just how serious the problem is.

There are many sources for the deterioration of the U.S. public
transportation systems . These include lack of maintenance because of
a lack of adequate funding, heav
ier traffic volumes, and heavier loads
particularly as truck traffic strives for increased efficiency by using
larger axle loads and longer strings of trailers . In addition, certain
environmental factors have effected a faster deterioration .
Especially w
ithin the context of bridge and pavement maintenance, the
increased use of deicing chemicals that began in the sixties has led
to more rapid deterioration of reinforced concrete bridge decks and
pavements .

The infiltration of chloride ions into the concr
ete causes the
pH surrounding the reinforcing steel to become acidic. This change in
pH allows the steel to oxidize. The resulting iron oxide crystals expand
as much as 16 times the volume of the source steel [Crumpton, 1985].
The internal expansion produc
es high tensile stresses in the concrete
. This leads to cracking near the top surface and spalling of the
concrete follows . The direct exposure of the underlying reinforcement
to the environment and traffic loads hastens the deterioration of the
slab. Un
less the damaged area is repaired, a significant loss of
strength and/or service life of

a pavement or deck will occur .

I
1

Most efforts to control the corrosion of deck and pavement
reinforcement have been directed toward protection of the steel bars.
Additional concrete cover, surface sealants for the concrete,
corrosion inhibitors mixed with the concrete, reduced concrete
permeability, cathodic protection, epoxy coating, and galvanizing are
examples . The use of fusion epoxy coated bars has become sta
ndard in
the effort to protect concrete reinforcing steel from corrosion,
certainly in the state of Arkansas. However, epoxy coating may not be
the final answer since small cracks in the coating may hasten local
corrosion [Clear, 1992] . Epoxy coating is a
lso being used with
pavement dowel bars. Few other alternatives have been proposed for the
protection of steel reinforcement apart from the suggestion of using
more expensive stainless steel [Black, et al, 1988], or to search for
another more effective coa
ting .

An alternate effort has attempted the development of other forms
of reinforcement that are not susceptible to corrosion. Fiber
reinforced polymer (FRP) bars provide one such option . This
"composite" material consists of thin high
-
strength syntheti
c fibers
embedded within a hardened polymer matrix. FRP bars have already been
used for slabs on grade, as prestressing tendons [Preis and Bell, 1987;
Nanni, 1991], in marine environment structures, and in structures
wherein non
-
magnetic properties are imp
ortant such as magnetic
resonance imaging installations [Roll, 1991], and large transformer
foundation pads . The bars are not susceptible to corrosion and have
high tensile strength.

The attention of the nation was brought vividly to focus on the
problem

of bridge deterioration in 1967. The Silver Bridge over the
Ohio River between Kanauga, Ohio and Pt. Pleasant, West Virginia failed
under afternoon rush hour traffic. The bridge was a 40 year old steel
suspension bridge of a total length of 1750 feet . It

had been inspected
as recently as April of 1965. But on that day it was ready to fail and
let 75 cars and trucks fall into the river, killing some 46 people.
Later investigation showed that the combination of a lack of adequate

2

maintenance and the use

of deicer chemicals had led to the rapid
deterioration of the main suspension "cables ." However, they weren't
cables
per se,
but eyebar links that are often susceptible to
deterioration and fatigue fracture .

The Silver Bridge failure led directly to th
e establishment of
the Federal Bridge Inspection program. Working through the state
departments of transportation, the program instituted an inspection
survey that intended initially to increase the frequency of
inspections so that every federal, state, an
d smaller local
jurisdiction bridges would be inspected at least once every two years.
The results of this inspection were to be included in a national data
base, or bridge inventory system. In Arkansas, the Arkansas Highway
and Transportation Department

(AHTD) has worked to increase the inspection frequency, especially for
those bridges whose

condition is problematic. Some bridges are inspected every year if not
more often .

This federal inspection program led to the common usage of phrases
such as "str
ucturally deficient" and/or "functionally obsolete ." The
results of the annual inspections are kept in a national bridge
inventory and are published periodically . The author first noticed
a typical summary of results in an annual November issue of Better

Roads
magazine which began to publish such data in 1978 . In the 1989 issue,
for example, of the some 588 thousand bridges in the fifty states and
the District of Columbia, a little over 38 percent, 225 thousand, were
still "structurally deficient" and/or

"functionally obsolete ." Of
that number, almost exactly two
-
thirds, 151 thousand, were on rural
highways or city streets, off the federally funded system.

Tables 1 and 2 below summarize the results of the Better Roads
data for the entire nation

and for

the state of Arkansas from the year when essentially all the
states reported complete

results for their jurisdiction until the present. The following Figures
1 through 4 present the

same data in a graphical format . Each figure plots a total number of
b
ridges from 1981 through

to the present, the total number substandard in that category, and the
percent substandard for

3

Table 1 Data published in "Better Roads" magazine from the National Bridge
inventory for the fifty United States and the District o
f Columbia

Reporting

Total

Total

Total

Total

Total

Total

Total

Total

Total

Year

Interstate
&

Substandard

Percent

City/County/

Substandard

Percent

All

Substandard

Percent


State
Bridges


Substandard

Township
Bridges


Substandard

Bridges


Substandard


1981

264,894

53,464

20.2

309,637

123,141

39.8

574,531

176,605

30.7

1982

263,303

58,379

22.2

307,292

154,171

50.2

570,595

212,568

37.3

1.983

264,078

62,830

23.8

302,775

165,928

54.8

566,853

228,758

40.4

1984

266,686

71,607

26.9

316,189

176,487

55.8

582,875

248,094

42.6

1985

269,129

71,584

26.6

317,112

177,618

56.0

586,241

249,202 .

