Designing Suspention Bridge: The Tacoma Narrows Collapse

1916 Words Jan 31st, 2018 8 Pages
According to the Board of Experts, the pivotal event in the bridge’s collapse, was the change from the vertical waves to the destructive twisting, torsional motion. This event was associated with the slippage of cable bands at mid span. Normally, the main cables are of equal length where the mid-span cable band attaches them to the deck. When the cable band slipped, the north cable became separated into two segments of unequal length. The unbalance translated quickly to the thin, flexible plate girders, which twisted easily. Once the unbalanced motion began, progressive failure followed.
For the cable bands, the transverse contraction of the cable section is of the utmost importance. It causes the friction between cable bands and cable to decrease and, as a result, the load carrying resistance goes down. Precautions should be taken to measure the relaxation in the bolts and to tighten them during the service life.
Based on the maintenance histories of suspension bridges, it is recognized that the tension in cable band bolts decreases with time due to creep of the zinc coating on the wires and gradual compaction of the cable, therefore, it is desirable to monitor the in-situ bolt tensions periodically in order to assess the reliability against band slippage. A method to address the above specific concerns is by the use of non-destructive evaluation techniques…
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