CA-FLAP-SR89(1)_Fanny_Bridge_SSR.pdf

Federal Highway Administration

Central Federal Lands Highway Division

Structure Selection Report

CA FLAP SR 89(1)

State Route 89/Fanny Bridge

Community Revitalization Project

January 12, 2015

Prepared For:

CENTRAL FEDERAL LANDS HIGHWAY DIVISION

12300 West Dakota Ave.

Lakewood, CO 80228

Prepared By:

CH2MHILL

9193 South Jamaica Street

Englewood, CO 80112-5946

CA FLAP SR 89(1) - FANNY BRIDGE STRUCTURE SELECTION REPORT JANUARY 12, 2015

i

Contents

Section Page

1.0 Introduction ........................................................................................................................ 1-1

2.0 Project Background ............................................................................................................ 2-1

3.0 Proposed Design Criteria ................................................................................................. 3-1

4.0 Proposed Bridge ................................................................................................................. 4-1

4.1 General Location ................................................................................................... 4-1

4.2 Typical Section ....................................................................................................... 4-1

4.3 Geometry ................................................................................................................ 4-1

4.4 Bridge Length ........................................................................................................ 4-2

4.5 Hydraulics .............................................................................................................. 4-2

4.6 Utilities.................................................................................................................... 4-3

4.7 Traffic Control ....................................................................................................... 4-4

4.8 Construction Access and Staging ....................................................................... 4-4

4.9 Right-of-Way ......................................................................................................... 4-4

4.10 Environmental ....................................................................................................... 4-4

5.0 Geotechnical & Substructure Considerations .............................................................. 5-1

5.1 Geology ................................................................................................................... 5-1

5.2 Types of Soil/Rock ................................................................................................ 5-1

5.3 Subsurface Conditions .......................................................................................... 5-1

5.4 Seismic ..................................................................................................................... 5-2

5.4.1 Deterministic Scenario........................................................................................... 5-2

5.4.2 Peak Ground Acceleration (PGA)........................................................................ 5-2

5.5 Preliminary Foundation Recommendations ...................................................... 5-3

5.5.1 Standard Spread Foundations ............................................................................. 5-3

5.5.2 Deep Foundations ................................................................................................. 5-3

6.0 Evaluation Factors .............................................................................................................. 6-1

6.1 Construction Cost .................................................................................................. 6-1

6.2 Visual Resources .................................................................................................... 6-1

6.3 Site Geometry & Clearances ................................................................................. 6-2

6.4 Constructability ...................................................................................................... 6-2

6.5 Service Life and Maintenance .............................................................................. 6-2

7.0 Bridge Alternatives ............................................................................................................ 7-1

7.1 General Discussion ................................................................................................ 7-1

7.2 Alternative 1 – Single Span Precast Concrete Bulb T-Beam Bridge ................ 7-2

7.3 Alternative 2 – Single Span Precast Concrete Adjacent Box Beam Bridge .... 7-3

7.4 Alternative 3 – Two Span Precast Concrete Spread Box Beam Bridge ........... 7-4

8.0 Accelerated Bridge Construction .................................................................................... 8-1

9.0 Bridge Elements ................................................................................................................. 9-1

9.1 Approach Slabs ...................................................................................................... 9-1

9.2 Bridge Railing ......................................................................................................... 9-1

9.3 Deck Drains ............................................................................................................. 9-1

9.4 Expansion Joints ..................................................................................................... 9-1

10.0 Construction Costs ........................................................................................................... 10-1

11.0 Evaluation and Recommendation ................................................................................. 11-1 ii

Appendices

A. Summary of Opinion of Probable Cost B. Bridge TS&L Drawings

Figures

1. Project Map

2. Existing Fanny Bridge looking downstream from Lake Tahoe Dam

3. Existing Bridge Typical Section

4. SR 89 (West Lake Boulevard) looking west over the Fanny Bridge

5. Proposed Bridge Typical Section

6. SR 89/SR 28 Wye Intersection on the west side of existing Fanny Bridge

7. Geologic map of Fanny Bridge area

8. Aesthetic treatment of barrier

9. Precast Concrete Deck Bulb-Tee Girders

10. Precast Concrete Adjacent Box Girders

11. Precast Concrete Spread Box Girders

Tables

1. Hydrologic and Hydraulics Summary

2. Existing Bridge Freeboard Results

3. Scour Results for the Existing Fanny Bridge

4. Fault Data

5. Bridge Construction Cost Estimates

1-1

1.0 Introduction

The proposed State Route 89/Fanny Bridge Community Revitalization Project (CA FLAP SR

89(1)) is being undertaken by the Tahoe Transportation District (TTD) and the Central Federal Lands Highway Division (CFLHD) with funding by the California Federal Lands Access Program (FLAP). The project includes a number of improvements to reduce traffic congestion and improve the safety and operations of the existing State Route 89 (SR 89) and State Route 28 (SR 28) interchange located in Tahoe City, California. The project addresses present and future automobile traffic demand, pedestrian and bicycle mobility, public transit needs, the structural integrity of the existing Fanny Bridge (Br. No. 19-0033), and emergency access to the West Shore of Lake Tahoe within the Fanny Bridge influence area. The project includes the realignment of SR 89, construction of a new roundabout and a new bridge over the Truckee River downstream from the Fanny Bridge and the replacement of the existing Fanny Bridge with a new structure.