42.5

1986

269,125

76,160

28 .3

315,752

169,657

53.7

584,877

245,817

42.0

1987

271,125

77,179

28 .5

315,555

166,201

52.7

586,680

243,380

41
.5

-
06

1988

272,337

77,787

28.6

314,606

161,915

51 .5

586,943

239,702

40.8


1989 .

274,678

74,910

27.3

313,039

150,552

48.1

587,717

225,462

38.4


1990

275,202

75,367

27 .4

310,134

145,654

47.0

585,336

221,021

37.8


1991

280,817

75,069

26.7

312,399

132,995

42.6

593,216

208,064

35.1


1992

281,670

74,424

26.4

319,080

132,480

41 .5

600,750

206,904

34.4


1993

279,073

69,473

24.9

309,077

121,951

39.5

588,150

191,424

32.5


1994

280,575

68,910

24.6

308,610

117,928

38.2

589,185

186,838

31.7


1995

281,840

70,784

25.1

309,365

116,720

37.7

591,205

187,504

31
.7


1996

281,398

70,126

24.9

307,845

112,281

36.5

589,243

182,407

31
.0


1997

280,898

68,810

24.5

309,142

110,645

35 .8

590,040

179,455

30.4


1998

279,543

68,466

24.5

309,792

109,626

35.4

589,335

178,092

30.2


Table 2 Data published in "Be
tter Roads" magazine from the National Bridge Inventory
for the State of Arkansas

Reporting
Year

Total
Interstate
&

Total
Substandard

Total
Percent

Total
City/County/

Total
Substandard

Total
Percent

Total
All

Total
Substandard

Total
Percent


State
Bridges


Substandard

Township
Bridges


Substandard

Bridges


Substandard


1981

6,539

1,100

16.8

7,671

5,930

77.3

14,210

7,030

49.5

1982

6,539

1,100

16.8

7,671

5,930

77.3

14,210

7,030

49.5

1983

6,691

1,777

26.6

8,017

6,522

81 .4

14,708

8,299

56.4

1984

6,628

2,005

30.3

7,707

6,691

86.8

14,335

8,696

60.7

1985

6,639

2,011

30.3

7,656

6,456

84.3

14,295

8,467

59.2

1986

6,649

2,008

30.2

6,330

4,296

67.9

12,979

6,304

48.6

1987

6,644

1,786

26.9

6,307

4,175

66.2

12,951

5,961

46.0

1988

6,667

1,511

22.7

6,325

4,147

65.6

12,992

5,658

43.5

1989

6,687

1,425

21 .3

6,196

3,927

63.4

12,883

5,352

41.5

1990

6,719

1',647

24.5

6,146

3,588

58.4

12,865

5,235

40.7

1991

6,750

1,617

24.0

5,990

3,159

52.7

12,740

4,776

37.5

1992

6,768

1,554

23 .0

5,925

2,821

47.6

12,693

4,375

34.5

1993

6,782

1,151

17.0

5,822

2,642

45 .4

12,604

3,793

30.1

1994

6,797

1,142

16.8

5,730

2,576

45 .0

12,527

3,718

29.7

1995

6,838

1,152

16.8

5,672

2,487

43 .8

12,510

3,639

29.1

1996

6,850

1,136

16.6

5,586

2,354

42.1

12,436

3,490

28 .1

1997

6,882

1,134

16.5

5,470

2,158

39.5

12,352

3,292

26.7

1998

6,941

1,109

16.0

5,405

2,048

37.9

12,346

3,157

25.6



National Interstate and State Bridges



3
Uv


2
70


40


10


80


a
c


cu

50


0



t




20

z





90



60

~~



30




0




i i

0

-
~"

Total Interstate &State
Bridges

91
81



71


61


51

Percent
Substandard

I

41 31

Total Substandard


02


1(


0


80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

Year of October Reporting Figure 1 National Interstate and State Bridge Data

National City/County/Township Bridges

-
tvv 1
0

1

360
91

Total City/County/Township Bridges

320
81


280
71

N
a)
>)

240




6(


200
160
120


"
Total Substandard

ro

Q4

~I

-

Percent
Substandard


5(
-
4(
3(


80









2C


40









1C


0 80

81

82

83

84

Year of October Reporting
85 86 87 88 89 90 91 92

93

94

95 96

97

98

990




Figure 2 National City/County/Township
Bridge Data




-

-
l



Arkansas Interstate & State Bridges

>
1000





0
>,

900





91

800





81



Total Interstate & State Bridges




700


1



71

rn

600





61

00







O

500





51

00

m`








n








E

400





41


Z









1
300


Percent Substandard



0
3





N






200





2(



100


Total Substandard



1(





1



0



80

81

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

97

98

99





Year of October Reporting








Figure 3 Arkansas Interstate & State Bridge
Data





Arkansas City/County Bridges

900C
800C

_
Pit,

,

91 81

700C

F
~~
---
P~

V.
-
Mm
~

Total City/County
Bridges

71
-
61

500C


400C

1

4(

300C

Total Substandard

3(

200C


2(

1000


i(

0


0

80

81

82

83

84

85

86

87
88

89
90

91

92 93 94 95 96

97

98 99


Year of October Reporting


Figure 4 Arkansas City/County Bridge Data


that category. A cursory examination of Figures 1 and 3 for Interstate and State Bridges fo
r the
nation and for Arkansas leads one to the same conclusions . The total number of bridges in this
category seems relatively stable, but with some decided increase as the general network of roads
is enlarged and upgraded. The percent of those national b
ridges that are substandard is relatively
low in both cases, but far higher than they should be . Fortunately, the percent substandard of
Interstate and State Bridges is lower in Arkansas than the national average, and


