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This installment of “A Bridge Moment” is the first in a series titled Everything You Always Wanted to Know About Concrete Box Culvert Bridges* (*But Were Afraid to Ask)

When is a box culvert a bridge?

Concrete box culvert bridges are one of the most common bridge types used by bridge engineers for water crossings, pedestrian tunnels, and wildlife crossings. The National Bridge Inspection Standards (NBIS) and most states define a bridge to be a structure that is longer than 20 feet measured along the centerline of the roadway. Therefore, a 3-cell box culvert structure consisting of 3 – 7 foot spans is a bridge.

What are the common types of concrete box culverts?

The majority of reinforced concrete box culverts are 4 sided rigid frames where the top and bottom slabs are cast monolithically with the side walls. Box culverts are constructed using cast-in-place concrete on site or cast in molds at the precast plant. The maximum span recommended by FDOT is 24 feet for 4 sided box culverts, but the author has designed several precast box culverts with spans up to 30 feet.

Multi-cell cast-in-place concrete box culvert bridge

FDOT requires precast box culverts to be constructed monolithically in one piece or segmented in 2 pieces with a U shaped bottom section and a separate simple span top slab. In other states and on private sector projects in Florida, precast 4 sided boxes are also supplied as 2 identical U shaped sections with a keyed joint in the center of the walls. ASTM C1786 – 18 provides standard specifications for segmental precast box culvert elements.