Construction of the proposed Fanny Bridge will have minimal impact on SR 89 traffic flow since traffic will be detoured to the new Truckee River Bridge and roadway that will be constructed prior to and approximately 1800 feet downstream from the existing Fanny Bridge. The construction of the proposed Fanny Bridge can occur in a single phase without the need for staged construction. This Structure Selection Report is for the new Fanny Bridge. The new Truckee River Bridge was the topic of a separate report.

Figure 1 – Project Map

Project Site Project Site

2-1

2.0 Project Background

The Tahoe Transportation District and CFLHD are proposing to make capital improvements to alleviate traffic congestion within the vicinity of the existing Fanny Bridge, Bridge No. 19-0033, on SR 89 in Tahoe City and Placer County. Local legend has it that the Fanny Bridge received its name from all of the fannies that can be seen from the road as visitors and locals alike stare over the edge looking at fish and into the cool waters spilling through the dam on Lake Tahoe.

Existing traffic congestion is created by high seasonal traffic, pedestrians, bicycles, vehicular turning and parking movements, and limited roadway geometrics. Additionally, the existing Fanny Bridge has been determined to be structurally and seismically deficient by the California Department of Transportation (Caltrans) Bridge Inspection Program and, as such, the Project will replace the existing bridge. To address traffic congestion, SR 89 will be realigned and a new Truckee River Bridge will be constructed over the Lower Truckee River downstream from the existing bridge. SR 89 at the northern end of the Project area will cross the new bridge structure just east of the Caltrans Maintenance Yard. SR 89 will then continue south on a new alignment across U.S. Forest Service (USFS) property and connect to the existing alignment of the SR 89 north of Granlibakken Road. The existing SR 89, also known as West Lake Boulevard, over the Fanny Bridge will be placed under Placer County jurisdiction and minor improvements will be made to the existing SR 89/SR 28 wye intersection.

Figure 2 - Existing Fanny Bridge looking downstream from Lake Tahoe Dam

2-2

Figure 3

EXISTING BRIDGE

The existing Fanny Bridge (Bridge No. 19-0033) was constructed in 1928 and was rehabilitated in 1988. The structure, which currently carries SR 89 (West Lake Boulevard) over Truckee River, is a four span reinforced concrete T-beam bridge supported on reinforced concrete abutments at each end and three – 2 column reinforced concrete piers in the stream channel. The bridge includes nominal wingwalls to retain the fill at the approaches and the structure is founded on spread footings. The overall length of the bridge is 120’-0” with four spans of 30’-0” each. The structure is 32’-0” curb-to-curb and carries one 17’-0” wide travel lane in the eastbound direction and one 15’-0” wide travel lane in the westbound direction. There is a 5’-0” sidewalk along the north side of the structure with a 1’-1” wide open concrete parapet. There is a similar 1’-8” wide parapet along the south side of the bridge, giving the structure an overall width of 39’-9”. The existing superstructure depth is approximately 3’-3”, which includes 2’-10” for the concrete deck and tee beam, 2” for the crown in the roadway and a 3” asphalt overlay. The bridge is located in Tahoe City at the outlet for Lake Tahoe Dam on the west shore of Lake Tahoe. The existing structure has deteriorated over the years and currently has a bridge sufficiency rating of 52.7 and is classified as “structurally deficient”. The existing structure also does not meet current seismic design standards and is vulnerable to failure in an earthquake.

Figure 4 - SR 89 (West Lake Boulevard) looking west over the Fanny Bridge

3-1

3.0 Proposed Design Criteria

The bridge design will be prepared in accordance with the 6th Edition of the AASHTO LRFD Bridge Design Specifications and interim revisions with modifications as required by the Federal lands Highway (FLH) Project Development and Design Manual (PDDM). Specific design criteria include:

Design Method: Load and Resistance Factor Design

Live Load: HL-93

Permit Load: Caltrans P15

Pedestrian Load: 75 psf

Design Flood: Q50/Flood of Record for Controlled Release with 2.0 ft freeboard

Bridge Railing: TL-2 required (Caltrans Type 80SW Modified)

Overlay: 30 psf

Seismic: Seismic Zone 2

Site Class D

PGA = 0.36g

Short-period Spectral Acceleration (0.2-sec return), SDS = 1.006g

Long-period Spectral Acceleration (1.0-sec return), SD1 = 0.549g

4-1

4.0 Proposed Bridge

4.1 General Location

The proposed bridge will be located at the same location as the existing Fanny Bridge.

4.2 Typical Section

The proposed bridge will symmetrical about the roadway centerline with an 11’-0” travel lane, a 2’-1½”shoulder, a 5’-0” wide sidewalk and a 1’-9” wide barrier, giving an out-to-out bridge width of 39’-9” to match the width of the existing bridge. The western approach to the bridge comes from a wye intersection with four travel lanes, two in each direction, and single lane ramps joining from the left and the right to and from Route 28, all merging into two lanes just before the bridge on the west. The eastern approach to the bridge consists of three lanes, one travel lane in each direction and a continuous center turn lane that transitions into two lanes just before the bridge on the east side.

