OVERVIEW Our approach when examining options for the future West Seattle Bridge’s main bridge spanning over the river includes consideration of two types of structures. The first structure option involves replacement of the compromised superstructure of the main bridge utilizing a cast-in-place concrete segmental design (Baseline 1). This would be very similar to the existing structure type matching the same width. A comparison cost analysis has been performed on a precast variation of the above option (Baseline 2) but no significant economies were observed. Our proposed structure option (MT-Steel Design) is focused on replacing the West Seattle Bridge with a re-imagined mass timber and steel composite design.
Assumptions in both designs: • Assessment is relevant only to the structural unit of the main bridge spanning over the river (590 feet) and the two adjacent smaller spans (375 feet, ea.). No consideration has been given to any improvements associated with the approach span portion of the bridge. • The current bridge foundations and substructure for the three spans will be replaced in all options. Baseline 1 and 2 will require new, stronger piers and foundations to support a wider drive deck that includes LRT and bicycle lanes. The MT-Steel Design is a light-weight structure; new piers and foundations are less substantial than in the Baseline design structure
BASELINE 1: CAST-IN-PLACE CONSTRUCTION Cast-in-place is a technology of construction where structures are cast at the site in formwork. This differs from precast concrete technology (Baseline 2) where elements are cast elsewhere and then brought to the construction site and assembled. Following existing bridge demolition, the construction sequence would be as follows: • • • • •
Construct new foundations and piers at four pier locations. (Twin piers for Baseline 1 and 2) Install traveling form systems (8 required) on completed pier tables Construct balanced cantilever sections Construct 80-foot backspan tie in sections on falsework Complete joint connections and remaining deck works
Benefits: • the • •
General type and appearance of structure would mirror that which currently serves local community This design type affords long span crossings, thus limiting ground disturbance Process enables reduction in number of closures required in the maritime channel
This option costs less upfront, but the linear nature of cast-in-place construction yields slower overall production rates and is not preferred for projects requiring fast-track construction.
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