Mattole Road Bridge (No. 4C-55)
Over Mattole River at Honeydew
Bridge Type Selection Study
Prepared for
County of Humboldt
Department of Public Works
Prepared By
MORRISON STRUCTURES, INC.
1890 Park Marina Dr., Ste 104
Redding, California 96001
January 2013
BRIDGE TYPE SELECTION STUDY
SUMMARY
Based on conditions at the bridge replacement site, analysis of viable bridge types, and
our experience with similar projects, there are three viable alternative bridge types for the
Mattole Road Bridge over the Mattole River at Honeydew. The three alternatives are:
1. Two-span steel Camelback through truss
2. Two-span composite welded steel girder
3. Two-span precast, prestressed concrete spliced girder
Based on preliminary cost comparisons for these alternative bridge types, the Alternative
3, two-span precast, prestressed concrete spliced girder bridge is the most economical
The estimated construction cost for this alternative is $4,499,000.
The estimated construction cost for bridge type Alternatives 1, and 2 are $5,737,000, and
$5,609,000 respectively.
INTRODUCTION
The purpose of this study was to determine the most suitable bridge type for crossing the
Mattole River on Mattole Road at Honeydew in Humboldt County. The steps taken to
achieve this goal were as follows:
 Determine the selection criteria based on conditions at the Mattole River site
(geometric constraints, etc.).
 Select suitable bridge types through a fatal flaw analysis.
 Prepare preliminary cost estimates for selected bridge types for economic
comparison.
 Prepare a report outlining the work and summarizing the findings.
PROJECT DESCRIPTION
The Mattole River Bridge (Honeydew Bridge No. 4C-55) on Mattole Road crosses the
river at Honeydew north of the junction with Wilder Ridge Road. The bridge is posted for
15 miles-per-hour and a vertical clearance of 14 feet. Built in 1920, the existing bridge is
a single-lane, two-span, 386-foot-long structure consisting of two 190-foot-long riveted
steel Camelback truss spans.
The existing deck, curbs, and rails are timber plank and the clear roadway width is less
than 15 feet. As-built plans indicate both abutments and the pier are founded on spread
footings. The structure is structurally deficient and functionally obsolete. The bridge is
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eligible for listing in the National Historic Register under Criterion C as an excellent
example of its type, period, and method of construction as a rare and significant bridge
type, the Camelback truss.
Approach Roadways are two lane with minimum 2 foot gravel shoulders on each end of
the existing bridge. Horizontal curved alignments at each end limit speeds to 25 miles per
hour. Estimated average daily traffic on Mattole Road is 250 vehicles.
The north approach is on a horizontal curve and fairly steep climbing grade leading away
from the bridge. Burrell Road intersects Mattole Road from the west, only a few feet
north of the existing bridge and a design exception for shortened length metal beam
guardrail will be required. County right-of-way varies from 50 feet to 45 feet centered on
the roadway.
The south approach is on sharp curved horizontal alignment, bounded on each side by
fairly level ground. Wilder Ridge Road intersects Mattole Road about 100 feet south of
the bridge. A residence and driveway are located to the east of the roadway. A shortened
metal beam guardrail will be necessary along the east edge of the roadway to avoid
existing trees. The Honeydew Post Office and Store parking, and fuel facilities are
located just west of the bridge. Shortened metal beam guardrail adjacent to the Store
parking may be necessary and the roadway cannot be raised significantly without
impacting the use and access to the property. Utility poles are located along the south
edge of the roadway intersection, and likely will not be impacted by the project. County
right-of-way narrows to 40 feet at the south span and approach of the bridge.
County supplied mapping indicates the existing bridge deck is at about Elevation 347.0.
The as-built plans indicate the superstructure depth is about 3 feet 9 inches from the top
of the deck to the bottom of the steel truss chords, placing the soffit at about Elevation
343.25.