4.3 Geometry

The western approach to the bridge comes from the wye intersection of West Lake Boulevard (SR 89) with West River Road (SR 28). The main portion of the roadway is on a tangent alignment across the bridge. The eastern approach follows a relatively large radius curve that becomes tangent just before the bridge. The vertical grade along the approaches and across the bridge is relatively flat and will match the existing grade.

Figure 6 – SR 89 / SR 28 Wye Intersection on the west side of the existing Fanny Bridge

4-2

4.4 Bridge Length

The proposed bridge length of 132’ will be slightly longer than the existing bridge so that the new abutments can be constructed behind the existing abutments. The existing bridge will be removed except for the abutment walls and wingwalls, which will remain in place. Portions of the existing abutment wall will be modified to accommodate a deeper structure depth. The upper portions of the existing wingwalls will be modified to accommodate a new barrier that will be similar to the one used on the new bridge.

4.5 Hydraulics

The Fanny Bridge is located immediately downstream of the spillway for the Lake Tahoe Dam.

In order to satisfy the hydraulic requirements of the stream channel, the structure depth needs to be kept to a minimum to avoid raising the vertical profile of the roadway at the river crossing. The flow in the stream channel was modeled based on the maximum flow from the historic data for the controlled releases that have occurred over the years. There is a USGS stream gage located at the bridge and historic data is available for the past hundred years of operation. In that time frame, the dam has never been overtopped and the bridge has never been underwater. Based on the available data, the maximum flow from the historic controlled releases is 2690 cfs with a water surface elevation of 6225.97 feet on the upstream side of the bridge. This is considered to be the Q50/Flood of Record and the goal is to provide a new bridge that will provide a minimum of a 2-foot freeboard at this flow rate which meets CFLHD and Caltrans criteria. Neither agency requires freeboard for the Q100 flow rate. However, this bridge is immediately downstream of a dam so additional criteria needs to be considered. With the presence of the dam, the Bureau of Reclamation (BOR) requested the new bridge be designed to pass to the flow rate of 3530 cfs and suggested additional freeboard be available for this flow rate due to debris potential. The BOR also requested that no damage occur to an existing concrete weir on the upstream side of the existing footings.

A summary of the preliminary hydrology and the hydraulic results for the existing Fanny Bridge have been included in Table 1 (below).

Table 1 Hydrologic and Hydraulic Summary

Drainage Area: 505 Square Miles

Design Flood/

Flood of Record Base Flood

Maximum Flood

Frequency (years) 50-yr 100-yr 500-yr

Discharge (cfs) 2,690 3,350 4,350

Water Surface Elevation at Existing Fanny Bridge (feet, NGVD 29)

6225.97 6226.80 6228.03

Velocity at Existing Fanny Bridge (ft/s)

3.14 3.52 4.02

NOTE: Flood plain data are based upon information available when the plans were prepared and are shown to meet federal requirements. The accuracy of said information is not warranted by the Tahoe Transportation District, Central Federal Lands, State, and County and interested or affected parties should make their own investigation.

4-3

The resulting freeboards have been summarized for the proposed Fanny Bridge in Table 2 (below).

Table 2 Proposed Bridge Freeboard Results

Flow

WSE

(feet, NGVD 29)

Minimum Soffit Elevation

(feet, NGVD 29) Freeboard (feet)

Q50 6225.97 6227.22 1.25

BOR 6,227.03 6227.22 0.19

Proposed bridge solutions in this report will result in no additional increase in the water surface elevation since the number of piers will be less than currently present. The proposed bridge does not meet the minimum freeboard requirements mentioned above, but does provide at least 1.25’ for freeboard for the 50-yr storm/Flood of Record.

The preliminary results for the scour analysis for the proposed Fanny Bridge are summarized in Table 3 (below). The results show that contraction scour is minimal. The scour depth due to contraction is 0.07 feet. The scour depth due to piers is 0.07 feet. The scour depths due to the left and right abutments are 2.44 feet and 8.55 feet, respectively.

Table 3

100-year Scour Results for the Proposed Fanny Bridge

Combined Scour Depths Pier Scour + Contraction Scour Ys (ft):

-- 0.07 --

Pier Scour + Contraction Scour Minimum Elevation (ft):

-- 6215.76 --

Abutment + Contraction Scour Ys (ft):

2.44 0.07 8.55

Abutment + Contraction Scour Minimum Elevation (ft):

6223.28 6215.76 6215.61

4.6 Utilities

The utilities on the existing bridge will be relocated onto the new Truckee River Bridge that will be constructed downstream from here. There will be no utilities supported on the proposed Fanny Bridge.

4-4

4.7 Traffic Control

Construction of the proposed bridge will have minimal impact on SR 89 traffic flow. Traffic will be detoured to the new Truckee River Bridge that will be constructed over the Lower Truckee River approximately 1800 feet downstream of the existing Fanny Bridge. The new Truckee River Bridge and connecting roadway will be constructed before the new Fanny Bridge.

Pedestrian and bicycle traffic will be temporarily diverted to cross at the walkway on the Lake Tahoe Dam immediately upstream of the Fanny Bridge. The construction of the Fanny Bridge can occur without the need for staged construction.