the last decade has shown a consistent decrease in that percentage both at the
national and Arkansas levels. In Arkansas during that time period the

rate of decrease
has been even more pronounced. When attention is focused on a problem, and the seriousness
and importance of the problem is understood, the American people respond. Arkansans are
a particularly self
-
reliant and practical people, and it is

not surprising that they
have responded in a more intense fashion. Consideration of the corresponding Figures
2 and 4 for "City/County/Township" Bridges at the national level and for Arkansas is
even more dramatic, and, in places, puzzling . First, in bot
h cases, the absolute numbers
of substandard bridges and the corresponding percentages are much higher than for the
Interstate and State bridges . This is understandable for a variety of reasons. The
bridges of the Interstate system and many of the bridges

on U .S. highways within the
separate states are part of a newer system. Also, the federal government has typically
more power to tax for maintenance monies than the individual states, especially if the
state is predominantly rural and less affluent. The
obverse of such a situation is that
the "state
-
aid" bridges, as they are called in Arkansas, are less well funded. In times
of financial distress the

first item to be neglected is maintenance and so the bridges suffer. Also, the audience for a substandard

bridge on the Interstate and state system is larger since the daily traffic count on these bridges is
typically larger . The larger audience can bring much more political pressure

I
10


for repairs than say the small population of a poor county concerned with a local bridge

I

.


Despite these factors, the changes in the "City/County/Township" category have been

dramatic both at the national and Arkansas levels . At the national level, after a peak in
1984,
there
has been a steady reduction in the absolute number and percentage of substandard bridges. The same
trend is evident in the Arkansas "City/County" bridge
data but the reduction is even more dramatic.
The peak value is again in
1984,
but the percent substandard is
86.8. By 1998
percent substandard has been
brought to a much lower value,
37.9,
but that is still higher than the national average for this
catego
ry,
35.4
percent. Arkansas has made major improvement in its state
-
aid bridges, but still has
a way to go to catch up with the nation in this category.

Some of the data is curious for the state of Arkansas . The total number of bridges in the

1

City/Coun
ty system has also been dropping . One major reason for this may be the increased popularity
in substituting systems of multiple culverts for bridges. It
could
be interesting in the future to
do a more detailed study of the history of the changes in Arkans
as's off
-
federaljurisdiction bridges.
Despite the dramatic reduction in the absolute number of Arkansas'


substandard state
-
aid bridges, the reduction of its percentage substandard for

the same category is
not as pronounced.

1.2 Timber as an Alternative

Part of the motivation for this study is the conviction that timber, as a bridge structural
material, can make a significant contribution to bridge replacement needs in the United Stat
es,
particularly for shorter span bridges in the "City/County/Township" jurisdictions. The other part
of the motivation is the recognition that this conviction is not widely shared by many people both
in and out of the bridge engineering community.

1
The
first ways for humans to cross streams were either to ford them at shallow points,

1

1


1

1


or to make use of convenient exposed stones in the stream bed . Perhaps the use of a naturally fallen tree
inspired our ancestors to intentionally fell trees for

similar use . Later masonry arches were also used.
It has been the author's experience in teaching structural design of both masonry and timber, that the
two materials, although the oldest and most natural of structural materials, are also the least under
stood
and the most maligned. Actually, because they are both natural materials they are, therefore, more random
in their behavior, more difficult to model mathematically, and more complicated than either steel and/or
reinforced concrete . This complexity h
as delayed the development of their adequate and complete
"engineering." Their lesser strength, when not adequately "engineered, " has led to a poor reputation for
both materials .

Nevertheless, the early history of bridges in the world and in the United States is a history of the
use of timber as a structural material . The effort to recall these long lasting and previous successes
has become a project for the timber industry . The
reader is directed to the

first two chapters of Mike Ritter's Timber Bridges : Design, Construction, Inspection, and Maintenance
[Ritter, 1990]. Another good example is a recent article in the magazine Public Roads [Duwadi and Ritter,
1997] that traces th
e history of timber bridges from the beginnings of the United States to the present,
and describes the development of the technologies of lamination and pressure treatment that are the basis
of "modern" timber bridges, and the source of the current competi
tiveness of timber with other bridge
structural materials.

Despite the major technological developments in the latter half of the twentieth century with respect
to timber bridges, there is still a basic current mind
-
set in the bridge design

community aga
inst the use of timber as a bridge structural material . Ritter [1990, p. 1
-
19] offers his
own explanation to that hesitancy . "Perhaps the biggest obstacle to the acceptance and the use of timber
has been a persistent lack of understanding related to desi
gn and performance

of the material ." Ritter, in turn, quotes Johnson [1986] as to the causes of this
"lack of understanding . " The timber industry is one of those industries that has
not made a substantial unified effort to generate and distribute techn
ical
information. This has been interpreted by some engineers as a reflection on the
suitability of the material itself, and not as an indictment of the industry for
failing to provide the information. The reason the timber industry has not met the
challen
ge is quite

obvious once one looks at the respective industries . Johnson goes on to say that whereas the
steel and cement industries have both separately and, on occasion, together actively promoted
structural steel and reinforced concrete as structural
bridge materials, the multiple parts of the
timber industry have not .

That is a dated statement, because in 1989, under the auspices of the Department of

Agriculture's U.S. Forest Service, the National Timber Bridge Initiative Program was established,
d
omiciled at their Northeastern Area office in Morgantown, West Virginia . The project is now called
the National Wood in Transportation Program . Part of the program is a competitive cost sharing
arrangement for encouraging the design and construction of i
nnovative demonstration timber bridge
projects, with an annual national budget that varies each year, but is in the order of 1.0 to 1.5
millions dollars .

