Posted by Alison Trost on December 3, 2010 at 9:39 AM.
Erection of the “gate” segments completing the 370-ft mainspan for the new IL-170 Bridge over the Illinois River in Seneca, Illinois. The existing four-span through-truss in the background was built in 1932 and was demolished by explosives on November 18, 2010.
The above photo shows erection earlier this year of the final three “gate” segments connecting the cantilevered ends of the six steel plate girders that complete the 370-ft mainspan for the new IL-170 Bridge over the Illinois River in Seneca, Ill. These “gate” segments which are 110-ft long and 9-ft deep are one of 11 segments required to create the 283-ft / 370-ft / 283-ft continuous steel plate girder unit over the river. Three smaller units consisting of 5-ft-deep plate girders and wide-flange sections complete the structure, which totals 1,700 ft between abutments.
Project challenges included reducing the depth of the plate girders by more than a foot to accommodate an increase in navigational clearance, while maintaining the established vertical profile of the proposed roadway. Other challenges included designing the river piers for barge impact and erecting the structure without shoring towers in the river to satisfy U.S Coast Guard requirements. Temporary lateral bracing between the new structural steel and existing truss was installed during erection to provide additional stability prior to hardening of the deck. The new structure uses six closely spaced plate girders of Grade 50W steel, with HPS Grade 70W steel over the piers. The total project cost around $24 million.
Photo and Text by Chris Stine, S.E., P.E.