4.8 Construction Access and Staging

The contractor will be able to access the proposed Fanny Bridge along West Lake Boulevard at either approach to the new bridge. Staging can occur adjacent to the proposed bridge.

Boundaries for such staging will be cleared in the Environmental Assessment (EA) and defined on the plans.

4.9 Right-of-Way

The proposed structure will be constructed on existing California Department of Transportation (Caltrans) Right-of-Way and ownership of West Lake Boulevard and the new Fanny Bridge will be transferred to Placer County upon completion of construction. While temporary construction easements may be needed, no additional right-of-way will be required for the construction of the new Fanny Bridge.

4.10 Environmental

The demolition of the piers within the river channel, if a two span option is selected, the construction of a new pier in the middle of the stream channel, will be subjected to regulatory oversight and associated permits, including but not limited to the following: Tahoe Regional Planning Agency (TRPA), Lahontan Regional Water Quality Board, United States Army Corp of Engineers (USACE), and the California Department of Fish and Wildlife (CDFW). The required permits will provide working windows for work within the water, dewatering requirements and clear water diversion requirements (if needed).

Considering environmental constraints, the schedule for bridge construction within the river channel has a window of May 1 to October 15.

5-1

5.0 Geotechnical & Substructure Considerations

5.1 Geology

Lake Tahoe is located on the California-Nevada state line at the eastern edge of the northern Sierra Nevada Geomorphic Province. The Sierras are a tilted fault block approximating 400 miles in length. The eastern slope forms a high rugged scarp, while the western slope is much gentler, dipping mildly under the sediments of the Great Valley. The Tahoe Basin is underlain at depth by the Sierra Nevada Batholith consisting of Mesozoic “granitic type” rocks, predominantly granodiorite (Hyne, 1972). Tertiary volcanic rocks, including basalts, andesites, latites, trachytes, and various tuffs and mudflows commonly overlay the granitics and are often exposed at the surface (Grose, 1985). In addition to the igneous rock units, deposits of alluvium, colluvium, and glacial sediments are locally abundant.

Lake Tahoe is dammed near the Fanny Bridge site by andesitic mudflow breccias. During the Pleistocene the lake level rose and dropped dramatically due to recurrent damming by alpine glaciers (Hyne, 1972). This rise in lake level, as much as 450 feet, subsequently resulted in a rise in the ground water table, which led to extensive physical weathering of the granitic rocks present along the perimeter of the lake and to a lesser degree extending toward the Fanny Bridge site.

5.2 Types of Soil/Rock

The Lake Tahoe Geologic map, prepared by the California Geological Survey, Figure 7, indicates the bridge site as being founded in undifferentiated Quaternary aged (less than 10,000 to 12,000 year old) lake deposits. Previous works indicate soils typically consist of clayey gravel with sand fills overlying layers of sand, clay, and silt. Sands and gravels were typically present at depths on the order of 50 feet.

5.3 Subsurface Conditions

A subsurface investigation has yet to be completed for the Fanny Bridge. A boring program will be completed in the summer and the Geotechnical Report will follow shortly thereafter. It is anticipated that a minimum of two borings will be drilled at each abutment and two boring in the stream channel at the proposed center pier location. For the structure selection report, we have included some general information from the Geotechnical Report of the proposed Truckee River Bridge that will be constructed approximately 1800 feet downstream of the Fanny Bridge.

FIGURE 7 – Geologic Map of the Fanny Bridge Area (http://www.quake.ca.gov/gmaps/RGM/tahoe/tahoe.html)

5-2

Any recommendations included in this report are subject to change once the site specific geotechnical information becomes available.

5.4 Seismic

Seismic hazards present some unique considerations primarily as they relate to the response of Lake Tahoe to Quaternary Faults trending north-south along the lake bottom. In addition to the potential for strong ground motion, research has indicated that there is also a potential for the development of a seiche. The development of a seiche could cause localized flooding that would manifest itself in ways significantly different from the flood stages modeled for the Truckee River. Because of the type of soils present at the site, preliminary estimates of liquefaction place the potential as low.

5.4.1 Deterministic Scenario

The deterministic spectrum is determined as the average of median response spectra calculated using ground motion prediction equations developed under the “Next Generation Attenuation” (NGA) project. These equations are applied to all faults considered active in the last 750,000 years (late-Quaternary age) that are capable of producing a moment magnitude earthquake of

6.0 or greater. Based on Caltrans ARS Online and Caltrans Fault Database, the controlling deterministic seismic source is the West Tahoe Fault, assigned the values in Table 4.

TABLE 4 – FAULT DATA

Fault Parameters West Tahoe Fault

Fault Identification Number (FID) 77

Maximum Moment Magnitude (Mmax) 7.0

Style of Faulting Normal

Fault Dip (degrees) 50

Dip Direction East

Deterministic Spectral Acceleration 0.352g

5.4.2 Peak Ground Acceleration (PGA)

The PGA determined from probabilistic spectrum data provided by Caltrans ARS application results in a site-specific acceleration of 0.44g. Based on 2009 AASHTO Guide Specifications for LRFD Seismic Bridge Design (USGS), a mapped PGA of 0.36g is determined.