Each state of the Union has received benefits from the program . The author has been shown several
s
uch bridge projects in Arkansas. He also witnessed and filmed the installation of an innovative bridge
project in Washington County. That bridge was a "stress laminated box girder" structure that
incorporated all of the current lamination developments in t
imber structural materials. The timber
bridge initiative program was been the source of a number of significant solutions to local bridge
replacement needs across the United States, but it has not caused a

major revision of attitude toward timber bridges
.


Another part of the Forest Service's information strategy was a series of timber bridge

I
design conference
s . The author has attended several of these conferences . He remembers vividly
the opening address at one such conference held in Birmingham, Alabama . The speaker

was the then Secretary of State for Alabama, a man who was also a licensed professional ci
vil engineer.
His primary point was that the potential economic advantage of the use of timber bridges for his state
was two
-
fold . On the one hand they promised a relatively cheap solution to the problem of replacing
substandard spans on Alabama rural roa
ds . On the other hand they gave promise to promoting the growth
of the important timber industry of his state. The increased use of timber bridges has an identical
two
-
fold potential for the state of Arkansas . The sections for the Washington County stres
s
-
laminated
box girder timber bridge mentioned earlier had been manufactured of mixed oak from Southern Illinois
. They could just as well have been manufactured by and contributed to the economy of northwest Arkansas
.

It would be a mistake, however, to
think that this mutually contributive economic solution is
without problems. At this writing the onset of global warming is being taken with increased
seriousness. The world weather is being threatened by the most significant El Nino of several decades.
A
five hundred year flood in North Dakota and southern Canada was been preceeded by numerous summers
of hundred year floods throughout the world . The contribution of forests

1

in exchanging oxygen for carbon dioxide becomes exceedingly important . The conf
lict between human
use that can be made of forest products as fuel, paper, structural material, and raw material for
the chemical industry has to be balanced with values provided by forests remaining

1
intact, i.e., flood protection, erosion control, wildlife habitat, oxygen manufacture, soil humus,
and human recreation. Even intense reforestation is not necessarily an answer if the method of

it defies the need for biodiversity in the

forest . Obviously, this is an area needing the wisest of human
decisions, and the ability to compromise on goals that include values that are not just short
-
sighted
immediate human values. Trade
-
offs are inevitable, but the author is still of the

belief

that the use of well
-
engineered and constructed timber bridges will have some significant part to
play in the real solution of Arkansas' rural bridge replacement needs .

1.3 Timber Bridges in Arkansas

The common American mind
-
set that views the design of timber bridges as a waste of money is widespread
in Arkansas as well . It is the author's experience and opinion that this is true not only among the general
public but also at all echelons of the bridg
e design
-
constructionmaintenance community as well.

This negative mind
-
set does not have as significant a discouraging effect on creativity and flexibility
in the "Interstate and State" system, because the public is accustomed to seeing steel stringers un
der
concrete decks for most major bridges and overpasses on the Interstate, federal, and state highways of
Arkansas . Timber superstructures could be a viable option for many of these bridge structures. But the
bridge design section of the AHTD has honed t
he design of concrete
-
deck
-
over
-
steel
-
stringers bridges to
the point that it is very easy and therefore very economical for the AHTD to continue their use for both
short and long spans . Nevertheless, there is some flexibility emerging in the bridge depart
ment of the
AHTD that is probably caused as much as anything by the need to modify designs in terms of life
-
cycle costs
instead of initial construction costs. The issue of bridge superstructure and deck deterioration plays
a large role in these changes .

Several years ago the author attended a one
-
day short course sponsored jointly by the AHTD, and the
"Arkansas Area Prestressed Concrete Council." The latter was at that time a new organization unknown to
the author . The membership of the organization cons
ists of

precast prestressed concrete element producers who are interested in the potential Arkansas
1
market.
The vast majority of the Council's members are domiciled in states bordering Arkansas because there
are very few such producers inside the borders

of Arkansas . The primary selling point of the conference
was the superior durability performance of precast stringers as described

1

in a presentation given by a staff member of the Portland Cement Association (PCA) [Rabbat, 1993].
The address was a com
parative study of the durability of certain types of bridge superstructures
using data taken from the National Bridge Inventory . The primary point of the article was that
bridges with prestressed concrete stringers were longer lasting . The structural mat
erial with
the poorest record in the study was timber. The author's own reaction to this was

1

that the development of the tech
nology that underlies "modern" timber bridges is so relatively new
and seldom used that one could believe that the study was not a fair comparison with respect to
timber.

The author in previous years kept lists provided by the AHTD of the distribution of
various
structural materials for the superstructures of state
-
aid bridges in Arkansas . His recollection
is that approximately half of the superstructures of those bridges in an era about a decade ago
were made of timber.

Negative reaction to the decay of

traditional timber bridges has led many county judges

and their road and bridge departments to make a commitment to find inexpensive alternatives to
timber bridges. Used railroad flatbeds have been used. These were cheap at first, but their price
has ris
en with their popularity . They are difficult to "load rate" because their strength reduction
due to previous fatigue loading is not easy to evaluate . Moreover, sometimes they are "modified"
in an unsafe manner in order to be fitted to a particular bridge

site. Also, corrosion

of these all
-
steel superstructures is not easy to prevent .

Another popular program for some counties has been the use of side
-
by
-
side precast

concrete channel sections for use in various span lengths for county bridge replacement
s . The

author is not certain when these plans were developed. The copies he has for both the bridge

sections and the plans for the forms list the University of Arkansas Division of Agriculture

Cooperative Extention Service in the title block . He belie
ves, however, that the design of the

sections was developed initially by AHTD for the sake of state
-
aid bridges in the mid
-
60's .

