Michael G. Barker, P.E., a professor in the College of Engineering and Applied Sciences at the University of Wyoming, collaborated with Missouri bridge engineering consultant, John Mann, P.E., to perform a true "apples-to-apples" comparison of superstructure costs for steel versus precast concrete in short span applications. The study also included the total construction costs for the project.
"We have one steel beam bridge and one hollow core slab precast concrete bridge that are nearly identical in all aspects," said Barker. "They were both built in 2012 with the same location and topography and with nearly identical roadway length and width, abutments, structural depth, and guardrail systems. Even the same local work crew was used to build both bridges."
The results? In a side-by-side comparison of construction square footage costs, the steel short span superstructure provided a 25.8% cost savings, with an overall 19.3% savings in the total cost of the structure. Here's a look inside the study.
Built in 2012, Audrain County Steel Bridge 411 has a 47.5-ft span and 24-ft roadway width with 2-ft structure depth plus slab and no skew. The basic superstructure design consists of four weathering steel stringers fabricated by Oden Enterprises in Wahoo, Nebraska. The total cost for the project was $111,853 ($97.48 per sq ft).
Also built in 2012, Audrain County Concrete Bridge 336 has a span of 50.5-ft with 24-ft roadway width and 2-ft structural depth on a 20-degree skew. It consists of six hollow core precast slab girders. The total cost for the project was $154,035 ($120.83 per sq ft). Concrete was selected in this case because the county engineer believed the structure would experience occasional water inundation and assumed that concrete would allow the bridge to reopen to traffic sooner. In both cases, Audrain County engineers tracked detailed costs for all components of the bridges, from the beginning of design to the end of construction. All superstructure-only costs were tracked separately for the "apples-to-apples" comparison.
When all of the actual costs were tallied, the short span steel bridge superstructure-only construction savings were 25.8% in comparison to the precast concrete superstructure. The steel Bridge 411 superstructure cost—including material (girders, deck panels, reinforcing steel and concrete), labor, and equipment—came to $37.54 per sq ft. The precast Bridge 336 superstructure costs—which included material (slab girders, reinforcing steel, and concrete for parapet walls and grout), labor, and equipment—came to $50.61 per sq ft.
Mann noted that the lower price per square foot for the steel bridge was driven by the fact that the steel girders were less than half the price of the slab girders. Crane costs also created some of the savings.
Another advantage to steel is the potential use of simple Geosynthetically Reinforced Soil (GRS) bridge abutments to handle lighter loads. In the comparison above, the county could have saved additional dollars on the project if the abutments had been designed for the lighter steel bridge. GRS abutments are innovative foundation systems available at a lower cost than other conventional foundation materials. The installation process is simple and they can be rapidly constructed—in some situations, in five days or less.
According to the Short Span Steel Bridge Alliance (SSSBA), there are many benefits in using steel in the construction of all types of crossings. According to Mike Engestrom, chairman of the SSSBA, steel provides sustainable, accelerated, durable and costeffective design solutions for engineers, architects, builders, code officials and other construction professionals.
Audrain County has a long history of selecting steel for bridge construction. According to Mann, "In our experience, a short span bridge in the range of 50 ft is almost always the best value when constructed of steel. At 70-ft. or longer, we'll perform a more in-depth analysis to evaluate variables, such as proximity to the precast plant and overall cost. There are some cases where we consider precast hollow core slabs for a super-fast turnaround, but we know we'll have to pay a premium. Overall, I've found that steel is the better buy."
Since 2008, Audrain County has constructed five short span steel bridges with an average length of 53 ft for an average total project construction cost of $86.09 per sq ft In that same time, the county has constructed four concrete bridges with an average length of 37.5 ft for an average total project cost of $96.32 per sq ft.