5-3

5.5 Preliminary Foundation Recommendations

Based on the currently available information, the preliminary foundation recommendation is to construct abutments supported by large size CISS and CIDH piles at the abutments and, if needed, to construct a center pier supported by the same foundation type. It is anticipated that new abutment pile caps will be constructed immediately behind the existing abutments. The bridge span length was increased to allow sufficient room to install piles without hitting the footing at the base of the existing abutment.

5.5.1 Standard Spread Foundations

A center pier could be founded on spread footing at an elevation below the anticipated scour depth. The center pier would need to be constructed in a manner that would not damage the existing concrete weir in the channel. Spread footings at the abutments are not being considered due to the amount of excavation required and the surcharge they would apply to the existing abutment and wing walls. Allowable bearing capacity will be a function of the supporting soils and the planned depth of embedment for scour protection. Typically, deepening the footing can significantly offset the reduced loads driven by lesser quality soil conditions. All bridge foundations shall extend a minimum of 3 feet below lowest adjacent grade for frost protection. Preliminary bearing capacity values of 5,000 pounds per square foot can be used for design. This value may be increased by a factor of 1.33 when considering wind and seismic forces. Associated settlement would likely be on the order of 1 inch, differential settlement between adjacent structure components would likely approach ½ the total amount.

Although spread footings are possible, for a two span option, the structural engineers do not recommend that piers be constructed on spread footings to minimize the possibility of differential settlement.

5.5.2 Deep Foundations

It is recommended that the new abutments will consist of concrete abutments supported by piles placed immediately behind the existing abutments. Deep foundations will derive their support from both skin friction/adhesion and end bearing. For the purposes for this report, we have assumed that the abutments will be supported on 4’-0” CIDH piles. The 4’-0” CIDH piles were assumed to have a service capacity consisting of 130 psf skin friction and 40 psf end bearing. The pile length used to determine the quantities for the cost estimate was 60 feet.

It is also recommended that the center pier for the two span alternative be founded on 4’-0” CIDH piles. This will eliminate any issues with both scour in the stream channel and differential settlement between the pier and the abutments.

It is worth noting that these recommendations are based on preliminary geotechnical information and that the recommendations included in this report are subject to change once the site specific geotechnical information becomes available. Final corrosion results of the soil and water in the final geotechnical report may also affect the final deep foundation type.

6-1

6.0 Evaluation Factors

The bridge type selection process is an evaluation of the functional, project, and site specific parameters with respect to the economic and practical constraints for various bridge types and span arrangements. Generally, the bridge types and span configurations that are thought to be economical, practical, serviceable, and aesthetically pleasing for a specific site are evaluated with items such as constructability, construction sequencing, traffic control, roadway geometrics, structural capacity/performance, and bridge site constraints taken into consideration. Economics and constructability typically govern but on occasion other parameters have a significant influence. Several of these items are evaluated on a quantitative basis while others require engineering judgment. The final recommended bridge type and span configuration is the one that best meets all of the applicable constraints. In general, construction cost, visual resources, site geometry and clearances, constructability, and service life and maintenance were the controlling factors for the proposed Fanny Bridge. These factors are discussed in detail in the following sections.

6.1 Construction Cost

Construction cost is generally the most dominant evaluation factor but does not always determine the recommended structure type. The cost is influenced by factors such as project location, material supply, shipping and constructability. The project is located near Lake Tahoe, so traditional materials such as concrete and reinforcing steel will be economical compared to more remote projects. Structural steel and precast girders would likely be supplied from elsewhere in California or nearby Utah that may be more costly than if supplied locally.

Influencing factors such as these will be considered in establishing unit prices. A summary of estimated construction costs for the evaluated alternatives is contained in Section 10, while detailed cost estimates are contained in Appendix A.

6.2 Visual Resources

The Fanny Bridge is located in Tahoe City adjacent to Lake Tahoe Dam and the Truckee River Outlet California State Parks Gatekeeper Museum. One of the considerations for the visual impact is to make the appearance of the new structure match the surrounding environment. The most visible feature to the public is the bridge railing. The existing bridge railing is a concrete railing with slender pointed arch openings. The proposed structure will use a modified Caltrans Type 80 SW bridge railing. Consideration will be given to the use of tinted concrete and formliners or stone facing to improve the appearance of the parapets while providing the required minimum test load category for crash protection. Barrier type may change prior to final design. Figure 8 – Aesthetic treatment of barrier

6-2

6.3 Site Geometry & Clearances

In general, site geometry and clearances can be classified as both quantitative and qualitative.

The minimum vertical clearance is generally quantitative and is based on site requirements. For the Fanny Bridge, the minimum vertical clearance is determined by the hydraulic requirements of the stream channel. The deck elevation for the proposed structure will be placed as close to the same elevation as possible to the existing roadway. The allowable superstructure depths are established by providing the required freeboard above the design flow while matching the vertical elevation of the existing roadway.

The horizontal alignment for the proposed structure will match the alignment of the existing roadway, which is tangent across the Truckee River at this location.

6.4 Constructability

Constructability (ease of construction) is always a key consideration in evaluating a bridge type and layout. Generally, this is tied to construction cost, as a premium is paid for bridges that are more difficult to construct. This includes items such as site access, staging and traffic control as well as specific elements such as foundations.