Several counties in the state made early use of these plans and have produced the sections for their
own bridge replacement
program for quite some time . Washington County is an example of such early
use. Craighead County, with Jonesboro as County Seat, and Jefferson County, with Pine Bluff as County
Seat, have newer and more advanced production facilities for year round produc
tion of the channel
sections .

The plans allow varying standard lengths of 19, 25, and 31 feet, depending on whether the main
girder reinforcement consists of #9, #10, or #11 rebars respectively . Most counties with which the
author is familiar use a 30 foot span length and #11 rebars .

Seven of the 3'
-
7.5" wide channel sections
side by side provide sufficient width for two standard lanes and space for precast curb units at the
two outside edges .


Counties that use
this system have found it very economical . Some other counties purchase similar
units from a few precast manufacturers in the state. All in all, this has been a very useful and
successful program for short span bridge replacement on counties in the state.

1
The scope for
this project will be described in more detail later. The initially intended scope included surveying
some 21 counties in the southern third of the state for help in identifying bridge sites where economic
comparison could be made of altern
ate bridge
1
replacement schemes including as many as three types
of timber superstructure bridges . The response to a questionnaire sent by the author to the county
judges in those 21 counties was so discouraging in terms of the positive response to the us
e of timber
bridge superstructures yet so interesting as to the variety of types of bridges systems used, that
the author decided finally to
1
send the questionnaire to all 75 of the counties in the state . Table
3 following gives the results

TABLE 3 RESP
ONSES TO QUESTIONNAIRE

Decision

Bridge Types Used In the Past

Preferred Material For

Against



County

Timber

Sawn
Timber

Glulam
Timber

Railroad
Flatcar

Precast
R/Concrete

R/C Deck, Steel

Culverts

Pilings



Beams

Beams

Beds

Sections

Beams





ARKANSAS

YES

no

no

50',89'

no

no

steel pipe


ASHLEY

YES

no


no

yes (p)

no

galvanized


BAXTER

no

no

no

no

yes (M)

no

precast R/C


BENTON


no

no

no

yes (M)

no

steel

10" I
-
bms


BOONE

no


no


yes (p)


corrugated metal

cast R/C


BRADLEY










CALHOUN

YES

no



yes (M)

no

plastic, metal

cast R/C


CARROLL

no

no

no

yes

yes (p)

no

concrete

cast R/C


CHICOT

no



yes

yes (p)


concrete, steel


00











CLARK

no


no

yes

yes (p)

timber deck




CLAY

YES

yes

no

yes

no

no

black steel pipe



CLEBURNE


no

no

no

no

no

corrugated plastic



CLEVELAND

YES

yes

no

yes

yes (p)

yes




COLUMBIA

no

yes

no

yes



metal



CONWAY


no

no

no

yes (p)

no

steel pipe, conc. box



CRAIGHEAD

YES




yes (M)


corrugated pipe

precast R/C


CRAWFORD

no


no

yes

yes (p)

yes

plastic double lined



CRITTENDEN

no

yes

no

yes

yes (p)


corrugated metal



CROSS

no

yes

no

yes

no

no

galvanized steel



DALLAS


yes

no

yes

no

yes

galvanized metal

steel or R/C


DESHA


no

no

yes

yes (p)

no

steel



TABLE 3 RESPONSES TO QUESTIONNAIRE (continued)

County

Decision
Against
Timber

Sawn
Timber
Beams

Bridge Types Used In the Past
Glulam Timber Beams Railroad
Flatcar Beds Precast R/Concrete
Sections

R/C Deck, Steel Beams

Preferred Material For
Culverts

Pilings


DREW


yes

no


yes (p)


metal

FAULKNER

YES

yes

no

yes

yes (M)

yes

precast R/C

FRANKLIN

YES

no


no

yes (p)

yes

steel tile culverts

FULTON

no

no


no

no

no

aluminum box

GARLAND

YES

yes

no

yes

yes (p)

yes

steel pipe

steel H
-
piles

GRANT

YES

yes

no

yes

no

no

galvanized or steel


GREENE


no

no

yes

yes (M)

yes

cut tanker cars

concrete in pipe

HEMPSTEAD

no

yes

no

yes

yes (p)

no

galvanized steel


r

HOT SPRING

no



yes

yes (p)


steel

steel pipe


HOWARD

no

yes

no

yes

yes (M,p)

yes

plastic, steel

steel, timber


INDEPENDENCE

YES

yes

no

yes

yes (p)

yes

metal



IZARD

no

no

no

yes

no

yes

corrugated metal

precast R/C


JACKSON

YES



yes



galvanized pipe



JEFFERSON

no

no

no

yes

yes (M)

yes

cast R/C, timber?



JOHNSON

no

yes

no

yes

yes (p)

yes

galvanized pipe



LAFAYETTE

YES



yes


no

tank car sections

some timber
piles


LAWRENCE

YES

no

no

no

no

no

corrugated



LEE

no

yes

no

yes

no

no

tank car sections



LINCOLN


yes

no

yes

yes (p)


galvanized steel



LITTLE RIVER


yes

no

yes

no

yes

steel pipe



LOGAN

YES

no


yes

yes (M)

yes

steel tubing



TABLE 3 RESPONSES TO QUESTIONNAIRE (continued)

County

Decision
Against
Timber

Sawn
Timber
Beams

Bridge Types Used In the Past
Glulam Timber Beams Railroad
Flatcar Beds Precast R/Concrete
Sections