Existing condition hydraulic analysis prepared by Pacific Hydrologic indicates that the
water surface elevations for the design floods (50- and 100-year) are 332.0 and 333.0,
respectively. For the purpose of this study we have assumed the minimum bridge soffit
elevation will be 335.0 to provide 3 feet of freeboard above the 50-year flood water
elevation at the crossing. Review of historical river bed elevations at the site indicate that
the river bed has raised nearly 20 feet in the main channel since the existing bridge was
constructed due to deposits of alluvium. It appears also that the recorded elevation of
high water during the 1964 Flood for the bridge site now matches the current estimates of
high water elevation for a 5-year flood.
The proposed bridge will be a multi-span structure on existing alignment, supported on
pier walls and cantilever abutments with an overall length of approximately 375 feet.
The proposed vertical alignment is a declining grade with only a slight overall increase in
profile grade elevation compared to existing in order to minimize impact to the south
approach intersection at Honeydew. Clear roadway width on the bridge will be 26 feet
(two 12-foot travel lanes with 2-foot shoulders) between Type 117 metal tube bridge
railings. Construction funding is to be provided by Humboldt County and the Federal
Highway Administration under the Highway Bridge Replacement and Rehabilitation
(HBRR) program.
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Based on the preliminary findings of the geotechnical exploration at the bridge
replacement site, a layer of fractured, weathered, decomposed rock was encountered at
depth at anticipated pier and abutment locations.
For the purpose of economic
comparison, it was assumed that foundations would consist of driven piles at the north
abutment and cast-in-drilled-hole piles at the south abutment and piers.
Other aspects of the project will include a temporary detour crossing the Mattole River,
removal of the existing bridge, construction of approximately 600 lineal feet of roadway
approach, and installation of rock slope protection. Estimated construction costs for the
detour, existing bridge removal, approach roadway alignments, guard railing, and rock
slope protection were not included in the economic comparison.
Due to the short time frame allowed for construction (June to October 15), work in the
channel will likely be required to be conducted over two seasons. The first season phase
of work would be to construct deep foundations and support walls required for new piers
and south abutment. The second season phase of work would be to construct the detour,
dismantle and remove the old bridge and construct new pier walls, abutments,
superstructure, and approaches.
BRIDGE TYPE SELECTION
SELECTION CRITERIA
The initial step in the bridge type selection process was to determine the criteria
appropriate for the Mattole River Bridge site. The following criteria were used in the
type selection process:
 Feasibility of falsework
 Environmental Issues
 Configuration (span lengths, number of substructure supports, and superstructure
depth)
 Hydraulic efficiency and clearance over high flows
 Durability and Maintenance
 Most economical construction
Falsework and Environmental Issues
For the Mattole River Bridge site, falsework and temporary support bents are considered
feasible for removal of the existing bridge and for construction of the new bridge
superstructure. Falsework that is necessary in the water will likely be provided by driven
piles that will be removed at completion of superstructure construction. For falsework on
dry gravel bars and river bank, steel and timber trestles on timber crib footings can be
utilized.
Environmental constraints will limit the time period when piles can be installed and/or
removed, but once piles are in place, work above the water is typically allowed by permit,
provided the work does not involve additional channel disturbance. Similarly, time
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constraints may be placed on installing and removing cofferdams for foundation
construction, but once the cofferdam is closed off from the live channel, work is typically
allowed to proceed within the enclosure, provided dewatering operations deposit the
water into a temporary sedimentation basin prior to discharge back into the stream. An
area of the overbank should be designated for the anticipated sediment basin and the
contractor’s staging area. If the cofferdam work and substructure can be completed
during the low flow months, the sedimentation basin can typically be located in a nearby
area of the dry gravel flood plain portion of stream channel.
In addition to falsework, gravel pads pushed out from the stream bank and a temporary
pile-supported work bridge is anticipated for driving falsework and temporary bent piles,
constructing the falsework over the main (low flow) stream channel, removing the
existing bridge, and constructing the new bridge. Access to the gravel pads and/or work
bridge will be from the south stream bank and gravel bar floodway area. The work bridge
will be supported by temporary driven piles and will be removed at completion of the
superstructure.
FATAL FLAW ANALYSIS
The following alternative bridge types were considered as potential candidates for the
Mattole River Bridge site:
1.