As discussed in Section 4, site access, staging, phased construction and traffic control will not be an issue at this location. Pedestrian and vehicular traffic will be detoured during construction and there is sufficient space for the contractor to stage equipment and materials along the approaches to the bridge.

The bridge alternatives being considered are conventional in nature and do not require special skills or skills not offered by the local market. As discussed in Section 5.5.2, from a constructability standpoint, large size CISS and CIDH piles are options at this location.

6.5 Service Life and Maintenance

Generally, following the AASHTO LRFD Bridge Design Standards will set the structure up to achieve the minimum desired service life. The FLH Project Development and Design Manual (PDDM) have additional provisions, such as the use of epoxy reinforcing in the deck, which will enhance the chances of achieving the expected service life. At this project location, a long service life is expected for structures constructed out of conventional materials such as cast-in-place concrete and prestressed concrete. Material and design detail selection for the proposed bridges will be such that the design service life can be readily achieved.

Proper maintenance, particularly on critical items that are known to have a reduced service life (i.e. expansion joints), needs to be performed in order to preserve the overall bridge service life.

The selection of the bridge layout and associated features will give consideration to frequency and ease of maintenance.

7-1

7.0 Bridge Alternatives

7.1 General Discussion

The proposed bridge will, more or less, match the horizontal and vertical alignment of the existing roadway. The horizontal and vertical alignments will be adjusted slightly to match the centerline of the new bridge. The overall length of the proposed bridge is 132 feet. For single span alternatives, the span length is 128’-0” center to center bearing. For the two span alternative, the span lengths are 64’-0” center to center bearing. The new bridge will be slightly longer than the existing bridge so that new integral abutments can be constructed behind the existing abutments. The existing abutments and wingwalls will remain in place and be modified as needed to accommodate the new superstructure.

The other main constraint on the design of a replacement bridge at this location is related to the hydraulics of the stream channel. The goal is provide a new structure will need to provide a minimum of 2 feet of freeboard above the maximum water surface elevation for the Q50/Flood of Record and must also pass the Maximum Flood (Qmax). The existing superstructure depth is approximately 3’-3”, which includes 2’-10” for the concrete deck and tee beam, 2” for the crown in the roadway and a minimum 3” asphalt overlay.

Based on the available hydrologic and hydraulic information provided by others, the low chord elevation for the existing bridge is Elevation 6229.58 feet. This yields 3.59 feet of freeboard above the Q50/Flood of Record (2690 cfs) at water surface elevation 6225.97 feet and 2.55 feet of freeboard above the Qmax Flood (3530 cfs) at water surface elevation 6227.03 feet.

7-2

Figure 9 – Precast Concrete Deck Bulb-Tee Girders

7.2 Alternative 1 – Single Span Precast Concrete Deck Bulb-Tee Girder Bridge

This alternative consists of 7 - 50” UDOT deck bulb-tee girders with an asphalt wearing surface over top of a waterproofing membrane single span bridge supported on seat type abutments.

The existing abutment and retaining walls will be left in place and modified as needed to accommodate the new superstructure. The new pile supported abutments will be constructed behind the existing abutments. This alternative includes sidewalks and the use of the Caltrans Type 80 SW modified bridge railing system. The upper portion of the existing abutment walls will be removed to accommodate the deeper superstructure. Also, the upper portion of the existing wingwalls will be removed and new parapets will be constructed to complement the existing railings adjacent to the bridge.

The deck bulb-tee girders have the advantage of eliminating the need for a concrete deck. The top flange of the bulb-tee girder serve as the deck and a waterproofing membrane and asphalt provide the wearing surface and deck protection. In this case, the asphalt thickness will vary from 2” minimum to 5” maximum to provide the required cross slope on the bridge deck. A polyester concrete overlay would not be recommended for this option due to the potential for reflective cracking in the overlay. Intermediate diaphragms at the quarter points help to ensure that the all of the beams deflect together as a unit. The concrete sidewalks will be physically connected to the top flange of the exterior beams providing additional stiffness to the superstructure. The maximum superstructure depth for this alternative is approximately 4.58 feet (4’-7”) from PGL to low chord elevation which gives a freeboard dimension of 1.25 feet for the Q50 Flood of Record and 0.19 feet for the Qmax Flood.

The estimated construction cost for Alternative 1 is $1,502,000. See Appendix A for further details.

7-3

Figure 10 – Precast Concrete Adjacent Box Girders

7.3 Alternative 2 – Single Span Precast Concrete Adjacent Box Girder Bridge

This alternative consists of 6 - 44”x 70” precast concrete adjacent box girders with a minimum 6” concrete deck single span bridge supported on seat type abutments. The existing abutment and retaining walls will be left in place and modified as needed to accommodate the new superstructure. The new pile supported abutments will be constructed behind the existing abutments. This alternative also includes sidewalks and the use of the Caltrans Type 80 SW modified bridge railing system. The upper portion of the existing abutment walls will be removed to accommodate the deeper superstructure. Also, the upper portion of the existing wingwalls will be removed and new parapets will be constructed to complement the existing railings adjacent to the bridge.

