SCCRTC- MP San Lorenzo River Bridge Walkway Widening Feasibility Report

Transcription

1 SCCRTC- MP San Lorenzo River Bridge Walkway Widening Feasibility Report May 19, 2016 INTRODUCTION Jacobs Engineering Group was selected to provide a feasibility report for the Santa Cruz County Regional Transportation Commission (SCCRTC) for a 9-10 walkway, attached to the northern side of bridge MP over the San Lorenzo River. PURPOSE The Monterey Bay Sanctuary Scenic Trail Network is a two-county bicycle and pedestrian pathway project. In Santa Cruz County the Trail Master Plan has the Coastal Rail Trail running parallel to the Santa Cruz Branch Line within the rail right of way. This walkway alignment runs adjacent to the existing San Lorenzo River Bridge. The bridge structures already support a multi-use trail about 4 feet wide, for which this feasibility would examine a 9-10 replacement. PROCESS Jacobs gathered as much information as was available for the study. Existing site conditions, bridge plans, rating reports, inspection reports and photos of the current walkway and utilities were used to determine the options for a wider multi-use trail. Perform a project review to identify options and constrains for the proposed widening and constructability. Perform preliminary engineering calculations for the necessary structural elements based on a proposed multi-use trail width of 9-10 to match the existing sidewalk on the west end of the bridge. Develop a schematic for the proposed multi-use trail. Develop an order of magnitude cost. EXISTING CONDITIONS The exiting San Lorenzo Bridge is a 3 span bridge made up of 2 structure types. Span 1 and 2 are 120 steel through-truss spans. Span 3 is a deck girder span. The superstructure was rated and found adequate for the walkway widening under a separate study by others. The walkway connections to the bridge are detailed different for each bridge type. In Spans 1 and 2 the walkway rests on overhang steel brackets that are riveted to the vertical members of the truss. Timber stringers support the walkway deck and span between the brackets. In Span 3 the walkway rests on overhanging timber beams that rest on one deck girder. The walkway further extends west (Railroad North) past Abutment 4 on newly constructed timber stringers that rest on a concrete abutment on the west end of the span and rest on the deck girder on the east end of the Span. See Pictures 3-5

2 The walkway is supported by steel tube frames at Abutment 1 and at the piers. PROPOSED PLAN AND PRELIMINARY DESIGN Design of the proposed multi-use trail would include similar structural elements to those that are already in place, i.e. timber planking, steel posts, chain link fencing. Some of the updated elements would include the steel stringers, the waterline supports and a possible aluminum grating for the multiuse trail instead of the timber planking which will be examined closely during future phases. Conceptual schematics of the proposed multi-use trail at various spans can be seen in the last two sheets of the report. The schematics are superimposed on the plans of the existing structure. Spans 1 & 2: The proposed design for Spans 1 and 2 is to replace the existing brackets with a trapezoidal beam supporting both the existing 10 waterline and the new multi-use trail. The concept is that the beam would anchor to the truss using high strength bolts. The waterline would continue to run between the multi-use trail and the truss using an updated support system and will be supported in place during construction. See Sheet Schematic 2- Exhibit A. The walkway support frames at Abutment 1 and at the piers would be replaced to accommodate the wider multi-use trail by adding diagonal posts anchored into the concrete. Span 3: The proposed design for Span 3 is to replace the existing timber beams, spaced at 2 feet, with steel beams capable of supporting the 9-10 multi use trail. Steel beams would anchor to the track deck girder on the inside and to the existing deck girder under the multi-use trail. Diagonal support brackets may be needed to counter load vibrations of the overhanging beams. The waterline would continue to run between the multi-use trail and the girders resting on the steel beams and will be supported in place during construction. See Sheet Schematic 2- Exhibit B. The overhanging beam under the girder at Abutment 4 would need to be modified to handle the larger loading. The anchoring of this beam would need to be upgraded and a post may need to be added under the end. There are existing pipe valves under the multi-use trail at this location. There is also a light pole adjacent to the pipe valves. All utility relocation and support in place will require close coordination with utility owners. This will assure timely review during design and minimum interruption during construction. Span 4: Span 4 of the existing walkway would need to be replaced with steel beams and new planking or grating. The existing abutment at the west end would be saw cut and the bridge seat would be widened to

3 accommodate the new walkway. The bridge handrails would tie into to the existing ones at the sidewalk on the west end. See Sheet Schematic 2- Exhibit C. DESIGN APPROACH Preliminary design of the multi-use trail structure included a live load of 90 pounds per square foot per AASHTO LRFD Guide Specifications for Design of Pedestrian Bridges, Article 3.1. In Spans 1 and 2, the loads in the paneling/grating transfer to the stringers, then on to the trapezoidal brackets. The brackets tie into the hangers and posts of the truss via steel angles and high strength bolts. In Span 3, the loads on the paneling/grating transfer to the overhanging steel beams, then on to the existing deck girder. Span 4 has a similar simple span to those on the brackets of Spans 1 and 2. ORDER OF MAGNITUDE COST Because of the nature of this study; the order of magnitude cost for this project which includes design, construction, construction support services, utilities and demolition is estimated with appropriate percentage for contingencies to be $1,550,000. CONCLUSION It is determined that widening the walkway on the San Lorenzo River Bridge by replacing the existing brackets of Spans 1 and 2, upgrading the beams in Span 3 and upgrading the stringers in Span 4 is feasible. The next steps would be: 1. Project scoping with stakeholders to establish parameters 2. Design survey 3. Develop preliminary level design schematics and obtain buy off from stakeholders 4. Design development and permitting 5. Construction

4 Picture 1- Aerial of MP San Lorenzo River Bridge Picture 2- Spans 1 & 2 Profile Looking South

5 Picture 3- Steel Brackets Supporting Existing Walkway at Spans 1 & 2 Picture 4- Timber Beams and Steel Deck Girder Supporting Existing Walkway at Span 3

6 Picture 5- Timber Stringers Supporting Existing Walkway at Span 4 Picture 6- Existing Wide Walkway at the West Abutment