Phase 1: Development of County Bridge Standards for Single Span Concrete Slab Bridges TR-812

(2022) Phase 1: Development of County Bridge Standards for Single Span Concrete Slab Bridges TR-812. Transportation, Department of

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Abstract

Nearly 10.5% of all highway bridges in the United States are classified as concrete slab bridges according to the U.S. 2020 National Bridge Inventory (NBI). Concrete slab bridges are typically single spans (20-50 feet) or multiple spans for relatively short stream crossings. The Iowa Department of Transportation (Iowa DOT) currently has three-span continuous concrete bridge standards (J-series) with lengths between 70-150 feet, but there are no single span concrete slab bridge standards. The objective of this project was to investigate the current practices, need, benefits, economy, constructability, and design criteria for cast-in-place (CIP) single span concrete slab (SSCS) standard bridge plans. A preliminary analysis was conducted to determine slab thickness and reinforcement requirements for various spans and roadway widths to compare costs with other bridge types and concrete box culverts in Iowa. Examples of existing SSCS bridges and a summary of the bridge inventory for bridges less than 70 feet were presented to show the extent of short span bridge design and use in Iowa. Currently, Iowa has a lower inventory of concrete slab bridges (5.1%) compared to other bridge types less than 70 feet in length, such as concrete box culverts (33.1%) and steel stringer bridges (25.0%). The cost analysis shows that SSCS bridges with integral abutments have about the same costs per square foot of deck area (avg. $195/ft2) compared to standard concrete box culverts with 4 foot tall sidewalls (avg. $203/ft2) and steel stringer bridges with integral abutments (avg. $194/ft2) but are less expensive than standard precast box beam bridges with integral abutments (avg. $235/ft2). SSCS bridges with high abutments are less expensive (avg. $294/ft2) than standard concrete box culverts with 12 foot tall sidewalls (avg. $431/ft2) and standard precast box beam bridges with high abutments (avg. $314/ft2). SSCS bridges have several advantages compared to concrete box culverts. Based on a survey submitted to Iowa county engineers, SSCS bridges have less right-of-way requirements, reduced streambed disturbance, and may more easily satisfy Army Corps of Engineers 404 permit requirements. Other advantages identified in the survey include improved hydraulic performance and less siltation/debris. Disadvantages include the need for a guardrail, longer construction, and potential maintenance/durability issues. It is recommended to proceed with final design and development of the SSCS standard bridge plans following the design criteria recommendations provided in this report. The Iowa DOT Bridges and Structures Bureau (BSB) will maintain oversight and updates for the SSCS standard plans. The standards should be published to the BSB website to make them available to the county engineers. Based on the findings, future work is recommended, as outlined at the conclusion of this report.