The precast concrete adjacent box girders have the advantage of providing a slightly shallower superstructure than the deck bulb-tee girders. For this span length, the precast concrete adjacent box beam superstructure requires a minimum 6” composite concrete deck to meet the maximum live load deflection requirements. The thickness of the concrete deck will vary from 6” minimum to 9” maximum to provide the required cross slope on the bridge deck. A 1-inch polyester concrete overlay would be provided for deck protection and to act as a wearing surface. Shear keys between the box beams and the installation of transverse post-tensioning bars or strands will ensure that the box beams deflection together as a unit. The maximum superstructure depth for this alternative is approximately 4.42 feet (4’-5”) from PGL to low chord elevation which gives a freeboard dimension of 1.42 feet for the Q50/Flood of Record and

0.36 feet for the Qmax Flood.

A disadvantage for this alternative is the extra time needed to pour and cure the concrete deck.

The estimated construction cost for Alternative 2 is $1,713,000. See Appendix A for further

7-4

Figure 11 – Precast Concrete Spread Box Girders

7.4 Alternative 3 – Two Span Precast Concrete Spread Box

Girder Bridge

This alternative consists of two equal spans of 4 - 30”x 66” precast concrete spread box girders made continuous for live load with a minimum 8” composite concrete deck two span bridge supported on seat type abutments and central reinforced concrete pier. The existing abutments and retaining walls will be left in place and modified as needed to accommodate the new superstructure. The new pile supported abutments will be constructed behind the existing abutments. The center pier will also be supported on piles. This alternative includes sidewalks and the use of the Caltrans Type 80 SW modified bridge railing system. The upper portion of the existing abutment walls will be removed to accommodate the deeper superstructure. Also, the upper portion of the existing wingwalls will be removed and new parapets will be constructed to complement the existing railings adjacent to the bridge.

The precast concrete spread box girders have the advantage of providing the shallowest superstructure of the three alternatives. A 1-inch polyester concrete overlay would be provided for deck protection and to act as a wearing surface. Intermediate diaphragms will ensure that the spread box girders deflect together and act as a unit. The maximum superstructure depth for this alternative is approximately 3.58 feet (3’-7”) from PGL to low chord elevation which gives a freeboard dimension of 2.34 feet for the Q50 Flood of Record and 1.28 feet for the Qmax Flood.

The main disadvantage of the two span alternative is that it requires substantial work in the stream channel and the associated requirements for erosion & sediment pollution control and protection of the stream environment. In addition, the deck for this alternative requires the installation of formwork for the construction of the concrete deck and requires additional time for curing of the concrete deck.

The estimated construction cost for Alternative 3 is $1,808,000. See Appendix A for further

8-1

8.0 Accelerated Bridge Construction

Accelerated Bridge Construction (ABC) is gaining popularity due to the FHWA’s Every Day Counts Initiative. ABC reduces construction time, which benefits the travelling public. To fully understand the benefits of ABC, it is necessary to evaluate commuter travel delay and impacts versus the cost of utilizing ABC techniques.

The reduced construction time and associated benefits are generally achieved by utilizing prefabricated bridge elements or by introducing expedited erection/placement techniques such as lateral bridge slides or SPMT bridge moves. In addition, GRS abutments can be constructed in applicable situations to expedite abutment construction. GRS abutments were not considered for this bridge site due to the large amount of excavation and temporary shoring that would be required.

For the construction of the proposed Fanny Bridge, vehicular traffic will be detoured to the new Truckee River Bridge and roadway that will be constructed downstream. The construction of the Truckee River Bridge and associated roadway improvements will be completed before work begins on the Fanny Bridge. While there will be some inconvenience for local traffic because of the detour, the majority of traffic will see reduced travel times as a result of the new Truckee River Bridge. In addition, pedestrian and bicycle traffic will be detoured to the walkway over the crest of the Lake Tahoe Dam immediately upstream from the Fanny Bridge.

The main features for ABC related to the construction of the Fanny Bridge include retaining the existing abutments and wingwalls and constructing new pile supported abutments behind them and the use of precast concrete girders.

By retaining the existing abutments and wingwalls, the amount of work and construction time associated with constructing the abutments is greatly reduced. In addition, it eliminates the need to demolish and remove the abutments and the associated environmental impacts and additional mitigation measures that would be needed in the stream channel. Instead of constructing new full height abutments and wingwalls, the new bridge will have relatively shallow abutments supported on piles. The shallow pile supported abutments require much less excavation and materials to construct when compared to full height abutments. The savings are doubled because similar construction is proposed for both abutments.

The use of precast concrete girders reduces the amount of time needed to construct the bridge superstructure. In terms of ABC elements, the precast concrete deck bulb-tee girders provide the biggest advantage in that they do not require the construction of a reinforced concrete deck.

The top flange of the deck bulb-tee girders are designed to serve as the deck with the use of a waterproofing membrane under an asphalt wearing surface. The precast concrete adjacent box girders provide the second biggest advantage because they eliminate the need for forming the reinforced concrete bridge deck. The precast concrete spread girders beams rank last of the three alternatives. While the spread box girders do require the placement of deck forms and the construction of a reinforced concrete deck, they offer the advantage of improved constructability with the simple placement of precast concrete girders. Precast deck panels could be considered in lieu of using deck forms for the interior bays but overhang forming would still be required.

