Context-Sensitive Designs: Testing of Multi-Performance Level Box Beam Standards
Start date: 09/01/14
End date: 06/30/17
- Federal Highway Administration State Planning and Research Funding
- Iowa Department of Transportation
- Iowa Highway Research Board
About the research
Adjacent concrete box beam bridges constitute more than 15% of bridges built or replaced each year. This type of bridge is generally constructed by placing box beams next to one another, grouting a shear key, applying a transverse post-tensioning force, and then placing either a thin (~3-in.) wearing surface or a thick (~6-in.) structural deck. In some cases, the top of the box beams are left bare to serve as the riding surface. These bridges are attractive because of their relatively shallow superstructure depth, ease of construction, and simple aesthetic attributes. Adjacent precast, prestressed box beam bridges have been used by multiple departments of transportation (DOTs) with varying levels of success.
Historically, these, and other similar adjacent precast elements, have suffered from differential displacements, which cause cracking in adjoining joint material (or, in some cases, cast-in-place topping material). Sources of differential deflection can come from a variety of conditions including live loads, temperature effects, and others. Generally, these reflective cracks in-and-of themselves do not pose a safety hazard. However, these cracks provide a direct path for water (plus chlorides) to enter the structural system causing corrosion of the mild and prestressing steel. Ultimately, this situation can lead to significant maintenance costs and/or safety concerns. Some early users of adjacent box beams now only allow them on low-volume roads where salt application does not occur.
Even with the known issues associated with adjacent box beams, they can still result in an economical short- to medium-span bridge that is generally quick and easy to construct. For example, the Missouri DOT (MoDOT) recently completed their Safe and Sound bridge replacement program in which 554 bridges were replaced over three construction seasons. The design/build team made extensive use of adjacent box beams, constructing more than 170 bridges using this system, for spans up to 90 feet in length.
The Iowa DOT (in principal cooperation with HDR, Incorporated) has been working to develop a new set of bridge standards particularly targeted toward use by counties. As is widely known, counties have a large number of bridges on their system that must be constructed and maintained. As such, they are increasingly in need of low-cost bridge concepts. For many counties, the ideal concept is one that could be constructed using county forces. For counties to be able to fabricate and install bridges themselves, it seems clear that the bridge concept cannot be prestressed and the weight must be reasonable.
During a meeting on May 2, 2014, interested parties (DOT, Federal Highway Administration/FHWA, designers, counties, and academia) discussed possible concepts for the desired bridge standard. Because the decision had previously been made (based upon preliminary work completed by HDR) to use a box beam shape, the discussion principally centered on needs associated with this concept. Of particular importance was information presented by Ben Graybeal of the FHWA who has been conducting testing on adjacent box beams at the Turner-Fairbank Highway Research Center. This work has resulted in a connection detail that appears to perform well. The main drawback associated with it (from a county perspective) is the fact that ultra-high performance concrete (UHPC) is needed. This material tends to be very expensive and requires a high level of expertise for proper mixing, placement, etc.
Thus, it was discussed that perhaps work could be done to develop alternatives that would have sufficient performance when placed in the right location, which is frequently called context-sensitive design. The idea is that counties could select the desired level of performance based on the need for the site. For example, a county highway with a high traffic volume might need the highest level of performance (the UHPC alternative); whereas, a bridge on a gravel road with fewer than 50 vehicles per day would not require such performance (e.g., due to the lack of deicing salts, etc.).
Although there was not full agreement, the idea of three performance levels was discussed. We'll call them Level I, Level II, and Level III. Level I performance—the highest—would basically constitute the connection system tested by the FHWA. Level II performance—the middle—would use a connection detail similar to the Level I system but the UHPC would be replaced with a more widely available material. Level III performance—the lowest—would be a concept where the adjacent box beams are not connected at all.
This work seeks to conduct testing and other work to evaluate details deemed important to the Level II and Level III concepts. The objective of the project is to provide information on how the two performance-level box beam concepts will perform by conducting on-site inspections of two box beam bridges previously constructed on the Iowa county road system, laboratory testing of drip edge details important to the performance of the Level III concept, and laboratory testing of two potential Level II connection details.