TIMBER BRIDGES IN
A Report by the
Area Councils
made possible by a grant from the
State and
Private Forestry Branch, Northeastern Area
September 2005
Analyzing Cost/Benefits of Modern Timber Bridges
Installed in NH
Since 1990
By Johanna Turley,
Introduction
In early 2001,
Objectives
The initial goal
of the NH RC&D when developing this project was to create a greater demand for
NH wood products and to help communities correct failing transportation
infrastructure. As the project
progressed, more specific goals and objectives developed. Primarily, the project will help attain a
better understanding of the economic, cultural, and resource utilization
advantages and disadvantages of using modern timber bridge structures. A second goal of this project is, if timber
is indicated advantageous by the study, to create a greater awareness and demand
for modern timber bridges.
Methods /Procedure
In early summer,
2001, a NH RC&D project team created a survey to inventory the timber
bridges built in NH since 1990. The
survey asked a variety of questions, including:
Type of bridge
Length/Span/Width
Use
Abutment Type
Timber Species used
Preservative used
Length of construction
Superstructure cost
Total cost
Reasons for constructing
Estimated concrete/steel alternative cost
Local opinions
Benefits/Problems with bridge
Who financed the bridge
Approximately 25 surveys were distributed
in late July/early August, 2001. By the
end of December, eleven towns had returned surveys, two of which provided
information on more than one bridge. Two
of the bridges, one pedestrian and one covered, were omitted from the analysis
because the principal focus of this study was on modern vehicular bridges. In January of 2002, when the final analysis
was done, eleven bridges were used.
Results
Thorough examination of the eleven
surveys proved to be successful in identifying trends in the modern timber
bridges built in
1. Why Choose Timber
One of the most important questions to
be answered in this project is why timber was chosen as a building
material. Most towns chose timber for
reasons including:
aesthetics
lower cost
ease of construction
longer expected life span
local crew available
want to support local wood product
manufacturers
Figure 1. Reason
for constructing a modern timber bridge.
Upon completion of each structure, it
is important to assess the town's level of satisfaction with the bridge. Ninety-six percent of the towns were extremely
satisfied with the results and one official even said "the general public
continue to be very pleased with the bridge and would unhesitatingly recommend
that other townships consider the use of timber for bridge building." Expected life span of these bridges is 50-70
years.
From the survey, the
only problem that occurred with timber was with one of doweled bridges in which
water penetrated the membrane and weakened the bridge. The benefits of timber, according to the
towns surveyed, are aesthetics, cost, upgraded load limits, eliminated
sedimentation problems, improved fish habitat, and improved access for
recreational purposes.
2. Bridge Construction
This project shows
how each bridge was constructed, the overall cost, and how it was
financed. The construction type that six
out of the eleven towns chose was glue laminate, which involves laminating
together, face‑to‑face, individual pieces of solid‑sawn
lumber. Figures 2 & 3 show a stress
laminate and glue laminate bridge, respectively.
Figure 2. Stress laminate constuction Figure 3. Glue laminate construction
Three out of the
eleven used doweled, one town used stress laminated and one town used nail
laminated. Eight out of the eleven bridges
were prefabricated rather than constructed on‑site. The company that was used by five out of the
eleven towns for the prefabricated bridge was Wheeler Consolidated, Inc. based
out of
Other survey
questions that are important in assessing the value of timber bridges are type
of abutment, timber species, and preservative.
Based on the eleven surveys, most towns used new, rather than existing
abutments made out of cast‑in‑place concrete. Other options for abutments, although not as
commonly chosen, are dry laid cut granite, timber crib/tie back, and sawn
timber. Type of timber species used for
the superstructure of the bridge were Douglas fir, southern yellow pine, and
hemlock. Douglas fir and southern yellow
pine were the most commonly used likely due to the fact that eight out of the
eleven bridges were prefabricated outside of
3. Cost Analysis
After thorough
examination of the results of the surveys, it was decided that the best cost
analysis would be done by using the superstructure cost as the figure of
comparison. Cost analysis compares the
length of the bridges to the cost of the superstructure. The results are more conclusive when the
bridges are grouped according to length rather than graphed individually. Figure 4 shows the results of this
comparison.
Figure 4.
Superstructure Cost vs. Length of Bridge.
As expected, the graph shows that the
cost of the superstructure of the bridge is dependent on the length. The only variation in the data pattern
appears between the 50-59 foot and 60-69 foot bridges. The average cost of the superstructure of
bridges with a length of 60-69 feet is slightly lower than those with a length
of 50-59. This variation may be a result
of a small sampling of bridges and one of the bridges in the 60-69 foot range
cost $87,599 while the other in the same range cost $102,662. This extreme difference may have been caused
by the fact that the first bridge was built in 1990, while the latter was built
in 2000 and inflation was not taken into account.
The next comparison was done between
the cost of the superstructure and the construction type. Of the four types of construction, only two
could be used in the comparison because stress laminated and nail laminated
were not used by more than one town.
Therefore, doweled and glue laminated are the two types of construction
used to compare. Figure 5 shows this
data.
Figure
5. Superstructure Cost vs. Construction
Type
This graph shows that
when glue laminated is used, the cost is lower than when doweled is used. This may be one of the reasons why glue laminate
construction was used most commonly.
The last item on the
survey compared the cost of differing timber species. Douglas fir and southern yellow pine were the
two species used most commonly. The data
shows that there is no significant difference between the cost of the
superstructure when built using Douglas fir or southern yellow pine. Therefore, other factors, such as
availability of a local wood substitute, access to treatment facilities,
trucking costs, and prefabrication costs must be taken into consideration when
choosing timber species.
Another important
factor to consider, when deciding what material to use for construction, is
annual maintenance costs. According to
this study, nine out of the eleven towns were unable to identify regular
maintenance costs since the installation, and the other two towns reported
their only costs to be regular bridge inspection costs every two years.
A majority of the
town initiated bridge projects utilized the State of
Note: Based on the
unavailability of estimated concrete/steel alternative costs, this part of the
survey could not be used.
Conclusion
This study indicates
that there are many advantages and few disadvantages to constructing bridges
out of timber. It is apparent from the research
conducted and analyzed that by using timber, cost is reduced, life span of the
bridge is increased, and overall satisfaction from the public is extremely
high.
This research also
proves effective in indicating some of the most important factors to consider
when constructing a timber bridge. By
looking at how previous bridges were constructed, what types of materials were
used for construction, and the length of construction, other towns interested
in using timber have a better understanding of what is effective. Finally, the cost analysis proves successful
in portraying that length and construction type do affect cost, while timber
species used does not.
Resources
1. Donnelly, Chris. User Friendly Guide to Timber Bridges.University
of
2. Rick DeMark, Lindsay Carter, Robert Spoerl, James Moore ‑
3. Dave Powelson ‑ New Hampshire Department of Transportation.
4. Lee Murray‑ DPW Director, Town of New
5. Peter Beard ‑ Road Agent, Town of
6. Louis Grandpre ‑ Selectmen, Town of
7.
Rick Forcier ‑ Highway Supervisor, Town of
8. Keith Noyes ‑ Public Works Director, Town of
9. Gene Chandler ‑ Chairman, Board of Selectmen, Town of
10. Mary Gebhart ‑
11. Ed Bergeron ‑ H.E.B. Engineers,
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