9-1

9.0 Bridge Elements

9.1 Approach Slabs

Approach slabs will be used at each abutment to transition and accommodate differential settlement between the abutments and approach fill. This will help mitigate against the traditional bump that can form at the beginning/end of the bridge when approach slabs are not used. The approach slabs are planned to be Caltrans standard type EQ(10) and will have a length of 10 feet.

9.2 Bridge Railing

As previously stated in Section 6.2, the most visible feature to the public is the bridge railing.

The proposed structure will use a Caltrans Type 80 SW modified bridge railing. The modifications include the use of stained or painted concrete with formliners or stone facing to improve the appearance of the parapets and the addition of steel hand railings along the top and a steel grillage in-fill between the curb and the bottom of the concrete railing. The Caltrans Type 80 SW modified bridge railing is designed to satisfy TL-2 crash test load requirements.

Additional public outreach is expected to finalize the bridge railing appearance. Based on public input, the bridge railing type may be changed during final design.

9.3 Deck Drains

There will be no deck drains on the bridge structure. There are currently inlets at each end of the bridge. The existing roadway inlets will be replaced with new inlets and moved as needed to accommodate the longer bridge structure.

9.4 Expansion Joints

The new bridge will utilize compression seals that require little maintenance and are expected to last up to 30 years. Snow plow deflectors will be installed at the expansion joint locations.

10-1

10.0 Construction Costs

Unit costs were established taking into consideration recent/historic CFLHD cost data with Caltrans cost data used as a cross reference. Determination of unit costs also considered project location, bridge type, construction operations (i.e. constructability, access and staging, traffic control, etc.), magnitude of quantities, material supply, and shipping among others. The cost of the precast concrete deck bulb-tee girders is based on direct communication with a fabricator in Utah.

The total costs presented in the table below include a 20% contingency. General project costs such as mobilization are not included. See Table 5 below for a summary of costs for the various alternatives considered. Detailed cost estimates for all of the alternatives are contained in Appendix A.

Table 5 – Bridge Construction Cost Estimates

Location Fanny Bridge

Alternative 1 $1,502,000 Alternative 2 $1,713,000 Alternative 3 $1,808,000

Least Cost $1,502,000

The least expensive bridge alternative to construct is the precast concrete deck bulb-tee girder bridge. With an estimated construction cost of $1,502,000, this bridge alternative is 14% less than the adjacent box girder bridge and 20% less than the spread box girder bridge.

11-1

11.0 Evaluation and Recommendation

All three alternatives for a new bridge at this location provide a reliable, long-term replacement structure for the existing Fanny Bridge. The existing bridge has deteriorated over the years and currently has a bridge sufficiency rating of 52.7 and is classified as “structurally deficient”. The existing structure also does not meet current seismic design standards and is vulnerable to failure in an earthquake. Because of these shortcomings, an alternative for the rehabilitation of the existing bridge was not included in this study.

All of the alternatives make use of the Caltrans Type 80 SW Modified bridge barrier and all allow the existing abutments and wingwalls to remain in place, thereby reducing both short-term and long-term stream impacts. And all three alternatives require the removal of the upper portion of the existing wingwalls and the construction of new barriers at the four corners of the bridge.

Only the two span option (Alternative 3) requires additional work in the stream channel for the construction of a new center pier. In addition to having the highest construction cost, the biggest disadvantage of the two span option (Alternative 3) is the additional work required in the stream channel for the construction of a center pier. This alternative also requires the construction of a concrete deck. This requires longer construction durations to place the deck forms in addition to placing and allowing the concrete deck to cure.

With Alternative 2, the single span precast concrete adjacent box girder bridge, it also requires a concrete deck but does have the benefit of no formwork between the girders as required in Alternative 3. However, this still requires longer constructions duration to place and cure the concrete deck.

The biggest advantage of Alternative 1 is that it does not require a concrete deck. The top flange of the deck bulb-tee beams provide the bridge deck, so all that is needed is a waterproofing membrane and an asphalt wearing surface, which is relatively easy and quick to install and maintain. Because of this advantage, the estimated construction cost for the deck bulb-tee girder alternative is $211,000 less than Alternative 2 and $306,000 less than Alternative 3.

The biggest disadvantage to Alternatives 1 and 2 are that they do not provide the 2.00 freeboard for the 50-yr storm, however, they do provide a 1.25’ and 1.42’ freeboard, respectively, and both pass the Qmax storm.

Therefore, based on the relative advantages and disadvantages of the three alternatives, we recommend Alternative 1, the single span precast concrete deck bulb-tee girder bridge, as the replacement structure for the existing Fanny Bridge.

Appendix A

CH2M HILL Opinion of Probable Construction Cost Project: CFL Lake Tahoe Design: S. Mittal Revisions: R. Luebbers Subject: Fanny River Bridge with Seat Type Abutment Date: 6/27/2014 Date: 7/1/2014

Structure: Alternative 1 - Single span Deck Bulb Tee UDBT50 Girder

Cost Estimate Bid Item Number Item Description Unit Quantities Unit Price Cost

15214-1000 Survey and Staking, Bridge LS 1 $8,000.00 8,000$

20301-0400…