About the research
The secondary road system in Iowa is vitally important to the movement of goods and people throughout the state. In some cases, secondary roads serve as feeders to the primary road system and then on to the National Highway System. In other cases, the secondary road system serves as a critical link for farmers as they move their crops.
Unfortunately, with approximately 20,000 bridges on the secondary system, county engineers are faced with the ever more difficult task of maintaining and replacing the bridges. While notable strides have been made in identifying more affordable bridge systems, there continues to be a need to lower replacement costs even more—especially for bridges in the shorter span lengths.
In the relatively recent past, a company based out of Omaha, Nebraska has developed a proposed bridge girder/deck section that may be a cost-effective option for county engineers. In very simple terms, the shape is a single T-shape that is approximately 8 ft wide, with a top flange/deck that is 7.5 in. thick, and has a stem that is 1 ft 5 in. wide at the narrowest point.
These individual sections are connected together using one of two proposed connection details: cast-in place ultra-high performance concrete (UHPC) or a bolted connection in combination with traditional joint fill materials. The proposed beam section appears to be easy to precast with no internal voids. Estimated weights for the cross-section are on the order of 19 to 31 tons for span lengths ranging from 40 to 60 feet. Additionally, the proposed section uses the commercially available, corrosion-resistant reinforcing steel known as MMFX, which has tensile strengths reported to be 100 ksi.
Preliminary cost estimates for producing these beams (not including materials) is on the order of just under $20,000 for four 52 ft long beams (enough for a 32 ft wide bridge). Anecdotal evidence suggests that the total cost of these beams may be even less than the low-cost railroad flat car option used by some counties.
Even in light of all of the positive attributes mentioned, a number of questions remain, some of which are more concerning than others. The most important question is centered on the previously mentioned connection details. While the use of connection details between adjacent precast concrete units is not new, in this particular case, ensuring the performance of the detail is particularly important because of two geometric features: 8 ft top flange width and single stem nature of the cross-section.
With the 8 ft top flange width, it is physically possible for a standard 6 ft wide truck to be placed entirely on a single beam. Thus, if the joint is not effective at distributing the load to other units, each single beam section must be capable of resisting an entire truck load. In a very worst-case scenario, should the joints not perform over the long-term, the single-T geometry may create a situation where a single beam becomes unstable, even if it otherwise has sufficient strength.
Buchanan County Iowa intends to build a bridge using the proposed shape during the summer of 2016. Thus, a unique opportunity exists to document the design, construction, and performance of this bridge. Furthermore, the fact that four beams will be fabricated offers the opportunity to have additional beams fabricated such that the connection detail can be tested in the Iowa State University Structural Engineering Laboratory.
The objective of this work is to evaluate the performance of this proposed short-span bridge alternative. This project will also document the design and construction of the bridge to provide valuable information to other counties interested in constructing similar bridges. In addition, laboratory testing will be completed to verify the efficacy of the proposed joint between adjacent precast beams.