2.
3.
4.
5.
6.
7.
Two-span steel Camelback through truss
Two-span composite welded steel girder
Two-span precast/prestressed concrete spliced girder
Three-span composite welded steel girder
Two-span steel tied arch
Two-span prestressed, cast-in-place concrete box girder
Two-span conventionally reinforced concrete box girder
Based on a fatal flaw analysis, two of the above alternative bridge types were eliminated
from further consideration. Given environmental constraints Alternatives 6 and 7 were
eliminated due to the excessive time required to construct the bridge. Therefore, only
alternatives 1 through 5 were selected for economic comparison.
ECONOMIC COMPARISON OF ALTERNATIVE BRIDGE TYPES
The economic comparison of the remaining alternative bridge types included determining
the configuration and superstructure depth and hydraulic efficiency. The economic
comparison addressed only the estimated cost of the superstructure and substructure.
For bridge type alternatives 1, 2, 3, and 5, the bridge configuration considered in the
economic comparison was a two-span configuration with equal span lengths, and an
overall total bridge length of 375 feet.
For bridge type alternative 4, the bridge
configuration considered in the economic comparison was a three-span configuration
with an overall total bridge length of 375 feet. The two-span configuration minimizes the
number of obstructions in the channel, increasing passage of flood debris and hydraulic
conveyance, while a three span configuration minimizes the superstructure depth.
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Additionally, each alternative selected for comparison can be constructed in the available
length of construction season.
Unit prices used in the economic comparison were estimated based on review of
construction cost data in District 1 and throughout the State, and our experience with
similar projects. Unit prices used for the different bridge types were consistent
throughout the economic comparison.
Presented below in Table 1 is a summary of preliminary cost comparisons for the bridge
configurations considered.
Table 1
Summary of Substructure and Superstructure Costs
Bridge Type Configuration Superstructure Minimum Hydraulic
Depth
Clearance
Alternative No. 1
10.96 feet at the south
43 inches
Two-span, Steel Camelback
abutment
Through Truss
Alternative No. 2
125 inches at
Two-span,
Composite
5.07 feet at the pier
pier and varies
Welded Steel Girder
Alternative No. 3
Two-span,
Precast- 118 inches and
5.66 feet at the pier
Prestressed Concrete Spliced varies
Girder
Alternative No. 4
92 inches at pier
Three-span,
Composite
7.38 feet at Pier 3
and varies
Welded Steel Girder
Alternative No. 5
10.46 feet at the south
Two-span, Steel Tied Arch
49 inches
abutment
Comparative
Cost
$5,737,000
$5,609,000
$4,499,000
$5,609,000
$7,024,000
As shown in Table 1, the estimated comparative cost for the steel tied arch alternative is
significantly higher than that for the steel Camelback through truss, the plate girders, or
the precast concrete girder. Therefore, no further consideration will be given to the steel
tied arch alternative.
The steel Camelback through truss has a thin structure depth that will provide at least 4
more feet of freeboard than any of the girder type alternatives, while maintaining a twospan configuration for hydraulic conveyance, and is therefore the most desirable
alternative for the passage of flood debris. Also, the steel Camelback truss alternative is
the same truss type and span configuration, and has similar overall length to the existing
bridge, which will minimize the impact to aesthetic setting and historic character at the
site.
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The precast-prestressed concrete spliced girder will provide the most economical option,
but will also affect the aesthetic setting and historic character at the site. The 2-span steel
girder option is less economical than the precast-prestressed concrete spliced girder but
may be constructed in about 4 less weeks. The 3-span steel girder option is similar in cost
but requires a support to be located in the main low flow channel and therefore will not
be considered further.
In summary, Alternatives 1 through 3 are each viable options for Mattole Road Bridge
over the Mattole River at Honeydew..
Cost comparisons were prepared based on our experience and judgment and on current
available data. Morrison Structures, Inc. has no control over future changes, competitive
bidding procedures, market conditions, and other factors affecting cost. The final bridge
configuration and actual construction cost may vary.
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