When the Iowa DOT set out to replace the eastbound bridge over the Cedar River in Linn County, the challenge was clear: build something better, stronger, and longer-lasting without putting everyday drivers through excessive delays. Schemmer was brought in to provide final design services for the eight-span, 1,134-foot-long PPCB (precast, prestressed concrete beam) bridge, and we got to work with that goal in mind.

To keep traffic moving as long as possible, our team designed drilled shafts for the piers that could be installed outside the footprint of the existing bridge, saving as much as six months of traffic disruption. That’s a big deal for local drivers, nearby communities, and the flow of goods along this important corridor.  Instead of the typical three or more drilled shafts per pier, we designed for just two to facilitate the installation while the existing bridge remained in service.

This unique substructure configuration meant a deeper dive into structural modeling and performance for the large-diameter drilled shafts:

• Schemmer performed a resiliency study, determining the most economical configuration for the drilled shafts for the high flood and ice loading common in the Cedar River.  We ultimately designed for about 10% excess capacity for the channel shafts to ensure this bridge had similar resilience to the typical structure that has additional supporting elements.
• With drilled shafts spaced 45 feet apart to span the existing bridge, the pier design called for heavier reinforcement across the shafts, columns, and pier caps to support the long-span BTE beam superstructure.
• With an eye on ease of construction to also aid in narrowing the required construction window, Schemmer modeled and detailed the reinforcing in the piers to avoid conflicts in the field, which helped eliminate RFIs and allowed for the pier cap reinforcement to be pre-assembled and lifted into place in one piece, further decreasing construction time.

This project is a strong example of how smart design and early collaboration can ease construction impacts and keep infrastructure working for the people who rely on it every day.