R/C Deck, Steel Beams

Preferred Material For
Culverts

Pilings


LONOKE

YES

yes

no


yes

yes (p)

yes

corrugated steel

MADISON

YES

no

no


yes


yes

galvanized pipe

MARION

YES

no

no


yes

yes (p)

no

galvanized metal

concrete
pilings

MILLER

YES





yes (p)


tank car sections


MISSISSIPPI

no

yes

no


no

no

yes

tank car sections

timber piling

MONROE

YES

yes

no


yes

yes (p)

no

pipes, rail cars


MONfGOVERY

YES

no

no


no

no

R/C strgrs

double wall plastic


NEVADA

no

yes

no


yes


yes

steel

timber, metal

NEWTON










QUACHITA

no

yes

no


yes

yes (p)

yes

treated culvert


PERRY

no

no



yes

no

no

CMP


PHILLIPS


yes

no


no

no

no

"everywhere we can"


PIKE

no





yes (p)

timber deck

galvanized pipe

metal

POINSETT

YES

yes

no


yes

no

no

galvanized steel

timber piles

POLK

no

yes

no


yes

yes

yes

double w
all plastic


POPE!

no

no



yes

yes (p)


plastic


PRAIRIE

no

yes

no


no

no

no

steel

timber piles

PULASKI

YES

yes

YES


yes

yes (p)

yes

R/C, aluminum box

concrete

RANDOLPH


yes

no


yes

yes (p)


steel


SALINE SCOTT

YES no

no no

no no

I

yes yes

yes yes (p)

no

galvanizd stl, plastic
tank car sections



TABLE 3 RESPONSES TO QUESTIONNAIRE (continued)

County

Decision
Against
Timber

Sawn
Timber
Beams

Bridge Types Used In the Past
Glulam Timber Beams Railroad
Flatcar Beds Precast R/Concrete
Sections

R/C Deck, Steel Beams

Preferred Material For
Culverts

Pilings


SEARCY

no

no


no

no

no

corrugated metal

concrete piers

SEBASTIAN

YES

no

no

yes

yes (p)

yes

cast R/C, plastic

cast R/C, stl H

SEVIER

no

yes

no



yes

plastic

cast R/C

SHARP




yes

no

yes

galvanized metal tile


ST.FRANCIS

YES

yes

no

yes

no

no


treated timber

STONE

no

no


yes

no

no

steel


UNION

no


no

yes

no

no



VAN BUREN









WASHINGTON

no

yes

YES

no

yes (M)

yes

precast R/C, metal

cast R/C

N

WHITE

no

no


no

no

timber deck

steel tile

timber


WOODRUFF







corrugated steel pipe



YELL

YES

no


no


yes

steel pipe



1

1


1


1

t


of the questionnaire in tabular form. The questionnaire was modified twice as the early responses indicated
difficulties the counties experienced in understanding the intent of some of the questions. The three
separate versions of the one
-
page questionnair
e sent to the county judges appear in the Appendix. If blanks
occur in Table 3 it is because the person responding from the individual county did not include a response
to that question . Three lines in the table are completely blank because the county jud
ge and/or road and
bridge department director chose not to respond not only to the initial mailing but to as many as three
follow
-
up mailings . All this is indicative of busy schedules, but the responses (or lack thereof) also
indicate a general disinteres
t in timber as a bridge superstructure material . Nevertheless, that 72 counties
out of 75 eventually responded makes the answers useful .

One of the primary interests of the author was whether counties had made a conscious policy decision
to not use timb
er as a superstructure material . Sixty
-
four of the 72 questionnaires received responded
to this question . The 32 "YES" responses are indicated in similar bold capitals in the table . Thirty
-
two
responded "no," but this may be misleading because most of t
hem were making a concerted effort to replace
their bridges by some means other than timber superstructures . Of the eight counties that returned a
questionnaire but did not respond to this question only two indicated that they had not made use of wood
in
the past, but six were using alternate types for their standard method of bridge replacement presently.
Although at one time about half of the "state
-
aid" bridges in Arkansas had timber superstructures, no county
responding is using any type of timber brid
ge as its preferred method of bridge replacement. That would
indicate a majority disenchantment with timber presently .

Fifty
-
nine counties responded to the question regarding the use of solid sawn stringers . Thirty
-
one
of them admitted to using solid sa
wn timber stringers at one time or another . The author believes that
many of the 28 who responded "no" to that question were thinking of recent

I

I


use. But only two counties of the 52 responding had ever used laminated
preconstructed

I


stringers or decks, one of the many applications and techniques that we
would now include as



part of a "modern" timber bridge. One wonders if the remaining twenty
did not reply to the



question because they did

not know what a "glulam timber beam" was .



Forty
-
eight of the 64 counties giving a response to the question said
that they had used



railroad flat cars as bridge superstructures . Such use is a recent
development and not easily

1


forgotten. The seeming economy of
such
use has obviously been very
convincing. Also, forty



counties of 64 responding said they had made use of precast reinforced
concrete bridge sections,



and 10 said that they had man
ufactured the units themselves .
Interestingly, although the most



typical form of bridge superstructure on the interstates and federal
highways are steel stringers



acting in composite fashion with a cast concrete deck, only 30 of the
58 counties that responded



to that question had ever used this method on county roads . Three
counties responded that they



used timber decks over steel stringers and one modified the question to
the use of reinforced



concrete stringers.



Many counties indicated they they were interested in replacing
deteriorated short span



bridges by single or multiple culverts . The preferred culvert materials
included plain and



galvanized steel, and plastic lined pipe . A few used tank car sections.
Some used precast



reinforced concrete sections . As many as three indicated the possible
use of treated timber



culverts, which would certainly be an option, especially if the streams
contained elements



corrosive to the other alternatives .



