Bridge Design
The bridge evaluation procedure for this feasibility study involved trail or path bridges adjacent to railroad bridges. No evaluation was performed on the highway bridges where trail segments have been recommended on roadways.
Photographs were taken of all bridge locations during the field survey work. One of Rizzo Associates’ structural engineers visited the sites of several bridges along the 132 miles of study corridor.
New trail bridge design concepts were developed using the following procedure:
| 1. |
The trail bridge span over a waterway was estimated to be the span of the adjacent railroad bridge plus 20 feet (10 feet on each end of the span). The 20 foot criteria was applied to reduce environmental impacts, to prevent construction directly in the river or stream waterway, and to allow construction of substructures on railroad embankments out of water and wetlands. |
| 2. |
Bridge approach spans and/or retained embankments were estimated by observing photos and the inventory mapping prepared for this study with its wetland overlay. Lengths and types of approach structures (bridge or retaining walls, see Section I) were established by considering the height of railroad embankment and the 15-foot requirement for separation of rail and trail, together with the length of wetlands adjacent to each site as indicated on the mapping. |
| 3. |
For purposes of this study, bridge types were considered by spans as follows: |
0 – 15 foot spans – Single span, treated timber, standard dimension lumber, with timber crib type abutment. (see Bridge A)
16 – 35 foot spans – Multiple span, treated timber, standard dimension lumber, or laminated timber superstructure components with timber crib or timber pile bent substructures. (see Bridge A)
36 – 90 foot spans – Prefabricated metal superstructure, or steel or prestressed concrete girders with timber or concrete decks, reinforced concrete on piles or spread footing substructure component. (see Bridges B and C)
90 foot plus spans – Prefabricated metal superstructure and reinforced concrete on piles substructure component. (see Bridge D)
| 1. |
Estimated costs were developed for square feet of bridge deck to account for superstructure and substructure costs for a bridge and square feet of exposed surface was developed for retaining walls (see List of Trail Bridges in Cost Estimate Chapter).
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| 2. |
Items that will assist in refining these cost estimates during a design phase of a trail segment include borings and soil exploration; hydrologic and hydraulic reports; accurate field survey; wetland identification and mapping; floodplain/floodway studies on each stream or river; engineering bridge type studies; and assessment of construction access and construction sequencing. |
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