Bridge over River Wa Umiam at Bara Pani

Existing two-lane NH-40 connecting Guwahati and Shillong cities is proposed to be four-lane. It crosses river Wa Umiam over the crest of a dam which cannot be widened to four lanes. New two-lane road shall be built crossing the river 165m down-stream of the dam over a 50m deep gorge requiring 110m long bridge. Constructability of bridge is the main issue as depth of gorge and perennial flow in river require overhead construction. A steel truss bridge of variable depth allowing through traffic with 20m end spans and 70m central span was provided. End spans of 20m could be erected from both sides by using props supported on ground. Central span was to be erected as 35m cantilever truss of 5m panels progressing from both sides and joined in middle. Piers and abutments were in RCC construction. The bridge design was checked for service stage loads and erection stages. Pre-cambers were provided for every stage of span erection.