Relatively few responsed with their preferred material for pilings . Most
that did included



either cast
-
in
-
place or precast reinforced concrete piles or steel
I
-
shapes . Only eight indicated


1

that timber piles were used routinely .



All in all, the responses given were only somewhat surprising to the
author . The lack


1

1


f

of understanding of "modern" timber construction as a viable alternative again seemed to mirror the
general lack of knowledge of the public with respect to technological alternatives including treated
timber. But it is obvious that counties are actively se
arching amidst new technology to find some
alternative bridge replacement technique that fits their budget, the level of training and capabilities
of their crews, and their actual bridge site conditions .

Despite the current lack of interest in timber as
a superstructure material for short span bridge
replacement, it is the conviction of the author that timber bridges are a viable alternative . Obviously
the demonstration of their viability rests in the hands of those organizations and industries that are
intimately connected to and knowledgable of this alternative . It is probably a matter of time before
their potential may be known and realized. In the meantime, continued progress can be made in developing
further improvements in timber bridges. For examp
le, consider the recent development of fiber composite
reinforced glulam stringers that adds significantly to the moment carrying capacity of a typical glulam
cross section . Such research should be continued in those universities equiped for it . It is ho
ped
that this publication may add to the interest and use of at least the three types of timber bridge
superstructures chosen for inclusion in this study .

I




I

2. PURPOSE AND SCOPE



1
2.1 Project Purpose


The previous chapter sought to justify this proj
ect by setting it in the general context of

1
the current need of repair of the nation's infrastructure . Although the notion of using
timber as the structural material for short span bridge replacement in the state of
Arkansas is not popular, nevertheles
s it could very well be the wisest decision for
certain actual spans and sites . A major aid for counties in deciding on the use of
a particular timber bridge type would be the development of standardized designs that
could be easily adapted to specific si
tes, and could be quickly provided by timber
suppliers and/or glulam manufacturers . This idea is consistent with the current status
of timber bridge design and the intentions of the timber industry . The accomplishments
of the National Timber Bridge Initi
ative and continued improvement in the technology
of timber bridge construction has renewed interest in some quarters in using modern
timber bridges to replace many structurally deficient short span bridges on county
roads. In a recent joint publication of

the U.S. Forest Service and the U.S . DOT FHwA,
Development of a Six
-
Year Research Needs Assessment for Timber Transportation Structures
[1992],
the twentieth highest priority of 118 total needs was to "develop prefabricated,
modular timber bridge systems

that are easily transported . " One of the basic
assumptions for such bridges in the state of Arkansas would be that the material used
would be Southern Pine . It is the dominant structural timber species for the

1

Southeastern United States and is, ther
efore, readily available . Moreover, it has certain other
distinct advantages over other structural softwoods . Southern Pine is a very strong and dense
material yet it also has a physiology that makes it more amenable to pressure treatment with

wood preservatives. The proportions
-

of lateral "rays" that allow the deep penetration of


25 treatment fluids into the body of the wood are larger in Southern Pine than they are in s
ay Douglas
Fir. This physical property makes the potential durability of
properly designed, treated, and constructed Southern Pine timber
bridges longer than would normally be the case .

Part of the emphasis of the project was to simply the design and con
struction of timber bridges
as much as possible. In some states, such as Iowa, the head of a county road and bridge department
is required to be a registered professional engineer . That is not the case in the state of Arkansas.
The people of Arkansas have

fewer high school diplomas per thousand than most other states, let
alone university degrees. The heads of the road and bridge departments are more often than not
experienced persons of high intelligence and capability, but their training

has been by exp
erience rather than formal education . Therefore, although they can adequately

construct a bridge they may not have been trained to understand the structural principles that

underlie those construction procedures . Sometimes the lack of formal training m
ay inhibit the

vision of the departments to try something new . Sometimes the same lack may allow risky

procedures vis
-
a
-
vis some popular new trend . Therefore, the emphasis in this project has been

on the use of simple designs that can be easily adopte
d by county crews, designs which can be

easily constructed .

This emphasis on ease of construction and simplicity can best be achieved by

standardization and "preconstruction. " By standardization is meant limiting the number of types
of bridges being d
esigned and planned to a small number. The initial and final intention was only
three types, although one of the final types were different from that initially intended . Using
more than one type would provide a wider range of span lengths, more than the r
anges inherent in
say the use of the precast reinforced concrete channel
-
sections previously described .

By "preconstruction" was meant the notion of that all timber suppliers and glulam manufacturers
would have access to the same set of standardized plan
s . If the program were

1


1

1


to gain popularity then some of the elements could be pre
-
manufactured and be waiting in storage for a
quick response to a county's needs. This feature was later reconsidered since it would not be currently
advantageous eco
nomically for the timber suppliers and/or glulam

manufacturers to "stockpile" sections . And the time necessary for site and foundation preparation at a
particular site would usually be sufficient for the preparation of the elements by the supplier includ
ing
pressure treatment .

The actual design of the bridge sections made use of a computer program written and revised a few
years previous to this project. The program was written originally to design seven different types of timber
bridges . As part of th
is project the program was revised for the second time to bring it up to date with
the most recent LRFD edition of the AASHTO bridge design manual [1994] and the American Forest and Paper
Association's
National Design Specification

for Wood Construction
[
1991] wherever the latter superseded the former.

2.2

Scope of the Project

The immediate focus of this research was the improvement of Arkansas' rural bridges by developing
a series of standard designs for timber bridges that could easily be used to replace county bridges that
were "functionally obsolete" and/or "structurally def
icient ." Therefore, it had primarily to do with rural
highways.

