Performance Evaluation of Inverted Tee (IT) Bridge System

In the state of Nebraska, the inverted tee (IT) girder system accounts for over 100 bridges. The IT girder system is an efficient construction technique for short to medium spans (up to approximately 80 feet), and was originally developed by University of Nebraska-Lincoln (UNL) and Nebraska Department of Roads (NDOR) engineers in 1996 (Kamel and Tadros, 1996; Jaber, 2013). Advantages of the IT girder system compared to traditional cast-in-place (CIP) slab systems include: 1) reduced costs and ease of construction due to its reduced weight; 2) shorter construction times (and roadway downtime) as no temporary formwork is required; 3) increased span-to-depth ratio; and, 4) increased structural capacity on a superstructure rehabilitation project when the depth is constrained. As a result, IT girder bridges have been successfully utilized in bridge designs throughout the state for both the state highway and local (county) systems. Current design practices aim to establish an efficient and quick construction of IT bridges. The selection of IT girder heights varies from 13.3 to 36.9 inches (IT 300 to 900), however there currently are no IT 900 girders in place within Nebraska. Current design specifies a maximum of 22 - 0.5 inch prestressing strands per girder. Before the concrete is placed, stay-in-place forms are utilized consisting of ¾ inch plywood sheets for a maximum girder spacing of 37 inches for construction ease. The cast-in-place deck is often 6 inches thick with a single layer of reinforcement. This reinforcement is typically specified as No. 5 bars spaced at 6 and 10 inches for the transverse and longitudinal directions, respectively. The castin-place deck is then completed using concrete with 28-day compressive strength of 4 ksi. Deck thickness shall be 8 inches with two layers of reinforcement on interstate bridges or when 42 inch NU rail is used. Despite its efficient construction, the flexibility of the IT girder system prior to deck placement (particularly for smaller sections) has caused some challenges during construction. Also, the use of single layer of reinforcement in a 6 inch thick deck may result in increased cracking as bridge ages especially in the transverse direction over the piers when the system is made continuous for live-load (Ambare and Peterman, 2006; Larson et al., 2013). Also, the effects of girder section, span, spacing, deck thickness, and skew angle on the distribution of live load are not fully understood. While IT girder systems have become widespread in Nebraska, little knowledge exists regarding their degradation and performance due to the relatively young age of these bridges. Therefore, a field assessment of existing IT system bridges is warranted to assess the current health of these structures and make recommendations for maintenance and future designs. For example, an updated design recommendation in Kansas incorporated post-tensioned strands to increase the systems durability (cracking) and span-to-depth ratio (further shape refinement) to combat some of the aforementioned issues (Nayal et al., 2007). Herein, the purpose of this proposed study is to assess the performance of this relatively young system to determine its structural performance, constructability, durability, and economy compared to other systems, such as slab and NU-girder bridges. This assessment will also recommend whether further enhancements are needed to improve the IT system performance. To accomplish these tasks, the current condition and cost data of all IT bridges will be collected and a detailed visual inspection of a large pool of IT bridges across the state of Nebraska will be conducted. Several interviews will be conducted with bridge producers from Precast Concrete Association of Nebraska (PCAN) and bridge contractors from Associated General Contractors (AGC) to understand their experience with IT production, as well as to collect their feedback on construction. Analytical investigations will be also conducted on select IT designs to further understand the effects of certain variables, such as: deflection under construction loads, live load distribution, loading of exterior girders, and re-decking of IT bridges.