The tasks associated with this project were initially planned to include the following steps

in essentially the order listed, although there was much overlapping of effort.

1)

Literature s
earch for the most recent improvements in the design procedures of modern timber bridges.
This would include background in the most recent changes in AASHTO design procedures for timber bridges
and the AFPA's NDS .

2)

Updating and improvement in the exist
ing computer program to incorporate any necessary changes.

3) Consultation with glulam and solid sawn timber producers and pressure treaters in the

southern half of Arkansas regarding grades and sizes of timber readily available, their
particular pricing

structures including transport, etc .

4)

Consultations with four counties in the southern part of the southeastern half of the state
. A site needing bridge replacement conformable to either a single span or a series of short
spans would be selected in e
ach county for comparitive designs .

5)

Development of standard designs for three types of timber bridges with accompanying plans.

6)

Economic study of the relative costs, both immediate and long
-
term, among the four sites
with respect to possible precon
structed timber bridges, the use of commercially available
concrete channels, and the usual methods of bridge construction in the counties .

7)

Report writing and organizing of meetings of interested parties for discussion of the results
.

The three type
s of timber bridges originally to be included in the project included : solid
sawn stringers under a transverse plank deck, glulam stringers under transverse glulam deck
elements, and longitudinal glulam deck elements. All of these timber bridge superstruc
ture systems
have been proven in their applicability. As the proposal was examined by federal authorities the
author was urged to include the use of a stress
-
laminated system . The author was initially hesitant
to use stress
-
lamination technology because i
t added a significant level of sophistication in
construction procedures for the county bridge crews . However, the use of glulam sections that
stretched from one abutment to the other rather than the use of individual solid
-
sawn segments
of partial length

solved the problem . In the fourth chapter a description of each of these types
will be given both graphically and by text within the context of the


historical development of lamina
tion techniques . With respect to tasks numbered 4)
and 6) it was obvious that the lack of interest shown by most county road and bridge
departments for this project meant that the author chose to eliminate those tasks .
They could well be the subject of a

later larger study that would seek to document and
examine the economic effects of the bridge replacement choices of counties

1

28


throughout the state of Arkansas .


Since there was little interest in timber superstructure bridges among the counties it will probably
be of little use to insist on the organization of "meetings of interested parties" as mentioned in the
task numbered 7)

until such time as that interest demonstrates itself .

1

29


3. LITERATURE REVIEW


Masonry and timber are our most ancient of structural materials . They have been essentially replaced
as our major structural materials by structural steel and reinforced

concrete from the middle of the 19th
century when portland cement concrete was invented and in subsequent years as stronger steels replaced
iron both as rolled structural shapes and as reinforcement for concrete . In the meantime, both older
materials con
tinued to be used primarily for small scale construction, and their design was more an art
than a science . Both masonry and timber are natural materials . The resulting random nature of the materials
delayed the development of appropriate engineering prin
ciples for their structural design. As natural
materials they are much more complex in behavior. Therefore, they were to some extent misunderstood and
only recently has the engineering community developed or is in the process of developing increasingly
ade
quate design procedures for both materials .

In the modern period between the beginnings of the industrial revolution in Europe and the development
of structural steel and reinforced concrete as we know it our society made use of timber and masonry for
ma
ny sophistocated structures . One illustration of this was the use of masonry and timber for major bridges
in this country . This use is well documented in the Ritter chapter [1990] and the Public Roads article
[Duwadi and Ritter, 1997] mentioned earlier .

These documents at one and the same time illustrate technology
that we need to relearn, and point to recent movements in the direction of remembering and reapplying that
technology .

The loss of the previous technology and the use of poor practice in the

design, treatment, and
construction of timber bridges in the early decades of this century led to the use of timber bridges that
gained a reputation for lack of durability . The address by Basile Rabbat [1993] previously cited enjoyed
pointing to the use
of prestressed concrete stringers as the most durable

I
30


design for bridges. His article also listed timber bridges as the least durable. All of this has
contributed to the negative mind
-
set vis
-
a
-
vis the use of timber as a structural material for brid
ge
superstructures descibed in Chapter 1 .

Recent improvements in the technology of "modern" timber bridges has in turn lead to the
development of increasingly sophisticated timber design codes . This includes primarily both the
National Design Specificat
ion for Wood Construction [AFPA, 1991] with its supplemental Design
Values for Wood Construction, and the LRFD Bridge Design Specifications [AASHTO, 1994]. The former
publication is also moving in the direction of an LRFD design format with the publication

of the
Load & Resistance Factor Design Manual for Engineered Wood Construction [AFPA, 19961 .

Any university trained engineer knows that one does not learn primarily from codes.
Unfortunately lack of interest in timber design in the mid
-
half of this cent
ury meant that there
were relatively few textbooks available in timber design. When the author was an undergraduate
at LSU in the mid
-
fifties he had an opportunity to study timber design only because his undergraduate
instructor in structural steel design
elected to teach his class timber design in the structural
steel design lab until such time as the class had learned enough steel design procedures to have
something to design. We used what was then the only text available, the fourth edition [1954] of
Sco
field and O'Brien's Modern Timber Engineering, published by the Southern Pine Association. By
the fifth edition [1963] it was in the hands of Dr. William A. Oliver of the University of Illinois.
As Dr. Oliver retired the revision and development of the tex
t was given to Dr. German Gurfinkel
of the University of Illinois . Under his direction this text reached a second edition [1973] as
Wood Engineering. It was for a number of years subsidized by the then Southern Forest Products
Association, but has since g
one out of print. Chapter 9 of that edition contains much on the "Design
of Wood Bridges" that is still of value to the interested

I
engineer. In the meantime, recent years have seen the development of several new timber